CA2094183A1 - Atmospheric pressure calibration systems and methods - Google Patents

Abstract

ABSTRACT

Systems are provided to derive a value of barometric pressure at a defined location in the atmosphere at a computed geometric height. By using the geometric height in a look-up table of pressure/height values representing a standard atmospheric profile, atmospheric deviation data indicative of the difference between measured and reference pressures at that atmospheric location (C) is derived.
Ground-based interrogators (10,20) located at spaced positions are used to initiate response signals from airborne transponders commonly installed in transient aircraft (C).
Using resulting range data based on round-trip timing differences in signals sent to (12,16) and received from (16,14) the airborne transponder (C), geometric analysis and computation is used (18) to determine the geometric height of the transponder representing a specific atmospheric location.
The height, together with barometric pressure data transmitted by the airborne transponder, are used (28) for the look-up of reference pressure data and derivation of atmospheric deviation data for that atmospheric location.
Systems and methods provide such data for inflight altimeter calibration, height determination, atmospheric pressure profile development, weather forecasting, transponder calibration and other purposes.

Description

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Dockat R4476 E40:cf 1ATMQSPHERIC PRESSURE CALIBRATION SYSTE~S AND METHODS

2This inven~ion relates to determination of 3 atmospheric pressure deviation at points in the atmosphere by 4 interrogation of equipment commonly present in transient aircraft. Such atmospheric deviation data is useful for 6 calibrating th~ atmosphere against a stanclard atmosphere, 7 determining aircraft height from indicated barometric 8 altitude, calibrating barometric altimeters, weather 9 forecasting and other purposes.

BACKGROUND OF THE INVENTION

11 Systems exist for measuring the height or 12 altitude of an aircraft. An airborne radar altimeter can be 13 used to measure geometric height (i.e., distance between the 14 aircraft and ground). An airborne barometric altimeter can be used to provide a measure of barometric altitude (i.e., an 16 estimate of altitude above mean sea level based on comparing 17 measured barometric pressure to a standard atmosphere value).
18 However, even if such measurements are 19 accurately performed, the radar altimeter provides readings during level flight which will vary widely depending on 21 whether the aircraft is passing over a valley or a mountain.
22 While a ground-based precision radar might be used to 23 accurately determine height of a passing aircraft above a 2`~ 8 3 1 defined datum level (based on xadar measurements adjusted for 2 the elevation of the radar site), such radars are expensive 3 and not available in many geographical areas. Similarly, even 4 if a barometric altimeter accurately measure3 barometric pressure and converts the pressure reading to a corresponding 6 altitude, such conversion merely provides an altitude value 7 from a pressure/altitude chart or table representing standard 8 atmosphere data, such as provided by the International Civil 9 Aviation Organization ~"ICAO"). Fig. 1 is an example of such a chart, in which P i9 a scale of pressure in millibars, ~ is 11 a scale of al.titude in thou~ands of feet and S is a standard 12 ICAO pressuxe/altitude profile for a temperature lap~e rate 13 of two degrees centigrade per one thousand feet and a 14 temperature of 15 degrees centigrade at mean sea level (MS1).
A problem in using such charts is that an aircraft does not 16 fly in a standard atmosphere, but in the real atmosphere 17 which is subject to temporal and spatial weather differences 18 affecting the barometric pressure measured at any aircraft 19 altitude. As a result, since there will virtually always be a discrepancy between the actual pressure as measured at the 21 aircraft location and the standard pressure for the aircraft 22 elevation, there will virtually always be a discrepancy in a 23 barometric altimeter reading.
24 Thus, even with all equipment accurately calibrated, a radar altimeter can provide relative height 26 above the ~arth's surface, but when operating over land it 27 cannot reliably measure height above a reference datum like 28 MS~. Also, a barometric altimeter measures altitude based on 2~133 1 sensing of barometric pressure, but pressure varies in an 2 unpredictable manner for a given geometric height and does 3 not provide a repeatable reference relative to a datum like 4 MSL. The preceding discussion does not address calibration difficulties and resulting error. For example, a barometric 6 altimeter on an aircraft must be arranged to attempt to 7 measure static pressure in a moving air stream subject to 8 variations in aircraft speed and altitude, temperature and 9 humidity, and subject to possible changes in aircraft con~iguration, damaged or blocked sensors, and baromet;er 11 decalibration over time with no adequate means of 12 recalibration.
13 A ~pical practical problem is the requirement 14 for continuous data acquisition to permit evaluation of changing weather conditions or control oF civilian air 16 traffic, weather forecasting and a variety of other civilian, 17 commercial and military applications. For weather 18 forecasting, as well as for air traffic control, it will be 19 apparent that there is a continuing need for current clata on actual atmospheric conditions at different geometric heights 21 on a local, national and global basis. While many types of 22 relevant data can be gathered, and sophisticated analysis and 23 plotting of data can be provided, one particular need has 24 continued unanswered. That is the need to know, for different geometric heights at different geographic points on 26 a continuing basis, how the measured barometri~ pressure at a 27 particular height above MSL, for example, differs from a 28 standard barometric pressure for that height abo~e MSL.

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1 That difference between measured barometric 2 pressure and a re~erence barometric pressure, for that 3 geometric height, location and time, can be termed an 4 "atmospheric deviation~. It can be shown that if accurate S atmospheric deviation data could be made available it would 6 be valuable for many purposes. Weather fore~asters, with 7 knowledge of the atmospheric de~iation between currently 8 measured and reference pressure values, can analyze 9 atmospheric conditions and forecast developing conditions.
Air traffic controllers can apply derived information 11 regarding changes in vertical separation of Elight paths 12 resulting from atmospheric pressure changes. Aircraft fllght 13 crews can be supplied with altimeter calibration information 14 and data correlating barometric altitude with geometric height~ Atmospheric deviation data may also be applied in 16 current calibration of aircraft landing systems, for 17 monitoring developing conditions which may identify wind 18 shear in the vicinity of airports, and for a variety of other l9 civilian, commercial and military purposes. In Fig. 1, curve C is a representation of the use of atmospheric deviation 21 data to calibrate the atmosphere against a standard 22 atmosphere represented by curve S. Thus, with availability 23 of accurate deviation data, it would become possible to 24 provide profile C based upon differences between standard or reference pressure and pressure values based on current 26 barometric measurements, at different altitudes.
27 While instrumented weather balloons 28 (radiosondes), as well as highly-equipped dedicated aircraft, 2~4~l~3 1 have been used to gather atmospheric data, these and other 2 existing devices and systems have been expensive, inaccurate 3 and/or used only at a few geographic locations, so that 4 sufficient quantitiss of current, accurate data have not been available. Thus, it should be noted, that regardless of what 6 forms of theoretical and other systems have been proposed or 7 implemented, a need has continued to exist or a practical, 8 accurate and economical system able to provide a continuing 9 volume of currently updated atmospheric deviation data for dispersed geographical areas of interest, without 11 necessitating specially equipped or dedicated aircraft, 12 development of new forms of equipment or new types of ground 13 installations.

14 SUMMARY OF_THE INVBNTION

In accordance with the invention, a system for 16 deriving atmospheric deviation data for a location in the 17 atmosphere, in cooperation with an airborne transponder which 18 provides response signals including current data based upon 19 barometric pressure, includes one or more transmitting means for transmitting first signals to the airborne transponder 21 and a plurality of receiving means, positioned at spaced 22 positions, for receiving response signals transmitted by the 23 airborne transponder from an atmospheric location in response 24 to such first signals. The response signals include current data based upon barometric pressure in the vicinity of the 26 atmospheric location. The system includes signal processing ~o9~3 1 means, coupled to the receiving means and responsive to 2 timing differences between the transmitting of such first 3 signals and receiving of response signals, for utilizing 4 range data derived ~rom such timing differences and geometric data regarding the spaced positions of the receiving means 6 ~or deriving data representative of geometric height of the 7 atmospheric location. Comparison means are included for 8 utilizing the data representative o~ geometric height and the 9 current data based upon barometric pressure for deriving atmospheric deviation data representative of deviation 11 between such current data based upon baxometric pressure and 12 barometric reference data applicable to the atmospherLc 13 location.
14 Also, in accordance with the invention, a method for deriving atmospheric deviation data for a 16 location in the atmosphere, in cooperation with an airborne 17 transponder providing response signals including current data 18 based upon barometric pressure, comprising the steps of:
19 (a) transmitting first signals to the airborne transponder;
21 (b) receiving, at a plurality of spaced 22 positions, response signals transmitted by the airborne 23 transponder from an atmospheric location in response to such 24 first signals, the response signals including current data based upon barometric pressure in the vicinity of such 26 atmospheric location;
27 (c) utilizing range data derived from timing 28 differences between the transmitting of such first signals in ~4~8~

1 step (a) and the receiving of response signals in step (b), 2 with geometric data regarding such spacecl positions, for 3 deriving data representative of geometric height of the 4 geometric location;
~d~ comparing geometric height as derived in 6 step (c) with the current data based upon barometric pressure 7 as rsceived in step (b) from the airborne transponder; and 8 (e) utilizing the results of the step (d) 9 comparison to derive atmospheric deviation data representative of deviation between current data basecl upon 11 barometric pressure and atmospheric reference data applicable 12 to said atmospheric location.
13 For a better understanding of the invention, 14 its operating advantages and specific objects attainecl by its use, reference should be had to the accompanying drawings and 16 descriptive matter in which there is illustrated and 17 described a preferred embodiment of the invention.

19 Fig. 1 is a chart showing a standard pressure/altitude profile at S and an illustrative current 21 calibration profile derived using atmospheric deviation data, 22 as shown at C.
23 Fig. 2 is a block diagram of a system for 24 deriving atmospheric deviation data in accordance with the invention, including two ground stations for interrogation of 26 an airborne transponder.

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1 Fig. 2A is a block diagram illustrating a 2 specific emhodiment of interrogator unit 10 of Fig. 2.
3 Fig. 3 is a block diagram of a system in 4 accordance with the invention, which includes three Fig. 1 systems arranged to derive atmospheric deviation data for a 6 wider geographic area.
7 Fig. 4 is a block diagram of a system in 8 accordance with the invention including three stations at 9 spaced locations.
Fig. 5 is a block diagram of a system in 11 accordance with the invention showing certain specific sub-12 systems.
13 Fig. 6 is a one-dimensional conceptual diagram 14 useful in describing the invention.
Fig. 7 is a two-dimensional diagram, useful in 16 describing the invention.
17 Fig. 8 is a three-dimensional di~gram useful 18 in describing the invention.
19 Fig. 9 is a diagram useful in describing a two ground station system in accordance with the invention.
21 Fig. 10 is a diagram useful in describing a 22 three ground station system in accordance with the invention.

24 A simplified block diagram of a system for deriving atmospheric deviation data in accordance with the 26 invention is shown in Fig. 2. A basic objective in use of 2 ~ 3 1 the system is to derive deviation data representing the 2 difference, at or about a point in time, between measured 3 ~arometric pressure and reference or standard atmospheric 4 pressure given by a standard pressure/altitude chart or data, for example. By determining such deviation between 6 measured and reference pressure at a specific atmospheric 7 location (i~e., in the vicinity of a point at a known 8 geometric height above a reference datum level at an 9 identified geographic position~, data can be made available -for many uses. For example, an aircraft crew can be advised 11 of the relation between barometric altimeter readings and 12 actual geometric height. Also, by gathering such atmospheric 13 deviation data ~or a slgnificant number of altitudes and 14 geographic areas, the data can be used for weather forecasting and generation of current pressure/altitude 16 profiles.
17 The Fig. 2 system is arranged to derive 18 atmospheric deviation data in cooperation with an airborne 19 transponder carried by the aircraft shown at point C.
Civilian, commercial and military aircra-ft commonly carry 21 transponders used for identification purposes. These 22 transponders are arranged to automatically transmit response 23 signals, commonly including barometric altimeter data, upon 24 being interrogated by a signal from any interrogator able to transmit the proper form o-f signal for this purpase. Such 26 transponders are commonly provided and used for applications 27 including air traffic control and military Identification 28 Friend or Foe (IFF) systems. Thus, while the Fig. 2 system 2 ~ 3 1 operates in cooperation with the airborne transponder carried 2 in the aircraft at point C, in fact, the transponder responds 3 automatically with no requirement for action by the aircraft 4 crew, so long as the transponder is turned on. In this way, the invention makes use of this automatic response including 6 barometric altimeter data, which is widely available from a 7 variety of transient aircraft flying at different altitudes 8 over varying geographic locations at many hours of the day 9 and night. In accordance with the invention, a wide range of atmospheric deviation data can be made available using 11 existing types of equipment, including ground-hased 12 interrogator equipment already in operation at many 13 locations, without requiring any dedicated aircrat ~lights 14 or new airborne equipment.
The system, as illustrated in Fig. 2, includes 16 transmitting means for transmitting first signals and 17 receiving means positioned at known spaced positions for 18 receiving responss signals. As shown, the system includes 19 two interrogator sets lO and 20, each having a transmitter, a receiver and an associated antenna, shown as units 12/ 14 and 21 16, and 22, 24, and 26, respectively. Interrogators 10 and 22 20, which may be identical units of the commonly used AN/TPX-23 54(V~ military Interrogator Set or other suitable equipment, 24 include transmitter circuitry shown as transmitter 12, for example, for sending first interrogation signals to the 26 airborne transponder located at point C in Fig. 2. Upon 27 receiving a properly encoded interrogation signal, the 28 airborne transponder transmits response signals, which 2 ~ 8 3 1 include current altitude data based upon barometric pressure 2 as sensed by the barometric altimeter of the airaraft and 3 converted to barometric altitude data, and may also include 4 aircraft identification and other data. Interrogator 10 also includes receiver circuitry shown as receiver 14, which is 6 arranged to receive the response signals by use of an antenna 7 16, which is used for both first signal transmission and 8 response signal reception in an appropriate shared manner.
9 In this embodiment, units 10 and 20 are identical and each independently interrogates and receives response signals from 11 the airborne transponder at point C, via the respecti~e 12 antennas 16 and 26. It will be appreciated that 13 interrogators such as 10 and 20, for example, may be 14 positioned at ground locations whose geographia positions, elevations and separations are known on the basis of 1~ appropriate surveying techniques, or may be positioned on 17 ships whose positions are known by use of available satellite 18 location systems, or may be otherwise positioned as 19 appropriate. Interrogators may also be positioned on aircraft, such as U.S. Air Force AWACS airborne warning and 21 control system aircraft, remaining aloft in an assigned 22 station area for extended periods of time, which are equipped 23 for position determination by satellite location systems and 24 radar altimeters, for example.
The system, as shown in Fig. 2, also includes 2~ signal processing means 18 coupled to the transmitters and 27 receivers of interrogators 10 and 20. Signal processing 28 means 18 is responsive to the timing difference between the 2 ~ 8 3 1 transmitting of a first signal by transmitter 12 to the 2 transponder at point C and the receiving by receiver 14 of a 3 response signal from the airborne transponder, such timing 4 differences being representative of the slant range to point C. The signal processing means 18 is thus effective to 6 utilize range data derived from timing differ~nces ~the 7 round-trip time required to send and receive a signal) from 8 each of the interrogators 10 and 20, which independently 9 interrogate the transponder in close time proximity with only relatively slight movement of the aircraft between the times 11 of such independent interrogations. The signal processing 12 means 18 also utllizes geometric data regarding the spaced 13 positions of the receivers 14 and 24. For this purpose, the 14 positi~ns of receivers 14 and 2~ are effectively the positions A and B, of the respective antennas 16 and 26, 16 which are spaced apart by the known distance E. Data as to 17 the positions, elevations and spacing of the receivers may be 18 coupled to signal processing unit 18 with the data 19 r~presentative of slant ranges or, where receivers such as 12 and 14 are fixed at predetermined positions, may be initially 21 stored in unit 18 for use as required. By conversion of the 22 timing differences, representing round-trip signal 23 transmission time, into the distances travelled by 24 electromagnetic signals in such time periods, the distances to the aircraft, shown as ranges Rl and R2 at the time of 26 signal transmission, can be determined. Aircraft 27 identification data included in the response signals enables 28 the identification and correlation of data received from 21D~183 1 speci~ic aircxaft. As shown, the input data supplied to 2 signal processing means 18, as described above and which may 3 be adapted for both signal and data processing, is coupled 4 from units 10 and 20 via input means, shown as terminals 17 for coupling input data to unit 18.
6 Antennas 16 and 26 typically are constantly 7 rotating antennas used for interrogating all aircraft within 8 a volume of airspace around the antenna position, permitting 3 information also to be made available as to the azimuth angle at which the response signals were recsived from the airborne 11 transponder. In Fig. 2, the angles ~1 and ~2 represent the 12 angles at which response signals are received by receivers 14 13 and 24, respectively, from the airborne transpondex at point 14 C which, as shown, is directly above point D in the vertical direction. As will be further described below, the geometric 16 data regarding the predetermined spacing E of the receivers 17 14 and 24 positioned at points A and B, the distances Rl and 18 R2 and the angles ~1 and ~2~ are utilized in signal processing 19 means 18 to derive data representative of the geometric height of the transponder, i.e., the vertical height H of the 21 point C which is the atmospheric location of the aircraft 22 when the transponder response signals were transmitted.
23 The Fig. 2 system for deriving atmospheric 24 deviation data also includes comparison means, shown as data comparison unit 28, for deriving atmospheric deviation data.
26 Comparison me~ns 28 utilizes the geometric height data from 27 signal processing unit 18 and current data based upon 28 barometric pressure in the form of the previously discussed 2~94183 1 barometric altimeter data provided by the airborne 2 transponder, as included in the response signals~ It will be 3 appreciated that since the data comparison unit 28 now has 4 available to it both the geometric height H of the atmospheric location C, as well as current data hased upon 6 the barometric pressure as measured at point C, a variety of 7 forms of atmospheric deviation data can be provided. A
8 comparison of geometric height and indicated barometric 9 altitude provides a deviation factor usable to convert barometric readings to approximate geometric height. Since 11 the barometric altimeter reading from the aircraft represents 12 a barometric pressure reading conversion to altitude based 13 upon data such as found in a standard pressure/altitude 14 table, knowledge of the actual geometric height and its respective standard pressure value permits development of a 16 factor representing the deviation between measured barometric 17 pressure and standard atmospheric pressure. Such factors are 1~ usable for weather forecasting and atmospheric profiling, for 19 example. Similarly, if the airborns transponder is configured to provide the current data in the response 21 signals in the form of actual barometric pressure readings 22 (rather than altitude data based upon barometric pressure~, 23 the geometric height data could be used to access atmospheric 24 pressure/altitllde reference profile data to provide a reference pressure value to be compared to the measured 26 barometric pressure to derive a current pressure/standard 27 pressure factor usable for weather forecasting, atmospheric 28 profiling and other purposes.

2 ~ 8 3 1 It should be noted that the even-numbered 2 elements 10-28, as described, effectively comprise a basic 3 system for deriving barometric deviation data which can be 4 used alone or in combination with other similar or comparable systems at separated geographic sites to develop data 6 covering a more extensive geographical area.
7 As illustrated in Fig. 2, the system also 8 comprises profile means, shown as profile data unit 32, 9 coupled to the comparison unit 28, for using the atmospheric deviation data for deriving atmospheric profile data. The 11 atmospheric profile data derived by unit 32 may be used in 12 the form of data relating to one or more iso:Lated atmospheric 13 locations, or may be combined with other data to provide 14 comprehensive profiles ~or a gaographic local, national or global area, depending on actual requirements and 16 availability of data. Government and other weather bureaus 17 and atmospheric study and analysis groups already employ 18 capabilities for generation of atmospheric profiles and for 19 other applications and it is a primary objective of the invention to provide practical and economical sources of 21 data, rather than to address specific end-use implementations 22 which can be carried out by those skilled in such fields.
23 The Fig. 2 system is also shown as including 24 data distribution means, shown as data distribution unit 34, for transmitting to an aircraft, via antenna 36, an altitude 26 correction factor usable for conversion of barometric 27 altimeter readings to approximate geometric height.
28 Distribution unit 34 can be configured to the particular 2 ~ 3 1 application. With connection to data comparLson unit 28, 2 distribution unit 34 can be used to provide a specific 3 correction factor to an aircraft which has just provided 4 response signals including barometric altimeter data. With connection to pro~ile unit 32, distribution unit 34 can be 6 used to provide general barometric altitude correction 7 factors, in response to an aircraft request, by using 8 pressure/altitude profile data developed from data previously 9 provided through prior interrogation of other aircraft.
Thus, even if the aircraft requesting such barometric factor 11 information doe~ not carry a transponder, in~ormation can be 12 provided based on analysis of data previously provided by 13 transponder-equipped aircraft flying through the same general 1~ airspace. Data distribution unit 34 can also be used to transmit data to a central point, by microwave link or any 16 appropriate means, for accumulation and application of data 17 relevant to a more extensive geographical area.
18 Fig. 2A provides a more detailed block diasram 19 of unit 10 or 20 of Fig. 2, in the form of a simplified functional diagram of an ~/TPX-54 (V) Interrogator Set which 21 is widely used as a basic collocated transmitter/receiver 22 arrangement in military and modified configurations.
23 Interrogator 10, as illustrated in Fig. 2A, includes 24 transmitter 12, which receives timing and control signals ~rom timing means 11 and provides interrogation signals to 26 output terminal 13a via transmit/receive unit 13~ T/R unit 27 13 is typically a diplexer for coupling output signals to 28 output terminal 13a, while substantially isolating receiver 2~9~183 1 14 from output interrogation signals. For reception of 2 response signals, T/R unit 13 provides an effective receive 3 signal path to receiver 14. As shown, receiver 14 receives 4 timing and control signals from timing unit 11 for control of range gates for signal reception, for example. Upon 6 receiving response signals from a transponder, as shown at 7 point C in Fig. 2, receiver 14 provides a video signal 8 representation of received data at terminal 17c, which is 9 usable to display information on a screen for an air traffic controller and also ~or other purposes, and also provides a 11 representation of received data to target data extract;or 12 means 15. In this configuration, the TDE unit lS of t:he 13 interrogator derives digital target reports including data as 14 to the range, azimuth, identification and barometric altitude of the aircraft carrying the transponder providing response 16 signals from transmitter 12. These digital target reports 17 are provided in a signal format readily transmittable to 18 other locations, via terminal 17b, for further processing in 19 accordance with the invention. In embodiments which clo not include a target data extractor as such/ video type signals 21 provided at terminal 17c can be processed in known manner in 22 preparation for transmission or distribution to other local 23 or remote units of the system. As shown in Fig~ 2A, timing 24 signals for reference and time base synchronization axe provided at terminal 17a. In this configuration, terminals 26 17a and 17b would be coupled to signal processing unit 18 of 27 Fig. 2, via suitable ones of terminals 17 in Fig. 2.

~9~83 1 The AN/TPX-54 (V) interrogator, set as 2 illustrated in simplified form in Fig. 2A, operates basically 3 as follows. Encoded interrogation signals are transmitted at 4 1030 MHz to transponder~equipped aircraft to elicit response signal replies at 1090 MHz consisting of identification and 6 barometric altitude data. The received signals are 7 subsequently processed by the target data extractor to derive 8 a single digital target report for each aircraft within 9 system coverage on each rotation of an associated directional antenna. Elapsed time difference between interrogation 11 transmission and reply detection provides a value of slant 12 range distance to the responding aircraft, which is quite 13 accurate for intended purposes. ~ircraft a2imuth relative to ~4 the rotating antenna is derived by an algorithm used to estimate the "center-of-gravity" o~ the reply sequence 16 representing a number of interrogations and responses during 17 antenna beam passage across the aircraft position. The 18 target report data, which can either be used locally or sent 19 to remote locations, comprises range data in 1/64 mile increments, azimuth data in 0.044 degree increments, one of 21 4,096 discrete identification codes, and barometric altitude 22 data in 100 foot steps from -1,000 faet to more than +100,000 23 feet. Timing data, basically providing interrogation signal 24 transmission reference timing data, can be combined with the digital target report data or distributed separately. In the 26 operation of interrogator equipment as currently employed, 27 all signals transmitted through space are in accordance with 28 U. S. national and international standards and ~ystems which 2~4 3L83 1 are variously known as Air Traffic Control Radar ~Beacon 2 System, Mark X SIF, Mark XA, and Secondary Surveillance 3 System. This specific description of the AN/TPX 54 (V) and 4 its application to systems in accordance with tha invention represents a currently contemplated best mode of 6 implementation and will provide an understanding of the 7 invention effective to enable those skillecl in the field to 8 implement various other embodiments~
9 Referring now to Fig. 3, there is illustrated a system for providing atmospheric profile data which 11 includes a plurality of systems 30A, 30B and 30N, each of 12 which is similar to system 30 in Fig. 2. One difference i~
13 that, whereas in Fig. 2 data comparison means 28 is lncluded 14 as a component unit of system 30, in Fig. 3 data comparison means 28A comprises a single unit arranged to receive data 16 from signal processing means (as shown at 18 in Fig. 2) 17 included in each of the systems 30A, 30B and 30N. The actual 18 configuration in a particular application is considered a 19 matter of choice for those skilled in the art once having received the benefit of the teaching of the invention. Units 21 28A, 32, 34 and 36 of Fig. 3 may be generally as shown and 22 described with reference to corresponding units in Fig. 2.
23 It will be appreciated that by having individual systems 30A, 24 30B and 30N dispersed to separated geographic sites, the Fig.
3 system has the capability of deriving atmospheric profile 26 data, such as pressure/altitude profiles, covering a more 27 extensive geographical area than could be covered by a single 28 system as in Fig. 2.

~9~83 1 Fig. 4 shows a system in accordance with the 2 invention which is arranged for operation with receiving 3 means at three spaced positions for receiving response 4 signals rom an airborne transponder at point C. The Fig. 4 system includes three antennas 16, 26, and 26A at spaced 6 positions to determine a triangle, as will be discussed below 7 in the description of operation with reference to Fig. 10.
8 In the Fig. 4 embodiment., as illustrated, unit 10 is an 9 interrogator as shown at 10 in Fig. 2, including a transmitter and a receiver. Units 20A and 20B in Fig. 4 11 include receivers, such as recaiver 24 described with 12 reference to Fig. 2, but may exclude transmitters, such as 13 shown at 22 in Fig. 2. Signal processing unit 18A in Fig. 4 14 is generally identical to unit 18 in Fig. 2, except that it is arranged to receive inputs from the three units 10, 20A
16 and 20B. In this configuration, the geometric 17 characteristics of a base triangle (points A, B and E in Fig.
18 10~ are determined by the spaced positions of antennas 16,26 19 and 26A. With transmission of a first interrogation signal from interrogator 10 and reception of the resulting response 21 signal at all three of units 10, 20A and 20B, range data to 22 point C from three positions is made available by the 23 provision of a reference or timing signal from unit 10, which 24 represents the time of transmission of the interrogation signal. As described further with reference to Fig. 10, with 26 the availability of data on range from each of the three 27 receiving positions and the geometric data defining the base 28 triangle, the geometric height of the atmospheric location C

2 ~

1 can be determined without requirement for availability of 2 azimuth data from units 10, 20A and 20B. This serves to 3 point up the fact that once the present invention is 4 understood by skilled individuals, different geometric models and methods of implementing the necessary geometric 6 determinations will be a matter of choice in view of 7 particular applications and objectives in use of the 8 invention. In Fig. 4 data comparison unit 28 is provided as 9 described with reference to Fig. 2 and units 32 and 34 and data transmission antenna 36 may be similarly included by 11 connection to terminal 29.
12 Fig. 5 illustrates a system ~or deriving 13 atmospheric deviation data which does not specifically 14 include the transmitter, receiver and antenna means o~ Fig.
2. Instead, there are provided input means, shown as 16 terminals 17, for coupling input data. The input data will 17 desirably include data based upon barometric pressure 18 measured in the vicinity of an atmospheric location and range 19 data representative of approximate distances to the atmospheric location from a plurality of refersnce po.ints at 21 spaced positions, and which may include azimuth data 22 reprssentative of approximate azimuths of said atmospheric 23 location relative to those reference points. Thus, such 24 data, which has been discussed with reference to the Fig. 2 embodiment, may be developed by a Fig. 2 type 26 transmitter/receiver/antenna combination or in other suitable 27 manner and provided to an appropriate arrangement of input 28 terminals such as represented at 17 in Fig. 5.

2~4183 1 In the Fig. 5 embodiment, units 18, 28, 32, 34 2 and 36 may be basically as shown and described with reference 3 to Fig. 2. As shown, signal processing unit 18 has been 4 modified to incorporate smoothing means 40 for enhancing the accuracy of range data by combining successive portions of 6 the data representative of measurements of the distance to 7 said atmospheric location from the same one of the reference 8 points. Thus, for example, range data representing 9 successive transmitting of first signals and receiving of response signals by a speaific receiver are averaged using 11 known techniques in order to smooth or average out responder 12 signal response tolerance errors so as to enhance the 13 accuracy o the range data. Such smoothing of the range data 14 i8 carried out separately for range data relating to each of the reference points, for example, to enhance subsequent 16 determination of the geometric height of the atmospheric 17 location.
18 In Fig. 5, data comparison means 28 is shown 19 as additionally comprising look-up means 42 and deviation derivation means 44. Unit 42 may be any suitable form of 21 equipment or system of a known type usable for storing a 22 quantity of data values, in this case a table of pressure 23 values for different atmospheric heights representing a 24 standard pressure/altitude profile, such as provided by ICAO.
In operation, when data representing the geometric height of 26 an atmospheric location is provided by unit 18, look-up unit 27 42 provides reference data representing the ICAO standard 28 pressure for that height. Deviation derivation unit 44 then 2 ~ 3 1 receives inputs representing both that standard pressure and, 2 from unit 1~/40, the data representing barometric pressure 3 actually measured in the vicinity of the atmospheric 4 location. Unit 44 is thus enabled to compare such inputs in order to derive atmospheric deviation data representative of 6 the difference between the measured and refer~nse data 7 applicable to the atmospheric location of interest.
8 The Fig. 5 embodiment also includes storage 9 means 46, coupled between unit 28 and profile data unit 32, for storing atmospheric deviation data. Storage unit 46 may 11 be any suitable form of data storage unit for permitting the 12 storage and retrieval in known manner of atmospheric 13 deviation data derived with respect to a plurality of 14 atmospheric locations representing geometric heights at one or more geographic locations to permit accumulation and use 16 of such data for development of atmospheric pressure/height 17 profiles and other purposes.

18 Geometric Determinations and Methods of Oe~ration 19 The following addresses determination of height using geometric relationships and introduces the 21 factor of signal turnaround delay. When an airborne 22 transponder of the type referred to receives an interrogation 23 signal, it must decode such signal and formulate and ~ransmit 24 a response signal. The elapsed time required to perform those functions results in a delay between the time the first 2~ signal reaches a transponder and the time at which the 27 response signal leaves the transponder. That elapsed time 2~ 83 1 will be termed the ~turnaround delay~. Ideally, the round 2 trip time between transmission of a first signal by a ground-3 hased interrogator and receipt of the response signal by that ~ interrogator would depend solely upon the distance between the interrogator and airborne transponder. In actuality, the ~ turnaround delay introduces an error by making the round trip 7 time longer. As a practical matter, however, it is 8 considered that the turnaround delay introduced by common 9 types of airborne transponders does not reduce system accuracy below the accuracy level required ~or presently 11 contemplated applications. ~evertheless, in accordarlce with 12 the present invention, systems and methods are provid0d which 13 include determination and/or correction for turna~ound delay, 14 so that barometric deviation data may be derived with or without correction for turnaround delay, depending on 16 requirements in particular applications.
17 Fig. 6 shows a one-dimensional conceptual 18 model of the Fig. 2 system. This is basically an academic 19 case provided for purposes of heuristic development. In Fig.
6 the position of the transponder at point C lies along the 21 line between the positions A and B of the interrogators 10 22 and 20 of Fig. 2. As shown, point A is located at position x 23 = o and point B is located at position x = Xb. In this 24 example, the interrogator is assumed to introduce a turnaround delay, with the result that the ranges to point C
26 from points A and B (as determined by the timing differences 27 between transmitting interrogation signals and receiving 28 response signals) each include an incremental range error 2 ~ 3 1 representing the effect of the turnaround delay, shown as the 2 distance AR, which has bean exaggerated for purpose of 3 illustration. Thus, the respective timing differences, as 4 initially measured at points A and B, correspond to the distances R1' and R2', respectively. In this illustration, 6 the turnaround delay is shown as increasiny each of the range 7 values by ~R, whereas R1 and R2 should equal the known 8 baseline value xb. The baseline value between points A and B
9 represents a predetermined distance as measured by a satellite positioning system or other method.
11 Thus, the difference between the points A and 12 B as determined using signal timing difference~ and the 13 measured distance between those points is equal to twice the 14 error introduced by the turnaround delay. As a result, ( Rl ' ~R2 ' ) -Xb ( 1 16 which can be solved ~o determine ~R, and thereby R1, which is 17 equal to Rl' less ~R, and R2. This is straightforward, 18 particularly since we already know that on an error-free 19 basis R, + R2 = xb.
The two dimensional case illustrated in Fig. 7 21 is considered next. In Fig. 7, it will be seen that, whereas 22 the distances as determined by the round trip signal 23 transmission and reception timing differences are indicated 24 by the total vectors R1' and R2', the transponder is actually located at point C. Point C is again separated by the actual 26 ranges R, and R2 from the points A and B respectively. It 2~9~3 1 follows that, R1'sinal+R2Sina2>xb (2) 3 and the excess is ~R (sin ~, + sin ~2 ) which can be solved 4 deterministically or by incrementally reducing the values of the measured distances by equal amounts until the inequality 6 in equation (2) is equali~ed. The values for the included 7 angles are provided by the angles of reception of the 8 response signals at the interrogators.
9 A three dimensional case assumed to not be subject to errors introduced by turnaround delay~, as 11 illustrated in Fig. 8, will now be consldered. The azimuth 12 data obtained from the interrogator units lO and 20 at points 13 A and B is used to determine the position of point D in the 14 horizontal xy plane. As shown, h is a vertical line representing the geometric height of the airborne transponder l~ at point C, lying directly above point D. With point B at Xb 17 and point D located at the coordinates xO, yO then, R12=xO2+yO2+h 2 ~ 3) 18 and R22=(x~-xo)2+y2+h2 (4) which are the equations for two spheres that, absent 21 turnaround delay, are tangent at the point of solution and h 22 is thereby determined.
23 When errors are introduced as a result of the 24 presence of transponder turnaround delays, the two sph~res 2~9~

1 intersect and form a circular locus. One approach to 2 solution for the actual ranges is to incrementally reduce the 3 values of Rl' and R2' until the two spheres become tangential.
4 This may be more readily seen if the pair of simultaneous equations (3) and (4) are subtracted to cancel h (which will 6 still be inherent in the range values) to provide, Rl2-R2a=2xbxo-xb (5) 7 Alternatively, the three dimensional model, 8 shown in Fi~. 9 as including range errors ~R resulting from 9 turnaround delays, may be addressed as follow~ To ~ummarize the arrangement, interrogators 10 and 20 are positioned at 11 points A and B, respectively, with a known baseline distance 12 saparating them. Each interrogator actively and autonomously 13 interrogates passing aircraft having airborne transponder~
14 and derives target reports at a rate of one item of report data per aircraft per 360 degree antenna revolution or sector 16 scan, including information as to range, azimuth, aircraft 17 identification and barometric pressure/altitude data. It 18 should be noted that each such item of report data will 19 normally encompass a plurality of interrogations and responses occurring in a short period during which the main 21 beam of the radiation pattern of the rotating antenna is 22 directed at an aircraft during each antenna revolution or 23 scan and the resulting repetitive range data can be used for 24 smoothing purposes as discussed above with reference to Fig.

5.
26 As between the two interrogators at points A
27 and B in Fig. 9, a common time reference, including provision 2 ~ 8 3 1 for processing and data transmission delays, permits 2 correlation of data received from a specific airborne 3 transponder as the result of interrogation by both 4 interrogators with only minimal time-spacing between the two separate interrogations, so that the aircra~t is in 6 substantially the same atmospheric location when responding 7 to each interrogator. Such common time reference also 8 facilitates conventional track-smoothing and use of 9 prediction algorithms for time-consistent positional extrapolation for increased accuracy in determining geometric 11 height. Thus, the turnaround delay for a given airborne 12 transponder (which may have a specification value o~ 3 ~
13 seconds, for example) may actually vary within a tolerance (i 14 0.5 ~ seconds, for example) on dif~erent interrogations, so that smoothing to average the range value over a plurality of 16 interrogations can provide increased accuracy of measured 17 range data.
18 ~n Fig. 9, the total slant range vectors R,' 19 and R2' include both the actual ranges to the point C, as well as the ~R errors introduced by the turnaround delay in the 21 airborne transponder. The angles ~, and ~2 are determined by 22 the measured azimuths from the respective points A and B.
23 The triangle in the xy plane is completely defined by the 24 baseline distance x~ and the included angles, thereby determining the distances Rgl and Rg2, which are the ground 26 plane distances to point D lying vertically below airborne 27 transponder location C. As shown, points A, D, and C define 28 a first vertical triangle lying in a vertical plane which 2~9~L~83 1 intersects airborne transponder location C, and points B, D
2 and C define a similar vertical triangle. The geometric 3 height h is common to both of these triangles, so that, R12 _Rg,.2 = ~22 -R 2 (6) 4 for the case in which ther0 is no turnaround delay. With the presence of turnaround delay, (R~ R) 2 _R~2 = (R2t -~R) 2 _R~2 ~7) 6 and ~R= ( gl Rg2 ) (Rl -R2 ) (8) 7 These equations can be solved in known manner in order to 8 determine the geometric height h of the atmospheric location ~ of the airborne transponder at the time the data was provided to the two interrogators. It should be noted, however, that ll in two special cases the height cannot be determined. The 12 above expressions are indeterminate when R2~ and R1~ are 13 equal, and lack a meaninyful solution when point D lies along 14 the baseline between points A and B. In the operation of a system for deriving barometric deviation data according to 16 the invention, data collected in these very specific 17 circumstances can generally be discarded without seriously 18 constraining the overall data collection process.
19 In view of the foregoing, it will be apparent that other known analysis techniques, such as use of conical 21 sections, can be applied for determination of values of 22 geometric height. In addition, data as to the elevation of 23 the receiving points can be utilized in the computations so 299~183 1 as to relate determination of g~ometric height to a defined 2 datum level, such as MSL. Also, once the foregoing examples 3 and analysis are understood, arrangements applying the 4 invention to other geometric models will become obvious to those skilled in the art. For example, the indeterminate 6 points discussed with reference to the Fig. 9 system can be 7 avoided by a three station system, such as illustrated in 8 Fig. 4 and represented by the Fig. 10 diagram. Briefly 9 considered, in the system of Figs. 4 and 10, three ground-based receivers are located at points AJ B and E, with all 11 three of the interconnecting baseline distances and included 12 angles determined by appropriate measurements and 13 calculations. In this arrangement, the geometric height of 14 an airborne transponder at point C is repxesented by line h between points C and D. The side triangles of the resulting 16 pyramid, as shown, do not lie in vertical planes, except in 17 special cases in which point D falls along one of the 18 baselines. The orthogonal projection from the apex C of the 19 pyramid to the xy plane defines an angle between the x axis and the point D lying vertically below the apex C.
21 In the Fig. 10 embodiment, the station at 22 point A includes an active interrogator/receiver unit, while 23 the stations at points B and E comprise receivers operating 24 independently of co-located transmitters. In this arrangement, the active station at A distributes accurate 26 timing reference data to the two passive receiver stations at 27 points B and E. Following transmission of a first 28 interrogation signal from station A, the resulting response 20g~3 1 signal transmitted by the airborne transponder at point C is 2 received by the individual receivers at each of points A, B
3 and E. Upon determination of the slant ranges to the point C
4 from each of the three receivers, using the time reference data as to the time of transmission of the single ~ interrogation signal, the geometric dimensions of the pyramid 7 are completely defined so as to permit determination of the 8 height h. This discussion of the Fig. 4 system, as 9 represented by Fig. 10, has ignored the presence and effects of turnaround delays. In Fig. 10, it will be seen that a 11 vertical plane is defined by the vertical height line h and 12 the line joining points A and C. By comparin~ the computed 13 angle ~ between this plane and the baseline A-B, with the 14 measured azimuth angle ~ as included in the data from station A, an indication of the error introduced by turnaround delay 16 can be derived. I~ the angle as determined by each method 17 has substantially the same value, it may be concluded that no 18 correction in the range data is necessary. If the angle 19 values are different, the time-delay measured ranges between point C and points A, B and E can be equally incrementally 21 shortened until equality between the measured and computed 22 angles is achieved.
23 With respect to the determination of 24 turnaround delay, it is expected that the turnaround delay for a specific airborne transponder, or for a specific model 26 of airborne transponder, may represent a fixed delay of a 27 value which, within equipment design tolerances, does not 28 vary appreciably over time or between units of a specific 1 model, as the case may be. This being so, it may be 2 desirable to apply an embodiment of the invention arranged to 3 effectively determine the turnaround delay associated with a 4 specific unit or a speci~ic model of airborne transponder.
Such delay, once determined, may then be stored in a manner 6 so that each response signal from a particuLar airborne 7 responder includes data as to its turnaround delay, along 8 with data representing barometric pressure/altitude and 9 aircraft identification~ Alternatively, data specifying the turnaround delay for a particular transponder unit or model 11 may be stored in data storage facilities of a ground system 12 embodiment in accordance with the invention, for retrieval 13 and application each time response signals from the 14 identified transponder unit or model are received for proces~ing to derive barometric deviation data.
16 As a general matter, it will be understood 17 that the objective is to determine a close estimate of the 18 actual geometric height of the atmospheric location of the 19 airborne transponder, so that such height can be used in comparisons to measured barometric pressure/altitude at or 21 about the same atmospheric location to derive atmospheric 22 deviation data. Also, it is expected that in most 23 applications the magnitude of error introduced by turnaround 24 delay will be of such relatively small significance to the overall required accuracy of the desired data, that it will 26 not be necessary to determine or correct measured range 27 values for turnaround delays.

Claims (34)

1. Claim 1. A system for deriving atmospheric deviation data for a location in the atmosphere, in cooperation with an airborne transponder which provides response signals including data based upon barometric pressure, comprising:
   one or more transmitting means for transmitting first signals to said airborne transponder;
   a plurality of receiving means, positioned at spaced positions, for receiving response signals transmitted by said airborne transponder from an atmospheric location in response to said first signals, said response signals including data based upon barometric pressure in the vicinity of said atmospheric location;
   signal processing means, coupled to said receiving means and responsive to timing differences between said transmitting of said first signals and said receiving of said response signals, for utilizing range data derived from said timing differences and geometric data regarding said spaced positions of said receiving means for deriving data representative of geometric height of said atmospheric location; and comparison means, coupled to said signal processing means, for utilizing said data representative of geometric height and said data based upon barometric pressure for deriving atmospheric deviation data representative of deviation between said data based upon barometric pressure and atmospheric reference data applicable to said atmospheric location.
2. Claim 2. A system for deriving atmospheric deviation data as in claim 1, additionally comprising atmospheric profile means, coupled to said comparison means, for using said atmospheric deviation data for deriving atmospheric profile data, representing atmospheric conditions above a geographic area, based upon response signals received from one or more airborne transponders at a plurality of atmospheric locations within a predetermined time period.
3. Claim 3. A system as in claim 2, additionally comprising data distribution means, coupled to said barometric profile means, for transmitting data for further use or analysis.
4. Claim 4. A system as in claim 2, wherein said atmospheric profile means uses data as to the elevation of each transmitting means and receiving means in order to normalize data to a desired datum plane.
5. Claim 5. A system for providing atmospheric profile data comprising:
   
   a plurality of systems as described in claim 1 positioned at separated geographic sites; and atmospheric profile means, coupled to the comparison means of each of said plurality of systems, for deriving atmospheric profile data representing atmospheric conditions above a geographic area, in response to atmospheric deviation data provided by said comparison means of said plurality of systems representing response signals received from one or more airborne transponders at a plurality of atmospheric locations within a predetermined time period.
6. Claim 6. A system for deriving atmospheric deviation data as in claim 1 wherein one of said transmitting means and one of said receiving means are collocated as units of an interrogator system which additionally comprises:
   timing means, coupled to said transmitting means, for providing timing reference signals; and target data extractor means, coupled to said receiving means, for deriving digital target report data including range, azimuth, identification and barometric altitude data relating to said airborne transponder;
   and wherein said digital target report data and timing reference signals from said timing means are coupled to said signal processing means and utilized with said geometric data regarding said spaced positions of said receiving means for deriving geometric height data, and said barometric altitude data and said geometric height data are coupled to said comparison means for deriving atmospheric deviation data.
7. Claim 7. A system for deriving atmospheric deviation data as in claim 6, additionally comprising atmospheric profile means, coupled to said comparison means, for using said atmospheric deviation data for deriving atmospheric profile data, representing atmospheric conditions above a geographic area, based upon response signals received from one or more airborne transponders at a plurality of atmospheric location within a predetermined time period.
8. Claim 8. A system for deriving atmospheric deviation data as in claim 1, wherein said data based upon barometric pressure in the vicinity of said atmospheric location comprises barometric altimeter data and said comparison means utilizes said barometric altimeter data and said data representative of geometric height for deriving atmospheric deviation data representative of deviation between barometric altimeter readings and geometric height in the vicinity of said atmospheric location at or about the time of said transmission of said response signals by said airborne transponder.
9. Claim 9. A system for deriving atmospheric deviation data as in claim 8, additionally comprising data distribution means, coupled to said comparison means, for transmitting to an aircraft an altitude correction factor representative of atmospheric conditions in the vicinity of said atmospheric location, said factor being usable for conversion of barometric altimeter readings to approximate actual geometric height.
10. Claim 10. A system for deriving atmospheric deviation data as in claim 1, wherein said data based upon barometric pressure in the vicinity of said atmospheric location comprises barometric altimeter data and said comparison means utilizes said barometric altimeter data and said data representative of geometric height for deriving atmospheric deviation data representative of deviation between measured barometric pressure in the vicinity of said atmospheric location and reference atmospheric pressure for an altitude similar to the altitude of said atmospheric location.
11. Claim 11. A system for deriving atmospheric deviation data as in claim 1, wherein said comparison means comprises means for utilizing said data based upon barometric pressure and said data representative of geometric height, together with atmospheric pressure/altitude reference profile data, for deriving atmospheric deviation data representative of deviation between measured barometric pressure in the vicinity of said atmospheric location and reference atmospheric pressure for said atmospheric location from said atmospheric pressure/altitude reference profile data.
12. Claim 12. A system for deriving atmospheric deviation data as in claim 1, wherein there is included only one transmitting means and said transmitting means is co-located with one of three spaced, fixed-site receiving means.
13. Claim 13. A system for deriving atmospheric deviation data as in claim 1, wherein there are included a plurality of transmitting means, each of which is co-located with one of a like-plurality of spaced, fixed-site receiving means.
14. Claim 14. A system for deriving atmospheric deviation data for a location in the atmosphere, in cooperation with an airborne transponder providing response signals which may be subject to signal turnaround delay and which include current barometric altitude data, comprising:
    one or more transmitting means for transmitting first signals to said airborne transponder;
    a plurality of receiving means, positioned at spaced positions, for receiving response signals transmitted by said airborne transponder from an atmospheric location in response to said first signals, said response signals being subject to a signal turnaround delay and said response signals including barometric altitude data based on current barometric pressure in the vicinity of said atmospheric location;
    signal processing means, coupled to said receiving means and responsive to timing differences between said transmitting of said first signals and said receiving of said response signals, for utilizing range data derived from said timing differences and geometric data regarding said spaced positions for adjusting said timing differences to compensate for said signal turnaround delay to provide adjusted timing differences, and for utilizing said adjusted timing differences for deriving data representative of geometric height of said atmospheric location; and comparison means, coupled to said signal processing means, for utilizing said data representative of geometric height and said barometric altitude data from said airborne transponder for deriving atmospheric deviation data representative of deviation between said barometric altitude data and atmospheric pressure reference data applicable to said atmospheric location.
15. Claim 15. A system for deriving atmospheric deviation data for a location in the atmosphere comprising:
    input means for coupling input data including data based upon barometric pressure measured in the vicinity of an atmospheric location and range data representative of approximate distances to said atmospheric location from a plurality of reference points at spaced positions, and which may include azimuth data representative of approximate azimuths of said atmospheric location relative to said reference points;
    signal processing means, coupled to said input means, for utilizing said range data, available azimuth data as may be selected, and geometric data regarding said spaced positions of said reference points for deriving data representative of the geometric height of said atmospheric location; and comparison means, coupled to said signal processing means, for utilizing said data representative of geometric height and said data based upon barometric pressure for deriving atmospheric deviation data representative of deviation between said data based upon barometric pressure and atmospheric reference data applicable to said atmospheric location.
16. Claim 16. A system as in claim 15, wherein said comparison means comprises:
    look-up means, coupled to said signal processing means, for utilizing said data representative of geometric height to identify and make available atmospheric reference data pertinent to the vicinity of said atmospheric location; and deviation derivation means, coupled to said look-up means, for comparing said data based upon barometric pressure to said atmospheric reference data to derive atmospheric deviation data representative of the difference between barometric pressure measured in the vicinity of said atmospheric location and reference data applicable to said atmospheric location.
17. Claim 17. A system as in claim 15, wherein said signal processing means includes smoothing means for enhancing the accuracy of range data by combining successive portions of said data representative of the distance to said atmospheric location from one of said reference points.
18. Claim 18. A system as in claim 15, additionally comprising:
    storage means, coupled to said comparison means, for storing atmospheric deviation data derived with respect to a plurality of atmospheric locations representing geometric heights at one or more geographic locations; and atmospheric profile means, coupled to said storage means, for using said barometric deviation data for deriving atmospheric profile data representing atmospheric conditions above a selected geographic area.
19. Claim 19. A system, for determining geometric height in cooperation with an airborne transponder providing response signals, comprising:
    one or more transmitting means for transmitting first signals to said airborne transponder;
    a plurality of receiving means positioned at spaced positions, for receiving response signals from said airborne transponder in response to said first signals; and signal processing means, coupled to said transmitting means and responsive to timing differences between said transmitting of said first signals and said receiving of said response signals, for utilizing range data derived from said timing differences and geometric data regarding said spaced positions for determining geometric height of said airborne transponder.
20. Claim 20. A system as in claim 19, wherein said signal processing means includes smoothing means for enhancing the accuracy of determinations based upon said timing differences by combining timing difference values representing successive cycles of transmitting of a said first signal by said transmitting means and receiving of a said response signal by the same one of said receiving means.
21. Claim 21. A system, for determining geometric height in cooperation with an airborne transponder providing response signals which are subject to signal turnaround delay, comprising:
    one or more transmitting means for transmitting first signals to said airborne transponder;
    a plurality of receiving means positioned at spaced positions, for receiving response signals, which are subject to a signal turnaround delay between reception of a first signal and transmission of a response signal, from said airborne transponder in response to said first signals; and signal processing means, coupled to said transmitting means and responsive to timing differences between said transmitting of said first signals and said receiving of said response signals, for utilizing range data derived from said timing differences and geometric data regarding said spaced positions for adjusting said timing differences to compensate for said signal turnaround delay to provide adjusted timing differences and for utilizing said adjusted timing differences for determining geometric height of said airborne transponder.
22. Claim 22. A system as in claim 21, for use with an airborne transponder providing response signals which include barometric altitude data, additionally comprising:
    comparison means, coupled to said signal processing means, for comparing said geometric height as determined by said signal processing means with said barometric altitude data included in said response signals from said airborne transponder to derive barometric altitude deviation data representative of current atmospheric conditions in the vicinity of said airborne transponder; and means, coupled to said comparison means, for transmitting said geometric height to an aircraft in which said airborne transponder is located;
    whereby, said barometric altitude deviation data is usable for calibrating a barometric altimeter.
23. Claim 23. A system, for deriving an approximate value of signal turnaround delay between reception of an interrogation signal by an airborne transponder and transmission of a response signal by said airborne transponder, comprising:
    one or more transmitting means for transmitting interrogation signals to an airborne transponder;
    
    a plurality of receiving means, positioned at spaced positions, for receiving response signals, which are subject to said turnaround delay, from said airborne transponder in response to said interrogation signals; and signal processing means, coupled to said receiving means and responsive to timing differences between said transmitting of said interrogation signals and said receiving of said response signals, for utilizing range data derived from said timing differences and geometric data on said spaced positions for determining an approximate value of said turnaround delay between reception of an interrogation signal by said airborne transponder and transmission of a response signal by said airborne transponder in response to said interrogation signal;
    whereby, said approximate value of said turnaround delay is usable for calibration of transmission and reception timing characteristics of said airborne transponder.
24. Claim 24. A system as in claim 23, additionally comprising means, coupled to said signal processing means, for transmitting said approximate value of said turnaround delay to an aircraft in which said airborne transponder is located, whereby said approximate value of said turnaround delay is usable for encoding into subsequent response signals to provide increased accuracy in use of said response signals for timing purposes.
25. Claim 25. A method, for deriving atmospheric deviation data for a location in the atmosphere, in cooperation with an airborne transponder providing response signals including current data based upon barometric pressure, comprising the steps of:
    (a) transmitting first signals to said airborne transponder;
    (b) receiving, at a plurality of spaced positions, response signals transmitted by said airborne transponder from an atmospheric location in response to said first signals, said response signals including data based upon barometric pressure in the vicinity of said atmospheric location;
    (c) utilizing range data derived from timing differences between said transmitting of said first signals in step (a) and said receiving of said response signals in step (b), with geometric data regarding said spaced positions, for deriving data representative of geometric height of said geometric location;
    (d) comparing said geometric height data as derived in step (c) with said data based upon barometric pressure as received in step (b) from said airborne transponder; and (e) utilizing the results of said step (d) comparison to derive atmospheric deviation data representative of deviation between said data based upon barometric pressure and atmospheric reference data applicable to said atmospheric location.
26. Claim 26. A method as in claim 25, additionally comprising the step of:
    (f) developing atmospheric profile data using atmospheric deviation data derived in step (e) based upon response signals received from one or more airborne transponders positioned at a plurality of heights.
27. Claim 27. A method as in claim 25, additionally comprising the step of:
    (f) transmitting said atmospheric deviation data to an aircraft for use in calibration of a barometric altimeter.
28. Claim 28. A method as in claim 25, additionally comprising the following steps between steps (b) and (c):
    (i) utilizing range data derived from timing differences between said transmitting of said first signals in step (a) and said receiving of said response signals in step (b), with geometric data regarding said spaced positions, for deriving an approximate value of a signal turnaround delay between the receiving of first signals and transmission of response signals by said airborne transponder;
    (ii) applying said approximate value of said signal turnaround delay to adjust said timing differences to provide adjusted timing differences; and (iii) supplying said adjusted timing differences for use in step (c) in substitution for said timing differences recited therein.
29. Claim 29. A method as in claim 25, wherein said data based upon barometric pressure as received in step (b) comprises barometric altimeter data and step (e) comprises utilizing the results of said step (d) comparison to derive atmospheric deviation data representative of deviation between barometric altimeter readings and geometric height in the vicinity of said atmospheric location at or about the time of said receiving of said response signals from said airborne transponder.
30. Claim 30. A method as in claim 25, wherein said data based upon barometric pressure as received in step (b) comprises barometric altimeter data and step (e) comprises utilizing the results of said step (d) comparison to derive atmospheric deviation data representative of deviation between barometric pressure in the vicinity of said atmospheric location and reference atmospheric pressure for a height similar to said geometric height of said atmospheric location.
31. Claim 31. A method as in claim 25, wherein step (e) includes utilizing the results of said step (d) comparison together with atmospheric pressure/altitude reference profile data, to derive atmospheric deviation data representative of deviation between measured barometric pressure in the vicinity of said atmospheric location and relevant atmospheric reference pressure data from said atmospheric pressure/altitude profile data.
32. Claim 32. A method, for deriving atmospheric deviation data for a location in the atmosphere, comprising the steps of:
    (a) receiving input data including data based upon barometric pressure measured in the vicinity of an atmospheric location and range data representative of approximate distances to said atmospheric location from a plurality of reference points at spaced positions, and which may include azimuth data representative of approximate azimuths of said atmospheric location relative to said reference points;
    (b) utilizing said range data and/or said azimuth data, together with geometric data regarding said spaced positions of said reference points, for deriving data representative of the geometric height of said atmospheric location above a datum level;
    (c) comparing said geometric height data as derived in step (b) with said data based upon barometric pressure as received in step (a); and (d) utilizing the results of said step (c) comparison to derive atmospheric deviation data representative of deviation between said data based upon barometric pressure and atmospheric reference data applicable to said atmospheric location.
33. Claim 33. A method as in claim 32, additionally comprising the step of:
    (e) developing atmospheric profile data using atmospheric deviation data derived in step (d) based upon input data received in step (a) with respect to a plurality of atmospheric locations at a plurality of heights and/or geographic positions.
34. Claim 34. A method for inflight calibration of a barometric altimeter located in an aircraft, comprising the steps of;
    (a) transmitting first signals to an airborne transponder located in said aircraft;
    (b) receiving, at a plurality of spaced positions, response signals transmitted by said airborne transponder in response to said first signals;
    (c) utilizing range data derived from timing differences between said transmitting of said first signals and said receiving of said response signals and geometric data regarding said spaced positions for determining geometric height of said airborne transponder;
    (d) transmitting data representative of said geometric height to said aircraft; and (e) utilizing said data to adjust the calibration of said barometric altimeter as necessary to achieve desired consistency between said geometric height and altitude measurements provided by said barometric altimeter.