WO2002004978A1

WO2002004978A1 - Positioning survey system

Info

Publication number: WO2002004978A1
Application number: PCT/DK2001/000453
Authority: WO
Inventors: Kai Borre
Original assignee: Gps Survey Aps
Priority date: 2000-07-06
Filing date: 2001-06-29
Publication date: 2002-01-17
Also published as: DK200001053A; AU2001268952A1

Abstract

A method for reducing the influence of multipath delay on signals in position survey systems using received signals from GPS satellites, wherein the position based constellation of said GPS satellites is periodic with a periodicity of D hour. Computed position vectors are compared to corresponding position vectors as computed D hours earlier. A relative position of less than one mm can be obtained for objects that are less than 1 km apart and with a height difference of less than 20 m.

Description

Positioning Survey System

FIELD OF THE INVENTION

The present invention relates to a method for reducing the effect of multipath in survey systems using signals received from positioning satellites as described in the preamble of claim 1. The invention relates further to an apparatus for obtaining position vectors between a reference location and a geographic relative point of interest.

BACKGROUND OF THE INVENTION

In order to obtain precise position coordinates or relative position coordinates, position vectors, a number of techniques have been developed to extract optimum information from satellite-based navigation systems like GPS, GLONASS, or Galileo. Signals from a plurality of satellites are combined in a positional computation for the receiver to reduce orbit errors of satellites as well as errors in signal propagation. A number of models can be used for reducing signal propagation delays caused by the ionosphere and the troposphere.

However, the main effect of these signal delays are eliminated by using double differenced observations, i.e. a linear combination of the four observations involved. By this procedure all correlated, systematic errors are eliminated, except multipath. Only in case a baseline is longer than 1000 meters or the height difference is larger than 20 meters we consider applying well known reduction models.

An overview of different effects and possible approaches for correction are given in US patent 5 323 322, where a world-wide network of differential GPS reference stations is disclosed which accomplishes an accuracy at sub-decimetre level over a given area. One of the additional error-inducing effects is the multipath delay, which occurs due to reflections of the signals at objects or surfaces in the vicinity of the signal receiving antenna.

Multipath errors can be filtered from the received signals if the difference in distance between the direct and the reflected signals are larger than 10 m. Furthermore, for signals that have been reflected an odd number of times, the polarisation of the signal is different from the original signal and can also be filtered out. However, for errors that are introduced by multipath where the distance difference between the direct and the reflected signals is less than 10 m, no model for reduction is known. However, any reduction of this error would result in a higher precision of the position.

The highest precision is desirable when detecting movements of for example buildings, bridges, or off-shore platforms. Using a high precision may indicate at an early stage, whether precautions have to be taken to avoid catastrophes.

It is an object of the invention to provide a method and an apparatus that considerably reduce the effect of signal multipath in positioning survey systems.

SUMMARY OF THE INVENTION

This object is achieved by a method as mentioned by way of introduction and as described in the characterising part of claim 1.

Despite the fact that the method itself is not too evolved, the literature does not describe that omnipresent multipath errors may be corrected in this special manner. The method is based on the fact that the configuration of position satellites repeats with a certain periodicity, which in the following is called D hours. For GPS this is 23 hours 56 minutes, and for GLONASS 22 hours 32 minutes. Therefore, signals from positional satellites received D hours apart should in principle be identical. After making the double differenced observations, the multipath correction is the final step for obtaining more precise positions.

For baselines shorter than 1000 meters and with differences of height less than 20 meters the ionospheric delay is put equal to zero. If the ionosphere cannot be ignored we apply ionospheric weighting which is somewhere in between ionosphere fixed and ionosphere float solutions. This results in a shorter observation time span required to resolve the carrier ambiguities.

The tropospheric delay is computed according to the Goad-Goodman model. This model may be calibrated by simultaneously estimating the tropospheric zenith delay.

To measure a relative movement of an object with respect to a reference point, an an- tenna is located at the object and a reference antenna is located at the reference point.

The observation data from these two locations are transmitted to a computer where the signals are double differenced.

The basic system architecture consists of a network of GPS sensor nodes and a central data processing station (personal computer). Raw GPS data in the form of pseu- dorange and phase observations are collected by the sensor nodes and transferred to the central processing station.

At each epoch we thus compute the position of the antenna from the observations, preferably more than four. The computation is made as a least-squares estimation; this yields a residual for every observation. This procedure can be used at the reference point as well as at all other object points. Actually the residuals build up a table of multipath corrections at every antenna for every satellite. There are as many sets of residuals as there are antennas each containing corrections to every satellite and for every epoch. These residuals are to be applied to any observation taken D hours later. This means that all residuals are to be stored for at least D hours. If a large residual occurs, we ask: is the residual too large to be accepted as being random or shall it simply be deleted ? The software acts appropriately according to the answer.

However, other effects may cause a substantial residual, for example a change of the satellite configuration which occurs from time to time due to replacement of satellites or strategic rearrangements. Such a change of satellite configuration will cause large deviations and be readily recognised by the system.

Slowly changing factors in the near vicinity of the antennas, for example temperature changes from summer to winter or changes in the vegetation in the near surroundings will not influence the residual, if it is computed from signals that result from observations taken D hours earlier.

However, moisture in the air, snow-fall, changes on the foliage of trees due to wind, or objects that pass by may change the residual on a day to day basis. By storing computed position vectors from a plurality of satellite cycles, it is possible by averaging techniques to filter out the influence of these factors. Furthermore, by storing not only the computed position vectors on a daily basis, but by also storing the corresponding weather conditions, the change of the residuals may be attributed to certain weather conditions wliich may be helpful for a quantitative interpretation of the residuals. If a tropospheric delay correction is used, the following meteorological data: surface pressure, humidity and temperature are to be taken. This is partly important when meas- urements are done using long baselines, i.e. that the at least two GPS ground stations are relatively far removed from each other.

The computations necessary are not more demanding than they all can be done in near real-time on a desk top computer. The present invention will be explained more in detail with reference to the drawings. SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of one embodiment of the invention, FIG. 2 is a flow diagram illustrating the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a system diagram of one embodiment of the invention. A first - reference - antenna 1 is located at a reference point 3, whereas a second - object - antenna 2 is located at a different object location 4. The number of object antennas 2 is in principle not limited to one antenna, but several different object locations 4 with antennas may be surveyed relative to the reference point 3. The first 1 and the second 2 antenna receive signals from a plurality of positional satellites 5, preferably more than four. In order to obtain the highest precision, it is necessary that antennas 1, 2 have the same specification and that they are mounted with identical orientation and on very stable foundations that are in stable connection with the structure to be monitored.

The signals from the antenna 1, 2 are transmitted by first transmission links 11, 11 ' to GPS receivers 7, 8. The observations from receivers 7 and 8 are then transmitted by further wired links or radio transmission links 10, 10' to a data receiver 9 connected to a computer station 6. Here the coordinate differences between the object location 4 and reference point 3 are computed.

In the case that the transmission links are wired links, it is preferable to use identical cables of equal length for not introducing any uncontrolled time delay which may decrease the accuracy of the total system.

Biases between the receiver channels and between the LI and L2 observations are small of the order of 0.2-0.3 mm. It is possible to model and estimate any receiver- dependent offsets in the computer software if so desired. The distance between the antennas limits the accuracy that can be obtained, such that a larger distance between antennas results in a less accurate determination of the baseline vector. Therefore, it is preferred that the distance between antennas 1, 2 is less than 1 km; additionally the second order correction term for the ionospheric delay restricts the height difference to be less than 20 meter. Obeying these restrictions it is possible to achieve a relative position accuracy less than one mm if the method according to the invention is employed.

At the computer station 6, the observations are double differenced to eliminate errors like satellite clock errors, eventual delays in cables, ionospheric delays, tropospheric delays. Finally, the observations are corrected for the remaining multipath delay by the method according to the invention, wherein the computed relative positions are computed from observations corrected by residuals estimated D hours earlier. This final step reduces the position precision from cm level to mm level. The residuals corre- sponding to each satellite are stored in memory at the computer station 6 for use in the forthcoming D hours.

The number of satellites being tracked are at least four, but it is preferable to use signals from more satellites, for example six or eight in order to achieve an accurate and reliable solution. The computer station 6 may receive observation data with a rate of 1

Hz or 0.1 Hz.

In FIG. 2 a flow diagram shows the method according to the invention. The first D hours period is used as an initialisation period where position data are obtained (1- obtaining position data in initialisation period) and where the computed three dimensional position vectors and corresponding residuals (2-calculating position vectors and residuals) during that period are stored (3 -storing position vectors and residuals in a memory). These observations are taken at predefined intervals, for example every second.

After a D hours initialisation period, a new set of data is obtained (4-obtain a new set of data after D hours) and the computed position vectors (5-compute position vectors and residuals) are compared to the vectors that were obtained D hours earlier (6- compare with vectors D-hours earlier, are they the same? Yes/No). If the vectors are identical within a predetermined threshold value, an OK indication is given (7-indicate OK), the computed vectors and residuals are stored (8-store vectors and residuals) and the procedure 4 is started again.

However, if the vectors differ by more than the selected threshold value, an indication is given (9-indicate difference), for example as a message on a computer display, and different routines are run to identify the cause of the difference.

Two different types of gross errors may occur

- when comparing observational residuals D hours apart, a given threshold value, say 3 mm, may be violated. This leads to deselection of the said satellite and recomputa- tion of position from the day before in order to compare to the present epoch. If more residuals violate the threshold the procedure is repeated. If several residuals show this pattern, it is time to consider whether the reference antenna is stable.

- when comparing differences of vector components D hours apart, the components in all three dimensions have to satisfy another given threshold value, for example 1 mm. If the threshold is violated, it leads to an alarm written on the computer screen. If not violated an OK statement is given. The latter should be the normal situation.

Claims

1. A metliod for reducing the effect of multipath delay in survey systems using signals received from positioning satellites, wherein the position based constellation of said positioning satellites repeats with a periodicity of D hours, characterised in that the metliod comprises the steps of

- at at least two different locations antenna means are located for receiving satellite signals in order to obtain at a first time a first set of position vectors computed from signals from at least four positional satellites and storing said position vectors in a storage medium,

- obtaining at a second time, after an integer number of D hours, a second set of position vectors computed from signals from the same at least four satellites,

- transferring said position vectors obtained from the at least two locations to a central processing station, - subtracting said second set of position vectors from said first set of position vectors.

2. A method according to claim 1 characterised in that the periodic cycle is 23:56 hours.

3. Method according to any of the previous claims characterised in that said computation of position vectors comprises a correction of the signals for effects due to ionospheric delay and due to tropospheric delay.

4. A method according to any of the previous claims characterised in that said method further comprises storing in a table calculated multipath errors between any satellite and any antenna for any measured epoch.

5. A method according to any of the previous claims characterised in that said first set of position vectors are stored together with data characteristic for weather conditions, in particular data relation to surface pressure, humidity and temperature.

6. An apparatus for obtaining position vectors between a reference location and a geographic relative point of interest

- a first antenna located at a reference location, operable to receive signals from positional satellites - a second antenna located at the relative point of interest operable to receive signals from positional satellites,

- a computer operable to receive said signals from said first and said second antenna and operable to compute position vectors from said signals, characterised in that said computer is operable - to store said computed position vectors between reference location and object location, and

- to compute residuals between position vectors that have been computed according to received satellite signals having a mutual time delay which is a multiple of D hours.

7. Apparatus according to claim 6 characterised in that said computation of position vectors comprises a model based correction of the signals for ionospheric delay and for tropospheric delay.

8. Apparatus according to claim 6or7characterised in that the computer is operable to store in a table calculated multipath errors between any satellite and any antenna for any measured epoch.