CA2086304A1 - Communication satellite network - Google Patents
Communication satellite networkInfo
- Publication number
- CA2086304A1 CA2086304A1 CA 2086304 CA2086304A CA2086304A1 CA 2086304 A1 CA2086304 A1 CA 2086304A1 CA 2086304 CA2086304 CA 2086304 CA 2086304 A CA2086304 A CA 2086304A CA 2086304 A1 CA2086304 A1 CA 2086304A1
- Authority
- CA
- Canada
- Prior art keywords
- satellites
- substantially equal
- orbit
- hours
- orbits
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64G—COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
- B64G1/00—Cosmonautic vehicles
- B64G1/22—Parts of, or equipment specially adapted for fitting in or to, cosmonautic vehicles
- B64G1/24—Guiding or controlling apparatus, e.g. for attitude control
- B64G1/242—Orbits and trajectories
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64G—COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
- B64G1/00—Cosmonautic vehicles
- B64G1/10—Artificial satellites; Systems of such satellites; Interplanetary vehicles
- B64G1/1007—Communications satellites
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64G—COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
- B64G1/00—Cosmonautic vehicles
- B64G1/10—Artificial satellites; Systems of such satellites; Interplanetary vehicles
- B64G1/1085—Swarms and constellations
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
- H04B7/15—Active relay systems
- H04B7/185—Space-based or airborne stations; Stations for satellite systems
- H04B7/195—Non-synchronous stations
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64G—COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
- B64G1/00—Cosmonautic vehicles
- B64G1/10—Artificial satellites; Systems of such satellites; Interplanetary vehicles
- B64G1/1014—Navigation satellites
Abstract
ABSTRACT OF THE DISCLOSURE
A network of communication satellites, for example direct broadcast satellites, in elliptical orbit comprises five or six satellites distributed over elliptical orbits having the same track on the ground.
The orbits each have a period of eight hours and in each period of 24 hours three apogees situated over three predetermined regions of the globe. The orbits have an inclination substantially equal to 63.4° and are highly elliptical to produce high angles of elevation over each region and quasi-geostationary behavior during the active part of their orbit.
A network of communication satellites, for example direct broadcast satellites, in elliptical orbit comprises five or six satellites distributed over elliptical orbits having the same track on the ground.
The orbits each have a period of eight hours and in each period of 24 hours three apogees situated over three predetermined regions of the globe. The orbits have an inclination substantially equal to 63.4° and are highly elliptical to produce high angles of elevation over each region and quasi-geostationary behavior during the active part of their orbit.
Description
2~3~
1' COMMUNICATIOy-sATELLITE NETWORK
BACKGROUND OF THE INVENTION
Field of the invention The present invention concerns a method of forming a network of communication satellites, especially broadcast satellites.
Description of the prior art Direct broadcast satellites (D~S-R) have extremely interesting prospects; a market study carried out on behalf of NASA and VOA (Voice of America) has shown that a system of this kind could have 50 million users by the year 2000, this figure being likely to increase to 150 million by the year 2005. This market study also suggests that direct broadcast satellites could provide an effective replacement for existing shortwave broadcasting facilities and could achieve better coverage of the planet combined with improved quality.
Another potential of direct broadcast satellites is compact disk quality digital audio broadcasting ~DAB) targeted on users of top of the range ser~ices which are not available through the normal terrestrial channels.
The European Broadcasting Union is currently evaluating the potential advantages of DAB services.
The feasibility and the success of direct broadcast satellites are based on the ability to receive the satellite signal at all points of the network using small portable or mobile receivers. The commercial interest of direct broadcast satellites is essentially due to this possibility of obtaining a signal that is available at all times and is easy to use.
- The use of direct broadcast satellites and small re~ceivers leads to the requirement for a high transmitted power flux density (PFD) in the coverage region of the broadcast. This power flux density requirement can be satisfied by only one satellite only if high angles of .. . .~ , , .~ ,. ~ . . . .
2~3~
elevation are obtained so as to rninimize marginal effects due to shadow areas and to prevent total obstruction of the satellite line of sight (LOS).
- Geostationary (GEO) satellites are able to cover an ext:ensive reception area but have small angles of elevation at latitudes above 30. A geostationary satellite typically has a period of 23.934 hours, an angle of inclination of 0 and is in a circular orbit at ; an altitude of 35 786 km.
Geostationary satellites are not able to provide - satisfactory service to countries in the Northern hemisphere as this would require very powerful spacecraft, in the Intelsat category, and large and complex antenna systems. Because of this, the use of non-geostationary orbits must be considered for direct broadcast satellites and for communications via mobile ~ satellites (MOB SAT).
; The reception and types of service objectives are very similar.
Satellite networks have been proposed using hyper-elliptical (HEO) orbits. The well-known "LOOPUS", ~SYCOMORES~ and "ARCHIMEDES~ projects in this field concern regional systems covering Northern Europe. The ARCHlMEDES project is the only one that has taken into consideration direct broadcast satellite services.
A first version of the ARCHIMEDES system proposes to cover Northern Europe with high angles of elevation using a network of four satellites with a period of revolution of 12 hours in a MO~NYA type orbit. This *ype of orbit has an apogee at 39 375 km and a perigee at 1 000 km with an angle of inclination of 63.4. The 40 000 km apogee entails a very high launch cost. What is more, this cost cannot be amortized by the provision o~ other services in regions outside Europe because only Europe is covered by this service.
2~8~
A second version of the ARCHIMEDES system uses TOUNDRA type orbits. It has the same drawbacks and requires even greater launch power because the apogee is ;~ at 47 000 km.
An object of the present invention is a methcd of forming a network of communication satellites offering the following features in combination:
- high elevation angle over extensive regions, - apogee altitude much lower than that of MOLNYA
type orbits, and ' - continuous coverage of several regions of the globe.
SUMMARY OF THE INVENTION
In one aspect the present invention consists in a method of forming a network of communication satellites, for example direct broadcast satellites, comprising a step of placing in elliptical orbit a plurality of communication satelli-tes in which said satellites are distributed over elliptical orbits having the same track on the ground, said orbits each having a period of eight hours and in each 24 hour period three apogees situated above three predetermined regions of the globe at an altitude of substantially 27 000 km, said orbits having an angle of inclination substantially equal to 63.4, said satellites having a perigee argument of 270 and a highly elliptical orbit chosen to produce high angles of elevation over each of said regions and quasi-geostationary behavior during the active part of their orbit.
The satellites may advantageously be distributed so that two adjacent satellites are always in line of sight visibility from ground stations in said predetermined regions and activated only from a given altitude in the vicinity of their perigee.
In a first embodiment there are five satellites : ~ . . ,; . . ~, : . . : : .
2~3~
:' distributed over the orbit with a time offset substantially equal to four hours in order to obtain 16 consecutive hours service per day in each region.
In a preferred embodiment there are six satellites distributed over the orbit with a time offset ` substantially equal to four hours in order to obtain continuous service 24 hours a day in each region.
The apogee altitude may be substantially equal to 26 800 km. The perigee altitude may be substantially ]0 equal to 1 000 km.
The zooming ratio between two satellites is advantageously less than 1.5 and preferably substantially equal to 1.32.
The coverage area of the satellites is optimized by choosing an angle of approximately 270 for the orbit perigee argument.
In another aspect the present invention consists in a network of communication satellites, for example direct broadcast satellites, in elliptical orbit comprising five or six satellites distributed over elliptical orbits havin~ the same track on -the ground, said orbits each having a period of eight hours and in each 24 hour period three apogees situated above three predetermined regions of the globe, said orbits having an angle of inclination substantially equal to 63.4 and being highly elliptical to produce high angles of elevation over each of said regions and quasi-geostationary behavior during the active part of their orbit.
In a first embodiment there are five satellites distributed over the elliptical orbit with a time offset substantially equal to four hours to obtain 16 conseclltive hours service per day in each region.
In a preferred embodiment there are six satellites distributed over the elliptical orbit with a time ofifset substantially equal to four hours to obtain continuous 2 ~
service in each region.
The apogee altitude may be substantially equal to 26 800 km.
The perigee altitude may be substantially equal to 1 000 km.
The perigee argument may be substantially equal to 270.
The zooming ratio between two satellites is advantageously less than 1.5 and preferably substantially equal to 1.32.
Other features and advantages of the invention will emerge from the following description given by way of ~; non-limiting example with reference to the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the track on the ground over the entire globe of a network of six satellites in a preferred embodiment of the invention.
Figure 2 shows the iso-elevation contours of the figure 1 satellite network in three reception areas namely North America, Europe and the Far East.
Figure 3 shows the iso-elevation contours for Europe.
Figure 4 shows the iso-elevation contours for the Far East.
Figure 5 shows the iso-elevation contours for North America.
Figure 6 shows zooming losses as a function of the zooming ratio.
Figure 7 shows the gain of the satellite antenna expressed in dB as a function of the coverage area.
Figure 8 shows the iso-PFD curves at -3 dB and -5 ds gain at apogee for Europe.
Eigure 9 shows the iso-PFD curves at -3 dB and -5 ds gain at apogee for the Far Rast.
I
2~8~3~
Figure 10 shows the iso-PFD curves at -3 dB and -5 dB gain at apogee for North ~merica.
; Figures 11 through 20 show the track on the ground of the satellite during the four-hour service period for a plane polar receive antenna in the respective cities of Madrid, Oslo, Athens, Amsterdam, Tokyo, Seoul, Beijing, Vancouver, San Francisco and New York.
DETAILED DESCRIPTION OF THE INVENTION
The invention concerns a network of satellites in eight-hour elliptical orbits with parameters enabling the results outlined above to ba achieved. The various elliptical orbits have the same track on the ground so that for an observer on the ground the satellites follow the same apparent orbit with an offset in time. A
hyperelliptical orbit wi-~h a period of eight hours provides three apogee loops which define three service regions wi-th a satellite clltitude in the order of 27 000 km. As will emerge from this description, three major regions can be served with angles of elevation greater than 50. These are Western Europej Japan and Korea and the ma~or par-t of North America. Compared with an ARCHIMEDES type project usi.ng MOLNYA type orbits~ five or six satellites are used rather than four, but there are ~hree service areas ra-ther than one. Because the apogee altitude is much lower, the transmit power required is reduced by a factor of 2.5. The unit cost of a sat~llite and launching it is lower than for the ARCHIMEDES projects and the cost of putting the satellites into orbit could be shared between users in the three main service areas.
A general study of -the possibilities of multi-satellite networks using non-geostationary orbits can be found in the proceedings of the second European conference on satellite communications (ESA SP-32 October 1991) in a con~ribution by G.PERROTTA entitled "A
:, .... ,. ............. ' .,,: ,.. :,., .. ;.. ' ~ . .; ,:.
2~6~
comparison Between Low-Circular and Elliptical Inclined Orbits for Small Satellites Applications". This paper concludes that ~he use of elliptical orbits with a revolution period of eight hours would be of no benefit.
The basic idea of the invention is specifically to use satellites having a revolution period of eight hours to form a network of communication satellites to provide multiregional communication systems which are economically viable and which use a minimum number of satellites whilst providing optimum elevation.
According to the invention an orbital period of eight hours and the ec~entricity of orbits having the same apparent orbit are selected so that a large territory can be covered in three geographical areas whose longitudes are spaced by 120 around the e~uator (Europe, Far East, North America) with a high angle of elevation, continuously (24 hours a day) and with a minimum number (5~1) of satellites in orbit.
The perigee altitude is preferably l 000 km or greater to prevent the satellites at perigee being affected by the upper layers of the atmosphere.
As shown in figure 1, each spacecraft or satellite is operational for a period of four hours in each of its three apogee loops above the three service regions. Each satellite hands over to the next satellite on the same apparent orbit at the end of four hours. Figure 1 shows the track on the ground o~ the various orbits. They constitute a single apparent orbit on which the six satellites numbered 1 through 6 are disposed, the ~irst satellite being shown at an apogee altitude over Europe, satellite 2 which fo]lows it at an interval of four hours at its perigee over the South Pacific to the West of South America, satellite 3 at its apogee over the Far East, satellite 4 at its perigee over the South Atlantic, satellite 5 at its apogee over ~orth America and ..
, .
:~
2~63~
satellite 6 at its perigee over Southern ~ustralia.
The orbital inclination is chosen to minimize the fuel requirement for orbit control maneuvers. The stable orbital inclination of 63.45~ is chosen in this example.
Note that it would be possible to increase the angle of elevation in the regions served by the satellite by slightly reducing the orbital inclination but this would greatly increase the fuel requirement to counteract the drift affecting the peri~ee argument due to the oblate shape of the Earth (J2 effect).
Perigee argument drift must be avoided because the perigee argument value is crucial to optimizing the coverage area in the three service regions. An optimum value of the perigee argument is 270.
~ The optimum parameters of the satellite network are given below.
number of satellites: 6 (5 apogee altitude: 26 800 km perigee altitude: 1 000 km orbital period: 8 hours orbital inclination: 63.435 perigee argument: 270 usable period per orbit: 4 hours Satellite relay conditions for LOS visibility at 20 500 hn altitude:
ascendant node: ~ 60 mean anomaly: + 180.
A multiregional broadcast satellite natwork of this kind using six satellites enables a user to receive broadcast signals a~ an elevation better than 50 over Europe, South East Asia and the major part of the North American continent. Figures 2 through 5 show the curves for equal angles of elevation in these regions.
Figure 2 shows the curves for regions on the ground having angles of elevation o~ 40, 50 and 60 in the .
: 2~8~3~ ~
: g three reg,ions served by the satellite networks.
Figure 3 shows the curves for elevations of 50, ~; 60~, 65 over Europe.
- Figure 4 shows the curves for elevations of 40, 50 , 55 and 60 over South East Asia.
, Figure 5 shows the curves for elevations of 40, 50, 60~ over the North American continent.
The curves in figures 2 through 5 were obtained by superposing 50 elevation coverage curves at apogee and at the satellite handover points. The curves for an elevation of 40 cover major population areas in Taiwan, Hong-Kong and the Eastern seaboard of the United States.
Each of satellites l through 6 covers the three service regions at the same local time bu~ this time drifts slowly over the course of a year because of sidereal drift. The annual drift is approximately four minutes. This has repercussions on satellite r,eplacement strategy and on the performance and lifetime of the system, which are interdependent.
Five satellites correctly synchronized and offset relative to each other in orbit at four hours defining a ~'train~ of satellites spaced at four intervals of ~our hours with a no service interval of eight hours are capable of providing continuous service in the three areas between 8 am and midnight local time. The interval of eight hours between the fifth satellite and the first s,atellite is chosen to coincide with the hours of darkness during which interruption of service is acceptable. Because of the drift in local time on crossing the apogee, it is necessary to resynchronize the five satellites approximately every three months to re-align the service to the required local time. This reduces the system lifetime because i~ is necessary to provide fuel on the spacecraft in orbit to carry out this maneuver.
-2~3~
Using a network of six satellites spaced in orbit by four hours achieves continuous service 24 hours a day with no penalty in terms of satellite lifetime given that no resynchronization maneuver is required. Furthermore, the sys-tem is redundant to some degree as it is ~olerant of failure of one satellite.
The design of the satellite depends on the capabilities of the launch vehicle and this means that the system in accordance with the invention has a major advantage as compared with the prior art MOLNYA or TOUNDRA type hyperelliptical system because of its significantly lower apogee altitude. It has been estimated to a first approximation ~hat the satellite weight saving is approximately 150 to 300 kg.
The relatively low apogee altitude also means that the satellite power output can be reduced, losses in transmission at l.S GHz being 4 dB less than for a satellite in a MOLNY~ type orbit and 5 dB less than for a satellite in a TOUNDR~ orbit along the direction of the nadir.
It will therefore be understood that the system in accordance with the invention enables savings in te.rms of the satellite payload and the power output required to broadcast under satisfactory conditions.
Each service area is delimited by the guaranteed minimum elevation t40~, the coverage of the satellite antenna and the power availa~le onboard the spacecraft.
An optimum compromise be~ween the coverage area and the signal fading margin leads to the selection Of a service region for which the guaranteed minimu~ angle of elevation is not less than 50 at any time. ~s mentioned above, an angle of elevation of 50 can only be achieved by a geostationary satellite in regions of low latitude (less than 30). The 50 angle of elevation avoids most obstacles obstructing the line of sight visibility of the ~
~.
:: , , , ' , , ' ' ' " ' ~ , ,',' ', ' ~ , ' ,, , ' 20~6~0~
: 11 ~:' satellite. This makes it possible to minimize the satellite handover margins.
As indicated above, the iso-elevation contours for a 50~ angle of elevation include all of Europe in service region 1, all of the Far East in area 2 and the major part of the North American continent in service area 3.
The satellite antenna dimensions are dictated by the requirement~ of the three service areas to achieve a balance on average in respect of the 50 angle of elevation contours and the coverage of the more densely populated regions.
It is possible to use on the satellite a very simple antenna making the system feasible with a spacecraft of reasonable cost. Specificallyj it is possible to use the same single-beam reflector to serve the three service areas.
Also, it is possible to activate the satellites only when they are required for communication functions, that is to say only in the service area.
Nevertheless, it is possible to use more sophisticated antenna systems. A multibeam antenna can extend the usaful service region into areas in which the s~atellite line of sight has an angle of elevation of 40, which is of particular benefit in respect of the Eastern seaboard of ~he United States.
An optimum combination of the iso-elevation contours at apogee and the antenna radiation diagram is obtained with an antenna having a 2 m diameter reflector and a half-power beam divergence of 7 at 1.5 GHz. As the satellite travels over its operational arc across the orbital apogee the antenna beam is assumed to point at all times towards a reference point on the terrestrial surface. The antenna gain is approximately 25.5 dBi.
An important ~eature of non-geostationary systems is the variable altitude which causes "zooming" of the 2~3~ ~
~, .
track in the service area. The system in accordance with the invention is calibrated at apogee and the lower altitude at which handover between satellites occurs combats the effects of this zooming.
; 5 Figure 6 is a graph on which the zooming losses ZL
in dB are plotted as a function of the zooming ratio ZR.
The zooming ratio is defined as the apogee altitude divided by the altitude at handover. The satellite having an orbit service period of four hours, the handover altitude is the altitude two hours before or after apogee. For an eight-hour orbit with an apogee altitude of 27 000 km the handover altitude is 20 500 km so that the zooming ratio is 1.32. This ratio of 1.32 is at the maximum in figure 6. Note that the zooming effect does not cause any signal level loss and that there is even a slight gain because, at the starting point, the curve rises slightly above the 0 dB point. For an apogee altitude of 26 800 km the zooming ratio is 1.34, which is extremely close to the maximum. Zooming losses are not regarded aS significant if the zooming ratio Z~ does not exceed 1.5, representing a zooming loss ZL of approximately l dB.
Figure 7 shows the loss of gain of the onboard antenna as a function of the beam half-width, that is to say the depointing angle DEP in degrees. The operating point corresponds to a loss of -3 dB and a depointing angle of 3.5 and a beam aperture angle of 7. A point H
producing a loss of -5 dB corresponds to an angle of 9.
Figures 8 through 10 show -3 dB constant gain curves at apogee in the service regions. The dashed line contours represent 5 dB ~ons-tant gain curves at the two satellite handover points. Figures 8 through 10 show that the -3 dB contours a-t apogee and the -5 dB con-tour at handover are approximately coincident, so that it is possible to define a common coverage area for the -3 dB ~
-:
:. . : ;: . .: : : . " . , :,, ~
2~3~
constant gain contour at apogee and for the -5 dB
constant gain contour at handover. As the mean distance to the terrestrial surface near apogee is in the order of 27 Ooo km and at handover is 21 000 km there is a path difference of approximately 6 000 km between these two points, representing a difference of approximately 2 dB.
Because of this the -5 dB constant gain curves at handover and the -3 dB constant gain curves at apogee, which correspond to approximately equal coverage areas, also correspond to a transmitted power flux density (PFD~
which is approximately identical. In other words, figures 8 through 10 show that taking mean values of inclination into consideration the loss of gain due to the antenna zooming effect at handover is compensated by the reduced signal loss due to the lower altitude of the satellite at handover. This explains the trend of the curve shown in figure 6.
, The a~ove considerations highlight the important advantage that all system ratings may be decided at the apogee point without loss of general applicability.
It is therefore suf:ficient to define the service region as the region in which, irrespective of the position of the satellite in a six-satellite configuration, a user receives a guaranteed minimum power flux density (PFD) 24 hours a day with a guaranteed minimum elevation. These regions are those shown in continuous line in figures 8 through 10.
From the user point of view the xeceiver comprises a receive antenna, an RF part, a baseband demodulator and a signal decoder. The receive antenna must be simple, must not require any satellite tracking system and must meet satelli~e visibility criteria. It must be also easily accommodated in portable and mobile receivers.
The specifications of the receiver antenna have been drawn up by analyzing variations of azimuth and :: , , . . : : ~ . . .
~ !, ~ , ' . ' 2~3~
elevation at test points in the three service areas.
The results are shown in figures 11 through 20.
These figures are polar diagrams of the track of the satellite on the ground in the plane of the antenna. It is assumed that the plane of the antenna is coincident with the local horizontal plane. The center of the polar diagrams represents an elevation of 90. The concentric circles represent points with the same elevation.
Each of the diagrams represents the useful region of the track of the satellites on the ground, that is to say the regions between the two handover points. At Madrid (figure 11) the elevativn is between 55~ and 75, approximately, at Oslo (figure 12) the alevation is between 50 and 80, at Athens the elevation is between approximately 55 and slightly more than 70, at Amsterdam (figure 14J the elevation is between appro~imately 65 and approximately 78, at Tokyo ~figure 15) the elevation i~ between approximately 55 and 68, at Seoul the elevation is between 58 and 65, at Beijing (figure 17) the elevation is between approximately 55 and 73, at Vancouver (figure 18) the elevation is between approximately 60 and 85, at San Francisco (figure 19) the elevation is between approximately 58 and 65 and at New York (figure 20) the elevation is between approximately 45 and 72.
Figures 11 through 20 indica~e that an antenna providing coverage of the Northern hemisphere can have an omnidirectional azimuth diagram and a -3 dB aperture of approximately 90 and can be used in each of the three service areas, including those in which the satellite line of site is at a small angle of elevation (40).
An antenna having the above specifications is able to achieve a maximum gain of 5.5 dBi at 1.5 GHz. Because of the symmetry in azimuth and the aperture width, the antenna can also accommodate movements in the horizontal 2~8~3~
plane typical of mohile and portable receivers. Losses caused by movement of the satellite antenna in its plane are considered to represent additional losses of approximately 2.5 dB.
Estimating losses due to shadinq effects at 3 dB, ~his represents a 99.9~ possible coverage in rural ar0as for angles of elevation greater than 40.
. . .. . .: . :. ,.
1' COMMUNICATIOy-sATELLITE NETWORK
BACKGROUND OF THE INVENTION
Field of the invention The present invention concerns a method of forming a network of communication satellites, especially broadcast satellites.
Description of the prior art Direct broadcast satellites (D~S-R) have extremely interesting prospects; a market study carried out on behalf of NASA and VOA (Voice of America) has shown that a system of this kind could have 50 million users by the year 2000, this figure being likely to increase to 150 million by the year 2005. This market study also suggests that direct broadcast satellites could provide an effective replacement for existing shortwave broadcasting facilities and could achieve better coverage of the planet combined with improved quality.
Another potential of direct broadcast satellites is compact disk quality digital audio broadcasting ~DAB) targeted on users of top of the range ser~ices which are not available through the normal terrestrial channels.
The European Broadcasting Union is currently evaluating the potential advantages of DAB services.
The feasibility and the success of direct broadcast satellites are based on the ability to receive the satellite signal at all points of the network using small portable or mobile receivers. The commercial interest of direct broadcast satellites is essentially due to this possibility of obtaining a signal that is available at all times and is easy to use.
- The use of direct broadcast satellites and small re~ceivers leads to the requirement for a high transmitted power flux density (PFD) in the coverage region of the broadcast. This power flux density requirement can be satisfied by only one satellite only if high angles of .. . .~ , , .~ ,. ~ . . . .
2~3~
elevation are obtained so as to rninimize marginal effects due to shadow areas and to prevent total obstruction of the satellite line of sight (LOS).
- Geostationary (GEO) satellites are able to cover an ext:ensive reception area but have small angles of elevation at latitudes above 30. A geostationary satellite typically has a period of 23.934 hours, an angle of inclination of 0 and is in a circular orbit at ; an altitude of 35 786 km.
Geostationary satellites are not able to provide - satisfactory service to countries in the Northern hemisphere as this would require very powerful spacecraft, in the Intelsat category, and large and complex antenna systems. Because of this, the use of non-geostationary orbits must be considered for direct broadcast satellites and for communications via mobile ~ satellites (MOB SAT).
; The reception and types of service objectives are very similar.
Satellite networks have been proposed using hyper-elliptical (HEO) orbits. The well-known "LOOPUS", ~SYCOMORES~ and "ARCHIMEDES~ projects in this field concern regional systems covering Northern Europe. The ARCHlMEDES project is the only one that has taken into consideration direct broadcast satellite services.
A first version of the ARCHIMEDES system proposes to cover Northern Europe with high angles of elevation using a network of four satellites with a period of revolution of 12 hours in a MO~NYA type orbit. This *ype of orbit has an apogee at 39 375 km and a perigee at 1 000 km with an angle of inclination of 63.4. The 40 000 km apogee entails a very high launch cost. What is more, this cost cannot be amortized by the provision o~ other services in regions outside Europe because only Europe is covered by this service.
2~8~
A second version of the ARCHIMEDES system uses TOUNDRA type orbits. It has the same drawbacks and requires even greater launch power because the apogee is ;~ at 47 000 km.
An object of the present invention is a methcd of forming a network of communication satellites offering the following features in combination:
- high elevation angle over extensive regions, - apogee altitude much lower than that of MOLNYA
type orbits, and ' - continuous coverage of several regions of the globe.
SUMMARY OF THE INVENTION
In one aspect the present invention consists in a method of forming a network of communication satellites, for example direct broadcast satellites, comprising a step of placing in elliptical orbit a plurality of communication satelli-tes in which said satellites are distributed over elliptical orbits having the same track on the ground, said orbits each having a period of eight hours and in each 24 hour period three apogees situated above three predetermined regions of the globe at an altitude of substantially 27 000 km, said orbits having an angle of inclination substantially equal to 63.4, said satellites having a perigee argument of 270 and a highly elliptical orbit chosen to produce high angles of elevation over each of said regions and quasi-geostationary behavior during the active part of their orbit.
The satellites may advantageously be distributed so that two adjacent satellites are always in line of sight visibility from ground stations in said predetermined regions and activated only from a given altitude in the vicinity of their perigee.
In a first embodiment there are five satellites : ~ . . ,; . . ~, : . . : : .
2~3~
:' distributed over the orbit with a time offset substantially equal to four hours in order to obtain 16 consecutive hours service per day in each region.
In a preferred embodiment there are six satellites distributed over the orbit with a time offset ` substantially equal to four hours in order to obtain continuous service 24 hours a day in each region.
The apogee altitude may be substantially equal to 26 800 km. The perigee altitude may be substantially ]0 equal to 1 000 km.
The zooming ratio between two satellites is advantageously less than 1.5 and preferably substantially equal to 1.32.
The coverage area of the satellites is optimized by choosing an angle of approximately 270 for the orbit perigee argument.
In another aspect the present invention consists in a network of communication satellites, for example direct broadcast satellites, in elliptical orbit comprising five or six satellites distributed over elliptical orbits havin~ the same track on -the ground, said orbits each having a period of eight hours and in each 24 hour period three apogees situated above three predetermined regions of the globe, said orbits having an angle of inclination substantially equal to 63.4 and being highly elliptical to produce high angles of elevation over each of said regions and quasi-geostationary behavior during the active part of their orbit.
In a first embodiment there are five satellites distributed over the elliptical orbit with a time offset substantially equal to four hours to obtain 16 conseclltive hours service per day in each region.
In a preferred embodiment there are six satellites distributed over the elliptical orbit with a time ofifset substantially equal to four hours to obtain continuous 2 ~
service in each region.
The apogee altitude may be substantially equal to 26 800 km.
The perigee altitude may be substantially equal to 1 000 km.
The perigee argument may be substantially equal to 270.
The zooming ratio between two satellites is advantageously less than 1.5 and preferably substantially equal to 1.32.
Other features and advantages of the invention will emerge from the following description given by way of ~; non-limiting example with reference to the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the track on the ground over the entire globe of a network of six satellites in a preferred embodiment of the invention.
Figure 2 shows the iso-elevation contours of the figure 1 satellite network in three reception areas namely North America, Europe and the Far East.
Figure 3 shows the iso-elevation contours for Europe.
Figure 4 shows the iso-elevation contours for the Far East.
Figure 5 shows the iso-elevation contours for North America.
Figure 6 shows zooming losses as a function of the zooming ratio.
Figure 7 shows the gain of the satellite antenna expressed in dB as a function of the coverage area.
Figure 8 shows the iso-PFD curves at -3 dB and -5 ds gain at apogee for Europe.
Eigure 9 shows the iso-PFD curves at -3 dB and -5 ds gain at apogee for the Far Rast.
I
2~8~3~
Figure 10 shows the iso-PFD curves at -3 dB and -5 dB gain at apogee for North ~merica.
; Figures 11 through 20 show the track on the ground of the satellite during the four-hour service period for a plane polar receive antenna in the respective cities of Madrid, Oslo, Athens, Amsterdam, Tokyo, Seoul, Beijing, Vancouver, San Francisco and New York.
DETAILED DESCRIPTION OF THE INVENTION
The invention concerns a network of satellites in eight-hour elliptical orbits with parameters enabling the results outlined above to ba achieved. The various elliptical orbits have the same track on the ground so that for an observer on the ground the satellites follow the same apparent orbit with an offset in time. A
hyperelliptical orbit wi-~h a period of eight hours provides three apogee loops which define three service regions wi-th a satellite clltitude in the order of 27 000 km. As will emerge from this description, three major regions can be served with angles of elevation greater than 50. These are Western Europej Japan and Korea and the ma~or par-t of North America. Compared with an ARCHIMEDES type project usi.ng MOLNYA type orbits~ five or six satellites are used rather than four, but there are ~hree service areas ra-ther than one. Because the apogee altitude is much lower, the transmit power required is reduced by a factor of 2.5. The unit cost of a sat~llite and launching it is lower than for the ARCHIMEDES projects and the cost of putting the satellites into orbit could be shared between users in the three main service areas.
A general study of -the possibilities of multi-satellite networks using non-geostationary orbits can be found in the proceedings of the second European conference on satellite communications (ESA SP-32 October 1991) in a con~ribution by G.PERROTTA entitled "A
:, .... ,. ............. ' .,,: ,.. :,., .. ;.. ' ~ . .; ,:.
2~6~
comparison Between Low-Circular and Elliptical Inclined Orbits for Small Satellites Applications". This paper concludes that ~he use of elliptical orbits with a revolution period of eight hours would be of no benefit.
The basic idea of the invention is specifically to use satellites having a revolution period of eight hours to form a network of communication satellites to provide multiregional communication systems which are economically viable and which use a minimum number of satellites whilst providing optimum elevation.
According to the invention an orbital period of eight hours and the ec~entricity of orbits having the same apparent orbit are selected so that a large territory can be covered in three geographical areas whose longitudes are spaced by 120 around the e~uator (Europe, Far East, North America) with a high angle of elevation, continuously (24 hours a day) and with a minimum number (5~1) of satellites in orbit.
The perigee altitude is preferably l 000 km or greater to prevent the satellites at perigee being affected by the upper layers of the atmosphere.
As shown in figure 1, each spacecraft or satellite is operational for a period of four hours in each of its three apogee loops above the three service regions. Each satellite hands over to the next satellite on the same apparent orbit at the end of four hours. Figure 1 shows the track on the ground o~ the various orbits. They constitute a single apparent orbit on which the six satellites numbered 1 through 6 are disposed, the ~irst satellite being shown at an apogee altitude over Europe, satellite 2 which fo]lows it at an interval of four hours at its perigee over the South Pacific to the West of South America, satellite 3 at its apogee over the Far East, satellite 4 at its perigee over the South Atlantic, satellite 5 at its apogee over ~orth America and ..
, .
:~
2~63~
satellite 6 at its perigee over Southern ~ustralia.
The orbital inclination is chosen to minimize the fuel requirement for orbit control maneuvers. The stable orbital inclination of 63.45~ is chosen in this example.
Note that it would be possible to increase the angle of elevation in the regions served by the satellite by slightly reducing the orbital inclination but this would greatly increase the fuel requirement to counteract the drift affecting the peri~ee argument due to the oblate shape of the Earth (J2 effect).
Perigee argument drift must be avoided because the perigee argument value is crucial to optimizing the coverage area in the three service regions. An optimum value of the perigee argument is 270.
~ The optimum parameters of the satellite network are given below.
number of satellites: 6 (5 apogee altitude: 26 800 km perigee altitude: 1 000 km orbital period: 8 hours orbital inclination: 63.435 perigee argument: 270 usable period per orbit: 4 hours Satellite relay conditions for LOS visibility at 20 500 hn altitude:
ascendant node: ~ 60 mean anomaly: + 180.
A multiregional broadcast satellite natwork of this kind using six satellites enables a user to receive broadcast signals a~ an elevation better than 50 over Europe, South East Asia and the major part of the North American continent. Figures 2 through 5 show the curves for equal angles of elevation in these regions.
Figure 2 shows the curves for regions on the ground having angles of elevation o~ 40, 50 and 60 in the .
: 2~8~3~ ~
: g three reg,ions served by the satellite networks.
Figure 3 shows the curves for elevations of 50, ~; 60~, 65 over Europe.
- Figure 4 shows the curves for elevations of 40, 50 , 55 and 60 over South East Asia.
, Figure 5 shows the curves for elevations of 40, 50, 60~ over the North American continent.
The curves in figures 2 through 5 were obtained by superposing 50 elevation coverage curves at apogee and at the satellite handover points. The curves for an elevation of 40 cover major population areas in Taiwan, Hong-Kong and the Eastern seaboard of the United States.
Each of satellites l through 6 covers the three service regions at the same local time bu~ this time drifts slowly over the course of a year because of sidereal drift. The annual drift is approximately four minutes. This has repercussions on satellite r,eplacement strategy and on the performance and lifetime of the system, which are interdependent.
Five satellites correctly synchronized and offset relative to each other in orbit at four hours defining a ~'train~ of satellites spaced at four intervals of ~our hours with a no service interval of eight hours are capable of providing continuous service in the three areas between 8 am and midnight local time. The interval of eight hours between the fifth satellite and the first s,atellite is chosen to coincide with the hours of darkness during which interruption of service is acceptable. Because of the drift in local time on crossing the apogee, it is necessary to resynchronize the five satellites approximately every three months to re-align the service to the required local time. This reduces the system lifetime because i~ is necessary to provide fuel on the spacecraft in orbit to carry out this maneuver.
-2~3~
Using a network of six satellites spaced in orbit by four hours achieves continuous service 24 hours a day with no penalty in terms of satellite lifetime given that no resynchronization maneuver is required. Furthermore, the sys-tem is redundant to some degree as it is ~olerant of failure of one satellite.
The design of the satellite depends on the capabilities of the launch vehicle and this means that the system in accordance with the invention has a major advantage as compared with the prior art MOLNYA or TOUNDRA type hyperelliptical system because of its significantly lower apogee altitude. It has been estimated to a first approximation ~hat the satellite weight saving is approximately 150 to 300 kg.
The relatively low apogee altitude also means that the satellite power output can be reduced, losses in transmission at l.S GHz being 4 dB less than for a satellite in a MOLNY~ type orbit and 5 dB less than for a satellite in a TOUNDR~ orbit along the direction of the nadir.
It will therefore be understood that the system in accordance with the invention enables savings in te.rms of the satellite payload and the power output required to broadcast under satisfactory conditions.
Each service area is delimited by the guaranteed minimum elevation t40~, the coverage of the satellite antenna and the power availa~le onboard the spacecraft.
An optimum compromise be~ween the coverage area and the signal fading margin leads to the selection Of a service region for which the guaranteed minimu~ angle of elevation is not less than 50 at any time. ~s mentioned above, an angle of elevation of 50 can only be achieved by a geostationary satellite in regions of low latitude (less than 30). The 50 angle of elevation avoids most obstacles obstructing the line of sight visibility of the ~
~.
:: , , , ' , , ' ' ' " ' ~ , ,',' ', ' ~ , ' ,, , ' 20~6~0~
: 11 ~:' satellite. This makes it possible to minimize the satellite handover margins.
As indicated above, the iso-elevation contours for a 50~ angle of elevation include all of Europe in service region 1, all of the Far East in area 2 and the major part of the North American continent in service area 3.
The satellite antenna dimensions are dictated by the requirement~ of the three service areas to achieve a balance on average in respect of the 50 angle of elevation contours and the coverage of the more densely populated regions.
It is possible to use on the satellite a very simple antenna making the system feasible with a spacecraft of reasonable cost. Specificallyj it is possible to use the same single-beam reflector to serve the three service areas.
Also, it is possible to activate the satellites only when they are required for communication functions, that is to say only in the service area.
Nevertheless, it is possible to use more sophisticated antenna systems. A multibeam antenna can extend the usaful service region into areas in which the s~atellite line of sight has an angle of elevation of 40, which is of particular benefit in respect of the Eastern seaboard of ~he United States.
An optimum combination of the iso-elevation contours at apogee and the antenna radiation diagram is obtained with an antenna having a 2 m diameter reflector and a half-power beam divergence of 7 at 1.5 GHz. As the satellite travels over its operational arc across the orbital apogee the antenna beam is assumed to point at all times towards a reference point on the terrestrial surface. The antenna gain is approximately 25.5 dBi.
An important ~eature of non-geostationary systems is the variable altitude which causes "zooming" of the 2~3~ ~
~, .
track in the service area. The system in accordance with the invention is calibrated at apogee and the lower altitude at which handover between satellites occurs combats the effects of this zooming.
; 5 Figure 6 is a graph on which the zooming losses ZL
in dB are plotted as a function of the zooming ratio ZR.
The zooming ratio is defined as the apogee altitude divided by the altitude at handover. The satellite having an orbit service period of four hours, the handover altitude is the altitude two hours before or after apogee. For an eight-hour orbit with an apogee altitude of 27 000 km the handover altitude is 20 500 km so that the zooming ratio is 1.32. This ratio of 1.32 is at the maximum in figure 6. Note that the zooming effect does not cause any signal level loss and that there is even a slight gain because, at the starting point, the curve rises slightly above the 0 dB point. For an apogee altitude of 26 800 km the zooming ratio is 1.34, which is extremely close to the maximum. Zooming losses are not regarded aS significant if the zooming ratio Z~ does not exceed 1.5, representing a zooming loss ZL of approximately l dB.
Figure 7 shows the loss of gain of the onboard antenna as a function of the beam half-width, that is to say the depointing angle DEP in degrees. The operating point corresponds to a loss of -3 dB and a depointing angle of 3.5 and a beam aperture angle of 7. A point H
producing a loss of -5 dB corresponds to an angle of 9.
Figures 8 through 10 show -3 dB constant gain curves at apogee in the service regions. The dashed line contours represent 5 dB ~ons-tant gain curves at the two satellite handover points. Figures 8 through 10 show that the -3 dB contours a-t apogee and the -5 dB con-tour at handover are approximately coincident, so that it is possible to define a common coverage area for the -3 dB ~
-:
:. . : ;: . .: : : . " . , :,, ~
2~3~
constant gain contour at apogee and for the -5 dB
constant gain contour at handover. As the mean distance to the terrestrial surface near apogee is in the order of 27 Ooo km and at handover is 21 000 km there is a path difference of approximately 6 000 km between these two points, representing a difference of approximately 2 dB.
Because of this the -5 dB constant gain curves at handover and the -3 dB constant gain curves at apogee, which correspond to approximately equal coverage areas, also correspond to a transmitted power flux density (PFD~
which is approximately identical. In other words, figures 8 through 10 show that taking mean values of inclination into consideration the loss of gain due to the antenna zooming effect at handover is compensated by the reduced signal loss due to the lower altitude of the satellite at handover. This explains the trend of the curve shown in figure 6.
, The a~ove considerations highlight the important advantage that all system ratings may be decided at the apogee point without loss of general applicability.
It is therefore suf:ficient to define the service region as the region in which, irrespective of the position of the satellite in a six-satellite configuration, a user receives a guaranteed minimum power flux density (PFD) 24 hours a day with a guaranteed minimum elevation. These regions are those shown in continuous line in figures 8 through 10.
From the user point of view the xeceiver comprises a receive antenna, an RF part, a baseband demodulator and a signal decoder. The receive antenna must be simple, must not require any satellite tracking system and must meet satelli~e visibility criteria. It must be also easily accommodated in portable and mobile receivers.
The specifications of the receiver antenna have been drawn up by analyzing variations of azimuth and :: , , . . : : ~ . . .
~ !, ~ , ' . ' 2~3~
elevation at test points in the three service areas.
The results are shown in figures 11 through 20.
These figures are polar diagrams of the track of the satellite on the ground in the plane of the antenna. It is assumed that the plane of the antenna is coincident with the local horizontal plane. The center of the polar diagrams represents an elevation of 90. The concentric circles represent points with the same elevation.
Each of the diagrams represents the useful region of the track of the satellites on the ground, that is to say the regions between the two handover points. At Madrid (figure 11) the elevativn is between 55~ and 75, approximately, at Oslo (figure 12) the alevation is between 50 and 80, at Athens the elevation is between approximately 55 and slightly more than 70, at Amsterdam (figure 14J the elevation is between appro~imately 65 and approximately 78, at Tokyo ~figure 15) the elevation i~ between approximately 55 and 68, at Seoul the elevation is between 58 and 65, at Beijing (figure 17) the elevation is between approximately 55 and 73, at Vancouver (figure 18) the elevation is between approximately 60 and 85, at San Francisco (figure 19) the elevation is between approximately 58 and 65 and at New York (figure 20) the elevation is between approximately 45 and 72.
Figures 11 through 20 indica~e that an antenna providing coverage of the Northern hemisphere can have an omnidirectional azimuth diagram and a -3 dB aperture of approximately 90 and can be used in each of the three service areas, including those in which the satellite line of site is at a small angle of elevation (40).
An antenna having the above specifications is able to achieve a maximum gain of 5.5 dBi at 1.5 GHz. Because of the symmetry in azimuth and the aperture width, the antenna can also accommodate movements in the horizontal 2~8~3~
plane typical of mohile and portable receivers. Losses caused by movement of the satellite antenna in its plane are considered to represent additional losses of approximately 2.5 dB.
Estimating losses due to shadinq effects at 3 dB, ~his represents a 99.9~ possible coverage in rural ar0as for angles of elevation greater than 40.
. . .. . .: . :. ,.
Claims (15)
1. Method of forming a network of communication satellites, for example direct broadcast satellites, comprising a step of placing in elliptical orbit a plurality of communication satellites in which said satellites are distributed over elliptical orbits having the same track on the ground, said orbits each having a period of eight hours and in each 24 hour period three apogees situated above three predetermined regions of the globe at an altitude of substantially 27 000 km, said orbits having an angle of inclination substantially equal to 63.4°, said satellites having a perigee argument of 270° and a highly elliptical orbit chosen to produce high angles of elevation over each of said regions and quasi-geostationary behavior during the active part of their orbit.
2. Method according to claim 1 wherein said satellites are distributed over said orbit in such a way that two adjacent satellites are always in line of sight visibility from ground stations in said predetermined regions and said satellites are activated only from a given altitude in the vicinity of their perigee.
3. Method according to claim 2 wherein there are five satellites distributed over the orbit with a time offset substantially equal to four hours in order to obtain 16 consecutive hours service per day in each region.
4. Method according to claim 2 wherein there are six satellites distributed over the orbit with a time offset substantially equal to four hours in order to obtain continuous service 24 hours a day in each region.
5. Method according to claim 1 wherein said apogee altitude is substantially equal to 26 800 km.
6. Method according to claim 1 wherein said perigee altitude is substantially equal to 1 000 km.
7. Method according to claim 1 wherein the zooming ratio between two satellites is less than 1.5 and preferably substantially equal to 1.32.
8. Method according to claim 1 wherein the orbit perigee argument is substantially equal to 270°.
9. Network of communication satellites, for example direct broadcast satellites, in elliptical orbit comprising five or six satellites distributed over elliptical orbits having the same track on the ground, said orbits each having a period of eight hours and in each 24 hour period three apogees situated above three predetermined regions of the globe, said orbits having an angle of inclination substantially equal to 63.4° and being highly elliptical to produce high angles of elevation over each of said regions and quasi-geostationary behavior during the active part of their orbit.
10. Satellite network according to claim 9 comprising five satellites distributed over the elliptical orbit to obtain 16 consecutive hours service per day in each region.
11. Satellite network according to claim 9 comprising six satellites distributed over the elliptical orbit with a time offset substantially equal to four hours to obtain continuous service in each region.
12. Satellite network according to claim 9 wherein said apogee altitude is substantially equal to 26 800 km.
13. Satellite network according to claim 9 wherein said perigee altitude is substantially equal to 1 000 km.
14. Satellite network according to claim 9 wherein said perigee argument is substantially equal to 270°.
15. Satellite network according to claim 9 wherein the zooming ratio between two satellites is less than 1.5 and preferably substantially equal to 1.32.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR9116180A FR2685833B1 (en) | 1991-12-26 | 1991-12-26 | COMMUNICATION SATELLITE NETWORK. |
| FR9116180 | 1991-12-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2086304A1 true CA2086304A1 (en) | 1993-06-27 |
Family
ID=9420536
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2086304 Abandoned CA2086304A1 (en) | 1991-12-26 | 1992-12-24 | Communication satellite network |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPH05268131A (en) |
| CA (1) | CA2086304A1 (en) |
| FR (1) | FR2685833B1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6577864B2 (en) | 1995-03-24 | 2003-06-10 | Virtual Geosatellite, Llc | Elliptical satellite system which emulates the characteristics of geosynchronous satellites |
| US6611683B1 (en) | 1998-04-06 | 2003-08-26 | Virtual Geosatellite Holdings, Inc. | Elliptical satellite system which emulates the characteristics of geosynchronous satellites |
Families Citing this family (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| IT1261940B (en) * | 1993-09-24 | 1996-06-04 | Alenia Spazio Spa | TELECOMMUNICATIONS AND REMOTE SENSING SYSTEM VIA SATELLITE BASED ON THE USE OF SHORT-TERM ELLIPSY ELLIPTICAL ORBITS. |
| JP2693708B2 (en) * | 1993-09-30 | 1997-12-24 | 株式会社宇宙通信基礎技術研究所 | How to set up a satellite communication line |
| FR2735304B1 (en) * | 1995-06-12 | 1997-07-11 | Alcatel Espace | SCROLLING SATELLITE COMMUNICATION SYSTEM, SATELLITE, STATION AND TERMINAL INCLUDED |
| US6695259B1 (en) | 1997-05-21 | 2004-02-24 | Hitachi, Ltd. | Communication system, communication receiving device and communication terminal in the system |
| JP3477115B2 (en) * | 1999-03-16 | 2003-12-10 | 株式会社日立製作所 | Method for broadcasting via satellite, satellite broadcasting system, satellite broadcasting base station, and satellite broadcasting terminal |
| JP3153496B2 (en) | 1997-05-21 | 2001-04-09 | 株式会社日立製作所 | Communication service providing method using artificial satellite with long stay time in zenith direction |
| US6556808B1 (en) * | 1998-12-30 | 2003-04-29 | The Boeing Company | Fixed ground track satellite constellation and user terminal |
| US20040211864A1 (en) * | 2003-04-25 | 2004-10-28 | Less Gregory P. | Efficient communications utilizing highly inclined, highly elliptic orbits |
| CA2716174C (en) * | 2010-10-01 | 2019-11-26 | Telesat Canada | Satellite system |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3145207A1 (en) * | 1981-02-28 | 1982-09-23 | Siemens AG, 1000 Berlin und 8000 München | TELECOMMUNICATION SATELLITE SYSTEM WITH GEOSTATIONAL POSITION LOOPS |
| US4809935A (en) * | 1985-07-31 | 1989-03-07 | Analytic Services, Inc. | Satellite continuous coverage constellations |
| DE3607931A1 (en) * | 1986-03-11 | 1987-09-17 | Siemens Ag | TELECOMMUNICATION SATELLITE SYSTEM ON QUASISTATIONAL RAILWAYS |
| FR2628274B1 (en) * | 1988-03-02 | 1990-08-10 | Centre Nat Etd Spatiales | COMMUNICATIONS SYSTEM WITH MOBILES USING SATELLITES |
-
1991
- 1991-12-26 FR FR9116180A patent/FR2685833B1/en not_active Expired - Fee Related
-
1992
- 1992-12-24 JP JP34435992A patent/JPH05268131A/en not_active Withdrawn
- 1992-12-24 CA CA 2086304 patent/CA2086304A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6577864B2 (en) | 1995-03-24 | 2003-06-10 | Virtual Geosatellite, Llc | Elliptical satellite system which emulates the characteristics of geosynchronous satellites |
| US6611683B1 (en) | 1998-04-06 | 2003-08-26 | Virtual Geosatellite Holdings, Inc. | Elliptical satellite system which emulates the characteristics of geosynchronous satellites |
Also Published As
| Publication number | Publication date |
|---|---|
| JPH05268131A (en) | 1993-10-15 |
| FR2685833B1 (en) | 1994-02-11 |
| FR2685833A1 (en) | 1993-07-02 |
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