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DORIS (Doppler orbitography and radiopositioning integrated from space): System assessment results with DORIS on SPOT 2
Acta AstronauticaVol. 25, No. 8/9, pp. 497-504, 1991 Printed in Great Britain.All fights reserved
0094-5765/91 $3.00+ 0.00 Copyright~) 1991 PergamonPress pk
DORIS (DOPPLER ORBITOGRAPHY AND RADIOPOSITIONING INTEGRATED FROM SPACE): SYSTEM ASSESSMENT RESULTS WITH DORIS ON SPOT 21" M. Dop.g~, B. LASORDeand P. ~ M P S CNES, 18 Avenue Edouard Belin, 31055 Toulouse Cedex, France (Received 13 March 1991)
Ahstraet---CNES designed the DORIS system, based on Doppler measurements from a satellite receiver of a worldwide network of ground beacons. This system will be used to determine the precise orbit (with an accuracy of < 10 cm) of the altimetric satellite TOPEX-POSEIDON, and also to compute precise coordinates (subdecimetric accuracy) of additional unknown beacons. The beacon network has been deployed, and a receiver is operating on-board SPOT 2, launched on 22 January 1990, to evaluate the instrument characteristics and improve processing techniques and models. The main system characteristics are flescribed, and the results are presented: instrument performances, data quality and availability. Then the first processing results are described: orbit determination, earth gravity field model, positioning, ionosphere modelling. These results are preliminary, since many iterations will occur in late 1990, for a continuous improvement of the precision.
1. SYSTEM DESCIHlrrlON AND CHARACrERISTICS
of colocation with other precise positioning systems; they will be adjusted to l0 cm by processing of the first DORIS data. The GLBs have the same characteristics, but work by a solar panel and a battery power source, and can operate outside. To save power, 'they compute approximate satellite visibilities and transmit only during them. To minimize mutual jamming (since all beacons transmit on the same frequencies), it is necessary to sequence the transmission of close beacons: each one has its "time slots" of 10 s/rain. After decommutation, preprocessing, and time tagging at the control centre, DORIS data are stored in a data base and then entered into the orbit determination process. This process includes three steps:
1.1. Principle
The DORIS system is based on the measurement, by a satellite-borne receiver, of the spacecraft velocity relative to various ground beacons: ODBs (orbit determination beacons) and GLBs (ground location beacons). Every beacon transmits very stable frequencies: 2.03625 G H z and 401.25 MHz. The last one is also used to transmit housekeeping and meteorological data. Omnidirectional antennas are used for the beacons and the receiver. The useful radiofrequency visibility has been determined to be 10° above horizon. In this case, the receiver processes one beacon: it measures, every 10 s, the Doppler shifts on both frequencies, and also collects the transmitted data. This data is stored in the receiver telemetry memory, and dumped twice a day to the ground. A control centre preprocesses the telemetry, monitors the behaviour of the receiver(s) and the beacons. It also computes the "work program" for the following days, to tell the receiver which beacon is to be received, and when. This program is sent every day to the receiver through the "master beacon", linked to the control centre. The master beacon is also slaved to the international atomic time, which gives the possibility to determine the time tagging for all measurements in the same time scale. The planned ODB network is 50 beacons (Fig. 1). Their coordinates were known with an accuracy of 10 cm-5 m (or more), depending on the possibilities tPaper IAF-90-366 presented at the 41st Congresa of the International Astronautical Federation, Dresden, Fed. Rep. Germany, 8-12 October 1990.
--preprocessing, i.e. edition of wrong data, comparing actual data with theoretical ones obtained using an extrapolated orbit. Bad data may be due to excessive measurement noise (jamming) or telemetry errors. Too short visibilities are also eliminated; ----orbit determination, i.e. filtering of measurements, using a least square adjustment of orbital parameters, average frequencies, and many other parameters. A dynamic orbit restitution is used, with 1-4 day arcs; --postprocessing, i.e. interpretation of residuals (difference between actual data and theoretical ones obtained with the computed orbit). Depending on their value and spectrum, residuals may be considered as a combination of instrument or propagation errors, or of imperfect orbit determination (due to force model errors, or to the OD strategy). The simulations performed before the
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Fig. I. The network of DORIS orbit determination beacons (October 1990). I, Planned; II, installed. launch showed that the limiting item is the force models, and especially the Earth gravity field, much more than the instrument. This has been confirmed by real data. This makes it possible to tune models by processing actual data. Of course, several simultaneous satellites may be equipped with a DORIS receiver, and use the same beacon network.
1.2. Main features Rationale of the system choices, a n d their consequences, are summarized: Full-there, all-weather system: because radiofrequencies and automated beacons are used, there a r e no constraints of cloud cover, rain, operators availability. Geographic coverage: the main error source being the Earth gravity field model, significant efforts have been made to have an almost complete coverage, in order to constrain the force models everywhere by accurate measurements. As beacons are easier to accommodate than lasers, VLBIs, it has been possible to have an isotropic coverage, including the southern hemisphere. Accuracy: the Doppler avoids any bias and any calibration. The main counting is made at the 2 GHz frequency and provides a typical 0.3 mm/s accuracy. This is roughly equivalent to a 5 cm range determination over a satellite pass. E~icicnt ionospheric correction: the main effect (in I / F 2) is corrected to centimetric level by use of the
2 GHz-400 MHz couple. Other terms may be neglecU~i. Tropospheric correction: all beacons transmit pressme, temperature and humidity data so that a wet and dry tropospheric correction can be made. One way, uplink: one way simplifies the system design; it has been made possible by improvements in technology of crystal oscillators, whose stabilities in the short term (10 s) and the medium term (500 s) are comparable to, or better than, the atomic clocks. The average frequency during a satellite pass has to be identified during processing. The choice of uplink (instead of downlink like TRANSIT or GPS) also simplifies the system, since all data are present in the satellite telemetry: there is no need for ground links (telex, tapes, electronic mail etc. and their operators) to collect ground data. Nevertheless there are some drawbacks: there is only one on-board receiver (on a given satellite); random access is not possible because a long counting time (10s) is necessary to get precision, so the receiver can only process one beacon at a time. The system is adequate for up to 100 or 150 beacons (orbit determination requires 30-50). DORIS is a high accuracy, but saturable system. Total system control: another consequence of uplink is that all the system is managed from the same place (network monitoring, operations, processing). This is a quality factor, as well as perfect knowledge of the hardware, specified, developed and extensively retted by the same team. Users of orbit determination and point:positioning will obtain fully processed and validated results. As a counterpart, it must be noted
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that DORIS is not, and will never be, a navigation system. 2. SYSTEM GOALS
2.L DORIS for altimetry Oceanographic altimetry satellite missions, such as TOPEX-POSEIDON (NASA--CNES project, to be launched in 1992), aim at determining the ocean altitude with respect to a reference surface. Processing of these data will improve the current, eddy and tide studies and modelling, give a better knowledge of the energetic exchanges between oceans and atmosphere, and thus participate in the Global Change studies. Ocean altitude determination requires (a) a radar altimeter which measures the satellite--ocean range, (b) many geophysical corrections, and (c) a full-time precise knowledge of satellite ephemerides, especially the altitude. The last item has been the limiting factor as far as worldwide precision (with a low degree of geographic correlation of remaining errors) is required for loworbit satellites, and this led CNES to design a new orbit determination system: DORIS. DORIS data processing will also provide a better geoid, by means of the Earth gravity field computation. It will be easier to determine, in the altitude information, what is the contribution of the ocean circulation, and what is the constant effect of Earth gravity field irregularities. In addition, many DORIS beacons are located close to coastal tide gauges; height information from these gauges will be used to validate the altimeter measurements, and to obtain a time continuous (but only local) information. 2.2. DORIS for positioning Once a precise orbit has been computed, based on the Doppler measurements from the (known) ODBs, it is possible to insert "unknown" beacons in the system, and to position them, i.e. determine their coordinates. Positioning may be absolute or relative: --absolute, taking advantage of the worldwide beacon distribution and thus of the good orbit quality--everywhere--within 10 cm; --relative, between two or more beacons, with an expected accuracy of 10-7 . Applications concern all aspects of precise geodesy, mainly science studies (seismology, volcanology, plate tectonics, glaciology) and engineering (tunnels, pipelines, monitoring of landslides). Key advantages of DORIS for these applications are the automated beacons (low operating cost), and the end-to-end processing (system operations, orbit determination, positioning, and quality control). The ODBs network itself may be continuously studied using DORIS measurements: this will provide useful and long-term data about continent drift and sea level variations.
2.3. Modelling DORIS processing provides many by-products, as for any precise orbit determination system: Earth gravityfleld: 3 months of DORIS data have been included in the new GRIM4 model, computed by the French GRGS (Groupe de Recherche en Gb3d~sie Spatiale) together with the German DGFI (Deutsches Geoditisches Forschungslnstitut). The high orbit inclination (98°) and the good beacon coverage lead to a significant improvement of the model for sun-synchronous orbits (like SPOT, ERS1), but also for other satellites, such as TOPEXPOSEIDON. Geoid determination: though not being a competitor with dedicated geodetic missions like Aristoteles, the Earth gravity model computed with DORIS will provide a better geoid solution for the medium range of wavelengths (order 40). Ionosphere model: from the hifrequency measuremerit, with a ratio of 5 between both frequencies, an excellent ionospheric error correction may be obtained: ionosphere is not a problem for DORIS. Furthermore, the correction is a measurement of the derivative of the TEC (total electronic content) swept by the beacon-satellite line. It will be used to obtain a TEC model, to correct for the ionospheric propagation of the CNES (monofrequency) altimeter onboard TOPEX-POSEIDON: the raw error could reach 25 cm.
Other propagation and forces modelsmEarth rotation: DORIS orbit perturbations, as other precise OD techniques, but especially due to the worldwide beacon network, will provide useful data for the International Earth Rotation Service. 2.4. SPOT pictures quality improvement DORIS, being a passenger on SPOT, is not considered part of the mission. However, on a non-operational basis, DORIS orbits could be used by the SPOT ground system to make easier, or to improve, geometric corrections of the pictures, especially when dealing with stereoscopy. A decimetric or metric accuracy is easily obtained with DORIS, compared to the decametric or hectometric orbits commonly used. Also, absolute reference points may be quickly determined with DORIS beacon positioning, and further used to correct geometrical deformations of pictures.
2.5. Real-time orbitography Studies are in progress to compute satellite orbits in real time using the DORIS receiver and its onboard microprocessor. This could be used on future science missions which would require an on-board knowledge of medium quality (50 m) ephemeris. 3. FIRST ar.SULTS OF P O r e S ON SPOT 2
To be ready for the ambitious objectives of DORIS, it was necessary to validate the instrument
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and system performances and to develop the operational processing algorithms. CNES decided to deploy the ODB network, and to put a first receiver on-board SPOT 2. This Earth observation satellite was launched on 22 January 1990, on a sun-synchronous orbit (832 kin, 98.6 °, 26 day repeat cycle), almost 4 years after SPOT 1. After satellite manoeuvres to reach the operational orbit, the DORIS receiver was switched on, its software was uploaded, and the first Doppler measurement was made on 3 February. The first orbit was computed 2 weeks later. The first months were devoted to the system validation (DORIS is a system with some "loops", namely the generation of onboard work program and its transmission through the master beacon), and to the instrument performances assessment. All the first year is experimental, with permanent improvements of the ODB network, of the orbit determination and positioning strategies, references and force models. The results mentioned hereafter have to be considered as provisional; we are still far from the ultimate accuracy of the system. 3.1. Link budget (Fig. 2)
Though the DORIS frequencies are registered, we are not a primary user in these busy frequency bands. The DORIS uplinks were dimensioned with some assumptions on spurious levels, leading to a 10 dB margin with respect to the thermal noise. The spurious level depends on the geographic area: it will be analysed by processing the 10 days in "jamming mode". The beacons transmit 10 W on the 2 GHz channel, and 5 W on the 400 MHz. The received power is
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measured by the receiver and sent in the telemetry stream, and has been compared with the theoretically received power vs elevation angle. Figure 2 illustrates this very good comparison, with the measurements of 1 day (crosses), and the solid lines indicating for one channel the nominal budget and the expected dis. persion (due to beacon transmitters and antenna dispersion, propagation, ground reflections). Some plots are well below the line, due to masking effects or very bad conditions of ground reflections. The expected pre-launch useful elevation angle was considered to be 17°, an average value between a "no spurious" case and an assumed spurious level. Measurements have been made (probably with high noise) below 1°, in a special mode called "waiting" mode, where the receiver is autonomous and tries to track any incoming signal. This illustrates the high receiver sensitivity. The useful elevation angle was finally adjusted to 10°, and kept at 17° for the Toulouse master beacon. The analysis of orbit deterruination residuals shows that measurements below 10° are degraded (increased noise, tropospheric errors) and must be discarded. An experiment will be made with a 6 dB reduction on the beacon transmitted power. This would allow some simplifications of the design of GLBs. 3.2. Beacon network
The network deployment began in 1986 (at that time, SPOT 2 launch date was supposed to be 1987). At system setup, 33 ODBs were installed in many countries; 12 h later, 28 beacons were received by the on-board instrument. Typically 3 beacons had important failures, and some had obsolete back-up batteries, or meteorological sensor failures. These
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DORIS problems have been solved, and the network extended: 38 beacons were transmitting by the end o f October 1990, and negotiations are continuing to still improve this figure. The critical areas remain the South Pacific (unavailability of islands), the Australia-New Zealand region and South East Asia (due to long and complicated negotiations to install beacons). The average visibility ratio (time percentage with actual data) in August 1990 was 54%, against a theoretical value of 61% (geometrical visibility at 10° above horizon). Differences come from beacon failures, bad measurement due to spurious signals, telemetry errors, and from imperfections of the "work program" determination (the choice algorithm is when many beacons are visible). The theoretical ratio will be 72-80% when new beacons are incorporated into the network. It must again be noted that the non-visibility areas are rather uncorrelated; furthermore, the orbit determination process is a dynamic one, globally working on thousands of measurements, and thus spreads the orbital /
errors.
Initially, O D B coordinates were known v~ith an
accuracy between 10 crn and 5 m, depending whether the ODB is colocated with a laser or VLBI station, or if only a Doppler determination had been made. These positions have been improved by the DORIS data processing. 3.3. Instrument performance
It is specified in terms of noise (rms on 10s measurements) and medium term (mainly the slope during a satellite pass, up to 10 rain, i.e. 60 measurements). The long term stability does not matter, since the average value of frequencies is identified during processing.
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Medium term instrument errors are due mainly to the natural drift of oscillators (Allan variances 5.E-13 to 1.E-12 in the medium term), and to tropospheric propagation mismodelling during the satellite pass. Vertical tropospheric values are about 2.4m, measured with an accuracy of 2-5 cm by the three meteorological ground sensors. To convert to Doppler values, models assume a perfectly stratified troposphere, and classical law to rink the integrated value with the ground values. Globally, medium term error was assumed to be, at first order, a slope, rms 5.E-13/min. This slope is random from pass to pass. It is not possible at present to confirm this value by the analysis of orbit residuals, because medium term errors look fike the orbital noumodellized signal, and are included in it. Trials to identify the slopes give value in the - 3 0 to 30.E-13/min range, but highly correlated with the orbit determination conditions; one may only conclude that the instrument contribution is not significantly worse than the expected value, but it is probably better. Noise: the typical value of measurement noise is 0.3 mm/s, or 1.E-12. Theoretical values, from o n board and ground oscillators, thermal and spurious (jamming) noises, quantization and other minor instrumental errors, were computed in a 0.2-0.9 mm/s range, depending on satellite elevation and spurious level. This noise is easy to evidence in the orbit residuals, since it has only high frequencies (unlike the orbit errors), and may be isolated by filtering. Some very quiet areas show a 0.25 mm/s rms value on one pass. The average value for a 2-day arc is
0.4-0.5 mm/s. This filtering has also been done by the Guier technique (which fits the actual data to the best "orbit-like" Doppler curve) (Fig. 3). This gives somewhat higher results (0.8 mm/s), because the high-pass
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filtering is not so efficient: medium-term instrument and propagation errors are not filtered by Guier processing. Measured values also include possible instabilities of propagation (high order ionosphere, troposphere). This exactly confirms the instrument expected characteristics, and the hypothesis on the amount of spurious signals. 3.4. Data flow and availability The first DORIS mission, on SPOT 2, was designed without constraints of operationality. For instance, there are no redundancies in the ground system, and the telemetry reception may be lost for maintenance periods or due to higher priority satellites. Moreover, the processing delay is not a priority for this preparatory program: three or four different computers or teams are involved from the control centre to the orbit data bank. After the first 6 months, the results are quite satisfactory: the orbit is computed within 1 or 2 weeks after telemetry reception. Orbit losses, in this 180-day period, are due to several reasons: --the on-board receiver has a "jamming" mode, in which it measures the spurious spectrum in DORIS bandwidths, to understand the possible signal losses. This technological mode was activated on purpose during two SPOT subcycles, i.e. 10 days, --the telemetry station was unavailable for about 5 days, for maintenance activities, and during launch of the TDF2 satellite, - - D O R I S operational errors lost another 5 days. SPOT 2 orbit control manoeuvres last about 3 h once or twice a month. The orbit availability is about 95%. It will (TBC) be improved with a back-up of the telemetry receiving chain. r
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3.5. Ionosphere modelling Studies have been performed to obtain a total electronic content model daily, to validate this approach before TOPEX-POSEIDON launch, and compare it with the statistical Bent model. All corrections from 1 day are input into a mathematical model, based on spherical harmonics or on a grid. After a least square fitting, a TEC model is obtained, and can be compared to other results: --comparison with the Bent model gives a r m s error ffi 4 em (converted in altitude error at the altimeter frequency, 13.65 GHz), ----comparison with GPS bifrequency phase measurements at the same time as SPOT 2 overflights gives 2 crn, ----comparisons with EISCAT ionospheric radar are in progress. For that purpose, a DORIS beacon has been installed in Tromso~, and EISCAT measurements have been done when SPOT 2 was passing over the radar. These results, obtained during a high solar activity period (March 1990) are very promising for future
altimetric correction. The present difficulty is to find a precise worldwide reference to assess the accuracy. 3.6. Orbit determination Orbits are computed with l--4-day arcs. There are 4000- 5000 Doppler measurements per day. This large number and the low noise of the measurements make the preprocessing phase easier with several steps. --Obviously bad measurements (only one channel, or parity errors, or errors in the beacon number) have already been deleted by the control centre. - - T h e too short passes (less than 10 measurements), or too asymetrical ones, are discarded. - - B a d quality data, due to spurious signals, or sometimes the first and last in a pass, are eliminated by the Guier filtering: it may delete complete passes, or isolated noisy measurements (1-2% of data). Data are discarded mostly on geometrical criteria; it is planned to improve the "work program" deterruination function, to make it more homogenous with the OD strategy. The remaining good data correspond to 45% of the time. This value will be increased by adding new beacons. After the preprocessing, the CNES-developed ZOOM software is run to adjust the orbit parameters, the average frequencies per pass, and some other parameters such as air drag coefficients. It is not the purpose of this paper to detail the OD algorithms. Postprocessing follows, with use of different graphic and statistical tools to evaluate the residuals and OD quality. Many parameters may be analysed, for instance received power, residuals, average frequency, time tagging, propagation and other corrections, and this vs time, elevation angle etc. The residual analysis is of prime interest to assess the instrumental performances, the force models and the OD strategies. As SPOT 2 carries no laser reflector, altimeter or other more accurate orbit determination system, all performance evaluations have to be internal. ----overlapping techniques between two arcs with a common part, - - r m s of individual measurement residuals in mm/s (they include noise, ODB positioning errors, and remaining orbit errors), --interpretation in terms of mean anomaly residuals in m: statistical analysis of the along-track satellite error (one point per beacon visibility), --repositioning of stations, and analysis of drifts and variances. None of these techniques gives a direct evaluation of the orbit error, but they may be compared to other systems, indicating that the present rms radial error is 20-30cm, and I m for the along-track error. Measurement residuals are 1.8 mm/s. For instance, the effect of an eccentricity error (height variation between -t-1 and - 1 m) would be 5 mm/s. It is expected that analysis of a set of DORIS data by
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other teams will allow external check, essential at this level of accuracy. Experimental techniques of orbit determination, based on a development in Fourier series of the remaining acceleration errors, with identification of 120 coefficients per day, have led to a 0.8 mm/s rms residual.
3. 7. Earth gravity field Pre-launch simulations indicated that the limiting factor in the orbit precision is the forces applied to the satellite, mainly gravity (and also air drag and other surface forces). The first orbits were computed with the NASA-GSFC-provided GEM T2 model. From July to September, a new NASA model, PGS 3520, was used. Typical residuals were 5 mm/s. In parallel, the GRGS and DGFI have been working to develop a new model, GRIM4. This model-like any gravity model--includes data from several satellites and ground measurements. A preliminary solution including 3 months of DORIS data (in the March-June period) was obtained in August, and used by the Orbit Determination team. Together with this new model, a set of ODB coordinates was computed. Measurement residuals, for the same arcs, dropped to 1.8 mm/s with the preliminary solution. This value is also obtained for the time period not used in the GRIM model determination. This model is optimized for the SPOT orbit, but also improves the results obtained with other satellites, sun-synchronons or not. The definitive GRIM4 solution will require processing of another satellite (GEOSAT), and adjustment of weighting strategies, and will be issued in late 1990.
3.8. Positioning Beacon position determination, or "positioning", may be performed in several ways. The synthesis of the results is being done by IGN (French Institut G6ographique National, partner of CNES in the DORIS processing). Adjustments will be necessary to evaluate the results and compare them with other precise positioning systems. Inputs will come from:
Orbit computation: we had a poor knowledge of the coordinates of two beacons; their positions were identified in the OD process. Precision appeared to be I m for 2-day arcs. Earth gravity field determination: this determination also provides a set of coordinates; precision is estimated to be 10-20era for the 3-month period. Geometrical processing: this is the nominal algorithm which will be used by the DORIS positioning service. It assumes perfect computed orbits, and adjusts the station coordinates to have the best fit with the Doppler data. Positioning results were produced either in absolute, or in relative: Absolute positioning was tested by using some ODBs (considered as unknown), and ground location
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beacons. The accuracy, based on the laser or VLBI enlocations, has been determined to be 70 cm, with a monthly variability of about 30cm. First results obtained with the first solution of the new gravity field model GRIM4 show a 20 em accuracy. Relative positioning has been tested with a pair of ODBs (300 m apart in Toulouse) and has given a 6 em accuracy in only 2 days. A long-term positioning compaign has taken place in Djibouti, with one ODB and two prototypes of ground location beacons (ranges between 15 and 45 km). Values of 10cm rms were obtained on 6-day periods (5 em in longitude, 9cm in latitude and 15 em in altitude). Other experiments have started, in a volcanic area (Hawai), and on a landslide hazard zone in south-east France, but have not been fully processed yet. It must be pointed out that these accuracies and precisions, obtained in July 1990 with the geometrical technique, have been limited by the orbit quality results, themselves hardly depending on the gravity model (PGS3520, not including DORIS data). Figures will be improved when the new GRIM model is used.
4. CONCLUSION: STATUS AND EVOLUTION
The first DORIS measurement was done on 3 February 1990. During the first 6 months, about 6 weeks were spent checking the system behaviour and technological modes. Most of the time was devoted to collecting Doppler data and compute orbits. Orbit quality has been continuously improved with optimization of the strategies (preprocessing, weighthag), and of the force models: especially air drag, and Earth gravity field. Residuals went down from 7mm/s (initial orbits) to 1.8 mm/s (in September 1990) with the preliminary GRIM4 model, This will be improved further with the final gravity model, and expected adjustments in orbit determination algorithms. The absolute point positioning results are of 70-100 cm accuracy. The relative point positioning gave a 10 cm precision on 6 days. These (July !990) results are also likely to be improved. The rest of 1990 will consist in further adjustments of the orbit and positioning techniques. In 1991, the pilot positioning experiments will continue, with their demonstrative and scientific character. Moreover, operational positioning compaigns will start, during the whole SPOT 2---and DORIS--life. A DORIS receiver is already integrated in SPOT 3 (1993+), and another one is planned for SPOT 4 (1996+). High-accuracy positioning with DORIS, and ODB network maintenance are ensured for the nineties. DORIS receivers for the TOPEX-POSEIDON altimetric satellite will be integrated in late 1990. Launch is planned in mid-1992, for a 3-5-year life. This satellite is higher than SPOT (1335 km), so the orbit
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determination is easier: still increased visibility ratio, reduced drag, smoother effects of Earth gravity anomalies. Results already obtained from SPOT 2 guarantee that the requested orbit determination will be obtained for TOPEX-POSEIDON: "10cm on the satellite height". Moreover, the modellization of the ionospheric content has validated the principle o f mono-frequency altimetry. For that purpose, a DORIS receiver can be carried out on future altimetric satellites. REFERENCES
1. M. Dorrer, Le syst~me DORIS. Cours International de Technologie Spatiale, CEPADUES Editions, Toulouse, France (1989).
2. M. Dotter, DORIS stirSPOT 2--DORIS on board SPOT 2, SPOT newsletter.S P O T I M A G E No. 13,June 1990. 3. F, Nouel, A. Piuzzi and Ch. Jayles,DORIS experiments with SPOT 2. At~mees in Spacecraft Dynamics, XXVII COSPAR Meeting (1990). 4. J. J. Valette and A. Cazenave, Precise positioning in the Afar resion with the DORIS system. 2nd International Symposium with the GPS System, Ottawa, Canada 099O). 5. P. Willis et a/., Positioning with the DORIS system: present status and first results. 2hd International Symposium with the GPS System, Ottawa, Canada (1990). 6. For more information on the DORIS system status and results, a "DORIS newsletter" is regularly published, starting December 1990. Contact CLS-Argos, 18 Avenue E. Iklin, 31055 Toulouse.
0094-5765/91 $3.00+ 0.00 Copyright~) 1991 PergamonPress pk
DORIS (DOPPLER ORBITOGRAPHY AND RADIOPOSITIONING INTEGRATED FROM SPACE): SYSTEM ASSESSMENT RESULTS WITH DORIS ON SPOT 21" M. Dop.g~, B. LASORDeand P. ~ M P S CNES, 18 Avenue Edouard Belin, 31055 Toulouse Cedex, France (Received 13 March 1991)
Ahstraet---CNES designed the DORIS system, based on Doppler measurements from a satellite receiver of a worldwide network of ground beacons. This system will be used to determine the precise orbit (with an accuracy of < 10 cm) of the altimetric satellite TOPEX-POSEIDON, and also to compute precise coordinates (subdecimetric accuracy) of additional unknown beacons. The beacon network has been deployed, and a receiver is operating on-board SPOT 2, launched on 22 January 1990, to evaluate the instrument characteristics and improve processing techniques and models. The main system characteristics are flescribed, and the results are presented: instrument performances, data quality and availability. Then the first processing results are described: orbit determination, earth gravity field model, positioning, ionosphere modelling. These results are preliminary, since many iterations will occur in late 1990, for a continuous improvement of the precision.
1. SYSTEM DESCIHlrrlON AND CHARACrERISTICS
of colocation with other precise positioning systems; they will be adjusted to l0 cm by processing of the first DORIS data. The GLBs have the same characteristics, but work by a solar panel and a battery power source, and can operate outside. To save power, 'they compute approximate satellite visibilities and transmit only during them. To minimize mutual jamming (since all beacons transmit on the same frequencies), it is necessary to sequence the transmission of close beacons: each one has its "time slots" of 10 s/rain. After decommutation, preprocessing, and time tagging at the control centre, DORIS data are stored in a data base and then entered into the orbit determination process. This process includes three steps:
1.1. Principle
The DORIS system is based on the measurement, by a satellite-borne receiver, of the spacecraft velocity relative to various ground beacons: ODBs (orbit determination beacons) and GLBs (ground location beacons). Every beacon transmits very stable frequencies: 2.03625 G H z and 401.25 MHz. The last one is also used to transmit housekeeping and meteorological data. Omnidirectional antennas are used for the beacons and the receiver. The useful radiofrequency visibility has been determined to be 10° above horizon. In this case, the receiver processes one beacon: it measures, every 10 s, the Doppler shifts on both frequencies, and also collects the transmitted data. This data is stored in the receiver telemetry memory, and dumped twice a day to the ground. A control centre preprocesses the telemetry, monitors the behaviour of the receiver(s) and the beacons. It also computes the "work program" for the following days, to tell the receiver which beacon is to be received, and when. This program is sent every day to the receiver through the "master beacon", linked to the control centre. The master beacon is also slaved to the international atomic time, which gives the possibility to determine the time tagging for all measurements in the same time scale. The planned ODB network is 50 beacons (Fig. 1). Their coordinates were known with an accuracy of 10 cm-5 m (or more), depending on the possibilities tPaper IAF-90-366 presented at the 41st Congresa of the International Astronautical Federation, Dresden, Fed. Rep. Germany, 8-12 October 1990.
--preprocessing, i.e. edition of wrong data, comparing actual data with theoretical ones obtained using an extrapolated orbit. Bad data may be due to excessive measurement noise (jamming) or telemetry errors. Too short visibilities are also eliminated; ----orbit determination, i.e. filtering of measurements, using a least square adjustment of orbital parameters, average frequencies, and many other parameters. A dynamic orbit restitution is used, with 1-4 day arcs; --postprocessing, i.e. interpretation of residuals (difference between actual data and theoretical ones obtained with the computed orbit). Depending on their value and spectrum, residuals may be considered as a combination of instrument or propagation errors, or of imperfect orbit determination (due to force model errors, or to the OD strategy). The simulations performed before the
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Fig. I. The network of DORIS orbit determination beacons (October 1990). I, Planned; II, installed. launch showed that the limiting item is the force models, and especially the Earth gravity field, much more than the instrument. This has been confirmed by real data. This makes it possible to tune models by processing actual data. Of course, several simultaneous satellites may be equipped with a DORIS receiver, and use the same beacon network.
1.2. Main features Rationale of the system choices, a n d their consequences, are summarized: Full-there, all-weather system: because radiofrequencies and automated beacons are used, there a r e no constraints of cloud cover, rain, operators availability. Geographic coverage: the main error source being the Earth gravity field model, significant efforts have been made to have an almost complete coverage, in order to constrain the force models everywhere by accurate measurements. As beacons are easier to accommodate than lasers, VLBIs, it has been possible to have an isotropic coverage, including the southern hemisphere. Accuracy: the Doppler avoids any bias and any calibration. The main counting is made at the 2 GHz frequency and provides a typical 0.3 mm/s accuracy. This is roughly equivalent to a 5 cm range determination over a satellite pass. E~icicnt ionospheric correction: the main effect (in I / F 2) is corrected to centimetric level by use of the
2 GHz-400 MHz couple. Other terms may be neglecU~i. Tropospheric correction: all beacons transmit pressme, temperature and humidity data so that a wet and dry tropospheric correction can be made. One way, uplink: one way simplifies the system design; it has been made possible by improvements in technology of crystal oscillators, whose stabilities in the short term (10 s) and the medium term (500 s) are comparable to, or better than, the atomic clocks. The average frequency during a satellite pass has to be identified during processing. The choice of uplink (instead of downlink like TRANSIT or GPS) also simplifies the system, since all data are present in the satellite telemetry: there is no need for ground links (telex, tapes, electronic mail etc. and their operators) to collect ground data. Nevertheless there are some drawbacks: there is only one on-board receiver (on a given satellite); random access is not possible because a long counting time (10s) is necessary to get precision, so the receiver can only process one beacon at a time. The system is adequate for up to 100 or 150 beacons (orbit determination requires 30-50). DORIS is a high accuracy, but saturable system. Total system control: another consequence of uplink is that all the system is managed from the same place (network monitoring, operations, processing). This is a quality factor, as well as perfect knowledge of the hardware, specified, developed and extensively retted by the same team. Users of orbit determination and point:positioning will obtain fully processed and validated results. As a counterpart, it must be noted
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that DORIS is not, and will never be, a navigation system. 2. SYSTEM GOALS
2.L DORIS for altimetry Oceanographic altimetry satellite missions, such as TOPEX-POSEIDON (NASA--CNES project, to be launched in 1992), aim at determining the ocean altitude with respect to a reference surface. Processing of these data will improve the current, eddy and tide studies and modelling, give a better knowledge of the energetic exchanges between oceans and atmosphere, and thus participate in the Global Change studies. Ocean altitude determination requires (a) a radar altimeter which measures the satellite--ocean range, (b) many geophysical corrections, and (c) a full-time precise knowledge of satellite ephemerides, especially the altitude. The last item has been the limiting factor as far as worldwide precision (with a low degree of geographic correlation of remaining errors) is required for loworbit satellites, and this led CNES to design a new orbit determination system: DORIS. DORIS data processing will also provide a better geoid, by means of the Earth gravity field computation. It will be easier to determine, in the altitude information, what is the contribution of the ocean circulation, and what is the constant effect of Earth gravity field irregularities. In addition, many DORIS beacons are located close to coastal tide gauges; height information from these gauges will be used to validate the altimeter measurements, and to obtain a time continuous (but only local) information. 2.2. DORIS for positioning Once a precise orbit has been computed, based on the Doppler measurements from the (known) ODBs, it is possible to insert "unknown" beacons in the system, and to position them, i.e. determine their coordinates. Positioning may be absolute or relative: --absolute, taking advantage of the worldwide beacon distribution and thus of the good orbit quality--everywhere--within 10 cm; --relative, between two or more beacons, with an expected accuracy of 10-7 . Applications concern all aspects of precise geodesy, mainly science studies (seismology, volcanology, plate tectonics, glaciology) and engineering (tunnels, pipelines, monitoring of landslides). Key advantages of DORIS for these applications are the automated beacons (low operating cost), and the end-to-end processing (system operations, orbit determination, positioning, and quality control). The ODBs network itself may be continuously studied using DORIS measurements: this will provide useful and long-term data about continent drift and sea level variations.
2.3. Modelling DORIS processing provides many by-products, as for any precise orbit determination system: Earth gravityfleld: 3 months of DORIS data have been included in the new GRIM4 model, computed by the French GRGS (Groupe de Recherche en Gb3d~sie Spatiale) together with the German DGFI (Deutsches Geoditisches Forschungslnstitut). The high orbit inclination (98°) and the good beacon coverage lead to a significant improvement of the model for sun-synchronous orbits (like SPOT, ERS1), but also for other satellites, such as TOPEXPOSEIDON. Geoid determination: though not being a competitor with dedicated geodetic missions like Aristoteles, the Earth gravity model computed with DORIS will provide a better geoid solution for the medium range of wavelengths (order 40). Ionosphere model: from the hifrequency measuremerit, with a ratio of 5 between both frequencies, an excellent ionospheric error correction may be obtained: ionosphere is not a problem for DORIS. Furthermore, the correction is a measurement of the derivative of the TEC (total electronic content) swept by the beacon-satellite line. It will be used to obtain a TEC model, to correct for the ionospheric propagation of the CNES (monofrequency) altimeter onboard TOPEX-POSEIDON: the raw error could reach 25 cm.
Other propagation and forces modelsmEarth rotation: DORIS orbit perturbations, as other precise OD techniques, but especially due to the worldwide beacon network, will provide useful data for the International Earth Rotation Service. 2.4. SPOT pictures quality improvement DORIS, being a passenger on SPOT, is not considered part of the mission. However, on a non-operational basis, DORIS orbits could be used by the SPOT ground system to make easier, or to improve, geometric corrections of the pictures, especially when dealing with stereoscopy. A decimetric or metric accuracy is easily obtained with DORIS, compared to the decametric or hectometric orbits commonly used. Also, absolute reference points may be quickly determined with DORIS beacon positioning, and further used to correct geometrical deformations of pictures.
2.5. Real-time orbitography Studies are in progress to compute satellite orbits in real time using the DORIS receiver and its onboard microprocessor. This could be used on future science missions which would require an on-board knowledge of medium quality (50 m) ephemeris. 3. FIRST ar.SULTS OF P O r e S ON SPOT 2
To be ready for the ambitious objectives of DORIS, it was necessary to validate the instrument
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and system performances and to develop the operational processing algorithms. CNES decided to deploy the ODB network, and to put a first receiver on-board SPOT 2. This Earth observation satellite was launched on 22 January 1990, on a sun-synchronous orbit (832 kin, 98.6 °, 26 day repeat cycle), almost 4 years after SPOT 1. After satellite manoeuvres to reach the operational orbit, the DORIS receiver was switched on, its software was uploaded, and the first Doppler measurement was made on 3 February. The first orbit was computed 2 weeks later. The first months were devoted to the system validation (DORIS is a system with some "loops", namely the generation of onboard work program and its transmission through the master beacon), and to the instrument performances assessment. All the first year is experimental, with permanent improvements of the ODB network, of the orbit determination and positioning strategies, references and force models. The results mentioned hereafter have to be considered as provisional; we are still far from the ultimate accuracy of the system. 3.1. Link budget (Fig. 2)
Though the DORIS frequencies are registered, we are not a primary user in these busy frequency bands. The DORIS uplinks were dimensioned with some assumptions on spurious levels, leading to a 10 dB margin with respect to the thermal noise. The spurious level depends on the geographic area: it will be analysed by processing the 10 days in "jamming mode". The beacons transmit 10 W on the 2 GHz channel, and 5 W on the 400 MHz. The received power is
et al.
measured by the receiver and sent in the telemetry stream, and has been compared with the theoretically received power vs elevation angle. Figure 2 illustrates this very good comparison, with the measurements of 1 day (crosses), and the solid lines indicating for one channel the nominal budget and the expected dis. persion (due to beacon transmitters and antenna dispersion, propagation, ground reflections). Some plots are well below the line, due to masking effects or very bad conditions of ground reflections. The expected pre-launch useful elevation angle was considered to be 17°, an average value between a "no spurious" case and an assumed spurious level. Measurements have been made (probably with high noise) below 1°, in a special mode called "waiting" mode, where the receiver is autonomous and tries to track any incoming signal. This illustrates the high receiver sensitivity. The useful elevation angle was finally adjusted to 10°, and kept at 17° for the Toulouse master beacon. The analysis of orbit deterruination residuals shows that measurements below 10° are degraded (increased noise, tropospheric errors) and must be discarded. An experiment will be made with a 6 dB reduction on the beacon transmitted power. This would allow some simplifications of the design of GLBs. 3.2. Beacon network
The network deployment began in 1986 (at that time, SPOT 2 launch date was supposed to be 1987). At system setup, 33 ODBs were installed in many countries; 12 h later, 28 beacons were received by the on-board instrument. Typically 3 beacons had important failures, and some had obsolete back-up batteries, or meteorological sensor failures. These
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DORIS problems have been solved, and the network extended: 38 beacons were transmitting by the end o f October 1990, and negotiations are continuing to still improve this figure. The critical areas remain the South Pacific (unavailability of islands), the Australia-New Zealand region and South East Asia (due to long and complicated negotiations to install beacons). The average visibility ratio (time percentage with actual data) in August 1990 was 54%, against a theoretical value of 61% (geometrical visibility at 10° above horizon). Differences come from beacon failures, bad measurement due to spurious signals, telemetry errors, and from imperfections of the "work program" determination (the choice algorithm is when many beacons are visible). The theoretical ratio will be 72-80% when new beacons are incorporated into the network. It must again be noted that the non-visibility areas are rather uncorrelated; furthermore, the orbit determination process is a dynamic one, globally working on thousands of measurements, and thus spreads the orbital /
errors.
Initially, O D B coordinates were known v~ith an
accuracy between 10 crn and 5 m, depending whether the ODB is colocated with a laser or VLBI station, or if only a Doppler determination had been made. These positions have been improved by the DORIS data processing. 3.3. Instrument performance
It is specified in terms of noise (rms on 10s measurements) and medium term (mainly the slope during a satellite pass, up to 10 rain, i.e. 60 measurements). The long term stability does not matter, since the average value of frequencies is identified during processing.
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Medium term instrument errors are due mainly to the natural drift of oscillators (Allan variances 5.E-13 to 1.E-12 in the medium term), and to tropospheric propagation mismodelling during the satellite pass. Vertical tropospheric values are about 2.4m, measured with an accuracy of 2-5 cm by the three meteorological ground sensors. To convert to Doppler values, models assume a perfectly stratified troposphere, and classical law to rink the integrated value with the ground values. Globally, medium term error was assumed to be, at first order, a slope, rms 5.E-13/min. This slope is random from pass to pass. It is not possible at present to confirm this value by the analysis of orbit residuals, because medium term errors look fike the orbital noumodellized signal, and are included in it. Trials to identify the slopes give value in the - 3 0 to 30.E-13/min range, but highly correlated with the orbit determination conditions; one may only conclude that the instrument contribution is not significantly worse than the expected value, but it is probably better. Noise: the typical value of measurement noise is 0.3 mm/s, or 1.E-12. Theoretical values, from o n board and ground oscillators, thermal and spurious (jamming) noises, quantization and other minor instrumental errors, were computed in a 0.2-0.9 mm/s range, depending on satellite elevation and spurious level. This noise is easy to evidence in the orbit residuals, since it has only high frequencies (unlike the orbit errors), and may be isolated by filtering. Some very quiet areas show a 0.25 mm/s rms value on one pass. The average value for a 2-day arc is
0.4-0.5 mm/s. This filtering has also been done by the Guier technique (which fits the actual data to the best "orbit-like" Doppler curve) (Fig. 3). This gives somewhat higher results (0.8 mm/s), because the high-pass
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filtering is not so efficient: medium-term instrument and propagation errors are not filtered by Guier processing. Measured values also include possible instabilities of propagation (high order ionosphere, troposphere). This exactly confirms the instrument expected characteristics, and the hypothesis on the amount of spurious signals. 3.4. Data flow and availability The first DORIS mission, on SPOT 2, was designed without constraints of operationality. For instance, there are no redundancies in the ground system, and the telemetry reception may be lost for maintenance periods or due to higher priority satellites. Moreover, the processing delay is not a priority for this preparatory program: three or four different computers or teams are involved from the control centre to the orbit data bank. After the first 6 months, the results are quite satisfactory: the orbit is computed within 1 or 2 weeks after telemetry reception. Orbit losses, in this 180-day period, are due to several reasons: --the on-board receiver has a "jamming" mode, in which it measures the spurious spectrum in DORIS bandwidths, to understand the possible signal losses. This technological mode was activated on purpose during two SPOT subcycles, i.e. 10 days, --the telemetry station was unavailable for about 5 days, for maintenance activities, and during launch of the TDF2 satellite, - - D O R I S operational errors lost another 5 days. SPOT 2 orbit control manoeuvres last about 3 h once or twice a month. The orbit availability is about 95%. It will (TBC) be improved with a back-up of the telemetry receiving chain. r
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3.5. Ionosphere modelling Studies have been performed to obtain a total electronic content model daily, to validate this approach before TOPEX-POSEIDON launch, and compare it with the statistical Bent model. All corrections from 1 day are input into a mathematical model, based on spherical harmonics or on a grid. After a least square fitting, a TEC model is obtained, and can be compared to other results: --comparison with the Bent model gives a r m s error ffi 4 em (converted in altitude error at the altimeter frequency, 13.65 GHz), ----comparison with GPS bifrequency phase measurements at the same time as SPOT 2 overflights gives 2 crn, ----comparisons with EISCAT ionospheric radar are in progress. For that purpose, a DORIS beacon has been installed in Tromso~, and EISCAT measurements have been done when SPOT 2 was passing over the radar. These results, obtained during a high solar activity period (March 1990) are very promising for future
altimetric correction. The present difficulty is to find a precise worldwide reference to assess the accuracy. 3.6. Orbit determination Orbits are computed with l--4-day arcs. There are 4000- 5000 Doppler measurements per day. This large number and the low noise of the measurements make the preprocessing phase easier with several steps. --Obviously bad measurements (only one channel, or parity errors, or errors in the beacon number) have already been deleted by the control centre. - - T h e too short passes (less than 10 measurements), or too asymetrical ones, are discarded. - - B a d quality data, due to spurious signals, or sometimes the first and last in a pass, are eliminated by the Guier filtering: it may delete complete passes, or isolated noisy measurements (1-2% of data). Data are discarded mostly on geometrical criteria; it is planned to improve the "work program" deterruination function, to make it more homogenous with the OD strategy. The remaining good data correspond to 45% of the time. This value will be increased by adding new beacons. After the preprocessing, the CNES-developed ZOOM software is run to adjust the orbit parameters, the average frequencies per pass, and some other parameters such as air drag coefficients. It is not the purpose of this paper to detail the OD algorithms. Postprocessing follows, with use of different graphic and statistical tools to evaluate the residuals and OD quality. Many parameters may be analysed, for instance received power, residuals, average frequency, time tagging, propagation and other corrections, and this vs time, elevation angle etc. The residual analysis is of prime interest to assess the instrumental performances, the force models and the OD strategies. As SPOT 2 carries no laser reflector, altimeter or other more accurate orbit determination system, all performance evaluations have to be internal. ----overlapping techniques between two arcs with a common part, - - r m s of individual measurement residuals in mm/s (they include noise, ODB positioning errors, and remaining orbit errors), --interpretation in terms of mean anomaly residuals in m: statistical analysis of the along-track satellite error (one point per beacon visibility), --repositioning of stations, and analysis of drifts and variances. None of these techniques gives a direct evaluation of the orbit error, but they may be compared to other systems, indicating that the present rms radial error is 20-30cm, and I m for the along-track error. Measurement residuals are 1.8 mm/s. For instance, the effect of an eccentricity error (height variation between -t-1 and - 1 m) would be 5 mm/s. It is expected that analysis of a set of DORIS data by
DORIS
other teams will allow external check, essential at this level of accuracy. Experimental techniques of orbit determination, based on a development in Fourier series of the remaining acceleration errors, with identification of 120 coefficients per day, have led to a 0.8 mm/s rms residual.
3. 7. Earth gravity field Pre-launch simulations indicated that the limiting factor in the orbit precision is the forces applied to the satellite, mainly gravity (and also air drag and other surface forces). The first orbits were computed with the NASA-GSFC-provided GEM T2 model. From July to September, a new NASA model, PGS 3520, was used. Typical residuals were 5 mm/s. In parallel, the GRGS and DGFI have been working to develop a new model, GRIM4. This model-like any gravity model--includes data from several satellites and ground measurements. A preliminary solution including 3 months of DORIS data (in the March-June period) was obtained in August, and used by the Orbit Determination team. Together with this new model, a set of ODB coordinates was computed. Measurement residuals, for the same arcs, dropped to 1.8 mm/s with the preliminary solution. This value is also obtained for the time period not used in the GRIM model determination. This model is optimized for the SPOT orbit, but also improves the results obtained with other satellites, sun-synchronons or not. The definitive GRIM4 solution will require processing of another satellite (GEOSAT), and adjustment of weighting strategies, and will be issued in late 1990.
3.8. Positioning Beacon position determination, or "positioning", may be performed in several ways. The synthesis of the results is being done by IGN (French Institut G6ographique National, partner of CNES in the DORIS processing). Adjustments will be necessary to evaluate the results and compare them with other precise positioning systems. Inputs will come from:
Orbit computation: we had a poor knowledge of the coordinates of two beacons; their positions were identified in the OD process. Precision appeared to be I m for 2-day arcs. Earth gravity field determination: this determination also provides a set of coordinates; precision is estimated to be 10-20era for the 3-month period. Geometrical processing: this is the nominal algorithm which will be used by the DORIS positioning service. It assumes perfect computed orbits, and adjusts the station coordinates to have the best fit with the Doppler data. Positioning results were produced either in absolute, or in relative: Absolute positioning was tested by using some ODBs (considered as unknown), and ground location
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beacons. The accuracy, based on the laser or VLBI enlocations, has been determined to be 70 cm, with a monthly variability of about 30cm. First results obtained with the first solution of the new gravity field model GRIM4 show a 20 em accuracy. Relative positioning has been tested with a pair of ODBs (300 m apart in Toulouse) and has given a 6 em accuracy in only 2 days. A long-term positioning compaign has taken place in Djibouti, with one ODB and two prototypes of ground location beacons (ranges between 15 and 45 km). Values of 10cm rms were obtained on 6-day periods (5 em in longitude, 9cm in latitude and 15 em in altitude). Other experiments have started, in a volcanic area (Hawai), and on a landslide hazard zone in south-east France, but have not been fully processed yet. It must be pointed out that these accuracies and precisions, obtained in July 1990 with the geometrical technique, have been limited by the orbit quality results, themselves hardly depending on the gravity model (PGS3520, not including DORIS data). Figures will be improved when the new GRIM model is used.
4. CONCLUSION: STATUS AND EVOLUTION
The first DORIS measurement was done on 3 February 1990. During the first 6 months, about 6 weeks were spent checking the system behaviour and technological modes. Most of the time was devoted to collecting Doppler data and compute orbits. Orbit quality has been continuously improved with optimization of the strategies (preprocessing, weighthag), and of the force models: especially air drag, and Earth gravity field. Residuals went down from 7mm/s (initial orbits) to 1.8 mm/s (in September 1990) with the preliminary GRIM4 model, This will be improved further with the final gravity model, and expected adjustments in orbit determination algorithms. The absolute point positioning results are of 70-100 cm accuracy. The relative point positioning gave a 10 cm precision on 6 days. These (July !990) results are also likely to be improved. The rest of 1990 will consist in further adjustments of the orbit and positioning techniques. In 1991, the pilot positioning experiments will continue, with their demonstrative and scientific character. Moreover, operational positioning compaigns will start, during the whole SPOT 2---and DORIS--life. A DORIS receiver is already integrated in SPOT 3 (1993+), and another one is planned for SPOT 4 (1996+). High-accuracy positioning with DORIS, and ODB network maintenance are ensured for the nineties. DORIS receivers for the TOPEX-POSEIDON altimetric satellite will be integrated in late 1990. Launch is planned in mid-1992, for a 3-5-year life. This satellite is higher than SPOT (1335 km), so the orbit
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determination is easier: still increased visibility ratio, reduced drag, smoother effects of Earth gravity anomalies. Results already obtained from SPOT 2 guarantee that the requested orbit determination will be obtained for TOPEX-POSEIDON: "10cm on the satellite height". Moreover, the modellization of the ionospheric content has validated the principle o f mono-frequency altimetry. For that purpose, a DORIS receiver can be carried out on future altimetric satellites. REFERENCES
1. M. Dorrer, Le syst~me DORIS. Cours International de Technologie Spatiale, CEPADUES Editions, Toulouse, France (1989).
2. M. Dotter, DORIS stirSPOT 2--DORIS on board SPOT 2, SPOT newsletter.S P O T I M A G E No. 13,June 1990. 3. F, Nouel, A. Piuzzi and Ch. Jayles,DORIS experiments with SPOT 2. At~mees in Spacecraft Dynamics, XXVII COSPAR Meeting (1990). 4. J. J. Valette and A. Cazenave, Precise positioning in the Afar resion with the DORIS system. 2nd International Symposium with the GPS System, Ottawa, Canada 099O). 5. P. Willis et a/., Positioning with the DORIS system: present status and first results. 2hd International Symposium with the GPS System, Ottawa, Canada (1990). 6. For more information on the DORIS system status and results, a "DORIS newsletter" is regularly published, starting December 1990. Contact CLS-Argos, 18 Avenue E. Iklin, 31055 Toulouse.