A spaceborne orbit determination system is being developed by the French Space Agency (CNES) for the SPOT 4 satellite. It processes DORIS measurements to produce an orbit with an accuracy of about 50O meters rms. In order to evaluate the reliability of the software, it was combined with the MERCATOR man/machine interface and used to process the TOPEX/Poseidon DORIS data in near real time during the validation phase of the instrument, at JPL and at CNES. This paper gives an overview of the orbit determination system and presents the results of the TOPEX/Poseidon experiment

Berthias, J.-P.

Jayles, C.

Pradines, D.

English

NASA Technical Reports Server

H.2 
Centre National d'Etudes Spatiales 
18 Av. Edouard Belin 
3 1055 TOULOUSE CEDEX 
FRANCE 
Abstract: A space borne orbit determination system is being developed by the French Space 
Agency (CNES) for the SPOT 4 satellite. It processes DORIS measurements to produce an orbit with 
an accuracy of about 50 meters rms. In order to evaluate the reliability of the software, it was 
combined with the MERCATOR man/machine interface and used to process the TOPEX/Poseidon 
DORIS data in near real time during the validation phase of the instrument, at JPL and at CNES. 
This paper gives an overview of the orbit determination system and presents the results of the 
TOPEX/Poseidon experiment. 
Introduction: 
Autonomous control of a satellite's orbit is an 
attractive concept which offers many advantages. 
Under the control of an on-board program, the 
optimum firing of the thrusters maintains the 
spacecraft on an orbit which has been pre- 
programmed. Meanwhile on-board instruments use the 
knowledge of the position and velocity of the 
spacecraft to orient themselves or pre-process their 
measurements. On the ground, operators at a 
simplified control center use telemetry data to verify 
the proper behavior of the spacecraft. 
Partial autonomy is already achieved by all satellites. 
Attitude control is a nearly completely automated task 
which is under the control of an on-board program. 
Since orientation measurements are performed by the 
spacecraft itself, and a short response time is 
necessary, the automation of attitude control is not 
only convenient but practically indispensable. On the 
other hand, until the last few years, the data necessary 
for precise navigation could only be obtained on the 
ground and required lengthy processing. Thanks to 
advances in the stability of on-board clocks, new on- 
board tracking systems including GPS and DORIS are 
now available. With these instruments the accurate 
measurements needed to compute the position and 
velocity of the spacecraft are available on-board, thus 
opening the door to automated navigation. 
As a first step in this direction, the French Space 
Agency, CNES (Centre National d'Etudes Spatiales) 
decided to pursue the subject of on-board orbit 
determination. First, a feasibility study was conducted 
in collaboration with ABrospatiale and Dassault 
Electronique. It demonstrated the possibility of 
computing the orbit of a low altitude satellite equipped 
with a DORIS receiver, using only a small program 
and limited computing resources. Following this 
success, it was decided to fly an on-board orbit 
determination system on the Earth observation satellite 
SPOT4, and the DIODE project was thus created. 
Some details of this system will be given in this paper. 
The presence of a DORTS receiver on 
TOPEX/Poseidon provided us with an opportunity to 
test the current version of the software in a new 
environment and validate some of our assumptions. In 
return, it offered a quick-look approach to the 
TOPEX/Poseidon DORIS orbit and a immediate 
appraisal of the performance of the instrument. 
DORIS: 
The DORIS system, developed jointly by CNES, the 
French National Geographic Institute, IGN (Institut 
Ghgraphique National) and the GRGS (Groupe de 
Recherches en G&d&ie Spatiale), uses a global 
network of ground beacons which broadcast 
omnidirectionally on two frequencies, 2036.25 and 
401.25 MHz. Each beacon contains an ultrastable 
quartz oscillator with a stability of about 5 for 
periods of 10 to 100 seconds. In addition, two master 
beacons, one in Toulouse and one in Kourou, are tied 
to atomic clocks. The DORIS instrument carried by 
https://ntrs.nasa.gov/search.jsp?R=19940019446 2020-06-16T15:36:40+00:00Z
the spacecraft contains a dual frequency receiver and 
an ultrastable oscillator. It computes the Doppler 
shifts and the of arrival of the time 
synchronization si The overall system noise is 
lower than 0.3 d s  [l]. 
The instrument computes the Doppler count at both 
frequencies every 10 seconds when a beacon is in 
view. In nominal operation mode, the receiver is 
programmed to listen to specific beacons, one beacon 
at a time. 
The orbital coverage provided by the extensive beacon 
network is over 70 R for the SPOT satellites (800 km 
altitude) and reaches 85 W for TOPEX/Poseidon 
(1330 km altitude). 
Implementation: 
To accommodate the on-board orbit determination 
software, Dassault Electronique, which manufactures 
the DORIS receiver, has added a new card to the 
instrument. This new card carries a Marcolli MAS 281 
microprocessor, which follows the MIL STD 1750A 
standard, and 48 kwords of available memory. The 
clock rate is 10 MHz. EDACs are used to detect and 
eliminate memory upsets. 
The memory of the new card is seen by the DORIS 
receiver as part of its memory. This allows for easy 
loading of the software while in flight, and permits the 
use of reserved memory locations for an interface 
between the receiver and the orbit determination 
system (cf Fig. 1). 
The orbit determination software is being developed 
by CNES. It runs under the supervision of the 
navigation management software written by Dassault 
Electronique, which is responsible, among other 
things, for the interrupt driven interface with SPOT 4. 
Both sets of software are written in Ada. The orbit 
determination program is designed following the 
Hierarchical Object Oriented Design (HOOD) 
method. With the help of the CNES Ada group, 
STOOD, a HOOD implementation tool developed by 
TNI, has been used to design and write the software. 
The result is a 2400 line long code which is easy to 
maintain and update. When compiled for the 1750 
target microprocessor using the VAX based TLD 
cross-compiler, the code and data occupies about 
32 kwords. 
. -  - I  
Fig. 1 - System architecture 
Principle of operation: 
Once an initial state vector has been uploaded, the 
function of the on-board orbit determination program 
is to update it every ten seconds. As the DORIS 
measurements are performed on-board, they are dated 
in instrument time, and a correspondence between 
instrument time and TAI has to be maintained to 
provide properly dated state vectors. 
Two separate Kalman filters are used to fulfil these 
functions. The first filter computes the difference 
between instrument time and TAI. This value, 
modeled as a constant, is updated when 
synchronization measurements are performed while 
flying over a master beacon. The second filter updates 
a state composed of the spacecraft position and 
velocity and a frequency bias for the current pass 
when measurements are performed. This filter is 
written using Bierman's UD formulation. 
The tuning parameters for'both filters can be modified 
by ground commands. 
When the navigation management software activates 
the orbit determination function, it provides the date 
at which the state is to be computed and the list of 
uploaded parameters to process. The date is equal to 
the TAI time of the end of the preceding cycle, 
rounded to an even multiple of 10 seconds. 
After activation of the orbit determination function by 
the navigation management software, the schedule of 
operations is as follows: 
1) If uploaded parameters are present, they are taken 
into account, and the values of the corresponding 
parameters are modified. 
2) The current state is extrapolated to the desired date. 
The extrapolation is performed by a fourth order 
Runge-Kutta integrator using the space efficient Gill 
formulation. 
580 
3) If measurements are performed over a master 
, the difference between instrument time and 
TAX is updated. 
Earth orientation model is almost non-existent: no 
correction is applied for precession, nutation, or polar 
motion. The Earth rotation rate is assumed constant 
4) If measurements are performed, the predicted value 
and the residual are computed. If the residual is lower 
than the acceptance threshold, the measurement is 
used to update the state. 
All these operations are performed in less than five 
seconds. 
If for some reason the orbit navigation function has 
not been activated over a few cycles, at the next call 
the difference between the date at which the state 
should be computed and the date of the current state 
can be large. As a maximum of five steps can be 
performed per call, it can take many calls for the 
system to catch up. During this interval, no 
measurement is processed and no result is provided to 
the user. 
Models: 
DORIS, a Doppler based tracking system, measures 
relative velocities. To relate the measured data to the 
spacecraft position, an integration of the equations of 
motion is required. Hence, a force model is necessary. 
Due to the excellent orbital coverage, periods during 
which the extrapolated position is not corrected by a 
measurement are very short. Therefore errors due to 
limitations in the force model cannot grow large. The 
current force model is limited to a dedicated sixth 
order and degree gravity field provided by the GRGS. 
This choice satisfies the accuracy requirements and 
reduces the memory requirement to a minimum. Third 
body perturbation and drag models have been 
developed, and could be easily included in the 
software if the mission justified it and if space was 
available. 
As the acceleration model is rudimentary, the model 
covariance input to the filter is large. This in turn 
gives a lot of weight to the measurements. The orbit is 
data driven, and this can be useful when an 
unexpected force is present. Least squares based 
standard orbit determination software cannot handle 
unmodeled forces, while this system can. In this 
respect, this system produces results similar to those 
of a partly dynamic and partly geometric analysis. 
Measurement models are also very simple. Dual 
frequency ionosphere correction is applied to the data. 
A basic troposphere delay correction, assuming a 
constant vertical electron content, is also applied. The 
and UTC is used in place of UT1 to compute the hour 
angle. Beacon locations are memorized in EKI on-board 
table which can be updated by ground commands. 
The choice of these models has been validated by 
processing many samples of SPOT 2 DORIS data. In 
particular, studies proved that the accuracy 
requirements are met even in the presence of the 
increased drag due to high solar activity or of frequent 
thruster firing such as periods when the spacecraft 
goes into safehold mode [2]. 
The TOPEX validation experiment: 
The launch of TOPEX/Poseidon on 10 August 1992 
provided an excellent opportunity to test the behavior 
of the orbit determination component of the system. 
Processing data from a new instrument is useful to 
ensure that no assumptions were made that are unique 
to the SPOT 2 configuration. 
Also, the high level of spacecraft activity between the 
launch and the transfer into nominal orbit requires the 
transmission to the software of many parameters, such 
as maneuver characteristics. 
It was therefore decided to run the software in near 
real time to process the TOPEX/Poseidon DORIS data 
both at JPL and at CNES. The environment for the 
orbit determination program was provided by 
MERCATOR. 
MERCATOR is regularly used by CNES to put 
geostationnary satellites in orbit [3]. It runs on SUN 
workstations and offers a framework within which 
technical modules can easily be included and 
activated. It was successfully employed for ten 
missions between April 1989 and September 1992. It 
is therefore a solid and reliable software which is 
ergonomically adapted to the needs of orbital 
mechanic engineers. 
For the TOPEX/Poseidon experiment MERCATOR 
was used to 
* decommutate the telemetry and supply the data in 
a form similar to that coming out of the DORIS 
instrument 
provide a user friendly interface 
permit the rapid analysis of results using auxiliary 
calculations and graphical tools 
- 
* 
581 
Fig. 2 - Time synchronization 
Telemetry from the DORIS instrument on the 
TOPEX/Poseidon spacecraft is recorded on-board and 
then played back to Earth via TDRSS. From this data 
the TOPEX/Poseidon Ground Segment generates a 
Selected Telemetry Record (STR) file daily. These 
files are then copied to the CNES control center via an 
electronic network. 
For our experiment, we could access these files both 
at JPL and at CNES. After the first few days, once a 
file was copied to our SUN, it took about half an hour 
to produce an ephemeris. 
In addition to this regular operation mode, we also 
processed special request STRs at JPL, using initial 
state vectors provided by the TOPEX/Poseidon 
navigation team. We were then able to produce short 
arc ephemerides only one to two hours after the 
acquisition of the data. This was especially helpful to 
check the state of the instrument after major 
operations (reset of the clock, change in the mode of 
operations, etc . ) . 
Overall, the quality of the results exceeded our 
expectations. In one instance, we had to integrate over 
an eight hour data gap, and were amazed to see the 
filter locked back on the right orbit on the first 
subsequent pass. Note that this would probably not be 
possible for the lower altitude SPOT spacecraft. 
In order to more accurately reproduce the conditions 
of on-board operations, we have reprocessed in a 
single batch the first 28 days of DORIS data following 
the second reset of the instrument clock. The uploaded 
parameters were the initial conditions, the date of the 
reset of the clock, the value of UTC minus TAI, and 
the pre-execution values of the five maneuvers (CAL, 
INC, IPM1, IPM2, IPM3) [4]. With this limited set 
of inputs, the orbit determination software produced 
the results shown below. 
Figure 2 shows the difference between DORIS time 
as computed by our orbit determination 
software using synchronization  reme men^ 
performed over the Toulouse master beacon. The 
curve clearly shows the clock adjustment of Aug. 19 
followed by the slow drift of the DORIS time. Note 
that at first the oscillator drifts, but, as the oscillator 
reaches a stable state, the difference becomes linear, 
due to the fact that the oscillator frequency is not 
exactly nominal. On the 18th day, an automated clock 
correction brought the DORIS clock back to TAL The 
error level in the time determination is at the level of a 
few tenths of milliseconds. This corresponds to a 
meter size error on the orbit in the along-track 
direction. Therefore, the time synchronization is not a 
limiting factor on the orbital accuracy. 
Fig 3 - Radial difference 
100 - 
6 1 50 
s o  
k 
P 
3 -50 
- * 00 
-,so 
0 5 20 SO 
-200 
Fig 4 - Along-track difference 
582 
show differences between the “on- 
a reference orbit obtained 
processing the DORIS data with the ZO 
precision level of the reference is 
The standard deviations corresponding to figures 3 
and 4 are 20 and 44 meters respectively. The overall 
standard deviation of the distance to the reference 
orbit is 53 meters. These results are therefore within 
the requirements of the SPOT 4 project. 
However, a few accidents contributed greatly to 
increase these statistics. In particular, this test showed 
us that our software does not properly handle forced 
clock resynchronizations, such as the one which 
happened on August 19. As these occur only after a 
restart of the instrument, we had never previously 
experienced one of those in our tests. 
It should be noted that maneuvers do not appear on 
these curves, except when there is a data gap, and 
therefore a reference orbit gap. On the 15th day of the 
arc, the apparent dip in the along-track curve is due to 
a lack of reference. This is repeated in the radial curve 
on the 22nd day. 
A periodic term with a daily period and an amplitude 
of about 20 meters can be observed in the along-track 
direction. This is probably due to the over- 
simplification of the Earth rotation model. 
Conclusion: 
The TOPEXlPoseidon experiment demonstrated the 
good operational behavior of the on-board orbit 
determination software and confirmed that the orbit 
accuracy requirements of SPOT 4 can be met. 
This experiment also demonstrated the interest of near 
real-time ground based processing of the DORIS data 
to assess the performance of the instrument, and 
eventually, to conduct spacecraft operations. 
Current tests on 1750 emulators confirm that results of 
similar quality should be obtained in real-time on- 
board of the SPOT 4 spacecraft. If the DORIS 
instrument on-board of TOPEXPoseidon had been 
equipped with the orbit determination card, position 
information with an accuracy of about 50 m rms 
would have been available to the spacecraft and all of 
its instruments since the turn-on of DORIS. 
Therefore, mission designers who plan to use DORIS 
can count on the availability of reliable position 
information on-board of the spacecraft. 
The authors wish to 
apply the concepts of object Oriented Design to this 
project and for guiding them through STOOD. They 
also wish to thank E. Cazala-Hourcade and 
L. Maisonobe who cheerfully provided them with the 
support needed to use MERCATOR, both at CNES 
and at JPL. 
References 
1. 
2. 
3. 
4. 
5. 
NOUEL F .  et al, DORIS: A Precise Satellite 
Positioning Doppler System, Astrodynamics 
1987, in Adv. Astron. Sci. 65, Solder et a1 (eds), 
BERTHIAS J.-P., JAYLES C., Real-time orbit 
determination on board of SPOT 4, proceedings 
of the IFAC workshop on spacecraft automation 
and on-board autonomous control, Darmstadt, 
Germany, 14-16 Sept. 1992 
TAVERNIER G., CAMPAN G., MERCATOR: 
MEthods and Realization for Control of the 
ATtitude and the ORbit of spacecrafi, 
SpaceOps 92, this volume 
FRAUENHOLZ R., Operational Navigation 
Support for the TOPEX/Poseidon Mission, 
SpaceOps 92, this volume 
NOUEL F. et al, Precise orbit determination with 
the DORIS/SPOT 2 system: first results, 
proceedings of the ESA symposium on Spacecraft 
Flight Dynamics, Darmstadt, Germany, 30 Sept.- 
4Oct. 1991 
311-320, 1988 
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Real-time on-board orbit determination with DORIS

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