Satellite Navigation Using GPS
T.J. Martín Mur & J.M. Dow
Orbit Attitude Division, European Space Operations Centre (ESOC), Darmstadt,
Introduction these systems could also be used for a wide
The launch of the first Sputnik triggered the range of scientific and other civil applications.
first challenge in satellite navigation, the de- New tracking methods that were not even
termination of the characteristics of the orbit foreseen by the original developers of the
of the satellite, using the variations in the sig- systems, like carrier tracking, were proposed
nal that was being radiated by the satellite. and, as soon as it was possible, successfully
Very soon the idea of using the inverse proc- tested and used.
ess was developed: if by knowing your posi-
tion you could determine the orbit of the One of the applications that was soon envi-
sioned was the use of GPS for navigation of
spacecraft. The first on-board receiver was
installed and flown in a Landsat satellite even
The Global Positioning System (GPS) is currently being used for a before the complete GPS constellation was
wide variety of applications. A GPS receiver on-board a spacecraft deployed. Since that time more receivers
can provide the means for autonomous navigation and it also have been flown on satellites, first as a dem-
allows a very accurate reconstitution of the trajectory of the onstration of increasingly precise uses and
spacecraft when on-board recorded measurements are combined now as the main operational means of navi-
with ground based measurements. gation.
This article outlines some of the basic concepts involved and The NAVSTAR Global Positioning System
presents the activities that the European Space Operations Centre The NAVSTAR Global Positioning System,
is carrying out in the field of satellite navigation using GPS. usually called GPS, consists of three compo-
nents: a space segment of GPS satellites, a
control segment that monitors and operates
those satellites and a user segment that em-
satellite, then it should also be possible to use ploys GPS receivers to observe and record
the signal transmitted by a satellite in a known transmissions from the satellites and perform
orbit in order to determine your own position. position, velocity, attitude and time calcula-
This concept was implemented in a series of tions.
satellites sponsored and operated by the US
Armed Forces. First the Transit satellites The GPS Space Segment
were deployed, then the Timation and finally
The space segment is based on three-axis
the NAVSTAR GPS system.
stabilized satellites orbiting in near-circular
orbits with a period of half a sidereal day and
The focus of these programs was to provide
an inclination of 55 degrees. There are six or-
the military forces of the US and its allies with
bital planes, each of them with four satellites.
precise positioning capabilities. In response
This constellation provides global coverage
the Soviet Union also developed and de-
with more than four satellites in view at all
ployed similar Global Navigation Satellite
Systems (GNSS): Tsikada and GLONASS.
The significance of the visibility of at least four
Right from the beginning it was realized that
satellites is that the GPS system is intended The GPS Control Segment
to allow instantaneous real time determina- The GPS control segment tracks and moni-
tion of the user position (3 variables) and the tors the signal from the GPS space segment
time of the fix (one more variable). Previous and estimates the orbits and clock behaviour
positioning systems, like the methods used in of the satellites. This information is uploaded
the Transit and Tsikada systems, were based to the satellites so it can be transmitted to us-
on the processing of several passes of data ers.
(requiring hours to days) and did not provide
the instantaneous solutions that GPS (or The GPS User Segment
GLONASS) offers. The GPS user segment can perform two ba-
sic measurements of the GPS signals. It can
The GPS satellites carry very stable atomic compare the C/A or P code that it is receiving
clocks that are used to derive the ranging sig- with a locally generated copy in order to
nals. The basic signal for civil use, L1, has a compute the transmission delay between the
frequency of 1575.42 MHz and it is modulat- satellite and the receiver. This measurement
ed with a Clear Acquisition (C/A) Pseudo is called pseudorange. Pseudoranges to four
Random Noise (PRN) code at 1.023 MHz that or more satellites can be used to determine
is different for every satellite. The signal is the position of the user once the position of
also modulated with a 10.23 MHz Precise (P) the GPS satellites has been obtained using
code that is usually encrypted and only avail- the ephemerides of the navigation message.
able to authorized users. On top of this there
is a 50-bit-per-second modulation which is The second and more precise method is to
used to transmit the satellite ephemerides obtain the difference in phase between the
(predicted orbit and clock) and other informa- received carrier signal and a receiver gener-
tion. Authorized users have also access to ated signal at the same frequency. This
the Precise code on a second frequency L2, measurement is known as the carrier phase
that allows users to correct for ionospheric observable and it can reach millimetre preci-
propagation delays. Some receivers are able sion, but it lacks the accuracy of the pseudor-
to measure the delay between the signal in ange because the phase when the tracking
the L1 frequency and the L2 frequency with- is started can only be known with an ambigu-
Figure 1. Common observ- out access to the P code. There are plans to
ability of GPS satellites by ity of an unknown number of times the carrier
add in future satellites another frequency for wavelength (about 19 cm for L1)
ground and onboard re-
civil users so they can easily correct for iono-
ceivers allows a better de-
termination of the satellite spheric delays.
GPS Satellite GPS Satellite
Use of GPS for spacecraft navigation data is processed together with ground
The number of applications and users of the based data, GPS provides possibly the best
GPS system has exploded in the last years, accuracy that can be achieved in precise or-
well beyond any expectations. The latest re- bit determination.
ceivers are cheap, small, with very good per-
formance and easy to use. The first user satellite to fly a precise GPS re-
ceiver was the TOPEX/Poseidon altimeter
One of the first scientific applications of GPS satellite. After that other satellites have been
was to precisely determine the position of flown with different types of receivers. Table
fixed ground antennas in order to study the 1 details the different applications for a GPS
dynamics of the Earth surface. Very soon it receiver on-board a spacecraft.
was realized that in order to obtain the best
results it would be necessary to compute very ESA has been involved in GPS activities
precise orbits of the GPS satellites. A number from the late eighties with the development
of groups started to do this and in this way the of space qualified GPS and GPS/GLONASS
first orbits that were precisely obtained using receivers, the ESOC activities to support
GPS were those of the GPS satellites them- spacecraft navigation and, lately, within the
selves. ARTES program, the European Geostation-
ary Navigation Overlay Service (EGNOS).
GPS has many advantages for the tracking of EGNOS will complement the GPS system in
satellites orbiting the Earth. It provides unsur- order to provide European users with in-
passed observability, because low-Earth sat- creased availability, integrity and accuracy
ellites are able to track six or more GPS for real time applications such as aircraft
satellites, with tracking arcs amounting to navigation.
about half of the user satellite orbit. This can
not be achieved by any ground based track- It is foreseen that ESA will participate in the
ing station. This very good observability also development of future Global Navigation
makes the method robust. There is a high lev- Satellite Systems (GNSS) that may replace
el of redundancy because orbits can be fitted GPS, GLONASS and their augmentations
even with only two GPS satellites being for such purposes in the future.
tracked at any time. When four satellites are
tracked GPS allows for real-time autonomous GPS has been proposed as tracking or sci-
determination of the position of the satellite, entific instrument for several ESA space-
with an accuracy equivalent to that obtained craft. It is the main positioning instrument
with non-precise ground tracking methods. If envisioned for the Automated Transfer Vehi-
a precise dual frequency receiver is used and cle (ATV), both for absolute navigation and
for navigation relative to the International
Table 1. Applications of a GPS receiver for space navigation
- To determine the position and velocity of a satellite.
- To accurately determine the time of observations from other tracking or scientiﬁc instruments.
- To determine the attitude of a satellite. This can be accomplished by comparing the measurements
obtained from different antennas.
- To collect GPS measurements that will allow a precise reconstitution of the orbit of the satellite.
- To collect GPS measurements that can be used to reconstitute the characteristics of the medium trav-
elled by the signal: ionosphere and troposphere.
- The relative navigation of two spacecraft (currently being validated).
- The tracking of the launch and early orbit phases of rockets.
- The tracking of re-entering spacecraft, even to the point of autonomous landing.
Space Station. The ATV Rendezvous Pre-de- for the GPS satellites that would be used by
velopment (ARP) program is being carried geodesists in regional deformation studies.
out to validate methods for relative navigation Within the terms of reference of the IGS was
that will be used for ATV, including relative also provision of support to other applica-
GPS navigation. Other spacecraft for which a tions including scientific satellite orbit deter-
GPS receiver has been proposed include mination. The assets which ESOC could
several of the Earth Explorer candidates and contribute to the IGS were its network of
other future observation and scientific satel- ground stations in which receivers could be
lites (Metop, STEP). installed and its expertise, supported by in-
house developed software, in precise orbit
ESOC involvement in GPS activities and geodetic parameter estimation.
ESOC has within the European Space Agen- Our first receiver was installed in Maspalo-
cy the responsibility to operate ESA space- mas (Spain) in June 1992 (Fig. 4). Receivers
craft. This includes the Flight Dynamics have also been installed in Kourou (French
activities needed to achieve and maintain Guyana) in July 1992, Kiruna (Sweden) in
their desired orbit and attitude. That is the July 1993, Perth (Australia) in August 1993,
reason why, as soon as GPS started to be Villafranca (Spain) in November 1994 and
proposed for spacecraft navigation, ESOC Malindi (Kenya) in November 1995. Our pre-
began to get ready to support ESA missions cise estimation software was extended to in-
that might use the GPS system. clude GPS measurement types for both
ground based and spacecraft borne receiv-
ESOC had an excellent opportunity to do so ers. We have been providing data and in-
by contributing to the success of the Interna- creasingly precise GPS products for the last
tional GPS Service for Geodynamics (IGS). five years. Currently we provide:
Scientists were proposing to install a perma-
nent network of precise ground based GPS - Raw measurement data from our six
receivers that would allow the monitoring of ground stations.
the movement of the Earth surface in order to
better understand plate tectonics and local - Precise orbits of the GPS spacecraft.
deformations that are the cause of earth- - Earth orientation parameters (polar
Figure 2. Current configu-
quakes. The data from these receivers could motion, length of day).
ration of the GPS-TDAF
be processed in order to obtain precise orbits
ESOC dedicated lines
Kourou GPS receiver
TurboRogue ESOC, Darmstadt
Malindi Perth cesium
- Station coordinate solutions for those sta- one-frequency ranging and altimeter data. Figure 3. Remote Station
Control panel of the GPS-
tions included in our analysis. - The implementation of a sequential ﬁlter to TDAF. This panel is used to
- GPS satellite clock information. estimate spacecraft trajectories using the monitor the daily retrieval
ESOC is currently an active IGS Analysis and precise products obtained by the IGS tasks.
Operational Data Centre and is specially in- analysis activities.
volved in current discussions to extend the
IGS to use space-borne receivers. Role of the ESA operations ground seg-
ment in GPS navigation
The ESA GPS TDAF The driving reason for implementing a GPS
The ESA GPS Tracking and Data Analysis TDAF in ESOC was to provide ESA the ca-
Facility (GPS TDAF) has been developed in pability to support the navigation of satellites
order to support the GPS activities carried out equipped with GPS receivers. This support
by ESOC. It includes our network of ground involves the following activities:
GPS receivers, the necessary communica-
tion interfaces to allow the remote operation Support of critical real time GPS applica-
and data downloading from ESOC and the tions
processing and analysis software needed to GPS has been proposed as the absolute and
format the data and to obtain the precise relative positioning system for spacecraft go-
products (Fig. 2). The system is highly auto- ing to the manned International Space Sta-
mated and includes an easy to operate inter- tion. For this application is clear that the
face for the retrieval and the processing of the ground segment cannot be in-the-loop for
data (Fig. 3). The GPS TDAF is currently be- the calculation of real-time trajectories of the
ing extended to process GLONASS data and spacecraft involved, because of the unavoid-
to include real-time monitoring capabilities able delays that this will create. Still the
that may be needed to support critical opera- ground segment has a role in monitoring the
tional phases like rendez-vous. integrity of the signals that are to be used for
Other recent developments are:
This can be accomplished using a ground
- The calculation of global and local iono- network of GPS receivers that is able to track
spheric models that can be used to correct all the satellites that the orbiting spacecraft
may use. The navigation data and observa- the continuous monitoring of the GPS space-
tions of these precisely located ground sta- craft visible from these ground receivers.
tions can be processed in order to check their
integrity and to estimate the error in the signal Precise Orbit Determination using GPS
for each GPS satellite. Badly performing sat- GPS is one of the best tracking types for Pre-
ellites can be identified and it can be predict- cise Orbit Determination of Low Earth Orbit
ed how many healthy GPS satellites will be satellites because it combines high accuracy
observable by the user spacecraft during the with unsurpassed observability. The high ac-
critical operations. This can also be done in curacy is obtained using the GPS carrier
real time in order to detect satellite failures phase observable, free of ionospheric errors
that can affect the navigation solution of the when dual frequency data is used. The un-
user spacecraft, so that the information can surpassed observability is provided by the
be delivered to Mission Control and the bad high number of GPS satellites that can be si-
GPS satellites can be excluded from the on- multaneously tracked by an orbiting receiver.
board computed navigation solution.
ESOC has incorporated models for the most
Another role of the ground segment will be to widely used GPS measurements in its Pre-
support the validation of the receivers and cise Orbit Determination software. This has
their correct functioning before critical opera- been done both for the determination of the
tions are started. It can also assist in the fast orbit of the GPS spacecraft and for the deter-
re-start of receivers by providing up-to-date mination of the orbit of user spacecraft
almanacs and other initialization data. (spacecraft carrying a GPS receiver). The
implementation has been validated using
ESOC has installed GPS receivers in six TOPEX/Poseidon data and the software is
Figure 4. GPS antenna and ground stations and it is developing a real- currently being used to support the ARP
pillar in Maspalomas, Spain. time communication system that will allow for Flight Demonstrations. More information on
these activities is given below.
Determination of the orbits and clocks of
the GPS satellites
For some applications it is not needed to si-
multaneously solve for the orbits of the GPS
spacecraft and the user spacecraft. The or-
bits and clock biases of the GPS spacecraft
can be precisely computed and then held
fixed for the computation of the orbit of the
ESOC has been participating in the Interna-
tional GPS Service for Geodynamics (IGS)
since it started and we have been producing
precise orbit and clocks solutions for the
GPS satellites. These ephemerides are esti-
mated to be accurate to about 10 cm.
Our GPS orbit determination software is also
being used for feasibility and validation ex-
periments for the ARTES-9 EGNOS project.
Operational Orbit Determination using
The facilities implemented for Precise Orbit
Determination can also be used for Opera-
tional Orbit Determination to produce a very
accurate orbit prediction and to calibrate ma-
noeuvres. This on-ground determined orbit
can also be used during the spacecraft
check-out to assess GPS-based on-board
For some applications it is not needed to error budget for the radial position. In order
have a very precise orbit prediction. In this to satisfy this challenge an unprecedented
case the GPS-based on-board generated po- effort was made to improve the gravity model
sitions can be used on the ground as observ- of the Earth and, to further guarantee the
ables in order to determine the orbit that will best possible results in orbit determination,
be used for orbit control, mission planning several tracking systems were placed on-
and station visibility predictions. This process board: retroreflectors for Satellite Laser
can also assess the quality of the on-board Ranging (SLR), a DORIS receiver, and an
generated positions. experimental precise GPS receiver. This ef-
fectively has made the T/P spacecraft a ver-
In this context our GPS orbit determination itable orbit determination laboratory that
software is going to be used operationally to allows the intercomparison between different
determine the orbit of the Danish Ørsted geo- tracking techniques.
magnetic research microsatellite.
T/P has been of unique importance for the
Geophysical Parameter Estimation validation of techniques for GPS-based sat-
Most of the activities listed before are possi- ellite navigation. It is equipped with a high
ble because networks of precise geodetic re- precision dual-frequency GPS receiver pro-
ceivers are currently deployed to support ducing long cycle-slip free carrier phase
these and other applications. For the most passes as well as pseudo-range measure-
accurate applications the position of the re- ments. It was launched when the GPS con-
ceivers in these networks has to be precisely stellation was almost complete and when the
determined, together with a number of other IGS network of high precision GPS receivers
geophysical parameters. The accurate deter- had started to provide continuous globally
mination of the position of the ESA ground distributed tracking data.
stations, the determination of Earth orienta-
tion parameters and the calculation of iono- For our evaluation to intercompare the orbit
spheric calibrations can also support other restitution capabilities of the three tech-
projects that are not using directly GPS but niques: SLR, DORIS and GPS a 10-day pe-
need accurate location of the position of riod was selected. For the GPS processing,
tracking antennas and correction for iono- T/P observations were used together with
spheric delays. data from about 20 ground receivers from
the IGS network. The orbit of the T/P space-
ESOC is contributing to the estimation of very craft was then solved simultaneously with
precise station coordinate solutions that in- the orbits of the GPS spacecraft. The chosen
clude the ESA ground stations and through data type was double difference phase
the IGS also to the activities of the Interna- measurements involving two GPS satellites
tional Earth Rotation Service (IERS). We are and two GPS receivers.
currently testing the use of GPS derived ion-
ospheric models in order to correct one-fre- The ephemerides generated using GPS and
quency ERS altimeter measurements and the using SLR and DORIS were compared with
S-band ranging and doppler measurements each other and with external solutions ob-
used for the routine control of most space- tained by the Delft University of Technology
craft. (DUT) and the Jet Propulsion Laboratory
(JPL). The orbits show a remarkable agree-
ment, with the difference in radial direction in
TOPEX/Poseidon precise orbit determina-
the order of 2 cm, and along track and cross
track differences in the order of 5 to 10 cm.
TOPEX/Poseidon (T/P) is a joint US/French
altimetric spacecraft launched in August These results demonstrate the capability of
1992. The main scientific goal of this mission the GPS-TDAF to produce very accurate re-
is to produce sea level maps to study ocean sults when precise data collected on-board
circulation and its variability. Very accurate can be combined with on-ground collected
orbit determination is vital to the success of data from a network of high-precision GPS
this or any other altimetric mission. In fact, the receivers.
fundamental quantity measured is the geo-
centric height of the sea surface, and this is
The ARP ﬂight demonstrations
obtained as the difference between the radial
orbit position and the altimeter measurement ESA is developing the un-manned Automat-
proper. The orbit determination requirements ed Transfer Vehicle (ATV) that will serve as
for T/P were set to a very demanding 13 cm a logistic / resupply vehicle for the Interna-
tional Space Station (ISS). The ATV will per- - Precise measurement models that use the
form a number of manoeuvres in order to GPS orbits and clocks computed by
rendezvous and dock with the ISS. GPS is ESOC. The models include a centre of
baselined as the main positioning system for mass correction that is performed using
ATV. It will be used for autonomous absolute the location of the particular antenna in
position determination and autonomous rela- the body-ﬁxed axes and the attitude data.
tive position determination with respect to the - A multi-satellite orbit propagator that
ISS. For autonomous absolute position deter- includes precise dynamic models and
mination the ATV will be equipped with a one- empirical accelerations.
frequency GPS receiver that will provide po-
sition, velocity and time solutions. For auton- - A Square Root Information Filter that proc-
omous relative position determination the ISS esses all the information and produces ﬁl-
will also be equipped with a GPS receiver that tered and smoothed estimates of the
will provide GPS observables for transmis- parameters.
sion to the ATV. The ATV will process them We are currently processing data from the
together with its own GPS observables in or- first ARP Flight Demonstration and we ex-
der to determine its position and velocity rela- pect to achieve absolute positioning results
tive to the ISS. with about 1 metre of accuracy and even bet-
ter relative positioning accuracy.
The ATV Rendezvous Pre-development
(ARP) project covers the pre-development of Conclusion
rendezvous technologies critical to ATV. One
of the aspects that are covered by this project The GPS system, originally deployed to al-
is the validation of relative navigation using low very precise delivery of weapons, has
GPS observables. For this three Flight Dem- demonstrated an incredible versatility of use
onstrations (FD) are planned, in which the for civil applications. GNSS systems are ide-
Space Shuttle will act as chaser and another al to support many aspects of the navigation
spacecraft (Astrospas for FD1 and the MIR of spacecraft orbiting the Earth. They can
station for FD2 and FD3) will be the target. support increased spacecraft autonomy and,
These spacecraft will carry one-frequency when used together with ground collected
GPS receivers and will collect GPS data dur- measurements, they provide unsurpassed
ing the proximity operations. The data will be accuracy. The ESA GPS-TDAF is already
post-processed on the ground to validate the supporting validation activities for navigation
relative navigation algorithms. of spacecraft using GNSS systems and will
as well be able to support preparations for a
The role of ESOC in these three ARP FDs is European contribution to future Global Navi-
to compute reference trajectories (relative gation Satellite Systems.
and absolute) for the spacecraft involved us-
ing all available measurements. These trajec- Additional information on these activities, as
tories will be then used to compare with the well as links to other related World Wide
results of the relative navigation filter. ESOC Web sites, can be found under http://nng.es-
will obtain the trajectories using the following oc.esa.de/.
- GPS observables (pseudorange and The development and operation of the GPS-
phase) and on-board derived positions TDAF has been possible thanks to the im-
from the two ﬂying receivers. portant contributions of C. García Martínez
- The ESOC precise orbit and clock solu- (GMV), J. Feltens (EDS) and M. A. Bayona
tions for the GPS satellites. (GMV), as well as those of S. Casotto and P.
- Attitude data derived from the spacecraft Duque and several trainees. Assistance of
Guidance, Navigation, and Control (GNC) station and communications experts at
system. ESOC and the ground stations is also grate-
The data will be decoded and converted to an
engineering format that then will be fed to a
program which will produce the best estimate
trajectories for the spacecraft. This program
is called GPSBET (GPS Based Estimator of
Trajectories) and it includes the following: