On-board GPS

The driving reason for implementing a GPS Tracking and Data Analysis Facility in ESOC was to provide to ESA the capability to support the navigation of satellites equipped with GPS receivers.

This support can be separated in five main activities:

These applications are explained in the following.

Support for critical real time GPS applications

GPS has been proposed as the absolute and relative positioning system for spacecraft going to the manned International Space Station. For this application is clear that the ground segment can not be in-the-loop for the calculation of real-time trajectories of the spacecraft involved, because of the unavoidable delays that this will create. Still the ground segment has a role in monitoring the integrity of the signals that are to be used for critical operations.

This can be accomplished using a ground network of GPS receivers that is able to track all the satellites that the orbiting spacecraft may use. The navigation data and observations of these precisely located ground stations can be processed in order to check its integrity and to estimate the error in the signal for each GPS satellite. Badly performing satellites can be identified and it can be predicted how many healthy GPS satellites will be observable by the user spacecraft during the critical operations. This can also be done in real time in order to detect satellite failures that can affect the navigation solution of the user spacecraft, so that the information can be delivered to Mission Control and the bad GPS satellites can be excluded from the on-board computed navigation solution.

ESOC has installed GPS receivers in six ground stations (Kiruna, Kourou, Malindi, Maspalomas, Perth and Villafranca) and it is developing a real-time communication system that will allow for the continuous monitoring of the GPS spacecraft visible from ESOC ground receivers.

Precise Orbit and Trajectory Determination using GPS

Precise orbit determination for Earth Observation Satellites equipped with GPS receivers

GPS is one of the best tracking types for Precise Orbit Determination of Low Earth Orbit satellites because it combines high accuracy with unsurpassed observability. The high accuracy is obtained using the GPS carrier phase observable, free of ionospheric errors when dual frequency data is used. The unsurpassed observability is provided by the high number of GPS satellites than can be simultaneously tracked by an orbiting receiver.

ESOC has incorporated models for the most widely used GPS measurements in its Precise Orbit Determination software. This has been done both for the determination of the orbit of the GPS spacecraft and for the determination of the orbit of user spacecraft (spacecraft carrying a GPS receiver). The measurements that have been incorporated include:

For very precise work it is necessary to simultaneously solve for the orbit of the GPS spacecraft and the user spacecraft. The implementation has been validated using Topex/Poseidon data, for which very precise orbits based on DORIS, Satellite Laser Ranging (SLR) and GPS have been obtained by ESOC and other centers. These orbits show remarkable agreement, with the rms in the radial direction varying between 1 and 3 cm and in the along track and normal directions in the 5-10 cm rms range. This level of accuracy allows for the calibration and full exploitation of high precision on-board instruments such as altimeters. It also allows for the estimation of very precise geodetic and dynamic models that facilitate very precise prediction of satellite orbits.

ESA is planning to install precise dual-frequency GPS receivers in all the proposed Earth Explorer missions. This will make possible the precise determination of the orbits of these spacecraft.

Precise Trajectory Determination for other Spacecraft equipped with GPS receivers

The term "orbit determination" is well suited for the computation of the orbit of a satellite subjected to a dynamic environment that is well known and predictable. Sometimes spacecraft fly in modes that are not so predictable, as when they are performing frequent manoeuvres or when they are flying at very low altitudes. GPS is very well suited for the determination of these trajectories because its continous coverage and great accuracy. Another important application for GPS is its use on relative or differential modes in order to compute the relative position of two spacecraft. This is the case in the rendezvous of an active chaser spacecraft that wants to dock with or berth to a passive target spacecraft.

ESA is developing the un-manned Automated Transfer Vehicle (ATV) that will serve as a logistic / resupply vehicle for the International Space Station (ISS). In this case the ATV will be the chaser spacecraft and the ISS will be the target. The ATV will perform a number of manoeuvres in order to rendezvous and dock with or berth to the ISS. GPS is baselined as the main positioning system for ATV. It will be used for autonomous absolute position determination and autonomous relative position determination with respect to the ISS. For autonomous absolute position determination the ATV will be equipped with a one-frequency GPS receiver that will provide position, velocity and time solutions. For autonomous relative position determination the ISS will also be equipped with a GPS receiver and it will transmit GPS observables to the ATV. The ATV will process them together with its own GPS observables in order to determine its position and velocity relative to the ISS.

The ATV Rendezvous Pre-development (ARP) project covers the pre-development of rendezvous technologies critical to ATV. One of the aspects that are covered by this project is the validation of relative navigation using GPS observables. For this three Flight Demonstrations (FD) are planned, in which the Shuttle will act as chaser and another spaceraft (Astrospas for FD1 and the MIR station for FD2 and FD3) will be the target. These spacecraft will carry one-frequency GPS receivers and will collect GPS data during the proximity operations. These data will be post-processed on the ground to validate the relative navigation algorithms.

The role of ESOC in these three ARP FDs is to compute a reference trajectory for the spacecraft involved using all available measurements. These trajectories will be then used as a reference to which the results of the relative navigation filter will be compared. ESOC will obtain these trajectories using the following data:

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:

• GPS pseudo-range observables
• GPS carrier phase observables
• GPS-derived  Earth-fixed position and clock bias solutions
• GNC-derived inertial positions

• Spacecraft position and velocity
• Clock bias, drift and ageing for each GPS receiver
• Empirical accelerations acting on all spacecraft
• Empirical accelerations specific to each spacecraft (differential accelerations and thrustings)
• Air drag and radiation pressure coefficients for each spacecraft
• Residual signal error for each GPS spacecraft
• Phase ambiguities
• Parameters for an ionospheric correction model
• Center of mass offsets for each spacecraft

The program has been successfully tested using one- and two-frequency data from Topex/Poseidon. In a typical test six hours of GPS data have been processed and the resulting orbit agrees with the NASA POE (Precision Orbit Ephemeris) to an rms of about 15 cm for each component.

Determination of the orbits and clocks of the GPS satellites

For some applications it is not needed to simultaneously solve for the orbit of the GPS spacecraft and the user spacecraft. The orbits and clock biases of the GPS spacecraft can be precisely computed and then held fixed for the computation of the orbit of the user spacecraft.

ESOC has been participating in the International GPS Service for Geodynamics (IGS) since it started and we have been producing precise orbit and clocks solutions for the GPS satellites. IGS orbits are estimated to be accurate to about 10 cm (see plot). This processing is described in the ESOC IGS Analysis Centre page.

Operational Orbit Determination using GPS

Orbit determination using raw GPS observables

The facilities implemented for Precise Orbit Determination can also be used for Operational Orbit Determination to produce a very accurate orbit prediction and to calibrate manoeuvres. This on-ground determined orbit can also be used during the spacecraft check-out to assess a possible GPS-based on-board orbit determination.

Orbit determination using on-board obtained positions

For some applications it is not needed to have a very precise orbit prediction. In this case the GPS-based on-board generated positions can be used as observables in order to determine the orbit that will be used for orbit control, mission planning and station visibility predictions. This process will also allow to assess the quality of the on-board generated positions.

Geophysical Parameter Estimation

Most of the activities listed before are possible because networks of precise geodetic receivers are currently deployed to support these and other applications. Two of these networks are the ESOC GPS receiver network and the IGS network. For the most accurate applications the position of the receivers in these networks has to be precisely determined, together with a number of other geophysical parameters.

The accurate determination of the position of the ESA ground stations, the determination of Earth orientation parameters and the calculation of ionospheric calibrations can also support other projects that are not using directly GPS but need accurate location of the position of ground stations and correction for ionopheric delays.

Our work in the estimation of geodetic and geophysical parameters is described in the ESOC IGS Analysis Centre page. Our work in ionospheric monitoring is described in the Ionospheric Monitoring page.

[ nng home | gps pages | gps tdaf | data collection | data analysis | ionosphere ]