Laurea Specialistica in Ingegneria Spaziale

2° anno

Corso ‘Stazioni di Terra’

Appendice 8

Applicazioni spaziali del GPS/GNSS

Giorgio Perrotta
Anno Accademico 2007

Introduction: Galileo and GPS

  • The bulk of the revenues are expected to come from Earth-based uses of GPS today, and of GNSS tomorrow, through the development of innovative applications serving the needs of the land, aeronautical and maritime mobiles.
  • However, it should be borne in mind that navigation satellite systems play an outstanding role in supporting other space assets, thus considerably contributing to render their operation more performant and cost-effective. Furthermore, the availability of the signals emitted by the satellites of the navigation constellation can be exploited to serve many scientific needs, leading to a significant fallout for the general public.
  • The Space applications of GPS/GNSS can be divided in two classes:

a) engineering applications;

b) scientific applications

  • For these applications the signal emitted by the GPS/GNSS satellites either constitute a unique data source or else are characterized by a low cost per data sample. In brief, the GPS/GNSS represent an economic and cost-effective way of providing solutions to old and new problems.

Engineering applications of GPS/GNSS (1)

On-orbit orbit determination

  • Performed in real time directly on board the spacecraft carrying a GPS/GNSS receiver
  • The Orbit Determination software is also incorporated into the receiver
  • The absolute orbit determination in real time is used to support satellite orbital-position related mission events, to drive spacecraft orbit control manoeuvres, and to implement spacecraft attitude reconstitution by joint operation with other spacecraft attitude sensors;
  • On-orbit determination is particularly useful on board LEO spacecraft, specially those carrying scientific or Remote Sensing payloads;
  • Real-time orbit determination is also being increasingly considered on board launch vehicles, supporting or replacing other means (e.g. ground based radars) used for rocket trajectory determination. In this application the rocket position and velocity vector would be relayed in realtime to ground using a simple transmitter;
  • The GPS/GNSS spaceborne receiver must be compatible with LEO typical parameters: orbit altitude between 300 and 1500 km; orbital speed around 7 km/sec; doppler shifts of the order of 100 kHz. These values are greatly different from those encountered in typical Earth-based applications, therefore the majority of commercial GPS chips are unfit for the task;
  • Recently, a few Houses have made available chips and associated software for making COTS-based spaceborne GPS receivers overcoming the previous limitations;

Engineering applications of GPS/GNSS (2)

Attitude determination
  • The GPS/GNNS signals can be exploited by a suitably equipped receiver to provide attitude and attitude rate data to the actuators of the attitude control subsystem
  • The benefits consist in the elimination of several different attitude sensors thus reducing costs, mass, DC power and improving reliability
  • Though in principle a GPS/GNSS attitude measurement Unit can entirely replace all other sensors (Sun, Earth, Horizon, gyroscopes, star trackers..) some spacecraft can opt to integrate the GPS/GNSS receiver with back-up or fine sensors for acquisition and reacquisition purposes or for normal mode operation. In any case a net saving w.r.t. a ‘conventional approach’ is achieved
  • The GPS/GNSS receiver is equipped with four antennas suitably spaced (since the sensitivity increases with the distance between antennas) and a software controlled multiplexer
  • The operation is based on the measurement of the differential phase of the incoming signal as received by the antennas of each pair
  • The performance of the GPS/GNSS attitude determination Unit is baseline-length limited (i.e. by the distance between the two antennas of each pair). Small spacecraft are thus bad candidates for efficiently implementing a GPS/GNSS attitude determination system, while large bodies will benefit most from its availability

Engineering applications of GPS/GNSS (3)

Relative positioning

  • GPS/GNSS receivers on board spacecraft can be used for relative position and velocity determination, in a multiple satellite or multi-body systems
  • A known application involving two bodies concern, for example, the rendezvous and docking between a satellite and a space station. The relative position and velocity determination requires that the position and speed of one body be transmitted, wirelessly, to the other body which also selfdetermines its position and velocity using a GPS/GNSS receiver and, by comparing the two data sets, performs the differencing
  • Relative positioning determination becomes more important for constellations and, in the near future, for satellite formations. The latter are characterized by much tighter mutual position constraints, this implying both a tighter orbit control of the individual satellites and a more accurate real-time orbital position determination of each body and of the relative position and speed between the multiple bodies for implementing an effective formation control
  • Noteworthy, relative positioning in orbit will require establishing efficient intersatellite links, though with modest capacity requirements. Depending on the mutual distance between the satellites of the constellation or formation several technologies can be considered spanning from quite simple to most advanced ones. Both RF and optical technologies can find uses where they can exhibit optimum cost-effectiveness and operational flexibility

Engineering applications of GPS/GNSS (4)

System Clock Synchronization

  • A GPS/GNSS receiver on board a satellite provides a precise time for all spacecraft subsystems, time intervals for science or remote sensing instruments’ measurements, precise and common timing for the operation of telecommunication payloads cooperating with ground based infrastructures (e.g. TDMA burst synchronization)
  • A satellite or satellite system benefits from the availability of GPS/GNSS emitted signals in that it can derive precise spacecraft timing in a simple and inexpensive way. If a GPS/GNSS receiver is already installed on board for other purposes ( orbit or attitude or relative position determination) the timing for spacecraft clock synchronization is simply achieved at zero incremental cost
  • Deriving precise clock from GPS/GNSS spacecraft emitted signals reduces the spacecraft functional and operational dependence from ground. This is of utmost importance for the autonomy, considering that a high autonomy level is increasingly required not only for military satellites but also for scientific and many application satellite systems
  • Concerning performance, the GPS/GNSS receivers are capable of achieving synchronization accuracies of the order of 100 nanoseconds or less, which is quite sufficient for many satellite missions

Scientific Applications of GPS/GNSS (1)

Gravity field modelling

  • The inhomogeneities in the Earth’s gravity field affect the spacecraft orbit. By precise orbit determination such anomalies can be measured and the Earth’s gravity field knowledge can be improved. This leads to a better estimate of the long and medium wavelengths of the spherical harmonics of the representation of the gravity field.
  • A better modelling of the Earth’s gravity field will be used, for example, to improve the design and operation of LEO spacecraft, specially concerning the critical remote sensing applications

Altimetry

  • Satellite altimetry can be based either on radars or on lasers. The purpose is to determine the spacecraft altitude above the Earth’s surface; and from this to derive the geoidal height to improve the resolution of the global gravity field. Errors in the altimeter height determination and orbit errors, (e.g. resulting from use of conventional ground-based means) contributing to the deterioration of the estimates, can be reduced if a GPS/GNSS receiver is instead used for precise orbit determination.

Scientific Applications of GPS/GNSS (2)

Geocoding remote sensing data

  • Geocoding of remote sensing data refers to the determination of terrestrial coordinates of the projection of the satellite sensor on the Earth’s surface at time of image taking. Geocoding w.r.t. Earth’s centre of mass requires two vectors: from the centre of mass to satellite ( from orbit determination) and from satellite sensor to the Earth’s surface. The latter implies the knowledge of satellite attitude and sensor range.
  • The availability of a spaceborne GPS/GNSS receiver can considerably improve the determination of the first vector through Precision Orbit Determination (POD) techniques and contributes to improve the sensor range estimate, thus leading to an overall better geocoding accuracy

Interferometric SAR remote sensing

  • Interferometric SAR uses the phase of the echoed signal to compute differential range and range rate in two or more SAR images of the same surface. Spacecraft sensor absolute position, relative position, attitude and ground-based geocoding are required for Interferometric SAR operation.
  • GPS/GNSS receivers on board the satellites can provide the required information.

Scientific Applications of GPS/GNSS (3)

Atmospheric occultation

  • Radiofrequency signals travelling to a LEO spacecraft through the Earth’s limb are subjected to attenuations and phase shifts: this is called ‘occultation’ and relies on multiple sources of RF signals in a given frequency band. However multiple satellites with these features are rather uncommon with one notable exception: the GPS/GNSS constellation.
  • In the GPS/GNSS frequency band the occultation takes the form of an alteration in amplitude and phase, which depends on the water vapour content of the lower atmospheric layers and the electron content in the ionosphere.
  • Since the main purpose is to assess the water vapour pressure and temperature for meteorology and climate change studies, the electron content-dependence must be eliminated from the measurement data. This requires a dual-frequency operation of the GPS/GNSS receiver whereby the electron contribution, which is frequency sensitive, can be isolated and discarded
  • The contribution of atmospheric occultation to meteorology is outstanding. With the advent of operational constellations of microsatellites, a transition is expected from scientific uses to routine data acquisition and information retrieval for worldwide added value services

GPS/GNSS space missions distribution

  • Spaceborne GPS receivers have been launched since 1980 for experimental, demonstration or preoperational purpose. Launches involved both large and small satellites and a mix of engineering and scientific applications. The majority of these missions did carry receivers built by American companies with a minor contribution by european houses and a quasi-negligeable one by Japan.
  • However, starting from 1996, the presence of European developers has considerably increased w.r.t. the previous years, this implying a growing interest and capacity in this application area.

Time period (years) / Mission carrying american GPS receivers / Missions carrying non-american GPS receivers
1980-1985 / 2 / -
1986- 1990 / - / -
1991- 1995 / 24 / 1
1996-2000 / 38 / 11
2001 onwards / n.a. / n.a.
  • The growth of European spaceborne receivers is expected to further expand with the realization of constellations and formations of nano, micro and minisatellites.

European spaceborne GPS/GNSS receiver developments

  • To contrast the massive presence of USA products, some European companies have started to develop and qualify performant GPS spaceborne receivers, mostly under contract to ESA. Both Laben and Alcatel are present, internationally, in high-performance high-cost space missions.
  • On the other hand, and till recently, few European companies have developed COTS-based receivers for the lower-cost market segment typical of minisatellites or even smaller spacecraft. However the situationis improving, partly due to the initiative of Research Centers.
  • Commercial chipsets – and there are many built by European houses characterized by excellent performance- are nevertheless attractive for building low cost space receivers, but the ITAR limits the access to the chips’ firmare to modify their operation for use on board satellites.
  • The above stresses the need for disposing of chips not subjected to export restrictions: and this is whithin the scope of the current technology developments sponsored by the EC and ESA.