SGPS APPLICATIONS

SGPS applications can be divided into two classes: engineering applications and scientific applications.

The so-called engineering applications involve the use of GPS-derived information for spacecraft operations. These applications (for example, orbit and attitude determination) are discussed in the "hyperlink options" given in the main page of this site.

The scientific applications utilise the GPS-derived information as data for further processing. A number of these applications are described below.

The rationale for using GPS in these applications is that GPS provides a sole means of collecting the required data, or at the very least provides a data collection means that has a low cost per data sample ratio and high accuracy.

Gravity field modelling

Recalling Newton's law of gravitation, any inhomogeneities in the earth's gravity field will be apparent in a spacecraft's earth orbit; the lower the orbit the more pronouced the effect. Therefore via precise orbit determination, our understand of the gravity field can be improved by computing the perturbations to a satellite orbit caused by the earth's gravity. That is, the long and medium wavelengths of the spherical harmonics gravity field representation can be estimated.

Augmenting GPS precise orbit determination with satellite-to-satellite tracking, a gradiometer, or an altimeter will aid in improving these estimates

Mission examples:

- TOPEX/Poseidon

- ARISTOTELES

- Gravity Probe B

- CHAMP

- GRACE

Related Internet Sites:

- Pieter Visser at TU Delft: Modeling the earth's gravity field

- NASA GSFC and NIMA: EGM96

- GFZ: Kinematics and Dynamics of the Earth Division

- CSR, University of Texas: Center for Space Research

Altimetry

Satellite altimetry comes in two flavours: laser and radar. They both consist of using the satellite as a fast-moving, global platform for a sensor that transmits and receives signals reflected off of the earth's surface to measure the altitude of the satellite above the earth's surface.

The basic mathematical model is: h = dh + a + H + dH + N, where

h -> ellipsoidal height of the altimeter (from orbit determination)

dh -> orbit error

a -> altimeter measurement

H -> sea surface topography

dH -> instantaneous tidal effect

N -> geoidal height.

Therefore SGPS orbit determination plays a significant role in h and dh, if GPS is used. Also, the sea surface topography can be used as an approximation of the geoidal height, and as such can be used to improve the resolution of the global gravity field (especially over the oceans).

Mission examples:

- TOPEX/Poseidon

- OrbView-2 (formerly SeaStar)

- GFO

- Jason-1

- ICESat

Related Internet Sites:

- NOAA: Laboratory for Satellite Altimetry

- Thomas Moody at University of Texas: Radar Altimetry

Atmospheric occultation (troposphere and ionosphere)

GPS signals travelling from a GPS satellite to a low earth orbiter (LEO) through the earth's limb are occulted (altered in amplitude and phase) by the atmosphere. Techniques have been developed to recover such profiles as temperature and water vapour pressure from the occulted signals in the troposphere and electron content in the ionosphere.

Mission examples:

- OrbView-1 (formerly MicroLab-1)

- ORSTED

- SUNSAT

- SAC-C

- CHAMP

- GRACE

Related Internet Sites:

JPL: GENESIS - GPS ENvironmental and Earth Science Information System

GPS/MET: GPS/MET Preliminary Report

Geocoding remote sensing data

The geocoding or geolocation of remote sensing data refers to the determination of the terrestrial coordinates of the projection of the satellite sensor on the earth's surface at the time of data capture. Geocoding with respect to the earth's centre of mass is performed by determining the following two vectors in the appropriate reference frames: the vector from the centre of mass to the satellite (from orbit determination); and, the vector from the satellite sensor to the earth's surface (from satellite attitude determination and sensor range).

Mission examples:

- TOPEX/Poseidon

- GFO

- SRTM

- Jason-1

- VCL

- ICESat

Related Internet Sites:

CSR, University of Texas: ICESat/GLAS Algorithm Theoretical Basis Documents (ATBD)

Interferometric SAR remote sensing

Imaging radar is an active illumination system. The amplitude of a echoed signal is used to construct the radar image. Synthetic aperture radar (SAR) uses the Doppler of the radar echoes created by the movement of the spacecraft to synthesise a large antenna. Interferometric SAR (InSAR) uses the phase of the echoed signal to infer differential range and range change in two or more SAR images of the same surface. Spacecraft sensor absolute position, relative position, attitude, and subsequently geocoding are required for InSAR, which GPS can provide.

Mission examples:

- SRTM

Related Internet Sites:

JPL NASA: SAR Interferometry and Surface Change Detection report

Due to the rapid developments in the field of spaceborne GPS and the aerospace industry in general, any comments, information or corrections pertaining to information on this site are welcome and encouraged.