The Potential Use of GPS Signals as Ocean Altimetry Observables

This paper investigates a potential application of GPS signals for ocean altimetry. The altimetry information is derived from dual-frequency GPS signals reflected from the ocean surface and received at a low-altitude satellite. Such altimetry would be superior to conventional radar altimetry in three respects: ( 1 ) Thanks to the global GPS constellation, sea surface heights at up to 12 points can be determined instead of a single nadir point at any one time; (2) The user satellite only need be equipped with a GPS receiver which also serves as the receiver acquiring direct GPS signals for orbit determination; and (3) The delay spread of the reflected signal is a function of the seasurface roughness, so that this technique may allow scatterometry-like measurements of surface weather conditions. The paper addresses several criteria for the GPS altimetry to k viable. These include the signal strength, delay characteristics and polarization of the reflected GPS signals, the receiver capability of discriminating and tracking these signals from the direct signats, the determination of the reflection points on the sea surface, and the information content of these signals. INTRODUCTION Satellite onboard radar altimeter has been used exclusively for precise measurement of ocean surface height in modem years [ 1–3]. ‘I%e radar signal is transmitted vertically downward by the satellite, usually at low altitudes. The signal, upon reflection back from the ocean surface, is received by the same satellite. The ocean surface height can then be derived from the radar signal delay and the precise height of the independently determined satellite orbit. Since only a single shot of measurement directly below the satellite is made at any one time, the satellite orbit has to be properly designed and measurements over a prolonged period of time are required for a global coverage of the ocean. GPS measurements have demonstrated to be capable of precise positioning of users on earth as well as on low altitude satellites. Positioning to cm-level accuracy has been reported [4-6]. Can a highly-modified GPS receiver be used to derive ocean topography by tracking the GPS signals as they retlect from the ocean surface, leading to low-cost altimetry missions? During a July 1991 JPL measurenlerlt tracking a setting GPS satellite from the top of the Mauna Kca volcano, a beat frequency was noticed in the measured phase and “amplitude which was consistent with a signal reflected from the ocean, about 3 to 13 dB down in amplitude, adding to the direct signal. The investigators noticed that the height of the receiver above the ocean could be crudely determined as a function of the beat frequency between the direct signal and the one reflected from the ocean surface. Glazman [7,8] has demonstrated a form of bistatic radar to measure sea height using a tower-mounted transmitter and receiver. He used the amplitude modulation caused by the addition of direct and reflected signals, as a function of transmit frequency, to deduce the sea height. His work demonstrated a technique for the separation of sea height from surface roughness, leading to sea-height accuracy of about 3 cm. A group of investigators at NASA I,angley and at the University of Colorado is investigating the use of reflected GPS signals for ionospheric calibration of a single-frequency altimeter. They have successfully tracked the reflected GPS signals from an air-borne platform+. NASAS Office of the Mission to Planet Earth has funded an investigation at the JPL that will plan how GPS Altimetry could be used to supplement more traditional ocean altimetry missions. ADVANTAGES GPS altimetry would be superior than conventional, vertical only, radar altimetry in three respects: ( 1 ) Sea surface heights at up to 12 points can be determined instead of a single nadir point at any one time; (2) The user satellite only need be equipped with a GPS rweiver which can also serve as the receiver acquiring direct GPS signals for orbit determination; and (3) The delay spread of the reflected signal is a function of the sea-surface roughness, so that this technique may allow scatterometry-like measurements of surface weather conditions. To provide a feel of the potential geographic coverage of the altimetry measurements, Fig. 1 shows the geographic coverage of the reflection points where sea surface height is to be determined. Here, a 24-satellite GPS constellation is assumed; Topcx satellite (1,336-km altitude) is used as the user satellite. Altimetry measurements are made for atl GPS ‘ satellites above Topex’s local horizon (-22° elevation at the reflection point on sea surface) over 1 day at 2-minute intervals. For comparison, Fig. 2 shows the Topex ground track which represents the geographic . — . — + Jim Garrison, Quarterly NASA GPS Users Council Videxxonference, January 8, 1997 90