The Delay-Doppler Altimeter: More Precision and a Smaller Footprint
¹ Johns Hopkins University Applied Physics Laboratory;
² NOAA Laboratory for Satellite Altimetry
Abstract.
The sea surface slope measurements provided by the Geosat radar altimeter (together with selected ERS-1 data) are the state-of-the-art for geodetic observations from space. However, those results fall significantly short of the potential resolution and sensitivity set by physical limits. Spaceborne radar data can approach these limits only if the instrument is substantially improved, and if the orbit is optimized. (Further information on the scientific goals, requirements, and benefits of improved data may be found in a companion paper by W. H. F. Smith at this meeting.) A delay-Doppler radar altimeter can deliver the required height precision and spatial resolution. This innovative satellite altimeter uses signal processing strategies borrowed from synthetic aperture radar to improve height measurement precision by a factor of two, and to reduce along-track footprint size by a factor of five or more in marked contrast to a conventional radar altimeter. The signal processing can be performed on-board in real-time, resulting in a modest data downlink rate. The delay-Doppler altimeter has been built and flight-tested on NRL and NASA P-3 aircraft. The airborne instrument-the D2P altimeter-also included cross-track angle measurement using a pair of interferometric receive antennas. Results verify the predicted along-track resolution and delay-Doppler signal properties. In addition, it was demonstrated that angular offsets in the along-track (pitch) plane are measured uniquely in the Doppler domain, and that angular offsets in the cross-track (roll) plane are measured by the interferometer. If a dedicated free-flying implementation were available, the orbit should be non-repeating, and have an inclination relatively near 55 degrees. From such an orbit, the required measurements would be completed in less than five years by a solo delay-Doppler radar altimeter satellite, or less than twenty months by a constellation of three co-planar satellites. As an alternative, the International Space Station (ISS) is also a candidate. The ISS has a non-repeating orbit at an inclination of 51.6 degrees. Although its orbit is nearly optimal, the angular motions of the ISS could disqualify it as a host platform. On the ISS, a D2P-style altimeter could be mounted on a mechanical gimbal. Angle measurements from the altimeter could be used to servo-steer the gimbal to maintain nadir pointing, regardless of the attitude of the ISS. The ISS was the nominal satellite of choice in the ABYSS ESSP project recently proposed to NASA. Further details are at http://fermi.jhuapl.edu/abyss and http://fermi.jhuapl.edu/d2p
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