The GPS Ocean Reflection Experiment onSpartan 251

James L. Garrison, Angela Russo, Melvin J. Ferebee, Dave Mickler, Michael Armatys

Abstract:	It has recently been demonstrated that the GPS signal which has reflected from the ocean surface contains useful geophysical data from which the sea surface wind speed and other parameters can be extracted. This can be used for remote sensing, similar to present day use of radar altimeters or scatterometers, but with significantly smaller instrumentation because of the utilization of the existing GPS broadcast signal for illumination. Several campaigns of aircraft exper-imentation have been completed demonstrating this technique and reflected GPS data has been collected from 25 kms altitude on a balloon. However, there has not yet been a demonstration that the reflected GPS signal can be detected from orbit with sufficient signal to noise ratio (SNR) to make useful remote sensing measurements. A technology demonstration experiment was planned for a Space Shuttle flight in early 2000 using the Spartan 251 recoverable car-rier. This experiment would also have been the first flight validation of the PiVo T GPS receiver developed in house at the Goddard Space Flight Center. The “open-architecture” design of this receiver would allow the software modifications to be made which control code-correlator spacing to map out the shape of the reflected signal waveform, which is the most basic data product generated by this instrumentation. A mod-erate gain left-hand circularly polarized antenna, con-structed from an array of off-the-shelf hemispherical antennas was to be used to give approximately 3 to 6 dB of additional gain. Preliminary SNR predictions have been done indicating that this antenna would offer sufficient gain to record waveform measurements. A system level description of the experiment instru-mentation, including the receiver, antenna and data storage will be given. The visibility of GPS reflections over the mission duration of several hours will be pre-sented, including the effects of the beamwidth of the antenna. Spartan 251 has now been postponed with the earliest opportunity in the year 2002. The results of this study, however, have been used to further define the requirements and expected performance of reflected GPS receivers in orbit. Several other space flight opportunities are being considered based upon this new information.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	885 - 894
Cite this article:	Garrison, James L., Russo, Angela, Ferebee, Melvin J., Mickler, Dave, Armatys, Michael, "The GPS Ocean Reflection Experiment onSpartan 251," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 885-894.
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