Title of Paper:  Developing technology for GPS altimetry in GOALS


Principal Author: Cinzia Zuffada


Abstract: The possibility of using the GPS signals scattered off the ocean and sensed by an air- or spaceborne receiver in a bistatic radar geometry, as a means of doing ocean altimetry, would greatly augment the existing capabilities for ocean remote sensing at significantly reduced costs. In fact, when considering the constellation of 24 GPS transmitters and one such receiver a multistatic system is obtained, capable of intercepting bounces from several areas of the ocean simultaneously,  and hence map the entire Earth surface in just 1 day. Such dense coverage can be translated into a higher temporal and spatial resolution than TOPEX/Poseidon or the proposed wide swath altimeter, thereby providing the ability to recover certain ocean topography features or processes, such as eddies, that are precluded with traditional altimeters.


JPL has been working to develop the technology needed to deploy such a system in space under the GOALS task awarded as part of the IIP, concentrating on a novel approach at beam steering with a GPS receiver, potentially more cost effective than current phased-array technology.  JPL has been leading in the development of high accuracy receivers for scientific applications, culminating in the current BlackJack design, intended primarily for flight applications. The current missions CHAMP and SAC-C carry a JPL receiver and a nadir looking antenna for reflections. Raw data will be acquired as soon as feasible and processed on the ground.


GOALS has developed all the basic components to allow us to acquire raw reflection data and extract sea surface heights, interpret the results via theoretical models and produce an altimetry measurement. We detected the first reflection from space and performed the first altimetry measurement from an airplane, demonstrating 10 cm height accuracy in 5 min (extrapolated from 1-sec data averages). We also obtained the most precise altimetry measurement to date. Specifically, from a fixed-site receiver we demonstrated < 1 cm accuracy in 1 sec. Other aspects of the research include estimates of the expected accuracy from space versus design parameters and the concept design of a constellation yielding spatio-temporal resolutions suitable for eddy monitoring.