This is an introduction to the August 2023 Innovation article, “Far Out: Positioning above the GPS constellation”
Innovation Insights with Richard Langley
On October 25, 1997, a defense satellite was launched from Cape Canaveral on an Atlas rocket with a Centaur upper stage. The Centaur went into an elliptical geosynchronous transfer orbit with an apogee close to geostationary orbit radius before releasing the satellite. Bolted to the side of the Centaur was an instrument package containing a GPS digital sensor. This piggyback device was part of an experiment by students at the U.S. Air Force Academy to test some of the concepts of GPS navigation for high-altitude spacecraft.
The sensor captured 40-millisecond samples of GPS L1 signals collected by a patch antenna. The digital samples were downlinked to a ground station in Colorado Springs where they were subsequently processed. The equipment successfully operated from November 3 until at least November 9. During that time, GPS signals were detected across a wide range of altitudes above the GPS constellation including at times when the Falcon Gold antenna was only in view of a GPS satellite’s transmitting antenna sidelobes. The downlinked data was carefully archived. The Falcon Gold experiment was discussed by Thomas Powell of the Aerospace Corporation in an article he wrote for this column in October 1999 entitled “The View from Above: GPS on High-Altitude Spacecraft.”
Fast forward a couple of decades. Researchers at the University of Minnesota are taking a fresh look at the Falcon Gold data using some innovative analysis tools, which may prove beneficial for processing GNSS data from receivers on other satellites flying above the GNSS constellations even all the way to the Moon. In this quarter’s Innovation column, they tell us about their work and its potential benefit.
This Falcon Gold data study is a great example of how archived GNSS data can be reanalyzed with fresh insight and new techniques to milk even more and better results from the data. Another important example is the wealth of data that has been acquired by the International GNSS Service since beginning operations in 1994. The data in the archive is reprocessed from time to time to produce a more consistent long product set for analysis of sources of systematic error and to improve its ultimate accuracy. This results in a better understanding of Earth system dynamics, for example, including plate tectonics. The data from many other GNSS instruments flown in space is also archived allowing look backs for further and more detailed analyses. This includes my GPS Attitude, Positioning, and Profiling (GAP) instrument on the CASSIOPE scientific satellite, now part of ESA’s Swarm constellation. Researchers continue to produce interesting scientific results from the GAP data. So, it’s not always necessary to generate fresh data for a study – useful data may already exist. What’s old can indeed be new again!