Satellite Laser Ranging
A Joint Agency Effort for Precise Satellite Positioning
G. Charmaine Gilbreath
Electro-Optics Center Naval Research Laboratory
About the Lecture
Satellite Laser Ranging is the accepted method of establishing “truth” for precise positioning of satellites. The orbits of geodetic satellites such as LAGEOS, for example, have been estimated to within centimeters. Precision in positioning of satellites can be used for any number of scientific and engineering applications. These include measurement of the motion of the tectonic plates, measurement of the displacement of the surface of the ocean to within 10 cm, and precise orbital determination for navigation. This lecture will provide an overview of the technique and will review the state-of-the-art capability recently developed under the leadership of the Naval Research Laboratory at the USAF Phillips Laboratory’s Starfire Optical Range. Notable in the design is the integration of a 300 millijoule laser with the 3.5 meter telescope at SOR. Returns have been obtained routinely from the NAVSTAR GPS satellites at 14,000 km as well as from low flying satellites at 400 to 600 km.
About the Speaker
G. Charmaine Gilbreath has been with the Naval Research Laboratory since 1980. She has conducted integration testing for intersatellite laser communications and served as principal investigator of a program in photorefractive research for adaptive optical wavefront control. Since 1992, she has been head of the electro-optics technology section. Her team has established a world-class satellite laser ranging capability in a joint effort between NRL, the USAF Phillips Lab, and NASA. They have re-established retro-array design within the government laboratory structure and have designed retro arrays for a number of DOD spacecraft. Recently, she has become involved in the NASA Origins program in its efforts to detect earth-like planets using space-based telescopes and interferometers. Ms. Gilbreath has a B.S. in physics from the Georgia Institute of Technology, a M.S.E. and a Ph.D. in electrical engineering from The Johns Hopkins University.
President Coates called the 2062nd meeting to order at 8:31 p.m. on October 4, 1996. The Recording Secretary read the minutes of the 2061st meeting and they were approved.
Mr. Coates introduced G. Charmaine Gilbreath, of the Electro-Optics Technology Section Naval Center for Space Technology, Naval Research Laboratory, to discuss “Satellite Laser Ranging: A Joint Agency Effort For Precise Satellite Positioning”.
In order to use the Global Positioning Satellite (GPS) System for precisely determining location on the earth's surface it is first necessary to know as accurately as possible the position of those satellites. The best method for establishing accurate satellite positions is Satellite Laser Ranging (SLR). Using SLR the positions of geodetic satellites such as LAGEOS can be estimated to within centimeters and the positions of other low orbiting satellites can be quickly and accurately determined with similar precision. This important capability has been developed by the Naval Research Laboratory, NASA and the USAF Phillips Laboratory at the Starfire Optical Range, Kirtland AFB, NM.
As mentioned, it is SLR technology that enables extremely accurate global positioning and navigation through the NAVSTARS GPS. Of the 24 NAVSTARS satellites, 4 to 5 are usually in the field of view from any point on earth. The SLR system must be capable of accurately and quickly redetermining the orbital parameters of all these satellites. In addition to its role in maintaining GPS accuracy, the precise positioning of satellites has a number of other scientific and engineering applications. By continuous monitoring of 44 sites around the world, motions of tectonic plates are being measured in “real time” to within 1 mm per year, and strains along active fault zones can be dynamically assessed for better earthquake monitoring and prediction. Using the TOPEX satellite system, displacements in the oceanic surface are being monitored globally to precisions within 10 cm.
The SLR at the Starfire Optical Range is located 1876 m above sea level near Albuquerque, NM. The optical system integrates a 300 millijoule Nd-YAG laser, a 3.5 meter telescope, and polarizing optics. The laser's green, coherent, vertically polarized light is aligned, circularly polarized and presented at the telescope arpeture with 100 microradian divergence. The beam focus is at a distance of 35 km. Calibration is maintained with both optic path internal and 46.8 m external reflector targets. The beam reflected from target satellites, now reverse circularly polarized, is directed back along the same optics path. Presently, the system does not use either multi-wavelength or atmospheric-adaptive optics.
Return signals are routinely obtained from the GFZ satellites in orbits as low as 370 km as well as the NAVSTARS GPS satellites at 20,200 km. For the very fast, low orbit satellites the ranging telescope must be agile enough to keep the beam on target with an azimuth velocity of more than 11 °/sec in both axes. At least for SLR, the 1 cm precision presently obtained is “good enough for government work”.
Ms. Gilbreath kindly answered questions from the audience. Mr. Coates thanked the speaker on behalf of the Society, announced the speaker for the next meeting, restated the parking policy, and adjourned the 2062nd meeting at 9:27 p.m.
John S. Garavelli