Philosophical Society of Washington

Minutes of the 2062nd Meeting

Speaker: G. Charmaine Gilbreath, Space Technology, Naval Research Laboratory
Topic: “Satellite Laser Ranging: A Joint Agency Effort For Precise Satellite Positioning”

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.


Respectfully submitted,
John S. Garavelli
Recording Secretary

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