Earth scale below a part per billion from Satellite Laser Ranging

Since the LAGEOS I satellite was launched in 1976, the systematic instrument error of the best satellite laser ranging observatories has been steadily reduced to the current level of only a few millimeters. Advances in overall system accuracy, in conjunction with improved satellite, Earth, orbit perturbation and relativity modeling, now allows us to determine the value of the geocentric gravitational coefficient (GM) to less than a part per billion (ppb). This precision has been confirmed by observations of the LAGEOS II satellite, and is supported by results from Starlette, albeit at a lower level of precision. When we consider observations from other geodetic satellites orbiting at a variety of altitudes and carrying somewhat more complex retro-reflector arrays, we obtain consistent measures of scale, which however must be based upon empirically determined, satellite-dependent detector characteristics. We arrive at an estimate of GM of 398600.44187 +/-.00020 km3/sec2, which lies within the 2 ppb uncertainty of the current standard, but differs from it by more than the error of the new estimate. Both the current standard and our recommended value fall comfortably within the ten ppb uncertainty of that determined from the most accurate alternative from lunar laser ranging observations. The precision of the estimate of GM from satellite laser ranging has improved by an order of magnitude in each of the last two decades, and we will discuss projected advances which will result in further refinements of this measure of Earth scale.