Laser ranging error budget for the TOPEX/POSEIDON satellite.
A laser ranging error budget is detailed, and a specific error budget is derived for the TOPEX/POSEIDON satellite. A ranging uncertainty of 0.76 cm is predicted for TOPEX/POSEIDON at 20 degrees elevation using the presently designed laser retroreflector array and only modest improvements in present system operations. Atmospheric refraction and satellite attitude effects cause the predicted range error to vary with satellite elevation angle from 0.71 cm at zenith to 0.76 cm at 20 degrees elevation. This a priori error budget compares well with the ~1.2-cm rms a posteriori polynomial orbital fit using existing data taken for an extant satellite of similar size and orbit.
Atmospheric Refractivity Corrections in Satellite Laser Ranging
Atmospheric refraction can cause significant errors in satellite laser ranging (SLR) systems. There are two techniques which can be used to correct for these errors. Atmospheric models based upon surface measurements of pressure, temperature, and relative humidity have been shown by ray tracing to be accurate to within a few centimeters at 20° elevation angle. The residual errors in the models are thought to be primarily caused by horizontal gradients in the refractivity. vity. Although models have been developed to predict the grnt effects, initial studies show that they can be sensitive to local topographic effects. Atmospheric turbulence can introduce random fluctuations in the refractivity, but only introduces centimeter level errors at elevation angles below 10°. Pulsed two-color ranging systems can directly measura the atmospheric delay in satellite ranging. These systems require mode-locked multiple-frequency lasers and streak-camera-based receivers and currently appear capable of measuring the atmospheric delay with an accuracy of 0.5 cm or better.
Correction of optical distance measurements for the fluctuating atmospheric index of refraction
One of the main limitations, at present, to the accuracy of measurements of long baselines by optical methods is the uncertainty in the average index of refraction over the optical path due to nonuniformity and turbulence of the atmosphere [Thompson et al., 1960]. This uncertainty is due primarily to lack of knowledge of the air density, although a small correction for water vapor is also required. Our purpose in this note is to point out that simultaneous measurements of the optical distance over the same path for blue and red light can be used to provide the required correction for the atmospheric index of refraction. The application of this method to interferometric distance measurements has previously been discussed [Erickson, 1962a].