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Utilizing the Hopfield model for the altitude variation of the refractive index, we have found a convenient approach to the necessary integrals. The approach leads to simple analytic forms. The forms clearly illustrate that the dry term correction is directly proportional to surface pressure and quite insensitive to surface temperature: For elevation angles above 5° the range correction is proportional to P·(Tc + 269.04)/(Tc + 273.16), where Tc is the surface temperature in degrees Celsius and P is the surface pressure. The resulting equations and their derivations and limitations are described.

Using a simple geometrical model, and one fitted parameter, tropospheric effects can be effectively removed from satellite Doppler data at microwave frequencies. Both the wet and the dry parts of the tropospheric r•efraction effect are removed. The technique works best for low (say, 1200 km or less) altitude satellites. For these satellites, the pass (transit) dui'ation limits the required atmospheric correlation time to about 20 min. The effective thickness of the neutral a•mosphere (10 km for the wet and 45 km for the dry) limits the required correlation distance to a few degrees in l•titude and longitude. These conditions are satisfied often enough to make the fitting technique highly useful. The fitted parameter together with minimal dependence on model structure appears to skirt a difficult problem, modeling the water vapor distribution in a poorly mixed • atmosphere . Experimental results (for a limited time period) confirm that the approach is valid on a global basis. In another context, this technique can be used to intensively sample the precipitable water vapor in the atmosphere without using balloons. I I i ß Vv I -10 ß ß --5 68 PTS, SATELLITE 30480, DAYS 198-199, 1982 Along-track (m)--2--ß v V ß v V ß ß ß ,vWv ß ß I Slant-range (m) Fig. 13. Station position changes associated with the fitting the tropospheric parameter.

Removal of the pole wander (Chandler wobble) as a source of bias in the Transit Navigation System has improved the precision available to a stationary user (surveyor). The precision associated with the mean of several (say 20) combined passes should be less than 5 meters. The dominant items in the error budget will remain (for a land‐based user) uncertainty in the satellite position and instrumentation errors. For an at‐sea, non‐stationary user; the errors are much larger and (still) dominated by uncertainties in the user's motion; typically 400 meters/knot of navigator's velocity error. The changes discussed here will not alter this situation in any way. We discuss, herein, just how the polar motion is accounted for in the system. No changes are required of any user's hardware or computer programs. The benefits accrue to him (or her) automatically via an improved precision ephemeris in the satellite which now contains the polar motion implicitly. The improvement will be implemented on Dec. 15, 1973. The Transit Coordinate System is reviewed and the recent performance (accuracy) of the system described.