THE LONGITUDEREFERENCE oftheNavyNavigation Satellite System (TRANSIT), published in 1971 has been recomputed based on a new geophysical model (WGS 72) and on an improved orbit determination program. The resulting geodetic longitude reference of the updated system agrees closely with the previous one.

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An experiment is described by which the ephemeris of a near-synchronous satellite was determined from passive range observations. The data consist of the measured times of reception at ground tracking stations of electromagnetic signals which are radiated from the satellite at regular intervals. A comparison of the ephemeris to one obtained from Doppler tracking indicates an accuracy of better than 4.9 mrad rms. This paper reports on an experiment to determine the ephemerides of a near-synchronous satellite from observations which are known as passive range measurements. A passive range tracking system consists of measurements taken at tracking sites of the times of arrival of electromagnetic signals transmitted at regular intervals from the spacecraft. The time interval between the instant the signal leaves the spacecraft and when it arrives at a tracking site is a measure of the range to the spacecraft. This system differs from the usual radar range measurement system in that it is open loop; the ground stations are radio passive. The typical radar range measurement system is closed loop, in which the ground station transmits a signal which is reflected (either actively or passively) from the spacecraft. Inherent to the passive range technique is the necessity to maintain time synchronization between the clocks at each of the tracking sites and the spacecraft. The calibration of the satellite clock is distinct from the station clock calibration, because the satellite is inaccessible and presumably at an unknown location. Consequently, it is necessary to estimate the satellite clock parameters as part of the orbit determination procedure.The DODGE (Department of Defense Gravity Experiment) satellite program was designed primarily to demonstrate gravity-gradient stabilization near synchronous altitudes and to determine the adequacy of theoretical analyses by correlation between digital simulation results and experimental attitude data [1]. A complete description of the mission, as well as the satellite and ground instrumentation, is given in [2]. The nominal prelaunch orbit characteristics were an altitude of 18 200 nautical miles and an inclination of 70. This implies an orbital period of approximately 22 hours and a period relative to an Earth-fixed meridian of approximately 13 days. The accuracy requirement for the ephemerides was arrived at after considering several aspects of the satellite mission. A posteriori ephemerides are necessary for the stabilization studies and the reduction of telemetry and video data, especially that from which the attitude of the spacecraft is to be determined. A maximum a posteriori position error of 100 nautical miles (4.6 mrad) was established as a realistic goal, which, by present day technology, is not a demanding criterion. However, the tracking methodology was formulated consistent with the accuracy goal, with particular emphasis on minimizing the total cost of obtaining tracking data. With this in mind, a subset of the TRANET tracking stations, which provide support for t...

The purpose of this experiment was to investigate the value of altimetry data in high‐precision satellite orbit determination. To accomplish this, software was developed to process laser, C‐band, doppler and altimeter data singly or jointly. Initially, orbit determination studies were undertaken using synthetic data to validate the software. As data became available, preliminary experiments were carried out. When all the data became available, an intensive study was made covering a 4‐day span in 1976. The results showed that even with sparse altimeter data it was possible to accurately determine the semimajor axis and eccentricity with altimeter data only. When altimeter data was supplemented with (as few as) two C‐band passes, high‐precision ephemerides were obtained. Using two laser passes to supplement the altimetry data did not achieve that same high precision. This is probably because the geographic location (mid‐Atlantic) of the highly accurate laser data were such that they did not ideally complement the available (south Atlantic and Indian Ocean) altimeter data.

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