Satellite tracking.

The tracking system is an essential part of a ground station for obtaining the orbital elements of any satellite. Since orbit degradation occurs under perturbing forces such as drag due to atmospheric density, earth oblateness etc., it is necessary to continuously track the satellite in order to update the orbital parameters.

Theoretically, six independent observations of a particular tracking parameter of a satellite are sufficient to determine its orbit completely. However, due to inaccuracies of measurement involved, more than one tracking parameter is usually measured and weighting for the data of a particular parameter is given depending upon the accuracy of measurement, for orbital computation purposes.

For Aryabhata, because of spacecraft constraints, no separate onboard tracking package was installed. However, with the available onboard communication packages (telecommand receiver and telemetry transmitter), three tracking systems were developed and are described below.

An interferometry system, to provide angular information of the satellite. It uses the carrier component of the telemetry transmission as the beacon for tracking purposes.

 A tone-ranging system, to provide range. It uses the onboard communication packages as transponder for the measurement of range.

A Doppler system, to provide range rate. It uses the onboard telemetry transmitter carrier frequency as the beacon frequency for tracking proposes. The three tracking systems were installed at SHAR and are being used to obtain the tracking parameters of Aryabhata having a near circular orbit of 600 km altitude

at an inclination of 51 ~ to the equatorial plane. The station at SHAR provides tracking data for about 10 min for the satellite pass with a maximum elevation of 90 ~ The data are recorded using a digital printer.

The tracking data are provided to the Post-Launch Operation Group for analysis. The analysis consists of smoothing the data by a curve fitting procedure and then using the smoothed data to up-date the orbital parameters of the satellite. The following sections deal with the requirements, principles, design considerations and performance of the tracking systems
 Choice of the systems

The systems conceived for Aryabhata are relatively simple compared to available systems due to the limitations of spacecraft configuration and availability of resources and time. The three tracking systems--interferometry, tone-ranging and Doppler tracking, the last being a supplement to the first two--were decided upon to meet the following requirements of accuracy:

direction cosine angle : 4-6' of an are;

range : 4-I km;

range rate : 4-6-6 m/s.

Sophisticated interferometry systems capable of giving an accuracy of better than -4-20" of arc in the direction cosine angles have been developed over the years by many nations. This involves huge antenna structures providing a directive gain, automatic electrical length measurement and control systems and an elaborate calibration procedure (Nollet et al 1974).

For Aryabhata, as the requirement of accuracy was only -t-6' of arc, it was decided to go in for a simple interferometry system consisting of six omnidirectional (turnstile) antennas with highly phase-stable foamflex cables to feed four pairs of receivers located in the operator's room.

The analysis indicates that with this simple system it is possible to meet the requirement of the angular accuracy.

There are three widely used systems for the measurement of the range of a spacecraft from the gound station:

        I.            a coded ranging system,

     II.            a multiple continuous-wave-tone ranging system,

   III.            a hybrid of the above two.