Estimation od Systematic Errors in Angles of Tracking Antennae


KNAGESWARA RAO,  P.SOMA
soma@istrec.ernet.in
ISTRAC

Abstract:
ISTRAC provides TT and C support in S-Band for Launch Vehicle and
spacecraft missions from a network of ground stations.  All the TTC stations employ
tracking antennae with Azimuth over elevation mount with pointing accuracies of
typically 0.03 deg.  During the Launch Vehicle phase, these antennae are designated
towards the Launch Vehicle using real time tracking date from Range in computer
designate mode. Target tracking is also achieved in program track mode using
predicted antenna pointing angles.  Antenna angles data is an important input for
Preliminary Orbit Determination immediately after satellite injection and also orbit
determination during tho LEO phase, This requires that the systematic errors In the
antenna look angles are estimated and corrected appropriately for proper target
pointing and also for the use of angles data in Orbit Determination.

The systematic errors are categorized as follow:

*   Zero set biases and deviations
*   Collimation
*   Deviations in alignment of the azimuth and elevation bearings
    (non orthogonality)
*   Nonlinearity of the encoders
*   Velocity and acceleration lags
*   Mislevel of the azimuth platform
*   Residual refraction effects

After a detailed sensitivity analysis, an error model that characterises the
systematic errors of azimuth and elevation angles well AvintvQd A software package to
estimate the error coefficients using Linear Least Squares in batch processing mode
was developed and operationally used.  The error coefficients are estimated with IRS
satellite tracking and operational orbit determination results with an established position
accuracy of better than 200 m as reference.  Systematic errors of each of the ground
station antenna are computed by conducting tracking campaigns designed to collect
data spread evenly In the Azimuth - Elevation domain.

This paper prosents the details of the error model employed, sofnare packaage
developed and the results obtained.  Improvements in the orbit determination results
using systematic error corrected angles data are reported.