4-14
Acquisition
The acquisition of satellite signals by a ground station equipped with large antennas and operated at
microwave frequencies places severe requirements on the system. Several factors must be considered.
These factors are discussed below:
SPATIAL-TIME FACTOR.—Very accurate antenna pointing information is available to earth
terminals from the satellite control facility located in Sunnyvale, California. Because of equipment
limitations, a small search about the predicted location of the satellite must often be conducted to make
initial contact. Either a manual or automatic scan is made around a small area close to the point where the
satellite appearance is predicted.
FREQUENCY CONTROL.—The frequency of a radio signal received from a satellite is not
generally the exact assigned down-link frequency. This variation depends upon the type of orbit of the
satellite. The greatest frequency variations in signals from satellites occur in medium altitude circular or
elliptical orbits. The smallest frequency variations occur in signals from satellites in near-synchronous or
synchronous orbits.
Tracking
When a particular satellite has been acquired, the earth terminal antenna will track that satellite for as
long as it is used as a communications relay. Several methods of tracking are in actual use; however, we
will explain PROGRAMMED TRACKING and AUTOMATIC TRACKING.
PROGRAMMED TRACKING.—In programmed tracking the known orbital parameters of the
satellite are fed into computation equipment to generate antenna pointing angles. The antenna pointing
angles are fed as commands to the antenna positioning servomechanisms. (You may want to review
servos in NEETS, Module 15, Principles of Synchros, Servos, and Gyros.) These point the antenna in the
required direction. The amount of data and computations involved in using programmed tracking is
extensive. These are a result of the antenna mount flexing and atmospheric and ionospheric bending of
radio waves. Because of these uncertainties, programmed tracking is not used extensively.
AUTOMATIC TRACKING.—In automatic tracking, the equipment generates antenna pointing
information by comparing the direction of the antenna axis with the direction from which an actual
satellite signal is received. Automatic tracking systems track the apparent position of a satellite. The
direction of arrival of the radio signal and the real position of the satellite is not required. The automatic
tracking system uses a servomechanism to move the antenna. Once the satellite has been located, the
servomechanism generates its own pointing data. This eliminates the requirement for continuous data
input and computation.
SATELLITE OUTAGE TIME.—The satellite outage time specifications allow for stewing
(moving) the earth terminal antennas, acquiring the satellite signal, and checking for circuit continuity at
HAND OVER. (Hand over is the period of time for one earth terminal to yield control to another as a
satellite moves out of its area of coverage.) This hand over period represents an outage time. If the control
terminal is unable to hand over to another terminal within a specified time, other arrangements are made.
For example, control may be retained or transferred to another terminal within the coverage area. There
are several reasons why a terminal may be unable to assume control on time; these reasons may combine
to increase the outage time. The difference of drift velocities of the satellites leads to bunching within a
coverage area. This causes gaps in coverage and increases outage times. When two or more satellites
simultaneously occupy the same space of the terminal antennas, they will interfere with each other. This
prevents reliable communications. Other factors leading to increased outage times are SATELLITE-SUN
CONJUNCTION (increased noise while the satellite passes near the sun), SATELLITE ECLIPSE
