|Real GSO satellites do not remain exactly stationary in orbit.
Movement will occur east/west and north/south due to:
- Small errors in placing them in orbit
- Disturbing forces in orbit
- Earth's gravity field is not uniform
- Gravitational forces of sun and moon
- Solar pressure
Stationkeeping on the satellite maintains the position within a small "box"
typically ±0.1° N-S and E-W.
Small antennas have a 3 dB beamwidth larger than the stationkeeping box so no difficulties
Large antennas have narrow beams smaller than the stationkeeping box so require a tracking and
steering system to remain pointing at the satellite.
In the early experimental days of satellite communications earth stations were manually
steered by operators using joysticks. Manual steering is not practical for commercial systems
and so automatic systems of two basic types are used:
- Programme track:
- Uses calculated theoretical pointing information
- Pointing data calculated from satellite orbital elements from tracking stations
- Data needs to be updated frequently to remain accurate
- Uses a signal from the satellite and a feedback control loop to track in real time
- Not dependant on predictions or orbital elements
Common types: Steptrack and Monopulse
Tracking errors result in loss of pointing accuracy and hence loss of earth-station gain in
the direction of the satellite. The aim is to keep the loss to a fraction of a dB. Reduction
in gain (dG) relative to the maximum (on-axis) gain of an antenna of half-power beamwidth (HPBW)
for a small angular offset (dq) is given approximately by:
dG = 12 (dq / HPBW)2 dB
The aim is therefore to keep pointing error (dq) to about 0.1 x HPBW.
For a large diameter antenna (~400*l) the HPBW is ~0.15°. Desirable pointing error is thus
< ~0.015° This is generally achieved by using an automatic tracking and pointing system
Smaller antennas (~ 25 to 100*l) the HPBW is ~ 0.5° à 2.5° and these do not need continuous
tracking but may need to be manually redirected from time to time.
Steptrack systems make frequent small changes in the pointing direction of the antenna in both
Does the signal increase or decrease? If it increases then the next step is in the same
direction, if decreases then the next step is in the opposite direction
Advantages of Steptrack systems:
- relatively cheap
- simple to implement
Disadvantages of Steptrack systems:
- Stepping results in loss of gain
- Easily confused by level changes (e.g. as a result of rain fade)
- "Wear and tear" on steering equipment
Monopulse systems. Multimode systems originated from developments in RADAR technology.
Antenna feed is designed so that higher order modes are generated in the feed when the signal
source is off the centre of the antenna beam. Signal processing of the higher order modes
generates pointing error signals used to steer the antenna
Advantages of Monopulse systems include very good accuracy.
Disadvantages include the relatively expensive feeds and recievers.
Both steptrack and monopulse systems need a beacon signal radiated by the satellite.
Improved steptrack (e.g. smoothed steptrack - SST) aims to improve the performance of
steptrack. This combines the elements of programme track and steptrack. It uses normal
steptrack approach to build a "model" of the satellite track. Once an accurate model
is established SST keeps it up to date (under normal circumstances) by sampling every 30
minutes or so. SST thus ignores any random variations which might cause tracking errors. If
the beacon signal is lost altogether (e.g. a beacon transmitter or receiver fails) then SST
can continue to predict the track using the programme track model. If SST detects a serious
departure from the model (e.g. because of an orbit correction) then the learning process is
restarted. Accuracy is claimed to be close to that of monopulse.