3-axis stabilized antennas have to be used on maritime vessels for many years to compensate for the ships movement to track a satellite signal. There are 4 main antenna manufacturers that the majority of VSAT (and TVRO) satellite service providers use for Commercial Maritime, Offshore Energy, Defence and Luxury Yachting.
The stabilized antenna system consists of the following elements:
- Balanced Pedestal
Stabilization is used to prevent any unwanted movement caused by friction and is not removed by balancing the antenna.
Friction will have negative effect on the balance and will cause miss pointing if not corrected.
Friction can be caused by:
- Belt tension
- Cable tension
- Air in the radome
- Shocks and vibrations
The antenna can track the satellite by sampling the RF energy in a circular movement called a Conical Scan or Conscan. It compares the signal at the left, right, top and bottom every second.
Tracking is needed to:
- Make sure, the pointing of the Main Reflector, stays within 0.2 degrees off the centre of the tracking transponder by using the received signal from the satellite.
- Compensate for inclinement of the target satellite.
Balance isolates ships motion from the main reflector with movable weights added to the azimuth, elevation and cross level. When the vessel is stationary the antenna should not move at any position. If the antenna is not balanced correctly then it will likely experience poor tracking due to increased load on the motors.
Ships Motion is influenced by the wind, water conditions and the tide.
Most antenna systems use the receive coax for communication between the Below decks Equipment (BDE) and Above Decks Equipment (ADE) via a FSK modem. The transmit coax goes straight to the antenna from the VSAT modem. A TVRO antenna obviously only uses receive coax.
Antenna Control Unit (ACU)
At the heart of every stabilized antenna system is the Antenna Control Unit (ACU). Located below decks the ACU has the configuration and satellite information loaded on to it.
Using the GPS located in the Radome and the vessel Gyro Compass the ACU can calculate the elevation, azimuth and polarization to the satellite and drives the antenna to the correct position. When the ACU has locked to the correct satellite signal it will start tracking and remain locked on position irrespective of the vessel movements.