Installation Manual

89-041 Section 4 Issue 5 S4-3
GNSS (GPS) antenna
The GNSS (GPS) antenna has to have a clear view of the sky, the objective is to see the
horizon freely through 360 degrees with a vertical view of 5 to 90 degrees above the horizon.
As the received GNSS signal is very sensitive to noise and interference generated by other
onboard transmitters ensure that the GNSS antenna is placed as far away as possible from
Radar, Inmarsat and Iridium transmitters. Ensure the GNSS antenna is free from direct view of
the Radar and the Inmarsat beam.
It is also important that the MF/HF and other VHF transmitter antennas are kept as far away as
possible from the GNSS antenna. It is good practice never to install a GNSS antenna within a
radius of 5 meters from these antennas.
The GNSS operates in the ultra high frequency band (1.575 GHz). The signal attenuation in
cables is therefore substantial and has to be taken into account when the coax cable between
the antenna and the UAIS unit is chosen. To compensate for this signal attenuation the supplied
GPS antenna includes a pre-amplifier with a gain of 36 dB.
The optimum combination of coax cable and GPS antenna gain will have a resulting installation
gain (pre-amplifier–cable attenuation) within 0 to 10 dB. The table below shows examples of
coax cables.
Cable description Attenuation / 100metre Maximum cable length
RG 58 70 dB @ 1.5GHz 40 metres
RG214 37 dB 90 metres
Connectors used must be TNC throughout.