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98-175666-D Chapter 3: Setup of the antenna 3-17
3.2.5 Operation in gyro-free mode
Heading input: none
If input from a gyro compass is not available, information from the GPS position is
used when searching for a satellite.
When the antenna does not have ship heading input from the vessel's gyro compass,
the azimuth direction of the satellite is not known. In this case the antenna will start a
360 degrees sky scan and scan until it finds a satellite. The satellite search time to find
the satellite and start tracking is therefore raised considerably. If the ship is on a
steady course and sails at a speed over ground above 5 kn, the system can use an
estimated heading from the current GPS position. This will reduce the search time, but
it will still be a longer search time than with heading input.
This mode can be difficult for inclined orbit satellites and elevations <5 and >70
degrees, see the following sections for details.
If a system loses the signal from the satellite, i.e. due to blockage, and the duration of
signal loss is longer than approximately 1 minute, a system without heading input
must do a new sky scan to find the satellite when the antenna is out of blockage.
Inclined orbit satellites
If the wanted satellite is an inclined orbit satellite, the system does not have any
information of the satellite latitude position but only information about inclination.
This means longer search times, depending on the maximum inclination. With the
increased search time for a system without heading input the search time can be so
long that it will be more or less useless in practice.
Tracking for satellite elevation between 5 and 75 degrees
When the system has found the satellite and is in pointing mode, the performance of a
system with heading input and a system without heading input will be very similar.
Note that this is only the case for a satellite elevation range from 5 to 75 degrees.
If the satellite is an inclined orbit satellite, the missing heading information introduces
a polarization error depending on the satellite elevation and the inclination. Normally
it is required that the polarization is controlled within 1 degree towards the satellite.
This gives the following limit for use of inclined orbit satellites (a purely physical limit),
and all systems without heading input have this limit.
Satellite elevation Maximum allowed inclination
<20 2.5
<50 0.7
<70 0.3
75 0
Table 3-13: Satellite elevation and maximum allowed inclination
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