Calibration
6-14 Chapter 6: Configuration 99-145912-A
6.2.6 About 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 Max allowed inclination
<20 2.5
<50 0.7
<70 0.3
75 0
Table 6-5: Satellite elevation and max. allowed inclination
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