GNSS Compass Reference Manual
Page 38 of 85
Version 1.1
13/01/2021
9
Operation
9.1
Initialisation
After power is applied to the GNSS Compass, it will take some time to initialise position, velocity,
time, roll, pitch and heading. This time will depend upon the status of the GNSS satellite
constellations but can be approximated using Table 21 below.
Model
Power Up Type
Position Initialisation
Heading
Initialisation
Low Cost
Hot Start
1 second
40 seconds
High Accuracy
Hot Start
3 seconds
120 seconds
Low Cost
Cold Start
30 seconds
60 seconds
High Accuracy
Cold Start
30 seconds
180 seconds
Table 21: GNSS Compass typical initialisation time
9.1.1
Hot Start
If the GNSS Compass has been powered off for more than 24 hours, when it is powered on it will
initialise with a cold start. If the GNSS Compass has been powered off for less than 24 hours it will
initialise with a hot start which is much faster.
9.2
Dual Antenna GNSS Heading
In the past most AHRS have relied upon magnetic heading. Magnetic heading has some
downsides in that it requires calibration and is prone to interference from ferrous metals.
GNSS Compass uses a new technique to determine heading by measuring phase differences
between two GNSS antennas mounted at a known separation inside the enclosure of the GNSS
Compass. The benefit of this technique is that it is not subject to magnetic interference and
provides more accurate heading than magnetic systems.
Dual antenna GNSS heading does require a clear view of the sky and sufficient spacing from RF
transmitters operating close to GNSS bands.
9.3
GNSS Outages
When the GNSS Compass has an obstructed view of the sky there may be loss of a heading fix,
loss of a position fix or both. This can occur when travelling under bridges, through tunnels or
between buildings.
During a loss of heading fix the GNSS Compass will continue to provide accurate heading for up to
10 minutes using its gyroscopes. A heading error will slowly accumulate during this period at a rate
of approximately 15 degrees/hour.
During a loss of position fix the GNSS Compass will continue to provide accurate position and
velocity for up to 2 minutes using its inertial sensors.
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