Spatial FOG Reference Manual
Page 43 of 144
Version 2.3
03/05/2018
10.3
Time
Spatial FOG was designed to provide a highly accurate time reference. When a GNSS
fix is available Spatial FOG's time is accurate to within 50 nanoseconds. When a GNSS
fix is lost, Spatial FOG's time accuracy typically remains within 10 microseconds over
extended time periods. When Spatial FOG hot starts the time accuracy is typically
within 1 second immediately on startup and corrected to within 50 nanoseconds as
soon as a GNSS fix is achieved. To synchronise with Spatial FOG's high accuracy time,
both the packet protocol and a 1PPS line must be used.
10.4
Heading Source
There are four diferent heading sources available for Spatial FOG. By default Spatial
FOG uses north seeking gyrocompass heading. This can be complemented with
velocity heading for forward driving vehicles such as cars. In applications where north
seeking gyrocompass heading is unusable and velocity heading is not an option, dual
antenna GNSS heading is possible with the internal GNSS receiver. It is possible to use
multiple heading sources and this will often provide performance benefits.
10.4.1
North Seeking Gyrocompass Heading
Spatial FOG's high accuracy gyroscopes allow it to detect the rotation of the earth and
seek north with a high degree of accuracy. When used as the sole heading source,
initialisation of the north seeking takes 15 minutes and requires that the unit remain
stationary in between three rotations. With the assistance of another heading source,
the gyrocompass heading can take as little as 60 seconds to initialise and will not
require any rotations or stationary time. For example on a ship that is moving at a
speed of over 1.15 metres/second with velocity heading enabled, the north seeking
algorithm will automatically initialise during the first 60 seconds of operation.
When used as the sole heading source, the north seeking initialisation procedure is:
1. Turn on Spatial FOG and leave stationary for 5 minutes.
2. Rotate Spatial FOG approxi90 degrees about the Z axis and leave
stationary for 3 minutes.
3. Rotate Spatial FOG approxi90 degrees about the Z axis and leave
stationary for 3 minutes.
4. Rotate Spatial FOG approxi90 degrees about the Z axis and leave
stationary for 3 minutes.
To assist users in getting accustomed to the north seeking procedure there is a north
seeking status dialogue in Spatial FOG Manager that provides feedback and guides the
user through the initialisation process, see section 11.7.12.
Spatial FOG's gyrocompass heading is fully automatic and always on. For all
applications, the gyrocompass heading requires approximate position. For moving
applications, the gyrocompass heading function requires continuous velocity updates
to provide accurate heading. This data is typically provided by the internal GNSS
receiver, however it can also be provided by an odometer or other type of speed
sensor.
