Time; Heading Source; Dual Antenna Heading; North Seeking Gyrocompass Heading - Advanced Navigation Spatial FOG Dual Reference Manual

A next generation backup battery system within Spatial FOG Dual provides the hot
start ability for more than 24 hours without power. When Spatial FOG Dual hot starts it
assumes that it is in the same state it was when it lost power and begins navigating
from that position. The hot start also provides ephemeris, almanac and time
information to the GNSS receiver which allows it to achieve a fix far more quickly than
it otherwise would. When the GNSS achieves it's first fix, if this position deviates from
the hot start position, Spatial FOG Dual will jump to the new position without causing
any side effects to the filter.
Whilst Spatial FOG Dual is without power it keeps track of the time accurately to within
1 second so that the time is immediately valid on a hot start.
Spatial FOG Dual's hot start is of particular benefit to vehicle tracking and robotics
applications. The primary benefits are immunity and fast recovery from power failure
as well as fast startup time.
10.4

Time

Spatial FOG Dual was designed to provide a highly accurate time reference. When a
GNSS fix is available Spatial FOG Dual's time is accurate to within 50 nanoseconds.
When a GNSS fix is lost, Spatial FOG Dual's time accuracy typically remains within 10
microseconds over extended time periods. When Spatial FOG Dual 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
Dual's high accuracy time, both the packet protocol and a 1PPS line must be used.
10.5

Heading Source

There are four different heading sources available for Spatial FOG Dual. By default
Spatial FOG Dual uses dual antenna heading. This can be complemented with velocity
heading for forward driving vehicles such as cars. In applications where GNSS is not
available, north seeking gyrocompass heading can be used. It is possible to use
multiple heading sources and this will often provide performance benefits.
10.5.1

Dual Antenna Heading

Dual antenna heading is the default heading source and provides very accurate
heading while GNSS is available. Dual antenna heading works under both stationary
and moving conditions and does not require any specific initialisation procedure. Dual
antenna heading only works when there is a good GNSS fix available. It requires a
clear view of the sky with minimal nearby sources of interference or multipath.
10.5.2

North Seeking Gyrocompass Heading

Spatial FOG Dual'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 once the system has initialised with either dual antenna heading or
Spatial FOG Dual Reference Manual
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Version 1.0
26/06/2015