Spatial Reference Manual
Page 51 of 158
Version 4.4
04/06/2019
10.3 Time
Spatial was designed to provide a highly accurate time reference. When a GNSS fix is
available Spatial's time is accurate to within 50 nanoseconds. When a GNSS fix is lost,
Spatial's time accuracy typically remains within 10 microseconds over extended time
periods. When Spatial 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's high accuracy time, both the packet protocol
and a 1PPS line must be used.
10.4 Heading Source
There are three different heading sources available for Spatial. The heading source can
be selected using the filter options dialogue in Spatial Manager, see section 11.8.3. It
is possible to use multiple heading sources and this can often provide performance
benefits.
10.4.1
Magnetic Heading
This is the default heading source and works well in the majority of cases. When using
magnetic heading, calibration is required every time Spatial's installation changes. The
downside of magnetic heading is that prolonged dynamic magnetic interference
sources can cause heading errors.
10.4.2
Velocity Heading
Velocity heading works by deriving heading from the direction of velocity and
acceleration. Velocity heading works well with cars, boats, fixed wing aircraft and other
vehicles that don't move sideways. Velocity heading does not work with helicopters
and other 3D vehicles. The downside of velocity heading is that heading can not be
measured until the vehicle moves at a horizontal speed of over 1.15 metres/second
with a GNSS fix. The benefits of velocity heading are that it is immune to magnetic
interference and no calibration is required when Spatial's installation changes.
10.4.3
External Heading
This can be used if there is some other way to derive heading that is external to
Spatial. Examples include dual antenna GNSS systems, north seeking gyroscopes,
reference markers and SLAM systems. The heading must be fed into Spatial using the
External Heading Packet or through the auxiliary RS232.
10.5 Magnetics
Static magnetic interference is resolved through magnetic calibration and dynamic
magnetic interference is compensated by a filter algorithm but should be minimised
where possible through installation location. Please see section 9.7 for more
information on magnetic interference. To compensate for static magnetic interference,
magnetic calibration should be performed any time Spatial's installation changes.
Spatial contains a dynamic magnetic compensation filter that is able to mitigate the
effects of short term magnetic interference sources while in operation. For example if
