Navman 11 - Manual (Page 66)

66 MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
These errors induce velocities in the receiver’s
state estimate even if the receiver is motionless.
The magnitude of these velocities depends on the
geometry and number of satellites in track. Typical
values are between one-half and two metres per
second. The position estimate of the receiver will
vary within a circle of approximately 100 m (the
2Drms position error specified in the GPS SPS
Signal Specification).
This varying of the position estimate while the
receiver is static is undesirable for users who
display their position on a map or for those with
similar applications. The receiver’s proprietary
navigation software is capable of “pinning” the
position output when the estimated velocity is low
and the navigation solution is of good quality. This
prevents the varying behaviour of the solution
in static situations, but introduces some risk
that actual platform motion will be mistaken for
deliberate SA induced error.
By default, the position pinning feature of the
Jupiter GPS receiver is enabled. The OEM can
turn it off using the ‘nav configuration’ message
(binary Message 1221).
4.6.3.5 Ground track smoothing.
Without the use of DGPS, satellite measurements
are corrupted by SA and do not generate a
consistent estimate for receiver position. Typical
range errors are on the order of 30 m. The receiver
processes all satellites above the mask angle to
minimise the effects of the range error.
Changes occur in the set of satellites being
processed because of blockage, and the rising
and setting of satellites. When a change occurs,
the position estimate formed from averaging these
errors is shifted. These shifts can be substantial,
typically 10 m to 20 m, but up to 100 m or more in
bad geometry.
The receiver has a proprietary compensation
technique for these constellation switch
effects which maintains a smooth estimate of
the platform’s ground-track and altitude. This
smoothing is accomplished without any loss of
dynamic response.
However, this method is not required and is
automatically disabled when DGPS is available.
Without DGPS, the method is enabled by default.
Most users should leave it enabled. One reason for
disabling it would be to compare the solution with
a point solution from another receiver or a solution
calculated off-line.
Ground-track smoothing can be disabled
using the ‘nav configuration’ message (binary
Message 1221). The configuration word is stored
in EEPROM, so the setting for ground-track
smoothing will be retained through power-off and
reset cycles.
4.6.4 Solution validity
The validity of navigation solution outputs is
determined by user-defined criteria. The OEM can
specify which criteria to apply for the particular
application. There are five possible criteria used to
validate a solution:
1 Was an altitude measurement used?
2 Was DGPS used?
3 How many satellite measurements were
used?
4 What is the Expected Horizontal Position
Error (EHPE)?
5 What is the Expected Vertical Position Error
(EVPE)?
The OEM cannot change the validity criteria in
NMEA mode. These criteria can be set using
only the binary ‘solution validity criteria’ message
(Message 1217) and they are retained through
power-off and reset cycles in EEPROM.
4.6.4.1 Altitude measurement validity criterion
The altitude measurement validity criterion allows
the OEM to specify if solutions that make use
of an altitude measurement should be marked
valid. Altitude is not used in the solution unless it
is necessary because of deteriorating EVPE or
untracked satellites. When it is required, the OEM
may wish this to be an indication that the solution
is invalid for purposes of the specific application.
The default is that solutions using altitude
measurements may be marked valid
4.6.4.2 DGPS used validity criterion
The DGPS used validity criterion indicates an
invalid navigation solution if DGPS is unavailable
after it has been required. The system default is
that DGPS is not required for a valid solution.
4.6.4.3 Number of satellites used validity criterion
The number of satellites used validity criterion
indicates an invalid navigation solution if the
minimum number of satellites required to be in the
solution is not met. The default for this test is zero.
A solution may be reported as valid with no
measurements used so long as the EHPE and
EVPE criteria pass and a Kalman filter solution
has been previously computed. The reason the
default is set to zero is to allow the receiver to
coast through brief outages without declaring the
solution invalid (e.g. for example, under a freeway
overpass).