67
MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
If the measurements are lost for a long time, the
EHPE and EVPE will grow until they surpass their
thresholds and the solution fails the validity test for
that reason. Some applications require a solution
to be marked invalid unless it uses three, four, or
more satellites. The OEM can set any of these
thresholds by sending a binary ‘solution validity
criteria’ message (Message 1217) with the number
of satellites required
4.6.4.4. Maximum EHPE validity criterion
The EHPE is the one sigma horizontal position
error estimate for the solution. The validity criterion
default is 10 m. The meaning of the reported
EHPE depends on whether or not DGPS is in
use. If DGPS is in use the EHPE is the estimated
one-sigma error in absolute position accuracy.
When DGPS is not in use, the EHPE and EVPE
are reported with the effects of the satellite User
Equivalent Range Error (UERE) excluded. This
excludes SA induced error from the EHPE and
EVPE.
So in SPS navigation, the EHPE and EVPE serve
as convergence indicators for the Kalman filter,
not as absolute accuracy limits for the reported
position. The EHPE validity threshold can be set
by the OEM in the binary ‘solution validity criteria’
message (message 1217).
4.6.4.5 Maximum EVPE validity criterion
The EVPE is the one sigma vertical position error
estimate for the solution. The default is 25 m.
The operation and meaning of this criterion is
analogous to the EHPE criterion in section 4.6.4.4.
The threshold can be set in the binary ‘solution
validity criteria’ message (message 1217).
4.6.5 Mean Sea Level (MSL)
MSL is a geoid, or surface of equal gravitational
potential. The height of the MSL geoid above or
below the reference ellipsoid (WGS-84 by default)
is called the geoidal separation. Positive geoidal
separation means that the geoid is above the
ellipsoid.
Values for the geoidal separation are computed at
any receiver position by interpolating on the table
of geoidal separation values provided in the U.S.
Government document, Department of Defense
World Geodetic System 1984.
Altitude or height computation is referenced to the
ellipsoid and is referred to as geodetic altitude.
Altitude referenced to the geoid (usually referred to
as altitude MSL) can be computed as the geodetic
altitude minus the geoid separation.
4.6.6 Magnetic variation
The magnetic variation model used in the receiver
is derived from the full International Geomagnetic
Reference Field (IGRF) 95 magnetic model.
Documentation, tabular data, and test programs
for the IGRF 95 magnetic model can be obtained
from the National Oceanic and Atmospheric
Administration (NOAA) National Geophysical Data
Centre (NGDC) web site (http:/ /julius.ngdc.noaa.
gov / seg/potfld/geomag.html).
The magnetic variation is used to convert true
heading to magnetic heading. It is output in
binary Message 1000. To convert the true course
provided in binary Message 1000 to magnetic
heading, the magnetic variation is added to the
true course.
4.7 Support functions
This section describes the support functions of the
GPS receiver.
4.7.1 Serial communication interfaces
Jupiter GPS receivers provide two bi-directional
serial communication interfaces: the host and
auxiliary ports (see Section 3)
4.7.1.1 The host port
The Host port is the primary interface to the
controlling application and provides all the services
for initialising and configuring the system as well
as for the reporting of the navigation solution and
receiver status.
By default, the Host port is configured for 9600
baud, no parity, 8 data bits, and 1 stop bit. The
Navman binary communications protocol is the
default selection for the Host port.
The default settings (configuration and protocol)
can be overridden with the use of the NMEA
Select control line of the interface. When this line
is asserted, the configuration defaults to 4800
baud, no parity, 8 data bits, and 1 stop bit, and the
communications protocol defaults to NMEA.
Note that the NMEA Select line will override any
previously stored selections for the Host port
configuration and communication settings.
While using the Navman binary communications
protocol on the Host port, a number of application
specific parameters can be configured to
customise the receiver for a specific application.
The ability to freely switch between Navman binary
and NMEA modes provides full capability to all
users.
Host port output messages can be configured
using the log control messages supported in both
Navman binary and NMEA message protocols.
Changes to the port configuration settings,
