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MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
4.6 Navigation
This section describes the operation of the
navigation software in the GPS receiver. It defines
many of the features of the navigation system with
emphasis on those that the OEM can control.
The navigation software initialises and maintains a
state vector which is used to report time, position,
and velocity to the user. The navigation software
uses pseudo-range, integrated carrier phase, and
Doppler measurements from the satellites, and
external altitude inputs if available. An eight-state
Kalman filter estimates position, velocity, and clock
errors.
The OEM has substantial control over the
operation of the navigation system to customise it
for a specific application.
4.6.1 Geodetic datums
Geodetic parameters are used in both input and
output messages. The receiver reports position as
a set of geodetic parameters (latitude, longitude
and altitude) in the geodetic position status output
message (binary Message 1000). Geodetic
parameters are also reported in the GPS fix
data message (NMEA Message GGA), with the
substitution of Mean Sea Level (MSL) altitude
for geodetic altitude, and in the recommended
minimum specific GPS data message (NMEA
Message RMC) as latitude longitude. The receiver
expects geodetic parameters as part of the
geodetic position and velocity initialisation input
message (binary Message 1200).
Whether geodetic parameters are used as input
or output data, they are always referenced to the
currently selected geodetic datum. Each geodetic
datum is defined by five parameters:
The semi-major axis.
Flattening of the reference ellipsoid.
Delta X component of the WGS-84 datum
origin offset.
Delta Y component of the WGS-84 datum
origin offset.
Delta Z component of the WGS-84 datum
origin offset.
The receiver has 189 pre-defined user datums
selectable by the OEM. These datums, together
with their identification codes, are listed in
Appendix E of this document. All of the pre-defined
datums are taken from the US Government
document, Department of Defense World Geodetic
System 1984.
4.6.1.1 User selection of geodetic datums
The default datum is WGS-84 (defined as datum
code zero for the receiver). Other datums are
selected using the Map Datum Select message
(binary Message 1211). The selected code
is reported back in the position status output
message 1000 or 1001.
For example, if the receiver was in Navman binary
mode and the OEM wanted to use a previously
recorded position to initialise the receiver, and then
get reported positions in WGS-84, the OEM would
send three messages to the receiver as follows:
1 Send a ‘map datum select’ message (binary
message (1211) with the desired datum code
(the new datum code is reported back in
message 1000 and/or 1001 if either or both
messages have been enabled).
2 Send a ‘geodetic position and velocity
initialisation’ message (binary Message 1200)
with the correct latitude, longitude, and altitude.
3 Finally, send another message 1211 with the
datum code 0 to set the datum back to WGS-
84.
The selected datum is always stored in EEPROM,
so that it is saved during power-off and reset
cycles. The Navman binary Map Datum Select
message must be used to change the geodetic
datum; the datum cannot be changed when in
NMEA mode. NMEA outputs will use the last
datum selected in binary mode or WGS-84 if a
datum selection has never been made.
4.6.1.2 User defined datums
Besides the 189 pre-defined datums, the OEM
can define five custom datums for a specific
application. Each datum is defined by the five
parameters described in section 4.6.1 and the new
datum definition is sent to the receiver using the
User-Defined Datum Definition message (binary
Message 1210).
There are five custom datum codes to choose
from, 300 to 304. All user-defined datums are
stored in EEPROM. Once the datum is stored, it
is selected using the Navman binary Map Datum
Select message in the same manner as the
pre-defined datums. If a custom datum is stored
and later re-defined, the existing datum will be
overwritten.