Navman 12 Designer's Manual

Also see for Jupiter 12: Datasheet

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Navman 12 Designer's Manual Manual pdf 64 page image

64MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.4.6 NavigationThis section describes the operation of thenavigation software in the GPS receiver. It definesmany of the features of the navigation system withemphasis on those that the OEM can control.The navigation software initialises and maintains astate vector which is used to report time, position,and velocity to the user. The navigation softwareuses pseudo-range, integrated carrier phase, andDoppler measurements from the satellites, andexternal altitude inputs if available. An eight-stateKalman filter estimates position, velocity, and clockerrors.The OEM has substantial control over theoperation of the navigation system to customise itfor a specific application.4.6.1 Geodetic datumsGeodetic parameters are used in both input andoutput messages. The receiver reports position asa set of geodetic parameters (latitude, longitudeand altitude) in the geodetic position status outputmessage (binary Message 1000). Geodeticparameters are also reported in the GPS fixdata message (NMEA Message GGA), with thesubstitution of Mean Sea Level (MSL) altitudefor geodetic altitude, and in the recommendedminimum specific GPS data message (NMEAMessage RMC) as latitude longitude. The receiverexpects geodetic parameters as part of thegeodetic position and velocity initialisation inputmessage (binary Message 1200).Whether geodetic parameters are used as inputor output data, they are always referenced to thecurrently selected geodetic datum. Each geodeticdatum is defined by five parameters:The semi-major axis.Flattening of the reference ellipsoid.Delta X component of the WGS-84 datumorigin offset.Delta Y component of the WGS-84 datumorigin offset.Delta Z component of the WGS-84 datumorigin offset.The receiver has 189 pre-defined user datumsselectable by the OEM. These datums, togetherwith their identification codes, are listed inAppendix E of this document. All of the pre-defineddatums are taken from the US Governmentdocument, Department of Defense World GeodeticSystem 1984.4.6.1.1 User selection of geodetic datumsThe default datum is WGS-84 (defined as datumcode zero for the receiver). Other datums areselected using the Map Datum Select message(binary Message 1211). The selected codeis reported back in the position status outputmessage 1000 or 1001.For example, if the receiver was in Navman binarymode and the OEM wanted to use a previouslyrecorded position to initialise the receiver, and thenget reported positions in WGS-84, the OEM wouldsend three messages to the receiver as follows:1 Send a ‘map datum select’ message (binarymessage (1211) with the desired datum code(the new datum code is reported back inmessage 1000 and/or 1001 if either or bothmessages have been enabled).2 Send a ‘geodetic position and velocityinitialisation’ message (binary Message 1200)with the correct latitude, longitude, and altitude.3 Finally, send another message 1211 with thedatum 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 resetcycles. The Navman binary Map Datum Selectmessage must be used to change the geodeticdatum; the datum cannot be changed when inNMEA mode. NMEA outputs will use the lastdatum selected in binary mode or WGS-84 if adatum selection has never been made.4.6.1.2 User defined datumsBesides the 189 pre-defined datums, the OEMcan define five custom datums for a specificapplication. Each datum is defined by the fiveparameters described in section 4.6.1 and the newdatum definition is sent to the receiver using theUser-Defined Datum Definition message (binaryMessage 1210).There are five custom datum codes to choosefrom, 300 to 304. All user-defined datums arestored in EEPROM. Once the datum is stored, itis selected using the Navman binary Map DatumSelect message in the same manner as thepre-defined datums. If a custom datum is storedand later re-defined, the existing datum will beoverwritten.