Navman 12 Designer's Manual

Also see for Jupiter 12: Datasheet

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Navman 12 Designer's Manual Manual pdf 69 page image

69MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.measurements before computing a positionsolution. This technique effectively eliminatesmuch of the error due to SA as well as errorsdue to unmodelled satellite clock errors, satelliteephemeris errors, and atmospheric delays.This ‘improved’ solution is present in all outputmessages.With a few minor exceptions outlined below,DGPS is enabled by default, but may be disabledby the OEM. Because SA changes with time,the corrections deteriorate with time as well.Therefore, DGPS operation will only occur whenenough current DGPS corrections are available.The Jupiter receiver accepts RTCM SC-104format DGPS correction messages directly on theauxiliary serial port. The receiver also acceptsDGPS corrections data formatted as a binary datainput message (Message 1351) over its primaryserial port. (Detailed information on the format ofthis message is provided in Section 3.5.2.19.)4.7.4.1 The RTCM protocolThe Jupiter will accept 6-of-8 RTCM SC-104 datadirectly from the auxiliary serial port. No externalformatting is required and the receiver handlesall parsing and verification of the data. The userneeds only to verify the integrity of the data sent tothe receiver to ensure that high bit errors are notpresent in the detected RTCM raw data stream.The user should be aware that RTCM SC-104data will be used only if, for every 30-bit word, thesyndrome (6-bit parity) exactly matches the onewhich should occur on the basis of the 24-bit dataseen in each word. No attempt will be made tocorrect single bit errors; any syndrome mismatchwill cause rejection of that data word and rejectionof the message in which it exists.The receiver will parse the incoming data bitsand decode all of the RTCM SC-104 messages.Those messages required for DGPS operation willbe used to fill in the DGPS database within thereceiver. Those messages which are not used willbe discarded.4.7.4.2 The RTCM message typesThe receiver accepts DGPS correction data as asubset of the 64 RTCM SC-104 messages foundin Table 4-2 of the RTCM SC-1 04 Version 2.1standard. Though the receiver will accept anddecode all RTCM messages, not all messages arenecessary for DGPS operation.The Data Sheet for each of the Jupiter GPSreceivers shows which of the messages definedin the RTCM standard are used by the receiverto form a DGPS position solution. Refer to thestandard for more detailed descriptions of theseand other RTCM SC-104 messages.Type 1 messageType 1 messages contain pseudo-range andpseudo-range rate corrections for a completeset of visible satellites. Currently, this is themost common type of message transmitted bycommercial RTCM providers and base stations.Type 2 messageType 2 messages contain delta corrections andare transmitted by reference stations to helpreceivers during ephemeris cutovers. These Table4-2. Parameters And Data Maintained In EEPROMmessages are used by the field receiver inconjunction with Type 1 or Type 9 messages untilboth the reference station and field receiver areoperating with the same set of ephemeris.Type 9 messageType 9 messages have the same format as Type1 messages, but usually only contain correctionsfor a subset of the visible constellation. Thesemessages are typically transmitted at a higherrate than the Type 1 messages. Beacons, suchas those operated by the U.S. Coast Guard, arecurrently the primary source for these corrections,but they are also available from some commercialservice providers and base stations.4.7.4.3 Compliance with RTCM SC-I04 requirementsThe Radio Technical Commission for MaritimeServices (RTCM) has a special committeenumbered 104 (SC-I04). Its charter is to createrecommended standards for the transmission ofDGPS correction data.4.7.4.4 DGPS initialisation and configuration.At power-on, the receiver initialises its internalDGPS database to indicate that no validDGPS data is available. If the user requeststhe Differential GPS Status message (binaryMessage 1005), the message will indicate that nocorrections have been processed. Some of theposition status messages (binary messages 1000and 1001, and NMEA message GGA) will alsoindicate that the receiver is not computing a DGPSsolution.As sufficient valid RTCM data is passed to thereceiver, it will automatically produce DGPSsolutions. Other than supplying RTCM data andensuring that DGPS operation is not disabled, noaction is required on the part of the user to causeDGPS operation.The receiver will compute DGPS solutionswhenever all of the following conditions aresatisfied: