Navman 12 Designer's Manual

Also see for Jupiter 12: Datasheet

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Navman 12 Designer's Manual Manual pdf 80 page image

80MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.Occasionally a type 2 message is sentinterspersed among the correction messages,which provides a secondary correction. This isdone to allow a user to operate with old (up totwo hours) satellite ephemeris and satellite clockdata while the reference station is operating withmost recent data. This correction, called the ‘deltacorrection’ is added to the normal correction forthat satellite. The reference station will usuallydecode the satellite data before the user doessince it is constantly monitoring the data.Data LinkThe data link, which communicates the correctionsfrom the reference station to the user receiver,can take a number of forms and operate at any ofseveral frequencies. The chief requirement is thatthe messages be communicated reliably at a datarate of at least 50 bps (continuous transmission).Figure C-5 shows the user data link functions. Inits simplest form, the data link continuously carriesthe DGPS data message without interruption at aconstant data rate of at least 50 bps. However, it istransparent to the GPS receiver whether the datais transmitted continuously or in bursts, or whetherprotocol overhead is added.For example, each message (or multiplemessages, or any fraction of a message) couldbe transmitted as a short burst at 2400 bps, alongwith a data link protocol preamble, parity, and evenerror correction bits. This would be stripped off atthe receiver end and the differential correction bitswould be stored in the buffer, to be transferred tothe receiver at will. DGPS broadcasts intended forgeneral public use require that the data link be astandard, published design.For non-public use, however, the referencestation, data link and receivers could be part ofan integrated DGPS system. In such a case, thedata might be encrypted to limit the service topaying customers. The format allows for suchoperation. At the minimum rate of 50 bps, thereis considerable robustness in the data link. Thecorrections are broadcast frequently enough sothat the loss of as many as three consecutivecorrections can be tolerated and still provide 5 maccuracy.GPS sensor display andoutputmeasurementprocessordifferential dataprocessorsatellite datanavigation dataposition coordinatesGPS receiverGPS satellitesignalsdata link receiver data formatterData linkDGPS datalinkdemodulatorFigure C-5 User equipment block diagram