Navman 12 Designer's Manual

Also see for Jupiter 12: Datasheet

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Navman 12 Designer's Manual Manual pdf 61 page image

61MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.used to identify previously healthy satellites andto generate “working” visible satellite lists, whilefrequency standard data minimises satelliteacquisition uncertainties.4.4.4 Frozen startThis state is entered if there are no valid datasources available (SRAM, RTC, EEPROM). Thisis considered to be a recovery mode becauseEEPROM should always contain valid information.An “out-of-the-box” board or a unit that has notoperated for a significant amount of time (months)may approximate this state because the data inEEPROM may be valid but expired or partiallycomplete.4.5 Satellite managementThis section describes the satellite managementfunctions of the Jupiter family of GPS receivers.4.5.1 Visible list generation.A list of satellites visible to the receiver antennais maintained whenever possible. A satellite isconsidered visible if its elevation in the sky isknown to be above the horizon, if its almanac andephemeris data indicate it is healthy, and if it hasnot been excluded by manual candidate satellitespecification. Note that although a satellite isvisible, its measurement is only available for use ifthe satellite is above the elevation mask angle.The receiver’s channel resources are directedtoward acquiring only those satellites which appearin this list except when the receiver is in cold startmode. Satellites within the list are ordered fromhighest to lowest elevation which, for sequentialacquisition, also dictates the order in whichacquisition attempts are made.Receiver position and current time are requiredto compute satellite positions from orbital data.If position and/ or time is not considered to bewell known (i.e. their expected errors are large),then the list is extended below the horizonand is filled to the maximum of 12 satellites. IfDGPS corrections are available, the satellitesrepresented in the corrections are used to set thelist membership instead, since they also representsatellites visible to a nearby transmitting DGPSbase station.New visible satellite lists are generated byevents that could cause a change in satellite listmembership or could indicate a significant changein a satellite position relative to the antenna. Theseevents include receipt of an elevation mask angleor candidate satellite specification command,downloading of a new satellite almanac, andchanges in satellite health status reflected in newalmanac or ephemeris data.In the case where DGPS corrections are used toestablish list membership, a change in the set ofsatellites reflected in the corrections also causes anew list to be generated. During initial acquisition,a new list is generated when the receiver makesstep adjustments to position and time. In theabsence of these events, the visible satellite list isupdated every 30 seconds. The visible satellite listis output in the Visible Satellites message (binaryMessage 1003)4.5.1.1 Dilution Of Precision (DOP)Geometric Dilution of Precision (GDOP) is ameasure of the quality of a satellite constellationgeometry. GDOP reflects the influence of satellitegeometry on the accuracy of user position andtime estimates. The best geometry is that whichproduces the lowest GDOP value. GDOP acts as amultiplier of the error in position and time estimatesdue to other sources.GDOP is a composite measure. It can beseparated into:• Position Dilution of Precision (PDOP)PDOP reflects the effects of geometry onthree-dimensional position estimates• Time Dilution of Precision (TDOP)TDOP reflects geometric effects on timeestimates. The relationship can be expressedas:GDOP =In turn, PDOP can be separated into horizontaland vertical components:• Horizontal Dilution of Precision (HDOP)• Vertical Dilution of Precision (VDOP)These components represent the effects ofsatellite geometry on two-dimensional horizontalposition and on vertical position (altitude)estimates, respectively.This relationship can be expressed as:PDOP =The receiver computes the best availableGDOP and each of its components in the VisibleSatellites message (binary Message 1003). Thebest available GDOP is that associated withthe satellite constellation consisting of all visiblesatellites above the mask angle (satellites whosemeasurements may be used).At least four satellites are required to estimateposition and time, and therefore to compute a(PDOP) 2 + (TDOP) 2(HDOP) 2 + (VDOP) 2