u-blox ANTARIS 4 - Basic Operation Cycle


1.2 Basic Operation Cycle
Whenthereceiverispoweredup,itproceedstroughasequenceofstatesuntilitcaninitiallydetermineposition,
velocity and time. Afterwards, the satellite signals are tracked continuously and the position is calculated
periodically.
Thisprocessisdepictedbelow:
Initialization
Search SVs
Track SVs
Decode Ephemeris
Data
Calculate Initial Position
Calculate Successive
Positions
Initialization
Pseudoranges
Carrie rphase
OrbitData
Onlyif Ephemeris
and Pseudoranges
foratleast3SVs

Figure2:BasicOperationCycle
Inordertoperformanavigationsolution(3Dsolution),thereceiverneeds
• Distances(PseudoRanges)foratleast4SVs(SpaceVehicles)
• EphemerisDatafortheSVsitwilluseinthenavigationsolution.
 Note If almanac navigation is enabled, the receiver can calculate a position without downloading
ephemerisdata(withasignificantpositionerrorcomparedtoanephemerisbasedsolution).
The initial position calculation is made using a Least-Squares Algorithm. Successive position calculations are
performed with a Kalman Filter. To generate a Position (3D solution) Calculation the receiver needs at least 4
measurements to
 different satellites, to calculate a position (Lat/Long/Height), for a 2D solution with an
estimatedaltitude3differentsatellitesarerequired.
PseudoRangeandCarrierPhaseinformationisavailabletothePositionDeterminationAlgorithmsifthereceiver
hasfoundaSV(Acquisition)andcantrackthesignalthereafter.
Ephemeris data for
a SV can be decoded from Orbit Data once the GPS signal has been acquired. Each SV
transmitsitsownephemerisdata,thetransmissionlastsfor18seconds,repeatingevery30seconds.
Thereceiverstoresephemerisdatainbattery-backupmemory.Thisdataisvalidfor2hoursandcanbeused
in
futurestartup’stoimprovethetimetofirstfix(TTFF).Ephemeriscanalsobesuppliedtothereceiverviatheserial
port.
1.3 Start-Up
Dependingontheavailabilityof lastposition,currenttimeandephemerisdata,thereceiverwillapplydifferent
strategiestostart-up,namely:
Coldstart
InColdstartthereceiverhasnoinformationofitslastpositionortime.ThisisthecasewhentheRTChasnot
beenrunningorwhennovalidephemeris
oralmanacdataisavailable.Suchasituationiswhenthereceiverhas
nevernavigatedorthebatterybackupmemoryislost.
GPSModules-SystemIntegrationManual(SIM)(incl.ReferenceDesign) GPSFundamentals
GPS.G4-MS4-05007-A1
  Page 12