2 - NAV PAGES57Each satellite has a 30-second data transmissionthat must be collected (hollow signal strength bar)before the satellite may be used for navigation (solidsignal strength bar). Once the GPS receiver has deter-mined your position, the 500W-series unit indicatesyour position, altitude, track and ground speed. TheGPS receiver status field also displays the followingmessages under the appropriate conditions:• Searching Sky —The GPS receiver is searching the sky for ANY vis-ible satellites. You are also informed of this statuswith a “Searching the Sky” message.• Acquiring Sat —The GPS receiver is acquiring satellites for naviga-tion. In this mode, the receiver uses satellite orbitaldata (collected continuously from the satellites) andlast known position to determine the satellites thatshould be in view.• 3D Navigation —The GPS receiver is in 3D navigation mode andcomputes altitude using satellite data.• 3D Differential Navigation— The GPS receiver isin 3D navigation mode and differential correctionsare being used.The Satellite Status Page also indicates the accu-racy of the position fix, using Horizontal Figure ofMerit (HFOM), Vertical Figure of Merit (VFOM), andEstimated Position Uncertainty (EPU). HFOM andVFOM represent the 95% confidence levels in hori-zontal and vertical accuracy. The lowest numbers arethe best accuracy and the highest numbers are worse.EPU is the horizontal position error estimated by theFault Detection and Exclusion (FDE) algorithm, in feetor meters.Dead ReckoningDead reckoning is the process of continuing navi-gation using your last known position and speed aftera loss of GPS navigation while on an active flight plan.CAUTION: Navigation using dead reckoning is there-fore only an estimate and should not be used as thesole means of navigation. Use other means of naviga-tion, if possible.Dead reckoning becomes active after a loss of GPSposition while you are navigating using an active flightplan.A pop-up message will appear and you must clearthe message before continuing operation.The ownship icon color is changed to yellow. The To/From flag is removed from the CDI. The Dead Reckon-ing annunciator (DR) appears on the left side of themap display when GPS position is unavailable and theunit is in Dead Reckoning mode. All external outputsdependent on GPS position are flagged.Terrain will be noted as not available and new terrainadvisory pop-ops will not occur. Traffic and Storm-scope information will not be shown on the Map page,but will continue to be available on their own dedi-cated pages. XM weather will still be available on theMap page.Dead Reckoning
