Version 2.0 189Engineering Mode Operator’s ManualGPS4 - GPS Position UncertaintyThis screen presents a bar graph representing the HDOP for the past23 hours. If you are trying to do precision work or navigation in thesame general area (within 100 miles) as the day before, you can look atthis screen to see when the best HDOP periods are. The GPS constel-lation shifts back 4 minutes per day. That means you can expect theGPS coverage to be virtually the same today as it was yesterday.The small 1 hour gap in the bar graph represents the 24th hour. Thegap is provided to ease the readability of the bar graph for the presenttime. The dashed line extending horizontally from the 4 represents thecurrent HDOP Limit Alarm, which is set in CFG Position. The currentHDOP and VDOP values are given in the upper window.MX Ant Reset & CSI Reset ToolsSpecial tools are available in the MX420 CDU that can be used to resetthe GPS and Beacon engines in the MX421. Use them only in extremecases when the GPS or beacon receiver fails or takes too long to lock-on. The “MX Ant Reset” softkey will clear the satellite almanac memoryof the GPS engine and reset all settings of the antenna to factorydefault conditions. The “CSI Reset” softkey resets the differential bea-con board.MX Ant Reset Procedure:This procedure will work only under Engineering level mode.1. Press the AUX key several times to bring up the AUX7 screen.2. Press the left-most softkey 3 times (to bring-up Eng’g level 3).3. Press the E key.4. Press the “MX Ant Reset” softkey to reset the GPS engine (orthe “CSI Reset” softkey to reset the beacon engine).5. Press the E key to exit.6. Press the GPS key to display the GPS or DGPS Status screen.
