GNSS Integrity and Dead Reckoning Fallback
You are on an oceanic leg at FL350, 400 NM from the nearest land, using GPS as the primary navigation source. The GPS receiver shows RAIM unavailable for the next 20 minutes due to satellite geometry, and the FMS has reverted to dead reckoning using the last known wind. Your last fix was 120 NM ago; since then you have flown heading 270°M at TAS 470 kt with an average wind of 240°/60 kt. Magnetic variation is 10°W. The next waypoint is 300 NM ahead on track 265°M.
1. Estimate your current position error from dead reckoning and the resulting cross-track error at the next waypoint.
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Wind 240/60, heading 270, TAS 470. Wind is 30° off the nose to the left: headwind ≈ 60 cos30° ≈ 52 kt, crosswind ≈ 60 sin30° ≈ 30 kt. Groundspeed ≈ 470 − 52 ≈ 418 kt. Over 120 NM, time ≈ 120/418 ≈ 0.287 h ≈ 17 min. Drift ≈ arcsin(30/470) ≈ 3.7° right, so track made good ≈ 273.7°M. Cross-track error after 120 NM ≈ 120 × tan3.7° ≈ 7.7 NM right of the intended track. If the wind forecast is wrong by 10 kt or 10°, the error grows by several NM per hour; over the remaining 300 NM at 418 kt (≈ 43 min), a 10 kt wind error can add 7–8 NM. Expect a total cross-track error of 10–20 NM at the next waypoint. Use the last known fix and a positive DR plot; do not rely on the FMS position without cross-checking.
2. State the operational actions and the navigation backup you should use while RAIM is unavailable.
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RAIM unavailable means GPS cannot guarantee integrity; you must not use it as the sole means of navigation. Fall back to the approved long-range navigation backup: IRS/INS with regular updates, or VOR/DME and NDB where range permits, plus a written DR plot every 30 minutes. Cross-check the FMS DR position against the last fix and against any ground-based aids. If the aircraft is equipped with a second GNSS or SBAS, use it with the same integrity caution. Advise ATC of the degraded navigation capability if required by the airspace, and plan a route that stays within the backup aid coverage. Resume GPS as primary only when RAIM is restored and the position is confirmed by an independent fix.