Daroga
First Officer
Flight distance : 3517287 ft
United States
Offline
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The Mavic uses multiple sensors, in differing importance, throughout your flight.
Only GPS is external to the Mavic. All other sensors are internal and self-contained within the Mavic. GPS altitude, measured as Above Sea Level (ASL) is acquired and use but tends to be less reliable than the internal sensors of the Mavic over the dynamic flight envelope of the Mavic.
So as you takeoff, the Mavic uses VPS to stabilize the aircraft. VPS is very similar to the radar altimeter on piloted aircraft - very accurate within 43 feet of the ground - but measures the relative distance above the ground or obstacle (AGL). The Attitude and Heading Reference System (AHRS) sensors, which include IMU gyros & accelerometers biased with compass and barometric altimeter are far more accurate than GPS altitude but again measured in AGL. So the primary altitude reference in the Mavic is always referenced relative to your takeoff location and is displayed as a differential above or below that takeoff spot, (+/- AGL).
So in your scenario, the Mavic is measuring realtime rate of change, biasing the barometric altimiter, to show the altitude differential between your takeoff spot altitude and the current aircraft altitude. It could care less about the terrain below it. If you decend into the valley, at 100 feet above the terrain, the altitude will show 0, go 25 feet lower and the alttude will show -25 feet as you are 25 feet BELOW your takeoff point.
If you climb back to 400 feet and proceed to point C, you can crash into the hill. But three systems, if enabled can protect you. The first and most important system is visual Line of Sight (LOS). See the hill and avoid it. With distance, depth perception becomes your enemy. So you should perform some risk assessment before each flight and know the obstacles, know their height, know the safe minimum altitude and set your RTH height.... The other two system are Mavic systems -> obstacle avoidance and VPS. At 43 feet above the rising terrain, the VPS system should start to show the VPS altitude and show it decreasing. If obstacle avoidance is enabled, it should stop the aircraft if and when it identifies an obstacle like the hill.
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