Customers who land helicopters on oil rigs need to know how high they are on the approach with respect to the deck height, and not with respect to the water. Of course, during the approach they are over the sea and the radio altimeter gives good readings for the sea and, as all good mariners will tell you, the sea goes up and down with the tides so we need an algorithm that will give us the right answer.
Which Height is Which?
We have two height references on the helicopter, namely barometric and radio altitudes. Neither is perfect, so we use each for the part of the job they are best at. (Oops – an apology to my English teacher who told me never to end a sentence with a preposition; “..at which they are best”).
For sitting on the deck itself, we know that the height is zero. As the aircraft lifts, the huge changes in airflow and hence pressure around the body of the helicopter result in changes in barometric altitude which we don’t want to use, but the radio altimeter will read the right values as the aircraft rises over the deck. Once in the hover and transitioning into forward flight, the barometric altitude is the more reliable source of height information. During this transition the radio altimeter is affected by the change in reference from the deck to the sea. After transitioning, either source can be used for measuring height and for the approach phase over water we use the radio signal as it is immune from the small pressure errors that arise due to sideslip, attitude changes etc.
Let’s take a look at the signals for an approach and landing to a rig. Here I have shown the radio altitude which clearly shows the sudden step change as the aircraft crosses the edge of the deck, and in yellow the spike in the differentiated signal. The negative spike is so large I have truncated it so that the other vertical speed signals can be seen.
The barometric, radio and inertial vertical speed signals (in ft/sec here) are similar during the approach. The wild spike in VS_radio can clearly be seen at the transition and thereafter the signals become more similar. The variations in pressure signals are more dominant during the in ground effect hover phase, then, as the power is reduced and the aircraft settles onto the deck, there is a significant pressure fluctuation giving the final spike in VS_Baro.
It may appear to be attractive to use the inertial vertical speed signal, however this uses the signals from three accelerometers, pitch and roll and so there is a significant chance that a sensor failure will result in an error.
The final step is to replace the spiked values with those from the pressure signal and integrate (backwards, knowing that we end at h=0) to get the height above the deck, AltitudeADH.
Takeoff from Rig
The same process is used to compute the height above deck height on takeoff. The algorithm is a mirror image of the landing process.
How High was the Rig?
We now know the radio altitude of the aircraft on the approach and the height with respect to the deck height. Simply subtracting one from the other gives an estimate of the height of the deck.