I saw The Perfect Storm over the weekend, and there was one scene that got me to thinking about something. There is a scene where a rescue helicopter is hovering over the ocean trying to save some people while the waves are rising from 50 feet down to 0 feet.
Here’s my question – is the altitude calculated from sea level, or from the highest point of the wave (if the helicopter happens to be over the highest wave at that particular moment)?
If I’m not clear enough, allow me to throw out an example. A helicopter is hovering over the ocean 200 feet above sea level. However, at the spot where it is hovering, a wave rises 50 feet. So, on the helicopter altimeter, would it read 200 feet, or 150 feet (taking into account that 50 foot wave)?
It would still read 200 ft. “Sea level” is an abstract concept, not the literal distance to the sea below you. I think it’s usually measured indirectly by measuring the air pressure, or using GPS. In either case, local height variations such as flying over a large wave (or building) don’t affect the measurement.
The altimeter reads the atlitude based on air pressure. There is a little window in the instrument that allows you to set the barometric pressure (29.92 in Hg on a standard day) based on local conditions. As stated, waves do not affect it.
Some aircraft have a radar altimeter; but not having used one, I can’t answer for them.
In WWII, the British needed to be an exact height over the water on their “dam-busting” missions. They used two spotlights that would converge at the appropriate altitude. Obviously, this would not be the best system if you have giant waves.
In a storm like this, a standard altimeter would be useless. The barometric pressure would be all over the place, and there would be no way to correct for it. If the nearest weather station were several hundred miles away it would be inaccurate to begin with.
A radar altimeter doesn’t measure altitude above sea level, it measures the absolute absolute from the surface. In the case of huge waves, obviously its readings would be all over the place. However, it’s the only altitude that really matters, right? Normally, a ‘sea level’ altitude reading is only useful when you are flying over unchanging terrain with published altitudes on a chart. In this case, your MK I eyeball is probably the best gauge you’re going to have.
The only accurate ‘sea level’ reading you could get in this case would be from GPS. GPS satellites know their altitude over the Earth, and therefore a receiver that can pick up 3 or more satellites can also learn its altitude.