I previously thought the minimum and maximum altitude reading for the average radar altimeter was 0 to some very large positive number in meters, however, I recently read altimeters (and even other avionics instruments) can display negative altitudes.
What is the minimum and maximum altitude reading for the average radar altimeter?
Radioaltimeter and radar altimeter are synonymous. I use radioaltimeter.
There are two points in your questions:
- What are altitude and an altimeter?
- What are height and a radioaltimeter?
This is the height above the mean sea level. It can be positive or negative, and altimeters can show negative values as required. Altimeters are based on a measure of the ambient pressure, which decreases with altitude. That's the trick. There are now other means to know the altitude, but for aircraft this is a measure of the pressure and an equivalence in a model.
There are many places on earth below the level of the sea, and some where aircraft can fly.
A height is a distance to a reference. Without other mentions, height is the height above the ground level (termed AGL). Altitude is a special case of height, where the reference is the sea level (termed AMSL).
A radioaltimeter measures the height above the "ground". Ground can be land or sea, depending on what you're flying over at the time of the measure.
A height is positive, as you can't fly under the ground or under the sea.
An interesting case for this answer is what instruments read when flying at a place below the sea level. Let's imagine a place where the ground is at an altitude of -150 ft, and the aircraft flies 100 ft above the ground:
- The altimeter will indicate a negative value, -50 ft, as we are 50 ft below the level of the sea.
- The radioaltimeter will indicate a positive value, +100 ft , as we are flying at a height of 100 ft above the ground.
Therefore a radioaltimeter will always show a positive value, and zero when on the ground. For accuracy sake, the exact value when on the ground depends on the location of the radioaltimeter antennas and how the instrument is calibrated, in some cases it can be a few meters positive or negative (values less than -20 ft will not be displayed), this detail is not relevant for the principle, you can assume zero.
You can see a radioaltimeter as a laser distance meter. Instead of using an optical ray it uses a radio ray. A radioaltimeter stops measuring height after about 2,500 (some manufacturers extend the limit, up to 6,000 ft). It's mostly used for landing.
To answer your questions:
Altitudes flown in commercial aviation vary from -1,240 ft to +60,000 ft (that was the altitude Concorde flew).
Heights displayed from -20 ft to 6,000 ft (usually 2,500 ft).