# What is the minimum and maximum altitude reading for the average radar altimeter?

The valid range for latitude is -90 to 90 degrees. The valid range for longitude is -180 to 180 degrees.

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?

• Why the downvotes? This question is copy/paste from a previous SO question with over 300+ upvotes except I'm asking for the valid range for altitude rather than something else. Commented Nov 30, 2021 at 17:15
• @mins I tagged air-traffic-control so I assumed (incorrectly) that readers would know I'm looking for the upper and lower bound altitude for an aircraft flying under ATC. I can edit the question to be more specific, perhaps What is the minimum and maximum altitude reading for the average radar altimeter? Commented Nov 30, 2021 at 17:22
• Thanks @mins. Question has been edited. Commented Nov 30, 2021 at 17:33
• Did you consider that maybe the downvotes are BECAUSE you copied and pasted?! Not all questions translate well across all subjects… Commented Dec 1, 2021 at 1:28

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?

There are two points in your questions:

• What are altitude and an altimeter?
• What are height and a radioaltimeter?

Altitude

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.

Height

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.

• 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).

• Note that the radioaltimeter can be slightly negative: Why does a radar altimeter show negative values on the ground? Commented Nov 30, 2021 at 18:36
• Hello mins, great eloborated answer, as usual. I want to add that the radio altimeter can read negative values. The thing is calibrated such that under typical slight nose-up attitude at landing it reads 0 when the main wheels touch the ground. When the aircraft is firmly on the ground, the landing gear struts are compressed so that the indicated height can be slighltly below zero. The radar antenna is usually under the wing and if the outgoing radio signal is reflected at the end of the antenna cable (e.g. because of corrosion) the radio altimeter will read quite a few feet negative. Commented Nov 30, 2021 at 22:03
• ... for example, in case of the Turkish Airlines flight 1951 crash, a corroded antenna connector caused a radio altimeter reading of -8 ft Commented Nov 30, 2021 at 22:11
• @DeltaLima: Always a pleasure to read your interesting additions. Thanks. I've read the related portions of the report. It's a frightening case actually. The cause of the -8ft indication was left undetermined, though connectors problems have been observed and highly suspected. I'm quite surprised the faulty height was still tagged "usable" by the computer, while this was a very sudden and improbable change.
– mins
Commented Dec 1, 2021 at 2:10
• @MichaelHall: Yes, and I wanted to thank him if he reads. Quite confusing figures without his fix.
– mins
Commented Dec 1, 2021 at 2:12