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This applies to the Airbus 320 or Boeing 737, among others:

Although it is useless on ground, the radar altimeter will repeatedly display a value (including negative) for a specific runway and airport on the PFD.

Why would a certain runway position be -4 radar altitude as opposed to 0?

Bianfable
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Sniper
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1 Answers1

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The zero reading is calibrated for main-wheel touchdown in a landing attitude. With the antenna forward of the main gear, it is, at that point, higher than it will be after the nosewheel is lowered to the runway.

So to put some rough numbers to that, in the landing attitude, mains on the ground, the antenna is roughly 8 feet above the runway, but it reads 0 -- a bias of -8 feet is applied to the display as compared to the measured value. With all wheels on the ground, the antenna is about 4 feet above the pavement (about that on the 737, a bit more on the A320 with its taller landing gear). Now, the bias of -8 gives the display value of -4 (i.e. 4' above pavement + -8' bias = -4' displayed).

Addressing issues mentioned in comments: Putting the antenna directly between the main gear would be difficult on a 737 because as the wheels retract inward, there isn't a lot of space between them. I'd suspect that as far as places go to mount an antenna, right there isn't a great one just because of all the dirt & hydraulic fluid & general grime in and around the wheel wells. Somewhat forward, where everything inside the aircraft (i.e. everything except the external structure of the antenna) is sealed off from all dirt & such is probably lots better for the longevity of sensitive electronics.

Also, even mounting an antenna directly in line with the gear wouldn't remove the need for a bias... it's still 4' or so off the ground when the wheels touch and you want a displayed "0". You'd just get rid of the effect that the displayed value goes slightly negative as the nose comes down.

As I understand the system, the rad alt beam isn't all that directional... if the deck angle is +10 degrees, the measured distance is the vertical distance from the antenna to the ground - not the distance of the (hypotenuse) line 10 degrees forward of vertical. You'd get the latter with a laser pointer aimed "straight down" from the aircraft, but the rad alt just transmits a pulse, and the first return it gets (i.e. from the nearest surface, irrespective of relative angle) is what's used to compute the height.

Ralph J
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    Those radar altimeters sound quite accurate. – rclocher3 Jun 02 '21 at 15:38
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    Absolutely, they are. In the flare, I can read the radar altimeter digital values in the heads-up display, and as it counts down from 1 to 0, the main wheels touch. – Ralph J Jun 02 '21 at 16:02
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    @rclocher3 Note that radar altimeters get more accurate the closer you are to the ground. You won't get 1 ft accuracy at 2000 ft. In fact, above 2500 ft the radar altimeter value is usually not shown at all. – Bianfable Jun 02 '21 at 17:10
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    Do you know where the radar unit physically is on the aircraft? At first blush the need for a bias could be eliminated by mounting the antenna in line with the main gear, but of course in a landing attitude the reading will still be the slant distance (hypotenuse of a right triangle) rather than the true height. – randomhead Jun 02 '21 at 19:43
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    I understand that the accuracy may be ±1% or so, with possible slant range issues as mentioned by @randomhead, but I'm impressed that one can be calibrated to be accurate to 1 foot on or near the ground, given that light travels 1 foot in about 1 ns. Of course it travels to the ground and back, so the radar pulse would take 2 ns to bounce off ground 1 foot away and return. – rclocher3 Jun 02 '21 at 19:50
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    @rclocher3 Wait until you see what laser distance meters used in construction can do. – SomeoneSomewhereSupportsMonica Jun 03 '21 at 00:22
  • @rclocher3 Not to mention laser doppler vibrometers (LDVs) which can measure changes in position of a few millionths of a meter. – alephzero Jun 03 '21 at 01:48
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    @rclocher3 there are clever ways to electronically measure time-of-flight down around 10-100 picosecond resolution without even any really exotic components :) – hobbs Jun 03 '21 at 02:17
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    +1, especially for addressing the sometimes silly comments. – Peter Kämpf Jun 03 '21 at 07:33
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    I'd think the point of the altimeter at low altitudes is you really want to know when any part of the plane is within likely range of touching the ground, so it makes sense to include a bit of fudge room to account for a range of reasonable angles of the plane. – Darrel Hoffman Jun 03 '21 at 13:48
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    @DarrelHoffman Yes - and a purely directional beam would show increasing height as the roll angle increases... not a desirable effect when approaching the flare, especially with a wing-low crosswind correction! – Ralph J Jun 03 '21 at 14:30
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    Sorry about the "silly" off-topic discussion of the physics of radar altimeters. For anyone interested in how the electronics work, see the electronics.SE question "How do IR rangefinders accomplish centimeter precision without high speed / high cost parts?" – rclocher3 Jun 03 '21 at 16:05
  • @alephzero indeed...ldv's are used to 'spy' on others by measuring the vibrations caused by speaking in a room on a window... – CGCampbell Jun 04 '21 at 12:59
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    @randomhead The B737 radalt LRUs are mounted on the aft bulkhead of the EE bay which is between the nose gear bay and the cargo bay. The transmit and receive antennas are mounted several feet apart along the keel about half way between the nose and main gear. Since the measurement is in the LRU, it has to account for the round trip time including the antenna cable lengths in addition to the antenna height above the ground in landing configuration. This 'installation delay' is configurable via programming pins. And the antennae have to be far enough apart to avoid receiving a side lobe signal. – Gerry Jun 04 '21 at 17:02
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    BTW the device in question is the Radio Altimeter. The radar altimeter is a device earlier in use to measure height above sea level/ terrain. I think it was used during pressure pattern navigation, which was used on oceanic flights. So it’s usage was at higher altitudes. – skipper44 Jun 07 '21 at 20:13