Radio Altimeters?

[Kelly Hopper]

A little bit of bar talk the other evening revealed something I had never thought of before:
Why are we calling radio altimeters radalts?
They do not read altitude. They read instantaneous height. Something quite different.
Radio Height Readers would be a far better suggestion?
It is a serious question as a crewmate (non English) got very confused over this purely because the word altimeter was in it!

[Endeavour]

Perhaps because years ago when the Radalt was first conceived, it was usual to set QFE when taking off/landing and hence the barometric Altimeter read height? Just a thought.

[DFC]

Look at your instrument pannel.

It has an Altimeter.

When you arrive in the cockpit and it is showing some random indication it is an Altimeter.

When you set QNH for departure, it is an Altimeter

When you set 1013 for enroute climb cruise and descent it is an Altimeter

When you set QFE for landing, it is an Altimeter.

Does your friend get confused and think that is should be called an Altimeter, a Flightlevelometer, and a Heighmeter? :D

The first uses of the radio altimeter was by the Navigator on long over-ocean crossings to work out "D" and over the ocean it most certainly measures Altitude.

Later uses for landing and (E)GPWS do not require us to call it something different.

RADio ALTimeter

[Kelly Hopper]

[QUOTE]The first uses of the radio altimeter was by the Navigator on long over-ocean crossings to work out "D" and over the ocean it most certainly measures Altitude./QUOTE]

Ah, I considered this but came to the conclusion that it actually still measures height above the water beneath you and not altitude.
Altitude is measured against a datum of "MEAN" sea level not the actual sea level.

[ImbracableCrunk]

The root altus means high. Altimeter is a height meter. Which height? Depends on the instrument and the setting.

[forget]

The root altus means high. Altimeter is a height meter.

:D Best Prune answer in a long long while. :D
Kelly, didn't you do Latin init?

[renard]

What is 'D' ?

[ImbracableCrunk]

What is 'D' ?

Fourth letter in the modern Latin/English alphabet. Also the third letter in raDar, raDio, and raDiationem.

[oxenos]

Heighmeter?
Presumably used for measuring how high the heigh is. Corn is of course measured in elephant's eyes. What is the preferred measure for heigh?

[forget]

What is D. Something to do with Pressure Pattern Navigation, after which, I'm lost. :bored:

[aterpster]

It indeed is an altimeter that reads absolute altitude.

[Escape Path]

Usually feet, although the Russians prefer metres :E

[grounded27]

Simply because only one radalt reports aural warnings/information. Safe thought to crosscheck vital information in TO and land modes of flight.

[Brian Abraham]

forget hasn't forgotten much and is on the money.

AP3340 (Air Ministry book of Aviation Meteorology) uses the term "D" to define the difference between the true height (Radalt) and the pressure altitude. "D" is applied to various formulas used in pressure pattern flying calculations. In the northern hemisphere if "D" is increasing drift is to port, if decreasing drift is to starboard.

[Saintsman]

Let's open this debate a bit more.

Is it a Radio Altimeter or a Radar Altimeter?

Or are they two different things?

[Wodrick]

Two different things

[forget]

Is it a Radio Altimeter or a Radar Altimeter? Or are they two different things?

Radio Altimeter is 'more correct'. Radar comes from RAdio Detection And Ranging and a Rad Alt doesnt really detect anything. (Pedants stay away.)

Wiki. In 1924, American engineer Lloyd Espenschied invented the radio altimeter. However, it took 14 years before Bell Labs was able to put Espenschied's device in a form that was adaptable for aircraft use.

PS. I've just seen Wodricks post - two different things. How so?

[Saintsman]

a Rad Alt doesnt really detect anything

Doesn't a Rad Alt transmit a signal and receive it back in order to determine the height?

[AerocatS2A]

It can't give a range to something without detecting it first surely? It detects the ground/water and displays a calculated range.

[forget]

I said - Radio Altimeter is 'more correct'. Radar comes from RAdio Detection And Ranging and a Rad Alt doesnt really detect anything. (Pedants stay away.)

The point of the discussion was - Radio Alt or Radar Alt. Every other ‘Radar’, Search, Fighter or Weather, needs to detect the direction and range of a target which may, or may not, be there.

Rad Alt doesn’t need to (first) detect anything to work, it bounces off something it knows is there - the earth. That’s the reason it’s Radio Alt rather than Radar Alt - and why I said - Pedants stay away. :)

[glum]

Nothing to do with the frequency of operation then?

[Brian Abraham]

AP3340 again.

Radar altimeter - operates on a pulse system, measures height over a range in the order of 1,000 to 50,000 feet. Error in the absolute measurement is within +-100 feet, but changes of height at a constant level are measurable to within about +-30 feet.

Radio altimeter - operates on frequency modulation and covers the range from zero to about 5,000 feet. Error is proportional to height and may amount to +-3%.

However the AP dates from 1960 and todays manufacturers seem to use the terms radar and radio interchangeably.

[forget]

Here's a remark from a web site which I daren't mention here. :hmm:

The radio altimeter is often erroneously described as a "radar altimeter". In fact it doesn't use radar principles beyond the fact that it responds to reflected signals.

I think that's what I was saying. OK, I accept that some 50s/60s kit may not fit that bill; but the guy who invented RadAlts did it before Radar was even thought of.

Conclusion, just call it a RadAlt and everyone's happy.

[BOAC]

Greetings from Mr PedantEvery other ‘Radar’, Search, Fighter or Weather, needs to detect the direction and range of a target which may, or may not, be there. - not true - a basic ranging radar produces only range, not direction.

NOTHING to do with the OP question, but hey, who cares!

[forget]

- not true - a basic ranging radar produces only range, not direction.

So what's the point? Range without direction? You know it's there - but you don't know where? You happen to have an example of this?

OK. Very Old AI.

[BOAC]

Gunsight ranging? Cue Pantomime mode "It's in front of you.........".......... "Oh no it isn't"

[aterpster]

forget:

So what's the point? Range without direction? You know it's there - but you don't know where? You happen to have an example of this?

I do, since I worked on them in a past life. The radar that was used to provide distance information to day-fighter optical gunsights provided only distance. The location was provided by the optical gunsight. An annunciator light would light when the ranging radar had a lock-on. There was no radar display. In radar parlence, this is known as an "A Scan" radar.

[forget]

Agreed. As I said - OK. Very Old AI; but we weren't talking about museum pieces. :)

Radar Ranging (http://www.ekco-electronics.co.uk/Ranging/Ranging.htm)

[BOAC]

oh dear - not 'AI', please

[galaxy flyer]

One "feature" of a radio altimeter is that it sends out a range only, no azimuth (think a near hemisphere below the plane). In theory, one could fly down the middle of a deep, but narrow, canyon and have a radalt of, say, 300' from the canyon wall, not the floor. Since nearly always the radio signal intercepts the ground first, not a problem, but this does solve low altitude bank angles.

GF

[aterpster]

F-100 Super Sabre optical gunsight.

The ranging radar had a fixed antenna.

http://www.f-100.info/images/cutshall_f-100f_fpanel1.jpg

[forget]

BOAC, What's wrong with AI? The stuff inside the sharpish end of the Hunter was always called that.

Page 17. Hawker Hunter 1951 to 2007 By David J. Griffin.

'The change of role to Ground Attack did not require the retention of the ranging radar, equipment meant for airborne aircraft intercept'. aka AI.

Hawker Hunter 1951 to 2007 - Google Books (http://books.google.co.uk/books?id=Ih5Vcy2PZXQC&pg=PA17&lpg=PA17&dq=hawker+hunter+AI+radar&source=bl&ots=BHa38k2gSU&sig=-gSY3w0d-6bl-0jBSPwcUwYlrX8&hl=en&ei=uYfuTL2wI4OohAfoo9SwDA&sa=X&oi=book_result&ct=result&resnum=4&ved=0CCkQ6AEwAw#v=onepage&q&f=false)

[galaxy flyer]

Didn't know there were TWO Hun Drivers, here!

GF

[BOAC]

Gosh! We have wandered a long way from RadAlts!The stuff inside the sharpish end of the Hunter was always called that. - not by most of us. AI means 'Airborne Intercept' and by the time you get to a guns kill the 'intercept' is over. 'Radar Ranging' is a more correct name. I believe the Google book to be incorrect in describing the RR in a Hunter as 'AI'.

Apart from some ex Navy two-seaters (T8?), Hunters, as far as I know (and no doubt John Farley will correct me) never had intercept radar fitted and were vectored into visual range on its targets by ground control. The exception was, I understand, a 'one-off' conversion to train naval Sea Harrier pilots in AI use.

[Brian Abraham]

The radio altimeter is often erroneously described as a "radar altimeter". In fact it doesn't use radar principles beyond the fact that it responds to reflected signals.Pedant on. Read his post and while he comes across as somewhat knowledgeable his brief of how they operate was not complete. He only talks of modulated frequency and not pulse. As to what they are called the manufacturers these days refer to them mostly as radar altimeters, although the radio is used infrequently. Radio/radar it matters not.

http://search.honeywell.com/search?proxystylesheet=ecm_en_frontend&client=ecm_en_frontend&baseSite=honeywell&output=xml_no_dtd&filter=p&site=default_collection&q=radar+altimeter&x=7&y=10

FreeFlight Systems Radar Altimeters and Indicators (http://www.freeflightsystems.com/prod_radalts.htm)

The manual of the aircraft I flew describes it as a radio altimeter. Depending on the particular piece of kit fitted it operates "to send a signal to the ground, receive an echo, and measure the phase or time difference between the transmitted and received signals. In the pulse system the time may be measured, and in the continuous wave systems the phase is measured".

Pedant off. (Sorry BOAC and forget)