Radio Altimeter question

[Double Back]

One for the RA experts here.
Suppose You are in an helo, equipped with a RA, and You are flying around 700 feet AGL.
The terrain below is not flat but undulating, and You are straight and level.
What will the RA indicate, referring to the terrain it "sees" in its cone, the lowest, highest or an average of the readings?
What would approximately be the dimension of the "circle" it paints on the surface from 700 feet?

Tks, Richard

[V_2]

As far as I am aware, there is no "cone". On my aircraft (and many others) there is a transmitter and a seperate reciever for each RA system(cpt and fo). The transmitter sends a beam of radio activity towards the earth, but only the ground directly below bounces the radio wave back to the reciever unit. This diagram below should help.Think of it like shinning a torch onto a mirror and back into your eyes.

https://www.google.co.uk/imgres?imgurl=http://lea.hamradio.si/~s53mv/radalt/principle.gif&imgrefurl=http://lea.hamradio.si/~s53mv/radalt/radalt.html&h=600&w=800&tbnid=_n4Gy4ywVqgjrM:&tbnh=144&tbnw=193&docid=R200Hd796NNEXM&itg=1&client=firefox-b&usg=__tS2xcNtKGOn_DEiDR7GXnZrheBc=&sa=X&ved=0ahUKEwjN56PF1-bMAhVDJcAKHf9WAZsQ9QEIIzAA

Over uneven ground the radio alitmeter will be all over the place, it reacts very fast. On approach into my base we often get the "1000ft" radio-callout 3 times. I hope that helps, (I could be a different system in a helicopter though to be fair...!)

[garyscott]

Whilst thinking of how to word it correctly, i thought sod it, copy and paste!

"Radar altimeter antennas have a fairly large main lobe of about 80° so that at bank angles up to about 40°, the radar detects the range from the aircraft to the ground (specifically to the nearest large reflecting object). This is because range is calculated based on the first signal return from each sampling period. It does not detect slant range until beyond about 40° of bank or pitch. This is not an issue for landing as pitch and roll do not normally exceed 20°." (From Wikipedia).
https://en.wikipedia.org/wiki/Radar_altimeter

[FE Hoppy]

They are RADIO altimeters not RADAR altimeters but apart from that garyscott is correct. You will see height above the highest terrain in the reflective area but the topology of the ground plays a part as you need enough reflective area in the correct plane. Hence the protected areas on cat11 /111 runway approaches.

[garyscott]

The disadvantages of "copy / paste", im afraid FE Hoppy :}

But Radio energy it is. :ok:

[abgd]

Why is it incorrect to refer to it as a radar altimeter, when it works by RADAR?

[NSEU]

"A rose by any other name would smell as sweet"

I remember the Lockheed-Martin Hercules being fitted with such named "Radar Altimeters" (although they had a longer usable range than that used on typical airliners*).

Radio altimeters operate on frequencies around 4.3GHz. C-band radar is 3.7 to 4GHz. X-band radar is 9~15.5GHz. They are all in the Super High Frequency Band (3Ghz to 30GHz)

As Gary Scott says, the highest land in the "cone" is displayed.

(EDIT) *Lesser known fact is that the Aux Fuel Tank Transfer system logic on the 747-400ER is tied into the Radio Altimeter system (well outside the normal range as seen by pilots on their displays).

[AerocatS2A]

Geez guys RAaDAR = RADIO Detection and Ranging.

Something that uses RADIO to detect and calculate range is by definition RADAR.

[stilton]

NSEU,


Why is the 747ER aux fuel tank transfer system tied into the RA ?

[Uplinker]

Yer, as Aerocat says, the RAD ALT is a RADAR like system. Both use radio waves to determine the range of an object, (in this case, the ground).

Actual RADAR measures the direction too, whereas the RAD ALT only looks in one direction - downwards.

[Capn Bloggs]

RAaDAR
Spelling! :}

[Double Back]

Outside the initial question but I as an ex B747-400ER driver would also like to know this strange tie up into the aux tank logic?

Anyhow, as far as the discussion indicates, You can more or less speak of a "cone", therefore some kind of circle it "paints" on the surface and that the highest area in that circle would be the indicated RAlt?

[msbbarratt]

RADAR vs Radio? Well if RADAR is Radio Assisted Direction and Ranging (a classic definition), then a radalt ain't radar; it doesn't tell you in which direction the nearest ground is.

Radalts assume that the first return (ie the nearest terrain) is the number you want to know about.

This is important to remember. The beam of a radalt is quite wide, about 80 degrees, so that aircraft manoeuvres don't hinder the radalt's operation.

Whilst 4.2 to 4.4 GHz is a good band in which to operate a radalt, it's not perfect. Some terrains, particularly coniferous forests under certain circumstances, act like radar absorbent material at this frequency, and you don't get a strong enough return off the trees below for the radalt to detect. This leads to an incorrect height reading, perhaps giving you the distance to, say, the lake over yonder instead. Probably not an issue in an airliner, but a bit of a fright for a fighter pilot flying fast and low!

4.2-4.4 GHz works well because it is high enough so that smallish terrain features are noticeable, you can make a useful beam shape for a reasonable antenna size, and get good precision. But it's not so high that, say, a ploughed field, causes too much scatter and poor returns. A ploughed field when observed at this frequency is 'electrically flat' (it looks like a mirror), but a building is not.

If you look at the history of bands used for radalts you'll see that the band of choice has gone up as the capability of electronic components has improved, and then stuck at 4.3 GHz for the past 30 years or so. Ground observation satellites are all up at 15GHz or so because at this frequency they can 'see' the furrows in a ploughed field, etc.

Most radalt waveforms (they're FMCW, usually a variable rate linear chirp) track the ground. They have good precision at low altitude, and less precision higher up. When first switched on they go through a ground acquisition search, and then reduce the chirp rate as the aircraft altitude increases, 'tracking' the ground. The tracking is rate limited, but it's quick, far quicker than your plane can change height.

So it all boils down to:

1) a radalt tells you the distance to the nearest ground, which isn't necessarily directly below you. RA approaching Kai Tak must have looked interesting! Over your undulating hills it's probably going to be to the ground directly below, depending on the exact geometry of beam width, terrain shape, etc.

2) that is rate limited by the ground tracking done by Radalts. Ultimately the needle on the gauge won't move fast enough to react to every single building passing by below, but it's quicker than your plane can manoeuvre.

Depending on the application some secondary rate limiting may be done. Radalts in hydrofoils need to track average wave height, not instantaneous wave height, to give a smooth ride.

[AerocatS2A]

RADAR vs Radio? Well if RADAR is Radio Assisted Direction and Ranging (a classic definition),

It isn't. It is RAdio Detection And Ranging. Not "assisted" and not "direction".

[Piper19]

To the original poster, it's not a cone or averaged. When we do a test on ground and someone drives underneath the antennas we can see the altimeters go down for a brief moment until he clears the antennas.

[NSEU]

When we do a test on ground and someone drives underneath the antennas we can see the altimeters go down for a brief moment until he clears the antennas.

I don't see how this disproves the cone theory. The "cone" is limited in angle otherwise the antennas would pick up objects well off to the side of the aircraft.

Neither does the system put a narrow pencil beam down to the ground, otherwise any tiny roll or pitch movement of the aircraft would produce an erroneous height due to slant angle.

Why would you see a variation during test. It feeds a (typically) fixed 40' test signal into the system.

Seems this topic has been visited before:

Radio Altimeter [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-240939.html)

Now, getting back to 747-400ERs and Rad Alts... There is 160 pages in my engineering training notes on Aux Tank fuel management alone, but the highlight is...

"Management operation...

During normal operations, the FSMC sends commands to open the valves for descent (Aux Tank) pressurisation when all of the following conditions are true:

*In the air
*Aicraft altitude decreasing (Cabin Pressure Controller detects descent) and
*Valid Radio Altitude data (8190 ft).

Note: Flaps entering the Landing range provide a back-up signal for descent pressurization."

There are numerous references to 10,000' altitude, but I can't figure out if the data is from the ADCs (perhaps via the cabin pressurisation controllers) or the RA's.

I couldn't quite believe what was written, but my instructor said it was valid.

[Smilin_Ed]

Although most practical radars operate in the higher frequencies, there are radars at many areas in the electromagnetic spectrum ranging from the HF through light (lasers). Engineers use laser range finding to accurately measure distances allowing them to start a bridge on both sides of a river and have them meet squarely in the middle. Navy ships use frequencies in the VHF part of the spectrum for Air Search. Short range fire control systems on ships and aircraft are in the gigahertz range.

Back in the 1950s, amateur radio operators operating in the HF bands detected echos of their own signals. It turned out that they were hearing their own transmissions bouncing off the moon. As I remember, the time delay was around a half of a second, something easily detected by the human ear/brain system.

[wiggy]

It turned out that they were hearing their own transmissions bouncing off the moon. As I remember, the time delay was around a half of a second,

FWIW a bit more than that - roughly, using imperial units

(240,000 *2)/186260 seconds, or about 2.6 seconds for cash...

[Chu Chu]

If I understand it right, the radio altimeter measures the distance to the nearest ground. A wide cone doesn't seem like it would normally be a problem -- high ground way off to the side will usually be further from the antenna than lower ground directly below the aircraft.

[megan]

Seems the names are interchangeable.

https://www.rockwellcollins.com/sitecore/content/Data/Products/Navigation_and_Guidance/Radio_Navigation_and_Landing/ALT10004000RadioAltimeter.aspx

http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-documents/MilitaryAC/HG8500_Radar_Altimeter.pdf
(https://aerospace.honeywell.com/en/search-results?search-bar=radio+altimeter)

[FE Hoppy]

Seems the names are interchangeable.

https://www.rockwellcollins.com/sitecore/content/Data/Products/Navigation_and_Guidance/Radio_Navigation_and_Landing/ALT10004000RadioAltimeter.aspx

http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-documents/MilitaryAC/HG8500_Radar_Altimeter.pdf
(https://aerospace.honeywell.com/en/search-results?search-bar=radio+altimeter)
I was in a meeting about ten years ago where the head of the Joint OEB told the manufacturer to go away and take his manuals with him and don't come back until they were amended to Radio Altimeter.

They were Honeywell units ;-)

[Snakecharma]

Talk about pole vaulting over mouse ****!

Who gives a flying crap whether they are called radar altimeters or radio altimeters!

And I thought I was a pedant.

[aterpster]

Snakecharma:

Who gives a flying crap whether they are called radar altimeters or radio altimeters!

RA means both except for the exceptionally anal retentive folks in this business.

[Snakecharma]

Aterpster, that's what I would have thought. I was amazed that the discussion about the "correct" nomenclature took place at all, let alone grew the legs that it seemed to.

Maybe I am just getting old :(

[megan]

I was amazed that the discussion about the pedant conversation took place at all, let alone grew the legs that it seemed to. And I am old. :)

[NSEU]

A wide cone doesn't seem like it would normally be a problem

I understand your point, but if the cone is too wide, it will start reflecting your own landing gear or low slung engines.
If I remember correctly, the Rad Alts on the 747SP were only a few feet behind the gear.

[Tourist]

In aircraft that use Radalt for height holding, the signal is smoothed to avoid bumps causing aggressive height changes.