What is this system/sensor? Underside UK military aircraft

[wellsprop]

Hi all,

 

I've always wondered what the sensor is that is often found beneath the nose of military aircraft.

 

See below, just behind the black nose cone is a sandy coloured square (that appears split into two parts).

 

Tonka (although the F3 doesn't have the same sensor)

 

Here it is on a HP Jetstream

 

And again on a Sea Harrier FRS1 (interestingly the FA2 doesn't have one)

 

It's obviously something to do with the ground and it appears much too large for a radio altimeter. I can only assume that it is a doppler velocity sensor for gauging ground velocity (hence the F3 wouldn't have one, although I would have thought the FA2 would for hovering - unless it uses an inertial sensor).

 

Can anyone confirm what this system is?

 

Cheers

Ben

[dov]

I do not know. But your question is superb!

Because you can ask about all details.

There are so many, most modeller have seldom a clue what is all the antennas, probes etc!

This topic should we blow up and widen!

Get on !

Or let me know and I do it!

Happy modelling

[Giorgio N]

It's a Decca Type 72 doppler navigation radar

[Hook]

The Decca doppler is not confined to UK aircraft. You can find the same antennae on some versions of the Mirage F.1 as well. 

 

Cheers,

 

Andre

[Jabba]

I thought that it was the Rad Altimeter or is that the simpletons name for what you have already said Giorgio..

[Doc72]

AFAIK, a doppler navigation radar measures the aircraft's movement relative to the ground. Thus it enables the crew to measure the true ground speed and ground track, taking account of wind drift. Based on the data from the doppler a navigation system can tell the crew more or less exactly where they are and in which direction to fly to reach a specific target (if the starting point is known).

 

BTW: An inertial navigation system (INS) does the same, doesn't it?, but without any radar emissions. What are the pros and cons of both systems? The Tornado probably has both. Of course, nowadays, using GPS is much simpler and more precise (as long as the system works).

[Niall]

Inertial navigation systems "slip" over time, so the radar system allows inertial errors to be corrected.

Edited September 3, 2021 by Niall
Corrected spelling

[Giorgio N]

43 minutes ago, Hook said:

The Decca doppler is not confined to UK aircraft. You can find the same antennae on some versions of the Mirage F.1 as well. 

 

Cheers,

 

Andre

 

Yes, it was widely used in other countries. One of the types equipped with it was the Viggen and maybe the best info on this specific unit on the web comes from this Swedish page, showing its use on the Saab aircraft

 

https://www.aef.se/Avionik/Notiser/Dopplerenhet.htm

 

Link is in Swedish but Google can translate it decently

 

 

Regarding the pros and cons of the INS compared to doppler systems, one other problem is the time needed to align the INS before take-off, time that in older units could be around 10 minutes. Today units are better but in any case it's been common for many years to use combined systemsm with INS and doppler together and later INS with GPS assistance.

Mind, the advent of GPS sure made navigation easier but this system on its own has the serious problem that can be jammed, while INS systems can not, reason why they are still used. Even doppler radar systems are much safer than GPS because while it's true that a radar can be jammed, the very short range nature of these systems makes jamming pretty difficult.

[Doc72]

Regarding the dis/advantages of doppler vs INS: Does it make a difference if you fly high or low, over water or over land? In addition, I imagine aligning the INS is difficult on an aircraft carrier that itself is constantly moving?

[Antti_K]

We tried a Doppler navigation in our Twin-Otter for low-level (100 ft AGL) work and weren't impressed by its performance. There is however, one feature that beats both INS and GPS: you can use Doppler also on Polar regions where older INS units and GPS won't work at all. On one flight to Svalbard in a Cessna Citation GPS was completely lost just a few miles after passing the legendary Bjornoya (Bear Island) roughly at latitude 70 N. From there on navigation was based on WX Radar on MAP -mode, because the Citation carried on integrated avionics suite that is solely based on GPS.

 

Running both an INS and GPS side by side will create (on long distance flights) a phenomenon where the two start to show an increasing difference between positions. Further on the two positions start get closer to each other again. The explanation is pretty simple: an INS "understands" only a perfect sphere whereas GPS "understands" only an ellipsoid (on which our aeronautical charts are based on). Therefore only the INS shows a correct Convergence between longitudes along a Great Circle route. GPS will show the correct Convergence only once during the Great Circle route when passing the Parallel of Origin (where the chart projection runs parallel with the Earth's surface).

 

On long distance flights INS is always affected by the Ground Speed over the Meridians. The reading must be corrected by the heading maintained and Ground Speed. The Apparent Wander, 15,04 degrees/hour x sin Mean Latitude, is only a theoretical basis for these calculations.

 

There is also one fundamental difference between these three systems: GPS and Doppler provide the navigator with Fix and INS a DR -Position (best estimate). DR -Position can also be calculated with some basic tools (plotter, ruler, compass and an aviation computer) and it is the basis for all Fixing techniques (radio, Pressure Pattern, Radar or Astro).

 

Sorry guys, an old Air Navigator broke loose😉

 

Cheers,

Antti

[Giorgio N]

49 minutes ago, Doc72 said:

Regarding the dis/advantages of doppler vs INS: Does it make a difference if you fly high or low, over water or over land? In addition, I imagine aligning the INS is difficult on an aircraft carrier that itself is constantly moving?

 

On carriers the INS of the aircraft was usually aligned by slaving it to the ship's own system. In this way the two were aligned the same way,

[72modeler]

@Antti_K

 

Hyvin tehty!

Mike

 

Or, and I hope I got this right:

Texan  syntymasta suomalainen kaltevuuden mukaan!

[Antti_K]

Kiitos Mike!

 

You got it almost right if you meant something like A Texan by birth, a Finn by heart.

 

Hopefully the OP will forgive me a slight "drift" from Original Topic. Mike, we got the first AT-6 "Texan" in Finnish registry in full USN colours. She was performing at the Kaivopuisto Airshow at Helsinki. Loved that sound by the seaside over the old Suomenlinna castle in a lovely summer evening...

 

Cheers,

Antti

[DougC]

Hey Antti, that’s a brilliant answer & an ATPL Theory-level summary of the three nav systems in question.
 

Interesting to hear you say that some older GPS units won’t work in Polar regions….. we found almost the opposite when we took our Twin Otters to the Pole (South). Some of the newer (at the time, in 2009) GPS units like the Garmin 296 & 530 would really struggle with the high update rates, especially as we got closer to the Pole and if we were crossing many lines of longitude rapidly. Whereas the older systems like our Garmin 150 would soldier on, seemingly unaffected. It was as if the slower update rate of the older GPS didn’t fry its brain.
 

Our solution was to try to fly down a constant line of Longitude for as long as possible before peeling off for a visual approach or setting up for an instrument approach to RW36 Grid. The latter gave some pretty wacky route depictions on the GPS screen (Mercator projection!) - I wish I’d taken a photo of it!

 

Apologies for the massive thread drift!

[e8n2]

15 hours ago, Antti_K said:

We tried a Doppler navigation in our Twin-Otter for low-level (100 ft AGL) work and weren't impressed by its performance. There is however, one feature that beats both INS and GPS: you can use Doppler also on Polar regions where older INS units and GPS won't work at all.

Antti

Yes and no.  The Doppler Nav Radar needs to be hooked up to a nav computer to give you your position over any place on the earth.  No nav computer it doesn't do much.  With the Carousel IV INS system and APN-218 Doppler Nav Radar on the KC-135s, the doppler allowed an airborne alignment of the INS if necessary.  You could also use Doppler to correct for inherent drift in the INS platform.  You could also use TACAN for the same thing and it was the preferred way to update the INS.  As stated in a previous post, the newer INS systems will align much faster than before.  The Laser Ring Gyro system that was developed for the F-20 would align in 30 seconds.  C-141s and C-5s until the turn of the current century also had the Carousel IV INS but no Doppler Nav Radar.  The Carousel system could however navigate over polar regions by going into what was called grid mode.  The Carousel system also used a rotating platform that would cancel out the affects of converging lines of longitude.  The much earlier Hound Dog missile used on B-52s also had a rotating platform for the same reason.  Of course its platform was a lot bigger than on the Carousel system (about 18" in diameter for the Hound Dog vs about 5" for Carousel).  My first stint with Lockheed was doing the FMS/GPS on C-5s.  It still had the Carousel iV system along with GPS.  The second stint we got rid of the Carousel INUs and the older GPS system and replaced them with much newer, more state of the art systems.  As to whether or not they still use grid mode for navigating over the polar regions, I have no idea.  Antti let out his old Nav training, and I let out my old INS/DNS training and work come out!

Later,

Dave

[alt-92]

9 hours ago, DougC said:

thread drift!

I see what you did there

[BillF67]

I once had Doppler explained as: It doesn’t know where you are but it calculates all the places you aren’t so by elimination it returns a location🥴

[exdraken]

All very interesting!

More explanations here;

https://www.baesystems.com/en-us/definition/what-is-doppler-navigation

 

[Antti_K]

9 hours ago, e8n2 said:

Yes and no.  The Doppler Nav Radar needs to be hooked up to a nav computer to give you your position over any place on the earth.  No nav computer it doesn't do much.  With the Carousel IV INS system and APN-218 Doppler Nav Radar on the KC-135s, the doppler allowed an airborne alignment of the INS if necessary.  You could also use Doppler to correct for inherent drift in the INS platform.  You could also use TACAN for the same thing and it was the preferred way to update the INS.  As stated in a previous post, the newer INS systems will align much faster than before.  The Laser Ring Gyro system that was developed for the F-20 would align in 30 seconds.  C-141s and C-5s until the turn of the current century also had the Carousel IV INS but no Doppler Nav Radar.  The Carousel system could however navigate over polar regions by going into what was called grid mode.  The Carousel system also used a rotating platform that would cancel out the affects of converging lines of longitude.  The much earlier Hound Dog missile used on B-52s also had a rotating platform for the same reason.  Of course its platform was a lot bigger than on the Carousel system (about 18" in diameter for the Hound Dog vs about 5" for Carousel).  My first stint with Lockheed was doing the FMS/GPS on C-5s.  It still had the Carousel iV system along with GPS.  The second stint we got rid of the Carousel INUs and the older GPS system and replaced them with much newer, more state of the art systems.  As to whether or not they still use grid mode for navigating over the polar regions, I have no idea.  Antti let out his old Nav training, and I let out my old INS/DNS training and work come out!

Later,

Dave

 

Hello Dave,

 

and thank you for an interesting and informative post.

 

we indeed had problems with the Doppler unit as it momentarily "lost its bearings" only too often. As we were in a hurry with the project, we simply reasoned that it wasn't suitable for such a low altitude and skipped the system. An old school INS was also problematic on survey missions (where long parallel lines are flown) but that was before my time. Do you have an explanation for this as it had caused me some head scratching over the years? We had a procedure for updating the INS using Astro Fixing but it was rather complicated. I never had the guts to try it myself. In Africa we were flying survey lines 200 NM in length and the best "aid" was oil burned in a drum placed at the end point of a flight line. A pillar of black smoke rose up thus creating a "Fix"... Nowadays a flight crew performing such an operation would end up in prison.

 

We still do instruct our pilots at Finnair for Polar operations and explain them the Tonta Grid -system and its use in detail.

 

Cheers,

Antti

 

 

[Bejay53]

Many years ago when I started out working for Raytheon, somebody sent me an audio clip explaining "missle Guidance systems".

If I could find a way to post it, I would.

Basically, "the missle knows where it is because it knows where it isnt........."

 

Edited September 4, 2021 by Bejay53

[e8n2]

14 hours ago, Antti_K said:

 

Hello Dave,

 

and thank you for an interesting and informative post.

 

we indeed had problems with the Doppler unit as it momentarily "lost its bearings" only too often. As we were in a hurry with the project, we simply reasoned that it wasn't suitable for such a low altitude and skipped the system. An old school INS was also problematic on survey missions (where long parallel lines are flown) but that was before my time. Do you have an explanation for this as it had caused me some head scratching over the years? We had a procedure for updating the INS using Astro Fixing but it was rather complicated. I never had the guts to try it myself. In Africa we were flying survey lines 200 NM in length and the best "aid" was oil burned in a drum placed at the end point of a flight line. A pillar of black smoke rose up thus creating a "Fix"... Nowadays a flight crew performing such an operation would end up in prison.

 

We still do instruct our pilots at Finnair for Polar operations and explain them the Tonta Grid -system and its use in detail.

 

Cheers,

Antti

 

 

If you were flying over a large lake or the sea when the doppler lost its bearings, that could be because either you didn't switch it into SEA mode, or it didn't have that capability.  The system is looking for a return signal bounced off the surface of the earth.  Over large bodies of water the return signal is scattered all over the place because the surface is too smooth.  On the SR-71s we had what was called the Astro-Inertial Navigation system (ANS).  The day before a mission we would load the latest star catalog and the mission co-ordinates to be flown.  Before going out to the aircraft we would hack the chronometer to WWV out of Colorado to get the correct time.  When it was flying on ANS guidance, which was most of the time, the system knew that based on the time and date at any given moment it should be able to find star A at such and such position in the sky.  If the system's present position from the INS portion was different, then it would correct the aircraft's position based on where the ANS found the star.  The more stars it could find and compare positions to, the greater accuracy it had.  From what I was told, the star trackers for the ANS were originally in BOMARC missiles, so we are talking about something from the 50s!  If you look at the area directly aft of the rear cockpit on a SR-71 model, or photo of the real thing, you will see a small window.  That was the hatch window for the star tracker.  Some kits have the area around the window as rounded, but it was in fact flat.  A rounded window would have messed up the star tracker too much.  Also having the boom from a KC-135 extended into the SR-71 for refueling also messed up the star tracker since the boom was right over the hatch window.  What you are talking about for Astro Fixing is basically the same thing as the ANS but I imagine you had to shoot the stars yourself, look up where they were supposed to be at that time, and hope that your time was correct.  Definitely very complicated!

Later,

Dave

[Antti_K]

Dave, Thank you for your post. More interesting stuff there.

 

Actually we were having problems with the Doppler over ground (hills, meadows, forest,...).

 

I most used to a British Mk.IX bubble sextant. It was easily obtainable, easy to overhaul and calibrate and so on. I've also "shot" celestial observations using a periscope sextant but I prefer the British hand-held model. On a periscope sextant the field of view is upside down thus complicating things further.

 

I've been wondering why the S-4 magnitude values (for the ANS) are still published in the Air Almanac. Is the system still used in ER-1s?

 

Cheers,

Antti

[e8n2]

14 hours ago, Antti_K said:

Dave, Thank you for your post. More interesting stuff there.

 

Actually we were having problems with the Doppler over ground (hills, meadows, forest,...).

 

I've been wondering why the S-4 magnitude values (for the ANS) are still published in the Air Almanac. Is the system still used in ER-1s?

 

Cheers,

Antti

Hi Antti,

When you were flying over the ground, was there a lot of snow on the ground?  I could see where that would cause things to smooth out and mess up with the signal.  Otherwise I couldn't really tell you why the doppler was messing up.  The TR-1s (now all re-engined and resdesignated as the U-2S, and the ER-2s, if equipped with an INS, would be using a totally different system.  What NASA is using with their ER-2s I don't know.  The Deuces, as we called them, never had an ANS system.  They used an INS system that was very accurate.  I don't really want to get into how accurate, but an inherent drift rate (cause by slight differences in the manufacture of the gyros.  No two are ever exactly identical and there will always be some errors that build up over time) in a KC-135 or even SR-71 that would be considered very low, would be considered unacceptably bad on the Deuce's INS and would require its replacement and repair.  Sorry but that is the best I can do to answer your question.  I left Beale at the end of `87 and did a one year assignment in the Philippines in a different career field and went to Travis to work on trash haulers (C-141s and C-5s) after leaving the Philippines.

Later,

Dave

[Antti_K]

Hello Dave,

 

and thank you for all the information. We were flying Aerogeophysical survey missions mainly on areas where there was no snow. On the other hand, some missions in Africa were flown over desert.

 

A couple of months ago I got home from the local hobby shop carrying the Italeri 1/48 scale TR-1 kit. I will build it as an U-2R. I'm still looking for information about the camera bay and camera equipment. Can you help me with that?

 

Cheers,

Antti

[NG899]

In passing, as radar altimeters were mentioned, the radar altimeter aerials on all Sea Harriers are the two square-ish items on the ventral fin’s underside edge. Despite being standard on USMC AV8A/Cs and Spanish AV8Ss, with different-shaped aerials to the later UK ones, they were only added to some Harrier GR3s from c.1984 onwards… Do check photos of the GR3 you are making a model of!!!

Cheers

 

Nick