Barometric Altitude vs Radar Altitude?

[goldfinger35]

Is there a way to have radar altitude shown on HUD all the time?

Currently, barometric altitude is always shown and radar altitude disappears after 5000.

Why do I need this? Well, for example, I want to bomb some targets but I want to stay away from AA fire (Shilka, ZU-23 etc.) which have effective firing altitude around 2.5 km. Therefore, I want to stay above 2.5 km (radar altitude, not baro!).

 

I have found only one workaround; to look at the TGP).

Is there any other way and why is barometric altitude primary source and not radar altitude?

 

In the picture below it is clear how misleading is barometric altitude:

[GGTharos]

Because barometric altitude is used for FLYING. You don't care about how far off the ground you are, other than avoiding it. People have gotten killed because they'd fly certain aircraft around high mountains, admiring their view and not minding their baro altitude since they could see the ground. Result - flying out at near-stall speeds, stalling in a turn, and not having the performance to get out of it at that altitude.

 

Find out what the elevation is at your AO and then you can look at your BARO altitude and still know how far up you are. Or you can just fly at 20000', works well.

Edited November 9, 2010 by GGTharos

[BlueRidgeDx]

All of that aside, the 5000ft limitation is a result of the radar altimeter's limited range. The reflected energy isn't sufficient enough for the receiver to acheive a reliable lock above 5000ft, so the display is limited to that value.

 

There's no way to force the radar altimeter to display data above this height.

[Frederf]

*gives BlueRidge some of Tharos's rep for actually answering the question*

[GGTharos]

Keep your hands off of that.

 

There were two questions in there, but I guess you missed the aviation one. ;)

[Gearbox]

If the nav system has a topo map loaded and knows your baro alt it should be able to calculate your AGL alt. Not saying that this exists in RL, but it should :)

[Kaiza]

If the nav system has a topo map loaded and knows your baro alt it should be able to calculate your AGL alt. Not saying that this exists in RL, but it should :)

 

Yep, I believe that is what it uses for weapon delivery data such as CCIP and GCAS etc

[timc]

\caveat on:

I'm not in the sim so cant verify this but my crusty brain thinks its true so therefore it is....

\caveat off.

 

Switch the Page dial on the AAP to Position (or use the UFCD) and I think that it displays height above ground.

[coolts]

I tend to use radar alt when down in the weeds / terrain masking. I dont want to trust my arse to someones map drawing skills, or my own fumble fingered baro pressure settings.

 

Mind you, i've never had a radar alt failure yet.

 

I may get some seaweed in the cockpit.

[hassata]

Setting Alt SCE to Delta for the difference between Baro and Radar helps.

[bogusheadbox]

Is there a way to have radar altitude shown on HUD all the time?

Currently, barometric altitude is always shown and radar altitude disappears after 5000.

Why do I need this

 

Just to expand on what GGtharos has said.

 

Firstly some corrections and definitions.

 

Correction. The number your radar displays is NOT an altitude its a HEIGHT

 

Height is defined as the vertical distance of a level, a point or an object considered as a point displayed from a given datum

 

Altitude is defined by the vertical distance of a level, a point or an object considered as a point measured from mean sea level."

 

From that we can move on.

 

Barometric altitude is most useful as in real life it will perform the following tasks.

 

1. Provide terrain seperation - Self explanatory

2. have everyone flying on the same datum. I will expand on both points below.

 

As in real life, your flight brief should include heights along the route for safety reasons e.g. terrain avoidance. Your map provides heights of terrain and/or obstacle elevation against sea level. Setting your altimiter to the correct pressure will correspond your altimiter to sea level allowing you to accurately judge your clearance from terrain.

 

Q. Why would you need that when you have a ground radar?

A. Ground radar looks below your aircraft. Ground radar cannot see the rising terrain in front of you that you will fly in to. Ground radar may not give you enough warning in the event of steeply sloped terrain (cliff) to outclimb the terrain.

 

The only way (especially in poor vis) that you can guarantee that you will not hit anything is to fly the appropriate altitude.

 

 

Now to the other point. Everyone flying on the same datum.

E.g. Your flight is due to rendezvous with another flight at 6000ft Altitude. But if you are reading radar HEIGHT and they are reading radar height? How do you meet up?

 

Flying from a common datum MEAN SEA LEVEL will ensure that you can make quick decisions regarding terrain seperation from your map and allow you to fly accordingly as tasked with respect to other aircraft.

 

If you are flying purely on radar alt (and that is fine when you can see terrain and need to get down and dirty) you really don't know your altitude as you are passing over terrain that is constantly changing in HEIGHT.

 

I hope that makes a little sense.

[Succellus]

I may get some seaweed in the cockpit.

 

And parachures ain t of much use in submarines.

 

So terrain for low and baro for high.

[pyates2104]

If the nav system has a topo map loaded and knows your baro alt it should be able to calculate your AGL alt. Not saying that this exists in RL, but it should :)

 

It does, not on an a10 though TAWS /TCAS II

[DiabloSP]

I hope that makes a little sense.

 

Makes complete sense, great post.

 

Expanding on the RALT thing, if I remember correctly, Tornados and some other planes have two TFR, one looking straight down to measure current RALT, and one looking a bit ahead so the plane can fly very close to the ground safely avoiding incoming elevations. Without that second radar keeping you safe, fliying close to the ground in low visibility is looking for trouble.

 

Always keep an eye on the flight plan and take notes on altitudes prior to takeoff. Especially if there are clouds expected.

[StrongHarm]

If you want to stay out of ground fire range, I would suggest making the aggressor the SPI (and I like to make it a distant markpoint as well) and checking it's alt. You can then increase your alt by that amount (i.e. ZSU is at 10k ft and you are at 15k, you need to go to 20k). It's good to plan from a stand off position anyway. It would do you no good to plan your alt while you're directly above the armor.

 

For low alt ops I like to enable my radar tape. It only shows up on the HUD below 1500ft(man p350):

- Switch IFFCC to test

- Sel down to DISPLAY MODES

- Sel down to RDALT TAPE

- Data rocker to switch to Y

- Exit

- Return IFFCC switch to ON

 

Also there is a altimeter source switch on the AHCP, but seems to have no effect on the altimeter. Perhaps it only changes the source when within 5000ft AGL (man p104):

 

5. ALT SCE. The ALT SCE switch selects the data source for altitude computations. This switch has three positions:

- BARO position (Up). Altitude data is derived from barometric.

- DELTA position (Middle). Altitude will be derived from difference between barometric and radar.

- RADAR position (Down). Altitude is derived from radar altimeter.

[bogusheadbox]

It does, not on an a10 though TAWS /TCAS II

 

Tcas and Tcas 2 is TRAFFIC collision avoidance system and only advises you of flying threats due to transponder interrogation

 

However you are correct that TAWS and EGPWS can advise in advance of possible terrain risks.

 

The only problem with TAWS is that it uses a known database of ground elevation. It has a more detailed terrain database within a certain area of larger airports and less "resolutoin" in other areas.

 

if I remember correctly, Tornados and some other planes have two TFR,

 

I am not sure if they have 2, but that would make sense. Even the australian old F111 uses a system to judge terrain elevation for high speed low level flight. IIRC it can autopilot you at low level avoiding terrain.

 

Magic plane BTW ;-)

 

- RADAR position (Down). Altitude is derived from radar altimeter.

 

The magic of computers. So your A/c can calculate an ALTITUDE for the RADAR HEIGHT Limited height use i am sure

Edited November 10, 2010 by bogusheadbox

[Kaiza]

It does, not on an a10 though TAWS /TCAS II

 

Yes the A-10c does, DTSAS.

 

TCAS II does not, T2CAS does.

[BlueRidgeDx]

Also there is a altimeter source switch on the AHCP, but seems to have no effect on the altimeter. Perhaps it only changes the source when within 5000ft AGL (man p104):

 

5. ALT SCE. The ALT SCE switch selects the data source for altitude computations. This switch has three positions:

- BARO position (Up). Altitude data is derived from barometric.

- DELTA position (Middle). Altitude will be derived from difference between barometric and radar.

- RADAR position (Down). Altitude is derived from radar altimeter.

 

The nomenclature "ALT SCE" is slightly ambiguous, but refers to Altitude Source, not Altimeter Source.

 

The ALT SCE is used for IFFCC weapon delivery computations, and is used to derive aircraft altitude above the target. The data is used internally within the IFFCC for CCIP/CCRP functions, and has no effect on the HUD displayed altimetry.

 

In Radar Mode, the IFFCC assumes that the height above target is equal to the radar height directly below the aircaft. As such, it's only useful over flat terrain, and it's subject to the 5,000ft AGL limitation of the radar altimeter.

 

The Baro and Delta modes are far more complex, and the specifics aren't really germane to the OP's question. Suffice it to say that these modes incorporate the CADC, EGI GPS altitude data, and various methods of updating and calibrating this data to derive an actual MSL target altitude.

 

Additionally, as someone already pointed out, the aircraft uses a DTED elevation database to determine target elevation. Using this database, the IFFCC can provide the elevation under the gun/bomb/maverick/depressible pippers. Alternatively, the TGP can also be used to determine target elevation provided you have actively lased the target.

Edited November 10, 2010 by BlueRidgeDx

[Scabbers]

Ok I am not an ATC or pilot but I have worked with RADALT and BARALT for awhile. Let me throw my 2 cents in here.

 

The radar altimeter has a low output power and the transmited pulse is calibrated to a maximum range of 5500 feet. The indicator will indicate to 5500 ft and then the out of range flag will display and the needle will max out and stay there.

 

The Barometric altimeter uses a reading from the static tube to read the ambient air pressure to determine altitude in reference to sea level. (0 pressure)

 

The reason for the two is best described as this.

 

A pilot flies along fat dumb and happy at 11,000 ft. He flies into a cloud bank and continues flying IFR. Well he is also flying toward a mountain range. As he gets closure to the mountains the ground begins to get closer to the aircraft. Once the air to ground distance is less that 5500 ft the radalt will begin to display the distance (AGL). Once the needle passes the bug or minimum distance setting the pilot receives a low altitude warning. The GCAS in the A10 will then issue a pull up, pull up command.

 

The reason I posted all of this is for systems familiarity. Maybe this will help. As for your question in reguards to AAA threats you need to do this in your head by reading you altimeter and having situational awareness.

 

The static tube is also intrsumental in continual altitude clabration for the Pitot's air speed reading provided to the airspeed indicator.

[goldfinger35]

Thank you all for your answers. They were really helpful.

 

 

All of that aside, the 5000ft limitation is a result of the radar altimeter's limited range. The reflected energy isn't sufficient enough for the receiver to acheive a reliable lock above 5000ft, so the display is limited to that value.

 

There's no way to force the radar altimeter to display data above this height.

 

OK but what does TGP show in upper right corner? It says it is Radar AGL altitude and it goes beyond 5000 (see pic in my 1st post).

[Scabbers]

Thank you all for your answers. They were really helpful.

 

 

 

 

OK but what does TGP show in upper right corner? It says it is Radar AGL altitude and it goes beyond 5000 (see pic in my 1st post).

 

Looks to be a delta indication. With out a track. it is difficult to disect. your delta would be a maximum radalt reception of 5880. SO I am not entirely sure but that is my best guess

[BlueRidgeDx]

OK but what does TGP show in upper right corner? It says it is Radar AGL altitude and it goes beyond 5000 (see pic in my 1st post).

 

That field will display the best available AGL altitude source. Radar altitude has priority, but DTSAS derived altitude will be displayed if outside of RADALT range, or if the RADALT is off/unserviceable.

[goldfinger35]

That field will display the best available AGL altitude source. Radar altitude has priority, but DTSAS derived altitude will be displayed if outside of RADALT range, or if the RADALT is off/unserviceable.

 

Good answer :thumbup:

So I will use that as a workaround to get a decent estimate of my height above 5.000 feet when radar altitude is not available.

[Frederf]

Maybe it's some calculated "delta" height between terrain database elevation and baro altitude? Good question.

 

EDIT: Hahaha, triple sniped.

[pyates2104]

Tcas and Tcas 2 is TRAFFIC collision avoidance system and only advises you of flying threats due to transponder interrogation

 

My silly mistake, I'm mixing my acronyms (MMA's)

:cry: