Barometric Altitude vs Radar Altitude?

[Rdiehl3]

Quote:

Originally Posted by StrongHarm

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.

 

When would you use Delta on the SCE switch?

Paulkrii guide says tgt elevation greater than 50ft(6.3.4)weapons deplyment guide. Ex)Mountains? Or is these modes a back up to the DTS system...Just confused a bit... not sure if this is fuctional in game. Anybody have any ideas or knowlege in using DELTA?

 

Also in IFFCC/Display mode/DELTA CAL:

RDR DELTA

RDR MSL

GPS DELTA CAL

GPS MSL CAL

SELECTED MODE

STORE

CANCEL

 

How and when and why would you use these in DELTA mode?

[Migo]

I can't answer you but can someone tell me how to find out the altitude to the ground ? Lets say you start an "instant action" mission. You only got the sealevel altitude 29.92. So instead of 300ft my HUD shows 3000ft I know the radar altitude can be displayed by using the RDR Tape but there must be another way.

[BlueRidgeDx]

When would you use Delta on the SCE switch? Paulkrii guide says tgt elevation greater than 50ft

 

Not when target elevation is above 50ft, but when Estimated Vertical Error (EVE) is greater than 50ft.

 

Here's a snippet from an internal email I sent a while ago that should shed a bit more light on the subject:

 

"To sum up it up in just a few paragraphs, the BARO mode captures the aircraft altitude during takeoff (with altimeter set to local altimeter setting (QNH)) because it is at a known MSL elevation (field elevation). It uses this as a starting point for various other calculations:

 

In flight, the pitot/static system passes information regarding ambient temperature and pressure to the CADC, which passes the info to LASTE/IFFCC, which then uses the information to determine the true altitude.

 

In addition, the EGI is able to provide GPS based altitude inputs to the LASTE/IFFCC in order to bias the platform altitude toward a "corrected" value. This depends on EGI FOM. If the EGI results in an Estimated Vertical Error (EVE) of more than 50ft (as displayed in the CDU), it's better to use DELTA mode.

 

In the DELTA mode, the pilot has to pick a point on the surface with a known elevation, fly directly over it at less than 5,000 AGL, and take a DELTA update in the IFFCC menu. The update process will simply add radar altitude to the known point's elevation to determine the starting point for the barometric calculations described above.

 

There is no GPS updating in DELTA mode.

 

Ideally, you'd want to perform a DELTA CAL in the target area, at the planned release altitude in order to have the best numbers. For obvious reasons, this is not always possible or desirable."

Edited August 10, 2011 by BlueRidgeDx

[nomdeplume]

I can't answer you but can someone tell me how to find out the altitude to the ground ? Lets say you start an "instant action" mission. You only got the sealevel altitude 29.92. So instead of 300ft my HUD shows 3000ft I know the radar altitude can be displayed by using the RDR Tape but there must be another way.

The radar altitude is always* displayed if you're below 5,000 feet, above the data block at the bottom right (e.g. 3120R). The tape only appears (if enabled) below about 1,500 feet. * - assuming your radar altimeter is working and you're not upside down.

 

The only way to determine height AGL is using a radar altimeter or similar, or by having a database that contains elevation information for all the terrain you're going to be flying over.

 

The A-10C does actually have such a database, and if you have the TGP equipped, it'll give a reading in the top right corner. This is presumably calculated by subtracting the terrain elevation at your current position (from the database) from your current barometric altitude. That means if your position isn't accurate, or your barometric altitude isn't accurate, or the stored data isn't accurate, then the readout won't be accurate.

[lubey]

 

To sum up it up in just a few paragraphs, the BARO mode captures the aircraft altitude during takeoff (with altimeter set to local altimeter setting (QNH)) because it is at a known MSL elevation (field elevation). It uses this as a starting point for various other calculations:

 

In flight, the pitot/static system passes information regarding ambient temperature and pressure to the CADC, which passes the info to LASTE/IFFCC, which then uses the information to determine the true altitude.

 

In addition, the EGI is able to provide GPS based altitude inputs to the LASTE/IFFCC in order to bias the platform altitude toward a "corrected" value. This depends on EGI FOM. If the EGI results in an Estimated Vertical Error (EVE) of more than 50ft (as displayed in the CDU), it's better to use DELTA mode.

 

 

So if you just leave the altimeter on 29.92 in Hg the whole time, this will make your ccip deliveries less accurate?? Interesting...

[Laud]

So if you just leave the altimeter on 29.92 in Hg the whole time, this will make your ccip deliveries less accurate?? Interesting...

 

Of course!!! It will make all your alt readouts less accurate. Local pressure is due to change (you know, this weather thingie... ;) )and of course you have to take this change into acount -> calibrate your altimeter to MSL (enter the briefed air pressure or set it to airfield elevation (if known) as long as you're on the ground).

[Rdiehl3]

If the EGI results in an Estimated Vertical Error (EVE) of more than 50ft (as displayed in the CDU), it's better to use DELTA mode.

 

In the DELTA mode, the pilot has to pick a point on the surface with a known elevation, fly directly over it at less than 5,000 AGL, and take a DELTA update in the IFFCC menu. The update process will simply add radar altitude to the known point's elevation to determine the starting point for the barometric calculations described above.

 

Where on the CDU would you find EVE being more than 50ft (what page/setting). Is this something that is common and something we should look for often?

 

2)How do you do a DELTA update? Is this procedure explained in the manual?

 

Thanks BLUE

[degoe]

thnx

 

 

Where on the CDU would you find EVE being more than 50ft (what page/setting). Is this something that is common and something we should look for often?

 

2)How do you do a DELTA update? Is this procedure explained in the manual?

 

Thanks BLUE

 

Thank you for the background story BLUE helps alot with understanding the systems and buttons.

 

I tried to find the EVE and delta update in the manual but I cant find it either. Are these things modeled in the sim?

I Would assume this delta update is a function used when there is no altitude information available from gps/maps like in an unknown territory.

Also I had some trouble with ccip bombing accurancy which is how I got here, assumed it had to do with my altitude settings.

So was hoping a switch to radar/delta would make it more accurate.

Would be a lot easier if I knew if there is an estimated vertical error.

[BlueRidgeDx]

I tried to find the EVE and delta update in the manual but I cant find it either. Are these things modeled in the sim?

 

Yes, you can find the EVE value in the CDU by pressing the SYS key, then selecting GPS.

 

I don't have access to the DCS manuals at the moment, so I can't check to see if/where the Delta update procedure is described there.

 

I Would assume this delta update is a function used when there is no altitude information available from gps/maps like in an unknown territory.

 

Not really; altitude source refers, literally, to the source of information displayed on the HUD altimeter. The choices, as you're aware, are BARO, DELTA or RADAR.

 

This is completely seperate, and has no bearing on the source of information used to display target/steerpoint elevation. Choices are AUTO (DTS), or HOT.

 

So your choice of altitude source is not driven by availability of DTSAS maps.

[funkster]

This is completely seperate, and has no bearing on the source of informahtion used to display target/steerpoint elevation. Choices are AUTO (DTS), or HOT.

 

How do you derive or enter HOT elevation? I know you can use the UFC Data rocker etc but this only gives DTS or a manually entered number? If it is the later how do you calculate the correct HOT target elevation?

[funkster]

Anyone? I can target elevation from TGP/LASER but what should be entered in the HOT function?

 

Does it make much difference? How in accurate I'd DTS?

[Flagrum]

Anyone? I can target elevation from TGP/LASER but what should be entered in the HOT function?

 

Does it make much difference? How in accurate I'd DTS?

this might be of help: http://forums.eagle.ru/showthread.php?t=118975

[funkster]

Yeah I had read that one. In fact it was one of the reasons for my questions :)

 

The thread just doesn't seem to be conclusive and I just wondered if there was a definitive way of using HOT and definitive proof it was working / worth while. So far I see little difference and it seems to be of little use and adding complexity in most cases.

 

I also noted that on the 476th weapon school on YT I.e using battle books sleds. They all show DTC.

[Flagrum]

Yeah I had read that one. In fact it was one of the reasons for my questions :)

 

The thread just doesn't seem to be conclusive and I just wondered if there was a definitive way of using HOT and definitive proof it was working / worth while. So far I see little difference and it seems to be of little use and adding complexity in most cases.

 

I also noted that on the 476th weapon school on YT I.e using battle books sleds. They all show DTC.

As far as I understood that thread, HOT is feasible when you have no (accurate) data from the DTS. This can be the case if you fly off-map (TAD all black?) or if the resolution of the DTS data is too coarse - like in mountainous(sp?) areas. Too coarse ... this is how I imagine it: the DTS data might have the elevation for 10m x 10m squares (example, have no proof for that), but the actual elevation of a point inside such a square can differ from that significantly on steep slopes.

[furball]

Convert Km in feet. A basic rule of thumb is to dive feet by 3 to get meter so 30.000 ft divided by 3 is around 10.000 m so 10 km (I know it is a round up). You can use it to convert km into feet. So 2.5 km is around 2500 m multiply by 3 is around 7.500 ft. Fly around 8.000 vertically and you should be out of AAA firing range.

 

If you want a perfect convertion simply use the right value 1 ft = 12 in = 30.48 cm

[Joni]

Hi, I'm resurrecting this old thread.

 

I'm studying the A-10C and noticed this about the ALT SCE switch. I tried using active pause while pointing CCIP reticle into the ground and at the same time messing with the altimeter setting and noticed that there is no computation errors and no correction from the computer. So this means that not only the switch is not modeled but also that the whole elevation computing is not modeled as well, am I right?

 

 

I'm also trying to understand what this switch would do (in case it's modeled) and how it would affect the IFFCC computations. Besides having the whole elevation database already loaded into the DTSAS, how does this switch work in relation to computing target's elevations?

 

 

Any insight will be appreciated.

 

 

 

THanks.

[Blech]

Hi,

 

Perhaps useful for you

 

https://www.fbo.gov/index?s=opportunity&mode=form&id=8b388d2fa254a683a486cbd0bad3a2bc&tab=documents&tabmode=list

 

1A10C120081110wChg1020120402 - Draft T.O.1A-10C-1

Page 1-423

Page 1-432

???

[Joni]

Amazing documentation, thank you.

 

I have one question: I read accross the forums that people think this switch is for target elevation computation (so did I) but it seems according to this document that the source is used for knowing the exact altitude of our aircraft so the computers can calculate a proper weapon delivery solution. I understand that target elevation only comes from DTSAS.

 

 

Feel free to correct me if I'm wrong on any of my statements.

[Joni]

:(

[DirtyFret]

Amazing documentation, thank you.

 

I have one question: I read accross the forums that people think this switch is for target elevation computation (so did I) but it seems according to this document that the source is used for knowing the exact altitude of our aircraft so the computers can calculate a proper weapon delivery solution. I understand that target elevation only comes from DTSAS.

 

 

Feel free to correct me if I'm wrong on any of my statements.

 

Yes I believe you are correct , as I understood by reading the T.O.1A-10C-1 flight manual , that switch between BARO, DELTA and RADAR just selects how the elevation of your aircraft is computed.This is then used to calculate you aircraft elevation above the intended target in CCIP, GUNS and CCRP modes.

 

Additionally it does a few other things:

 

"Selects altitude source to be used for CCIP/CCRP TDC generation, INS HUD and overhead updates, offset, and overhead marks elevation computations."

 

T.O.1A-10C-1 p. 1-423

[Joni]

Also, I read the section like 10 times and I still cant understand what happens when you change the kolsman window setting in flight due to weather changes. The manual says that it may cause a bias error up to 250 feet, but if that's the case then how would an A-10C pilot correct the altimeter setting without generating that bias error?

 

 

thanks

[DirtyFret]

Also, I read the section like 10 times and I still cant understand what happens when you change the kolsman window setting in flight due to weather changes. The manual says that it may cause a bias error up to 250 feet, [...]

 

 

I didn't either :D. I believe that the 250feet mentioned would be a worst case scenario - as in the pilot actually dialing a totally wrong QNH or actually trying to get a wrong altitude.

 

Additionally it seems that the LASTE creates a calibration point for altimeter setting and air mass on takeoff roll so the value you input on the ground also affects the calculation. (T.O p. 1-432)

 

So if you wanna test it, I'd suggest to go crazy on the ground with the pressure setting and then double crazy in the air with it while using CCIP in active pause mode, just as you described earlier. :laugh: :joystick:

 

[...]but if that's the case then how would an A-10C pilot correct the altimeter setting without generating that bias error?

 

thanks

 

I guess by switching to DELTA mode. Apparently you are able to create a "in-flight" calibration point with the DELTA mode. I would speculate that this becomes a necessity if the Estimated Vertical Error (EVE) becomes too great (>50feet) with BARO. I havent actually tried (yet) to degrade EVE. It would be interesting if someone could do it.

 

 

Disclaimer: I have no idea of what i am talking about , I just skimmed the manual, flew around in DCS and took a screenshot. ;)

Edited August 25, 2015 by DirtyFret

[Joni]

It doesnt work in DCS, iv'e tried it yesterday. No matter what you do the geometric altitude is always correct.

 

I was wondering this as in the real airplane.

[DirtyFret]

It doesnt work in DCS, iv'e tried it yesterday. No matter what you do the geometric altitude is always correct.

 

I was wondering this as in the real airplane.

 

 

I am not sure if I understand correctly what you mean, but just changing the QNH in-flight wreaks havoc on the altitude readings. Now it would be interesting if one could find out if this meddling with the BARO will actually change the EVE value in the GPS page and the effectiveness of the IFFCC in game.

[Joni]

Yes, the reading in the hud is changed, but there are no computing adjustments made after messing with the kolsman setting. I tested it on ccip and ccrp. The solutions are always the same.