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Baro altitude value for calculations not taken from cockpit altimeter


Harker

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Several systems in the cockpit use the barometric altitude for certain calculations. In my understanding, the altitude value for that should be the same value that the pilot sees on the barometric altimeter gauge in the cockpit and should change when the pilot adjust the altimeter pressure setting.

 

However, in DCS, the altitude shown in the gauge (and HUD) is not the same as the one used for calculations. Instead, the altitude used is the one you see when you go to the F2 view, which I will refer as "DCS altitude".

 

The three included tracks show that this bug occurs for at least the Radar page calculations, FPAS page calculations and for the barometric altitude floor warning ("ALTITUDE ALTITUDE" message), set through the HSI>DATA>AC subpage.

 

The screenshots depict a situation where it's clear that the Radar page just not adjust the target's displayed altitude or the displayed altitude difference between ownship and target.

 

First screenshot, I have set the pressure to 31.00, my altitude is showing as 23560 ft and yet the alt. difference between ownship and a target at 20000 ft is given as -1.6. My DCS alt. was ~21600 at that time.

 

Second screenshot, I have set the pressure to 28.10, my altitude is showing as 2900 ft and yet the alt. difference between ownship and a target at 20200 ft is given as -1.4. My DCS alt. was again ~21600.

 

Third screenshot, different setting. I'm at 29770, but showing as co-altitude with a contact at 30.2. And my DCS alt. was ~32000.

 

The bug has been present for some time now, possibly since the beginning.

 

All tracks and the first two screenshots were taken with the latest OB build, 2.5.6.52437.

Screen_200808_080806.thumb.png.b56a2abffca12ac25b7f332539da60e1.png

Screen_200808_080858.thumb.png.d451d4a45b0837ec6b84c1e14bc8dccf.png

Screen_200306_013704.thumb.png.7aec73e6aeb2a02df7e20c63a6508874.png

FA-18C_Barometric Alt for calculations not taken from cockpit_RADAR.trk

FA-18C_Barometric Alt for calculations not taken from cockpit_FPAS.trk

FA-18C_Barometric Alt for calculations not taken from cockpit_FLOOR.trk

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Above 18k ft, the altimeter has to be reset to 29.92 (at least back in the day that was the rule)

 

I have no idea how DCS handles this for calculations (automatically?:noexpression:)

 

From 29.92 to 31.00 that should be roughly 1.1 or 1.2k ft. lower, somewhere around that number.

 

Still, the numbers you're showing are a bit off. (?)

 

I haven't checked it yet. In VR I don't see the data in F2 view.

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Hey Harker, I just did a quick test just to see the difference between the in-cockpit altitude v. F2 view info bar.

 

I used 29.92 for the first test (set in ME weather and in cockpit)

On the second flight I used 31.00

 

The difference between in-cockpit (HUD, radar, altimeter) and the F2 view indication was the same regardless of altimeter setting.

 

At the field elevation (30ft) the difference was 2 feet.

At 10k feet the difference was ~145 feet (higher in F2)

At 20k feet the difference roughly doubled.

 

I didn't have time to verify the FCR calcs., will try later.

 

EDIT: This was done at the default temp of 20C. When I changed it to standard 15C, the difference at 20 k feet was ~ 65 feet (higher in F2 view)


Edited by Gripes323
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I tinkered some more in A/A mode and so far I haven't noticed any significant differences between ownship radar indications and altitudes set for AI, in ME.

 

Standard conditions set in ME (15C, 29.92)

 

But... I don't see the altitude difference number next to antenna elevation caret on my radar:noexpression: I'm also on the latest OB.

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Good find. The aircraft should definitely not be using true altitude for most, if not all, of what you listed there.

Exactly. Unless I'm mistaken, the aircraft cannot know its true altitude from another source, right? It's up to the pilot to set the correct pressure. As far as the aircraft is concerned, its altitude is the one calculated by the barometric altimeter.

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Above 18k ft, the altimeter has to be reset to 29.92 (at least back in the day that was the rule)

That's a convention and is up to the pilot. The aircraft doesn't care. And anyway, the problem exists below 18,000 ft as well.

 

 

Hey Harker, I just did a quick test just to see the difference between the in-cockpit altitude v. F2 view info bar.

 

I used 29.92 for the first test (set in ME weather and in cockpit)

On the second flight I used 31.00

 

The difference between in-cockpit (HUD, radar, altimeter) and the F2 view indication was the same regardless of altimeter setting.

That's strange. Are you saying that when flying straight and level, your displayed altitude does not change with a different pressure setting? Your true altitude doesn't change and is independent from anything you do in the cockpit. Thanks for testing, btw.

 

 

I tinkered some more in A/A mode and so far I haven't noticed any significant differences between ownship radar indications and altitudes set for AI, in ME.

 

Standard conditions set in ME (15C, 29.92)

 

But... I don't see the altitude difference number next to antenna elevation caret on my radar:noexpression: I'm also on the latest OB.

The differences aren't significant, but they're there. With the radar and SA page, you can only see 100 ft increments, so it could be that. The differences are smaller at lower altitudes.

 

 

 

As for displaying the altitude difference on the Radar page, go to DATA and either deselect Declutter or select Declutter 1.

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That's a convention and is up to the pilot. The aircraft doesn't care. And anyway, the problem exists below 18,000 ft as well.

 

I know a/c doesn't care... I was throwing some ideas as to how DCS might be handling this at high altitudes;

 

That's strange. Are you saying that when flying straight and level, your displayed altitude does not change with a different pressure setting? Your true altitude doesn't change and is independent from anything you do in the cockpit. Thanks for testing, btw.

 

As long as you set the same setting in ME and the baro altimeter window.

There is negligible difference between HUD/baro alt and F2 view.

 

 

 

The differences aren't significant, but they're there. With the radar and SA page, you can only see 100 ft increments, so it could be that. The differences are smaller at lower altitudes.

 

Agreed

 

 

 

As for displaying the altitude difference on the Radar page, go to DATA and either deselect Declutter or select Declutter 1.

 

Crap, sorry about my "multiquote" hehe. Read my responses within your quote.

 

Here's a very short clip at 3 different altitudes. Temp set in ME 15C, Baro 31.00

 

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Crap, sorry about my "multiquote" hehe. Read my responses within your quote.

 

Here's a very short clip at 3 different altitudes. Temp set in ME 15C, Baro 31.00

 

Yeah, the differences get bigger as altitude increases. But the point I'm making is that all the cockpit systems that provide calculation outputs based on the baro altitude, take the true altitude value instead. Which the aircraft shouldn't be able to know.

 

Another issue with this is that when you are given a holding altitude for CASE II/III, it refers to true altitude. While normally, it should be the altitude with your altimeter set for carrier landing. This creates an issue, especially in bad weather, that you end up very close to the holding altitudes of AI flights, since your 8,000 ft is 7,600 ft true, for example. Although this might be an issue with the SC and AI flights instead.

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I haven't checked this in MP yet but I'd think that with human players indicated v. true shouldn't be an issue since everyone gets the same altimeter setting from marshal/airfield controllers.

 

With AI, different story. I've just verified this flying in close form with an AI Hornet. My instruments were showing exactly 26k (in F2 view 25922 ft.)

AI was 78 ft. above me (in F2 view he was dead on 26k ft.)

 

So, it's close enough for the government but... you're right, ED should correct this and make all altitude assignments in ME 'indicated'. Well, calibrated;) and air data comps shouldn't use true no matter how small the difference.

 

Edit: I'd have to look it up or ask someone that knows... perhaps somewhere in the NAV setup pages there is a 'temp' input that feeds this into air data comp (?) so there's a good starting point for calculations.


Edited by Gripes323
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Exactly. Unless I'm mistaken, the aircraft cannot know its true altitude from another source, right? It's up to the pilot to set the correct pressure. As far as the aircraft is concerned, its altitude is the one calculated by the barometric altimeter.

GPS?

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GPS?
GPS could do that indeed, but isn't GPS only used for global positioning and INS drift correction though, in our Hornet?

 

And if we do have access to the GPS altitude, where do we see that value?

 

You can argue that the barometric one is only used for coordination with other aircraft, like is often the case for general aviation, but then why aren't radar targets included in that coordination?

 

One thing that's definitely a bug is that the aircraft will use the true altitude as a criterion for the barometric altitude floor warning message, which makes no sense. The field says BARO and then the true altitude is used.


Edited by Harker

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GPS could do that indeed, but isn't GPS only used for global positioning and INS drift correction though, in our Hornet?

 

And if we do have access to the GPS altitude, where do we see that value?

Afaik GPS always gives you all 3 coordinates in space as your position. There is probably a MFD format page for GPS information - which is not implemented, yet, iirc.

 

 

How the GPS information is actually used in the hornet - directly as input to various systems or just for automatic INS nav fixes, I don't know.

 

 

 

.

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Good find. The aircraft systems should be using the sensed altitude. However I wonder if the systems are calculating Density Altitude, which would be slightly different to Pressure Altitude as displayed by the Altimeter.

 

Someone with more knowledge of the ME might set up a standard ISO day and test if the variance is gone. Of course I don't know if you can adjust the temperature gradient in the ME.

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Afaik GPS always gives you all 3 coordinates in space as your position. There is probably a MFD format page for GPS information - which is not implemented, yet, iirc.

 

 

How the GPS information is actually used in the hornet - directly as input to various systems or just for automatic INS nav fixes, I don't know.

 

 

 

.

There is a GPS page available in HSI>DATA>A/C, but I don't know what it actually shows, a quick look at NATOPS wasn't very helpful.

 

So, even if the Radar, SA, and FPAS calculations use true altitude (still strange IMO, considering that the pilot can reference only his own baro altitude quickly, but I might be wrong), the Baro floor warning should definitely use baro alt and CASE II/III holding altitudes should use baro alt as well, at the pressure setting given by the carrier.

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Nicely put together @Harker.

 

Agree that the Baro altitude warning should use the same "BARO" altitude that is presented in the HUD.

 

It is unlikeley that the attack display shows Baro altitude for ownship and another reference for tracks but possible. Track correlation could be done with system altitude and Baro could still be the displayed altitude.

 

Right now you can't really know what the real altiude difference is between you and your track. That's a bummer at the merge.

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INS should also provide altitude information, shouldn't it?

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INS should also provide altitude information, shouldn't it?
In NATOPS, the DATA>A/C subpage only shows Lat, Long, WSpeed, WDir, MVar, GPS Hor/Vert Error and GPS time, just like in our DCS Hornet. No altitude info (unless that's accessible via the GPS button option, but I can't find that DDI page). That's what made me think that GPS is used only for Lat/Long corrections in AINS mode and the aircraft's altitude source is the barometric altimeter.

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Nicely put together @Harker.

 

Agree that the Baro altitude warning should use the same "BARO" altitude that is presented in the HUD.

 

It is unlikeley that the attack display shows Baro altitude for ownship and another reference for tracks but possible. Track correlation could be done with system altitude and Baro could still be the displayed altitude.

 

Right now you can't really know what the real altiude difference is between you and your track. That's a bummer at the merge.

Exactly, except right now, you know the altitude difference (if you don't have Declutter 2 enabled) and the target's true altitude. The problem is that you don't know yours. I agree that's it's strange that the radar page shows your own barometric altitude at the bottom, but everything else is in true altitude.

 

I'm also not sure if it's really supposed to always show the true heading regardless of the relevant option in HSI>DATA>A/C, but that's another question.

 

I'm also thinking that if you can only see your own barometric altitude, but your radar displays true, you cannot safely use it for altitude deconfliction. Like, your baro is 7,000, your true is 6200 (because of weather, or altimeter setting or whatever) and you get a contact at 6.0 on your radar, you think you have a 1,000 ft altitude separation, but in reality, you only have 200 ft.

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GPS Altitude is only accurate to 10-20 metres (35 to 70 feet) and that is with a modern GPS.

 

The standard for an IFR barometric altimeter varies around the wordl but 25ft accuracy is common. (with 50feet for VFR altimeter)

 

So a correctly set Altimeter should be far more accurate than the GPS altitude. Hence why Baro alt remains the requirement for RNP......

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By ICAO 8168 vol.3:

A serviceable altimeter indicates the elevation of the point selected, plus the height of the altimeter above this point,

within a tolerance of:

a) ±20 m or 60 ft for altimeters with a test range of 0 to 9 000 m (0 to 30 000 ft); and

b) ±25 m or 80 ft for altimeters with a test range of 0 to 15 000 m (0 to 50 000 ft).

 

GPS is not that simple. It provides WGS84 based altitude. You would need to apply corrections to get accurate altitude (or height).

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In NATOPS, the DATA>A/C subpage only shows Lat, Long, WSpeed, WDir, MVar, GPS Hor/Vert Error and GPS time, just like in our DCS Hornet. No altitude info (unless that's accessible via the GPS button option, but I can't find that DDI page). That's what made me think that GPS is used only for Lat/Long corrections in AINS mode and the aircraft's altitude source is the barometric altimeter.

Huh, interesting, thanks :thumbup:

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There are many comments on how good the aircraft can estimate its altitude. This is not really too important as we know that system altitude error is probably relatively small for the discussed application. We don't know what altitude is presented to the pilot on the attack display. We would need to know what NATO or US doctrine is.

 

FYI manuals for aircraft with the same navigation system show that the GPS can be used for updating the system altitude and GPS can be used as the primary navigation source and for computed deliverys. The error of the GPS is stated to be equally to 100 ft or less which is in line with e.g. the Vipers GPS.

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  • 2 months later...
  • 1 month later...
Bump. Any news on this?

I tried some workaround as well. It seems that not only the pressure but also the ambient temperature has impact to this. I assume that F2 (F10) altitude is recalculated altitude to ISA. Try this:

 

1) Set AI to 27000 [ft] MSL and the weather conditions in ME to 15 [°C] and 29.92 inHg. Fly a formation with the AI. Set BALT autopilot at 27000 (HUD) then jump to F2....you should see difference <100ft.

1) Set AI to 27000 [ft] MSL and the weather conditions in ME to 30 [°C] and 29.92 inHg. Set BALT autopilot at 27000 (HUD) then jump to F2....you should see difference >1000ft.

 

It seems that the AI ignore the weather conditions set in the ME and set its altitude based on ISA (15°C and 29.92).

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