barometric altitude vs. target height

[VTJS17_Fire]

Hi Razbam,

 

found another bug, comparing the altitudes in the barometric altimeter and VTB, while in PIC mode. There is no difference in target height, whether which QNH you set.

 

barometric altimeter is set to QNH (in this mission 1029 hPa); own altitude: 15.800 ft.; target altitude: 16.000 ft. (as set in the ME)

 

 

barometric altimeter is set to standard air pressure (1013 hPa); own altitude: 15.400 ft. (not changed); target altitude still: 16.000 ft. (as set in the ME)

 

 

Should the radar the target height not calculate from the own height? Or does the radar calculate the target height from other factors?

 

Mission (open alpha) attached.

Nevada-UniTraining_v20_nowind_dark.miz

[VTJS17_Fire]

And by the way: The radar altitude seems to be complete broken, but I don't know if it's a Razbam issue or a DCS issue.

 

As you can see, the tanker altitude is set to 16.000 ft. (in route and while orbiting), but flys ingame at 15.000 ft. But the radar shows it at 16.000 ft. (independent on the QNH, as mentioned above).

 

 

tanker altitude set at 16.000 ft. in the ME (route and orbit) - And no, that's not the altitude limit for the IL-78M

 

 

radar lock (PIC) shows 16.000 ft. for the tanker, but trailing at level flight, my own altitude says 15.000 ft.

 

 

These two observations suggest, that the radar altitude is calculated from the mission editor. At least, for the pre-set AI.

[Frederf]

There are several height infos possible in the Mirage:

1. Baro

2. Radio

3. Radar

4. INS

and perhaps a few more I don't know (like baro captured on takeoff or delta like the A-10).

 

I would think that the radar target displayed altitude would be dependent on the INS altitude and not changed by twisting the Kollsman knob.

[VTJS17_Fire]

The Radar altitude is it not, both are above 5.000 ft. AGL.

 

Any altitude shown makes no sense, because the tanker flew at 15.000 ft.

If every target is shown 1.000 ft. above its real altitude, how Do you want to Intercept targets? That makes no sense.

 

Something is broken, IMO.

[Frederf]

INS altitude seems very funny:

 

1. I cannot input alignment elevation in meters (it must be feet).

2. I lied to INS and baro on alignment by +3000' but only HUD and round gauge is affected.

3. In motion ALT dial position to monitor INS on 00 does not show current altitude, only "0".

4. Only when stationary does INS show aligned altitude in digital display (I did not try and land at a different elevation to see if it would change).

 

Either radar is cheating and getting true position directly from game engine or INS is knowing perfect altitude even when it should be incorrect. Yeah, something is wrong.

[Zeus67]

ME sets the altitude in "true" mode. That is, with SL = 0 and the aircraft, in your case, is at 16,000 feet above SL. This value is not affected by atmospheric pressure and temperature.

 

The M-2000C HUD and Baro Altimeter shows indicated altitude. It may be, but usually it isn't, equal to the true altitude used by the ME. The difference is based on the atmospheric pressure and temperatures as set by the game engine. This is also why the altitude reading changes when you change the Kohlsman setting in the altimeter.

 

If you click on F2, you will see the aircraft's true altitude as opposed to indicated.

 

INS altitude reading is bugged. If you used PREP 00 and put the knob in ALT, the reading will be wrong. INS ALT in PREP 00 should always returns aircraft true altitude, i.e.: not affected by atmospheric conditions.

 

I'll check on the alignment elevation in meters.

 

Radar target altitude is calculated from contact elevation with respect to the aircraft true altitude. So it should return contact true altitude.

[Frederf]

It must be possible to get the contact altitude to be wrong by feeding the airplane poor altitude data. There is no such thing as a "true altitude sensor" in the real airplane.

[Zeus67]

It must be possible to get the contact altitude to be wrong by feeding the airplane poor altitude data. There is no such thing as a "true altitude sensor" in the real airplane.

 

Actually there is, it is called the INS. The INS computes the aircraft's true altitude independently from the barometric altimeter. Of course it can also compute indicated altitude based on the barometric altimeter setting.

 

The "bug" regarding the differences between aircraft altitude, target altitude as reported by the radar and the altitude as configured using the ME are caused by these:

 

The ME does not know what will be the ambient atmospheric temperature and pressure at the point where the AI aircraft will be spawned since these are created at simulation time. So it must use the real altitude to place the AI aircraft.

 

The aircraft's baro altimeter reads atmospheric pressure and temperature at simulation time and uses the K setting to display altitude. But since this altitude depends on the K setting, it may be the real one or not.

 

The INS calculates the aircraft true altitude based on the altitude value entered at initialization/alignment time. For the INS that is an exercise not different from calculating lateral and longitudinal displacement. So at all times and as long as the INS is working, you have: Indicated Altitude (baro altitude) and True Altitude (INS calculated altitude).

 

Note: The INS calculated true altitude is also subject to INS drift error and thus it is updated every time you do an INS update.

 

The HUD reports Indicated Altitude as well, so there is no confusion between what the HUD displays and the baro altimeter.

 

Now, the radar locks a target and reports the target's elevation in relation to the aircraft. It cannot determine the target's altitude by itself, it can only detect range, angle of elevation and angle of azimuth. The attack computer uses the INS calculated altitude to compute the target's altitude based on it's elevation angle. Since the INS calculated altitude is not affected by the K setting, the radar will report back an altitude that is very close to the one configured in the ME (if the target is an AI aircraft flying steady).

 

Why use Indicated Altitude instead of True Altitude? Because with Indicated Altitude you can set the altimeter 0 with the K setting knob. So that when you update the K setting with an airfield's QFE value, so the altitude you read in both the HUD and the Altimeter is in relation to the airfield and not to the sea level. This is very important during landings. Specifically the AP Approach Hold function requires that the altimeter is set to the landing airfield's QFE setting.

 

The only issue that I am willing to review, but it is at the bottom of the list, is which aircraft altitude the radar uses to calculate a target's altitude.

 

Edited to add: I have reviewed the information that I have and the radar uses "Topographic Altitude" to calculate target altitude. In other words, it uses what I call true altitude. Since the radar already does that then no change is needed.

Edited September 2, 2016 by Zeus67

[Frederf]

We can temporarily uninstall the barometric altimeter from the airplane and our minds for the sake of clarity of discussion.

 

I repeat, the airplane does not have a true altitude sensor in the sense that it can read this information directly from the environment. The INS is not such a device. The INS merely evolves position based on a trusted initial value. If that trusted initial value is wrong the INS has no way of directly obtaining the truth.

 

The INS's knowledge of altitude is entirely dependent on the validity of the data it is given. If I align at sea level but tell the INS it is 1000' MSL at the time then must believe it. Thus when the airplane translates upward +9,000' it must think it is at 10,000' when it is only at 9,000'. When looking at a radar contact which is horizontal to it, it must conclude incorrectly that the contact is at 10,000' instead of its true position.

 

If the INS holds an incorrect assumption about ownship altitude it must make appropriately incorrect calculations based on that information. Currently the contact altitude is accurate despite deliberately misinforming the INS at align.

 

Maybe there is some mechanism that the INS is obtaining correct position despite my best efforts to misinform it. I cannot monitor it because the ALT knob position is not showing the 00 data as expected.

[VTJS17_Fire]

In other words, it uses what I call true altitude. Since the radar already does that then no change is needed.

Sorry Zeus, but when my radar says the target is at 16.000 ft. while it is at 15.000 ft., there is a change needed.

 

This is the first time, that I hear, that this should be the correct behavior. It means, that I have to place air objects 1.000 ft. above the altitude they should fly. It worked fine on all planes I flew in DCS/ FC for 10 years. The target is on the same altitude like my radar says, as they were placed in the ME.

 

How want you intercept a target with 1.000 ft. altitude difference? When you think you're level with your target, it's notching you.

[Frederf]

It is not at 15,000'. The tanker of group "Tanker Rus 251-Center" is at exactly 16,000'. It does all sort of crazy climbs and dives in the first 9 minutes or so but stabilizes at 16 kft in the attached mission.

[Azrayen]

I don't believe DCS simulates the standard QNH (1013/29.92/760) ; if done, this should help I guess ; perhaps with the next ATC management system (hope...)?

[Shadow_1stVFW]

I set my altimeter to field altitude to try and get the most realistic setting. I've seen what you're saying about the tankers being at different altitudes than I set. I always assumed they were operating off a QNE as given by the airfield they started from, which would zero their altimeter on deck there and be different from mine. Or some other value of they air started.

 

Sent from my SAMSUNG-SM-G900A using Tapatalk

[jojo]

I don't believe DCS simulates the standard QNH (1013/29.92/760) ; if done, this should help I guess ; perhaps with the next ATC management system (hope...)?

 

1013hPa isn't QNH. It's standard altimeter setting for IFR flights above transition altitude.

However, if your mission is set with default weather, which seems to be standard atmosphere, and your airport is at sea level, your QNH will be 1013hPa.

 

QNE is 1013hPa setting.

 

To be simple, QNH is the pressure that displays your airport altitude when you're on ground.

 

QFE is when you set altimeter to 0 on the ground.

 

The ME altitude should be QNH.

 

IRL you should land on QNH setting.

 

Being an "old" plane, the M-2000C use QFE setting for auto-ILS procedure.

Edited September 4, 2016 by jojo

[VTJS17_Fire]

I don't believe DCS simulates the standard QNH (1013/29.92/760)

Standard QNH (or standard pressure) doesn't have to be simulated, because it must not be a really air pressure. It is only a number which you set above a known altitude (transition level, TRL). With the standard pressure above TRL (in Nevada/ USA FL 180), all aircraft use the same altitude above MSL, which is very important for ATC and other agencies, such as GCI/ AWACS.

 

 

1013hPa isn't QNH. It's standard altimeter setting for IFR flights above transition altitude.

"Above" is (almost) always transition level, in USA flightlevel 180. If you descend, you have to be careful of the transition altitude, which is 17.000 feet, IIRC.

 

Above/ with transition level, you call your altitude as flightlevel (FL), with/ below transition altitude as feet.

[Frederf]

The number the ATC gives you is the QFE value and it should work in the sense that dialing in that value gives about 0 when on the airfield.

 

As for AI they always fly their flight plan as set in true altitude (or AGL) and true velocity (ground speed) and don't participate in any pressure altitude or other behavior based on cockpit instruments.

[VTJS17_Fire]

As for AI they always fly their flight plan as set in true altitude (or AGL)

Only if you set the option (drop down menue) in the ME. Otherwise they use altitude above MSL as standard.

 

It would be impossible to air refuel, if the tanker flies the set altitude with AGL. :D

[nomdeplume]

Pretty sure he meant they either fly true altitude or AGL (if you tick the AGL box). Not that AGL was a synonym for true altitude; I'm 99.999999% confident Frederf knows the difference.

[Azrayen]

Standard QNH (or standard pressure) doesn't have to be simulated, because it must not be a really air pressure. It is only a number which you set above a known altitude (transition level, TRL). With the standard pressure above TRL (in Nevada/ USA FL 180), all aircraft use the same altitude above MSL, which is very important for ATC and other agencies, such as GCI/ AWACS.

Yeah, standard pressure.

 

You point exactly what I was trying to say: All aircraft having the same setting.

That's what we don't have IIRC in DCS today, as:

- AI aircraft use ME altitude ("true" altitude) (if this is always equal to standard pressure altitude, then my point is moot, but the ΔZ we discussed here let me think it's not equal).

- human aircraft use standard (if they think about it) or let their altimeter set at their departing QFE/QNH (most often than not: people who don't fly IRL or on other sims like FS don't have the "setting altimeter at transition" reflex).

BTW I don't know what's the TL in Russia, but here in Europe it's much lower (and variable) than the FL180 of America: around 4-6000ft depending on the location & weather.

[VTJS17_Fire]

For Russia it's hard to get a valid source, because they did use QFE until last year and now adapt the QNH use.

 

In Georgia (Caucasus region), it is FL 070 (IIRC).