Just ARC is on, but the distance is counting...Hm?

[loscsaba86]

Hello Community

I have the newest beta and if I am flying with the MiG-21Bis and if I have turned on just the ARC (not the RSBN) the distance is counting. The needle does not show the direction (which is correct), but the distance left to the airport is counting. If I am correct, the ARC should not show the distance, just the RSBN.

And if I turn on the RSBN, the needle moves also, everything works well and the distance continues to count as before, this is fine.

I just wanted the report this strange behaviour.

Best regards
Csaba

[Dr_Arrow]

If you are just operating the switch ARC/RSBN then it is the correct behavior. This switch only selects what is present on the HSI if it is ARC signal or RSBN signal. The distance counter is independent on it and is connected to RSBN, so this means it always shows the distance to the currently selected RSBN navigation channel as long as you have valid signal and unless you switch the RSBN completely off by its own circuit breaker switch on the right-side panel. 

[Hiromachi]

Distance counter works for both ARK and RSBN. Distance counter was first introduced in MiG-21PFM (and retrofitted to some PFs) which is way way before RSBN. Obviously for ARK the principle of operation differs but tldr is that this is correct.

[Frederf]

In reality RSBN works without signal as a dead reckoning position keeper. Module doesn't quite recreate it. For a better simulation of this system look to L-39.

[Torbernite]

It's surely not a non-signal dead reckoning method like doppler navigation on Mi-8. It's functionally a red style TACAN. Ranging and azimuth measurement are both in pulse style.

If you only trust blue docs, then details in ADA044697/ADA044698. Azimuth in page 16, 20 and range in page 24.

Azimuth measurement also discussed in https://doi.org/10.1002/9781119163060.ch4

[Frederf]

RSBN is a non-signal dead reckoning system similar to Doppler. The inputs are true airspeed, true heading, and pitch. You can fly a whole mission by RSBN reference without external signal.

[Torbernite]

I see your firm but you are just repeating your former words and give no evidence or disproval. Could you give some more information?

[Frederf]

Here is section from pilot's operating instructions describing RSBN use both with and without signal. The DCS MiG-21bis does not operate as it does in reality. It will "coast" during signal dropouts but the operation of the spring-loaded azimuth and distance switches (for manually setting these values) does not function as it does in the real airplane. The DCS L-39 has similar RSBN equipment which does have much better modeled equipment that can be used with occasional or no signal from ground stations.

I have highlighted appropriate sections of the MiG-21bis and L-39 flight manuals.

I try to find L-39 instructional video that demonstrates no-signal navigation but I haven't seen one. It does have the ability but it is not a popular topic. I have done it in the simulator successfully. The DCS MiG-21bis module does not simulate the ability so this knowledge is academic to the DCS user.

MiG-21bis PFOI Section Six Air Navigation.pdf L-39 RSBN Section.pdf

[Torbernite]

Nice resources! So the system is actually capable in both radio and DR condition. I have only learned the beacon and signal design and know little about the onboard equipment.

Now I just wonder how much those two modes are used, as a TACAN with DR capability as backup in long range flight beyond radio coverage, or as a DR equipment with automatic updating function through beacons. Considering the intensity of beacon distribution (as the L-39 module provides) I'm afraid that it's the latter condition.

Edited April 19, 2022 by Torbernite

[portman]

vor 5 Stunden schrieb Torbernite:

I have only learned the beacon and signal design and know little about the onboard equipment.

 

Then why are you questioning the knowledge of others? Not everyone here claims things without appropriate background knowledge. Why does one have to bring evidence for everything when he simply knows a lot about Soviet aircraft?

[Torbernite]

43分钟前，portman说：

Then why are you questioning the knowledge of others? Not everyone here claims things without appropriate background knowledge. Why does one have to bring evidence for everything when he simply knows a lot about Soviet aircraft?

Frederf explained the system quite clearly in his third post, but his former two posts are misleading in some way suggesting that it should be a DR only system with no signals involved. My opinion is that at least RSBN is not a DR only system and I have given my information sources to show my opinion and he should give his if he means that way.

I admire Frederf for his patience and preciseness and apologize if offended him, but not your words. It's not the way of finding answers.

[Frederf]

"Should" is an interesting concept. It can be used and there is a whole chapter in the manual using it as such, at least while doing ARK navigation. It has the qualitative technical ability. Is it a good idea or it it practical? I dunno. RSBN is a DR-only system in technical capability. Does RSBN have DR-only capability operationally? Not sure. If you have the L-39 module you can test it out. It worked good enough for me in my test flight to get me within homing range of the destination outer beacon even at max range flight with no midcourse updates or ground visual reference.

The A-4E has a similar DR system. Granted it does have an optional Doppler system and the ability to set in a wind adjust component but that's close to apples to apples and that system is designed for DR flight for hundreds of miles.

I'm curious how the counter works with the ARK system like maybe the azimuth component gets bypassed so it assumes all motion is radial inward. The instruction in the manual for using the distance counter with ARK don't mention adjusting the azimuth component.

 

[Torbernite]

9小时前，Frederf说：

 

I'm curious how the counter works with the ARK system like maybe the azimuth component gets bypassed so it assumes all motion is radial inward. The instruction in the manual for using the distance counter with ARK don't mention adjusting the azimuth component.

 

I checked your MiG21 manual and found it lying in my disk for a long time. Thanks for your reminder and now I'm reading it carefully. Here is my understanding to that chapter:

 

When used with only ARK system (ADF-NDB, 197 and following paragraphs in the manual), it seems no azimuth but only direction considered, because ARK or NDB gives no azimuth but only direction to the station, unlike the RSBN station, which gives both. It doesn't tell us where "we" are (what the RSBN does), but only where "it" is. The ARK distance counter just believes the distance you set to it as initial value and does dead reckoning with that. Knowing TAS, heading and pitch is enough.

For example let's tune an NDB beacon in 30° from us and set the distance counter to 50km. Now the system only knows there's an NDB beacon in 30° direction from the aircraft, 50km distance. The 30° is not azimuth but just a direction, relative to the nose heading instead of true north. In other words, the navigation system knows the beacon is in 30° right of our nose and we are flying 30° left of the beacon, but not our azimuth from the beacon.

Then if we fly at 500km/h, the system knows at this point it should show the distance for us at a decreasing rate of sin30°*500km/h.

It don't need to know where is the true north to do that. No matter we are flying towards north and the beacon is in our 30° azimuth, or we are flying towards east and the beacon is in our 120° azimuth, it's in our 30° direction 50km anyway and no azimuth actually needed, the calculation is still correct. Under this principle you can see such a system fixes with the manual description. It's not a normal pure dead reckoning like INS or doppler in some way in this mode, for the heading or direction is measured from the ARK signal. But the signal contains no information and updates no position, so it could still be called a dead reckoning.

The manual says on leaving the coverage zone of a homing beacon the readings of the distance counter may become wrong. I think the system may assumes all motion in radial as you said, or it may do dead reckoning based on last known position which seems to be less possible, for the ARK is a more primitive system than RSBN, the direction is fully measured from the antenna and not likely to be continuously calculated after signal is out. The instrument just combines it with the heading indicator (the movable scale controlled by a heading gyro) to show the azimuth, but it's hard to infer that the GMK and ARK have more integration under the panel. If so, the dead reckoning would be possible but less accurate than RSBN dead reckoning for the ARK (ADF-NDB) is initially less accurate (degrees level). Very likely it's not designed to be used in this way.

 

Only the RSBN station, which works as a VOR+DME or TACAN, could tell us where "we" are. When used with RSBN, we can not only know where the station is from our view, the signal from the station would also tell us we are 195°, 50km from the station. This 195° is azimuth and means we are 15° from the south seeing from the station, and have nothing to do with our heading.

When signal lost, the RSBN system would go into a true dead reckoning mode. It could calculate the relative motion to the station. For example, if we are at 195°, 50km from the station, flying towards 300°, in other words, the station is in 15° azimuth and 75° direction of us, and the signal is blocked by terrain. Then the system could calculate our position in next second with TAS, heading and pitch. Even if we turn, the GMK would tell RSBN the heading is changed and give it new heading information to continue the calculation. Due to the accurate measurement of RSBN (±0.02° theoretically and ±0.25° in practice for azimuth, according to my former resources), this dead reckoning is more reliable than former imagine. 

The RSBN and ARK seem to do different calculation, just happen to use the same counter to show the results.

Edited April 20, 2022 by Torbernite

[some1]

Long story short, with ADF the distance counter should work a bit like a trip odometer, counting the kilometers up or down from the number initially dialled in the counter.

It doesn't know the real distance to the beacon. This is not a radio DME. 

Edited April 20, 2022 by some1

[Torbernite]

Totally correct. Only difference between the ARK distance counter and odometer is that it considers the angle between heading and NDB direction to produce a correct radial component of velocity.

And the radial velocity update has a little lag when direction changes on DCS L-39. As a result, this system is relatively accurate when flying straight inwards or outwards, or far from the beacon (beacon direction changes slowly), and the error becomes more significant near the beacon.

[Dragon1-1]

Would love to see RSBN being corrected as part of the next update. L-39 is an ED module, so presumably the system can be implemented in a similar way in the MiG, based on the functionality added for the L-39.

As for RSBN being a DR system, I don't know how it is in the MiG, but the Tu-154 flies primarily by a Doppler-based DR system which it can update from RSBN beacons on command. It's a manual procedure that's incredibly easy to mess up and hard to correct if you do, but you can basically fly an entire flightplan by DR, and as long as there's an RSBN on the way somewhere, you won't even deviate that much, though it's obviously no GPS. Useful in Siberia, where NDBs are few, VORs are nonexistent and even landmarks can be hard to come by.