Research on RSBN-4N, PRMG-76U(M)/PRMG-4K

[Frederf]

In the interest of further education and development below is a series of images and details about the RSBN and PRMG radio navigation systems as it may pertain to the MiG-21bis. The specifications and details of the PRMG-4K (older) and PRMG-76UM (modern) are similar enough that both are referred to interchangeably.

 

Ground Equipment

RSBN-4N Rho-theta navigational aid with distance transceiver center aerodrome

KRM-4 Localizer array, ~1km beyond runway end

GRM-4 Glide path antenna beside runway touchdown zone

RD-4 Distance transceiver co-located with GRM-4

Associated electrical power and control equipment

 

Airborne Equipment

RSBN-6S Short Range Radio Navigation System

NPP-type navigation instrument

 

Area Navigation and Letdown Function

Direction ±0.25°

Ranging ±200m±0.03% at least 250 km (5 000 m)

Top of letdown 132 km at 10 000 m

Floor of letdown 21 km at 600 m (barometric)

Gradient of letdown ~4.8°

 

Localizer

Range 45 km (>75km typical)

Total Signal Region (not less) ±15° (±18° typical) horz.; 0.85-7° vert.

Instrument Reading Zone 3-6° horz.

Approach Threshold Lateral Error < ±10.5 m (Cat I) ±7.5 m (Cat II)

 

Glide Path

Range 18 km (30 km typical?)

Total Signal Region (not less) ±8° horz

Glide Path Selection 2-4°

Instrument Reading Zone 0.3-1.75° vert.

 

Distance

Range (not less) 45km

Total Signal Region As for Localizer

Zero point offset 0-5km (approach threshold set as zero)

Ranging Error tolerance 250 m

 

Typical Setup

Localizer reading zone (4th dot to 4th dot) as well as distance beyond runway for antenna is adjusted as a compromise between accuracy of approach and ease of use. It depends on the dimensions of the runway, type of aircraft, expected approach patterns, and pilot skill level. The 1st dot to 1st dot span may ideally correspond to the lateral boundaries of the runway at the approach end such that maintaining within the central position circle assures approach to the middle one-third of the runway.

 

Glide path selection depends on aircraft type and terrain, typically between 2°20' and 3°00'. A glide path selection of 2°40' is typical for the MiG-21. Glide path width is a similar compromise between precision and ease of use. The glide path is often 4-5 times narrower than localizer for similar deflections on their respective position pointers.

 

Distance Repeater Offset

Note when the distance transponder aboard the aircraft is switched from RSBN equipment reference to referencing the RD-4 repeater the measured distance will change depending on the physical distance between the RSBN-4N equipment and the RD-4 repeater. The RD-4 signal additionally is internally modified to produce a reading of 0 km when the aircraft is at the runway threshold despite the unit being physically located several hundred meters farther down the runway.

 

Image Notes

The attached images are of a single scene with subsequent figures being insets. All distances and angles are drawn to scale including the 5 km RSBN box pattern. The angular limits of the instrument readable signal zones (4th dot or less) are not shown. They would likely be 3-4° total width for the localizer.

[foxbat155]

Great work Frederf, I really like your attitude to details. Some corrections, all data from RSBN-5 and RSBN-6 manuals.

 

Airborne Equipment

RSBN-6S Short Range Radio Navigation System

NPP-type navigation instrument

 

All Soviet short range radio navigation sets working with RSBN-4: RSBN-5, RSBN-36, RSBN-6S, A-312 etc.

 

Top of letdown 132 km at 10 000 m

 

Depend from aircraft type, for L-39 8000 m AGL, for most others 10000m AGL.

 

Floor of letdown 21 km at 600 m (barometric

 

600 m above antenna level ==> AGL.

 

Localizer

Range 45 km (>75km typical)

 

RSBN-4 range:

at 200 m AGL (flat terrain )-no less 40 km,

at 1000 m AGL - 120-140 km,

at 5000 m AGL - 240 km,

at 10000 m AGL - 360 km,

 

 

Instrument Reading Zone 3-6° horz.

 

Reading zone is 4-6 deg. each side ( total 8-12 deg. ). This 2 deg. difference is beacause of isntruments azimuth accuracy +-2 deg. .

 

Glide Path

Range 18 km (30 km typical?)

Total Signal Region (not less) ±8° horz

Glide Path Selection 2-4°

Instrument Reading Zone 0.3-1.75° vert.

 

Distance

Range (not less) 45km

Total Signal Region As for Localizer

Zero point offset 0-5km (approach threshold set as zero)

Ranging Error tolerance 250 m

 

Not less 18 km at 400 AGL, in most conditions 21-23 km.

 

PRMG-4 range data:

-azimuth channel distance:

at 300 m AGL - 20 km,

at 1000 m AGL - 80 km,

at 10000 m AGL - 180 km,

-landing path channel:

at 300 m AGL - 20 km,

at 1000 m AGL - 80 km,

 

 

RSBN-5/RSBN-6 system typical accuracy ( Navigation/Letdown mode ):

Autonomous mode ( without signal from RSBN-4 ):

distance error - no more 0,8 km/min at speed 800 km/h,

azimuth error - no more 0,5 deg/min at speed 800 km/h,

 

Radio-correction mode ( with signal from RSBN-4 ):

distance error - no more 200 m at speed 800 km/h,

azimuth error - no more 0,25 deg total at speed 800 km/h ( but instument indications accuracy 2 deg. ).

 

Whole system block scheme.

 

RSBN-4/PRMG-4 dislocation at the airfield.

[Frederf]

PRMG-2 1955

PRMG-4KM 1974

PRMG-5 (PRMG-76U) 1981

 

Very nice.

 

The 600m floor is above aerodrome level but still barometric, the manual takes great lengths to remind the pilot to be in AIR switch setting (not GROUND) and that the correct pressure setting is set in the avionics bay (DW-30?).

 

Interesting that the L-39 is a different top of letdown. It must be that airborne equipment is calculating top, bottom based on its own distance rules based on DME from station. Some aircraft may have different distances

 

Ah, you are correcting my PRMG Localizer info with ranges for RSBN but those are more detailed in any case. I think your full deflection and azimuth errors are also for RSBN. Surely PRMG Localizer cannot have +-2° accuracy instrument uncertainty; this must be RSBN bearing pointer maximum error.

 

I read for PRMG-4KM "range width adjustment for exchange rate sector is 4-6°" this is pleasantly comparable with Western ILS which has normal full deflection of +-2.5°. The sector width is adjustable for unusual runway geometry or equipment placement I think.

 

I found a layout for Samoletnaya airport and the airplane pictured is Tu-16/Tu-104 and the region II for full deflection is listed 2,7-3,6° total width. The GP is set 2,3° 0,6-0,8° full height (+-0,3-0,4).

 

It's good to have more data points for LLZ and GP signal at various distances and heights because that's one of the major limitations of the current implementation. As DCS L-39 and real world ILS experience shows the practical range of these signals is often several times what they are strictly required to do.

[foxbat155]

The 600m floor is above aerodrome level but still barometric, the manual takes great lengths to remind the pilot to be in AIR switch setting (not GROUND) and that the correct pressure setting is set in the avionics bay (DW-30?).

 

On DW-30 must be set runway air pressure mainly for appropriate glide path accuracy. Baro or Radalt will be use depend from pilot's preferences and approach terrain conditions. Generally this is secondary issue because pilot's attention is focused on glide path.

 

 

Interesting that the L-39 is a different top of letdown.

 

Yep it's bit strange, but I think this maybe have some connection with aircraft's flight properties i.e. is more stable at 8000 m in comparision to 10000 m level.

 

you are correcting my PRMG Localizer info with ranges for RSBN

 

Yes, sorry small mistake. KRM-4 range at 200 m is 40 km, at 1000 m 80 km.

 

Surely PRMG Localizer cannot have +-2° accuracy

 

Yes, KRM-4 antenna system itself have 0,25 deg accuracy, but from pilot's point of view ( NPP ) this is -+2 deg because of indication accuracy.

 

I read for PRMG-4KM "range width adjustment for exchange rate sector is 4-6°"

 

My manual ( for Su-20 aircraft ) does not mention about that.

 

I found a layout for Samoletnaya airport and the airplane pictured is Tu-16/Tu-104

 

I think those are data for different system. Soviet civilian aircrafts have used different landing system - SP-50. I have manual somewhere , but honestly I never read her.

[Frederf]

Of course for estimated line when descending from 1000 or 2000 m on LLZ/GP bottom the floor of letdown pattern is of no concern. For 600m fly over to RSBN box pattern maybe it matters more. It's just a fun detail and is also the same altitude source that slant range unit uses when dive bombing without radar.

 

SPS-50 uses KRM-2, this is definitely KRM-4 so probably RSBN-2 and/or PRMG-4KM or newer. I wonder what systems ED has modeled for the 4 systems built into the terrain.

 

I believe bearing pointer +-2° error but I still have trouble believing that localizer pointer on NPP center can have error equal to 50% deflection (to second dot) for a 4° total wide LLZ signal. A 2° bias to one side would be 45m at 1200m (inner beacon, 60m H). It's possible I guess and if that is maximum acceptable error it can be much better on a regular day.

 

Airplane manual would not mention adjustment of ground equipment I think because it is of no concern to the air crew. All air crew need to know is how wide in meters is signal at threshold, say 1-dot left to 1-dot right, if even that much.

[foxbat155]

I believe bearing pointer +-2° error but I still have trouble believing that localizer pointer on NPP center can have error equal to 50% deflection (to second dot) for a 4° total wide LLZ signal. A 2° bias to one side would be 45m at 1200m (inner beacon, 60m H). It's possible I guess and if that is maximum acceptable error it can be much better on a regular day.

 

In manual is written literally that each fourth dot ( in NPP center left and right side ) means 4 up to 6 deg. Because KRM-4 accuracy is 0,25 deg, this error is on NPP side and I think this is not typical error rather maximal error.

 

On your drawings Letdown zone center point not should be 21 km before runway, instead in middle of the runway?.

[Frederf]

Ah, my guess is that because the installation of the ground equipment can vary so what is full deflection for different airports can be different. E.g. Batumi is set up with 6° exchange rate width and Kyrmsk is 4°.

 

The center of the circle is supposed to be centered on the RSBN-4N equipment which is near midfield. I might not have made the diagram perfect but that was the intent.

[foxbat155]

So this means that Letdown mode is always finish over RSBN-4N antenna?. Pilot always must do RSBN box ( or some half box ) before he will start landing procedure, and is impossible perform PRMG landing straight from route?. I have to dig deeper about this.

[Frederf]

I understand letdown is an RSBN-4 function, not a PRMG function. For example if airfield has RSBN and 2 ARC (but no PRMG) then you can letdown near airfield on RSBN and fly 2 ARC approach. In letdown both RSBN azimuth correct and distance correct lights are shining.

 

But it is possible to letdown close to airport and then switch to PRMG reference before landing without the box. Bottom of letdown shape is 600m at ~20km. Top of glide path is ~13km at 600m so there is 7 km level so about 90s level flight to transition.

[foxbat155]

You right, I forgot that letdown cone is cut at bottom, and have flat 21 km circle around runway. Cheers.