NCTR modes

[overscan (PaulMM)]

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[Dilbert]

I would like to express that, from my limited experience with radio signals, I often find publically disseminated "explanations" of NCTR technology to strain the limits of credibility. Specifically, there are descriptions that refer to using a "blade count" from engine compressor faces to tell a MiG-29 from a Su-27, or 1D range profiles. A blade count may vary more greatly with the target speed and throttle setting than with aircraft type, assuming the compressor face is even visible at all, and the 1D range profile will vary with aspect, payload, and even the direction the onboard radar antenna is pointed. Many of these publically available descriptions are based on scientific papers that are currently or were recently being researched, that might not be at a stage where they are already part of a radar system that entered service in 1991. Rather, I wouldn't be too surprised if NCTR turns out to be mainly based on Combat Tree itself, or even if that's what the "CT" in the classified acronym "NCTR" really stands for (e.g. "Next-generation Combat Tree Receiver" or some such).

My uninformed 2c..

 

[overscan (PaulMM)]

F15C032 - NON-COOPERATIVE TARGET RECOGNITION (NCTR) UPDATES - MAX

(OPERATIONAL CAPABILITY)

DESCRIPTION: A software change to the APG-63(V)1 and APG-70 radar to identify approximately 14 target types. Includes improvements to NCTR algorithms to reduce incorrect identification and improve performance. This program requires changes to the radar Operational Flight Programs (OFPs) and the Central Computer OFP in F-15 Suite 6. Common with F-15E.

JUSTIFICATION: Failure to update the aircraft NCTR capability with these 14 target types will limit the pilot/aircrew to visual target identification, decreased survivability, and restricted weapon employment.

PROGRAM ELEMENT: PE 27130F.

ISSUES: NCTR upgrade MAX will be incorporated in Suite 6 OFP software. 164 F-15C/D aircraft will be retrofitted with APG-63(v)1 radars and receive the OFP update.

My take - NCTR is real, but its not infallible.

The 1970s "Musketeer" program did not "count blades" as such, but detected beat frequencies (?) induced by the rotating blade elements in fan and turbine. These presumably depend on (a) number of blades and (b) rotational speed of the engine.

The NIIP Bars radar determines the identity of targets with an (as yet unspecified) algorithm matching "the parameters of the signal reflected from the target". As supplied to India, the ID mode gives only general target class; large target, medium sized target, small target, group target, transport, helicopter, jet. They provided India with the ability to load in the spectral characteristics of individual types, after which the radar would be able to identify specific types, but Russia did not provide any of this data.

 

[overscan (PaulMM)]

IFF systems share the same blades as the UHF omni-comms in many instances and so the _response_ could theoretically be picked up from any direction -around the enemy aircraft-.

I doubt if this applies to the CTree equipped aircraft because they and indeed ALL the 'dedicated interceptor' types (excepting the F-14 I believe) used on-array interrogator dipoles that look like little T-bar handle grips.

This is both an effective and necessary means for isolating the azimuth and elevation returns of the incoming signal for display within the radar scope bearing/slant indications for steering (because ALL aircraft operating in Western Airspace are required to have a reply transponder even if they cannot actively interrogate and blades are usually 360` capable within their local phase tune lobe pattern, which is to say all-round but within a fixed up-down or vice versa, while the multiple [10 or 20] T-bars on the array are rotationally-neutral and can be amplitude compared within the scan pattern of the main radar dish or flat plate).

It should also be understood that there are multiple 'MODES' of IFF/SIF operation and not all of these will give precision data on their own. Some are height+airspeed, some are transponder-coded for range (probably something to do with measuring the carrier backwaves for timing between primary radar return and secondary beacon) some will ID your aircraft with specific series-numeric codes that are relative to the TYPE of civil-mil-commercial etc. aircraft or country you are coming from.

An intercepting aircraft can request (over radio) specific 'Squawk this' changes in the IFF settings and especially with certain coded commands ALL aircraft within a given area will flash-over their primary return if they think they are the ones intended for message receipt. While such can itself be confusing, a refusal (or ignorance) of the squawk authorization change can indicate an aircraft (or operator) unfamiliar with civil ATC rules or in deep NARDO trouble.

Depending on ADIZ location and warstate responses will then 'vary'.

And then there is 'Mode 4' which has encryption options that can ONLY supposedly be single-day read from a qualified (you have the equivalent transponder interrogation code or I don't reply) interrogator.

When you are in 'active' mode it is broadcast-squawk, when you are 'passive' it is selective-transponder, when you cease all replies (a good way to lose one's license if there is no accompanying emergency declaration in controlled airspace) it is called 'strangling the parrot'.

One other thing needs to be mentioned as I believe it is KEY to the way aircraft ID each other.

A transpondering system replies /within the scan interval/ of the interrogator. Thus if you are illuminated by any qualified radar, you amplify the skin return with your beacon response overlay. Indeed, many civil ATC radars have limited 'read' of skin paints without the additive transponder return.

More importantly for military use (where skin paint detection is critical), let's say you have a configuration of front-rear-spanwise 'spaced' antennae.

With modern processing on a monopulse and especially phased array antenna, there is a high likelihood that, to a military grade radar, the aircraft is amplifying not just a single 'blip' of return but the _scattering model_ of that total shape+antenna spacing.

Hence you are not 'just reading the EML address' of someones electronic (hackable) code but the _physical configuration pattern_ of the way that code is being sent, either in and of itself (a little bit of carrier modulation from X a little different from Y etc.) or as a function of the total signature return of the aircraft as a whole.

This is one of the key factors of variance between IFF interrogator mimicking (which goes back at least to 'Perfectos', a British spoofer used to counter intercept Luftwaffe nightfighters in WWII) and what I like to think of as 'true' NCTR or Non Cooperative Target Recognition whereby you are looking at the enemy's response with an eye towards analysis of imbedded data within the signal itself.

The former style of forced detection is basically NO DIFFERENT than sitting in front of your TV and finding that your neighbor starts screaming about his busted tele when you fiddle the remote.

The latter is the action of seeing a target regardless of it's 'intention' to reply to your interrogator.

There is SOME indication that the the F-15 APX-76 interrogator function was capable of such interaction with it's ALQ-128 'EWWS' or Electronic Warfare Warning System to discern elements of faked signals and signal-within-signal beaconing.

One source of mine suggests that this was done to help provide a doppler ranging on the command link of RFCG SAM's (SA-2/3/4 etc.) while another suggests that the reason the antenna groups are so highly separated (nose and tail) is SPECIFICALLY to assist with 'modelling' (in real time) this passive signal scatter effect between what the main array saw and what the fat cylinder of the ALQ-128 did, atop the port vertical tail.

Such a capability would likely provide 'positive deconfliction' between multiples and spoofing tagalongs or FEBA crashers in a NATO Europe horde of targets but it MIGHT NOT work if the IFF was turned off completely and there was NO amplification or secondary scatter of the base transponder return.

We all know how the F-117 retracts it's blade antennas and the F-22 masks or buries it's equivalent so it is even possible that the reemission pattern is not by user choice.

An effort called 'Project Musket' supposed brought full functionality to F-15 NCTR in the mid-late 80's, activating what had previously been labelled 'spare' or 'EWWS' on the throttle but which gained a new name 'NCTR' in late F-15C/D cockpit diagrams.

It is somewhat likely that this system was a bit clumsy in use due to the beam swath limits of the mechanical APG-63V1/70 arrays which is what led to the requirement for a 'dual tier' ID from AWACS and F-15's during Desert Storm and basically disqualified the F-14's from forward use altogether.

It is believed that the way THIS worked was to 'double bounce' the E-3's S-Band (duuuuh, IIRR) signal off the ground using the vertical slotted array quick-steer (like a primitive phased array for instantaneous beam relocation) and the aircraft and then literally 'halve the bearing error' (from the secondary target ground-shadow) as the IFF antenna dipoles swung into alignment a half-scan back on the mushroom rotation.

This allowed the AWACS to use it's Very High Power gear to interrogate the EXACT spatial volume which the onboard computers said was the point where the suspect aircraft was.

If the E-3 was also 'looking at' the F-15 vectoring in when the fighter pilot said he saw targets at X-bearing Y-range and Z-elevation, with an NCTR response of A when he was expecting B, then an experienced radar operator could cross confirm the target IDs as either non-emitting 'bogey' or (perhaps) a scatter-modelled 'bandit' and in turn open up free fire zones for range and azimuth so that F-15 could engage, FQ-BVR, as the (friendly aircraft in the surrounding airspace) situation dictated.

Most 'knowing' people believe that a similar or expanded system is the baseline of the NBILST or Narrow Beam Interleaved Search (and) Track gear on the F-22; whereby a much finer pencil beam (just a few phased array transmit-receive modules) is steered over the established track space and 'swept', very rapidly, over the target to establish the scatter algorithms required return states using a selected phase discontiguancy overlap.

Smaller wingroot antennas then 'stereographically' (like a split lens optical rangefinder) reintegrate the phase patterns to match what is called an E-Pulse model of expected sub-model (airframe) return spikes that remain present when you cancel or 'extinct' the baseline APG-77 waveform.

Since the NBILST arrays replace the exact same location of the AIRST/EOSS optical gear, you can probably safely assume they are effective to AT LEAST 70km of the European PIRATE/OSF/OTIS equivalents.

In any case, THIS is why you see /entire airframes/, from B-52 to UAV, in these great big huge anechoic halls at Rome and elsewhere.

Far from (or at least in addition to) being an 'own systems interference' isolation check for things like EW and comms and attack sensors, they want to KNOW how the airframe reacts to a radar wave impedence from 360` and within a few degrees of cohorizon planing (most scatter model analyzers are 2D limited).

So that, even if we DON'T 'know who goes there' by yelling at them to halt and be recognized, we can at least establish that it sure as hell ain't one of ours.

By the way the airframe itself talks back to us.

Kurt Plummer

Kurt on acig.org

 

[Pit]

Further development of NCTR (or similar aircraft class detection) technology is pursued on new Irbis set, but NIIP has claimed it would be ready after the basic set is done in 2009. Maybe on a similar approach to "Marks or Phases" on N-011M.

Software of N-001V and N-011M is being used on N-035 Irbis, acord to NIIP, up to 20 classifications would be possible when the system is ready plus assorted methods used on Bars.

 

[overscan (PaulMM)]

In the air-superiority mission, the addition of a Programmable Signal Processor (PSP) to the F-15 will provide raid assessment, track-while-scan, improved ECCM, and non-cooperative identification through the dual mode recognizer which analyzes target engine signature

AIAA-81-0932
Tactical Systems Anniversary Celebration
Maj. Gen. CARL H. CATHEY, Jr.
Tactical Air Command,
Langley AFB, VA

 

[Dilbert]

In the air-superiority mission, the addition of a Programmable Signal Processor (PSP) to the F-15 will provide raid assessment, track-while-scan, improved ECCM, and non-cooperative identification through the dual mode recognizer which analyzes target engine signature

Exactly. Whatever "NCTR" really is, it runs on some 16k (?) glorified calculator that was around in 1979, and appears to have changed little since. How complex can it be?

 

[Steve Davies]

NCTR and ALQ-128 EWWS are two separate systems.

NCTR is a technical system that uses compressor and turbine blades to ID the motor(s), and thus derive an educated guess at the the ID of the contact - I am not a scientist, so I won't attempt to explain how it does that! However, I have seen it in action first hand, and I have talked to various pilots about it. It is not infallible, it can be tricked, but it is reasonably accurate. NCTR is not related to Combat Tree.

Project Musketeer in the 1980s specifically used the MiGs of the USAF's 4477th TES, Red Eagles, to help iron-out NCTR's algorithms. Up until then, for example, the MiG-23 would often appear as a DC-10.

The EWWS is an electronic solution derived from Combat Tree.

Hope this helps.

 

[SOC]

NCTR is not a system per se but rather a group of techniques used to identify targets. What you describe is referred to as Jet Engine Modulation or JEM. See here for a simplified technical explanation: http://www.tscm.com/modulate.pdf

 

[Steve Davies]

I beg to differ.

NCTR in the F-15 (for example) is a technical, software-based system that analyses the returns bounced back from fan blades and turbine blades. Granted, in something like the F-22, NCTR may be altogether more sophisticated and involve multiple sensors and techniques, but that has nothing to do with JEM.

JEM, as described by USAF technical manuals, involves: "modulations from moving parts on the target, such as engine turbine blades, which can create higher order doppler returns from front or tail aspect targets which result in detections exhibiting false velocity information". JEM tracks are normally resolved automatically by the radar and data associated with them are discarded. Thus, JEM has nothing to do with EIDing a contact.

 

[lancer21]

I would like to express that, from my limited experience with radio signals, I often find publically disseminated "explanations" of NCTR technology to strain the limits of credibility. Specifically, there are descriptions that refer to using a "blade count" from engine compressor faces to tell a MiG-29 from a Su-27, or 1D range profiles. A blade count may vary more greatly with the target speed and throttle setting than with aircraft type, assuming the compressor face is even visible at all, and the 1D range profile will vary with aspect, payload, and even the direction the onboard radar antenna is pointed. Many of these publically available descriptions are based on scientific papers that are currently or were recently being researched, that might not be at a stage where they are already part of a radar system that entered service in 1991. Rather, I wouldn't be too surprised if NCTR turns out to be mainly based on Combat Tree itself, or even if that's what the "CT" in the classified acronym "NCTR" really stands for (e.g. "Next-generation Combat Tree Receiver" or some such).

My uninformed 2c..

My uninformed 2c is that i believe that aswell...i'm no scientist engineer etc, but counting turbine blades ??? i think( until proven otherwise-and i might be wrong , offcourse) that this turbine counting thing is an urban legend, disinformation , propaganda , you name it ...

I think its something as simple as the folks in the states getting into that RCS , stelath frenzy in the seventies and eighties, they figured out they can built nice models of Migs ,Mirages and so forth and they can test them and measure their radar reflections from different angles , diffrent distances etc and they summarized that info into some nice computer code lines and they put that database into the F-15's radar...

Simples! ;D

 

[BDF]

Some other known techniques are High Range Resolution (or HRR) and ISAR. This website shows some of this in surprising detail:

http://www.ottawa.drdc-rddc.gc.ca/html/rast_309_nctr-eng.html

These techniques benefit from better signal processing, precise beam control, and higher bandwidth radars that are available now in the form of AESAs. There just wasn't enough processing power (or perhaps beam control) in the APG-63/70 during the timeframe the MSIP II Eagles entered service during the late 80s to use these techniques. The newer APG-63(v)1 and 2 may have adequate signal processing but I'm not certain. JEM doesn't really "count" the blades, but looks for a specific "warble" pattern in the return. It can only be accomplished in STT as the beam needs to dwell on the target long enough to observe the modulation. Obviously it's aspect dependent and hence there's a requirement for other sources of ID (such as AWACS) as part of their EID matrix.

BDF