I read somewhere (it sounds like a tired old cliche) that if you emit you get found or words to that effect. Evading detection is as old a military goal as there is but especially crucial for aircraft. The word stealth should really be ‘low observable’ because that is what it is, an aircraft with passive stealth design, radar absorbent materials and other features can be very hard to detect if it is not transmitting.

Passive detection by infra red is much harder with the aircraft coming head on and can be limited by range, any reduction in detection range reduces the window of opportunity in which a successful defensive shot(s) can be taken.

Using active detection or radar exposes the radar to counter fire.

And so a game of cat and mouse ensues between aircraft designer and air defence system designer, pilot and defence system operator, with each employing a range of technologies and tactics to outfox the other.

Passive detection systems can use the aircraft’s radiated signals but obtaining meaningful targeting information beyond ‘it’s over there somewhere’ is complicated by a number of factors.

An example of a system that attempts to overcome these factors is the VERA-NG from the Czech Company ERA.

VERA-NG uses a network of receivers and measures the time difference of arrival of the targets emissions, IFF, radar, communications and data links for example.

The system comprises a main analysis module and a number of passive receivers. The receivers are usually geographically dispersed and networked into the analysis module. This dispersion allows the system to utilise the time difference of arrival technique to locate and identify transmitting aircraft.

This process is called ‘multilateration’, 2 sites can provide location in two dimensions and the third is used for height, a three location system can therefore be used to locate in 3D by triangulation. This information can then be passed into an integrated air defence system for further identification and attack using other systems.

The point of it is to remain passive and this very difficult to detect and destroy.

Of course, if the aircraft chooses not to transmit at all then this kind of passive ESM system becomes rather less useful!

As can be seen in the image below, the receiver need not be mounted on a ‘military looking’ vehicle and is difficult to distinguish from a cell phone transmitter for example, at least at a distance.

VERA Sensor
VERA Sensor

The system can be mounted on a green military truck as well though.

VERA System
VERA System
VERA System
VERA System
VERA System
VERA System

On the Czech Army website for the VERA it describes how passive detection systems were first developed in Czechoslovakia in the late fifties, they obviously a long track record in this kind of capability. Range is limited by radar horizon so they are often seen mounted on elevating masts, range for this version is quoted as 450km, the ability to simultaneously track 200 targets with a location accuracy of between 50m and 300m.

It is not a replacement for radar but a very effective addition and one of them has been purchased by the US DoD, wonder why!

Air Power Australia has a good overview of other passive locating systems, click here

So, passive locating systems are sophisticated and an effective partner for the more traditional radar systems but they don’t replace radar.

Which brings me on to the subject of Passive Bistatic Radar or PBR.

Bistatic Radar simply means the transmitter and receiver are separated, early radars such as the iconic WWII Chain Home system were bistatic, one site had the transmitter and another the receiver. The image below shows the transmitters on the left and receivers on the right

Chain Home Radar
Chain Home Radar

As technology improved these were eventually bought together on the same equipment and they were consigned to the dustbin. What has bought about a renewed interest is the vulnerability of radars to attack and stealth aircraft. Splitting the transmitter(s) and receiver(s) offers a whole variety of interesting options.

Another even more interesting technique is to take the dedicated transmitters out of the picture altogether and piggy back of the thousands of radio transmitters in any modern nation; FM radio, analogue and digital TV and mobile telephone transmitters.

Instead of transmitting a signal and receiving the return the latest techniques use ‘transmitters of opportunity’

Cutting out the distinctive radar transmitter and relying on electromagnetic noise already in the environment allows an active system to be passive, fiendish eh.

A couple of in depth papers on Passive Bistatic Radar can be viewed here and here.

Which brings me back to the Czech company Era.

They have recently developed a working PBS called the Silent Guard and has been designed primarily for detection of so called ‘non-cooperative flying targets’ Have a read at the link, it is good stuff.

ERA Silent Guard
ERA Silent Guard
ERA Silent Guard
ERA Silent Guard
ERA Silent Guard
ERA Silent Guard

What makes this all the more interesting is that ERA have looked at combining their VERA-NG and Silent Guard systems.

mmmm, are we seeing the beginnings of an integrated counter to ‘stealth’ or merely another step in the detection – counter detection journey that has been going on for years?

I suspect the latter, but technology does not stand still, even if passive detection systems might seem to be limited to detect aircraft ‘somewhere over there’

Who knows, no doubt the people that do, won’t be saying!


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May 30, 2013 11:23 pm


Said it before, but nothing that emits electromagnetic radiation is ‘stealthy’ and that incudes you and me!

Passive Millimetric radar is in in infancy, last time I looked its detection ranges passively stand at 5-7 miles in a ground role in test conditions. Pointed at an uncluttered sky it will get longer.

20 years ago the Norgies had optronic systems with a 45 mile range in anti air role (in perfect conditions) they wired them to their long range anti aircraft missiles.

The Chinese still maintain that 1950’s soviet era VHF radars with modern computing behind them could track the f117 due to turbulence tracking. I believe the RN did that trick with a T42??

As you have said bistatic radar has longer history and just needs the money and the development time put in.

The Chezc systems above look very interesting.

There remains the ‘600 mile an hour seagull problem’.

I did not think 15 years ago ‘stealth’ would confer operational invisibility for long, I am still of that opinion.

Rocket Banana
May 31, 2013 8:25 am

I said some time ago detection systems would advance quicker than the delivery of F35.

And this is just the tip of the technology iceberg.

May 31, 2013 9:10 am

PBR has one very basic problem, commercial transmitters are not sited with air defence in mind- they are instead located to give me a good 4G signal or make sure my digital TV does not cut out. There is also an inherent weakness in not being able to control the signals being transmitted, again commercial transmitters are not designed to track F-117s. PBR was all the rage about a decade ago and interest has since waned with focus instead being on very powerful and very sophisticated dedicated military radars.

VERA is nothing special either, passive detection has been used in Air Defence since the early 60s- on both sides of the iron curtain: See here:

That the US purchased a system means nothing, they have picked up all sorts of Soviet Origin technology over the years; not because it is game changing but because they can and it is nice to know what the opposition has.

Finally, the likea F-22/35 do not rely on stealth at all; they both carry very capable EW systems; there is more than one way to skin a cat. A reduced RCS is useful attribute for combat aircraft- but it is not and has never been one that can be relied upon against a genuinely well equipped and trained opponent. And that is the point of the F-35, it combines low RCS with advanced EW capabilities and outstanding situational awareness: If one is tasked with penetrating defended air space then F-35 is the obvious choice.

May 31, 2013 9:52 am

It is not just siting, the signals are not designed for the detection of flying objects, they are not hardened either electronically or physically (thus incredibly easy to jam compared to say a proper high power frequency hopping air surveillance radar) and as yet PBR has not been made to work in a multitude of geography types (hills and mountains have been particularly frustrating) or at low altitudes. Much more interesting are the new generation of high powered dedicated military radars- and there are a couple of US programmes worth following.

The whole point of Linesman/Mediator was to combine active and passive surveillance- it really is nothing new. The logic of course is simple, you present your opponent with a catch 22; they either don’t jam and they get seen by the radar or they do jam and get seen by the passive system- of course reality is more complicated than that but that’s the basic idea. For precisely this reason the fundamental balance does not change.

S-300’s deterrent potential has yet to be seen.

May 31, 2013 10:23 am

Not at all,

The problem with BPR is that it has actually been found to be of relatively little use. Passive detection is very useful, BPR not so much.

May 31, 2013 12:24 pm

There is in the broadest sense 5 areas of interest on LO aircraft emissions Heat, noise, visual, radio frequency and electro magnetic. As radar is the most used and efficient method of detecting aircraft stealth is often associated with radar detection but its much more than that.

F35 nor really any of the tactical fighter aircraft are optimised against survalliance radars they can be detected and I’m sure have been. These jets are optimised to varying degrees in certain aspects against the k band frequencies why because that is what fire control radars and radar guided missiles generally use, the idea being while you may know the aircrafts there you can’t get the missile to lock onto it. Hopefully this means the engagement zones of ground based missiles or opposition fighters cannot engage you before there within your weapons range. Of course knowing the aircraft is out there is useful if you have excellent c&c and practised tactics to defeat LO aircaft. The package is more important than one v one comparisons for this reason and to an extent is why there are questions about how much of the aircrafts performance, support costs ect should be sacrificed in tactial jets to allow for this advantage.

The survivability of the aircraft can be further helped by using an aircraft das, and its performance ( height, speed, g loading thrust, ect) as well as gd information and tactics. However should you wish to optimise an aircraft against the survalliance radars then you head towards the flying wing shape and the b2/uav planforms were seeing now.

Jeremy M H
May 31, 2013 3:07 pm

Mark really makes the best point here regarding the F-35 and other such aircraft. They are optimized to do certain things well, not to be undetectable in all regimes. The stealth on the F-35 is a force multiplier, not its only reason for existing like it was on the F-117. It multiplies the effectiveness of a tactical fighter by emphasizing on thing over another. Frankly having looked at the F-35 and other contemporary programs I don’t think it is giving up that much in terms of actual employment speed, range or combat agility in exchange for that. Who got the exact mix right is TBD I suppose but the F-35 is not making compromises on the scale of an F-117 for stealth.

I think the mantra that detection systems are advancing much more rapidly than an airframe is on some level true (it hardly cannot be if one really thinks about it) but is really overly simplistic. Computing power doubling rates are what are most often cited but there is nowhere near a linear relationship between computer power and radar detection capabilities. The relationship is much more theoretical and has many points of potential failure and many substantial engineering obstacles that have to be overcome when leaps forward are taken.

There is something to be said for the economics of things shifting in favor of the IADS but an IADS system generally has several critical failure points. More than that the end pieces for the engagement are not really cheap. An S-300 system will run you well over $100 million a pop for the radar and 8 launch vehicles. The economics are not nearly as one sided as they are often presented (generally as the cost of a missile vs the cost of a plane). Long-range radars and command and control add to that cost yet again.

However you choose to look at it things are incredibly expensive for both sides.

May 31, 2013 3:47 pm

S-300 has no deterrence value. The IDF uses Syrian airspace at will. With careful planning, the SAM umbrella gets tomahawked on day one.

The biggest obstacle for air ops is the Russian fleet in the eastern med.

And for detection by wonder weapons; I’ve written enough signal processing code to know, that the main problem is assurance. You can make best guesses analyzing sensor data, you can even come close to a theoretically perfect system, but it all is bonkers when the bad guy emulates false data. Multiple sources could mitigate to a point, but then you have a capable computational sensor grid against a capable computational ECM machine, fed by human nastiness.

Apart from this, being able to detect has nothing to do with getting and keeping a lock on target.

Rocket Banana
May 31, 2013 4:35 pm


“there is nowhere near a linear relationship between computer power and radar detection capabilities”

I’d suggest completely the opposite. The reason we have AESA is because of computer power. It was theorised years ago, but only now made a reality as we waited for computer power to solve the complex partial differential equations quickly enough.

Jeremy M H
May 31, 2013 5:54 pm


That is true but it begs the question. In terms of detection performance how much performance is really gained by an AESA? Sure, there are some raw performance gains over conventional sets of comparable power. But people really want AESA for its ability to do multi-mode things well, its LPI modes, beam-forming ability, self-designating ability and its jamming resistance. It is not an anti-stealth thing or a raw distance of detection issue.

Rocket Banana
May 31, 2013 6:15 pm


Guessing here but an AESA radar should be able to “image” the reflection, which means you don’t just get a “blip”, you get a picture of what it is you’re going to launch your Asters at ;-)

In addition, you forget that “beam shaping” really means “beam steering” which means you’re not just “blipping” as the radar rotates, it “sees” the whole area, this helps “image” the area which is why (I believe) Sampson rotates, it makes the maths and computation a little easier. A little like those apps you get that can “stitch” photos together to make a panoramic picture.

May 31, 2013 7:50 pm

Its not simple. There are no ‘wonder weapons’ and that includes ‘stealth’.

‘Stealth’ (or whatever you want to call it), clearly incurs costs. Perhaps not that much in ‘performance’ but hugely in manufacturing and maintenance costs.

Are those ‘costs’ worth it?

F35 is YEARS behind schedule, F117 has been going for 30 years. Low observability has been postulated and indeed used (iron ball paint on, and shape of SR71), since the 60’s.

Ideas of how to counter the idea have been flying around just as long. I do not suggest there will be a eureka moment and wonder detection system put in service. However lots of very clever people with lots of cash are working on this. I doubt much of it is in the public domain.

Somewhat ironically they will probably crack it just as we give up on manned aircraft!

June 1, 2013 12:31 am

Quick thought experiment:

Courtesy of one of Dr Norman Friedman’s books, here’s a description of what a fairly antiquated signals intelligence system (in an EP-3) could do with radar signals:

BRIGAND (bistatic radar intelligence generation and analysis, new development) is a technique for detecting the signals reflected by objects near a target radar; in effect, it picks up a distorted version of the targets own radar picture. It was developed by VQ-1 technicians in 1960, using a highly directional antenna of an APS-20 to feed a very sensitive radar receiver (an APR-9). The detailed view of the nearby ground clutter makes it possible to locate the target radar more precisely. BRIGAND also reveals nearby objects. For example, if even if only one ship in a group is radiating, back-scatter reveals the others. BRIGAND is also effective against airborne radar and IFF transponders.

Now when I think of stealth I always think of this sort of thing and what smart operators could do with weak radar signals in ye olden days. Imagine what they could do with 50 years worth of computer advances and some big arse antennas on the ground that aren’t limited by the size of the aircraft. I’m pretty sure it’s gonna be quite scary. I’m sure stealth developments have their applications, but I don’t imagine it’s ever going to be too one-sided.

June 1, 2013 3:43 pm

I also think that that the s-300 in Syria would have no military deterrence value. If it has a strategic impact, then I would think it would be a political impact. I think it’s apparent military value is really just a part of the media hype around it.

There are a lot of unknowns about a untested system like the s-300, but even if we ignored that it might not handle modern western ECM technology, it is still hard to see how a few batteries would make a difference, or even survive in Syria.

First from what I have read the deal with Syria is only for one battery? and about 100-150 missiles. It seems unbelievable that they would even bother with a single battery, because it just wouldn’t be able to add much to even the defence of Damascus. The S-300, like the US Patriot system uses a fixed face passive array for it’s targeting radar. In the S-300, the targeting radar covers a 60 degree sector. While it’s on a turntable so can be turned to face a different direction if the threat area changes. It isn’t meant for fast changes. So a battery is set up to cover a 60 degree threat sector. So with a single battery they might get lucky and catch someone unaware of where the battery is, but if the position of the battery is known, then it should be simple to avoid the direction it is covering and deal with the battery.

Maybe the deal is actually for more than one battery, but it seems unlikely that it would be for enough to even provide all around protection to Damascus.

Due to the 60 degree sector and due to the missile’s quite large minimum range, the S-300 is nearly always deployed with shorter range missile systems to protect it from cruise missiles and stand-off weapons. The only reported modern short range system that Syria has is the Pantsir-S1. It was reported that they did get between 36 and 50 of those, but who knows how many they have left, and how operational they are. So there is a good possibility than any S-300 batteries they did get would be without the normal short range proctection they need.

It also seems to be standard Russian practice to deploy a number of decoys with each battery. Both visual (those blow up decoys) and decoy transmitters (to draw anti radiation missiles) plus jammers. So those would add to the cost for Syria. From what I can tell, for them to get a S-300 system that could even attempt to protect Damascus, they would have to be looking at least $1 Billion to $1.5 Billion. Does Syria have that sort of money these days, or would Russia just give it to them?

Even if they get enough systems to provide all around defence, then they would still only have a limited number of missiles. Lets say they got 6 batteries, each battery had 6 launcher vehicles, plus a targeting radar. The batteries shared a long range early warning and acquisition radar, and each battery had 4 Pantsir-S1 vehicles for protection. Each battery would have 24 S-300 missiles ready to launch and up to 48 Pantsir missiles. Give each battery one or two reloads.

How hard, if it all other means of destorying them failed, would it be to play a game of attrition with them? Using cruise missiles and other stand off weapons, and UAVs, and decoys like the miniature air launched decoy. On a side note: I happen to think that weapons like Fire Shadow are useful for acting as decoys (just be cause they are slow, can a enemy afford to ignore them, and if fired on by a S-300 missile, then something like Fire Shadow is certainly cheaper than a S-300 missile). Or GMLRS or ATACMS. If it was Nato taking action then batteries around Damascus, would most likely be in range of ATACMS missiles fired from either Turkey or Cyprus. If it was Israel taking action, then they should be in range of their GMLRS; again even if a lot of GMLRS rockets were shoot down, they would still be cheaper than the S-300 missiles, and possible cheaper than the Pantsir missiles. Although I would have no idea if a Pantsir missile would even be able to hit a GMLR.

So in my opinion, the S-300 would basically have to try to hide and hope to be able to set up a ambush on unsuspecting aircraft. Now SAM ambushes worked to some extent for the Serbians, but that was by using shorter range tactical surface to air systems, that were easier to hide and move around. While the S-300 is mobile, its not really going to be too easy to hide in Syria. Also as far as I know, all current users of the S-300, only seem to deploy it to prepared fixed sites. With the situation in Syria, with ground changing hands between the rebels and government all the time, I would think the number of sites that they could safely deploy to would be limited.

Maybe I’m just saying things that are obvious, or maybe I’m completely wrong, but it just seems that all the media keep reporting the S-300 as some sort of wonder weapon. Now I’m not trying to say it isn’t a effective system (wouldn’t know because it’s untested), but even if it is, it needs a lot of supporting weapons and systems and trained operators, and to be part of a modern integrated air defence system. In Syria it is unlikely to have any of them, and to be very limited in number of batteries and missiles. As with basically all weapons, surface to air missiles need to be part of a combined arms, and proper tactics for defeating the enemy. Just deploying a few systems in a situation like Syria, seems like a recipe for them to fail.

Now where I think a S-300 deployment will change things is in the diplomatic/political. First if Russia does provide Syria with them, then will it provide some Russian personnel to train the Syrians and even help to operate them? If it does, then what will be the Russian response if they got hurt by a Western attack?

Also if Russia does go ahead with providing them, then they are betting a lot on them not being attacked and destroyed. How much damage would it do to Russia’s credibility, and future sales of the S-300, if they were destroyed by Nato/ Israel. I really can’t believe that Russia wants to see the S-300 tested in a situation like there currently is in Syria.

Jeremy M H
June 3, 2013 1:32 pm


Just wanted to say that was a great comment.