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