Performance & Technical Description of the Blipper Optimised Phase Modulating Helical Array

The performance and behaviour of a radar detection reflector in the marine environment is technically complex. The following text is an attempt to explain the issues concerned in plain English.  

The first thing to understand is the difference between a radar reflector and a radar detection reflector. Radar reflectors come in many forms; Single corner reflectors, Spheres, Plates and octahedral. Their primary function is to calibrate radar sets and systems. They have also been used for approach guidance. They tend to have performance in a single plane and are non-uniform in azimuth.

A radar detection reflector is specifically designed for the marine environment and is optimised to elicit a response from any illuminating radar under all operating conditions. To do this the response from the reflector must appear as a real target to the receiving radar, or it will be considered spurious and rejected by the receiver's discriminating circuitry and software. Rejection will occur particularly in poor conditions, when detection is most important. A radar detection reflector has a uniform capability both in roll and yaw.

Clearly a radar reflector is not best suited to the role of detection. However some manufacturers have grouped several reflectors together in one housing and made passable attempts at detection devices.  

Performance relates to the function of a radar detection reflector's capacity to increase the probability of being detected by the transmitting/receiving radar of a vessel at sea. The higher the probability of detection, the better the reflectors performance. Unfortunately there is no simple test that can establish the value of detectability. This is due to the large number of variables in the real world environment, (multipath returns, vessel dynamics, illuminating radar acceptance threshold and weather conditions etc), all factors that cannot be replicated on a test range.

As a responsible manufacturer of radar safety equipment Firdell fully supports official testing. However these tests will only establish the Radar Cross Section (RCS) at very close range and at specific angles of heel. There is no official requirement, and therefore no attempt is made, to measure the detail performance over the complete operating range. The official reason for this is that it would take too much time and effort to provide the data. The argument is that this simplistic test provides uniformity in testing a range of different products. Unfortunately this approach disguises some poor performers and discriminates against some excellent devices. The official test results should therefore be treated with caution when selecting a radar detection reflector.

There is a trade off between RCS and installed reflector size. A big RCS is desirable but will invariably require a large reflector. This can lead to installation problems on smaller vessels.

However, once an RCS is large enough to exceed the detection threshold of the illuminating radar’s discrimination circuitry and software, increasing size has little beneficial effect. This sets the minimum practical reflector size.

Bigger, however, can be of benefit when operating in crowded areas and when weather conditions are bad. There is then clearly a trade off between size, performance and operating conditions. Firdell offers a range of devices that cover most boat types and usage patterns.

Unfortunately a large measured peak RCS does not necessarily lead to a higher probability of detection as other conflicting issues may undermine the apparent performance. However, Firdell’s SYNTAR computer program can simulate the effect of real world variables allowing reflector designs to be evaluated and refined. Firdell has, over a 30-year period, evaluated many configurations of radar reflectors for many applications both civil and military. Firdell currently holds 28 related patents.

Of the multitude of configurations evaluated the Optimised Phase Modulating Helical Array has been shown to be significantly better at improving the probability of real world radar detection. To be detected it is necessary to make the reflected signal acceptable to the illuminating radar (a target representative return). The optimised phase modulating helical array has a number of unique properties. The most important is that it replicates the pattern of reflection of a real target. Real targets are made up of many small reflecting centres and are therefore spiky in character. The spikes are of similar magnitude and are quite densely packed.

The helical array has the same characteristic and is therefore not discriminated against by radar sets rejection circuitry and software. This single characteristic makes the Firdell Radar Detection Reflectors real world performance hard to beat, as  non-characterised returns run the risk of being rejected as spurious. However the helical array exhibits further important detection advantages.

The large number of reflection centres provides near uniform performance through 360° of azimuth and up to at least 20° of heel. The dynamic relationship between two vessels at sea means that an illuminating radar beam will be painting a different part of the reflector as the relative position of the vessels change. The helical array provides uniformity of return during all relative vessel movements. This uniformity replicates the behaviour of real targets again preventing rejection by the illuminating radar..

The large number of reflecting centres in the Helical Array make it less susceptible to the effects of Multi-Path performance degradation. This arises from part of the incoming radar energy being reflected off the sea surface. The incoming reflected signal interferes with the signal that has arrived directly. This interference alters the returning signal, from complete cancellation at one end of the spectrum through to doubling the return signal strength at the other. Due to vessel dynamics this phenomenon is always present. In reflectors with a small number of reflecting centres the returning reflection can be completely cancelled for long periods. With a large number of reflecting centres the period of cancellation is greatly reduced resulting in a high level of detectability.

It should be noted that one of the elements of stealth technology (to be invisible to radar) is to reduce the number of reflecting centres to the minimum. Firdell’s Blipper range of Detection Reflectors is the exact opposite, with reflection centres scientifically optimised to be as conspicuous to radar as possible.  
Measured Performance Visualisation Of Firdell Blipper Range

The fundamental requirement of a radar detection reflector is that it must have a uniform performance at all attitudes of roll and yaw. Uniformity means no gaps. To ensure absence of gaps Firdells reflectors are performance measured at increments of less than 1-degree in both roll and yaw (heel and azimuth). The measurements result in more than 60 polar diagrams one for each increment of heel. Clearly this amount of data in this format cannot be interpreted very quickly. Firdell has adopted the standard defence performance visualisation format, Target Pattern Map, to enable the large amount of data to be presented in an easy to interpret manner. 

For an explanation of Target Pattern Maps go to our Target Pattern Maps—A Description & Explanation page on this site.

Blipper 210-7

This shows the TPM of the Blipper 210-7 out to ±20º of heel. 

The choice of colours is set to show the region of performance most relevant to the detection of this target. There are of course many peaks above 3 sq.m (the maximum displayed here) that are not shown. These peaks in this case above 10sq.m are unimportant in the context of detection because they are limited in area.

Blipper 300- 5

This shows the TPM of the Blipper 300-5 out to ±20º of heel. 

The choice of colours is set to show the region of performance most relevant to the detection of this target.  There are of course many peaks above 8 sq.m (the maximum displayed here) that are not shown but these are unimportant in the context of detection because they are limited in area.


Blipper P-175

This shows the TPM of the Blipper P-175 out to ±30º of heel. 

The choice of colours is set to show the region of performance most relevant to the detection of this target. There are of course many peaks above 3 sq.m (the maximum displayed here) that are not shown but these are unimportant in the context of detection because they are limited in area.

















Firdell Radar Reflectors Ltd. - Shirebrook House - Buxhall - Stowmarket - Suffolk - IP14 3DQ - United Kingdom