Radar Reflectors

To maximise their visibility to ships navigating using radar as a primary means of identifying potential hazards, including collision situations, boats can be fitted with a radar reflector.

In fact this is a requirement under the SOLAS V regulations:

SOLAS V 19.2.1.7 requires vessels if less than 150 gross tonnage and if practicable, [shall have] a radar reflector or other means, to enable detection by ships navigating by radar at both 9 and 3 GHz.

Essentially this means that if it is possible to use a radar reflector on your boat you should do so, but you should also be aware of the limitations of some of the radar reflectors currently available to you.

X band and S band

Marine radars operate in two bands, the X band (9410 MHz ± 30 MHz) and the S band (3050 MHz ± 30 MHz). X band radars are heavily affected by sea or rain clutter, whereas S band radar assures large target detection in adverse weather. There are now radar target enhancers (RTE) available for recreational use that operate satisfactorily in both bands.

Some passive radar reflectors also respond to both bands but with reduced performance on S Band.

ISO standards

The ISO test standards, with which radar reflectors must comply, are ISO 8729-2:2009 (Active) and ISO 8729-1:2010 (Passive).   

Passive radar reflectors built to the current standard (few, if any, are available) are often too large to be practically fitted to smaller vessels and it is with this in mind that the MCA have issued their guidance. They consider it to be feasible for vessels of 15m and over to fit radar reflectors that comply with the standard, but advise that vessels of under 15m in length should fit a radar reflector with the greatest echoing area practicable.

With this in mind, the critical factors when selecting and fitting a radar reflector remain to ensure a device with the largest possible radar cross section is carried and that it is mounted in accordance with the manufacturer’s instructions. Generally speaking, the higher a reflector is mounted, the better, although vessel operators should take account of the possible effects of the mass of the reflector on the stability of the vessel.

The current ISO test standards in very basic terms require that a radar reflector has:

• a peak Radar Cross Section (RCS) of at least 10m
• an RCS of at least 2.5m over an azimuth angle of at least 240° when the reflector is vertical (i.e. not healed over)
• an RCS of at least 0.625m over an azimuth angle of at least 240° for angles of heal up to +/-15

Radar reflector trials

Over the past few years there have been several trials carried out by the boating press, on how effectively radar reflectors for recreational boats meet these requirements. All seemingly conclude that whatever their manufacturers might claim, some designs are little better than no reflector at all and even the most popular do not always come up to expectations, this is hardly surprising when the IMO requirement is fully understood. 

Laboratory test results on passive units have shown that average RCS values are often much lower than claimed and many reflectors have large nulls (areas where there are virtually no radar returns at all). The physics of radar reflection is complicated and it is notoriously difficult to conduct trials under real conditions at sea where conditions are less than perfect and reproduce test lab results. Given that the reflection from even the best recreational reflector is also affected by positioning, orientation, and angle of heel, you may be starting to get the picture that you might not always appear on the [radar] picture!

The current ISO standards resulted from the IMO requirement set out in resolution MSC 164(78). This IMO resolution recognises that consistency of response is more effective in raising the probability of radar detection than single high peaks. This is defined as a Stated Performance Level (SPL), which is required to be maintained at up to 10 or 20 degrees (two classes recognising the stability differences of power and sailing vessels) either side of the vertical, and limits weight to 5kg and volume to 0.05 m3.

Advice from the MCA

The Maritime and Coastguard Agency (MCA) has issued a Marine Guidance Note on the carriage and use of Radar Reflectors on small vessels. MGN 349 is a notice to all Owners, Operators, Masters and Skippers of small vessels under 150 tons including Pleasure craft. It can be read in full using the link provided, however in brief the recommendations made in section 4 are as follows:

It is strongly recommended that:

4.1 The requirements of SOLAS Chapter V Regulation 19 are complied with;

4.2 Yachtsmen permanently install not just carry on-board, a radar reflector, or RTE [radar target enhancer] that offers the largest Radar Cross Section (RCS) practicable for their vessel;

4.3 Small craft owners and operators are strongly recommended to fit the best performing radar reflector possible. It is also essential for skippers to be aware that, notwithstanding the type of radar reflector fitted, in certain circumstances their craft may still not be readily visible on ships' radars. They should navigate with caution.

4.4 The following reports published by the Marine Accident Investigation Branch are considered during the process of selecting a radar reflector:

http://www.maib.gov.uk/cms_resources.cfm?file=/radar%20reflectors%20report.pdf
http://www.maib.gov.uk/publications/investigation_reports/2007/ouzo.cfm

Active RTE

The revision of the test standards have resulted in the introduction from two UK based manufacturers of new active products (RTE’s) to the market, there were, previously a mediocre set of products to select from. The QinetiQ report on their "Performance Investigation of Marine Radar Reflectors on the Market" provides a useful insight into the effectiveness of the products they tested.