MAIN HEAD New Technology radar DECK HEAD Is it possible to make better radars for ships, and how well do they work? Dr Andy Norris looks at the situation BODY Every now and again, over the last twenty or thirty years, the early demise of the use of magnetrons (generators of simple microwaves) in marine radar has been incorrectly predicted. There have recently been some real awakenings in the maritime community concerning the possible introduction of new technology (NT) radar. Some new technology ideas include coherent microwaves, which allowing changes to the frequency, as well as the amplitude, to be measured, so a great deal more can be taken from the radar signal. Also pulse compression, doing clever computer processing things with the shape of the wave. This enables more power to be put in each radar pulse, without lengthening the radar pulse (which degrades the resolution of the radar because the radar is spread over a larger distance). Solid state transmitters, which generate the microwave signals with using a different technology to magnetrons. Also phased arrays, which send lots of electronically controlled radar beams at once. In order for something to change in the well-regulated maritime world a number of criteria have to be met: There has to be a need, either driven by shipping companies or by legislators; the need must be able to met at an affordable cost; and the appropriate legislation has to be in place Sometimes the 'need' is created by new technology offering a cost advantage, in which case, providing it is covered by existing legislation, the change is soon incorporated. This is presently not the case for NT radar, which is not covered by existing legislation. SUBHEAD Radar clutter The end user (seafarers) would definitely like to see a better performance of radar in detecting small targets in heavy clutter. There is a huge fear among navigators of running down small craft in poor visual and radar conditions. If such technology existed at an affordable cost it would be a foolhardy shipping company who did not seriously look at investing in such systems, if only to minimise the risk of litigation. After many years of development most modern marine radars are probably working at the pinnacle of their potential in this area. In fact this pinnacle was probably met some time ago and apart from radars being better at their automatic features of clutter reduction, they can probably do no better than the experienced operator could cajole out of radars 20 or more years ago. A technique known as "frequency diversity technique," with radio waves at two different frequencies at once, can extract better clutter performance from non-coherent radar systems (eg combining returns from 3 GHz and 9 GHz radars). Another way to improve clutter performance is to change to using coherent radars, which process radar reflections in frequency as well as amplitude. Some reports have stated that the detection probability of seeing a small target such as a buoy at 3 nautical miles in sea-state 5 is increased from about 20% to greater than 80% by using coherent radar techniques. Such a dramatic improvement will obviously help the end user significantly. An incoherent radar is incapable of receiving and analysing this additional component (frequency) of the signal. It is consequentially throwing away all possibility of doing sophisticated processing of the Doppler spectrum that can help to further the distinction between clutter and targets. An additional benefit of 'going coherent' is that pulse compression techniques can be applied, as discussed below. This avoids the ultra-short high-powered radar pulses given by the standard magnetron in today's systems, which are causing radio interference, which is of increasing concern to the International Telecommunications Union (ITU). Short high-powered pulses create a lot of mutual inference between radars; they prevent co-sharing of the radar frequency bands with other services in an ever-increasingly frequency-hungry world; but what is worse is that they interfere with other services not even in the radar band. This is because there is a lot of band spillover due to the extent and level of the frequency 'sidelobes' that such pulses produce. The expensive filtering that would be needed to reduce these frequency sidelobes to acceptable future limits could significantly eat into the performance of magnetron based radars. Therefore there is a potential legislative 'need' for coherent radars, in addition to the need for improved clutter performance. SUBHEAD Continuous wave techniques The pulse compression that can be applied to a coherent radar can even be extended to the use of continuous wave (CW) techniques, doing clever things with the shape of the radar wave. In fact there are existing naval navigation radars already using CW. One of the advantages of using either CW or very long pulses is that the mean power of the radar becomes equal or comparable to the peak power. It is the mean power that is important in determining the fundamental detection performance of a radar. For example, a 25kW magnetron system on medium range, medium pulse is radiating no more than 10W of mean power. Such power levels are well within the range of solid state technology transmitters at moderate cost. Also, today's digital technology allows a massive amount of digital signal processing to be applied at reasonable cost. This means that the clutter processing that was only affordable for significant radar defence system just a few years ago is now potentially available for the humble marine radar. Despite the potential affordability of coherent radars for marine use manufacturers will still have to invest much money and resources in developing these systems. At present they are reluctant to make this investment because of legislative obstacles. SUBHEAD Legislation IMO's current radar requirement includes the clause: Radars should be able to detect and display signals from radar beacons. Also 9 GHz radars should also be able to detect and display signals from SARTs. This is a killer for NT radar, because it means that ships have to have radar operating at 9 GHz. The radar modulation should be designed to allow the best clutter performance, albeit meeting any future ITU 'spurious emissions' requirements, which should be a natural consequence. Any restriction on the radar modulation to ensure operation with racons (radar beacons) will almost certainly jeopardise the development of effective NT radars. For instance, it is difficult to imagine that any racon could be designed to operate with CW (continuous wave) radars. This existing requirement therefore needs to be examined carefully. An IMO Correspondence Group is currently looking at radar requirements that may embrace both conventional and NT systems. Their work will be discussed in July by IMO's Safety of Navigation Sub-Committee (NAV 50). The racon/SART issue will inevitably be a discussion point. It is a difficult problem. In particular, and for good reason, the user community and 'lighthouse' authorities value radars' independence from satellite position fixing systems. If racons are replaced by AIS transponders a part of this independence is compromised. It has therefore been proposed that the use of NT radar will be initially restricted to the 3 GHz band. Ships having such systems would solely rely on their existing 9 GHz radar to detect racons. (SARTS only radiate at 9 GHz). Since the use of solid state transmitters at the lower band is more affordable and it is also the band that gives the best rain clutter performance the proposal also makes sense from NT radar practicalities. The decision as to what should be done in the future regarding 9 GHz radars must be left, possibly for a number of years, until there is a proper solution to the racon issue. Maybe, once GPS is properly supplemented with GLONASS and/or Galileo and when sophisticated integrated navigation systems can make good use of 'redundant' position information to improve the integrity of the navigation solution, then transfer of all racons to AIS transponder technology would be safe. It is probably best not to initially legislate for better clutter performance from 3 GHz NT radars until more experience is gained. They should therefore be type approved to the same standards as conventional marine radars, with the exception of racon detection. It will be up to manufacturers to create sales for the inevitably more expensive NT radars - prospective purchasers will need a good reason to invest in them. However, proven improved clutter performance would be at least one of the selling points! Future requirements from IMO and ITU might eventually make such radars compulsory.