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The Fundamentals of NT Radar

When New Technology (NT) radar appears on the market it will need a complete revision on how we look at radar specifications. For many years we have got used to thinking in terms of kilowatts of power, kilovolts of high tension supplies and bulky packaging for the radar transceiver.

We generally understand the performance differences that can be expected between a 4 kW and a 25 kW system. Likewise, we see the need for short, medium and long pulses and why we have different pulse repetition intervals for short and long range working. Depending on the particular design of NT radar, many of these long established concepts will change.

For instance, we may be comparing systems advertised with powers of 2, 10 or 100 watts, all with a similar target detection performance to a 25 kW conventional radar. How can this be true?

Since the advent of civil marine radar, which has been with us for around 60 years, radar transmitters have been based on the magnetron.

This is a device that generates electromagnetic waves at microwave frequencies by a special vacuum tube, needing very high voltages and strong magnetic fields.

The relatively low cost magnetrons used in marine radars produce pulses of microwave energy where the frequency and phase of radiation vary in relatively uncontrolled ways throughout the pulse and with random differences from pulse to pulse.

Coherent radars

NT radars transmit signals with controlled frequency and phase characteristics. Technically, they are known as coherent radars. The conventional marine radar is termed a non-coherent radar.

When electromagnetic energy is reflected by a moving target the frequency of the reflected signal is changed according to the precise movement of the target.

If the target is moving away from the transmitter it elongates the time frame of the oscillations, thereby reducing the frequency of the reflected signal. Conversely, if the target is moving towards the radar it increases the frequency.

This effect is known as Doppler, which is a commonly experienced phenomenon for everyday audible sources, such as the sound of a fast car passing a stationery listener - we notice that the sound is lower in frequency when the car recedes compared to the sound when it is approaching.

Because it can compare the received signal with the highly controlled transmitted signal, an NT radar can accurately measure the changes in frequency of the reflected signal.

It can then use fast digital processing techniques to separate targets from sea clutter because the variations in frequency due to the movement of the individual wavelets forming the clutter signal are different from the frequency variations given by targets. Conventional non-coherent radars cannot measure frequency and so they cannot use this mechanism to differentiate targets from clutter.

Other benefits

Coherency has other benefits for radar. A non-coherent radar system can only access half of the energy of the reflected signals. (This is tied up mathematically with its inability to be able to determine frequency).

This means that a coherent radar only needs to transmit half the amount of power compared to a non-coherent system in order to get the same target detection range.

An even more important advantage is that it removes the need to transmit very short pulses, which are clumsy to generate and produce undesirably high interference to communications systems that operate outside of the radar frequency bands.

On a non-coherent radar short pulses are required to split the returns into small 'range cells'. At 50 nanoseconds, which is the shortest typical pulse length used for conventional marine radar, the range cells are about 15 metres in length.

The ability to generate pulses of about this length is a necessary component of a non-coherent radar so that it achieves acceptable performance in adverse sea and precipitation clutter conditions. If shorter pulses were used it would give unacceptable out-of-band interference.

It can be shown mathematically that a short pulse is effectively comprised of a very broad range of frequencies in a precise combination. The mathematics behind this is known as Fourier analysis. It is this range of frequencies, known as the signal bandwidth, that is important in determining the fundamental range cell capability of a radar. In fact there are an infinite number of high bandwidth waveforms that can be used, other than that of a short pulse. Suitable processing of the received reflected waveform converts it into the equivalent signal that would have been produced by a short pulse. This is known as pulse compression, as it typically involves converting a long pulse into a short pulse.

Equivalent pulse lengths less than those available from marine magnetrons can potentially be provided by coherent systems, further improving performance in both sea and precipitation clutter. Proper choice of the transmit signal waveform gives far less out-of-band interference levels compared to that given by transmitting the equivalent short pulse. The potential benefits given by pulse compression and Doppler processing are significant, leading to immediate improvements that may give more than 10 times the visibility of targets in clutter, compared to today's systems. Over the years this could perhaps rise to over 100 times better. It is this potential capability that led IMO, in its recently revised radar performance standards, to encourage the use of coherent radars in the 3 GHz band.

Power

A conventional marine radar operating at 25 kW peak power, using a medium length pulse of 250 nanoseconds at a pulse repetition frequency of 1500 pulses per second, has an average power of only 9 watts. Since a non-coherent radar 'wastes' half its transmitted power, the equivalent range performance of a coherent system could be achieved with an average power of 4.5 watts. Some naval navigational radars already use a particular coherent technology known as FMCW, standing for frequency modulated continuous wave. These radars emit a non-pulsed, frequency varying signal. Since the signal is continuous, peak and mean powers are the same, therefore the transmitter needs a total capability of only 4.5 watts.

In practice, powers as low as 1-2 watts can meet the IMO range requirements. These days, affordable semiconductor devices are readily available with such a power capability. It can be difficult to achieve all the IMO radar performance requirements with an FMCW radar and so a pulsed system, where the pulses are relatively long but modulated in frequency or phase, may provide better solutions. This increases the flexibility of the design, since different pulse lengths can be utilised to optimise the radar performance for different conditions. Such systems would have pulses that have a peak power of several tens or perhaps hundreds of watts. Semiconductor devices with this power capability at 3 GHz are becoming increasingly affordable. For both FMCW and pulsed coherent systems the transmitted waveform has to be accurately constructed. Again, modern technology is beginning to allow this to be done digitally, at an affordable cost.

Reliable operation

Because of the small powers and the low voltages that are needed for semiconductor-based transmitter systems they not only become compact but are capable of very reliable operation. In particular, they use no components with a short operational life. The life expectancy of a magnetron in a conventional system is only 10,000 hours, which is only just over a year of continuous running. Furthermore, the very high voltages needed to drive the magnetron considerably stress the transmitter electronics and make magnetron-based transmitters prone to failure.

NT radar solutions will attempt to make savings on the transmitter technology in exchange for increasingly sophisticated digital processing systems and associated large software development programmes. A price premium can be expected but users should be more than adequately rewarded with performance improvements both in terms of clutter and in reliability.

About Andy Norris

Andy Norris works as a consultant in the international marine navigation sector. He is also Chairman of Technical Committee 80 of the International Electro-technical Commission (IEC), responsible for producing international standards for marine navigation and radio communication equipment in close cooperation with the International Maritime Organization. He holds the visiting position of Special Professor of Navigation Technology at the University of Nottingham, UK.