Developments in Small Craft Radar – Martin Quaintance FRIN
29 November 2009
Radar systems on small marine craft have come a long way in the past few years. In part, this has been related to the developments in networking and display systems, applicable to all the instrumentation on board, but some changes are specific to the radar systems alone.
One of the biggest advances came with the inclusion of signal processing within the scanner, so that cabling to the display was carrying digital data, rather than RF signals. This in itself opened up great flexibility of displays and controls, fitting with the general trend to offer integrated instrument systems.
The combination of chart plotters and radar offered easier interpretation, especially when most of the limitations suffered by the early attempts at overlaying radar images onto charts were solved. Where overlays were not available or desired, split-screen display could offer chart and radar pictures side-by-side.
Another major step was the inclusion of MARPA in the majority of small craft radars. This technology, which is standard and indeed mandatory on large ships, was first introduced as a premium feature on the top-end small-craft sets, but quickly spread down the price range.
Early versions of automatic setup controls were far from perfect, making necessary the manual adjustment of gain, tuning and clutter, except in ideal conditions. This circuitry has advanced rapidly to make manual adjustment a relative rarity, though it is still an important skill for all operators.
Marketing departments love to catch onto buzz-words and apply them to their products, so it should come as no surprise to see the latest radar systems described with the current TV cliché of “High Definition” and the internet’s “Broadband”. Fine words, but how do they differ and what do they actually offer to the user?
The essential difference between the HD tag applied to Raymarine and Garmin radar systems, and Simrad’s Broadband Radar, is that the former use conventional magnetron pulsed beams with advanced processing of the returned signals. Simrad on the other hand, use Frequency Modulated Continuous Wave (FMCW) transmissions in place of the pulse technology. (See video of this antenna at www.navigationuk.com/downloads/fmcw/fmcw.html
As you will be aware, a pulsed radar antenna emits a series of pulses of microwave energy, with a long enough interval between each for the pulse to travel out to the furthest targets in the chosen range and return to the scanner before the next pulse is generated. The time taken for the echo of a pulse to return is directly proportional to the round-trip distance to a particular target and can be used to draw a contact on the display at the right range. Pulse radars require circuitry which can switch the antenna between transmit and receive modes, in sync with the pulse timing.
The latest pulsed systems offer a higher scanning speed (48 r.p.m.), which helps keep the display updated fast enough for today’s high-speed craft, or can use a more conventional scan speed (24 r.p.m), while operating on two different transmission frequencies. Careful processing of these dual frequencies produces sharper images with less noise and clutter.
Continuous Wave radars by contrast, do not transmit pulses, but a continuous wave as the name suggests. There are normally two antennae, one to transmit and one to receive, mounted together, in one housing. Plain CW radar techniques are used in systems such as police radar guns, which are not concerned with range, but only the velocity of the target.
The returns from a static target will be at exactly the same frequency as the transmission, but any moving target will change the returned frequency in relation to its velocity. We are familiar with this frequency change – known as Doppler Shift – from hearing the change in pitch of the sounds from a car, motor bike or train as it passes us. As they approach, they “catch-up” with the sound waves already emitted, shortening the wavelength and increasing the pitch. Similarly, as they move away, the wavelength is “stretched”, resulting in a lower pitch.
The Doppler shift is proportional to the velocity towards or away from the observer. It should be noted that this is the Relative velocity. If the observer is moving, their movement can affect the frequency as well. I say “can affect”, since there will be no frequency shift if both observer and target are moving on parallel courses at the same speed. The transmitted frequency is subtracted from the returned frequency, and the result is proportional to the relative speed.
CW radar is fine if we wish to know speed rather than range, but in our normal marine radar situation, the range is the most important measurement. Enter FMCW. In this type of radar, the continuous wave is not at a fixed frequency. Instead, the carrier frequency is modulated (changed) by a lower-frequency “sawtooth” waveform as shown. The actual frequency of the transmission therefore “sweeps” across a range of frequencies, then returns rapidly to the initial frequency and starts the sweep again.
How does this enable the measurement of range? Look at the diagram showing both transmitted and returned waves. It will be seen that if we measure the time interval between the moment that one particular frequency was emitted, and when that exact same frequency is received, the delay is proportional to the round-trip distance to the target. We have our range measurement!
The strength of an echo is related to the energy transmitted and, when short pulses are used, they must be of high power to carry sufficient energy for a good return. Think of a garden hosepipe filling a paddling pool, compared to filling it from buckets. The longer the interval between bucket-loads, the bigger the buckets must be to match the hosepipe fill-rate. FMCW radar does not require such high transmission power as conventional radar, as its transmission is continuous, like the steady flow of the hosepipe.
Reports from journalists who have observed the demonstrations of FMCW confirm that this technology can give excellent results at relatively short ranges. The advantage is not so clear on longer ranges and we can see some reason for this is we return to the diagrams. What happens if the echo takes longer to return than the duration of one sweep of the modulating sawtooth wave? We would not know whether an exact frequency we receive came from the latest sweep past that frequency in the transmission, or from the equivalent point on a previous transmission. Either we need some extra trick in processing to tell the difference, or we must slow down the sweep rate so that the slowest echo for our chosen range is received before the sweep repeats. A slower sweep will give less precise timing.
For the purposes of this illustration only, suppose the combined limits of accuracy in frequency-stability of the transmission and frequency-measurement of the return add up to 50Hz. Suppose the change of transmission frequency at a fast sweep rate is 2000 Hz per microsecond. The limit of accuracy in timing would be 0.025 microsecond which is equivalent to a limit of range accuracy of 12 feet. If a slower sweep rate changed the transmission frequency by only 500 Hz per microsecond, our 50Hz limit of accuracy would now be 0.1 microsecond or 48 feet.
So FMCW radar appears to offer better resolution at short ranges, but cannot offer that advantage at longer ranges. Both “Broadband” and “HD” radar are new techniques and we are sure to see further refinements in the next few years. All these advances in radar are wasted if the skipper and crew do not know how to set up and correctly interpret the display, so it is crucial that customers are made aware of the importance of training.
The “Simulator” built into most radar sets can only offer a very limited amount of practice, as nothing actually moves. It is fine for learning where the manufacturer has hidden the controls for each function, but cannot offer assistance in learning collision avoidance or coastal navigation. One day radar operation courses are readily available, generally using PC-based simulators to give this vital hands-on experience. Simulated radar sets and vessel controls interact with a virtual world to provide unlimited experience in a wide range of scenarios and weather conditions.
Anyone can learn and practice all the essential skills of small-craft radar operation, using a simulator on their own PC. A single-user version of the Simulators used in most of these courses is available for home study from www.lightmaster.co.uk and is ideal to maintain a dealer’s knowledge, or as an add-on to the sale of any radar system. LightMaster’s Radar TutorPlus CD combines the Simulator with a step-by-step Tutor program to build confidence and explain the essential controls and techniques.
M. Quaintance, F.R.I.N. – Lightmaster Software - www.lightmaster.co.uk
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