The New Radar Technology – 'Broadband' Radar Explained

by Bill Johnson, Wiley Nautical on 28 Apr 2009 The landscape of small boat radar has just changed.

Until two months ago, all the radars for the leisure marine market worked in pretty much the same way, but earlier this year a significant innovation became available. (The technology has been developed by Navico who are releasing the technology under the Simrad, Northstar and Lowrance brands)

Named 'Broadband' (no connection with the internet use of the term), these radars operate using continuous transmission of microwaves – as opposed to the traditional pulse transmission. Clearly, anyone considering buying a radar system will want to know how the new technology differs from the old, how it works, and what advantages – and disadvantages – are likely.

How the New Technology Works
Those who have studied radar (and/or who have read Chapter 2 of my book Essential Boat Radar) will know that radars transmit microwaves, and detect returning echoes of those waves from objects in their path. They will also know that the radar needs to calculate the range of those objects.

Up until now, all small boat radars did this in the same way, using a method which dates to the earliest days of radar. What they do is use pulses of microwave radiation rather than a continuous transmission, and measure the time interval between sending out the pulse and receiving its echo from the object. The distance travelled by the pulse and its echo (out to the object and back) can be calculated by the formula:


distance = speed x time

where 'speed' is the speed of light: 3 x 108 metres per second. The range of the object is, of course, half this distance.

Now there is a second method for calculating the range. 'Broadband' – or Frequency Modulated Continuous Wave (FMCW) – radars use a continuous transmission of microwaves. They also listen continuously, for echo returns. But the frequency of the microwave transmission is not constant: it increases at a steady rate, in a 'sawtooth' pattern (see figure 1).

So even though there isn’t a pulse, we still have a method of timing the interval between the transmission of the microwaves and the detection of their echo. Once the waves have left the transmission antenna, their frequency doesn’t change. They continue to the object, reflect off it, and return to the radar’s receiver antenna. By then the radar is transmitting a higher frequency.

The radar looks at the difference between the frequency it is currently transmitting, and the frequency of the echo it is receiving: and knowing the rate at which the transmission frequency is increasing, it can work out the time delay. From then on, the calculation is exactly the same as before. (See Figure 2.)

Note that with FMCW, the transmitter and receiver operate continuously, requiring separate antennae contained in the same dome. Pulse radars switch from transmit to receive, so they can use the same antenna for both functions. The technology for producing the microwaves is also radically different, and this has several consequences which are described below.

What Are the Differences Between Pulse Radar and FMCW / 'Broadband'?
The first thing to note is that most of the radar system, and very nearly all of what you have learned about using radar, is entirely unchanged by the new technology. The picture will still look the same; you still use it in the same way; all the technical facts about beam width, side lobes, multiple echoes etc are the same; and the functionality available from integration with other instruments is identical (see Essential Boat Radar for this information).

The areas of difference that a user needs to know about are identified and discussed below. They are:

• warm-up time and tuning
• range discrimination and target detection
• sea clutter
• transmission characteristics

It’s also fair to note that, being completely new on the market, much is not yet known about how the new systems actually perform. We expect that they will benefit from the inherent advantages of the new technology, but there are also disadvantages to overcome. The unknown factor is how well the new technology has been implemented by the manufacturer.

There are plenty of extremely good radars using the 'proven' pulse (magnetron) technology, and I doubt they will all be swept aside by FMCW in the short term. (Perhaps a reasonable analogy is digital photography: undoubtedly the technology of the future, but it took quite a long time for it to equal or out-perform the well-established chemical film.)

Warm-up Time and Tuning
The device that does the microwave pulses is called a magnetron. One feature of the magnetron is that it takes time to warm up, and another is that its transmission frequency varies a bit.

For the user on a boat (see Chapter 3 of Essential Boat Radar) the warm up period means that when you turn the radar on, you have to wait a couple of minutes before you can use it. Radar systems generally have a low power-consumption 'Standby Mode', so that you can get the picture immediately by selecting 'Transmit' when you need it.

The variation of frequency means that the receiver has to be fine-tuned to the transmit frequency, and there is a Tuning control to do this. (This is not much of a burden to the user with modern systems, because they can perform the tuning function automatically.)

Range Discrimination and Target Detection Performance
a) Short Range
This is the most exciting benefit of FMCW technology.
The range discrimination of pulse radar depends on the pulse length (see page 71 & 72 of Essential Boat Radar for a full explanation of this), whereas there is no theoretical limit to the range discrimination available to a FMCW radar. This leads to pictures that are very much sharper in range with FMCW radars than the equivalent picture with pulse radar, particularly at short range. Added to this, pulse radars have a substantial minimum range below which they can’t detect anything (perhaps 40 – 50 metres) because they have to switch from transmit to receive, but FMCW radars can detect targets a very short distance from the boat.

Another factor is that, at short range, FMCW is able to transmit more energy to illuminate targets, and needs less receiver bandwidth, than pulse radar. This means that target detection is inherently better with FMCW at short range.

So the short range pictures are undoubtedly better with FMCW radars. There is an important point to note however: FMCW offers no intrinsic gain in bearing discrimination, because beam width will have the same relation to antenna size with both types of radar.

b) Long Range
At longer range, FMCW still discriminates range with extraordinary precision compared to pulse radars (particularly when the latter are using longer pulses, which they do for longer ranges). However, the difference isn’t nearly so obvious on the display when you are looking at targets several miles away. In any case, you are not so interested in precise range discrimination of a few metres when the target is a few miles away, is moving (a vessel), or perhaps partly obscured below the horizon (a coastline).

The chances of detecting a target depend on how much energy you can illuminate it with.

Pulse radars have a very high transmission power available (typically 2 or 4 kW for a small boat system) which they use for a very short period of time: the duration of the pulse. FMCW radars,on the other hand, transmit continuously at a much lower power (around 1 or 2W, i.e. about a thousand times less).

At short range, FMCW 'wins' over pulse radar – it can transmit more energy to illuminate targets. Pulse radar has to use very short pulses for range discrimination, and this means less energy per pulse. But at long range, pulse radar can use longer pulses (albeit fewer of them per second), and therefore, with its ve