How radar works
The practical application of radar as a navigational device is straightforward for anyone with traditional navigation skills – it’s interpreting the radar picture that’s challenging. Radar sees the world very differently from us. It stands for Radio Detecting And Ranging, which is a pretty good description of what it does: it tells you what’s out there and how far away it is.
In a similar way to an echosounder, radar works by sending out a wave of energy and receiving reflections from objects that it hits. By timing how long it takes for the echo to return, it calculates the distance to the object. It is both a transmitter and receiver. Like us, it can’t talk and listen at the same time, so an internal valve opens and closes to protect the sensitive receiver, making sure it doesn’t yell into its own ears.
Yacht radars operate within the ‘X band’ range of frequencies, which is around 9GHZ with a 3cm wavelength.
This allows the antenna to be relatively small and gives good resolution on the display. Merchant ships usually carry an ‘S-band’ radar too, operating at 3GHZ with a longer wavelength and lower frequency. This is more powerful, has a greater range and is less affected by environmental factors such as sea-state but the antennas are huge and impractical on yachts.
Most modern radars have a nominal range of 24 or 48 miles. However, owing to the curvature of the earth this range is limited by how high they’re mounted. The radar horizon is slightly better than line of sight, and can be calculated using this formula:
TARGET MATERIAL
It isn’t just size and distance that determines how easy it is to detect targets, however: the material of the target is equally important. Steel or aluminium make better targets than GRP and wood. What isn’t so well known is that the shape of the target matters too. Rough and uneven surfaces have a higher chance of reflecting some of the energy back to the antenna as the echo scatters in different directions. Smooth, flat surfaces will send all the radar waves off in the same direction, and not necessarily back to you. Stealth ships with their sleek, slanting hulls are designed to do just this. This means that rugged cliffs make good targets, but low-lying, sloping beaches make poor ones.
Water itself produces a very strong reflection, which is inconvenient given where we’re operating. Both waves and rain will return your radar pulse with the consistency of a welltrained Labrador; it’s just unfortunate that you didn’t want that particular stick back. ‘Seaclutter’ and ‘rain-clutter’ mask the targets lying behind them. There are controls to reduce these effects, but they need to be used with care: they’re powerful but somewhat crude adjustments. Being too heavy handed with them can wipe out much of the detail on screen.
OBSTRUCTIONS
Any obstructions close to the radar create a blind spot. A mast-mounted radar such as Wimsey’s will have a blind spot astern. The beam will bend around the mast, slightly reducing this effect. Practically speaking, this is the best place to have a blind spot as the closing speed of things approaching from astern will be less than that of those approaching from ahead. A narrow blind spot of less than 5° isn’t a huge concern on a sailing yacht as the course often fluctuates enough to pick things up in the wobble.