Radar
Navigation
Radar sends microwave pulses from a rotating antenna and shows echoes from boats, land, buoys and rain around you, day or night and in fog. It sees plenty that AIS can't, but small fiberglass and wooden boats can be weak targets, and it takes practice to read well.
How it works
The antenna, in a dome or an open bar, turns while sending pulses and timing their echoes, giving range and bearing to each target. Range is limited by the radar horizon, so a higher antenna or taller target is seen farther off. Gain sets how sensitive the receiver is; sea clutter hides echoes from nearby waves; rain clutter hides rain. Too much of any of them also hides real targets. Many older radars use a magnetron, a transmitter tube that wears out.
Using it
- Practise in clear weather, comparing the screen with what you can see, so you learn how coastlines, buoys and boats look on radar. Bowditch calls this experience invaluable.
- Set gain so a light speckle of background noise just appears, then back it off until the speckle disappears and real targets show steadily, as one maker's manual describes.
- Use sea and rain clutter sparingly: turning them up clears clutter near the boat but can remove nearby targets too. Shortening the range also helps in rain.
- Switch between short and long ranges; the Navigation Rules expect long-range scanning for early warning as well as close watch.
- Set a guard zone so the radar alarms when anything enters an area around the boat.
- MARPA lets you tag targets and see their course, speed, closest approach and time to it. One maker says it needs a heading sensor and a GPS fix.
- Wood and fiberglass reflect radar poorly. A radar reflector mounted as high as practical helps ships see you, and NOAA's Coast Pilot asks disabled boats being searched for to hoist one as high as possible.
- Keep people away from the antenna while it's transmitting; one maker warns the radio energy can harm eyes at close range.
- Licence (FCC, 47 CFR 80.13(c)): radar on a US-only recreational boat is licensed by rule; no individual licence is needed.
- Navigation Rule 7(b) (Inland and International, USCG): radar that is fitted and working must be used properly, including long-range scanning and systematic observation of detected objects.
- Rule 6(b): a vessel with working radar must weigh the radar's limits, sea state, interference and the chance of missing small craft when deciding a safe speed.
- Rule 7(c) forbids assumptions based on scanty information, especially radar information. Rule 19(d) says what to do in restricted visibility when you detect a vessel by radar alone.
Looking after it
- Every 3-6 months, one maker advises checking the antenna or dome's mounting bolts for corrosion and tightness and the dome for cracks, and cleaning it with a cloth dampened with fresh water (no household cleaners).
- Every 6-12 months, check cable connectors for tightness and corrosion.
- Clean the display gently with tissue and an LCD cleaner; no solvents.
- Keep an eye on the transmit-hours counter if yours has one. On a magnetron radar, fading long-range performance is the maker's sign that the magnetron needs replacing.
Troubleshooting
Radar shows nothing: no echoes and no background noise
- Check it has finished warming up and is transmitting, not sitting in standby. One maker's radar counts down about a minute before it can transmit.
- Turn gain up and sea and rain clutter down; too much filtering can blank the picture.
- Check the fuse and battery voltage.
- If the screen says 'no heading pulse' or 'no bearing pulse', check the antenna cable connection.
- Check the signal cable for damage, then call a technician.
Weak echoes or can't see distant targets
- Check gain and range settings.
- Clean and inspect the dome or antenna.
- On a magnetron radar, steadily fading long-range performance may mean a worn magnetron.
Odd echoes: arcs around a target, repeated echoes, or blank sectors
- Arcs either side of a strong target are side-lobe echoes; lower the gain or the range.
- Repeated echoes at double or triple a target's range come from a strong target close by.
- Blank sectors are usually shadows from masts, stacks or other structure near the antenna.
MARPA won't start
- Check a heading sensor is connected and sending heading (NMEA 0183 HDG/HDM or NMEA 2000 PGN 127250, in one maker's case).
- Check the GPS has a fix.
Buying a boat that has one
- Ask its age and look for a transmit-hours counter. Old magnetron radars eventually need a new magnetron.
- Ask the maker whether the antenna works with displays still on sale; replacing an old display can mean replacing the antenna too.
- Look for a cracked dome, corroded mounting bolts and damaged cable.
- Test it: power up, wait for warm-up, transmit, and compare the picture with landmarks you can see.
Sources (9)
en.wikisource.org · navcen.uscg.gov · ecfr.gov · www8.garmin.com · www8.garmin.com · www8.garmin.com · www8.garmin.com · svb-marine.it · nauticalcharts.noaa.gov
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.