Radar principles
Governing: STCW Code Table A-II/1 (radar fundamentals); IMO MSC.192(79); Bowditch Chapter 24. Builds on: OOW navigation.
Range and bearing
A marine radar transmits a short pulse and times the echo. The pulse travels out and back, so range is half the round trip. Bowditch uses 162,000 nm per second for radio waves, which gives about 12.3 microseconds per nautical mile of range. Bearing comes from the direction the antenna points when the echo returns.
MSC.192(79) requires range accuracy within 30 m or 1% of the range scale in use, whichever is greater, and bearing accuracy within 1 degree. One degree is about 97 m at 3 nm; worth remembering when you use a single bearing to clear a danger.
The two bands
| Band | Frequency (MSC.192) | Wavelength | Strengths |
|---|---|---|---|
| X (3 cm) | 9.2-9.5 GHz | 300,000 km/s / 9.4 GHz = 3.2 cm | Sharper beam for a given antenna, better discrimination, detects small targets, SARTs respond on X-band |
| S (10 cm) | 2.9-3.1 GHz | 300,000 km/s / 3.05 GHz = 9.8 cm | Less affected by rain and sea clutter; better detection in bad weather |
Longer S-band waves also diffract more, so an S-band set "sees" slightly further into the shadow behind an obstruction.
Resolution
- Range resolution is governed by pulse length. Two targets on the same bearing merge unless they are separated by more than half the pulse length, which Bowditch gives as 150 m per microsecond. A 0.05 microsecond short pulse separates targets about 7.5 m apart; a 1.0 microsecond long pulse only 150 m. That is why long ranges (long pulses) turn small boats near a breakwater into part of the breakwater.
- Bearing resolution depends on horizontal beam width and range. A wider beam, or a greater range, smears targets together sideways and spreads coastlines.
The radar horizon
Bowditch: D = 1.22 x square root of h, with h in feet and D in nautical miles. In metres the constant becomes 1.22 x square root of 3.281 = 2.21.
| Example | Working | Horizon |
|---|---|---|
| Antenna 16 m | 2.21 x square root of 16 = 2.21 x 4 | 8.8 nm |
| Target height 4 m | 2.21 x square root of 4 = 2.21 x 2 | 4.4 nm |
| Maximum theoretical detection | 8.84 + 4.42 | 13.3 nm |
Refraction bends the beam, usually a little beyond the geometric horizon; abnormal conditions (ducting) can extend it greatly or shorten it. Targets whose reflecting surfaces rise above the horizon may be detected beyond it.
CAUTION — Theoretical is not detection The horizon only tells you where the target could first appear. Whether it does depends on its size, material, aspect and the clutter around it. GRP tenders, jet skis and swimmers may not show at all.
Why it matters on a yacht
Yacht radar antennas are often lower than on ships of similar size, and mast-top domes share space with satcom and sensors that create blind and shadow sectors. Know your antenna heights, the blind sectors marked on the bridge, and which of your radars is X or S band. On yachts of 3,000 GT and over the REG Code requires a 3 GHz radar or a second 9 GHz radar (see L2.3).