Two Sites for One Coastal Radar: Height Versus Sea Arc
A state coastal security committee must choose one of two sites for a coastal surveillance radar. Site A is a 62 m headland, 1.2 km from a fishing harbour with 300 registered boats, 400 m from a village, with road access, grid power and a clear sea arc of 210°. It lies within 9 km of a naval air station's approach funnel and 3 km from a turtle nesting beach. Site B is a 210 m hill 18 km inland, on forest land, needing 250 m of new approach road and a power line; its sea arc is only 120° because the headland at A blocks the rest. The coast records 3,000 mm of rain and monsoon winds of 90 km/h for four months. The committee also needs a VHF set for fishermen and an automatic weather station. In 2019 a boat carrying contraband entered through the creek behind Site A undetected.
1. Which site should carry the radar? Use the radar horizon and the sea arc in your reasoning, not just the elevation.
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Radar horizon against a target at sea level is roughly 4.12√h km, where h is the antenna height in metres.
Site A: 4.12 × √62 ≈ 4.12 × 7.87 ≈ 32 km. A small boat with a 3 m antenna adds 4.12 × √3 ≈ 7 km, so about 39 km of usable range.
Site B: 4.12 × √210 ≈ 4.12 × 14.5 ≈ 60 km, or about 67 km against the same boat.
Height therefore favours B - but range is not coverage. B is 18 km inland and the headland at A masks 120° of its horizon, so the water it can actually see is a distant fan; small craft working close inshore, which is exactly what the 2019 intrusion was, stay hidden behind the headland. A's 210° clear arc and 1.2 km offset from a harbour of 300 boats give continuous, close cover where the threat lives.
Recommendation: primary radar at A. A cheap gap-filler on B is justified only if the road, the power line and the forest clearance are separately funded; otherwise B is a very tall mast that cannot see the near sea.
2. The 2019 intrusion came through the creek behind Site A. What non-radar measures must be attached to the project, and why?
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A radar detects; it does not intercept, and it cannot identify. Every contact it produces needs an asset and an identity, so the project must buy the whole chain.
- VHF sets and a harbour watch linking the 300 boats to the marine police, so that a legitimate boat can be identified in seconds and an illegitimate one is conspicuous.
- Boat registration tokens and a token register held at the harbour, checked at sea, so a radar contact can be matched to a name.
- A creek patrol schedule tied to tides, because the creek is navigable only at high water; patrols at low tide are decoration.
- CCTV or a watchtower at the creek mouth, giving the radar a visual confirmation capability.
- A fishermen's liaison meeting each season, since the fishermen are the cheapest and fastest sensor on that coast and will report an unfamiliar hull before any radar operator does.
- A joint standard operating procedure between the marine police, the Coast Guard and the radar station, naming who scrambles what, and in how many minutes.
Without this chain, Site A produces plots that nobody acts on - the 2019 failure repeated with better equipment.
3. Which clearances and siting constraints must be settled before construction at Site A, and what would you relocate?
Read the worked answer
Three of the constraints are not technical and can stop the project dead if left to the end.
- Air station approach funnel, 9 km away. A tall mast and, more importantly, radar emissions inside a funnel need a written no-objection from the naval air station and the civil aviation authority. Emissions can interfere with aircraft systems; the tower can infringe obstacle limitation surfaces.
- Turtle nesting beach, 3 km away. Coastal regulation zone and wildlife clearances will apply, with restrictions on lighting and on construction during the nesting season. A radar with floodlights and diesel generators on a nesting beach is a compliance problem.
- Village 400 m away. Land acquisition, access rights and generator noise must be settled before the foundation is dug.
- Climate. Wind load for 90 km/h gusts, 3,000 mm of rain, salt corrosion and lightning protection dictate a radome, marine-grade coatings and a raised plinth; power needs a diesel generator and solar backup, not grid power alone.
Relocations: the VHF set belongs at the harbour, not on the headland - that is where the fishermen are. The weather station is better on the open headland but must be sited away from the radome's shadow and the generator exhaust.