Satellites measure sea temperature and ocean colour; INCOIS uses these to mark likely fishing zones and sends daily advisories to India's coastal communities.
Fishing is more rewarding in some patches of sea than others, and the good patches move from day to day. Satellites can spot ocean features linked to them, and India turns this into daily advice for fishermen.
Two measurements matter most.
Sea surface temperature shows fronts, eddies, rings and upwelling, places where water masses meet or deep water rises. INCOIS, the Indian National Centre for Ocean Information Services, takes this from thermal channels on the US NOAA satellites and Europe's MetOp satellites.
Ocean colour shows chlorophyll, the green pigment of the tiny plants at the bottom of the food chain. INCOIS uses India's Oceansat-2 and the US Aqua satellite for this. India has flown ocean-colour satellites since Oceansat in 1999. The newest, EOS-06 (Oceansat-3), carries an ocean colour monitor, a sea surface temperature monitor and a wind scatterometer.
INCOIS combines these into Potential Fishing Zone (PFZ) advisories. They are issued every day, with reference to 586 fish landing centres along the Indian coast. Each zone is given as latitude and longitude, and as a distance and direction from a familiar landmark such as a landing centre or lighthouse.
Advisories reach fishermen in local languages by phone, radio, Doordarshan, websites, mobile apps and electronic display boards at major fishing harbours. They are not issued during the government's annual fishing ban or during dangerous sea conditions such as cyclones, high waves and tsunamis.
The aim, INCOIS says, is to increase the catch for each unit of effort, so boats spend less time and fuel searching.
Search for EOS-06: it circles the poles about 740 km up, measuring ocean colour and sea temperature on its daytime passes. Find EOS-06 (Oceansat-3)
A PFZ advisory gives a position like '35 km south-west of a lighthouse'. Turning latitude and longitude into distance uses the size of the Earth.
One degree of latitude is a fixed length:
2π × 6,371 km / 360 ≈ 111.2 km
One degree of longitude shrinks with latitude, by the cosine of the latitude. Off Kochi, at about 10° N:
111.2 × cos 10° = 111.2 × 0.985 ≈ 109.5 km
Off Veraval, at about 21° N:
111.2 × cos 21° = 111.2 × 0.934 ≈ 103.8 km
So a zone 0.3° west and 0.2° south of a landing centre near Kochi lies about 0.3 × 109.5 = 32.9 km west and 0.2 × 111.2 = 22.2 km south. The straight-line distance is √(32.9² + 22.2²) = √(1,082 + 493) = √1,575 ≈ 39.7 km.
Why temperature fronts? At a front, the sea surface temperature changes sharply over a short distance. In an SST map this shows up as a steep gradient; for example, 1.5 °C over 10 km is 0.15 °C per km. INCOIS identifies fronts, eddies and upwelling from SST and adds chlorophyll from ocean-colour sensors to refine the zones.
Sea surface temperature and ocean colour (chlorophyll).
INCOIS, the Indian National Centre for Ocean Information Services.
About 111 km.