How do satellites measure the ocean?

Satellites bounce radar off the sea to measure its height, read its temperature and colour, and gauge surface winds, covering oceans that ships rarely visit.

India has a coastline of thousands of kilometres and vast seas around it. Ships and buoys sample only tiny spots. Satellites see the whole ocean, again and again. Different instruments measure different things.

SARAL (AltiKa) Pulse Echo Round trip ≈ 5.21 ms Range = c × t / 2 Satellite height 781 km Sea height = height − range Sea surface Reference ellipsoid
The altimeter times its radar pulse to the sea and back: range = c × t / 2. Subtracting that range (and small corrections) from the satellite's precisely known height gives the sea surface height. For SARAL at 781 km the round trip takes about 5.21 ms (not to scale).

Height: radar altimeters

An altimeter points a radar straight down and times the echo off the sea surface. If you know exactly where the satellite is, you know how high the sea is. ISRO says SARAL's measurements are used to study ocean circulation and sea surface elevation.

SARAL, a joint Indo-French mission launched on 25 February 2013 on PSLV-C20, carries AltiKa, a Ka-band altimeter at 35.75 GHz built by France's space agency CNES. ISRO built the satellite's bus. SARAL flies at 781 km in a sun-synchronous orbit. It also carries ARGOS, which collects data from ocean buoys.

Temperature, colour and wind

EOS-06 (Oceansat-3), launched on PSLV-C54 in November 2022, continues the Oceansat series. It carries:

  • an Ocean Colour Monitor, whose data show chlorophyll and so the tiny plants in the sea;
  • a Sea Surface Temperature Monitor;
  • a Ku-band scatterometer, which provides wind vectors (speed and direction) over the ocean;
  • an ARGOS data-collection payload.

Oceansat data already feed INCOIS's daily fishing advisories.

See it live

Search for SARAL and EOS-06: both are low, near-polar satellites whose ground tracks sweep across the Indian Ocean many times a day. Find SARAL and EOS-06

Key facts

  • SARAL: launched 25 February 2013 on PSLV-C20; 781 km; 407 kg.
  • AltiKa altimeter: Ka-band, 35.75 GHz, built by CNES.
  • EOS-06: 1,117 kg; orbit about 738 km at 98.3° (PSLV-C54 plan).
  • EOS-06 payloads: OCM-3, SSTM, SCAT-3 scatterometer and ARGOS.

Go deeper

An altimeter measures range by timing:

range = c × t / 2

The satellite knows its own height above a reference ellipsoid very precisely (from tracking). Sea surface height is then:

SSH = satellite height − range − corrections

where the corrections remove delays in the atmosphere and effects such as tides.

Worked numbers for SARAL at 781 km. The two-way travel time is:

t = 2 × 781,000 / 299,792,458 ≈ 0.00521 s = 5.21 ms

How precise must the timing be? A 1 cm error in range means a two-way path error of 2 cm:

Δt = 2 × 0.01 / 299,792,458 ≈ 6.7 × 10⁻¹¹ s = 67 picoseconds

So measuring sea level to centimetres means timing echoes to tens of picoseconds, averaged over many pulses.

Wavelength of AltiKa's signal: λ = c / f = 299,792,458 / (35.75 × 10⁹) ≈ 0.0084 m, or about 8.4 mm.

Check yourself

What does a satellite altimeter measure, and how?

The height of the sea surface, by timing a radar pulse to the sea and back and subtracting that range from the satellite's known height.

Name the instruments on EOS-06.

Ocean Colour Monitor (OCM-3), Sea Surface Temperature Monitor (SSTM), Ku-band scatterometer (SCAT-3) and ARGOS.

Which countries built SARAL?

India and France: ISRO built the satellite bus and CNES built the AltiKa altimeter and ARGOS payload.

Sources

  1. SARAL — ISRO
  2. EOS-06 — ISRO
  3. PSLV-C54/EOS-06 Mission brochure — ISRO / VSSC
  4. Potential Fishing Zone Advisory — INCOIS