Temporal resolution

Geography glossary

Also called: Revisit time · Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"

Meaning

Temporal resolution (also called revisit time) is how long a satellite takes to image the same area again. It is counted in days. The orbit mostly sets it, and tilting the sensor sideways can shorten it.

It matters because only repeat images of one place show what has changed there, such as seasonal change in the Himalaya or tsunami damage at Banda Aceh in 2004. A short revisit also lets officials track a disaster while it is still happening.

Explanation

How it works: orbit → repeat pass → revisit interval

  • Cause: the orbit. Most Earth observation satellites fly in a Sun-synchronous polar orbit.
  • This is a near-polar orbit. The satellite passes over each place at the same local time on every pass.

  • So the sunlight is the same each time. Images from different dates can be compared fairly.

  • Process: the Earth turns below the satellite. Each pass images a strip of ground called the swath (the width of ground imaged in one pass). The satellite needs many passes to cover the whole Earth.

  • Result: a fixed repeat cycle. After a set number of days, the satellite is over the same place again. That number of days is its temporal resolution.

  • Landsat 8 on its own repeats every 16 days [3].

  • Fewer days means finer temporal resolution. An 8-day revisit is finer than a 16-day revisit.

Two ways to shorten the revisit

  • Off-nadir steering (tilting the sensor sideways):
  • The sensor tilts to look at a place to the side of its path. It does not have to wait until it passes straight overhead.
  • Resourcesat-2A LISS-4 can be steered ±26° across its track, which gives a five-day revisit [2].
  • This shortens the revisit without changing the orbit.

  • Cartosat-3 is an agile satellite, which means it can quickly tilt to point at a target [4].

  • Constellations (a group of satellites working together):

  • Landsat 8 + Landsat 9 together give an 8-day repeat, half of the 16 days Landsat 8 needs on its own [3].
  • India runs the Resourcesat, Cartosat and RISAT series for the same reason.

The trade-off with spatial resolution

  • A sharper sensor sees a narrower strip:
  • Finer spatial resolution (smaller pixels) usually means a narrower swath.
  • A narrow swath needs more passes to cover the Earth.

  • So the revisit interval gets longer.

  • Examples:

  • AWiFS (Resourcesat-2A, 56 m) has a wide swath and is used for regional crop watch [2].
  • LISS-4 (5.8 m) has a narrow swath and needs the ±26° tilt to reach a 5-day revisit [2].

  • Landsat 8 OLI (30 m, 185 km swath) has a 16-day revisit [3].

  • One satellite cannot be sharp, wide and frequent all at once. That is why constellations are needed.

What it is used for: change detection

  • Change detection: comparing images of the same area taken on different dates to find out what has changed. It needs good temporal resolution.
  • Step 1: image the area on date 1 and on date 2.
  • Step 2: line the two images up pixel by pixel.

  • Step 3: mark the pixels whose spectral values (brightness in each band) have changed. These show new growth, flooding, damage or new buildings.

  • Uses: crop monitoring, deforestation, urban sprawl, flood and tsunami damage, and glacier retreat.

Where it occurs

  • Himalaya, India (NCERT Fig. 6.9): IRS images from May and November were compared.
  • Coniferous forest stays green all year, so its tone does not change.

  • Deciduous trees (trees that shed their leaves) and crops change tone between the two seasons.

  • Banda Aceh, Sumatra, Indonesia (NCERT Fig. 6.10): images were taken before and after the 26 December 2004 Indian Ocean tsunami. They show coastal land, buildings and vegetation washed away and the shoreline changed.

  • India's satellites:
  • Resourcesat-2A was launched on 7 December 2016 by PSLV-C36 into a Sun-synchronous polar orbit. Its LISS-4 camera has a 5-day revisit [2].
  • Cartosat-3 was launched on 27 November 2019 by PSLV-C47 from Sriharikota into a Sun-synchronous orbit at 509 km. It is an agile satellite [4].

  • Their data is shared through Bhuvan and opened up under the National Geospatial Policy 2022.

  • World (USA): Landsat 8 was launched on 11 February 2013. It repeats every 16 days alone and every 8 days with Landsat 9 [3].

  • How it is used in India: repeat images are used for crop estimates, forest monitoring and disaster management (floods, cyclone landfall and forest fires).

Don't confuse with

  • Spatial resolution: this is how small an object the sensor can see (the ground size of one pixel, in metres). Temporal resolution is how often the sensor sees the same place (in days). A sharper sensor often has a longer revisit, not a shorter one.
  • Swath: this is the width of ground imaged in one pass. It is not a time. A wider swath usually helps the satellite come back sooner.
  • Sun-synchronous orbit: this is the type of orbit that makes each pass happen at the same local time. Temporal resolution is the number of days between passes over one place.
  • Change detection: this is the method of comparing images from two dates. Temporal resolution is the property of the sensor that makes those repeat images possible.

Prelims Hooks

  • Temporal resolution is set by the orbit. It is measured as a revisit interval in days. Fewer days means finer temporal resolution.
  • Landsat 8 alone: 16 days. Landsat 8 + Landsat 9: 8 days [3].
  • Resourcesat-2A LISS-4: 5-day revisit, reached by ±26° steering across its track [2].
  • NCERT change-detection cases: Himalaya (IRS, May vs November: conifers stay the same, while deciduous trees and crops change) and Banda Aceh, Sumatra, Indonesia (26 December 2004 tsunami).
  • Trap: finer spatial resolution usually means a narrower swath and a longer revisit. Statements claiming the sharpest sensor also revisits most often are wrong.
  • Trap: a constellation shortens the revisit without changing any one satellite's orbit (Landsat 8 + 9 cut it from 16 to 8 days) [3].

Mains Points

  • Disaster management (GS-III):
  • Before-and-after images (Banda Aceh 2004) show exactly where the damage is.
  • A short revisit (5 days [2], 8 days [3]) lets officials track floods, cyclone landfall and forest fires as they happen.

  • This speeds up relief and damage assessment.

  • Agriculture and forests (GS-I/GS-III):

  • Seasonal images (Himalaya, May vs November) separate evergreen conifers from deciduous trees and crops.

  • Repeat images help with crop monitoring and with tracking deforestation, urban sprawl and glacier retreat.

  • India's satellite strategy as a policy response:

  • Because of the resolution trade-off, one satellite cannot be sharp, wide and frequent at the same time.
  • So India builds series and constellations (Resourcesat-2/2A, Cartosat, RISAT) and agile satellites such as Cartosat-3 [4].
  • Their data is shared through Bhuvan and opened up under the National Geospatial Policy 2022. This cuts India's dependence on foreign imagery.

Related concepts

Read more

Sources

  1. 1Class 11, Ch 6 "Introduction to Remote Sensing" (primary)
  2. 2ISRO — RESOURCESAT-2A mission pageisro.gov.in · tier 1
  3. 3NASA Science — Landsat 8 missionscience.nasa.gov · tier 2
  4. 4ISRO — PSLV-C47/Cartosat-3 press releaseisro.gov.in · tier 1