Why is the NISAR (NASA-ISRO Synthetic Aperture Radar) mission designed to revisit the same location every 12 days?

Correct — D. NASA says NISAR observes Earth's land and ice-covered regions every 12 days from both ascending and descending orbits, an average revisit of about 6 days.

A fixed repeat is what temporal resolution means: how often the same spot is seen again. Radar images of one place taken a cycle apart show how the ground has moved or changed, which is land deformation monitoring. That is option (d).

Why the others are wrong

• (a)
  To support low-earth orbit operations at reduced altitude — The repeat cycle says nothing about altitude. NISAR flies a polar orbit at about 747 km, and the 12 days describe how often it passes over the same spot, not a lowered orbit.
• (b)
  To enable cost-efficient satellite passes through optimized fuel consumption — Fuel economy is not what a revisit interval is for. The 12 days fix how often the same ground is re-imaged, which serves the measurement of change, not the cost of each pass.
• (c)
  To ensure consistent caching of data before transmission to ground stations — Data storage and downlink are handled on board and through ground stations whatever the repeat cycle. The 12 days describe returning to the same place on the ground, not scheduling transmission.

NISAR is the NASA-ISRO Synthetic Aperture Radar satellite, with an L-band radar built by NASA and an S-band radar built by ISRO.

Radar supplies its own illumination, so it images through cloud and at night. Returning on a fixed 12-day cycle lets it compare the same ground over time: movement after earthquakes, landslides and volcanic activity, ice-sheet change and vegetation change.

Key facts

• NISAR was launched on 30 July 2025 on ISRO's GSLV from the Satish Dhawan Space Centre, Sriharikota.
• It flies a sun-synchronous polar orbit at about 747 km.
• It observes Earth's land and ice every 12 days from both ascending and descending passes, an average revisit of about 6 days.
• NASA built its L-band radar and ISRO its S-band radar.

Common traps

• Reading 'every 12 days' as a fuel or cost choice, when a revisit interval is about how often change can be measured.
• Taking global coverage to mean a geostationary satellite, when NISAR is a polar orbiter at about 747 km.

No directly related past PYQ was found.