Satellite imaging geometry: For across-track sensors, which statements are correct about orbit frame, Earth rotation, and image skew?

Introduction / Context:
Across-track (whiskbroom or pushbroom) satellite imagers build two-dimensional scenes line by line as the platform moves along its orbit while Earth rotates underneath. This geometry produces systematic effects on ground coverage and image orientation. The question lists three propositions and asks which are correct.

Given Data / Assumptions:

• (1) The satellite orbit is essentially fixed in inertial space over short time scales.
• (2) During successive scans, Earth rotates beneath the sensor.
• (3) The imaged ground area is skewed due to relative motion and projection.

Concept / Approach:
In an inertial frame, a near-polar, Sun-synchronous orbit maintains a nearly fixed plane, while Earth’s rotation causes the ground track to shift west-to-east (or vice versa) between passes, producing revisit patterns. As the sensor scans across track and integrates over finite time, the moving ground imparts a skew or shear to the projected swath unless corrected by onboard timing and ground processing.

Step-by-Step Solution:
Assess (1): Orbit plane is quasi-inertial (neglecting precession/perturbations) → acceptable for this context.Assess (2): Earth rotation under successive imaging is fundamental to revisit and swath displacement → true.Assess (3): Combined platform motion, scan timing, and Earth rotation yield skewed ground footprints unless rectified → true.Hence, all three statements are correct.

Verification / Alternative check:
Landsat and Sentinel-2 raw Level-1 products show predictable along-track/scan geometry and require georectification to remove skew and terrain distortions.

Why Other Options Are Wrong:
Any subset omits a true statement, making it incomplete.

Common Pitfalls:
Confusing ground-fixed tracks with orbit-fixed planes; assuming images are perfectly north-up without orthorectification.

Final Answer:
1, 2, 3.