Sensor Characteristics & The Four Remote Sensing Resolutions

Explanation

Remote sensor design involves technical trade-offs between the four resolutions. A satellite cannot simultaneously have sub-meter spatial resolution, hundreds of hyperspectral bands, 16-bit radiometric depth, and daily global revisit coverage due to signal-to-noise ratio (SNR) constraints and telemetry downlink limits. Pushbroom sensors (along-track) use linear CCD arrays, observing each ground cell longer (higher dwell time) than oscillating whiskbroom mirrors, producing superior signal-to-noise ratios.

Key points

• Spatial Resolution: Smallest spatial distance on the ground resolved by a single sensor pixel (Instantaneous Field of View - IFOV); e.g., 30m Landsat vs 0.3m WorldView.
• Spectral Resolution: Number and width of specific electromagnetic wavelength bands the sensor records (Multispectral has 4-15 broad bands; Hyperspectral has hundreds of narrow contiguous bands).
• Radiometric Resolution: Sensitivity to subtle variations in radiant energy; measured in bit depth (8-bit = 256 gray levels; 12-bit = 4096 levels; 16-bit = 65,536 levels).
• Temporal Resolution: Revisit time required for a satellite to re-image the identical geographic location on Earth (e.g., Landsat: 16 days, Sentinel-2: 5 days, MODIS: 1-2 days).
• Scanning Mechanisms: Across-track (Whiskbroom with oscillating mirror) vs Along-track (Pushbroom with linear CCD array).

Example

Code examples

/*
Sensor Scanning Architectures:
1. Whiskbroom (Across-Track):
   - Single detector element swept back and forth across swath by a rotating mirror
   - Shorter dwell time per pixel -> Lower signal-to-noise ratio
   - Example: Landsat 4/5 Thematic Mapper (TM)

2. Pushbroom (Along-Track):
   - Linear array of thousands of stationary CCD detectors aligned perpendicular to flight direction
   - Longer dwell time per pixel -> Higher radiometric sensitivity, no moving mirror parts
   - Example: Sentinel-2 MSI, SPOT
*/

Exam questions

• [10 Marks] Define and contrast the Four Resolutions of remote sensing (Spatial, Spectral, Radiometric, Temporal) with illustrative examples.
• [8 Marks] Explain the difference between Whiskbroom (across-track) and Pushbroom (along-track) scanning systems with neat sketches.