Sensors — SPOT & IKONOS

Three optical remote-sensing system types — distinguished by how much of the spectrum they sample and how finely:

• Multispectral — a few (5–15) discrete broad bands covering visible → SWIR. Example: Landsat TM/ETM+, SPOT, IKONOS.
• Thermal — one or more bands in the thermal-IR region (8–14 µm). Example: Landsat TIRS, ASTER thermal.
• Hyperspectral — hundreds of narrow contiguous bands (5–10 nm each). Example: Hyperion on EO-1 (220 bands, 0.4–2.5 µm).

Reference: Jensen, 2000.

Two scanning geometries used by multispectral sensors, 0.3–14.0 µm:

• Across-track (“whiskbroom”) — a single detector + rotating/oscillating mirror sweeps perpendicular to the flight path. Examples: Landsat MSS, TM, ETM+.

• Along-track (“pushbroom”) — a linear array of detectors (one per pixel in the swath) builds the image as the platform moves forward. Examples: SPOT HRV/HRVIR, IRS LISS, IKONOS, QuickBird.

• Why pushbroom is better: longer dwell time per pixel → higher SNR and finer resolution; no moving mirror.

SPOT (Satellite Pour l’Observation de la Terre) — French CNES program.

(The original slide listed SPOT 2 and 4 as “still going” — that was true when the slide was written, but both have since been decommissioned.)

SPOT scene interpretation practice. The example shows three land-cover classes you should be able to pick out from a SPOT multispectral image:

• Oxbow lake — dark sinuous water feature (river meander cutoff).
• Urban — bright, geometric patterns.
• Agriculture — regular rectangular fields, often in false color due to NIR vegetation response.

SPOT 1–3 orbit. Memorize these numbers — they show up on short answer.

• Orbit type: sun-synchronous, descending node crossing at 10:30 AM local.
• Inclination: ~98.7° (about 8° off polar).
• Altitude: 832 km.
• Revisit: 26 days at nadir (much less with off-nadir pointing).
• Swath: 60 km (single HRV) / 120 km combined (both HRVs).
• Radiometric resolution: 8-bit (256 grey levels).

SPOT 1–3 bands.

• Panchromatic — 0.51–0.73 µm (visible minus blue), 10 m resolution, 6 000 pixels/line.
• Multispectral — 20 m resolution, 3 000 pixels/line:
  • XS1 Green: 0.50–0.59 µm
  • XS2 Red: 0.61–0.68 µm
  • XS3 NIR: 0.79–0.89 µm
• No blue band on SPOT 1–3 (important — rules out natural-color composites and some atmospheric-correction products).

Scene-size comparison. The illustration contrasts the footprint of a single image:

• SPOT HRV: 60 km × 60 km

• Landsat TM / MSS: 185 km × 170 km

• Trade-off: SPOT’s smaller scene gives higher resolution; Landsat’s larger scene covers more ground per pass, so fewer scenes are needed to mosaic a region.

Two HRVs per satellite. Each SPOT carries two identical HRV (High Resolution Visible) sensors, mounted side-by-side. They can operate independently or jointly, which gives SPOT its 120 km combined swath and enables simultaneous stereo acquisition.

Off-nadir viewing (SPOT HRV).

• Each HRV’s viewing mirror can be commanded from the ground to tilt up to ±27° off vertical.
• Advantage 1 — stereoscopic imagery. Two images of the same ground target from different angles produce parallax, which lets you derive elevation (DEMs, 3D models).
• Advantage 2 — shorter revisit. At mid-latitudes, the effective revisit drops from the 26-day nadir cycle to 1–4 (5) days.

Off-nadir coverage math (know the numbers):

• The tilted view lets SPOT image any point within a ~900 km swath on either side of the ground track.
• Equator: a given point can be imaged 7 times per 26-day cycle.
• Latitude 45°: 11 times per cycle → about 157 times per year, average 2.4 days between views (minimum 1 day, maximum 4).
• With all three SPOTs flying, 95% of Earth was imageable every day.

SPOT 4 (1998–2013). Two changes from SPOT 1–3:

• HR-VIR replaced HRV — adds a mid-IR (SWIR) band at 1.58–1.75 µm at 20 m. Useful for vegetation stress, snow/ice discrimination, and geology.
• VEGETATION-1 instrument — a wide-field sensor flown alongside HR-VIR.
  • Swath 2 250 km, 1 km resolution at nadir.
  • Bands: Blue 0.43–0.47 µm, Red, NIR, Mid-IR.
  • Daily global coverage — enables vegetation time series similar to AVHRR/MODIS.

SPOT 5 (2002–2015). Highest-spec of the classic SPOT series.

• HRG (High Resolution Geometric), 2 sensors:
  • Panchromatic 2.5 m or 5 m (0.48–0.71 µm) via “Supermode”.
  • Green, Red, NIR at 10 m; Mid-IR at 20 m.
• HRS (High Resolution Stereoscopic) — fore-and-aft pair for DEM production at 10 m, 120 km swath.
• VEGETATION-2 — same specs as VEGETATION-1 on SPOT 4.

IKONOS overview. First commercial sub-meter Earth-observation satellite — a landmark.

• Operator / launch: Space Imaging, September 1999 (active to March 2015).
• Orbit: sun-synchronous, 10:30 AM equator crossing.
• Altitude: 682 km.
• Ground-track repeat: 11 days.
• Nadir swath / scene: 11 km × 11 km.

IKONOS off-nadir pointing.

• Can tilt up to ±45° from vertical in both along-track and across-track directions — much more aggressive than SPOT’s ±27°.
• Enables:
  • Frequent revisit of any given area (~1–3 days at mid-latitudes).
  • Stereo imagery — both side-by-side (across-track) and fore-and-aft (along-track).
• Example: the “1 m IKONOS pan image of Rome” from spaceimaging.com.

IKONOS bands.

• Panchromatic: 0.45–0.90 µm, 1 m resolution.
• Four multispectral bands at 4 m resolution:
  • Blue 0.45–0.52 µm
  • Green 0.51–0.60 µm
  • Red 0.63–0.70 µm
  • NIR 0.76–0.85 µm

• Radiometric resolution: 11 bits → 2 048 grey levels (vs. 8-bit = 256 on SPOT/Landsat TM). This extra bit depth captures subtle contrast in shadows and bright snow/clouds.

• Pan-sharpening of the 4 m MS onto the 1 m pan gives an effective 1 m color image.

Why IKONOS is so agile.

• The entire spacecraft pivots, not just a mirror or the sensor. This slews it onto targets in seconds and lets it follow curved features (rivers, coastlines) with continuous strips.
• Users can order arbitrary strips and mosaics — not just the fixed 11 × 11 km scenes.

Data-volume takeaway. The IKONOS image of Traverse Bay is 13 000 MB (13 GB) for a single high-resolution mosaic. High spatial + high radiometric (11-bit) + multi-band = huge files. Plan storage and processing for any high-resolution project.

High-resolution imagery — advantages and disadvantages.

Advantages - Urban / built-up — individual buildings, streets, parking lots, rooftops. - Agriculture — field-scale irrigation pivots, crop rows, damage assessment. - Forest — individual tree crowns, thinning patterns, plantation rows.

Question to be ready for: the difference between regular (plantation / agricultural) and irregular (natural forest) patterns — regular indicates planned / cultivated, irregular indicates natural growth.

Disadvantages - Very small footprint → many scenes needed for regional coverage. - Expensive / commercial tasking. - Data volumes are huge (see slide 17 — 13 GB for one mosaic).

Pan-sharpened example — Old Mission Point Orchards (2011). The 1 m panchromatic is fused with the 4 m multispectral to yield a 1 m color image. Typical things you can now detect:

• Individual trees and tree rows in the orchards.
• Buildings, driveways, vehicles, fences.
• Crop vigor differences via the multispectral component.

Pan-sharpening is a standard IKONOS workflow — trades a little spectral fidelity for major spatial detail.