EM spectrum, sensor types, and radiometry

Remote sensing is the science of measuring something from a distance. In space remote sensing, the "distance" is hundreds to tens of thousands of kilometers, and the "measurement" is almost always electromagnetic radiation that reaches a satellite's sensor. This week is the physics primer: what the EM spectrum is, what each region tells you, and why thermal infrared is the key to launch detection.

The electromagnetic spectrum

The EM spectrum spans an enormous range of wavelengths. Earth observation uses a sliver of it:

• Visible (0.4–0.7 µm) — what your eyes see. Sentinel-2 bands 2 (blue, 0.49 µm), 3 (green, 0.56 µm), 4 (red, 0.66 µm).
• Near-infrared (NIR, 0.7–1.4 µm) — invisible to humans but reflected strongly by healthy vegetation. The basis of NDVI.
• Short-wave infrared (SWIR, 1.4–3 µm) — sensitive to water content, mineral composition, and active fires.
• Mid-wave infrared (MWIR, 3–8 µm) — thermal emissive. Surfaces emit measurable radiation in this band based on their temperature. GOES Band 7 (3.9 µm) is here. This is the band LaunchDetect uses for plume detection.
• Long-wave infrared (LWIR, 8–14 µm) — also thermal emissive, but for cooler objects. GOES Bands 13–15 (10.3, 11.2, 12.3 µm) and Landsat 9 TIRS bands.
• Microwave (1 mm–1 m) — penetrates clouds and (somewhat) ground. Used by passive radiometers and active radar (SAR).

Passive vs active sensors

Passive sensors measure radiation that already exists — sunlight reflected (optical, NIR, SWIR) or thermal radiation emitted by Earth's surface (MWIR, LWIR). They don't send anything; they just listen. Most weather satellites are passive.

Active sensors emit radiation and measure what bounces back. SAR (Sentinel-1, RadarSat) emits microwaves; LiDAR emits laser pulses; altimeters emit short radar pulses. Active sensors see in the dark and through clouds; passive optical doesn't.

Radiometric quantities

Three quantities every remote sensing pipeline computes:

• Radiance (L) — the raw measurement: W/m²/sr/µm. How much electromagnetic energy is hitting the sensor per unit area, per unit solid angle, per unit wavelength.
• Reflectance (ρ) — for solar bands (visible, NIR, SWIR): the fraction of incoming sunlight reflected by the surface. Dimensionless, 0–1. Computed by dividing radiance by the solar irradiance at that wavelength.
• Brightness temperature (T_b) — for thermal bands: the temperature of a perfect black body that would emit the same radiance. Computed via the inverse Planck function. Kelvin units.

Why thermal IR sees plumes

A rocket plume is hot — combustion gases at 1,500–3,000 K. Earth's surface at "ambient" is ~290 K. The Planck curve says: the hotter an object, the more radiation it emits, and the peak emission shifts to shorter wavelengths. At 3,000 K, the peak emission is around 1 µm (still in the SWIR). At 290 K, the peak is around 10 µm (LWIR).

So why does GOES Band 7 (3.9 µm) work better than Band 14 (11.2 µm) for plume detection? Because at 3.9 µm, a 1,500 K plume emits roughly 5,000× more radiance than a 290 K background. At 11 µm, the ratio is much smaller. Band 7 has the highest thermal contrast for hot objects, which is why it's the band of choice for fire and plume detection.

The lab

You'll build a Matplotlib plot of the EM spectrum from 0.4 µm to 13 µm, annotated with: the human-visible range, the GOES-R ABI 16-band layout, Landsat 9's 11 bands, Sentinel-2's 13 bands, and the Planck curves for a 290 K (Earth) and 1,500 K (plume) emitter. The resulting plot is a one-image reference you'll refer back to all of Track 3.