Introduction
The Visible Infrared Imaging Radiometer Suite (VIIRS) has become a cornerstone instrument for Earth observation, particularly for applications such as wildfire detection, fire radiative power (FRP) estimation, and environmental monitoring. Deployed across multiple satellite platforms — Suomi NPP (SNPP), NOAA-20, and NOAA-21 — VIIRS provides a unique opportunity to combine continuity with redundancy in global observations.
VIIRS Instruments
Platform Summary
Suomi NPP, NOAA-20, and NOAA-21 all carry the VIIRS instrument, but they differ in mission timeline and operational role. NOAA currently lists Suomi NPP, NOAA-20, and NOAA-21 as part of the JPSS-related polar-orbiting fleet, with NOAA-20 formerly known as JPSS-1 and NOAA-21 formerly known as JPSS-2.
All three satellites carry VIIRS, which provides moderate- and high-resolution imagery across visible, near-infrared, shortwave infrared, midwave infrared, and thermal infrared spectral regions. Key characteristics include:
- I-bands at approximately 375 m spatial resolution at nadir, useful for higher-resolution imagery and active fire context.
- M-bands at approximately 750 m spatial resolution at nadir, used for radiometric measurements, including fire detection and FRP retrievals, notably through the M13 band near 4 µm.
- A wide swath of about 3,000 km, enabling near-global daily coverage.
- A Day/Night Band (DNB) designed for low-light visible imaging at night.
From a design standpoint, VIIRS is highly consistent across SNPP, NOAA-20, and NOAA-21. This consistency is critical for building long-term environmental records and ensures that differences between platforms are generally small and well characterized. However, exact spectral response functions and calibration details can still vary slightly from sensor to sensor.
| Satellite | Mission name | Launch date | Mission role | Status / role |
|---|---|---|---|---|
| Suomi NPP | SNPP | Oct 28, 2011 | First VIIRS platform and bridge mission before the operational JPSS series | Long-term VIIRS record and continuity |
| NOAA-20 | JPSS-1 | Nov 18, 2017 | First operational JPSS satellite | Primary operational JPSS observations |
| NOAA-21 | JPSS-2 | Nov 10, 2022 | Second operational JPSS satellite | Continuity and redundancy |
VIIRS Spectral Bands
VIIRS has 22 spectral bands: 16 moderate-resolution M-bands, 5 imagery-resolution I-bands, and 1 Day/Night Band (DNB). NOAA STAR summarizes VIIRS as covering roughly 412 nm to 12 µm, with ~375 m imagery bands and ~750 m moderate-resolution radiometric bands at nadir. NASA Earthdata provides the band wavelength ranges summarized below.
For SNPP, NOAA-20, and NOAA-21, the nominal VIIRS band set is the same. This makes the three platforms especially useful for multi-satellite fire monitoring, continuity studies, and cross-platform product comparisons.
VIIRS Moderate-resolution bands: M-bands
M-bands are the main radiometric bands used for many quantitative VIIRS products. They are especially important for ocean color, aerosols, clouds, snow and ice, sea surface temperature, land surface temperature, and active fire detection.
| Band index | Band | Center wavelength | Approx. wavelength range | Spectral region | Main use |
|---|---|---|---|---|---|
| 1 | M1 | 0.412 µm | 0.402–0.422 µm | Visible / reflective | Ocean color, aerosols, coastal water |
| 2 | M2 | 0.445 µm | 0.436–0.454 µm | Visible / reflective | Ocean color, aerosols |
| 3 | M3 | 0.488 µm | 0.478–0.488 µm | Visible / reflective | Ocean color, aerosols |
| 4 | M4 | 0.555 µm | 0.545–0.565 µm | Visible / reflective | Ocean color, vegetation, aerosols |
| 5 | M5 | 0.672 µm | 0.662–0.682 µm | Near infrared | Red / NIR transition; vegetation, ocean color, aerosols |
| 6 | M6 | 0.746 µm | 0.739–0.754 µm | Near infrared | Atmospheric correction, aerosol correction |
| 7 | M7 | 0.865 µm | 0.846–0.885 µm | Near infrared | Vegetation, water/land contrast, ocean color, aerosols |
| 8 | M8 | 1.240 µm | 1.230–1.250 µm | Shortwave infrared | Cloud particle size, cloud microphysics |
| 9 | M9 | 1.378 µm | 1.371–1.386 µm | Shortwave infrared | Cirrus detection, high cloud screening |
| 10 | M10 | 1.610 µm | 1.580–1.640 µm | Shortwave infrared | Snow/ice discrimination, snow fraction |
| 11 | M11 | 2.250 µm | 2.230–2.280 µm | Shortwave / midwave infrared | Cloud phase, snow/cloud/land discrimination |
| 12 | M12 | 3.700 µm | 3.610–3.790 µm | Midwave infrared | Sea surface temperature, thermal contrast, fire context |
| 13 | M13 | 4.050 µm | 3.970–4.130 µm | Midwave infrared | Active fire detection, hot spots, FRP support, SST |
| 14 | M14 | 8.550 µm | 8.400–8.700 µm | Thermal infrared | Cloud-top properties, atmospheric window |
| 15 | M15 | 10.753 µm | 10.260–11.260 µm | Thermal infrared | SST, land/cloud temperature, split-window retrievals |
| 16 | M16 | 12.013 µm | 11.540–12.490 µm | Thermal infrared | SST, split-window correction, cloud/atmospheric correction |
VIIRS Imagery bands: I-bands
I-bands provide finer spatial detail than M-bands. They are useful for visual interpretation, fire location context, cloud structure, snow/ice discrimination, vegetation, and thermal imagery.
| Band index | Band | Center wavelength | Approx. wavelength range | Spectral region | Main use |
|---|---|---|---|---|---|
| 1 | I1 | 0.640 µm | 0.600–0.680 µm | Visible / reflective | High-resolution visible imagery, vegetation, true-color composites |
| 2 | I2 | 0.865 µm | 0.850–0.880 µm | Near infrared | NDVI, vegetation/water contrast, land surface context |
| 3 | I3 | 1.610 µm | 1.580–1.640 µm | Shortwave infrared | Snow/ice discrimination, burn scars, land surface features |
| 4 | I4 | 3.740 µm | 3.550–3.930 µm | Midwave infrared | High-resolution thermal imagery, active fires, hot spots, clouds |
| 5 | I5 | 11.450 µm | 10.500–12.400 µm | Thermal infrared | Thermal infrared imagery, cloud-top temperature, surface temperature |
VIIRS Day/Night Band: DNB
The Day/Night Band is a broad panchromatic visible/near-infrared band designed for low-light imaging. It is especially useful for nighttime lights, moonlit cloud imagery, smoke or snow under illumination, aurora, and qualitative nighttime context around fire events.
| Band index | Band | Center wavelength | Approx. wavelength range | Spectral region | Main use |
|---|---|---|---|---|---|
| 1 | DNB | 0.700 µm | 0.500–0.900 µm | Visible / near infrared | Low-light visible imaging, nighttime lights, moonlit clouds, smoke, snow/ice, aurora, city lights |
Most Useful VIIRS Bands for Fire Applications
For fire detection and characterization, the most important bands are the midwave infrared and thermal infrared bands. The midwave infrared region around 3.7–4.0 µm is highly sensitive to sub-pixel hot targets, while the longwave infrared bands provide background temperature and atmospheric context.
| Fire application | Most relevant VIIRS bands | Why they matter |
|---|---|---|
| Active fire detection | M13, I4 | Strong sensitivity to hot sub-pixel fire sources near 4 µm |
| Fire radiative power / fire intensity | M13, M12, I4 | Midwave infrared radiance is strongly affected by high-temperature fire emission |
| Background thermal characterization | M15, M16, I5 | Helps estimate non-fire background temperature and split-window atmospheric effects |
| Cloud screening | M9, M14, M15, M16, I5 | Helps identify cirrus, cloud-top properties, and cloudy thermal backgrounds |
| Smoke / aerosol context | M1–M7 | Visible and near-infrared bands help characterize smoke, aerosols, and surface reflectance |
| Burned area / surface change context | I2, I3, M7, M10, M11 | NIR and SWIR bands are useful for vegetation, char, snow/ice, and land surface contrast |
| Day/night interpretation | DNB, thermal bands, solar zenith angle | DNB provides nighttime visible context, while thermal bands support day/night fire detection |
Orbit and Observation Timing
Even though all three satellites share a similar afternoon polar-orbiting configuration, they do not observe the same location at exactly the same time.
Local Overpass Timing Differences
| Order | Satellite | Relative time |
|---|---|---|
| 1 | SNPP | Reference time, t₀ |
| 2 | NOAA-20 | t₀ + ~50 min |
| 3 | NOAA-21 | t₀ + ~100 min |
Important considerations when comparing satellites:
Temporal Offsets Are Not Fixed
Although the three platforms — Suomi NPP, NOAA-20, and NOAA-21 — are often separated by roughly 50 minutes along similar orbits, this spacing should be understood as nominal rather than exact.
In practice, the time difference (Δt) between observations can vary due to several factors:
- Latitude effects: At higher latitudes, orbital convergence changes the apparent timing between overpasses.
- Orbit maintenance maneuvers: Small adjustments in satellite position can shift relative phasing over time.
- Viewing geometry: The exact location within the swath, from nadir to edge-of-scan, influences when a given point is observed.
As a result, the expected sequence:
SNPP → NOAA-20 → NOAA-21
is generally useful as a conceptual reference, but it is not strictly guaranteed at every location and time. The commonly cited separations of about 50 and 100 minutes should be treated as typical values rather than precise constants.
Why Temporal Differences Matter
For dynamic phenomena such as wildfires, temporal offsets are not a minor detail. They are often the dominant source of differences between satellite observations.
Fire behavior can evolve significantly over short time scales:
- Fire intensity, and therefore FRP, can increase or decrease rapidly.
- Fire fronts can expand, merge, or fragment.
- Sub-pixel fire conditions can change dramatically within minutes.
Because of this, a 50–100 minute delay between observations can lead to:
- Different measured fire intensity or FRP.
- Changes in fire extent and spatial structure.
- Apparent inconsistencies that may resemble sensor bias.
Key Interpretation
Differences in FRP between SNPP, NOAA-20, and NOAA-21 are often driven by temporal sampling and fire dynamics, rather than by intrinsic differences between the instruments themselves.
This is an important distinction. The VIIRS sensors onboard all three platforms are nearly identical by design, and their calibration differences are relatively small. However, the natural variability of fires at sub-pixel and short time scales can easily exceed these instrumental differences.
VIIRS Fire Products
NOAA EFIRE: Enterprise Fire Product
The NOAA Enterprise Fire product, often referred to as EFIRE, provides an operational fire detection and characterization framework applied across multiple VIIRS platforms. The goal is to provide consistent fire products from SNPP, NOAA-20, and NOAA-21.
EFIRE supports:
- A unified algorithm framework applied consistently across multiple satellites.
- Improved maintainability and scalability of the processing system.
- Harmonized outputs to support multi-sensor continuity and interoperability.
Rather than treating each satellite independently, EFIRE is designed to ensure that fire products from SNPP, NOAA-20, and NOAA-21 are directly comparable and consistent by design.
| Feature | Description |
|---|---|
| Multi-platform | Same general algorithm framework applied across SNPP, NOAA-20, and NOAA-21 |
| Consistency | Harmonized outputs across satellites |
| Scalability | Designed for operational processing and future missions |
| Continuity | Supports long-term fire records |
References
| Link | Site |
|---|---|
| NOAA STAR JPSS VIIRS instrument overview | star.nesdis.noaa.gov |
| NASA Earthdata VIIRS spectral bands | earthdata.nasa.gov |
| NOAA NESDIS Joint Polar Satellite System overview | nesdis.noaa.gov |
| NOAA JPSS VIIRS Active Fires Environmental Data Record from NDE | ncei.noaa.gov |
