Atmospheric sounders and gas spectrometers
Atmospheric sounders and gas spectrometers measure trace gases and vertical temperature-humidity profiles from orbit, turning sunlight scattered through the atmosphere into enforceable evidence. Coverage is continental; physics is the witness.
What the instrument actually measures, and how
Every gas absorbs and re-emits electromagnetic radiation at wavelengths determined by its molecular structure. A spectrometer aboard a satellite disperses incoming sunlight (or thermal emission from the surface) across a detector array and reads the depth of absorption lines at those characteristic wavelengths. The ratio of absorbed to unabsorbed light, corrected for surface albedo and aerosol scattering, yields a column-integrated concentration of the target gas. Do this across thousands of spectral channels simultaneously and you can separate overlapping signatures from methane, carbon dioxide, nitrogen dioxide, ozone, sulphur dioxide and water vapour in a single pass.
Two broad instrument classes dominate operational programmes. Passive nadir sounders such as TROPOMI (aboard Sentinel-5P) and IASI (aboard MetOp) observe reflected or emitted radiation without illuminating the scene themselves. TROPOMI covers 270 to 2385 nm across ultraviolet, visible, near-infrared and shortwave-infrared bands, giving it simultaneous sensitivity to NO2, CH4, CO, SO2, O3 and aerosol index. IASI operates in the thermal infrared (645 to 2760 cm⁻¹) and retrieves vertical temperature and humidity profiles in addition to trace gases, making it the workhorse of numerical weather prediction as much as air-quality monitoring. The distinction matters for mission design: shortwave-infrared channels need reflected sunlight and therefore fail over night-side passes and at high latitudes in winter, whereas thermal infrared works day and night but loses sensitivity near the surface where concentrations are highest.
Spectral resolution sets the detection floor
Spectral resolution is measured in nanometres or wavenumbers and determines which gases can be separated from one another and at what concentration threshold. TROPOMI achieves 0.25 to 0.55 nm resolution depending on band, which is sufficient to resolve the methane absorption feature near 2305 nm to a precision of roughly 1 part per billion over land under clear-sky conditions. That precision sounds modest until you note that background tropospheric methane sits around 1900 ppb: a 1 ppb sensitivity is enough to detect a large industrial leak at continental scale.
Finer spectral resolution generally means a narrower entrance slit, which reduces the étendue (the product of area and solid angle the instrument accepts) and therefore forces a trade against ground pixel size or signal-to-noise ratio. TROPOMI's ground pixel was 3.5 × 5.5 km at launch, later improved to 3.5 × 5.5 km across-track by operational adjustment. Purpose-built commercial methane imagers such as GHGSat's Wildfire instrument achieve pixels below 25 × 25 m by accepting a much narrower swath (roughly 12 km versus TROPOMI's 2600 km). The choice is not a matter of one being better: wide-swath instruments find the anomaly; high-resolution instruments attribute it to a specific facility. A complete monitoring architecture uses both.
The enforcement value of a gas map
Atmospheric concentration maps carry evidentiary weight that ground-based monitoring cannot easily replicate. A facility operator can restrict access to a ground sensor; they cannot prevent a satellite from overflying. When TROPOMI detected elevated NO2 columns over industrial zones during the early 2020 lockdowns, the spatial pattern was specific enough to attribute emission reductions to individual sectors. The same logic applies to treaty verification: the IAEA and national regulators increasingly cite satellite-derived SO2 and NO2 data as independent corroboration of declared facility activity.
For a government buyer, the practical enforcement question is revisit frequency versus detection threshold. TROPOMI on a sun-synchronous orbit at 824 km achieves global daily coverage at its 2600 km swath, but a single overpass under partial cloud may miss a transient leak event. Stacking multiple days of clear-sky retrievals reduces random noise but allows a short-duration release to go undetected. A constellation of smaller sounders, or a combination of wide-swath and targeted high-resolution assets, closes that gap. The methane monitoring architecture proposed under the Global Methane Pledge explicitly anticipates a mixed fleet for this reason.
Where the physics stops cooperating
Cloud is the dominant limit. TROPOMI's methane retrieval requires a cloud radiance fraction below roughly 0.5; in persistently cloudy regions such as the Congo Basin or maritime Southeast Asia, usable clear-sky observations may occur on fewer than half of all passes. Aerosol loading from biomass burning introduces retrieval biases that require auxiliary aerosol data to correct, and those corrections add uncertainty. Over bright desert surfaces, albedo variability can mimic absorption features at the 1 ppb level.
Thermal infrared sounders such as IASI have their own pathology. They are most sensitive to gases in the mid-to-upper troposphere because the surface emission contrast is highest there; near-surface concentrations, which are what regulators usually care about, are retrieved with larger uncertainty and require careful a priori assumptions about the atmospheric profile. On a practical level, the retrieval algorithms for IASI-class instruments are computationally intensive: near-real-time products typically lag the overpass by two to three hours, which is acceptable for climate monitoring but not for emergency response to an acute industrial release. Finally, instrument calibration drifts over mission life; without an on-board calibration source or regular vicarious calibration against reference sites, systematic biases accumulate and make trend detection unreliable. This is not a reason to avoid the instrument class, but it is a reason to budget for calibration infrastructure from day one.
Sizing a national sounder: what the numbers look like
A TROPOMI-class instrument is a large payload: the Sentinel-5P satellite masses approximately 820 kg and the instrument itself accounts for the majority of that. It is not a smallsat option. For a national programme with a more constrained budget, the realistic choices are either a hosted payload on a commercial or partner satellite, a compact shortwave-infrared spectrometer in the 50 to 150 kg class targeting a single gas (methane or CO2), or a data-purchase agreement against an existing operational constellation.
A compact methane sounder in the 6U to 12U cubesat class has been demonstrated at proof-of-concept level, but published precision figures for cubesat spectrometers remain an order of magnitude worse than TROPOMI for equivalent integration times. The physics is unforgiving: smaller aperture means fewer photons, and fewer photons mean noisier spectra. A 16U platform with a 10 cm aperture and a cooled shortwave-infrared detector can achieve roughly 10 ppb methane precision per pixel over a 200 km swath, which is useful for detecting major industrial point sources but not for diffuse agricultural emissions. Knowing that boundary before the mission design review saves a great deal of time.
Engineering parameters
| Instrument mass (TROPOMI class) | 200 to 400 kg (full satellite 700 to 900 kg) |
| Instrument mass (compact single-gas sounder) | 15 to 80 kg depending on aperture and cooling |
| Spectral range (typical) | UV to SWIR: 270 to 2400 nm; TIR sounders: 645 to 2760 cm⁻¹ |
| Spectral resolution | 0.25 to 0.55 nm (TROPOMI-class); 0.5 cm⁻¹ (IASI-class) |
| Ground pixel size | 3.5 × 5.5 km (TROPOMI); 12 × 12 km (IASI); <25 × 25 m (high-res commercial) |
| Swath width | 2600 km (TROPOMI); 2200 km (IASI); 12 to 300 km (compact/commercial) |
| Methane column precision (clear sky, land) | ~1 ppb (TROPOMI-class); ~10 ppb (compact cubesat-class) |
| Detector operating temperature | SWIR detectors: 140 to 200 K (requires passive or active cooling) |
| Raw data rate | 100 Mbps to 1 Gbps depending on spectral channels and pixel count |
| Revisit (single satellite, SSO) | Daily global coverage at wide swath; 2 to 5 days for high-resolution narrow-swath |
One contract, one accountable engineer
Commissioned as one programme, not a stack of contracts: spacecraft, launch, ground segment, mission control, training and handover are priced together. Source-access terms and audit rights are agreed in writing before signature. Request a sounder mission scoping call.