Satellites & Instruments

A remote sensing instrument sends out its own pulse of energy and times the echo. Is it active or passive?

Active: it supplies its own energy. A passive instrument only receives sunlight reflected by Earth or radiation that Earth emits.

Which listed instruments are active?

The lidar (laser radar) CALIOP on CALIPSO, a cloud and aerosol satellite, the Cloud Profiling Radar (CPR) on CloudSat, the precipitation radar DPR on GPM (Global Precipitation Measurement), and the radar altimeters on the Jason and Sentinel-6 sea level satellites.

Which kinds of instruments are passive?

Imagers, radiometers, spectrometers and sounders, which only receive reflected sunlight or emitted radiation. On the list, only the lidar, the radars and the altimeters are active.

What is the difference between a radiometer and an imager?

A radiometer measures how much radiation arrives in set wavelength bands. An imager builds a picture from such measurements, so most satellite imagers are 'imaging radiometers'.

What is the difference between a spectrometer and a sounder?

A spectrometer splits light into many narrow wavelengths to identify gases by their absorption lines. A sounder uses channels with different absorption strengths to build vertical profiles of temperature and humidity.

Is GRACE (twin satellites that map changes in Earth's gravity) usually counted as an active or passive sensor, and why is it a trick question?

Usually not active (keys call it passive or 'tracking'): it senses gravity's pull, not energy bounced off Earth. But the twins range each other with microwaves, so a few keys call it active.

Why can passive reflected-light bands (visible, near-IR or NIR, shortwave IR or SWIR) not image at night, while thermal IR and microwave bands can?

Reflected-light bands need sunlight. Thermal IR and microwave measure radiation that Earth and clouds emit themselves, day or night.

What is 'brightness temperature,' the quantity thermal IR and microwave images report?

The temperature a perfect blackbody would need to emit the measured radiance. It equals the true temperature only if emissivity (how well a surface emits compared with a blackbody) is 1 and the atmosphere adds or removes nothing.

What does 'nadir' mean, and what is a 'swath'?

Nadir is straight below the satellite. The swath is the width of ground imaged in one pass (about 2,330 km for MODIS, the main imager on NASA's Aqua and Terra satellites, but only about 0.1 km for a nadir lidar).

What does a 'curtain' image (height on the y-axis, latitude or distance along the track on the x-axis) tell you about the instrument that made it?

It came from a nadir-pointing active profiler (lidar or radar) such as CALIPSO or CloudSat, which slices a thin vertical cross section under its track instead of mapping a wide swath.

Why do nadir profilers like CALIPSO's lidar and CloudSat's radar show only a thin slice of a hurricane, not its whole circle?

They point straight down without scanning across track, so they sample only a narrow line (about 0.1 to 1.4 km wide) along the orbit.

What is 'backscatter'?

The part of an emitted lidar or radar pulse scattered straight back toward the instrument. Stronger backscatter means more or bigger particles at that height.

What is a sun-synchronous orbit, and why do most listed Earth-observing satellites use one?

A near-polar low orbit that crosses each latitude at the same local solar time every day. It gives consistent lighting for comparing images over time, plus near-global coverage.

Which listed satellite is NOT in low Earth orbit (LEO)?

GOES, the geostationary (GEO) weather satellites of NOAA (the US weather agency), 35,786 km above the equator. Everything else on the list is in LEO, including the instruments on the International Space Station (ISS).

GOES-East always views the same area. What orbit does that imply, and what is the payoff?

Geostationary: it orbits above the equator once per Earth rotation (23 h 56 min), so it stares at one hemisphere and can re-image it every few minutes.

What are the main drawbacks of a geostationary imager like ABI (the main imager on GOES) compared with a polar orbiter like MODIS?

Coarser pixels (0.5 to 2 km for ABI) because it is about 50 times farther away, and poor, very slanted views of high latitudes and the poles.

What was the A-Train, and why did satellites fly in it?

The 'Afternoon' constellation: sun-synchronous satellites flying minutes apart on the same track (about 1:30 pm equator crossing), so different instruments saw the same scene almost at once.

Which two satellites carry MODIS, and how do their overpass times differ?

Terra (morning, about 10:30 am, descending) and Aqua (afternoon, about 1:30 pm, ascending), NASA's two flagship Earth Observing System satellites. The rules list MODIS on Aqua, but Terra's MODIS is identical.

What makes MODIS the workhorse imager for climate questions?

36 bands from visible to thermal IR (0.4 to 14.4 µm), a 2,330 km swath that covers the globe every 1 to 2 days, and a record of 25+ years.

What are MODIS's three spatial resolutions?

250 m (bands 1-2, red and NIR), 500 m (bands 3-7) and 1 km (bands 8-36, mostly IR).

MODIS spatial vs temporal resolution: which is high and which is low, and in what orbit?

High temporal (global every 1-2 days), low to moderate spatial (250 m to 1 km), sun-synchronous polar LEO.

Is MODIS active or passive, and is it a pushbroom or whiskbroom scanner?

Passive. Whiskbroom (cross-track): a rotating mirror sweeps its view side to side across the swath. A pushbroom instead images a whole cross-track line at once with a detector array.

Which MODIS bands make a true-color image, and in what display order?

Bands 1 (red), 4 (green), 3 (blue), displayed as R-G-B = 1-4-3.

Which MODIS bands are used to compute NDVI, the normalized difference vegetation index?

Band 2 (NIR, 841-876 nm) and band 1 (red, 620-670 nm): NDVI = (B2 - B1)/(B2 + B1).

In a MODIS 7-2-1 false-color image, what colors are burn scars, healthy vegetation and snow, and why?

Burn scars red-brown (bright in SWIR band 7, dark in NIR), vegetation bright green (high NIR, band 2), snow and ice bright cyan (absorb SWIR, bright in red band 1).

In a MODIS 3-6-7 false-color image, what appears red-orange, and what is this composite for?

Snow, lake and sea ice, and ice clouds (they absorb SWIR bands 6 and 7 but reflect blue band 3). It separates snow and ice from liquid-water clouds, which look white or peach.

Why do snow and water clouds look the same in true color but different near 1.6 µm (SWIR)?

Both are bright white in visible light, but ice absorbs strongly at 1.6 µm while water droplets still reflect, so snow turns dark and clouds stay bright.

Which climate feedback can a season of MODIS 3-6-7 images help track, and how?

The ice-albedo feedback: the images map the area and duration of snow and ice cover, which controls how much sunlight is reflected.

Which MODIS spectral region gives cloud-top temperature, and why that one?

The ~11 µm thermal IR window (band 31): thick clouds emit there almost like blackbodies, and little gas absorbs above them.

How does MODIS on Aqua retrieve sea surface temperature (SST)?

From thermal IR near 11 and 12 µm (the 'split-window' difference corrects for water vapor), plus bands near 4 µm at night.

Why is MODIS's ~4 µm SST product night-only?

In daytime, reflected sunlight adds to Earth's emitted signal at 4 µm. At night only emission remains, giving a clean temperature.

Why do MODIS SST maps have scattered blank spots and solid black or gray areas?

Blank spots are clouds (thermal IR cannot pass through cloud, so those pixels are masked). Land is masked too, often shown black or gray.

If IR SST retrievals in the cloudy tropics are not cloud-masked, do they over- or underestimate SST, and why?

Underestimate (cold bias): the sensor measures the cold cloud tops instead of the warm sea surface.

What layer of the ocean does an IR-derived SST actually measure?

Only the skin, the top ~10-20 µm. Deeper structure like the thermocline (the boundary between warm surface water and cold deep water) must be inferred indirectly, for example from sea surface height.

Why are SST and ocean-color maps often shown as 8-day or monthly composites instead of single passes?

Clouds leave gaps in each pass, so averaging many days fills them in (a single pass is 'Level 2', composites are 'Level 3').

An SST map runs from about -2 to 32 on its color bar. What are the units, and is it an anomaly map?

Degrees Celsius (seawater freezes near -2 °C). It is absolute SST: anomaly maps are centered on 0 with diverging plus/minus colors.

In an August MODIS SST map there is data north of 60°N but none south of 60°S. Why?

It is Southern Hemisphere winter: the far south is in or near polar night, covered by sea ice (no open water), and persistently cloudy.

What does aerosol optical depth (AOD) mean, and what values are clean vs very hazy?

A unitless measure of how much the whole aerosol column dims sunlight (transmitted fraction = e^-AOD). Below ~0.1 is very clean, ~1 or more is heavy smoke, dust or pollution.

How do MODIS's Dark Target and Deep Blue aerosol algorithms differ?

Dark Target works over dark vegetation and ocean. Deep Blue uses blue and near-ultraviolet (UV) light near 412 nm, where bright deserts look darker, to retrieve AOD over bright land.

Why does MODIS struggle to retrieve aerosol over a bright desert?

The bright surface's reflectance swamps the small extra reflectance from the aerosol, biasing AOD. The Deep Blue algorithm helps.

Which MODIS bands detect active fires?

The ~4 µm mid-IR bands, compared with the 11 µm band.

Why is a ~4 µm (3.9 µm) band so much more sensitive to fires than an 11 µm band?

A fire (~800-1200 K) peaks near 2.5-3.5 µm (Wien's law), so its 3.9 µm radiance rises enormously compared with ~300 K ground, much more than at 10-11 µm.

Besides darker color, what property marks a fresh burn scar in MODIS thermal data?

Higher land surface temperature: the blackened ground absorbs more sunlight and has lost its evapotranspiration cooling.

In a MODIS true-color image of a fire region, what do red dots or outlines mark?

MODIS active fire detections (thermal hot spots found with the ~4 µm and 11 µm bands).

In true-color imagery, how do you tell smoke from cloud, and which way the smoke is moving?

Smoke is gray-brown and semi-transparent, streaming from fire points. Clouds are whiter with sharper edges. Plumes fan out downwind from their source.

What are the limits of true-color imagery for studying aerosols?

It is qualitative (no AOD number), has no layer height, smoke can be confused with haze or cloud, it needs daylight, and bright surfaces interfere.

What do MODIS product names starting with MOD vs MYD mean?

MOD = Terra, MYD = Aqua (for example MYD11 is Aqua land surface temperature).

Which listed instrument would you use to track day-to-day changes in surface albedo (fraction of sunlight reflected), snow cover or vegetation greenness across a continent?

MODIS (Aqua or Terra), for its wide swath and daily revisit.

What will continue the MODIS record as Terra and Aqua reach the end of their missions?

VIIRS, the imager on the NOAA/NASA weather satellites Suomi NPP, NOAA-20 and NOAA-21.

What does strong red-band (~645 nm) light coming up out of the water mean in MODIS coastal imagery?

Turbid, sediment-laden water such as river or storm-runoff plumes: suspended sediment scatters red light strongly.

Which five instruments does Suomi NPP carry?

VIIRS (imager), CrIS (infrared sounder), ATMS (microwave sounder), OMPS (ozone) and CERES (radiation budget).

What orbit is Suomi NPP in, and which satellites carry the same instruments?

Sun-synchronous polar LEO, about 1:30 pm crossing. NOAA-20 and NOAA-21 carry VIIRS, CrIS, ATMS and OMPS and continue its record (NOAA-20 also carries CERES).

What is VIIRS, and what older sensor does it replace?

A 22-band visible-to-thermal-IR imaging radiometer on Suomi NPP and NOAA-20/21, the successor to MODIS (and to the older AVHRR weather imager) for clouds, aerosols, SST, fires, snow and vegetation.

How does VIIRS compare with MODIS in pixels, swath and band count?

375 m imaging bands and 750 m moderate bands (its thermal bands are much sharper than MODIS's 1 km), a wider ~3,000 km swath with no gaps between orbits, but 22 bands vs MODIS's 36.

What is the VIIRS Day/Night Band, and what can it see?

An extremely sensitive low-light visible/NIR band (~0.5-0.9 µm, ~750 m) that images at night: city lights, gas flares, fires, fishing boats, aurora, and clouds or snow lit by moonlight.

Which instrument would you use to see clouds, snow or city lights at night in visible-like light?

The VIIRS Day/Night Band (lit by moonlight, airglow or the lights themselves).

Why can VIIRS detect smaller fires than MODIS?

Its 375 m fire channel near 3.7 µm has pixels about 7 times smaller in area than MODIS's 1 km fire bands.

Which type of band is used for SST from Suomi NPP?

Thermal IR window bands near 11 and 12 µm (split window), plus ~3.7 µm at night.

Multispectral vs hyperspectral: what is the difference, and which listed instruments are examples?

Multispectral: a few to a few dozen separate bands (MODIS 36, VIIRS 22, ABI 16). Hyperspectral: hundreds of narrow, touching channels forming a continuous spectrum (CrIS on Suomi NPP, the ocean color instrument OCI on PACE).

What does CrIS measure, and what kind of instrument is it?

A passive hyperspectral infrared sounder (a Fourier transform spectrometer with over 1,300 channels, 3.9-15.4 µm) that retrieves vertical profiles of air temperature and water vapor, plus some trace gases.

How can CrIS find temperature at different heights while looking straight down?

Channels where CO2 or water vapor absorbs strongly only 'see' emission from high, cold layers, while weakly absorbed channels see lower down. Combining many channels builds the profile.

What is CrIS's main limitation, and which partner instrument covers it?

Infrared cannot see through clouds. ATMS, its microwave partner on the same satellite, sounds through most non-raining clouds.

What does ATMS, the microwave sounder on Suomi NPP, measure?

Vertical profiles of temperature (from the oxygen band near 50-60 GHz) and humidity (from the 183 GHz water line). It is passive, with 22 channels from 23 to 183 GHz.

Why can ATMS's microwaves see through most clouds that block CrIS's infrared?

Microwaves (millimeters to centimeters long) are far longer than cloud droplets, so non-raining clouds are nearly transparent to them.

Which Suomi NPP instrument sounds temperature and humidity through non-raining clouds?

ATMS, the microwave sounder (CrIS, the infrared sounder, needs clear sky).

ATMS low-frequency window channels (~24-89 GHz) vs channels near the 165-183 GHz water line: what does each mostly see?

Window channels see the surface (strong land-water contrast, since water emits weakly in microwave and looks 'cold'). Water-line channels see upper-tropospheric water vapor and air masses, not the lakes or ground.

A 14 km CrIS pixel covers a 4.2 km wide, 14 °C cold filament in 18 °C water. What temperature does it report, and why?

About 16.8 °C. It is a mixed pixel that averages by area: 0.3 x 14 + 0.7 x 18 = 16.8.

What is the UV aerosol index (absorbing aerosol index), and what does a high value mean?

A unitless index comparing two UV wavelengths with a clean, gas-only atmosphere. Positive values mean UV-absorbing aerosol (smoke, dust, ash), and it works even over clouds and bright deserts.

What does OMPS on Suomi NPP measure?

Ozone (total column and vertical profiles) from backscattered UV sunlight, plus SO2 and the UV aerosol index.

What are OMPS's three parts?

Nadir Mapper (daily global total ozone, SO2, aerosol index), Nadir Profiler (coarse ozone profile) and Limb Profiler (fine vertical ozone and aerosol profiles, looking at the atmosphere's edge).

Which Suomi NPP instrument maps the ozone layer?

OMPS (Ozone Mapping and Profiler Suite).

Which instruments produce the UV aerosol index?

OMPS (on Suomi NPP), OMI (on NASA's Aura chemistry satellite) and TROPOMI (on Europe's Sentinel-5P pollution satellite).

What is the GOES-R series, and which parts of the world do GOES-East and GOES-West watch?

GOES-16 through GOES-19, NOAA's geostationary weather satellites. GOES-East (GOES-19 since 2025) covers the Americas and Atlantic, GOES-West (GOES-18) covers the western US and Pacific.

What are the two Earth-imaging instruments on a GOES-R satellite?

ABI (Advanced Baseline Imager, a 16-band passive imaging radiometer) and GLM (Geostationary Lightning Mapper).

How often does GOES ABI image the full disk, the continental US, and a mesoscale sector?

Full disk every 10 min, the continental US every 5 min, and small movable mesoscale boxes (about 1,000 km) every 60 s (30 s when the two boxes overlap).

What sensor type does GOES use for continuous weather imaging?

A passive imaging radiometer (ABI) measuring reflected visible/NIR and emitted IR radiance.

Which ABI band has the finest resolution, and what is the resolution of the IR bands?

Band 2 (0.64 µm red) at 0.5 km. Most IR bands are 2 km.

GOES ABI band 2 (0.64 µm, red visible): what does it show, and when?

Reflected sunlight (brightness of clouds and surfaces) in the sharpest ABI detail. Daytime only, with no temperature information.

True or false: you can get cloud-top temperature from a GOES visible band.

False. Visible bands measure reflected sunlight. Temperature needs a thermal IR band such as band 13 (10.3 µm).

GOES ABI band 3 (0.86 µm) is nicknamed the 'veggie' band. What does it show?

Healthy vegetation (bright, strong NIR reflection), burn scars (dark) and land-water boundaries (water very dark).

Which two GOES-16 bands would you use to compute NDVI?

Band 3 (0.86 µm NIR) and band 2 (0.64 µm red).

GOES-16 data show a large area of negative NDVI. What is it most likely?

A body of water (water reflects more red than NIR).

GOES ABI band 4 (1.37 µm) is the 'cirrus' band. Why does it show only high clouds?

Water vapor absorbs strongly at 1.37 µm, so light from the ground and low clouds never makes it back. Only high cirrus above most of the vapor reflects sunlight (daytime only).

Which band would you choose to spot thin, high cirrus in daytime imagery?

The ~1.37-1.38 µm cirrus band (ABI band 4, MODIS band 26).

GOES ABI band 5 (1.6 µm) is the 'snow/ice' band. What does it separate?

Snow, ice and ice clouds (dark, because ice absorbs at 1.6 µm) from liquid-water clouds (bright). Daytime only.

GOES ABI band 7 (3.9 µm) is the shortwave IR window. What is it best for?

Fire hot spots day and night, and at night, low clouds and fog (in a difference with band 13).

Which ABI band would you use to detect wildfire hot spots from geostationary orbit?

Band 7 (3.9 µm shortwave IR window).

What do ABI bands 8, 9 and 10 (6.2, 6.9 and 7.3 µm) show?

Water vapor in the upper (6.2), middle (6.9) and lower (7.3) troposphere: moisture plumes, dry slots, jet streams and atmospheric rivers.

Why can't the water vapor bands (6.2-7.3 µm) see the ground?

Water vapor absorbs so strongly there that the radiation reaching space is emitted by the vapor itself, high in the troposphere.

In a water vapor image (for example 6.2 µm), what do bright and dark areas mean?

Bright (cold) = moist upper air or high cloud, since the emission comes from high, cold vapor. Dark (warm) = dry upper air, letting the sensor see lower, warmer layers.

Which ABI bands would you use to track an atmospheric river, a long narrow plume of moisture feeding a storm?

The water vapor bands 8-10 (6.2-7.3 µm), often with total precipitable water products.

GOES ABI band 13 (10.3 µm) is the 'clean' longwave IR window. What does it measure?

The brightness temperature of whatever it sees: cloud-top temperature where cloudy, land or sea temperature where clear. Works day and night with very little water vapor absorption.

In GOES band 13 (10.3 µm) imagery, what do very cold brightness temperatures (often colored green, yellow, red) indicate?

High, cold cloud tops of deep convection (strong thunderstorms, hurricane eyewalls), because the troposphere cools about 6.5 K per km of height.

What do expanding, very cold cloud tops and overshooting tops in IR imagery imply about a storm?

Strong updrafts lifting air up to the tropopause, which means heavy rain and possibly hail and severe weather.

What is a thunderstorm's anvil, and why does it form where it does?

The flat, spreading ice-crystal cloud at the top of a cumulonimbus. Rising air hits the stable tropopause, stops rising and spreads sideways.

Why are IR satellite images usually displayed 'inverted' (cold = white)?

So high cold clouds look white as in visible images, while warm ground and ocean look dark. In ordinary thermal images warmer is lighter.

GOES ABI band 16 (13.3 µm) is the CO2 longwave band. What does it sense, and why?

CO2 absorbs at 13.3 µm, so the band sees emission mostly from the middle and upper troposphere. Only high, cold clouds stand out, which helps assign cloud-top and wind heights.

What is 'CO2 slicing'?

Comparing IR bands with different amounts of CO2 absorption (for example 13.3 µm vs the 11 µm window) to find the pressure, and so the height, of a cloud top.

Which ABI band is the 'dirty' longwave window, and why 'dirty'?

Band 15 (12.3 µm): water vapor absorption partly contaminates the window. Band 13 (10.3 µm) is the 'clean' window, and band 16 (13.3 µm) is the CO2 band.

Which ABI band near 9.6 µm is sensitive to ozone?

Band 12, the 'ozone band' (ozone absorbs there in the thermal IR).