CORE 2: Satellites & Instruments

Aqua is a satellite, not an instrument. Which two listed instruments does it carry, and what is each one for?

It carries MODIS, the 36-band daily global camera for clouds, aerosol, sea surface temperature and true color, and CERES, which measures Earth's energy budget. Tests ask in both directions, so remember that MODIS also flies on Terra and CERES also flies on Terra, Suomi NPP and NOAA-20.

Suomi NPP is a NOAA and NASA polar orbiter. Which listed instruments does it carry, and what is each one for?

VIIRS is the daily imager that continues the MODIS record and adds a Day/Night Band, CrIS and ATMS are infrared and microwave sounders for temperature and humidity at many heights, OMPS maps ozone, and CERES measures the energy budget. It is the most crowded platform on the list, so it is a common "which satellite" answer.

Aura is NASA's atmospheric chemistry satellite. Which two listed instruments does it carry, and how do they differ?

OMI is an ultraviolet-visible spectrometer that looks down and maps total ozone, NO2 and SO2 every day. MLS looks sideways through the edge of the atmosphere (the limb) and measures vertical profiles of ozone and ozone-destroying chlorine in the stratosphere.

Which platforms carry ASTER, TROPOMI and ECOSTRESS?

ASTER rides on Terra, the morning satellite that also carries MODIS and CERES. TROPOMI rides on ESA's Sentinel-5P, and ECOSTRESS is mounted on the International Space Station.

Which four listed instruments are active, and why do most answer keys not count GRACE as active?

The CALIOP lidar on CALIPSO, the CPR radar on CloudSat, the DPR radar on GPM and the Jason radar altimeters send out their own pulses and time the echo. GRACE only measures the changing distance between its two satellites to sense gravity, so it does not probe Earth with a pulse the way the others do.

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

Reflected-light bands need sunlight to bounce off the surface. Thermal IR and microwave measure radiation that Earth, the air and clouds emit themselves, which never stops, and active lidar and radar bring their own light.

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

It is a near-polar low orbit that crosses each latitude at the same local solar time every day. That gives consistent lighting, so images from different days, seasons and years can be compared fairly, plus near-global coverage as Earth turns underneath.

Five listed missions are not in sun-synchronous orbits: GOES, GPM, Jason, GRACE and ECOSTRESS. What do the LEO ones gain from that?

Their orbits drift through different local times of day, so GPM samples rain at all hours, the Jason altimeters avoid blurring the daily tides into their sea level record, and ECOSTRESS on the ISS sees plants at morning, noon and evening. GOES is the exception because it is geostationary.

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

It is geostationary, about 36,000 km above the equator, circling once per Earth rotation so it hovers over one spot. It can re-image the same hemisphere every few minutes, which is why it is the tool for watching storms develop.

What are the main tradeoffs between a geostationary imager like ABI on GOES and a polar orbiter like MODIS?

GOES gives far more frequent images of one hemisphere but has coarser pixels, because it is dozens of times farther away, and only slanted views of high latitudes. MODIS gives finer pixels, consistent lighting and coverage of the whole globe including the poles, but only once or twice a day.

A smoke plume from Canadian wildfires drifts across the US for several days. Why pair GOES with MODIS or VIIRS to study it?

GOES images the plume every few minutes, so it tracks where the smoke moves hour by hour. The polar orbiters pass only once or twice a day but have finer pixels and fire-detection bands that pinpoint the active fires and the thickest smoke.

What was the A-Train, and what was the point of flying satellites that way?

It was a line of sun-synchronous satellites flying minutes apart on the same track and crossing the equator around 1:30 pm (the "afternoon train"), including Aqua, Aura, CALIPSO, CloudSat and OCO-2. Different instruments saw the same scene almost at once, so their measurements could be combined. Terra (a morning satellite) and GOES (geostationary) were never members.

Which listed satellite is still flying at the front of the A-Train, and what happened to the others?

OCO-2. CloudSat and CALIPSO dropped to a lower orbit in 2018 and have since retired, and Aqua and Aura ran low on fuel and are drifting to later crossing times.

Describe MODIS's spatial and temporal resolution and its orbit.

It has high temporal resolution, covering the whole globe every one to two days, but only moderate spatial resolution, with pixels of 250 m to 1 km. Its wide swath from a sun-synchronous low orbit is what makes the frequent global coverage possible.

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

It came from a nadir-pointing active profiler, meaning CALIPSO's lidar or CloudSat's radar. These point straight down and slice a thin vertical cross section along their track instead of mapping a wide swath.

What is the difference between a multispectral and a hyperspectral instrument?

A multispectral instrument records a few to a few dozen separate bands, like MODIS, VIIRS and GOES ABI. A hyperspectral instrument records hundreds of narrow, touching channels that form a continuous spectrum, like CrIS and PACE's ocean color instrument, which lets it pick out subtle spectral fingerprints.

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

False. Visible bands measure reflected sunlight, which tells you brightness, not temperature. Temperature needs a thermal infrared window band near 10 to 11 µm.

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

It is the temperature a perfect blackbody would need to emit the measured radiation. It equals the true temperature only if the surface's emissivity is 1 and the atmosphere adds or removes nothing, so products correct for both.

In a GOES infrared window image (about 10.3 µm), what do the coldest brightness temperatures, often colored green, yellow or red, indicate?

They mark high, cold cloud tops of deep convection such as strong thunderstorms and hurricane eyewalls. The troposphere cools about 6.5 °C per km of height, so the colder the top, the taller the cloud.

Why can a water vapor image (about 6 to 7 µm) never show the ground, and what do its bright and dark areas mean?

Water vapor absorbs so strongly at these wavelengths that the radiation reaching space comes from the vapor itself, high in the troposphere. Bright (cold) areas are moist upper air or high cloud, and dark (warm) areas are dry upper air that lets the sensor see lower, warmer layers.

A cloud is bright in a visible image but warm (gray) in an infrared image. What kind of cloud is it?

A thick, low liquid-water cloud such as stratus, stratocumulus or fog. It is thick enough to reflect a lot of sunlight, but its top is low and therefore nearly as warm as the ground.

A cloud is bright in a visible image and very cold in an infrared image. What kind of cloud is it?

A deep convective cloud such as a cumulonimbus, a thunderstorm complex or a hurricane core. It is thick enough to reflect strongly and tall enough that its top is very cold.

A cloud is dim or see-through in a visible image but cold in an infrared image. What kind of cloud is it?

Thin, high cirrus. It is too thin to reflect much sunlight, but it sits high where the air is cold, so its emission is cold.

Why is a band near 3.9 µm much more sensitive to fires than an 11 µm band?

A fire is roughly 1,000 K, so by Wien's law its emission peaks near 3 µm, and its 3.9 µm radiance shoots up compared with roughly 300 K ground. At 11 µm the difference between fire and ground is far smaller.

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

It is an extremely sensitive low-light visible band on VIIRS that images at night. It sees city lights, gas flares, fires and fishing boats, plus clouds and snow lit only by moonlight, so a nighttime city-lights image points to VIIRS.

What does a "true color" image mean, and why does GOES have to simulate its green?

True color puts the red, green and blue bands into the red, green and blue channels so the scene looks as your eye would see it, which is how MODIS true color is made. GOES ABI has no green band, so its true color invents green from the neighboring bands.

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

Burn scars are red-brown because charred ground is bright in shortwave IR (shown as red) but dark in near-IR. Vegetation is bright green because leaves reflect near-IR strongly (shown as green). Snow and ice are cyan because they absorb shortwave IR but are bright in visible red.

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

Snow, sea and lake ice, and ice clouds appear red-orange because they absorb the shortwave IR bands but reflect blue. Liquid-water clouds stay white, so the composite separates snow and ice from ordinary cloud and is used to track snow and ice cover for the ice-albedo feedback.

A daily MODIS sea surface temperature map is speckled with blank spots over the ocean. What causes them?

Clouds. Infrared cannot see through cloud, so cloudy pixels have no sea surface reading. That is why monthly averages look complete while daily maps look patchy.

A MODIS map made from reflected sunlight in August has data far north of 60°N but none far south of 60°S. Why?

The Southern Hemisphere's high latitudes are in winter darkness in August, so there is no sunlight to reflect, while the Arctic has summer sun. The gap moves to the north in December.

What layer of the ocean does an infrared sea surface temperature actually measure?

Only the skin, the top fraction of a millimeter, because water absorbs infrared almost immediately. Deeper structure, like how much warm water is stored below, must come from other tools such as sea surface height.

On a MODIS sea surface temperature map with lettered spots, how do you decide where a hurricane could form?

Look for water at about 26.5 °C or warmer that is at least a few degrees of latitude away from the equator. Warm water supplies the storm's heat energy, and the Coriolis effect is needed to start the spin, and it vanishes at the equator.

Why are MODIS aerosol optical depth maps often blank or unreliable over bright deserts like the Sahara?

The method looks for extra brightness that aerosols add over a dark surface. A bright desert already reflects so much that the aerosol signal is hard to separate, so the retrieval fails or is biased.

How do MODIS and CALIPSO differ in what they tell you about aerosols?

MODIS passively measures the total aerosol optical depth of the whole column over a wide swath, which shows the horizontal extent. CALIPSO's lidar actively measures where the aerosol layers are in height along a thin line.

What does the polarization channel of CALIOP, the lidar on CALIPSO, let it do?

It measures how much the returning laser light is depolarized. Irregular particles like ice crystals, dust and ash depolarize strongly, while round ones like water droplets and smoke droplets barely do, so it can tell ice from water and dust from smoke.

In a CALIPSO backscatter curtain, what do a thin bright line along the bottom, strong patches high up, and black areas under a layer mean?

The bright line along the bottom is the ground or sea surface reflecting the laser. Strong patches high up are usually cirrus or other cloud, and black under a layer means the laser was completely blocked there, not that the air is clear.

Why does a lidar curtain go dark below thick clouds?

Thick cloud scatters and absorbs the laser light completely, the way fog swallows headlights, so no signal returns from below. There may still be more cloud or aerosol down there that the lidar cannot see.

Why can lidar see thin cirrus and fine aerosol that cloud radar misses?

Lidar's light is about 0.5 to 1 µm long, close to the size of these tiny particles, so they scatter it strongly. They are far too small to scatter a radar wave a few mm long, which slides past them almost unseen.

Months after a large eruption, CALIPSO shows a thin aerosol layer about 15 to 25 km up. What is it, and what does it do to climate?

It is stratospheric sulfate aerosol made from the volcano's SO2. It reflects incoming sunlight and cools the surface, and it lasts a year or two because the stratosphere has no rain to wash it out.

Why can CloudSat's radar see through thick clouds that block CALIPSO's lidar?

Its wavelength of about 3 mm is hundreds of times larger than cloud droplets, so the droplets scatter it only weakly and the pulse reaches cloud base and the ground. It still reflects off larger particles like ice and rain, which is what it maps.

Two cloud curtains of the same storm: one is fine-grained, shows aerosol layers and goes blank below thick cloud, and the other is coarser and reaches the ground. Which is which?

The fine-grained one that goes blank is CALIPSO's lidar, and the coarser one that penetrates to the ground is CloudSat's radar. Tests give partial credit if you name both satellites without matching them, so always say which is which.

Between CALIPSO and CloudSat, which gives finer vertical detail and which is better for tracking a thin wildfire smoke layer?

CALIPSO for both. Its laser pulses resolve layers tens of meters thick, and smoke particles are small enough to scatter its light strongly, while radar barely sees smoke.

Why were CloudSat and CALIPSO launched together in 2006 and flown in formation with Aqua?

They are complementary. Lidar sees thin cirrus and aerosol, radar sees thick cloud interiors and precipitation, and MODIS on Aqua adds the horizontal picture, so together they gave the full 3-D structure of clouds.

Order CALIOP (CALIPSO's lidar), CPR (CloudSat's radar) and DPR (GPM's radar) from shortest to longest wavelength, with the particles each sees best.

CALIOP uses light under 1 µm and sees aerosol and thin cloud. CPR uses about 3 mm and sees cloud droplets, ice and light precipitation. DPR uses about 1 to 2 cm and sees rain and snow. Longer waves see bigger particles and penetrate deeper.

GPM's radar can build a 3-D view of rain inside a storm, while CloudSat and CALIPSO only make flat curtains. Why?

GPM's DPR sweeps its beam from side to side across its track, so it maps a swath with depth, while CloudSat and CALIPSO point only straight down and record one line.

GPM's Core Observatory carries a radar (DPR) and a passive microwave radiometer (GMI). Why carry both?

The DPR gives detailed 3-D rain structure but only over a narrow swath. The GMI sees a much wider swath, and the two together calibrate the partner satellites whose data are merged into the near-global rain maps.

Why does GPM's DPR radar use two frequencies?

Comparing the echoes at two frequencies reveals the size of the raindrops and lets the radar correct for the signal being weakened by heavy rain. The higher frequency is more sensitive to light rain and snow, and the lower one handles heavy rain.

A GPM rain map shows little or no precipitation in some areas that a visible image shows are covered by cloud. Why?

Not all clouds produce rain. GPM maps precipitation falling through the cloud, not the cloud itself, so thin or non-raining clouds leave no signal.

What does CrIS on Suomi NPP measure, and how can it find temperature at different heights while looking straight down?

CrIS is a hyperspectral infrared sounder that retrieves air temperature and humidity at many heights. Channels where CO2 or water vapor absorb strongly only "see" emission from high, cold air, while weakly absorbed channels see lower down, so combining channels builds a profile.

Which of these is not blocked by clouds: ECOSTRESS, MODIS, CrIS or ATMS, and why?

ATMS, the microwave sounder on Suomi NPP. Microwaves pass through non-raining clouds, while the other three use infrared or visible light that clouds block, which is why ATMS fills in temperature and moisture profiles under clouds where CrIS cannot.

What does CERES measure?

It measures Earth's radiation budget at the top of the atmosphere in W/m², meaning the sunlight reflected back to space (shortwave), the heat Earth emits (outgoing longwave radiation, or OLR) and the net. It is a broadband radiometer, so it adds up all the energy in a wide range instead of splitting it into narrow bands.

Two CERES maps with cropped color bars look alike. How do you tell reflected shortwave from outgoing longwave?

Reflected shortwave is high where things are bright, like clouds, ice and deserts, while OLR is high where things are hot and clear. The giveaway is the cloudy tropical belt (the ITCZ), which is high in shortwave because thick cloud reflects sunlight but low in OLR because its high tops are cold.

Why is OLR highest over subtropical deserts and lower over tropical rainforests?

Hot, dry, clear desert air lets the warm ground emit straight to space, and emission grows as T⁴. Over rainforests, thick high clouds with cold tops and abundant water vapor absorb the surface's heat and emit less to space.

On July CERES maps, Antarctica is the lowest color on both the reflected shortwave and the OLR maps. Why, and which one can actually reach zero?

In July Antarctica is in polar night, so reflected sunlight is truly zero. Its OLR is low because the ice is very cold, but it never reaches zero because everything warmer than absolute zero emits, so only a shortwave map can read zero.

Do clouds warm or cool Earth overall according to CERES, and why?

They cool it overall, by about 20 W/m². Their reflection of sunlight back to space outweighs the heat they trap, and the cooling is strongest over dark oceans where clouds add the most brightness.

What has CERES revealed about Earth's overall energy balance?

Earth is absorbing slightly more sunlight than the heat it sends back out, so the net flux into the planet is positive, about 1 W/m² or a bit more. That extra energy is what is warming the oceans and the climate.

CERES vs MODIS: how do they differ, and why are they flown together?

CERES has a few broadband channels and coarse footprints about 20 km wide that measure total energy in W/m². MODIS has 36 narrow bands at 250 m to 1 km that identify the scene, such as cloud type, aerosol and surface, which tells CERES what it is looking at.

How does a Jason radar altimeter measure sea surface height?

It sends a radar pulse straight down and times the echo, which gives the distance to the sea. Subtracting that distance from the satellite's precisely tracked orbit height gives sea surface height, and an onboard microwave radiometer corrects for water vapor slowing the pulse.

Why does a high sea surface height anomaly on a Jason map usually mean extra heat stored in the upper ocean?

Warm water expands, and a thicker warm upper layer makes the sea surface stand taller. So a mound in sea level marks extra heat stored below, which infrared sea surface temperature cannot see.

How does El Niño appear on a Jason sea surface height anomaly map?

Sea level is well above normal along the central and eastern equatorial Pacific, toward South America, and below normal in the western Pacific. Weakened trade winds let the warm water that was piled up in the west slosh east, where it expands and raises the sea.

How do GRACE's two satellites, flying about 220 km apart, sense a region with extra mass such as a thick ice sheet?

The leading satellite feels the extra pull first and speeds up, so the gap grows, then the trailing one catches up as the leader moves past and slows. The gap is measured extremely precisely, and a month of orbits builds a gravity map whose month-to-month changes show where ice, groundwater or ocean water was gained or lost.

A GRACE time series over California's Central Valley drops steeply during droughts. What is being lost, and why can GRACE see it?

Groundwater, pumped out of aquifers when surface water runs short. Losing that much underground water reduces the local mass and gravity, which GRACE senses even though the water is hidden from any camera.

Jason shows sea level rising in a region while GRACE shows no added mass. What is the cause?

Thermal expansion of warming water, also called the steric change. It adds volume but not mass, so the altimeter sees the sea rise while GRACE sees nothing. If GRACE did show added mass, the rise would come from extra water instead.

How is the sea level budget closed with satellites?

Total rise from the Jason altimeters equals the ocean mass gained, measured by GRACE, plus thermal expansion, measured by Argo floats that profile ocean temperature. The difference between altimetry and GRACE is therefore the expansion part.

Both a Jason sea level map and a GRACE map use rainbow colors and length units. How do you tell them apart?

A Jason map covers only the ocean and is sharp enough to show eddies. A GRACE map colors land and ice too, because it shows stored water and ice anywhere, and its features are smooth blobs hundreds of km across.

What does PACE measure, and what does its hyperspectral ocean color instrument add?

It measures ocean color (phytoplankton and ocean ecosystems) plus aerosols and clouds. Because its instrument records a continuous spectrum instead of a few bands, it can tell which types of phytoplankton are present, not just how much chlorophyll there is.

Which reflectance ratio is used to estimate ocean chlorophyll, and why?

The ratio of blue to green light. Chlorophyll absorbs blue light, so more phytoplankton lowers the blue/green ratio and makes the water look greener.

Where are the high-chlorophyll areas on an ocean color map usually found, and why?

In coastal upwelling zones, such as off California, Peru and northwest Africa, and near river mouths. Nutrient-rich water brought up from below or washed off the land fuels phytoplankton blooms.

What does OCO-2 measure?

It measures XCO2, the average CO2 concentration in the whole column of air from the ground to space, in ppm. It also picks up solar-induced fluorescence, a faint glow from plants that tracks photosynthesis.

How does OCO-2 measure CO2?

Sunlight passes down through the atmosphere, reflects off the surface and comes back up, crossing the air twice. CO2 absorbs specific shortwave IR wavelengths along the way, and OCO-2's spectrometers measure how deep those dark absorption lines are, since deeper lines mean more CO2.

OCO-2 has two CO2 bands. What is its third band, and why does it need it?

An oxygen band. Oxygen is a fixed fraction of dry air, so its absorption tells OCO-2 how much air the light actually crossed, which turns the CO2 signal into a concentration and flags clouds or aerosol that changed the light path.

Why does OCO-2 use shortwave IR CO2 bands rather than the strong 15 µm thermal band that infrared sounders like CrIS use?

Reflected sunlight in the shortwave IR samples the whole column down to the surface, where CO2 sources and sinks are. Thermal emission at 15 µm mostly comes from CO2 in the middle troposphere, so it misses the air near the ground.

Why do clouds and thick aerosol cause OCO-2 to fail or give biased CO2 values?

OCO-2 needs sunlight that crossed the full column and bounced off the surface. Clouds block or reflect the light before it reaches the ground, and aerosol scatters it, changing the path length so the amount of air crossed is uncertain.

OCO-2 maps lose data at high latitudes, and the gap moves from one hemisphere to the other through the year. Why?

OCO-2 needs reflected sunlight, and each pole is dark or has very low sun in its winter because of Earth's tilt. As the seasons change, the dark zone shifts to the other hemisphere.

OCO-2 shows a large seasonal swing in CO2 in the Northern Hemisphere and a small one in the Southern Hemisphere. Why?

The Northern Hemisphere has much more land and forest, so plants draw CO2 down strongly in summer and release it in winter, the planet "breathing." It also has most of the fossil fuel emissions, so its CO2 is higher overall.

A test shows an OCO-2 solar-induced fluorescence map. What does it show, and how can it reveal the season?

Fluorescence is a faint red glow that chlorophyll emits while photosynthesizing, so it maps how actively plants are growing. If the Northern Hemisphere's land is glowing strongly, it is northern summer.

OCO-2 vs TROPOMI: which measures CO2 and which measures methane?

OCO-2 measures CO2 and has no methane band. TROPOMI measures methane and carbon monoxide, along with NO2, SO2 and ozone, but not CO2.

Why can MLS on Aura measure ozone chemistry in polar night when OMI cannot?

MLS measures microwave emission from the gases themselves, which needs no sunlight. OMI needs ultraviolet sunlight scattered back up, so its winter ozone maps are blank over the dark pole.

On trace gas maps, how do NO2, SO2 and CO look different, and why?

NO2 lasts only hours, so it forms tight hot spots over cities, highways and power plants. SO2 usually appears as a single plume from a volcano or smelter on a clean background. CO lasts one to two months, so fire plumes spread across whole continents and oceans.

TROPOMI and OMI both map NO2. How does a TROPOMI map look different?

TROPOMI's pixels are only a few km across, sharp enough to outline single cities and even highways. OMI's larger pixels blur cities into broad blobs.

How does ASTER on Terra build 3-D elevation maps of the land?

It has two telescopes, one pointing straight down and one looking backward, that image the same spot from two angles moments apart. The shift between the two views (parallax) gives the height, the same way your two eyes judge depth.

Where is ECOSTRESS mounted, and why there?

It is on the International Space Station, whose orbit is not sun-synchronous, so ECOSTRESS passes at different times of day and captures the daily cycle of plant temperature and stress. The tradeoff is that it never sees beyond about 52° latitude, the ISS orbit's limit.

Why does a plant under water stress look warmer to ECOSTRESS?

It closes its stomata to save water, which stops transpiration, the evaporative cooling of its leaves. Without that cooling the leaves warm by a few degrees, so a warm patch of crops is a sign of stress before it shows in color.

Why is ECOSTRESS better than MODIS for studying urban heat islands?

Its pixels are about 70 m instead of MODIS's 1 km, so it resolves individual neighborhoods, parks and roads instead of averaging hot pavement with cooler grass in one mixed pixel. Its changing overpass times also show how city heat builds and lingers through the day.

How do you tell an anomaly map from an absolute-value map?

An anomaly map shows the difference from the long-term average, so its color bar is centered on zero with plus and minus values, and its title often says "anomaly" or "departure." An absolute map's color bar starts at a real physical value, and its pattern mostly follows latitude.

A map shows sea surface height anomaly in cm. Which missions made it?

Radar altimeters, meaning TOPEX/Poseidon, the Jason series and Sentinel-6. They are the only listed tools that measure the height of the sea surface itself.

A map shows mass change in cm of "equivalent water thickness" or gigatonnes. Which mission made it?

GRACE or GRACE-FO. They weigh changes in ice, groundwater and ocean water through gravity, and express the mass as a depth of water