Meteorological satellites fall into two general classes in terms of their orbital characteristics. We can group most into the Polar Orbiting Environmental Satellite (POES) category. These launch into orbits at high inclinations to the Earth's rotation (at low angles with longitude lines), such that they pass across high latitudes near the poles. Depending on orbital altitudes, angular velocities, and inclinations, these satellites strive to be sun-synchronous, that is, to cross the equator southbound about 11° westward (as Earth rotates underneath) with each trip around the world (about 105 minutes long), so that they pass over some reference position (e.g., the equator) at the same local time. This time is usually between mid-morning and mid-afternoon on the sunlight side of the orbit. Thus they image their swaths at about the same sun time during each pass, so that lighting remains roughly uniform. Of course the clouds change with each orbit, but their broad patterns and positions remain mostly unchanged in the short orbital periods involved.
From this method, we can make a daily mosaic from the swaths, which is a good general summary of global weather patterns for that period. This same orbital configuration applies to Landsat, SPOT, and some of the other land observers. Other POES members have inclinations or other orbital constraints, such that they cross equivalent latitudes at different times of day, allowing observations over various times in the diurnal cycle. Most POES orbits are circular to slightly elliptical at distances ranging from 700 to 1700 km (435 - 1056 mi) from the geoid. At different altitudes they travel at different speeds.
The second type of Metsat is in the Geostationary Operational Environmental Satellite (GOES) class. These satellites are geosynchronous, meaning their orbits keep them synchronized with Earth’s rotation, i.e., they take 24 hours to complete one orbit. To have a 24 hour orbital period, they must keep an orbital altitude of 35,780 km (22,234 mi, or about 5.61 Earth radii), which sets their speed at 3.07 km/s (6,868 mph).An equatorial point travels underneath at a speed of about 0.465 km/s (1,040 mph).
When these satellites orbit above the equator, with zero inclination, they are also geostationary (fixed) relative to a point on the equator, so that they observe the Earth without any significant relative motion. At this distance, and with a wide FOV, they see the Earth as a full disk, but the area covered is less than a hemisphere, being about 1/4th of the planetary surface.
The Earth and satellite, synchronized, rotate relative to the Sun, so that part of each day, the satellite's sensors face the night side. But, because of this night effect, to view the Earth in daylight continuously requires at least three (preferably four) geostationary satellites located equidistantly around the globe above the equator. Under these orbital conditions, the sensors can look at the disk at any moment, gaining a synoptic image, in which it views all meteorological elements in real time. We acquire visible images typically 30 minutes apart and infrared images less often. GOES platforms usually operate in pairs, spacing about 75° apart. We refer to current GOES units above the western hemisphere as GOES-East and GOES-West.This next figure offers a quick look at most of the major Metsat systems (POES and GOES) now operating to provide worldwide or hemispherical coverage.
Most of these will be discussed later in the Section as we go through the various Metsats operating in both the past and present. Before taking this sojourn, a diversion is necessary to emphasize the three-dimensional nature of meteorological remote sensing.
Throughout this Tutorial so far, the various applications of remote sensing have dealt almost exclusively with obtaining information pertaining to two-dimensional targets - principally, the Earth's surface. (Several aspects of Geophysics, considered in the Introduction, are concerned with subsurface measurements.) But it is obvious that remote sensing of the atmosphere - a thick layer around Earth - must be three-dimensional (3-D) in its efforts. The concept of probing the atmosphere to determine its properties from surface upwards is embodied in the term "atmospheric sounding".
Sounding is a nautical term that applies to the ancient practice of dropping a line with weights overboard to determine the depth of waters being traversed by a ship. Atmospheric sounding is similar but is accomplished by releasing "weather balloons" that rise upwards carrying measuring instruments.
Temperatures, pressues, moisture content, wind speeds, and composition are the principal variables sought by a weather probe. Temperature is the most frequent parameter thus measured, as seen in this plot:
It would take a huge fleet of weather balloons constantly being sent aloft to get a good picture of the atmosphere's physical state over large areas (actually, volumes). Now, satellites can perform this job much better. Obtaining 3-D atmospheric data with satellite sensors is difficult but doable. These sensors are in effect sounders, which can look vertically through atmospheric levels. Limb sounders look off-nadir through longer paths nearly tangent to the Earth's surface. Most operate on this principle: various properties of atmospheric gases are temperature-dependent. Wavelengths of emissions and absorbances (peaks and troughs) will vary with temperature. If those wavelengths can be measured, variations of temperature with depth into the atmosphere (related to altitude above the surface) can be sounded. Since temperature is the prime parameter to be sensed, the thermal infrared and the passive microwave regions are used almost exclusively in atmospheric sounding.
The writer's attempts to surf the Internet to obtain a clear picture of how atmospheric sounders operate has proved difficult and unrewarding. One passage on a website maintained by CEOS (the Committee for Earth Observations by Satellite) is the best found. Three paragraphs from that site are reproduced here (in italics):
Atmospheric sounders generally make passive measurements of the distribution of IR or microwave radiation emitted by the atmosphere, from which vertical profiles of temperature and humidity through the atmosphere may be obtained. Oxygen or carbon dioxide is usually used as a �tracer� for the estimation of temperature profiles since they are relatively uniformly distributed throughout the atmosphere, and hence atmospheric temperature sounders often measure radiation at wavelengths emitted by these gases. For humidity profiling, either IR or microwave wavelengths specific to water vapour are used. Most measurements are conducted in nadir viewing mode.
Sounders are able to estimate profiles of temperature and humidity by identifying radiation coming from different levels in the atmosphere. This is achieved by observations of the spectral broadening of an emission line, a phenomenon which is primarily caused by intermolecular collisions with other species, and which decreases with atmospheric pressure (and therefore is a function of altitude).
Microwave sounders have the ability to sound through cloud and hence offer nearly all-weather capability; their spatial resolution (both vertical and horizontal) is generally lower than that of the IR instruments. IR sounders are routinely used to provide temperature profiles from a few km altitude to the top of the atmosphere with a temperature accuracy of 2-3K, a vertical resolution of around 10km, and a horizontal resolution of between 10 and 100km.
The way in which the AIRS sounder (see below) determines temperatures/water vapor throughout the atmosphere is described by T. Pagano (pers. comm.) in this way: The individual spectral channels have weighting functions that peak at different altitudes in the atmosphere. A Singular Value Decomposition retrieval (there are other methods too), combined with a radiative transfer program (computes the expected upwelling radiances given a temperature and water vapor profile) is used to solve for the temperature and water vapor profiles. It does this by minimizing the observed spectrum with the calculated spectrum.
Among early Metsat instruments, we can mention as typical: the High Resolution Infrared Sounder (HRIS - 20 channels that cover from the visible to 15 µm, with a coarse resolution at 42 km); the Stratospheric Sounder Unit (SSU); the Microwave Sounder Unit (MSU); the Visible Infrared Spin-Scan Radiometer (VISSR), a two-channel sounder on GOES, which also may have the GOES 1-M sounder; the Solar Backscatter UltraViolet (SBUV-2) radiometer; the Limb Infrared Monitor of the Stratosphere (LIMS); and the Stratospheric Aerosol and Gas Experiment (SAGE). Sounders are especially suited to obtaining temperature, water vapor, ozone, and other trace-gas data that we can plot in altitude profiles. Here is an example (somewhat degraded) of temperature (red) and water vapor (blue) profiles over Denver, Colorado, on January 13, 1997, as calculated from the GOES-8 Sounder (see page 14-7 for details); the ordinate replaces altitude with pressure (in mbars).
The sounder on GOES-8 uses different channels to sense atmospheric properties. This composite panel show images made by various channels:
One way in which a sounder can sense levels in the atmosphere is illustrated in the diagram below. Each temperature map was made at a specific wavelength that is capable of measuring the radiant temperature in the atmosphere at some particular altitude (or depth within the air column, which can also be expressed in terms of atmospheric pressures in millibars). The map progression from upper left to lower right follows a trend of decreasing height above the surface. Note especially in the lower pair that the lateral temperature variation is from cooler in the northwest to warmest in the southeast.
Soundings can also be made in the microwave region. At different frequencies (or wavelengths, especially in the millimeter range), the return signals give blackbody-equivalent temperatures at different depths.
However, because satellites sweep over much of the Earth, particularly those in near-polar orbits (see below), and thus, take data readings continuously as they orbit, we can present the sounding data as global maps. As an example, consider these maps of global temperatures at four different altitudes (specified as pressures in millibars), as obtained by the TIROS Operational Vertical Sounder (TOVS) on Nimbus 12 on April 15, 1997 (115 mbars is approximately at 15 km and 950 mbars is near the surface):
We display a number of similar maps made from atmospheric sounder instruments on Metsats elsewhere in this section. But, for now let us point out that the EOS Aqua satellite has three of the most sophisticated sounders yet flown in space (see Section 16). Of these, we mention here AIRS - the Atmospheric InfraRed Sounder, which is described on this JPL AIRS site. AIRS consists of a hyperspectral sensor that has 2378 individual channels sensing in the thermal spectrum. It can produce both temperature and moisture maps at various atmospheric levels and profiles into the atmosphere - all on a daily basis. (In practice, profiles are not produced routinely, only as needed. Here is a typical temperature map produced by AIRS:
Every day AIRS produces temperature maps for the surface and the 700 mm level and for total precipitable water. Here is a typical set for August 24, 2006:
Researchers working with AIRS data have prepared some novel illustrations. Here's one that shows temperatures above Europe in a quasi-3D mode:
With this digression into the topic of sounders as new background, we will now proceed to review the history of Metsats beginning with TIROS and Nimbus.