Box 3-D--NOAA's Polar-Orbiting Operational Environmental Satellite System

The POES satellites follow orbits that pass close to the north and south poles as Earth rotates beneath them. They orbit at about 840 kilometers altitude, providing continuous, global coverage of the state of Earth's atmosphere, including essential parameters such as atmospheric temperature, humidity, cloud cover, ozone concentration, and Earth's energy budget, as well as important surface data such as sea ice and sea surface temperature, and snow and ice coverage. All current and near-future POES satellites carry five primary instruments:

1. The Advanced Very High Resolution Radiometer/2 (AVHRR/2) determines cloud cover and Earth's surface temperature. This scanning radiometer uses five detectors to create surface images in five spectral bands, allowing multispectral analysis of vegetation, clouds, lakes, shorelines, snow, and ice.

2. The High Resolution Infrared Radiation Sounder (HIRS/2). HIRS/2 measures energy emitted by the atmosphere in 19 spectral bands in the infrared region of the spectrum, and 1 spectral band at the far red end of the visible spectrum. HIRS data are used to estimate temperature in a vertical column of the atmosphere to 40km above the surface. Data from this instrument can also be used to estimate pressure, water vapor, precipitable water, and ozone in a vertical column of the atmosphere.

3. The Microwave Sounding Unit (MSU) detects energy in the troposphere in four areas of the microwave region of the spectrum. These data are used to estimate atmospheric temperature in a vertical column up to 100 km high. Because MSU data are not seriously affected by clouds, they are used in conjunction with HIRS/2 to remove measurement ambiguity when clouds are present.

4. The Space Environment Monitor (SEM) is a multichannel charged-particle spectrometer that measures the flux density, energy spectrum, and total energy deposition of solar protons, alpha particles, and electrons. These data provide estimates of the energy deposited by solar particles in the upper atmosphere, and a "solar warning system" on the influence of solar fluctuations on the Earth system.

5. The ARGOS Data Collection System (DCS) consists of approximately 2,000 platforms (buoys, free-floating balloons, remote weather stations, and even animal collars) that transmit temperature, pressure, and altitude data to the POES satellite. The onboard DCS instrument tracks the frequency and timing of each incoming signal, and retransmits these data to a central processing facility. The system is able to determine transmitter location rather accurately.

Other instruments do not fly on every POES mission. Instruments in this category include:

The Stratospheric Sounding Unit (SSU), a three channel instrument, has flown on all NOM POES satellites except for NOM-12. It measures the intensity of electromagnetic radiation emitted from carbon dioxide at the top of the atmosphere, providing scientists with the necessary data to estimate temperatures through the stratosphere. The SSU is used in conjunction with HIRS/2 and MSU as part of the TlROS Operational Vertical Sounder System.

The Solar Backscatter Ultraviolet Radiometer/2 (SBUV/2) measures concentrations of ozone at various levels in the atmosphere, and total ozone concentration. This is achieved by measuring the spectral radiance of solar ultraviolet radiation "backscattered" from the ozone absorption band in the atmosphere, while also measuring the direct solar spectral irradiance. The SBUV is flown on POES PM orbiters only.

The Search and Rescue Satellite Aided Tracking System (SARSAT or S&R) locates signals from emergency location transponders onboard ships and aircraft in distress, and relays these data to ground receiving stations, which analyze them and transmit information to rescue teams in the area.

The Earth Radiation Budget Experiment (ERBE) was flown only on NOM-9 and NOM-10. This research instrument consists of a medium and wide field-of-view nonscanning radiometer, operating in four channels that view the Earth and one channel that views the sun, and an arrow field-of-view scanning radiometer with three channels that scan the Earth from horizon to horizon. ERBE measures the monthly average radiation budget on regional to global scales, and determines the average daily variations in the radiation budget.

NOM currently has four POES satellites in orbit. NOM-11 and NOM-12, launched in September 1988 and May 1991, respectively, are operational, while NOAA-9 and NOM-10, launched in 1984 and 1986, are essentially in a stand-by mode. However, the ERBE instrument on NOM-9 continues to return limited data on the Earth's radiation budget, and the SBUV/2 instrument on NOM-10 continues to return useful information on ozone concentration in the atmosphere. NOM plans to upgrade several of the POES instruments in the near future. The SSU and MSU will be replaced with the Advanced Microwave Sounding Units (aboard NOAA K-M), AVHRR will gain an additional channel, and the ARGOS system will have expanded capacity. NOM is planning additional improvements (in the latter part of the 1990s) to AVHRR, HIRS, and AMSU and expects to add a Total Ozone Mapping Spectrometer (TOMS) to the platform.

NOTE: The SSU is contributed by the United Kingdom; ARGOS is a contribution of the French Space Agency CNES; and the SARSAT instrument is a joint project of Canada and France.

SOURCE: Office of Technology Assessment, 1993.