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November 9, 1999 — NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) operates two kinds of environmental satellites: geostationary operational environmental satellites (GOES), and polar-orbiting operational environmental satellites (POES). Instruments to monitor space weather are flown aboard both types of satellites.

Geostationary Operational Environmental Satellites

GOES satellites circle the Earth in a geosynchronous orbit, which means they orbit the Earth 22,300 miles above the equator at a speed matching the Earth's rotation. The current operational GOES satellites are GOES-8, launched on April 13, 1994, and GOES-10, launched April 25, 1997. GOES-8, located at 75 degrees West longitude, provides coverage over North and South America and the Atlantic almost to the tip of Africa. GOES-10, at 135 degrees West longitude, overlaps the western part of North America and provides coverage of the Pacific, the Hawaiian Islands, and the Gulf of Alaska.

Polar-Orbiting Satellites

NOAA's two polar-orbiting satellites constantly circle the Earth in sun-synchronous orbit (450-nautical mile altitude). The satellites circle the Earth in an almost north-south orbit, passing close to both poles. One crosses the equator at 7:30 a.m. local time, the other at 1:40 p.m. local time. Currently, NOAA has two primary operational polar orbiters: NOAA-14, launched in December 1994, and NOAA-15, launched in May 1998.

The Space Environment Monitor System Aboard GOES

The Space Environment Monitor (SEM) consists of: a three-axis vector magnetometer; an Energetic Particle Sensor and associated High-energy Proton and Alpha Detector (HEPAD); and an X-ray Sensor (XRS). This set of instruments is designed to provide real-time measurement of space weather: solar activity, the charged particle environment, and the Earth's magnetic field at synchronous orbit.

The magnetometer measures the magnitude and direction of the Earth's ambient magnetic field with three orthogonal sensors, located in a sensor assembly and attached to a boom that places the sensor three meters away from the body of the spacecraft.

The Energetic Particle Sensor (EPS) and the High-Energy Proton and Alpha Detector (HEPAD) monitor solar protons and alpha particles, produced during large flares, which are a radiation hazard to manned and unmanned operations in space and the ionosphere at high latitudes. The HEPAD instrument covers the very high-energy protons and alpha particles that are produced in large solar flares.

The X-Ray Sensor performs real-time measurements of the solar X-ray emissions in two channels covering the spectral ranges of 0.5 Angstroms (shortwave) and 1 to 9 angstroms (longwave). The sensitivity of the sensor was chosen to permit quiet sun background measurements at as low a level of solar activity as possible while detecting events at the lowest practicable threshold for early event warning.

The Future: Solar X-Ray Imager

Under development for a future GOES is a Solar X-Ray Imager. Instead of detecting X-ray energy from the sun in a one lump (integrated) sum like the x-ray sensor, it will detect energy from 250,000 separately defined areas (pixels). The improvements in predicting solar flare activity and its effect on the near Earth environment are expected to be comparable to the improvements in our forecasters' ability to forecast weather with the launch of NOAA's first geostationary satellite in 1974.

The Space Environment Monitor Aboard POES

The Solar Environment Monitor aboard POES is used to determine the energy deposited by solar particles in the upper atmosphere and to provide a solar storm warning system.

The Space Environment Monitor aboard NOAA's polar-orbiting satellites is a multichannel, charged-particle spectrometer that measures the population of the Earth's radiation belts and the particle precipitation phenomena resulting from solar activity, both of which contribute to the solar/terrestrial energy exchange. The SEM consists of two separate sensor units and a common data processing unit. The sensors units are the Total-Energy Detector (TED) and the Medium-Energy Proton/Electron Detector (MEPED). The lower-energy sensors (the TED and the proton and electron telescopes of the MEPED) have pairs of sensors with different orientations because the direction of the particle fluxes is important in characterizing the energy interchanges taking place.

For more information contact: NESDIS Public Affairs, Suitland, Md., (301) 457-5005.

Updated April 1999