4. WEFAX, DCP, SEM, and SAR services on GOES I-M

The WEFAX service has been a feature of the GOES system since 1975. The service uses the spacecraft as a transponder to transmit low-resolution imagery sectors as well as conventional weather maps to users with low-cost reception equipment. There are hundreds of WEFAX reception stations throughout the western hemisphere that use the GOES WEFAX service. With the GOES-VAS system, the WEFAX service was provided at UHF frequencies and was not continuous since it interfered with other spacecraft functions. The WEFAX service with GOES I-M is a continuation of the prior service; however, transmissions are continuous, and the transmission frequency has changed from UHF to S band, 1691.0 MHz, with a signal bandwidth of about 30 kb s-1. Up to 13 maps or images can be transmitted through the GOES I-M every half hour, and it is the goal of the ground data handling system to transmit GOES I-M WEFAX imagery within a half-hour of its receipt.

DCPs, which relay environmental data through GOES satellites, have been an important part of the GOES system since its inception. The data collected at the DCPs provide a variety of observations, ranging from meteorological and oceanographic to remote hydrological and seismic station observations. Varying user requirements for each of those types of data led to a flexible GOES data collection system (DCS), which supports three basic types of DCP. They are (a) DCPs that relay data through the satellite at preprogrammed times, (b) DCPs that relay their data through the satellite when their sensors reach a predetermined threshold, and (c) DCPs that respond when interrogated through the GOES satellite. Currently, about 12 000 DCPs are assigned, serving about 300 users. The GOES I-M DCS, which can service up to 56000 DCPs, has three major components. The primary component is the DCP, which may be placed at remote locations from which they relay environmental measurements through the second part of the system, the GOES I-M spacecraft. The data are transmitted from the DCP on one of 266-UHF (401 MHz) channels to the spacecraft where the signal is converted to S band and transmitted to the third component of the system, the Wallops Island CDA station. At the Wallops CDA, the data are sent to the DCS automated processing system (DAPS). At the DAPS, a variety of functions are performed that culminate in the environmental data being provided to the user community in several ways: (a) in real time through the commercial domestic satellite system, (b) by means of dedicated synchronous communication lines as output messages to NMC, and (c) through dial in modems using asynchronous communication lines.

With GOES I-M, the SEM subsystem continues to provide data to the joint NOAA and U.S. Air Force Space Environmental Services Center in Boulder, Colorado. The SEM is composed of four basic instruments that are based on proven designs that have evolved with improvements in component technology. The instruments are (a) a solar X-ray sensor, (b) a magnetometer, (c) an energetic particle sensor, and (d) a high-energy proton alpha detector. The instruments provide 24 h day-' measurements of ambient magnetic field vectors, solar X-ray flux, and multiple measurements characterizing the charged particle population for protons of energy greater than O.8MeV, alpha particles of energy greater than 3.2 MeV, and electrons greater than 0.6 MeV. The data are used in Boulder to monitor and predict solar activity and its effect on an array of activities. The activities include prediction of ionospheric conditions that affect radio transmissions, effects on electric power transmission grids, and radiation levels that affect high-altitude aircraft operations and manned space flight.

The GOES I-M system provides operational SAR support from geostationary altitude for the first time. This important complement to the polar-orbiting part of the SAR function allows nearly instantaneous detection of 406-MHz distress signals from special emergency transmitters that are carried on some aircraft and marine vessels. For detection, the distress signal must originate from within the GOES satellite's field of view; this means that regions beyond 75 from the satellite subpoint are not covered. The GOES I-M SAR does not position locate, as can be done using the polar system; however, it serves as an almost immediate alert to users that a distress signal has been broadcast. This allows search and rescue points of contact to expedite their investigations and rescue responses.