1. Introduction

a. GOES background

Since the early 1960s, meteorological, hydrological, and oceanographic data from satellites have had a major impact on environmental analysis, weather forecasting, and atmospheric research in the United States and throughout the world. While polar-orbiting satellites provide snapshots of various phenomena once or twice daily, it was not until December 1966, when the National Aeronautics and Space Administration (NASA) launched the first geostationary Applications Technology Satellite (A TS- 1), that the ability to see weather systems in animation was realized. ATS-1 imaged the full-disk earth every half-hour with a spin scan camera conceived and designed by Verner Suomi (Suomi and Parent 1968). Operational use of ATS-3 imagery at the National Severe Storm Forecast Center (NSSFC) and the National Hurricane Center (NHC) followed in 1972. NASA research and development fostered the Geostationary Operational Environmental Satellite (GOES) program within the National Oceanic and Atmospheric Administration (NOAA). Five spin stabilized satellites were built and launched, introducing a new era of satellite service: NASA's demonstration of two Synchronous Meteorological Satellites (SMS) began with the launch of SMS-1 in May 1974 and NOAA's operation of a GOES series followed with the launch of GOES- I in October 1975. The Visible and Infrared Spin Scan Radiometer (VISSR) provided imagery from these original SMS and GOES satellites.

GOES significantly advanced our ability to observe weather systems by providing frequent interval visible and infrared imagery of the earth surface, atmospheric moisture, and cloud cover. GOES data soon became a critical part of National Weather Service (NWS) operations by providing unique information about existing and emerging storm systems both day and night. Subsequently, more spectral bands were added to the VISSR, enabling the GOES system to acquire multispectral measurements from which atmospheric temperature and humidity sounding could be derived: the VISSR Atmospheric Sounder (VAS) was introduced on GOES-4 in 1981. Although the addition of more spectral bands represented a major improvement in satellite capability, several compromises were necessary. First, imaging and sounding could not be done at the same time. Second, the spinning GOES-VAS viewed the earth only 5% of the time, so it was not possible to attain the instrument signal-to-noise ratios needed for either high-quality soundings or high spatial resolution image data. Thus, while the image data were used operationally, the sounding data were used only in special experiments. Recognizing the need for improved imaging and sounding, NOAA began development of its next generation of geostationary satellites, GOES I-M, in 1985.

b. GOES-I in 1994

In the spring of 1994, the first of NOAA's next generation of geostationary satellites, GOES-I, is scheduled for launch. The GOES I-M system has been developed by Space Systems/Loral, under NASA supervision, for NOAA. ITT is the subcontractor for the GOES I-M imager and sounder instruments. Each GOES I-M spacecraft is designed for a five-year lifetime. The GOES I-M series introduces improved capabilities to observe weather-related phenomena on all scales from geostationary altitude and represents the evolution of geostationary satellite technology in the United States during nearly a quarter of a century. It supports most U.S. geostationary environmental satellite requirements, both operational and research, into the early twenty-first century.

Responding to user requirements for improvements in the GOES-VAS system, the GOES I-M system promises (a) no conflict between imaging and sounding operations, (b) multispectral imaging with improved resolution and better signal to noise in the infrared bands, (c) more accurate atmospheric temperature and moisture soundings, (d) more precise image frame-to-frame registration, and (e) stable longterm calibration. The GOES I-M imager improves operational multispectral imaging capability; the GOES I-M sounder allows NOAA to begin operational geostationary sounding for the first time.

With the GOES I-M series, there are new designs for all major portions of the system: (a) to improve instrument performance, the satellite is earth orientedдhat is, three axes stabilized, so that the earth atmosphere is observed nearly continuously; (b) to avoid conflicts between sounding and imaging operations, separate instruments perform those functions; (c) to improve imaging capabilities for cloud and storm diagnostics and to enhance signal-to-noise characteristics for atmospheric sounding, new multispectral sensors were designed; (d) to accommodate constant earth observation, a different data format was devised for retransmission of raw data to direct-receive users; and (e) to handle the high data volume, a new ground data processing system was developed to distribute data and products to a variety of users.

In addition to the customary DCP (data collection platform) and SEM (space environment monitoring) services, the satellite has an improved WEFAX (weather facsimile) capability. It also provides dedicated SAR (search and rescue) support from geostationary altitude for the first time.

FIG. 1. Spacecraft on orbit configuration. The main body is 78 in. x 84 in. x 92 in., and the overall length is 96 ft, of which the solar sail boom constitutes 58 ft.

The earth-oriented spacecraft design (see Fig. 1 ) required new scanning, navigation, calibration, and thermal control systems. For more efficient imaging with near-continuous viewing of the earth, the scanning system moves in boustrophedon fashion, slewing back and forth east to west and then west to east; GOES-VAS sampling is always west to east. A "star sensing" system allows precise pointing of the imager and sounder systems; thus, pixels are located to within 4 km at nadir. Image and mirror motion compensation systems correct for satellite and instrument motions that affect image frame-to-frame registration. Improved navigation and gridding allow images to be presented in a fixed GOES projection. To improve calibration, reference blackbodies are external to the instrument telescopes so that space and blackbody looks are used directly to convert detector response into radiances; on the GOES-VAS, an internal blackbody offers less precise calibration because it requires adjustment for the stray radiation from the telescope optics in front of the blackbody. To accommodate the large daily thermal changes that occur with staring instruments, increased monitoring of thermal gradients and telescope focal properties is introduced.

The following sections describe various features of the GOES I-M spacecraft, data handling system, and products. The satellite sensor systems are discussed in section 2. Quality control procedures for converting data into products follow in section 3. WEFAX, DCP, SEM, and SAR services are presented in section 4. Section 5 outlines data flow to the users, probable imager and sounder schedules, and initial product suites. Section 6 presents some examples of simulated GOES-I imager and sounder data. Finally, section 7 looks at potential future products and services.