PC observation systems bring weather forecasting to the classroom
By John Haystead, Contributing Editor
Bob Maurais teaches industrial arts and technology to grades 5-8 at the Harrison Middle School in Yarmouth, ME. In 1986, he began experimenting with weather-satellite systems using ham-radio equipment. Now, thanks to the Environmental Research Institute of Michigan (ERIM; Ann Arbor, MI), his pupils have access to weather-satellite meteorological data and real-time imagery.
In developing the system, ERIM contracted GTI Electronics (Lehighton, PA) under NASA (Washington, DC) sponsorship to provide a PC-based weather-satellite downlink and display workstation. "The goal was to bring remote sensing to a larger portion of the population and to increase awareness of remote-sensing products," says Tom Wagner, manager of the Space Technology Education Program (STEP) in ERIM`s Earth Science Group. As part of the initial pilot program, ERIM provided a downlink and display system to 35 schools including Harrison.
"Students can see the latest satellite imagery while being introduced to computer science and image processing, says Maurais. "They can measure ocean temperatures and track the Gulf Stream, or measure the surface temperatures of cloud tops, interpolate their height, and determine the intensity of storms."
Images from space
The Harrison school`s APT/WEFAX system accesses both Weather Facsimile (WEFAX) signals from the National Oceanic and Atmospheric Administration (NOAA) geostationary operational environmental satellites (GOES) and Automatic Picture Transmission (APT) signals from its polar orbital environmental satellite (POES) .
Positioned in permanent equatorial orbits at 23,500-mile altitudes, the NOAA geostationary GOES-8 and GOES-9 satellites deliver pole-to-pole visible and IR WEFAX imagery at 8-km/pixel resolution (see Fig. 1). While visible imagery shows weather movement and storm fronts, IR-temperature imagery identifies and predicts severe weather activity. Because GOESs transmit WEFAX images every five minutes, 24 hours/day, images can be stored and replayed to show how weather patterns develop over time.
The NOAA POES-12 and POES-14 are in polar orbits at an altitude of 500 miles. APT images from them provide 2000-mile longitudinal coverage across a roughly 1500-mile latitudinal swath. Although the satellites` orbital planes remain constant, the earth`s rotation moves their relative position approximately 25 west on every 101-minute orbit. POESs also provide visible and IR imagery but at 4-km/pixel resolution. POESs observe sea-surface and ground temperatures to predict frosts or track ocean currents.
In addition to APT/WEFAX signals, NOAA`s weather satellites transmit higher-resolution digital images that can also be received by direct-readout stations. GOES variable (GVAR) mode is a high-speed, high-resolution (0.8 km/pixel) digital channel transmitted at 2.11 Mbit/s that satellite controllers use to vary their field of view and zoom in on key weather centers such as hurricanes. Because a complete pole-to-pole view of the western hemisphere generates up to 385 Mbytes of data and takes 20 minutes to transmit, live images are usually sent only once every three hours and require a 10-ft satellite dish to receive.
Polar satellites also transmit high-resolution, digital-format signals (see Fig. 2). High Resolution Picture Transmission (HRPT; 1.1 km/pixel) data are transmitted at 665,400 bits/s on two fixed frequencies, 1698 and 1707 MHz, with a standby frequency of 1702.5 MHz. HRPT receiving stations also require a dish antenna, tracking software, and an integrated GPS clock for time/position accuracy.
Maurais donated his fellowship award money to the Harrison school to purchase the equipment. Compared to the $2000-$3000 cost of a complete APT/WEFAX installation, the Harrison school`s HRPT/GVAR system, including the 5-ft HRPT and 14.5-ft GVAR dishes, costs $13,000. "With GVAR and HRPT`s higher resolution, students can focus on states or regions and evaluate local weather conditions or whether lakes are frozen," says Maurais.
GTI Electronics` APT/ WEFAX system comprises a 386 or faster PC, custom plug-in receiver and processor cards, antennas, cables, and DOS-based software. Although the APT and WEFAX signals require different antennas, both signal types are received and processed by the same AT-bus cards in the computer (see Fig. 3).
WEFAX images are transmitted as analog radio-frequency signals at 1.6 GHz and are collected by a 4-6-ft dish antenna. APT signals, transmitted from POES-12 at 137.5 MHz and from POES-14 at 137.62 MHz, are received by an omnidirectional VHF-FM antenna. Both antennas plug directly into the receiver card using a cable connector.
GTI Electronics manufactures the APT/WEFAX system receiver card, which, as described by George Islieb, GTI director of technical operations, is "basically a synthesized VHF scanning radio." Islieb says, however,
"Because of the range and 5-W power output of satellite signals, it is critical that the receiver`s intermediate frequency (IF) bandwidth be optimal. To eliminate signal clipping, the receiver card uses a GaAs FET amplifier and multiple RF filtering stages. The receiver must also compensate for Doppler-frequency shift caused by satellite movement from the ground station. Although APT data can be received by a relatively simple cross-dipole FM antenna, more complex quadrifilar (four-channel, 360) antennas can be used that are not affected by signal polarization changes as signals pass through the atmosphere.
Output from the receiver card is passed to a decoder card from OFS WeatherFAX (Raleigh, NC) over an exterior cable connection where it is filtered and amplified by bandpass filters and gain-setting amplifiers and delivered to an 8-bit flash ADC. The ADC extracts analog RF and converts it to a digital format. The 2400-Hz modulated APT carrier signal is sampled at 4800 samples/s. However, because the WEFAX signal is transmitted at 1.6 GHz, it must first be downconverted to VHF. Because this downconversion is performed at the antenna, the workstation can be located some distance away from the dish without causing significant signal attenuation.
WEFAX data are transmitted as a serial data stream at 240 lines/min (or 4 lines/s). Because the signal is also sampled at 4800 samples/s, this equates to 1200 samples/line. But because oversampling would produce an image too short and wide for a true 1:1 image aspect ratio, a software algorithm reduces it to 800 samples/line.
GTI`s DOS-based "Weather FAX" software provides zoom, color, contrast, histogram generation, range-bearing and distance measurements, temperature readout, and animation. Numerous off-the-shelf satellite-tracking programs are also available to predict when specific satellites will be in range of the receiving station. ERIM`s STEP program has developed Bird Dog--software that tracks the current locations of 16 satellites and shows their orbital track and observation areas.
GTI`s PC-based direct-readout systems also handle GVAR and HRPT signals, but additional receiver and frame formatter cards handle digital-format signals. According to Islieb, except for the higher resolution, the images will look similar.
Although both the APT/WEFAX and HRPT/GVAR boards can theoretically be integrated into the same PC, most PCs do not have enough slots and higher-data-rate systems require Pentium 100-based PCs. GTI`s HRPT/GVAR systems run on 166-MHz Pentium-based PCs under OS-2. HRPT and GVAR receiver cards incorporate additional DSP processors and software and are connected by external cables to a frame-formatter card that places the data in the correct format for the AT bus.
Maurais uses a 100-MHz 486 PC for his GVAR imagery system, with additional 386 PCs running the HRPT and APT/WEFAX systems. A third 386 PC is used to run the "Instant-Track" polar-orbiting satellite-tracking software from the Amateur Radio Satellite Corporation (Xenia, OH) needed by the HRPT station.
Although the satellite-imaging technology has been a great success, ERIM`s Wagner would like to see wider-scale implementation. According to Wagner, one major hurdle to widespread use of the systems has been the apprehension of many school teachers who are not computer literate.
Pupils at the Harrison Middle School in Yarmouth, ME, can access high-resolution weather satellite data thanks to low-cost PC-based decoder boards.
FIGURE 1. Geostationary Operational Environmental Satellites (GOES-8 and -9) provide visible and IR imagery and temperature and moisture profiles of the atmosphere. Using the data, forecasters can create composite visible images of the earth or study weather patterns of the continental USA.
FIGURE 2. IR image of the Great Lakes region from NOAA`s Polar Orbiting Environmental Satellites (POES) The Advanced Very High Resolution Radiometer (AVHRR) carried on satellites is a broad-band scanner, sensing visible, near-IR, and thermal IR wavelengths.
FIGURE 3. Satellite decoder boards act as synthesized VHF scanning radios that filter, decode, and convert satellite frequencies to displayable images on personal computers.
Add-in cards turn portable systems into weather workstations
Portable APT/WEFAX systems are now allowing satellite weather data and imagery to be acquired from laptop computers. Jerry Dahl, principal engineer at OFS WeatherFAX (Raleigh, NC), a developer of the Viking Satellite System, says, "More and more users are demanding a permanent base-station and a portable version of their APT/WEFAX systems."
Powered by a laptop computer, the Viking Satellite System integrates a discrete radio receiver unit and small satellite dish antenna. Currently the system uses a compact dish antenna with integrated downconverter, but the company is also evaluating a new 22-in. loop Yagi antenna that can be mounted on a camera tripod. The receiver unit plugs into the laptop using a removable PCMCIA decoder card that also can be plugged into a standard desktop-PC ISA bus using an adapter card.
In addition, the PCMCIA card incorporates a dual AM/FM decoder that allows it to receive another type of WEFAX signal called HF Marine FAX, carried on upper-sideband AM short-wave radio frequencies (RFs) with frequency-shift-key (FSK) FM modulation. The signal provides weather and sea-condition information specifically tailored for marine users.
The Viking receiver is designed for operation in high-RF-noise environments. With a sensitivity of 0.33 mV, -120 dBm, the receiver includes a tone detector to determine when satellite signals are present. Because all WEFAX image transmissions are preceded by a binary-coded header (similar to a barcode), which identifies the type of image being sent, the system can be programmed to automatically receive and store only those images of interest for later viewing in an animation loop.
OFS Weatherfax provides the Viking system with DOS-based software, but the system is also available through Ocean & Coastal Environmental Sensing (OCENS; Seattle, WA). OCENS supplies its SeaStation Mariner system with Windows-based software to the marine market. Additional software includes a geopolitical map overlay of the APT/WEFAX imagery. The system can also display the location of the user relative to the map using a GPS system interface.
Temperature profiling permits rapid identification of ocean or weather fronts. Changes in sea-surface temperature also indicate boundaries along which many species of fish accumulate.