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AMATEUR RADIO BY GARRY CRATT, VK2YBX Receiving weather satellite signals; Pt.2 Last month, we looked at the polar orbiting weather satellites and discussed the equipment necessary to receive and decode images transmitted by them. This month, we look at the SHF transmissions which originate from the Japanese GMS-4 satellite. The Japanese GMS-4 satellite is located in geostationary orbit at 140° east. This satellite transmits enhanced images on a frequency of 1691MHz, requiring the use of microwave techniques for good quality reception. When one considers the minute signal levels reaching the Earth's surface (typically .05µV), it becomes apparent that considerable care must be taken to receive and display weather images from this satellite. The GMS satellite uses a deviation of ±126kHz and so the receiver must have a minimum IF bandwidth of 260kHz. This increased bandwidth means that the received noise is about 10 times larger in amplitude than from a VHF polar orbiting satellite having a deviation of ±18kHz. In order to recover useable signals , the antenna must be capable of pro- This image from the Japanese GMS-4 satellite clearly shows the cyclone that eventually crossed the Queensland coast during March 1992. 86 SILICON CI-IIP viding sufficient gain between the incoming signal level of -134dBm and the typical receiver sensitivity of -1 lOdBm. This equates to a gain of 26dB, requiring a dish having a diameter of at least 1. 5 metres. Such dishes are often available from satellite TV dealers, who often have damaged units of no use at 12GHz but still quite useable at the frequency of the GMS satellite. Fig. l shows the gain that can be expected from dishes of various _diameters. The path loss between spacecraft and Earth at 1691MHz is approximately 188dB and as the output signal from the satellite is 5W (+37dBm), and the gain of the spacecraft antenna is around 17dBi, the calculated signal at the ground is -134dBm. Receiver requirements To obtain a good signal, the receiver should also have a noise figure of around 1.5-ZdB. A popular configuration is to use a microwave mixer and local oscillator chain feeding a VHF receiver. Particular care must be taken to ensure that the local oscillator is kept as stable as possible: it will be oscillating at about 1500MHz, to produce an IF of 137MHz (for example), and any drift in the oscillator will be multiplied by 15 or so (assuming the oscillator runs a 20MHz crystal). Thus, a frequency offset of lkHz will become a shift of 75kHz at 1500MHz. The most important parameter of the GMS receiving system is the IF bandwidth of the receiver. Some weather satellite enthusiasts have attempted to use scanning receivers for the purpose, as they cover the SHF frequency range. However, these receivers have a wideband FM bandwidth of 150-lB0kHz, causing poor signal-to-noise ratio and severe limiting of greyscale resolution. A correctly ing this distance for maximum signal. Fortunately, for those without the resources or time to construct a GMS system, specially designed and prebuilt components .are available. PH Communications - phone (07) 264 1575 - produces a built up GMS receiver, downconverter and dipole feed system. They also produce a suitable 1.7GHz LNA (low noise amplifier) for those who think they need it. S.CISAT Products are finalising a complete GMS receiver/downconverter. QUORUM Communications (address details in last month's issue) produce a suitable downconverter, model SDC16918. Polar orbiters Taken from one of the NOAA polar orbiting satellites, this infrared image of the United Kingdom shows quite a lot of detail, including many small towns. designed receiver must have an IF bandwidth of 260-280kHz. Unfortunately, most receiver designs featured in overseas magazines are suitable only for METEORSAT or GOES satellites, which have an IF bandwidth of 40kHz. With this kind of signal improvement over GMS transmissions (+8.25dB), signals can be heard on four phased Yagi antennas. While GMS transmissions can be heard using such equipment, pictures cannot be produced, due to the reduced signal levels. As 1691MHz is a frequency used exclusively for weather satellite transmissions, commercial feedhorns for this frequency are not readily available. Because the efficiency of the dish is related to the type of feedhorn arrangement used, the design of this component is also very important. The first step towards constructing a suitable feedhorn is to determine the focal point of the dish, so that the feedhorn can be correctly positioned. This can be calculated using the formula F = D2 /2c, where F is the focal point, D is the diameter and c is the depth of the dish. Designs for "coffee can" feedhorns can be found in the ARRL Antenna Book, and Jessop's VHF UHF Manual. A table in the latter book shows both the 3dB and lOdB beam width required for various values of F/D. This table enables constructors to select a suitable feedhorn design, once the F/D of the dish is known, ensuring that the dish is fully illuminated and operating at peak efficiency. As an example, a feedhorn suitable for a dish having an F/D ratio of 0.56 can be constructed using a 12cm long, 18cm wide can, containing a quarter wavelength monopole (3cm), mounted 3cm from the rear of the tube. The focal point is measured from the inside centre of the dish to the inside edge of the feedhorn. As is the case with all microwave receiving systems, the feed must be rotated to the correct satellite polarity, corresponding to maximum signal level, and the focal point should also be fine tuned by carefully adjust- Fig.1: Dish Diameter vs. Gain Diameter Gain (dBi) 0.6 18 1.2 24 1.5 26 1.8 27.5 2.0 29 2.4 30 3.0 32 There is yet another mode of transmission used by the polar orbiting weather satellites described in our first article. These polar orbiters produce extremely high resolution pictures and the data is transmitted on frequencies similar to GMS. In the case of NOAA 9 & 11, this frequency is 1707MHz. For NOAA 10 it is 1698MHz. Data is collected from spacecraft instruments such as the Advanced Very High Resolution Radiometer (AVHRR), the Operation Vertical Sounder (TVOS), the Space Environment Monitor (SEM), the Data Collection System (DCS) and the spacecraft telemetry system. The subject of HRPT signal reception is quite complex and extremely interesting. Dedicated newsletters for enthusiasts are available, such as the Journal of the Environmental Satellite Amateur Users Group. This is published by the Dallas Remote Imaging Group, 4209 Meadowdale Drive, Carrollton, TX 75010 USA. Several copies perused in our office indicate that this newsletter contains a high level of quality information. Further reading (1) "High Resolution Weather Satellite Pictures," M. L. Christieson, Wireless World December 1981 and January 1982. (2) "Tracking Low Earth Orbit Satellites At LIS Band", "Break In" NZART publication, March 1989. Acknowledgment I would like to thank Mr Brian Buckingham and Mr Fred Lehner for their time and assistance in supplying background information. SC MAY 1992 87