Armed with an array of telescopes and sensors, weather satellites provide visual and infra-red images of the earth's atmosphere. These images are transmitted in the form of a radio signal. Many of these signals are easily received and decoded.

Appearance of a NOAA satellite

A: Advanced very high resolution radiometer B: Stratospheric sounding unit C: High resolution infrared radiation sounder D: Microwave sounding unit E: Observation unit for proton and electron F: Alpha particle sensor G: Earth sensor H: Solar battery array I: S-band antenna J: VHF antenna K: Data collection system antenna L: X-band antenna

To the right is a photo of the Feng Yun 2B, the first geostationary satellite launched successfully by the China Meteorological Administration. It was placed into orbit at 105° East on 10-June-1997.

There are basically two categories of weather satellites: geostationary and polar orbiters, each having its own characteristic advantages and disadvantages.

These satellites are located at a specific altitude from the earth at which the rotation of the satellite follows exactly that of the earth. This is the so called geostationary orbit, at about 36,000 km (22,500 miles). Such satellites are available 24 hours per day, providing 24 hour a day weather imaging. These satellites cover the entire earth disk from edge to edge, top to bottom.

Signals from these satellites are on 1.691 GHz in the UHF band. Because geostationary satellites are so far away, their signals are weak, and consequently they are more difficult to receive. For reception of such signals, special high gain antennas are required. Signal losses must be kept to a minimum, through the use of good quality connections and low loss cable. An external low noise pre-amp (mounted near the antenna) is almost universally employed. Often, the pre-amp is also fitted with a down-converter, allowing for the use of cheaper cable on the down-lead and more widely available VHF receivers.

If a parabolic dish can be obtained ready made, then this antenna is the easiest to put together. Small inaccuracies in the curvature has minimal effect, and a warped dish works fine. The huge 'LNB' in front of the dish is actually a circular horn feed, which is nothing more that a metal can with an internal element.	At 1.7GHz, a loop yagi antenna provides better performance than a dish antenna, as dishes smaller than three metres are not too efficient at these frequencies. Construction of a loop yagi is quite straightforward, but the dimensions are critical and any inaccuracies will lead to degraded performance.

The Polar Orbiting satellites, so named because they travel in a North-South orbit that takes them almost over the North and South poles, are the easiest to receive. The plane of their orbit forms a constant angle to the sun, thus these satellite orbits are also referred to as being sun-synchronous. One reason for a sun-synchronous orbit is that the light illuminating the scene below the satellite is constant for a particular geographic region at a particular time of day.

The satellite passes over a particular region at almost exactly the same time each day. As the earth rotates under the satellite orbit we end up with a satellite pass ascending from the South and a later pass in that 24 hour period descending from the North. Currently there are four US and two active Soviet satellites providing us with picture gathering opportunities. Special software, such as InstantTrack, is available to predict and track these satellites.

The polar orbiting satellites are in low earth orbit, typically 500-1300 km (300-800 miles) up. This orbit height limits the width of the image swathe they are able to capture. The images returned by these picture takers show a 2700 km (1700 miles) width of ground and clouds below. As a polar orbiting satellite travels in its orbit it is scanning the earth below with a radiometer consisting of a mirror, telescope and both infrared and visible image sensors.

As each line is scanned by the radiometer, the line is transmitted to the ground below on VHF frequencies in the 137 MHz band. Typical frequencies include 137.30, 137.40, 137.50, 137.62, 137.85 MHz. The lines are sent at a relatively slow 2 lines per second. To receive a complete viewable picture on a computer requires that several minutes of the satellite orbit be received. A typical overhead satellite pass usually lasts 10 to 15 minutes depending on how good the antenna and radio are.

A turnstyle or crossed dipole antenna is quite easy to construct. It provides a reasonably smooth, omni-directional, circularly polarised response, which is what's needed, but it suffers from deep nulls at low elevation angles.	A quadrifilar helix sounds and looks complicated but it is not that difficult to construct. It is an inherently excellent antenna for ground station use. This is the type of antenna that is actually used on the satellites themselves for transmitting the images.

The signal from both types of satellite is a 25kHz wide FM signal, which carries a 2.4kHz video subcarrier. The video subcarrier is amplitude modulated.

Listen to a NOAA satellite transmission

The signal has a wider bandwidth than narrowband FM used for voice communication, and a narrower bandwidth than that used for commercial FM broadcast stations. This means that the signal will be distorted if received with a narrowband FM receiver. A wideband FM receiver works fine, but the audio output will be lower than normal.

Apart from using a specialised receiver or scanner, it is possible to use a standard FM broadcast receiver together with a frequency down-converter.

Receiving the signal is not enough to produce an image on your computer. Something is needed to convert the signal into a meaningful image.

FTV uses the SoundBlaster card as a front-end, and processes the signal in software, routing it through various demodulators and decoders modelled in software.

Other systems that make use of dedicated hardware are also available. Dedicated hardware often comes in the form of external boxes that connect to the serial or parallel port. Some products include a dedicated plug-in card for the PC.

A popular combination which started so many people, including me, on the road of HF-FAX is without doubt the software program JV-FAX, which itself supports a wide array of converters, together with the ubiquitous HAM-COM interface, which is an ingenious circuit that is very easy to construct. Note that for WEFAX use, yet another converter is needed to convert the 2.4kHz AM video subcarrier into an FM signal in the range 1.5-2.3kHz.