Weather Satellites

With some fairly simple, low-cost equipment (much of which you can build yourself), a modern PC with a sound card, and free software, you can receive live weather pictures from meteorological satellites in earth orbit. This page describes some of the hardware and software I use at home.

My system has grown and developed over the eighteen months or so I have been active in this hobby. I receive analogue APT (Automatic Picture Transmission) signals in the VHF band (around 137 MHz). I started with a Cirkit receiver kit and crossed dipoles aerial, and a free software package called WAVSAT written for Windows 3.1 by Steve Bonnett. WAVSAT 2.0 can be found here. This gave me recognizable pictures from satellites which passed fairly close overhead.
Subsequently I added a RIG Dartcomm pre-amplifier, which boosts the signal to give better results with satellites close to the horizon.

My next development was to design and build an RS-232 serial interface so that my computer could switch the receiver between channels - there are a total of five active channels used by different satellites. You can read about this interface, and the software I use to control it, in the RIG journal number 57. The text of the article is available here, while here is the controlling Korn Shell script.
If you wondered what happened to Figure 1. (the circuit diagram) in the published article, here it is in  Zipped PostScript form!

In one rather fraught day spent wrestling with unmanageable lengths of microbore copper central heating pipe, I built the excellent QHA aerial design by John Boyer & Steve Blackmore. This replaced my crossed dipoles, and gave better results, especially at low elevations, where the crossed dipoles tended to exhibit nulls which give interference bands in the images.

One big advantage of RIG membership is the chance to buy the excellent RIG RX-2 receiver kit. This is available (to members only) at the RIG shop. I bought one of these kits and was delighted at the performance. While setting it up, I was able to pick a signal from NOAA-15 on a piece of wire laid along the bench! The main advantages of this receiver over the Cirkit are twofold: better rejection of interfering signals (not a big problem at my rural location), and lower noise level on the strongest signals.

The image conversion features of WAVSAT are not its strong point, and I had been meaning to attempt to write something better myself, when David Taylor's excellent SatSignal program became available. In subsequent versions, this has developed truly impressive performance, and I now use it for all image conversion - WAVSAT is still unmatched in its abilities to schedule reception from multiple satellites and organise the resulting files.

Unfortunately, the RX-2 as supplied does not allow computer control of the received frequency. Instead it scans the known active channels, stopping when it recognises the characteristic 2.4kHz APT sub-carrier. While this works quite well in general, it can give rise to problems at times when more than one satellite can be received at the same time. Now that I can pick up signals right down to the horizon, this is happening more frequently.
The RX-2 is a synthesised receiver, which uses a PIC microcontroller to set the frequency and control the scan. RIG make the source code available, and based on this code, I was able to write a modified version of the firmware which allows computer control of the received channel. Like my Cirkit receiver modifications, this provides an RS-232 interface to the computer. The command set is very similar.

I am currently developing new software to take over from WAVSAT the control of the receiver based on the satellites predicted orbits, and to manage the conversion into images, using SatSignal. I will update these pages (fairly) regularly as this project progresses!