This project came about after I started playing about a little more with my Uniden UBC93XLT scanner. This is a toy I got a wee while ago from Dick Smith Electronics. I hadn’t really done too much with it apart from the usual listening to various coastguard, airport and Police transmissions (the number of people doing stupid things in cars who get stopped by the police who have no vehicle license, no WOF and suspended drivers licenses is amazing – you’d think they drive in a way that means they wouldn’t get stopped). That all gets boring pretty quickly so I wondered what else this little scanner could do.
I should point out that I am not a radio amateur, or HAM, as they are known. I am just an electronics tinkerer. Anyone with good, general electronics skills can do this.
Bender showing off my scanner.
One thing you can do is pick up pager frequencies. I pick up 157.950MHz easily at home which is a Telecom pager frequency. It sounds like a random collection of bleeps and bloops – R2D2 with his knickers in a twist. Apparently with a simple mod you can break out the unfiltered audio (called the discriminator output) in the scanner and then, using some software called PDW, decode the pager signals.
Even though the mod is easy to do (if you are electronically inclined) looking at other peoples pager messages strikes me as being terribly uninteresting, kind of like having to overhear other peoples one sided phone calls on the bus, so I haven’t tried this modification myself.
One other thing I could try (after doing the discriminator mod above) is receiving AIS signals. These are the identification signals used by ships. Since I work in the city and it’s not too far down to the port I should really try seeing if I can receive them. Perhaps the Northern Steamship Company pub might be a good place to try!
The frequencies used are 161.9750 for AIS1 and 162.0250 for AIS2. Apparently you can then feed the signals into a sound card on a PC and using some software like Shipplotter you can track the movements of shipping in your area. Something to try another day.
One thing I did want to try, being interested in all things to do with space and space technology, is receiving weather satellite transmissions. Now, before I start I should say that you cannot actually decode the weather satellite transmission using this particular scanner. You can HEAR the signals but you cannot generate any kind of useful pictures with them. Well, I have been unable to. But given that you can hear them was enough to start me off on this project.
Lets start with some background first. Up there, in space (well, low earth orbit 850km up), there are some weather satellites. Both the Americans and the Russians have them up there, the NOAA and the METEOR satellites respectively. There are others of course but the NOAA are the ones you can easily receive signals from so they are the ones I am listening to. These satellites are continuously transmitting signals down to earth using a system called APT. The Wikipedia Automatic Picture Transmission article gives you all the background you need but here are the basics.
The NOAA satellites are in a polar orbit around the earth and as they orbit they are continuously scanning a line beneath them 3000km wide and transmitting this line back to earth. Being in a polar orbit means each satellite will pass over the entire planet as the earth rotates beneath them. As a satellite comes up over the horizon and passes overhead a receiver on the ground can start receiving these lines, decode them in sequence and build up a picture. Due to their low altitude these satellites orbit rapidly and each satellite will make several passes over your location each day.
The signals are transmitted on a group of frequencies around 137MHz. The actual picture received contains two views of the earth below the satellite – one in visible wavelengths and another in infrared. A line is scanned twice every second and each line contains the image data as well as non image data. This system has been used since the 1960s so it is quite remarkable it is still in use today. You can get up to date information about the NOAA weather satellites from this status page.
To receive the pictures you need three things: a receiver, an antenna and a PC (with a sound card and some software to do the decoding).
First, the receiver. As I mentioned above my particular scanner (or indeed most scanners) isn’t suited for doing more than hearing the signals but it is enough to let me work on the second piece, the antenna.
The antenna obviously needs to pick up the 137MHz signal the satellites transmit on. It also needs a wide receiving pattern as it has to be able to pick up the signal from horizon to horizon. The signal from the satellites is circularly polarised so the antenna must be designed in a particular way to receive the signal. Two suitable antenna designs are the turnstile antenna or the quadrifilar helix antenna (QFH).
Looking around online I came across several references to the QFH antennas with very good instructions on how to make them specifically for receiving weather satellite pictures. The design I decided to use was from here: http://www.g4ilo.com/qfh.html
Because I didn’t know if I was going to be able to receive anything I made my first QFH using materials I had on hand, namely wooden dowels. I made an antenna using the instructions and dimensions given above. I used RG-6U coaxial cable as that was easy for me to get quickly in order to try things out. I first tried using the antenna indoors until a friend pointed out that the metal roof would block the signal (a duh moment) so instead I simply took the antenna outside and hand held it in order to try it out. To my amazement it worked! I could hear, albeit with a lot of noise and static, the satellites transmitting their signals.
It’s worth mentioning that because the satellites are constantly orbiting you need to know exactly when a satellite will be passing over. This brings us neatly to the third thing you require, a PC with a sound card and some software running on it.
The software I am using is called WXtoImg and it is available to download and use for free (you can also pay to register it for additional features). Interestingly it seems to be written here in New Zealand. This software does two things. It decodes the received audio signal and formats that into usable images and it also tells you when (and where) the next satellite will appear.
Another useful piece of free software is WXTrack by David Taylor. This software lets you see exactly where satellites are over the earth at any given time.
Both of these programs require you to download up to date satellite tracking information called Keplers but this is well explained and easy to set up.
Since I could now hear the signal and I had downloaded and installed the appropriate software I rigged up a temporary support to hold the antenna up outside my window were it would have a (reasonably) clear view of the sky and attempted to feed the signals into my PC.
The results were not good (it wooden work)! This I expected and the problem isn’t the antenna itself but the limitations of my scanner.
There are various problems with using scanners for receiving weather satellites. Other weather satellite sites explain the problems better than me but to summarise the main problem is due to bandwidth. Now bandwidth, as I understand it (and I could be wrong!), when talking about receiving weather satellite transmissions is the difference between the lowest frequency and the highest frequency that you need to be receiving signals over to get the full signal. A scanner like mine is designed for receiving voice signals. Voice signals don’t need a very wide frequency range so we say the scanner has a narrow bandwidth. The bandwidth of a scanner like mine is about 15kHz so the scanner will filter out any signal outside that range around the main frequency.
The satellites transmit their signal over a much wider range spread around the main frequency, i.e. is has a wider bandwidth. To accurately receive the entire weather satellite signal you need a receiver with a 30kHz to 50 kHz bandwidth. So in effect my scanner is blocking out part of the signal. We basically lose information from that lost signal so the software can’t build a good picture from it.
Other issues with a normal scanner are sensitivity, how strongly it picks up the weak satellite signal, and selectivity, how well it picks up only the frequency you want. Another, more subtle problem, is the doppler shift caused by the movement of the satellite as it passes overhead. This causes the frequency to shift as the satellite passes overhead.
All these issues mean my little scanner isn’t up to the job. To really demonstrate this here is what a properly recorded signal of a weather satellite should sound like: http://n8imo.com/APT/images/N14.WAV (from http://n8imo.com/wefax4.html).
What I get is this: http://www.asciimation.co.nz/misc/06250521.wav\
My friend Dave wanted to see a picture of what the scanner produced. This is the best that I ever managed to get. Mainly noise. With some vague picture in the middle as the satellite passed right overhead.
So when people say a normal scanner won’t work, they are right!
The next thing to do is sort out a proper receiver. There are a number of options here. You can buy a scanner that does have the correct bandwidth but these are very expensive. Or you can buy a ready made receiver just for receiving weather satellites. These are also expensive. Finally you have kit receivers you buy and assemble yourself. Being an electronics tinkerer this is what I was after. There are various kits on offer but I chose the Minikits weather satellite receiver.
This receiver is very reasonably priced, should do everything I want and the company is more or less local (well, Australia) so I was able to pay in Australian dollars rather than US dollars or Euros which tend to work against someone earning NZ dollars. Shipping was also cheaper and faster.
I ordered the kit last week and it should arrive any day now so I will be building that as soon as possible.
In the mean time, having proved that a QFH antenna would indeed work I set about making a more accurate, more weather proof version.
Will describe that in part 2 here.