Receiving weather satellite pictures in New Zealand – Part 3

July 4th, 2010

The other two parts of this are available here:  Part1 and here: Part2

Finally some results! But lets start with what I have done so far. Since my initial experiments with my scanner were somewhat successful and I had my new QFH antenna made and ready to go I just needed my Minikits weather satellite receiver to arrive and then I could try getting some decent images.

The kit arrived after just over a week and ended up costing me $NZ125 including shipping. This is far cheaper than any other kit I could find online, all of which you have to pay in Euros or US dollars.

IMG_8361_1 The Minikits kit.

The kit is rather nice. It comes with a tinned double sided plate through hole circuit board. Several little bags of components including all the coils and ICs you need. It also came with an F connector to solder onto the board but I have chosen to use BNC connectors instead. It also comes with the instructions and the description of how the circuit works.

The kit uses an MC13135P receiver chip which is a complete FM receiver on a chip basically. There is also a TL072 op-amp and an MC34119 audio amplifier. All of the components are standard leaded types except for a surface mount NTC thermistor used as part of the automatic frequency control circuit to account for temperature variations.

You do need a few other things so I added the speaker, the signal meter, a AFC switch (the kit comes with a jumper) and the audio out connector. I also used full sized pots for the volume and squelch control since I will mount them off the board. The kit comes with trimmers to use on the board though. The kit runs off a 12 volt supply which is regulated down to 5 volts on the board for most of the circuit to use. I made a small power supply board with a 12 volt regulator on it and powered that from an old power supply from a set of old computer speakers. The used speaker I am recycling came from the same set of old speakers. The signal meter was a nice find. The first place I look for odd electronic stuff, Surplustronics, didn’t have anything so I headed up the road to the top of Queen Street to a real electronics junk shop. I have no idea what it’s called but it’s been there for years now. It’s cram packed with junk. Most of it is useless but the odd interesting thing here and there. Nothing is sorted. It’s all just in piles. I dug about and came up with this nice signal meter with a 0-5 scale on it. He wanted $3 for it. I had $2.20 in change (good thing I hadn’t bought a Mars bar that day) so he said close enough. It was unused and still in the bag!

IMG_8363_1 Assembled kit set. The red thing is a plastic tuning tool.

Building the kit is easy. The instructions say it takes about four hours but I did it in two. All those years of working as an electronics tech to pay for my way through university came in handy! Once you have built it you need to set it up.

The way the receiver works is the incoming signal, which is around 137MHz, first goes though three bandpass filters. These eliminate any extra signals from getting through and just pass the weather satellite transmission frequencies that we want to receive. This is important in some places where there are strong pager frequencies nearby (people still use pagers – probably more reliable that Telecom’s XT network).

We then want to take the incoming signal and convert it down to a much lower frequency. This is called an intermediate frequency.  An intermediate frequency, or IF, is used because it is easier to work with and  filter on the lower frequency and because if a receiver converts all the frequencies you want to receive down to the same IF then all your circuitry from that point on can be optimised for just that IF frequency. This receiver uses the same IF most FM receivers use, 10.7MHz.

To get to the IF the receiver chip uses a mixer. The mixer takes the received signal and mixes it with another signal that we generate on the board. This signal needs to be 10.7MHz higher than the signal we wish to receiver. The mixer takes our signal and subtracts from it the received signal to get the 10.7MHz IF signal. This process is called Hetrodyning (which is probably what my friend Fing tells his friends he is doing when he comes to visit Auckland and we all go to the pub for lunch). The on board frequency is generated by a thing called a voltage controlled oscillator or VCO.

In this circuit the VCO is controlled by a coil, which gives a rough setting, and the tuning resistors, to fine tune to for each frequency.  The kit uses 25 turn trimmers for tuning. Each channel uses one trimmer. The kit only comes with one but you can simply add more and use a multi-pole switch to switch in the appropriate ones. For now I just have the one which I adjust for each different frequency when I need to.

The signal passes though various stages in the chip and eventually gets spat out as audio. There are two audio paths. One is to the audio amplifier chip which is controlled by the squelch circuit. The other is a line output to the PC sound card.

To set the receiver up you need to adjust the VCO to be 10.7MHz above the frequency you want to receive. The receiver needs to receive from 137MHz to 138MHz approximately. The tuning resistor needs to tune across that range.  If you set the tuning resistor to half way in it’s travel that would correspond to 137.500MHz. We therefore want the VCO to be oscillating at 148.200MHZ (148.2 – 10.7 = 137.5).

To set the VCO frequency you need to adjust one of the coils on the board. These coils have adjustable ferrite slugs in them that you screw up and down to set the frequency. They are fairly delicate and also metal near them will upset the frequency so you need to use a red plastic tool for adjusting these. This was available as an extra from Minikits cheaply.

The best way to know you have the right frequency is with a frequency counter. Unfortunately I don’t have one. So the next best thing is to use my scanner again. Because the coil is oscillating it is also radiating radio waves. If you hold the scanner close to the circuit board set to 148.200MHz when you adjust the coil and it hits that frequency you’ll hear it on the scanner.

With the VCO set this way you know the receiver should be receiving across the 137 to 138MHz frequencies over the 25 turn range of the trimmer tuning resistors.

Next the instructions say to attach a RF signal generator set to 137.500MHz to the antenna input and adjust the filters and output demodulator for the cleanest signal. Again, I don’t have a signal generator so instead I had to wait until I knew a satellite was overhead. With the antenna sitting out on the front deck railing I used the WXtoImg software to know when to listen and on which frequency. I used my scanner to be able to hear when the signal was there.  I then connected the antenna to the receiver and tuned the receiver to the right frequency and quickly adjusted the rest of the circuit while I still had a signal. This can take some time given the low number of passes per day and the short time the passes last. You have about 15 minutes in which to tune things.

To do this tuning I used my ocsilloscope on the audio output to see how clean the signal looked as well as by watching the signal meter and adjust it to give the highest signal as suggested in the instructions. This worked very well.

IMG_8362_1 Initial setup and testing.

One interesting part of the circuit is the automatic frequency control, or AFC. This is quite clever and it will allow the receiver to account for the Doppler shift in the satellite as it passes towards then away from the receiver by adjusting the VCO.

You will see that the EeePC is displaying an image here but I found that it can’t actually decode the pictures properly. I think this is because I was using the mic input instead a a line in (which the EeePC doesn’t have) and so I wasn’t getting a proper signal.

The next thing to do was transfer everything over to my main PC and to put the antenna in a better position (on top of the guttering on the edge of my roof for now). With everything set up like that I finally got my first real picture.

first image The first image!

And this is what I got. The noise and blank part at the bottom of the image was me trying to tune things in as the satellite passed (from south to north). The horizontal lines were when I was adjusting things. In between though you can see a good image.

With more tweaking and tuning I am now getting nicer images. I am just using the basic settings in the software for now. I think there are a number of different settings to show you different things but I haven’t played with them must so far. These images are using MCIR map colour IR and an Equidistant Cylindrical  settings.

07040210a First good image.

I still need to manually tune hence the missing bottom to the image but this was a imagine received from NOAA 18 on 137.9125 MHz (that’s why so much of the bottom is missing – I had to turn that 25 turn trimmer a lot) at about 2pm.

The same satellite did another pass about 1 hour 40 minutes later and I got the image below.

07040352 Second pass by the same satellite showing Australia.

You can see how the satellite follows different paths on different orbits so you end up receiving differing pictures. Using some (free) software such as WXTrack you can actually see the path the satellite will take overhead. If the satellite is very low on the horizon you will get noisy images. The antenna needs a clear line to the satellite so obstructions in the way will degrade the signal.

So, what’s next? Well, I am still only using the one tuning resistor so for each different satellite frequency I need to re-tune the receiver. This won’t do as a long term solution. Instead of the idea that is given in the kit of using 5 tuning resistors all switched in via a rotary switch I want to try something a bit more automatic.

The kit does provide connection points for adding a phase locked loop or PLL for short. This would allow scanning across the whole frequency range then have it stop once it detects the appropriate signals. You can buy suitable experimenters PLLs that are micro controlled but these are rather expensive and I am trying to do this on the cheap!

I considered using a micro controller to switch between five or so pre-tuned frequencies. Because there are only a few satellites transmitting on fixed frequencies (the status of the satellites and information about them is available on the NOAA POES status site) you don’t actually need to scan the whole range. You can just step through fixed channels looking for a signal.  I could use a micro to switch each channel in for a brief period, say 2 or 3 seconds, and if a suitable signal was detected stay on that channel until the signal drops out again.

Switching in a set of preset tuning resistors can be done using a bilateral switching chip. A micro could control the switching. I then though that’s perhaps over complicating things so perhaps I can do it entirely using normal ICs.

My current plan is to use a 555 timer providing a slow clock pulse. These I feed into a 4017 decade counter (one of the first ICs I ever experimented with so a favourite). The outputs of the counter feed into a 4066 bilateral switch to switch in each tuning resistor in turn. They also turn on various LEDs to show what channel is currently selected. When a signal is detected the counter stops on that signal until the whole signal is received. This part I haven’t worked out yet. I was thinking of using the squelch signal but I don’t think this will work too well. Instead I might make a tone detector to listen for the tone you hear when the signal is coming in. The APT system used to transmit the images used a 2400Hz sub-carrier it should be possible to detect easily.

IMG_8367_1 Progress so far.

So far I have got the clock, counter and switching working. You can see those on the breadboard in the picture above. I need to start on the tone detector. Perhaps using a LM567. My backup plan if I can’t get that working is to use an Arduino perhaps. Everyone loves Arduino!

Also I need a more permanent antenna installation. Perhaps a small mast mounted on the roof of my garage so I can put it there and get a nice, clear view of the sky.

And of course once done everything needs to be put into some nice housing.

Will update this with progress soon.


Posted in Projects | | Top Of Page

10 Responses to “Receiving weather satellite pictures in New Zealand – Part 3”

  1. Asciimation » Blog Archives » Receiving weather satellite pictures in New Zealand – Part 2 Says:

    […] The kit arrived! Read about it in Part 3. Posted in Projects | Trackback | del.icio.us | Top Of […]

  2. Conor Says:

    Great write up, and awesome project. I am sure you have inspired the meteorologist in many.

  3. Leif Says:

    You might want to try that pager mod only instead of tuning a pager frequency and using the pager decoding software use it for the weather satellites. I bet you get a better picture. But only if you feel like experimenting since you already ordered the new receiver.

  4. Leif Says:

    I wrote that last comment before seeing there was a part 3. Maybe you could get a free pll chip by asking for a sample? Analog Devices among other companies are pretty good about that.

  5. antoine Says:

    Very interesting project, thanks for sharing it with the www! I tought all that information was carefully encrypted but its great to know its not. I can definetly see some people putting toghether really neat weather stations with this.

    Is this what you plan to do with your project?

  6. W1RDB Says:

    Great description and great project. Thanks for all the nice pictures. Probably the best investment you could make at this point is a low noise RF pre-amplifier to boost the signal. Installed right at the antenna, it will make for a solid signal horizon to horizon. Best bang for the buck IMHO!

  7. ahooper Says:

    Just wondering where in AK you are located and if you would be interested in doing a few tests by setting up the receiver at a repeater site i have access to upneat the TV transmission station in wiatarua then feed the audio stream down vie an audio server over the internet from there?
    Flick me an email if you are keen

  8. viktor_au Says:

    Thanks for the good article. I bought the weather sat kit from Dick Smith Electronics but good to know other projects as well.

  9. W0MM Says:

    Great article!

    I created wide composite images with WXtoImg and different pictures taken in EU and NA. You can see those composites at http://widecomposite.w0mm.com.

  10. Asciimation » Blog Archives » Using RTL-SDR to automatically receive weather satellite images. Says:

    […] Part One, Part Two, Part Three. […]