Pictures from Above

...or, a Pentium is a nice thing to have.

I've been playing around with this stuff for some time. My first experiments with weather satellites used external hardware, an interface that rectified the APT subcarrier, low-pass filtered the proceeds and then digitized them to be pulled into the computer's parallel port and processed by a program like JVFax. This worked, but then I got curious about my computer's sound card...

My first program was the classic digital crystal set AM detector: rectify and low-pass filter. It actually worked well. But I've continued playing and trying things, and am now getting pretty good pictures. Here's one of B.C., now that I'm back on the west coast:

15 July 2000

...and here's a picture from when I was at school in Toronto:

4 September 1999

My current software uses a complex AM detector and an LMS adaptive filter for noise reduction. I claim no originality for any of the algorithms. They're all covered in standard DSP textbooks.

There are a couple of different standards for sending images. Both use a nominal 2400 Hz AM subcarrier.

The format used by the U.S. NOAA satellites is 240 lines per minute, with alternating visible and infrared scans. This sample is from NOAA-14. The "tick-tock" effect is the sync pulses. The beginning of the visible scan is a burst of 1040 Hz ("tick"), while the beginning of the IR scan is 832 Hz ("tock").
The older satellites launched by the Soviet Union and the newer ones launched by the CIS send 120 line per minute IR imagery only. The "tunk" sound is the sync pulses. This sample is from Resurs 01-N4. Older satellites like Meteor 3-5 had a higher-frequency subcarrier, closer to 2500 Hz. The newer ones seem to be spot on 2400 Hz.

All the signal processing is in floating point. Pentiums do it well: reading from a file, my program processes 280,000 samples per second on a 550 MHz Pentium 3, up from the 80,000 the old Pentium 233 MMX did.

Hardware and stuff

For an application like this you actually want the cheapest, nastiest sound card you can find. The cheapest card will record everything there is to record from a communications-grade audio channel with ease. Additionally, expensive cards have all those 3D surround effects and similar junk, which may make shoot-'em-up computer games sound better, but which raise hell with the amplitude and phase response that us DSP hacks work so hard to get right.

The hardware of my VHF APT weather satellite system is:

Home-made crossed-dipole antenna.
Home-made dual-gate MOSFET (BF961) preamp, essentially the front end of the Suffolk 2 meter transverter.
Modified Realistic (i.e. Radio Shack) Pro-43 scanner, with the 455 kHz IF filter replaced by a capacitor.
Home-made Linux software with complex AM detection and LMS noise reduction. Written with Gnu C and the SGI TIFF libary.
I now use GIMP for all the image post-processing: resizing, cropping, playing with the gamma curve, and so on.
550 MHz Pentium 3 system to run all this stuff on. Gross overkill, but it was cheap. :-)

I want more!

Here are some links that might be useful:

The terminally curious can find APT specifications here.
This is NOAA's main jumping-off point for weather satellite intormation.
The Remote Imaging Group in England has links to software, hardware information and lots more.
You can get satellite tracking software from AMSAT.
You can get Keplerian elements for your tracking software from Celestrak.

I plan no further APT development. It's time to get serious: HRPT. I've started on a receiver and am considering how to process the bit stream.

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