Okay, so here’s the deal: I’ve been working on my weather satellite decoder off and on for a few weeks, and been cleaning up the code, trying to make it simpler and better explained in preparation for eventual publication. I’m now at the point where I’m trying to do the whole “sync detection” part, so that each scanline is placed directly below the previous one. But if you look at the image on the right, you’ll see that the problem isn’t just finding the one, correct line length, the lines actually vary in length, becoming longer as the picture moves down, causing the image to drift right.

I immediately thought, “this has something to do with Doppler shift”, which is correct, but which I didn’t have any kind of real justification or mathematical basis to my understanding. It was, of course a guess. I was walking back from lunch, explaining this to Tom, and I said “the odd thing is that I don’t understand how the lines could be getting longer.” After all, the satellite emits 2 scanlines per second from its point of view, that doesn’t change when its received, right?

Tom looked at me like I was stupid.

Which was of course enough to spawn me to rethink, and realize that yes, I was being stupid. Let’s say that the satellite emits a 100hz tone for one second. The received signal will be Doppler shifted up or down based upon the relative velocities of the satellite and the ground station. If the satellite is moving towards the ground station, the Doppler shift will move the frequency up. The received signal will complete the same number of cycles, but at a higher frequency. That means it has to take less time. Similarly, for a satellite moving away, the frequency will be lower and each scanline will be greater than the nominal frequency.

In my case, at AOS, the satellite is approaching, and the line rate will be greater than the nominal 2hz. At max elevation, the satellite is moving perpendicular to the observation vector, and the line rate will be precisely (well, not precisely, as pointed out by Steve below, but close enough for my purposes…) 2hz. At LOS, the line rate will be less than 2hz. In each case, successive lines are longer than previous lines, which causes the nominal trend to the right we see in the images as decoded by my simple decoder.

Duh Einstein. I find again that when you think about things the right way, the answer seems obvious.

Addendum: if we could trust the sampling rate of the sound card, we could determine the exact point where the satellite passed max elevation, by finding the place where the line rate was precisely 2hz. I’m not sure how accurate your average sound card is. I’ll think about it some more.

Guy buys some cheap laser modules, scavenges parts from a couple of old scanners, and builds himself a laser stencil cutter. Uses Postscript to generate the cutting paths, an idea which Don Lancaster called a Flutterwumper.

Laser cutter, start slicing stuff for under 50 dollars

G6LVB has a really nifty website, which includes plans for a neat little PIC powered antenna tracker. The writeup says that it used the “Plan 13” algorithm for G3RUH, which I had never heard of before. A little judicious Googling reveals:

PLAN-13 Satellite Position Calculation Program

This algorithm seems much simpler and more straightforward than the SGP4 stuff that most other programs use. I might just code this up in Python (or C maybe) for future experimentation.

I got some time today waiting for the furnace guys to show up (they never did) and managed to figure out a few of the things that were wrong with the image normalization code and with the sync detection code. Here’s the latest automatically generated image from one of the recordings I made earlier.

There are a couple of things about these images which aren’t precisely correct, but overall the rectification works pretty well: the sync detection is working and reasonably reliable in the regions where the image is essentially nose free. With some additional work, I could be even better, and the occasional one pixel glitches in the position of the scanline would likely go away. The normalization isn’t perfect either. But still… pretty neat.

It’ll be apparent what’s going on halfway through the song.

Okay, I admit it. I have a special place in my heart for The Incredibles. It was my first opportunity to work for Brad Bird. I worked with a lot of really great people, many of whom I went on to work with again during Ratatouille, and let’s face it: I just love the whole comic book/superhero genre.

It’s hard to describe what I spend a lot of my time doing during movie production, but I’m often working in the rendering department. It’s our responsibility to make sure that each and every frame of each and every shot meets with the directors approval. We fix all sorts of minor technical issues, and even the occasional odd bit of animation or lighting. When the shot clears out department, the director has basically said that he’s approved the look of the film (minor changes can be made in image mastering, but hopefully they are minor).

That’s why it’s so painful to watch The Incredibles on the ABC Family Channel on Comcast.

To be honest, I don’t know whether it’s the Family Channel’s fault or Comcast fault, but sweet spirit of camphor, are they sending the movie down the pipe at 500kbps or something? The transfer is simply awful. Dreadful. Unwatchable. Words escape me how terrible it is. Pixelation. Colors are crushed. Mosquito noise. It’s just pathetic.

Please, please, please. Watch the DVD. It’s really quite good. Don’t judge it by the youtube experience you see on the Family Channel. I’ve seen better from pirate DVDs from Singapore.

Sorry, just had to get that off my chest.

Not getting your daily geek on? Try the PDP-1 playing a Christmas Carol Medley.
Via PDP-1 Music

I’m sitting up in bed, laptop buzzing away, with my wife sleeping next to me. Within the next hour, we’ll have showers taken, coffee brewing, and begin opening presents. I didn’t get the house cleaned as much as I would like and my wife’s Jeep still needs its wiper arm replaced (did they Loc-Tite the bolt or what?), but for today, the sun is shining, there are presents under the tree, and all is well with the world.

Best wishes from me and mine to all of you and yours. Merry Christmas.

Okay, I know I’m going overboard on this whole amateur radio kick I’ve been on, but with all the satellite stuff I’ve been dinking with, I couldn’t resist.
NORAD tracks Santa.

Google Maps for mobile phones apparently has a new feature that enables you to automatically determine your position by triangulating your location from cell towers (much like the Navizon client does). This is very cool! Unfortunately, you can’t update the google maps application on the iphone, so I can’t tell how well it works. Are you listening Apple? Get it on the iphone soon.

While discussing my experiments with weather satellite reception, I talked about how I made my recordings using the “WFM” or wide FM settings on my Radio Shack Pro 60 scanner. If you read up on this subject, you’ll find that the signals transmitted by these satellites have about a 50khz bandwidth. The normal “narrow” FM mode used by most HTs and scanners have about 15khz bandwidth. The wide FM settings have 200khz bandwidth. If you record a signal this way, you are about 6db down, and you have the potential to capture strong signals which would normally be out of band, but if it works, it works pretty well. But someone asked me “What happens if you record with the normal 15khz setting?”

So I did the experiment.

Narrow Versus Wide FM

The first bit of this run was recorded in the narrow setting. The majority was recorded in the wide setting. These were done using my VX-3R, which was handier than my Pro60. If you look at the top, you can see that the results are noisy, and we get no darks. If I thought really hard about this, I probably could tell you why this happens, but that’s what you get.

Addendum: I worked a bit on the image, cropped out the noisy bit, and adjusted the balance and color a bit. This is what you get.

Here’s a nice, concise overview of how to setup a conventional packet station to transmit via the ISS or via PCSAT-1. I might do a similar page of my own that shows how to do this using just the Kenwood TH-D7A at some point, but until then, this is quite helpful.

Basic Packet Setup for ARISS

Here is the Google satellite map of the area around the Occidental Grand Flamenco Xcaret where we stayed on our Mexico vacation. Toward the bottom is a man-made inlet with a small beach where you can actually snorkle amidst some kind ofneat fish. Just to the right of the inlet is a small ruin (pictures later), and if you trace the straight path to the northwest, it bisects the main entrance hall (standing at the midpoint, you could see the Mayan ruin at one end, and the christmas tree in the three story tall lobby at the other). You can also see the rather nice swimming pools.

Just thought it was neat.

View Larger Map

While touring at Chichen Itza, your guide will undoubtedly ask you to clap your hands while standing on the broad field in front of the main step pyramid. What you hear is an odd chirping echo, caused (I’m guessing) by the many vertical steps which run up the side of the pyramid. Our guide explained that during equinox and solstice celebrations, the plain would be filled with drummers and clappers, and the echo returns must have truly sounded unworldly.

I immediately thought of my friend Tom and the fact that we could probably simulate just what that might sound like.

Toward that end, I did a quick google for “Chichen Itza acoustics”, and uncovered this paper summary which had an interesting theory: that the reflected echos were designed to sound like the chirps of the Quetzal bird. Interesting, and not unconvincing. It appears that my Googling is turning up a lot of other interesting phenomena that might bear additional study, but my lunch hour is over. I’ll have to try again later.

The blog has had no updates in the last week. The reason is simple: I was on vacation. Far away from the land of the Internet, email and cell phones. The wife and I decided to spend a week in Mexico, at the Occidental Grand Xcaret on the Mayan Riviera.

While we were there, we visited some place I’ve always wanted to go: Chichen Itza.

Here’s a tease. I’ll upload some more photos tomorrow when I get some free time.