Predicting Satellite Orbits

I was interested in figuring out when the next pass of the ISS was going to occur, and was reminded of the interesting mathematics involved in predicting satellite orbits. While I’ll probably never use this, the code that everyone uses are derived from the original FORTRAN code, generally referred to as the NORAD SGP4 and SDP4 models. You can find these, as well as more modern revisions to fix bugs in the original codes at the Celestrak website.

CelesTrak: Documentation

Addendum: The original Spacetrack Report #3 linked to by Celestrak seemed to be trashed. Luckily, you can get a good copy from the Amsat website.

Juan Buhler / Morning Fun

Juan Buhler, fellow Pixarian and exceedingly talented photographer, has a new gallery of cool photos up today. He said in his email:

I was in Pamplona for San Fermin this year. I had a press pass, to shoot
pictures of the running of the bulls from the fence.

I figured there’s plenty of sharp pictures of this event, so I set my
shutter speed to 1/15-1/20 and shot that way.

Very, very cool.

Juan Buhler / Morning Fun

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Decades of NASA photos, video coming to the Web

All I’ve got to saw is awesome.

Decades of NASA photos, video coming to the Web | Tech news blog – CNET

The space agency and the Internet Archive said Tuesday that they plan to scan and archive more than 12 million NASA photographs and 100,000 hours of film and video footage for free access online, under an five-year agreement. As part of the deal, the Internet Archive will host the media album on a new Web site,

PSK31 Web receiver

I’ve got quite a bit going on lately, which is why I haven’t been posting all that much. For some reason, I’m back in a bit of a ham radio kick. I noticed that my license had lapsed, and so I renewed and then felt like doing some monitoring of the local repeater action. But really, I’m most interested in digital communications. A mode which has intrigued me for some time is PSK31, a 31.25 baud protocol for keyboard-keyboard transmissions, now gaining quite a following on 20m. Some time ago, I was working on my own code to demodulate the signals (in principle, not that difficult. but with a few subtlties) and am trying to dust it off. In the mean time, here’s a cool applet online to let you know what it is like:

PSK31 Web receiver
To see the picture, you need to get a browser that supports Java. Basically, this is a subset of the 20m band for ham radio. The spectrum is broken into red, green and blue sections. PSK31 signals appear as horizontal lines over time. Below the window, you can select to zoom in on either, the red, green or blue sections, or view all three side by side. If you see a signal, zoom in on the section its in, then position the arrow to the right of it, right in the middle.

Voila. The decode should appear right below.

From the receiver’s position in Kiev, I’ve seen strong signals using twenty to thirty watts from as places as far away as Spain. I’ve seen relatively little DX from the U.S. thusfar, but that might be a timing thing: I’d have to work out when the propagation works out the best. Still, very neat!

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P.S. PSK31 is a nifty protocol, but it suffers from what I feel are a couple of short comings. The most annoying is the variable length character coding they use. Instead of coding 7 bit ASCII directly (say, with a start and stop bit), PSK31 encodes each character as a varying number of bits, each beginning with “1” and up to ten digits long, but with no code patterns that have two adjacent zero bits. Characters are separated by two or more adjacent zero bits. Thus instead of characters being encoded by constant 9 bits, we get a variable encoding of between 3 and 12 bits. It probably does save a tiny bit of bandwidth, because common characters are encoded in less space, but I suspect the savings are minor due to the stilted distribution of characters in your typical QSO. (QSOs aren’t English, which varicode is obviously derived from). But you’ve bought this marginal increase in efficiency at the price of having to create a rather intricate state machine to pack characters. I’m using it as an exercise in programming, dusting off my knowledge of finite automata and trying to write a program which generates the code for the state machine directly from the specification. But (for instance) this added complexity means that you have to work harder, which isn’t bad if you are using a laptop, but might be less than optimal if you are trying to use something like an Atmel AVR to do decoding.

Oh well, just some thoughts.

Damaged shuttle Endeavour lands safely – Yahoo! News

This morning I was listening to a local 2m repeater on the way into work. They were rebroadcasting the NASA feed which happened to be covering the landing of Endeavor. Endeavor was forced to return to earth a day early because of fears that hurricane Dean would affect flight operations at Mission Control in Texas. Endeavor also had suffered a bit of a gash on its underbelly, which was the cause of a great deal of analysis, but NASA claims that Endeavor was not in any danger.

Welcome back to the crew.

Damaged shuttle Endeavour lands safely – Yahoo! News

Crystal Radio Patent

Crystal Radio Patent DiagramI was reading one of my books about crystal radios the other day, and they mentioned this patent. It’s hard to imagine the time period where this radio was considered innovative, but crystal radios are still pretty amazing little bits of electronic fun to hack together with kids. Winding coils, tickling crystals of galena… good stuff.


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NASA – Mira: A Star with a Comets Tail

Had you asked me what Mira was before this week, I would have (correctly) told you that it is the name of a variable star. It turns out that it is Omicron Ceti, a red giant star approximately 418 light years away in the souther constellation Cetus (the Whale). It is also a well known variable star: oscillating from magnitude 2.0 to magnitude 4.9, a variation in brightness of almost 15.

What’s new are ultraviolet studies done by NASA from the Galaxy Evolution Explorer telescope. Apparently the star is leaving a trail of material almost 13 light years long as it travels through space. Very cool!

Mira, leaving a trail behind itself as it hurtles through space…NASA – Mira: A Star with a Comets Tail

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Examining Will Crowthers Original “Adventure” in Code and in Kentucky

Digital Humanities Quarterly has published a fascinating article about Will Crowther’s original Adventure game. It turns out that it is closely modelled on a real cave, and the article includes a lot of interesting information and pictures about the real cave. Very neat! I’ve skimmed it, and will try to read it more in depth over lunch. Hope some of you will do so too.

DHQ: Digital Humanities Quarterly: Jerz. Somewhere Nearby is Colossal Cave: Examining Will Crowthers Original “Adventure” in Code and in Kentucky

Addendum: This article on also provides links to the original FORTRAN source code.

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Tears of St. Lawrence

Well, I wasn’t going to stay up past midnight to see the best of it, but I did wander outside tonight at around 11:30 for around 15 minutes to see if I could spot some of the peaking Perseid meteors.  My score was:

  • three fast moving bright Perseids
  • 13 planes
  • one polar  orbiting satellite
  • hints of half a dozen faint Perseids
  • oddly enough, the Milky Way was faintly visible from my house

Not bad.  But I’m going to bed.

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Harry Potter and the Order of the Phoenix

Well, the latest installment of the Harry Potter series was down to a mere two showings per day so I figured if I was gonna see it, this might be the last weekend. I know it is pretty late in the run, so a review is pointless (if you haven’t seen it already, you probably won’t, and if you have, it’s pointless to tell you what I thought) but I thought I’d mention a couple of points.

I thought it was actually pretty weak. It’s definitely the darkest of the movies so far, which in itself isn’t a bad thing, but the actual problem with the script is that nothing actually happens in the two hours in the middle of the film. It plods along almost completely bereft of any action, with only the barest hint of what is going on, from pointless scene to pointless scene. Honestly, a few hours later, I can’t think of a single scene in the middle of the movie that was in the least degree memorable.

Perhaps almost as disturbing was the rather odd color balance and focus of the print. I seriously thought that the projector was out of focus, because every scene seemed to have a very odd focus. But I noticed that parts of the scene were in focus: it was obviously intentional. But I found it it really irritating: often the person talking was out of focus, and it was actually stressful to my eyes to watch as they tried to force the image into focus. It was truly bizarre. I haven’t heard anyone else mention this kind of thing, which I think is kind of strange. Perhaps I’m more finely sensitive to this kind of thing since part of my job at Pixar is to stare at frames and deal with issues about focus, but it was truly distracting.

This movie got a lot fairly decent reviews (scoring a B from critics on Yahoo, and a B+ from Yahoo users) but frankly I think it isn’t that good. I’d rank it as a C.

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The LAMBDA-Method For Fast GPS Surveying

I spent some time today looking at technical information about GPS receivers. I had known for some time that it was possible to determine the relative position of two receivers to a very high degree of accuracy, but didn’t know the details. Here’s one method that appears to have popped up in my brief research, I’m mostly leaving it here as a bookmark for myself.

Using techniques like this, it is possible to determine the distance between two points several kilometers apart to just a few hundred microns.  This makes it possible to use GPS receivers to measure things like the movement of tectonic plates.

The LAMBDA-Method For Fast GPS Surveying (ResearchIndex)

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