Software GPS, Part 0 Introduction

I've always been fascinated by GPS. I got my first GPS unit in 1997. Stacy and I had just gone on a hike with another couple, and they took a wrong turn and started arguing with each other in Czech. So I got a Garmin GPS II. It didn't have maps; it just recorded a breadcrumb trail of your position, and let you enter waypoints on the map. But one cool feature it did have was a display of the GPS satellites' positions in the sky relative to your current location. You could watch specific satellites rise, transit and set. This amazed me, but the truth is even more amazing than that.

A lot of people think GPS works by triangulation, but it doesn't. It uses trilateration. Triangulation is measuring the angle to or from several known locations to your location. Trilateration is measuring the distance. You know the location of each GPS satellite because each satellite is broadcasting its precise orbital parameters, along with an extremely precise time signal. By taking the difference between the timestamp that the satellite broadcast and the time at your location and multiplying by the speed of light, you can draw a sphere around the satellite, the surface of which contains your location. Doing this for a second satellite gives you a second sphere, narrowing your location to somewhere the surfaces of the two spheres intersect. A third satellite should narrow your position down to a single point, but because of uncertainty in the signal, you typically get more of a blob. Typically, you need 3 satellites to get a good 2D fix (lattitude & longitude), but 4 or more to get a good 3D fix (lattitude, longitude and altitude).

Now consider this. In order to figure your location to within 10 meters, you need to know the satellite's position even more precisely than that. Anecdotal reports (by a guy sitting at the next table at No-Name) put the precision of the satellite's at under half a meter. Now, I rarely know where I am to within half a meter, and I'm not hurtling through space at 4 km/sec, having been shot there atop a rocket in the first place.

I've always wondered about the details of the GPS signal, and in particular the satellite orbits, which my more modern Bluetooth GPS doesn't support. If only there were some way to decode the raw satellite signal, so I could play with it...

3 For All in San Jose This Weekend!

I dabble in improv, but these guys are the real deal. Improv doesn't get any better than this. This Friday and Saturday at the Center for Spiritual Living in San Jose.

http://www.sanjosecenter.org/3forall.htm

I'm in Chicago, except I'm not.

I'm in Chicago from July 13th through August 17th for iO Chicago's month-long summer improv intensive.

Excecpt for this weekend, when I'm in Champaign-Urbana for the Second Worldwide Four-Square Invitational Conference (actually a multi-class reunion for University High School). I'll be here in Champaign until the afternoon of Sunday the 20th of July.

Today my life resembles George Carlin's "A Place for My Stuff" routine, as I had to pack up a subset of the stuff I took to Chicago, and bring it to Champaign. "The third version of my house," as he calls it.

If you're in Champaign or Chicago, and would like to get together, drop me an email.

Improv Performance at ComedySportz San Jose Sunday 6/29

Our Advanced Games Workshop at ComedySportz will having a class show this Sunday at 7:30 PM. Tickets are $5. If you're coming, please arrive by 7:15, to allow time to buy tickets and get seated by showtime. Click here for directions to the theatre.

Shuttle Launch

So, it’s been a few months since I went to see the shuttle launch.

It was okay.

I guess you could sum up my feelings with that Peggy Lee song “Is That All There Is?” The reason that I went to see the shuttle launch is because of the essay Penn Jillette wrote about it in Penn and Teller’s “How to Play in Traffic”.

“It’s 3.7 miles away, and your looking at this flame and the flame is far away and it’s brighter than watching an arc welder from across a room[….] The fluffy smoke clouds of the angels of exploration spill out of your field of vision. They spill out of your peripheral vision.”

“You don’t exactly hear it at first, it almost knocks you over. It’s the loudest most wonderful sound you’ve ever heard. […] You can’t really hear it. It’s too loud to hear. It’s wonderful deep and low. It’s the bottom.”

“This is a real explosion and it’s controlled and it’s doing nothing but good and it makes your unbuttoned shirt flap around your arms. It’s beyond sound,it’s wind. It’s a man-made hurricane.”

The key point there being, “3.7 miles away”. In the VIP section. I was in closer to 7 miles away, along the NASA Causeway, in the closest section open to to the general public. From there, the Shuttle is a tiny speck without binoculars, and the sound of the launch, when it hits you, is reminiscent of the sound of distant thunder in the midwest. And with the low clouds, the whole show was over in matter of seconds. I could tell you more, but just watch the movie. That’s pretty much what I saw and heard, and I’m nowhere near as good at words as Penn.

Next time, I’m bringing binoculars.

Watch This Space

I'm doing a little spring cleaning of my website, and as part of that, I've restarted my blog. This time I'm using Blogger. For those of you keeping score at home, previous attempts have used:

• iWeb
• Bliki
• GeekLog

But, hey, this time for sure!