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Delta 2 Rocket Launches 50th GPS Satellite
wetshoe writes "This CNN article reports that 'the 50th U.S. Global Positioning Satellite has lifted off aboard a Boeing Delta 2 rocket.' It was sent into space to replace an aging GPS satellite. One more reason why geocaching is so much fun."
Uh, No. SA has been turned off since the Clinton administration and hasn't been turned back on. Ten feet or so is the best accuracy you're going to get without using differential gps.
First.. Space Junk.
GPS is launched into an orbit some 12,000 miles above the Earth's surface. That orbit has a grand total of about 50 satellites split into 6 different, non-overlapping planes and slightly different altitudes. There are very, very few satellites that go out that far and none have a circular orbit within a few hundred miles of the GPS satellites. Very, very little chance of a collision.
Also, from that height, the satellites lack enough fuel to deorbit or be sent into the sun. In 1992, my Univ of Colorado aerospace engineering lab went down to the control center and we had a nice tour. I asked the officer giving the brief if they intended to establish some sort of parking orbit for dying satellies as they get phased out. He indicated that it was something they would consider as the constellation gets built out.
Secondly..
Paying the bill.
GPS was encrypted from Day 1. The lower resolution receivers we use just are allowed to decrypt the satellites. It is very difficult to get the higher resolution channel.
The US government is perfectly willing to let the other countries contribute to the costs associated with running GPS.
But..
You might want to consider why the other countries are willing to spend billions on a redundant system rather than pay into GPS or use it for free.
When someone spend billions rather than use a free service, something is up.
The US military adamantly refuses to free any of the control of the system up. It is a US *military* asset. As such, it has military utility. They have completely thrown off the commercial channels in the past while engaging in military activities in a region by jiggering with the output to cause the locations to be off. (They can also turn off all the commercial channels on satellites flying over Afghanistan, then turn them back on before the reach the US, for example).
The rest of the world seems to have some qualms about handing the world's major navigation system to a single provider, for some reason.
There are multiple bits of information buried in the data.
g ps /gps_f.html
There is the carrier frequency.
Then each satellite has a specific identifying signal for each channel called a psuedo random number.
Then, it layers in a telemetry data packet as part of the actual data transmitted
http://www.colorado.edu/geography/gcraft/notes/
2. The satellite launched was a Block IIR vehicle. Block IIR-M and IIF vehicles are still in a very low orbit (close to sea level..haven't been launched.)
3. We can't burn satellites in from semi-synchronous orbit (the GPS orbit) using today's technology. When they're disposed of we kick them away from the earth a couple of hundred kilometers. Orbital degradation is slight at semi-synchronous, but the satellites will interfere with each other in about 6,000 years. I hope we'll be able to clean it up before then.
4. GPS Signals arrive on two frequencies, L1 (L1 = 1575.42 MHz) and L2 (L2 = 1227.6 MHz). C/A code (which is FREE as in air to civil users) is modulated onto the L1 carrier signal. It has never been encrypted. It has been degraded (selective availability, the method of degradation, was turned off in 2000) but is now every bit as accurate as the military signal. The only significant advantage the military receivers have is the ability to correct for ionospheric defraction using both frequencies.
5. The major driver behind Galileo (EU GPS) is economics. Basically the US has a handle on a 12 billion dollar industry and the EU wants its share. They're expecting to charge money for the same service the US gives out for free! Somebody failed economics.
Feel free to respond with any questions, I'd love to answer them.