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GPS Test Successful From Outer Space
An anonymous reader writes: "AMSAT reports that the GPS experiment
on the international amateur radio spacecraft AO-40 has undergone successful testing." A note on the site reads in part: "This experiment supplied and sponsored by NASA, is to determine if it is possible to get positional data outside of the GPS ring of satellites.
There are two GPS receivers on AO-40, the A receiver for receiving signals around apogee and the B receiver for signal reception around perigee. ... A signal on the apogee receiver from about 52 Thousand Kilometres out with good signal levels has been received, further data is being gathered and those
downloaded so far are being analysed. If this experiment goes the way I expect, it will revolutionise the way we use GPS in Space. Many future HEO spacecraft will
be able to take advantage of GPS for autonomous navigation
and stationkeeping." This is one of the most interesting applications of GPS technology I've heard about -- nice way to reuse what was intended as a terrestrial navigation aid.
This opens up so many possibilities! Like the article mentioned, satellite station keeping for one. Also, we would know exactly where they were (well, down to a few dozen meters at least) for avoidane and things like that.
Also, so much is said about the problems of space debris. GPS recievers can be small, small enough to attach to debris. Yes, placing it on every little thing could suck, but on the larger things that pose a real hazard. I know there is a project to map the sky's debris, forgot the link. Now, astronauts could know in real time where this stuff is.
Using this for navigation might not work now. As far out as you are, the satellites are really close together, and any errors you see now are going to be much larger. But, we could place GPS satellites orbiting other bodies to use for solar system navigation. LIke i said, lots of possibilites.
http://www.google.com/search?hl=en&lr=&ie=UTF-8&o
Maybe I'm missing something here, but what makes the results so astounding? Did the experimenters think the GPS satellites transmitted/responded in one direction only (toward Earth)?
Bruce
Bruce Perens.
I had the pleasure of working on some of the first test satellites (circa 1991)... These worked at LEO orbits (LEO is below the GPS satellites, which is below geostationary), so they had the advantage that they operated in a manner relatively similar to earth-bound units.
Three major differences:
- speed and altitude limitations removed (the government doesn't want these guiding missles)
- satellite reacquisition time reduced. Going fast means that you'll have to change satellites used for the calculation much more often. Back in the day of single-channel receivers, this was a major concern.
- vehicle dynamics set to assume an orbit, not some low earth speed.
Here's a great page with some info & diagrams of what's going on. It also shows how a signal can be received from a higher orbit: it listens to satellites on the other side of the earth. This is refracted through the ionosphere, and a lot of math is probably used to compensate (actually the military version of gps uses two frequencies - the ionosphere modifies each one differently and, knowing this, can be corrected better).
The RADCAL satellite took measurements, but didn't use the GPS signal for navigation. REX-II actually used a closed loop system to stabilize the entire satellite. The attitude control system is an essential part of any satellite, since it points the antennas to the ground and the solar cells at the all-important sun. Usually, there are many different types of sensors (horizon sensors, magnetometers that compare the current field with a predetermied map of the earth, star sensors, and gyros), and typically none of these sensors alone provides a complete attitude. The fusion between all these sensors, with various levels of error and fault tolereance, is a really tough job! So, a small, light gps adds a lot of good information to the equation, and can serve as the primary sensor, or as a good backup.
We used a modified trimble gps unit with 4 antennas. This unit was originally designed to determine the attitude of fighter planes, but we used modified software to work in space. One antenna read the main GPS signal, while the other three measured the phase difference between themselves and the main signal to find the difference in distance to the satellites.
Side note about the fighter jet version of the software: The differential positions of the antennas were used to calculate the attitude. I know what you're thinking: why 4 antennas to solve 3 unknowns (pitch, yaw, roll)? It turns out that wing flex (since these were spread out as far as possible, which meant 2 were on the wings) had to be taken into account. Besides that, the extra antenna provided improved coverage in case the fuselage blocked an antenna.
We used these units sucessfully in many leo satellites...
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This would certainly offer a way of learning how to do efficient targeting. If we really are going to develop starwars, then targeting of high speed items becomes very important. Your approach would allow us to develop this and do something useful. I use to think that a net device with ion engine could do the trick, but it would take a long time to pick up very little. Too bad NASA and USAF move slowly on just about every thing.