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.

Comments

Re:umm...

[4444444]

the geosync gsp satelites

Umm the gps satelites are not in geo they are orbiting at about 10K miles

Re:Why _wouldn't_ if have been possible?

[spaceyhackerlady]

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)?

The antenna patterns of the GPS satellites are not in fact published. It's probably a safe assumption that they point at Earth, but the DoD have never published any specs.

There are a number of challenges to using GPS for satellite navigation. A terrestrial receiver isn't going to be in much trouble if it takes a few milliseconds to compute a fix. A satellite receiver will move an appreciable distance in the same time, rendering the fix meaningless.

AMSAT had to use a semi-custom GPS receiver to get around the anti-SCUD provisions that are mandated for commercial GPS receivers.

A propos DMCA: the GPS algorithms are all published. You can download them from the Internet.

...laura

This IS useful

[ZigMonty]

Most of our space craft ARE in orbit around the earth. This would be great for satellites, especially microsatellites. These are usually under 100kg and could be used for anything from satellite observation and diagnostics to cheap comm sats to HDTV mobile cameras. This experiment could help drive their cost down as it would mean that they could use a cheap GPS receiver for guidance instead of some clumsy, custom method. The US Air Force has some interesting ideas. As does this New Scientist article.

Re:This IS useful

[Anonymous Coward]

There are 24? GPS satellites currently in orbit. 3 to 4 are required to be able to get a position fix (really 4 for this application) depending on the geometry of their positions relative to your own.

The idea is that they transmit pseudorandom signals (transmitting some data) at a particular frequency (actually two) that receivers can lock onto. By having accurate locks onto several satellites and knowing the position of the satellites, it is then possible to determine your distance from each one and come up with a location.

Good receivers can lock on to 8 to 12 satellites at a time. Some receivers can also use the similar signals from the Russian equivalent GLONASS satellites. I think there are only 12 GLONASS satellites up there.

All that is from memory, so feel free to correct it or supplement it, but there is not really a single source of failure...the biggest problem is jamming from objects in your local area.

Re:This IS useful

[phil reed]

This sounds like it would raise the issue of single point of failure; if someone managed to knock one of the GPS satellites out of orbit, a bunch of other satellites that were relying on it would then follow it out of the sky. That doesn't sound like such a great idea to me.

Unless, of course, there were multiple GPS satellites, and each satellite relyed on the positioning information returned by the majority of them. The odds of all of them being knocked out at once are much lower than just one being knocked out.

There are 24 (plus or minus 1 or 2) GPS satellites in orbit at any one time. In order for your GPS receiver to work at all, it has to receive signals from a minimum of three satellites, four if you want to get a fix that includes altitude. My commercial-grade Garmin 12XL generally shows signals from 7-8, and I've seen as many as 12 at one time.

Still, if someone were interested in knocking out communications in the united states, they would still only have to knock out the GPS ones in order to knock out the rest.

Not hardly. Satellite designers are exceedingly paranoid, and they would never rely on a single method of positioning information.

Today's word is Triangulation

[Alien54]

how is this helpful?.. unless the spacecraft are in orbit around the earth, the geosync gsp satelites are going to rotate with it, so any approaching spacecraft won't really be able to use it effectivley 'cept to find out their distance from the earth, which can be done by easier means :P

So how does GPS work on Earth? The Geometry of Triangles, be it distance, angles or whatever. In GPS, it is easier to use the differance in distance for multiple satellites. Note You need more than one.

Secondary neat important fact: The Satellites orbit around the earth, and do not stand still at all relative to the earth. They are NOT geosync at all.

The Question is one of elementary geometry.

Imagine the orbits as a circle on a piece of paper, and satellites as points on the circle. If you can use triangles to find the location of a point inside the circle using those points, you can also use triangle to find locations outside the circle.

In the case of GPS and the Earth the problem is mostly one of signal strength.

With slight modifications, the logic also works in 3D.

;-)

Re:Rename it?

[kd5biv]

Maybe it's time to rename the thing to the "Galactic Positioning System."

I think that's a little optimistic. ;-)

Seriously, this thing seems to have the potential to make space probe control and navigation much easier and more accurate. It should be very interesting to see what applications come of this.

I think it changes all the rules. Any platform that can figure out for itself where it is in orbit and go where it wants to go without a team of engineers on the ground driving it 24/7 is going to be a major step up in technology. The AO-40 experiment shows SGPS is good for position data out to and possibly beyond GEO (it was launched into GTO from Kourou and didn't get much delta-V before the propulsion systems failed, so its apogee is still about GEO altitude), so you can count on future sats having at least SGPS receive capability on board now that the technology's been demonstrated.

I wouldn't be surprised to see some SGPS capability and perhaps onboard orbital management added to ISS and/or STS in the near future as well .. it's just too nice to have that independent capability on-orbit without having to do ground tracking. With enough hardware on the station and spacecraft, you could even do point-and-click rendezvous, sharing live data over TDRSS and computing plane changes and transfer orbits automatically, and of course you could continuously update your own Keplerian elements .. basically, you don't need nearly as much help from NASA or NORAD.

In short .. way cool. Way cool indeed.

Re:How useful is this, really?

[topham]

As someone else mentioned, GPS is one-way transmission. GPS received know what data they should expect to receive at a given time. BY knowing when the signal was sent, and how long it took to receive it they can work out how far they are from each satalite in range. Get 3 or 4 and you know where you are quite accuratly. (Interestingly enough it should be possible in space to get a much more accurate result than on the ground. No atmosphere to screw with the signal. Currently that is the most significant source of error.).

GPS Satalites orbit on a 12hr schedule (I think). But 2 new satalites used for GPS are geosynchronous. (used for WAAS enabled receivers. They send atmospheric correction data to increase the accuracy, and as bonus can act as GPS satalites as well.).

By the way, GPS is 'free', there is no charge to use the service above buying a receiver. And if your going to use the sun/stars for navigation your still going to have to have somehting interpret the data...

Re:Why _wouldn't_ if have been possible?

[LinuxHam]

(much less calculate) a position when the reciever is moving thousands of miles an hour. GPS literature refers to this as 'high dynamic' situations, a typical limit I've seen is around 950 MPH.

Think about AO-40 as its heading towards apogee. This is a high earth orbit satellite, peaking at 30,000km away from earth. I'm sure the 950mph was a speed rating in a flat plane. Lots of x and/or y, but very little z.

Rotate the frame of reference from a Concorde trying to get a GPS fix over the Atlantic at mach 2.0 to a Delta rocket lifting off from Kennedy trying to get one (pretending that liftoff is straight up from the ground with no arcing). My bet is the Delta rocket would get one while the Concorde would fail the 950mph limit you mention.

Just extend that Delta theory to 30,000km out and that's where AO-40 got its fix. I don't care how fast its going, in a straight line, the GPS sats see it as not changing x or y, just z. Track AO-40 on some sat tracking software sometime and you'll see that the orbit is so far out and so "vertical" to the earth at some points that the sat often appears to nearly stop right in its place on the map. That's when its moving directly away from or towards the surface of the earth.

Of course, I'm always willing to be wrong. It just seems like 17,000mph purely in the z direction would get the same sat exposure as a car sitting at a red light.

Re:I wonder

[wigger]

I wonder what class amateur Radio licence you need

I took the exam on the 22th

you took the exam but you don't know what class of license you need for 50MHz and above? WTF?!

frequencies and status of all amateur satellites, updated weekly

a technician class license will give you all the ham bands 50MHz and up.