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

Rename it?

[sessamoid]

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

Re:Rename it?

[kd5biv]

It should be very interesting to see what applications come of this.

Basically anything that requires live updating of on-orbit position data, which is a *lot* .. navigating in space just got a whole lot easier, for manned and unmanned travel alike.

Bear in mind, too, that even if you're going someplace where you *can't* get valid SGPS data, you still have to travel through a large region of space where you still *can* get valid data, which means your picture of your lunar transit or Hohmann transfer orbit is going to be that much more accurate. I'm still salivating over what this means for commercial lunar-earth orbit transits, now that getting into an accurate lunar transit doesn't take radar tracking and heavy CPU on the ground at JSC.

Now all we have to do is put a lunar orbit equivalent in place -- maybe with some telecom capability added in, call it something like Lunar Positioning and Communication System (LPCS) -- and you've got most of what it would take to get to the moon and back on a regular basis .. OK, yeah, yeah, except for fuel, but that's a logistics problem .. ;-)

Nice....

[forsaken33]

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.

Re:Nice....

[rice_burners_suck]

"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."

Several months ago, I listened to a radio show where they talked about space debris from all sorts of previous missions. As it turns out, they currently track objects that are quite large, as well as objects the size of a soda can. IIRC that's the smallest they can track. All space debris, even smaller than what they can track, poses a serious hazard to satellites and more so to larger vehicles. Imagine something the size of a golf ball hitting the space shuttle at the speed of a bullet and you've got an idea of the danger. Chances are, the shuttle is designed to get hit by all sorts of stuff, but it can still cause serious damage.

Destroying the space trash isn't the answer either, as it would simply smash larger pieces into smaller pieces, and that's an even bigger mess.

For some reason, I don't think a GPS receiver can be placed on all but the largest pieces of trash. It's like mice who wanted to place a bell around the cat's neck for advance warning. They all cheered about the great idea until an old, wise mouse came along and asked, "And who will put the bell on the cat?" Besides, to attach a receiver to a piece of space debris, you'd have to intercept the debris with a vehicle. If you're going to go through all that trouble, you may as well send a garbage truck^H^H^H^H^Hshuttle and collect all the pieces.

AAAAARGHHH

[dingo]

OH NO!!!!!
now what am I going to do?
now i am not safe anywhere from the government tracking me with GPS
They must have grown suspicous during my occasional trips to the mother ship
now we will be found out
*sigh*
i knew the anal probing days had to come to an end

Re:umm...

[4444444]

the geosync gsp satelites

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

Debian played a part

[Bruce Perens]

The developer of the GPS experiment is Bdale Garbee, a long-time Debian developer, who is presently working on the Debian IA64 port at HP's Linux lab. Bdale uses Debian to host development, I'm not sure if he uses it to run ground-station software but it's likely.

Bruce

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

[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.

The future of Geo Caching

[Quizme2000]

My Cache is located at N 37 50.047 W 122 13.809 A 56,000 km. It contains a compass, a dollar bill and a 5kg nuclear core. Please sign the log.

Re:Why _wouldn't_ if have been possible?

[crisco]

I don't think it is so much the directional antenna as it is the fact that the reciever is orbit and therefore moving very fast in relation to the GPS satellites. Your Garmin handheld just isn't designed to recieve (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.

Another consideration is the doppler effects from the movement, the GPS reciever might need a specially designed front end to account for the fun variations in timing signals.

GPS technology has some serious math and crypto 'hacks' going on. They've taken a technology designed in the 70's for military use and several meters accuracy and have gotten cheap consumer handheld recievers near that and they have enabled land surveyors to achieve sub centimeter accuracy with two recievers (enabling some unheard of accuracies in measuring the earth and making large scale engineering projects much easier to finish). I doubt that satellite tracking was in the original design documents either.

Wild idea: How to deal with space debris.

[Bruce Perens]

I wonder if space debris could be deorbited by using laser light as a sort of retro-rocket? Light exerts pressure, and although this is a very small amount of pressure, it will accumulate if you keep pushing. So, put up a satellite with lasers, not powerful enough to melt debris but powerful enough to give it a little push. Push on the debris with the laser light from ahead of its orbit. The satellite gains some orbital energy, the debris loses some. Eventually, the debris deorbits.

Bruce

Re:Wild idea: How to deal with space debris.

[MarkusQ]

Bruce --

Great idea. One quibble though:

Push on the debris with the laser light from ahead of its orbit. The satellite gains some orbital energy, the debris loses some. Eventually, the debris deorbits.

If the goal is to deorbit the debris, we can take the fact that it loses energy as given. However, depending on the relative orbits, it isn't clear that the laser platform will gain energy. If they are appoching each other head on, for example, they both lose energy. In most cases, I suspect the effect on the platform would be a mild course correction with little change in total energy. One strategy might be:

Rule 1) Don't shoot at targets that would reduce our orbital energy (eliminating 1/2 the potential targets).

Rule 2) Only shoot at half the remaining targets, chosen so that we consistantly precess our orbit, bringing previously ignored targets onto our active list.

Another thought is that we get the delta-V even if we don't hit the target, so we can always shoot at nothing (after confirming that there is in fact nothing there) to adjust our course.

-- MarkusQ

Great

[Ezubaric]

Now maybe NASA can tell whether their probes are 100 meters or 100 feet away from smacking into Mars.

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.

;-)

Wow!

[tcc]

Better start sending positionning signals into space TODAY, because when we'll reach warp drive capabilities, we'll travel faster than the radio signal, and I don't want any excuses for those people to be Lost and waiting for the signal to catch up on them.

radiation-hardened devices

[randal_hicks]

"...Testing this experiment has been a long time in coming, and we were worried that radiation may have damaged the GPS receivers."AMSAT President Robin Haighton, VE3FRH

Off-the-shelf components promise to decrease the cost of missions, but what risks are we taking by doing so?

Few companies use Gallium Arsenide (GaAs) as a substrate in the manufacture of their semiconductors due to its cost. It is brittle, and does not possess a native oxide (used as an insulating layer). Consequently most companies use good ol' Silicon (Si). One of the benefits of devices built on GaAs, is their capability to function normally in a high-radiation environment. They are said to be radiation-hardened.

Should a device need to be radiation-hardened, it would most likely need to be manufactured in another company's fab, using/developing a new process.

Even if the software was written today, and a satellite was ready to be deployed tomorrow, it'd be a long time before we had an autonomous and station-keeping satellite...simply due to the time needed to build a radiation-hardened device. It'd just be a matter of time (and statistics) before a Si-based device malfunctioned and took out several other satellites along with it.

Existing satellites will likely be fine, but as they are EOL'd I would expect their replacements to have this technology. In the interim, maybe satellites can be built with the capability for an upgrade module that NASA can intall for them at a later date? --providing another market and decrease TCO for companies/governmments that require satellites.

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...

Some info on the first satellite GPS tests...

[morcheeba]

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...

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.