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: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.

Hmmm...

[hendridm]

Attach GPS transmitters to space debris? Once you find the debris, why not clean it up :)

If they know where the debris is to tag it, how come they don't know where it is before it hits something?

Also, although I am no expert, this sounds as feasible as keeping inventory of a wheat field.

It seems more feasible to advance our radar technology and evolve computers to track and keep tabs on potential collisions with unknown objects...

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

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 .. ;-)

Re:How useful is this, really?

[Anonymous Coward]

I don't know much about how GPS works, but I have a basic idea. The system determines the time it takes to bounce signals to a few different satellites and then triangulates where you are based on the determined distance from each satellite.

That is not quite right. The don't bounce the signal to a few different satellites, they send the signal out to all the objects. The objects then take the signals from each satellite and use the varying differences in time (and knowledge of the satellites position) to calculate its position.

That works great when the distance between you and one of the satellites is the same or somewhat less than the distance between the satellites themselves. But out in space, even if they only go a small fraction of the way to the moon, the distance to ANY GPS satellite is going to be so much further that the difference in "ping" time from one satelitte to the next is going to be minimal.... which means your ability to triangulate where you are gets worse and worse. In fact, I don't think this would be reliable except for orbiting satellites. And I am sure orbiting satellites have been doing the same thing via ground stations (non-GPS signals) for years. So how useful is this? Unless we plan on setting up hundreds or thousands of GPS sattellites on the way to Mars (or to the moon for that matter)... but there's already this cool triangulation thing you can do with a few big natural satelittes... the sun, earth, and nearby planets. Last I checked that's totally free.

You are right the quality of the position fix degrades as you get further from the earth. Why is it useful? It is useful because most satellites orbit the earth and it works there. They have been doing similar things on the ground for many years, but if the satellite can take care of altitude and attitude adjustments itself, it can save a lot of money. The current ground station implementation is very expensive (a couple million?) and theoretically the cost of a space GPS receiver could get down to tens of thousands of dollars.

Most deep space missions will continue to use celestial bodies as their main navigation aids. But most space missions are not out of the Earth's orbit

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.