GPS in Space

[spacefire]

Would a GPS receiver work in LEO?<br />I know the satellites that make up the GPS constellation are also in LEO (right?) but the GPS does triangulation to determine lat long and alt so would it work from the same "level" as the signal generating satellites? <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>

 

[mlorrey]

GPS works in space. The GPS satellites are NOT in LEO, they orbit at 10,000 mile altitudes. So long as GPS sat antennae are omnidirectional, you should be able to accurately navigate out to GEO with similar accuracy as on the Earth's surface. Accuracy will decrease above GEO with altitude, however this can be remedied by orbiting more satellites at GEO and/or around Lagrange points.

 

[qso1]

GPS sats are orbiting at an altitude of 12,500 miles or something like that which is not considered LEO, LEO being up to about 500 miles. At current GPS altitudes, single elements of the system have a much broader coverage of the Earth than they would in LEO. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[tap_sa]

With multiple antennas you get the spacecraft orientation too.

 

[spacefire]

cool thanks for the answers. I guess the GPS satellites are somewhere between LEO and GEO.<br />So LEO would still be way below and comparable to being on the Earth's surface relative to the altitude of the satellites.<br />Any object in LEO would move much faster than a GPS satellite...would that create Doppler shift problems? <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>

 

[mikejz]

It would be worth adding that almost all conventional civilian GPS receives are locked from working at beyond a specific altitude and speed. <br /><br />Oscar-40 carried an experiment to determine the usefulness of GPS above the orbit of the GPS satellites; however, the satellite failed.

 

[henryhallam]

<font color="yellow"><br />It would be worth adding that almost all conventional civilian GPS receives are locked from working at beyond a specific altitude and speed.</font><br /><br />Yes, I ran into this while working on a tracking system for a high altitude balloon project (~80,000ft). The GPS module we have is limited to <1000kt and <60000ft. Fortunately you are allowed to exceed one of these two limits as long as you don't exceed the other one at the same time.<br />Obviously this is to stop people using them in ICBMs... I did think it was kinda cool to be working on a project where I had to worry about that!<br />

 

[propforce]

Yes the GPS are technically in MEO and, as Shuttle_Guy have verified, you can use it as guidance in LEO.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[mlorrey]

Yes, though this limitation is in the programming of your GPS unit, not the satellites, and is only limited to GPS units produced in the US. You can buy GPS units produced overseas that do not have this limitation. The limitation is asinine, as stupid as the old limitation on Americans using or exporting "weapons grade" encryption when such was commonly available overseas. The law only limits Americans, not anybody else. One more example of the absurdity of government policy.

 

[mlorrey]

BTW: the easy sleazy way around the programming limits is to have two different units: one that only measures altitude, the other only measures velocity...

 

[llivinglarge]

I love the fact that Endeavour gets all the cool gadgets... Being the youngest does have its benefits.

 

[josh_simonson]

I have trouble getting a gps lock in my car, let alone a spacecraft. You'd have to hold it up to the window to get it to function. Space windows might be too thick or block radiation to prevent a handheld unit from working. <br /><br />That'd be another thing to do on a suborbital flight. Mark a waypoint 100 km up. <img src="/images/icons/wink.gif" />

 

[mlorrey]

Handheld consumer GPS units are not made to handle operations in a car. The atmosphere, as well as passing trees and buildings, interfere with line-of-sight reception. In orbit, there are no such problems, and signals are strong.

 

[drwayne]

My handheld is surprising, in that, once it has acheived lock, it maintains most all satellite tracks in the car. I was quite surprised.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[mikejz]

Ditto:<br /><br /><br />I actually just picked a cheap USB GPS unit on ebay for $40 that plugs into my laptop. surprising I never have it lose the signal.

 

[henryhallam]

<font color="yellow">I have trouble getting a gps lock in my car, let alone a spacecraft. You'd have to hold it up to the window to get it to function.</font><br /><br />Try getting an external antenna with an active amplifier (the power is supplied by the handheld unit so it doesn't need extra batteries). $10 to $20 on eBay or various online shops. I bought one of these the other day and it works wonders in the car and in the aircraft. I just put the antenna on top of the dashboard and the signal went from 3 or 4 satellites to a steady lock on 9 satellites.

 

[scottb50]

It seems like the obvious advantages of GPS would make the conversion a no-brainer. The engineering costs are already spent and it's mostly a matter of stringing the wires and paperwork. <br /><br />I bet Travolta has GPS on his G-2 and it obviously didn't have it when they were built. <div class="Discussion_UserSignature"> </div>

 

[siarad]

Some glass (Sundim absorbing UV light) screen signals, an aerial came with my GPS for putting on the roof.

 

[erioladastra]

<br />ISS has GPS and uses it as its primary source for attitude and state determination.

 

[erioladastra]

<br />Generally ISS can see 4 satellites. However, you can also get blockage from vehicle structure. In addition, the software has been very tricky. High accuracy is doable but software and firmware on the antenna make it tricky.

 

[mikejz]

Found this concerning GPS on the shuttle<br /><br />http://klabs.org/DEI/References/avionics/papers/shuttle_gps_upgrade_062904.pdf

 

[mlorrey]

I have, at some times, locked on to as many as six satellites with my PDA GPS unit (a Magellan unit for my Handspring PDA). ISS should be able to lock onto at least 1/3 of the satellites out there.

 

[erioladastra]

"ISS should be able to lock onto at least 1/3 of the satellites out there."<br /><br />Depending on attitude.

 

[mlorrey]

Does ISS just drift in attitude, or is it tidally oriented with solar panels upward?

 

[nacnud]

The ISS is supposed to fly with the long pressureised axis pointing in the direction of motion and paralle to the ground, this minimised the effect of the gravity gradient on the experiments. If left to fly free this pressurised axis would end up in a gravity stabalised vertical position which whild being more stabe isn't a good for experiments.