GPS in Space

[mlorrey]

Okay then, a GPS antenna up on the solar panel mast should allow clear line of sight to at least 1/3 of the GPS constellation at all times.

 

[erioladastra]

<br />First, the solar arrays are always moving so they are not good places for GPS antennae. The ants are mounted on the forward facing truss segement (forward defined as the positive X axis or the long axis of the ISS facing into the velocity vector). However, before we get the external thermal system fully assembled, we have to fly attitudes like XPOP where a fixed face always face the sun as you orbit. In this attitude, much of the station will be between the ants and the satellites for large chunks of the orbit. Even outside of that orbit, ISS structure will block part of the sky reducing the number of satellites. But this doesn't mean ISS gets lost - once it gets an attitude fix, it can propagate that fairly accurately for some time. If the solutions gets too bad or old, we can use the Russian segment data for information.

 

[CalliArcale]

I think he meant on the end of the P6 truss, not the ends of the arrays themselves. The P6 is currently pointing "up", mounted to the zenith of the Z1 truss segment. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[mlorrey]

I take that into account. Without blockages, the GPS should be in line of sight with slightly less than 1/2 of GPS sats (because they are 12,560 miles out, not in LEO). My understanding of the antennae location came from this:<br /><br />"The GPS receiver assembly has already been installed on the ISS, and the antennas are on the center section of a 300-foot long truss that will connect to massive solar panels; the section is designated Integrated Truss Structure S0."<br />http://www.gsfc.nasa.gov/gsfc/technology/issgps/issgps.htm<br /><br />The SIGI GPS unit actually uses FOUR antennas mounted to the truss in different locations, which allows SIGI to calculate the stations attitude via interferometry of signals to the different antennae, and limits blockage of station components.

 

[erioladastra]

Correct, but with the earth blocking some, you really only see ~ 1/4. You need 4 satellites for good attitude and state determination. I am not explaining it well so let me see if I can find a picture. When we are in XPOP (which goes away at 12A) ISS really struggles to get good vectors.

 

[mlorrey]

You're not communicating well because you are trying to prove an unsupportable point. My point is: <br /><br />a) the four GPS antenna are on the upper side of the P6 truss, with an unobstructed view of a 360 degree view laterally, with at least 180 degrees in each axis (N-S, E-W). This means you should be exposed to pretty much all GPS sats not blocked by Earth. ISS is between you and Earth. Since you are at ~325-350 miles altitude above the Earth, you see slightly over the horizon in all directions, and given all GPS sats are in 12,250 mile altitude orbits, your optimum view should have line of sight to slightly more than 1/2 of the entire GPS constellation at all times. Minus some blockages for various parts of the station, you should see slightly less than 1/2 of the constellation at all times, EXCEPT when the shuttle is docked with the station. Shuttle will block a significant area when docked, but at any other time the SIGI antennas should have no problems with line of sight to 1/3-1/2 of the constellation.<br /><br />Now, I understand that at one point, there were two antennas which crapped out. The status reports don't note that anymore so I assume they got repaired.<br /><br />ISS also has GLONASS receivers (the russian version of GPS), which can lock onto both GLONASS as well as GPS satellites. Now, why US nav systems don't try to take advantage of the GLONASS sats to improve accuracy is beyond me, perhaps just arrogance.

 

[mlorrey]

Look, 1/4 of the 24 satellites in the constellation is 6, six satellites. <br /><br />*I* can see six GPS satellites with my consumer GPS unit from my porch. In fact, sometimes I can see as many as 8 or 9. The claim that the ISS SIGI GPS unit can only see a piddling *6* GPS satellites at a time is patently absurd and ludicrous.

 

[erioladastra]

<br />Dude, I am trying to prove anything. I am just trying to explain to you that when ISS is in XPOP you don't get very good lock from the SIGIs which are on the S0 by the way. I am not trying to debate you. I am not trying to say it isn't good. I am just trying to say that the software and firmware is not a trivial thing and the program has spent several years refining it. It is a wonderful thing! Nonetheless, it has limitations. In XPOP, we sometimes have to take in a Russian vector (which is uplinked, not real time from GLONASS generally). If I have confused you I am sorry but I am not trying to refute you.