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If GPS works on timing why doesn't having a cable ruin it?

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  1. Nov 4, 2015 #1
    This just out of curiosity because the system works either way, I just wonder why.

    I have a high precision GPS system, there are two antennas placed about 2 meters apart with cables that are about 8 meters long going back to an embedded computer. Besides giving you the position, it uses the relative positions to give you heading.

    Now as I understand it, GPS uses the time of flight of the satellite signals to to trilaterate your position. But wouldn't the time difference between the two antennas be thrown off by the time it takes the signal to travel through that 8 m cable? Does it use the timestamp in the signals to somehow compare them without using it's own reference time? Because presumably they hit the antenna with the correct timing. Does the extra time added by the cable simply cancel out, or is it a more active process than that?

    Just wondering how this works.

    Thanks
     
  2. jcsd
  3. Nov 4, 2015 #2

    Filip Larsen

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    Welcome to PF!

    The short answer is, that a GPS will calculate the position of it antenna, that is, the position is calculated based on the relative timing between signals from different satellites so the "common" time they spend from the antenna to the tuner/decoder cancels out. You could in theory record the signal from an antenna and then play it back to a GPS decoder later and get the position and time of the antenna at the time the signal was recorded.

    PS: My answer above is based on a one-antenna GPS. I would guess that your two-antenna GPS works by just calculating the position of each antenna more or less independently of each other (that is, the antenna signals are not mixed, but decoded separately), but there may be other effects at play that I am not aware of.
     
    Last edited: Nov 4, 2015
  4. Nov 9, 2015 #3

    davenn

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    hi there Paul

    curious to know what sort of GPS receiver gear you are using ?
    Are both of these receivers in a fixed non-moving location ?

    Just asking as I am a Trimble GPS tech servicing GPS gear for heavy industry and surveying

    Dave
     
  5. Nov 18, 2015 #4
    Paul, a couple things need to be addressed. Most of us learned to radio navigate from an older model based around Loran or Loran C. In this process radio transmitters sent out an identifiable individual signal. Due to changes in the signal dependent on distance it was possible to get a reasonably accurate distance from each transmitter. Each distance gives you an arc of position from a known point. If two or more signals can be read and interpreted a point of position can be inferred.

    With GPS each satellite checks it's position relative to earth and each other. The transmissions are coded data packets that say which satellite is transmitting, where it thinks it is, and what time it transmitted. The cable can make a difference, however, with the tremendous distance from satellite to antenna ( tens of thousands of miles) a couple extra feet makes little difference.

    Add to that now we are working in three dimensions. In the old land based systems it was assumed that one is on the surface and you have a map. Two arcs of position would give you a point on the geoid. In GPS 3 is a minimum number and will not give you high accuracy. You need several satellites to each contribute a sphere of position. The intersection of the spheres is where you are assumed to be. It often does not match perfectly and so the device can find a small pyramid of position given four satellites and the assumption is you are at center. With each added satellite the location is refined.

    Since the satellites are distributed around the earth your cable will shift you out of position slightly (metal signal conductance varies from air) but it adds directly on one side and indirectly on the opposite. By the time you normalize all feeds (assuming you can see enough satellites) the difference is miniscule.

    Atmospheric anomalies have a more pronounced affect and can sometimes shift you significantly out of place. These are sometimes corrected for when your device initially connects and downloads the almanac as it provides clues to the device to self recognize when a signal is aberrant.
     
  6. Nov 18, 2015 #5

    Filip Larsen

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    As mentioned earlier in this thread it is possible to say precisely what difference the cable makes. The receiver will calculate the position and time of the antenna at the time the signals from the satellites was received there. For the cable lengths the OP mentions this means the receiver will show a position that is potentially up to 9 meters off (8 m + 1 m, assuming the cables are attached to the middle of the antenna assembly) and its time will always lag around 30 ns compared to the time a similar receiver would calculate using an internal antenna when placed at the location of the first receiver.
     
  7. Nov 18, 2015 #6
    You are correct Filip in that the time delay is measurable and predictable. The additional time will shift the sphere of position to be slightly larger. Due to the geometry of multiple satellite positions this will make the overlapping area somewhat larger with a bias towards being lower in altitude. The device still assumes the actual position to be centered in whatever that relative area is. This will give in a worst case an error of position of approximately 4 meters. I do not know the particulars of this device but it sounds as if the signals are then averaged. Thus I am assuming the error to be closer to 2 meters.

    PaulMakesThings, I also do a fair amount of field work using a very high sensitivity GPS. I see minor errors in my work as well. The solution that I found to be the best is to find a known surveyed point such as a property corner or section marker. I then log that point for reference and go about log gin my other data. Yes what I do moves around bit but this also gives me a correction factor for the date, and atmospheric conditions. I can shift my data points if needed by comparing them to the known fixed point and where it was logged as. I am assuming you could use a similar test to experimentally determine the actual error in your setup.
     
  8. Nov 19, 2015 #7

    Filip Larsen

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    I am not really sure what you are trying to say, but it seems to me that you are still thinking that the length of the cable somehow change the calculation of position and time somehow. It doesn't. The cable means that the receiver will decode the exact same information from the GPS signals no matter how long the cable is. The calculation of position and time depends on the relative timing between signals from different satellites and this relative timing is not changing when the signals follow the same pathway (like a cable).

    You could in theory have a cable going half-way around the world and a identical receiver at the far end would still show same position as the receiver local to the antenna, albeit now the far receiver time would be around 70 ms behind the local receiver (assuming that the receivers start from same state and get all their information from the GPS signal alone).
     
  9. Nov 19, 2015 #8

    Averagesupernova

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    PaulMakesThings, I would be curious to know what 'high precision' means to you. Are you talking sub-inch accuracy? Ultimately it is ALWAYS about the position of the antenna(s).
     
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