Does GPS take longer to calculate speed more accurately?

Discussion in 'GPS Technical Discussion' started by David619, May 26, 2017.

  1. David619

    David619

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    I am using a battery powered GPS tracker which has the GPS sleeping most of the time.
    The unit wakes up the GPS, gets a position, speed & heading, transmits that information via satellite then goes back to sleep.
    Speed resolution is only 4 knots and heading resolution is only 45 degrees which isn't really sufficient for our needs.
    The manufacturer says that the reason for the lack of resolution is that they only wake up the GPS for a short time in order to gain a GPS fix, which is primarily to determine location and then the unit goes back to sleep.

    This is the first I have heard of this. Would a GPS have to wake up for a longer period in order to get a more accurate speed & heading?

    My suspicion is that the reason for the lack of resolution is more to do with the extra transmission cost - a few more bits need to be sent and also a bit more battery power to transmit those extra bits.

    Interested to know whether I have missed this and a first fix would not be able to have an accurate speed/heading. Seems to me if they can have 4 knots of resolution they must know the speed to better than 4 knots of accuracy. If it isn't accurate then how are they to know that it isn't actually moving at 50 knots?
     
    David619, May 26, 2017
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  2. David619

    Nuvi-Nebie Moderator

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    I would say the answer to this question is yes, you only have to look at the accuracy figures of any GPS to see that they get better with time after the first fix. The cost of 'air time' / battery time to send extra bits really would be negligible
     
    Nuvi-Nebie, May 26, 2017
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  3. David619

    David619

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    Ok, thanks for the information. I will have to research more. I did have a pretty good look for info on speed vs time after turn-on for a fixed GPS, or even position vs time after turn-on but couldn't find any graphs or other info to prove it one way or the other.

    For the transmissions, these devices typically transmit position, speed & heading in about 8 bytes - 64 bits. So if they had to transmit 2 more bits I guess we would be looking at 3% increase in time to send and 3% extra battery cost for transmissions. These devices are advertising battery lives up to 9 years so transmitting the extra bits could waste up to 3 months of battery life over 9 years.

    However, having to wait for a 2nd or even a 3rd fix to get better speed/heading accuracy would drain the battery a lot more than that. Pity these units don't have the ability to adjust TTFF so we could adjust to balance battery vs accuracy!
     
    David619, May 29, 2017
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  4. David619

    Nuvi-Nebie Moderator

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    Once the GPS has gathered useable data from a group of satellites it can calculate just it's position, it then needs to gather a new set of data and calculate a second position, speed and heading are then derived from the movement from one position to another, no single set of satellite data can provide your speed or heading, the problem is that the two positions that the GPS is working with are inaccurate, as time goes on each 'position' will be more accurate due to better / more satellite data.

    As far as the transmission of extra bits is concerned, I don't think you can assume that the time taken to transmit 64 (or 66) bit is the total 'on time', it may well be that the overhead time to establist the communications link is at least as long - if not longer than the time it takes to send the data
     
    Nuvi-Nebie, May 29, 2017
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  5. David619

    David619

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    Possibly the 'first fix' is actually the 2nd internal fix as the unit already puts out speed & heading on the 'first fix' and both parameters are always within 2 knots & 22.5 degrees of the actual vessel values. Makes me wonder how close it would be if we had better resolution.
    I think most GPS these days have doppler calculations to get instantaneous velocity but I have no idea how they would calculate heading (cmg) without at least 2 updates.

    There would definitely be other overheads in transmission time & battery life that I didn't take into account. But it seems these trackers are favouring battery life over accuracy.
     
    David619, May 31, 2017
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  6. David619

    Nuvi-Nebie Moderator

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    I would say that is the case, probably more suited to locating a stationary stolen vehicle in a nearby town rather than how fast it was driven there :)
     
    Nuvi-Nebie, Jun 1, 2017
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