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  1. #1

    CanDoo GPS vs GPSMAP 76

    Hello,
    I've been using a Garmin GPSMAP76 that came with my 2006 GTX ltd mainly to mark waypoints. Now that the modding bug has bit...I'm wondering how accurate the GPSMAP76 is when it comes to speed. It's running the latest software, but it is an older unit compared to what I see others on the board using.

    I'm considering a CanDooGPS, and will probably get one anyway, but I may hold off for a month or two if the GPSMAP 76 is considered accurate.

    Any thoughts on the 76? Thanks.


  2. #2
    Click avatar for tech links/info K447's Avatar
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    That older Garmin unit (I used to own one) will have decent speed accuracy, BUT you may need to maintain your maximum speed for multiple seconds before the displayed speed reading will settle down. During hard acceleration the Garmin speed display will lag behind your actual speed at each instant in time.

    The Garmin needs a clear view of the sky to achieve best accuracy. You can see this on the Garmin screen that shows all the satellites your unit is currently able to track. The higher you mount the Garmin the easier it will be to receive and lock onto the satellite signals.

    This is true for all GPS receivers, the less obstructed the skyview (from the GPS unit) in all directions the more accurate the satellite tracking and speed display accuracy.

    Newer and better GPS units update the speed reading more rapidly, multiple times per second. The CanDooPro GPS speedometer module is quite accurate.

  3. #3
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    Quote Originally Posted by Mobius101 View Post
    Hello,
    I've been using a Garmin GPSMAP76 that came with my 2006 GTX ltd mainly to mark waypoints. Now that the modding bug has bit...I'm wondering how accurate the GPSMAP76 is when it comes to speed. It's running the latest software, but it is an older unit compared to what I see others on the board using.

    I'm considering a CanDooGPS, and will probably get one anyway, but I may hold off for a month or two if the GPSMAP 76 is considered accurate.

    Any thoughts on the 76? Thanks.



    my personal gps and it lives in my bugout bag. The charts I have for it are pretty useless around here, the shoreline changes every year. Used it on black lake in upstate ny and it was a gem.

    the Candoo will use a far more advanced chipset for its gps then the garmin could have put in there back when gas was a buck, and its a clean install from what I've heard.

  4. +1 by:


  5. #4
    Let me add a bit of information, perhaps more that you really want, LOL. The GPSMAP 76 uses the SirfSTAR III chipset. It is a bit older technology, but was the most accurate chipset available when it was new. The SirfSTAR III uses mainly a doppler shift technology to compute speed, but also uses what is called a Kalman filter , to quote Wikipedia " also known as linear quadratic estimation (LQE), is an algorithm that uses a series of measurements observed over time, containing statistical noise and other inaccuracies, and produces estimates of unknown variables that tend to be more precise than those based on a single measurement alone, by using Bayesian inference and estimating a joint probability distribution over the variables for each timeframe ". Let me explain what this means in plain language.

    There are two ways to compute speed. As we all know, a GPS unit reports back a position, a lat/long point on a map, and updates this regularly. One way to compute speed is compare the distance between two positions over a set amount of time. However, this is not very accurate, because the positions that the GPS fixes can be off by anywhere from 16 to over 50 feet. So modern GPS chipsets like the SirfSTAR III rely primarily on a method called doppler shift. The GPS satellites are always moving in the sky, and transmit a set frequency. If a satellite is moving towards you, the frequency you receive is higher, if it is moving away from you, it is lower. Once the chipset acquires a fix on a number of satellites (typically they will fix on many satellites, and fixes a position, it can use the frequency shift changes to compute the velocity it is moving. However, per my statement above about using Kalman filtering, the chipset uses a combination of both position fixes and doppler shift, mainly depending upon the strength of the satellite signals.

    Another factor, pointed out by K447 is the update rate. The Garmin unit gets its updates from the SirfSTAR III chipset once per second, then it does some other smoothing on the data, so it updates very slowly, maybe taking a few seconds for the unit to "catch up" to the true speed it captures within the chipset. (the SirfSTAR III actually updates the tracking of doppler 10 times per second internally, even though the Garmin unit gets this data once per second from the chipset. )

    The Candoopro GPS module uses a newer, better chipset called the SirfSTAR IV. It has a much improved Doppler shift algorithm internally, is faster to compute the Kalman filter variables, and we take the actual data from the chipset at 5 times per second and update the pulse stream to the cluster at this rate. However, keep in mind that the clusters on all skis have some smoothing in the rate it updates what it displays. There are some guys out there that are taking the pulse stream from our unit and feeding this directly into their logger to get a very fast translation of the rate of acceleration, at 5X per second.

    CSR (the company that makes the SirfSTAR chipsets ) has recently updated to SirfSTAR V. The main thing this new chipset does is also adds GLONASS capability alongside GPS for more precise positions. (GLONASS is the Russian "GPS" system...) It also processes data internally faster. However, I don't see us upgrading to this new chip as it doesn't provide any faster or better speed reporting, and is more expensive. We already have the fastest updating GPS module on the market, so why should anyone pay more for it?

    There are some chipsets (and modules) available in the market that output data at 10 times per second, which is faster than our 5 times per second. However, these use an inferior computing scheme for velocity, so even though they are outputting data "twice as fast", they are not accurately updating the speed at this rate, so we are actually outputting data faster when you think in terms of reporting true speed.


  6. #5
    All GPS units that I have played with are good at steady state speed. Big differences show up when accelerating.

    The stock Sea-Doo piece is one of the best I have but is calibrated to read about 1.5 mph high.

    The piece from Candoopro is good but there is about a 1.5 sec delay in it. All of them have some delay.

    Andy
    Last edited by K447; 03-31-2017 at 07:37 AM. Reason: calibrated

  7. #6
    JT jpt7779's Avatar
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    I have 3 Gpsmap76's and love them, they just keep on going and going........

  8. #7
    mrsprinkles's Avatar
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    Quote Originally Posted by jpt7779 View Post
    I have 3 Gpsmap76's and love them, they just keep on going and going........
    I bought the Navionics USA App from the app store and it has really detailed sonar maps you can download ahead of time. I got a ram mount case for my iphone and have it on my handlebars. Its secured in two spots, the ram mount, and a tether just in case. I have a garmin gps unit thats a couple of years old and the maps were no where near as detailed. The GPS speed might be a little weird when accelerating, but overall its great.

    EDIT: Added Pic
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  9. #8
    Thanks everyone, this is all great info.

  10. #9
    Quote Originally Posted by whittlebeast View Post

    The piece from Candoopro is good but there is about a 1.5 sec delay in it. All of them have some delay.

    Andy
    Not true. The SirfSTAR IV chipset we use tracks at 100 ms, the Kalman filter outputs every 200 ms which is then output to our processer at the same rate, which is 2 tenths of a second. We adjust the pulse width every 2 tenths of a second. The maximum delay of our implementation is therefore 4 tenths of a second. If you are seeing a 1.5 second delay, it is because whatever logging device you are using to measure our frequency has some smoothing in it, probably a 1 HZ (1 second) smoothing which accounts for your 1.5 second delay comment.

  11. #10
    Quote Originally Posted by jpt7779 View Post
    I have 3 Gpsmap76's and love them, they just keep on going and going........
    Do you use your Gpsmap76 to monitor your top speeds? or do you use something else as your primary speedometer?

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