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(Which) External antenna for Garmin GPSmap 64s?

Hi,

I'm new to the mapping scene and next week I am headed to France for some field work. A friend of mine is giving me his Garmin device (GPSmap 64s) and I am wondering how I can improve the accuracy of my measurements.

The setting will be a light forest, with a lot of open space and a few boulders ranging from 1-5m height. Here are my questions:

- Would you recommend buying an external antenna for the mapping?
- Will the increase in accuracy be likely worth the investment (if seen antennas as expensive as the GPS device itself)?
- If so, which one would you suggest (I am looking for a good price/value not high-end performance)?

Thank you for your help! I am really excited about this and hope to hear from you.
George

Comments

  • Tim 1457 Points
    What accuracy are you expecting (or needing) to achieve?
  • g_org 0 Points
    Well, that's hard to answer. I lack experience in this matter, but I hope for the accuracy this device is capable of in near perfect terrain (little to no reflections & plenty satellites in sight).
    I need it to be within 5m (less would be better... but really, consistency is more of a concern), but again, this is a test run, so it is perfectly fine for me to tinker with this.
  • Boyd 1749 Points
    I would be very surprised if an external antenna makes a difference. In the consumer world, antennas are typically only used when the device is inside a vehicle that has obstructions. The 60 series (GPSMap 60/62/64) has a quad helix antenna that many people feel is the best in the business already.

    Tim's point about expectations is right on target. Consumer devices like the 64 are not likely to give you any better than +/- 5 meters regardless of what you do. The very expensive antennas you have seen are probably intended for use with specialized survey grade gps units that cost thousands of dollars. In the consumer space, spending more money is not likely to give you more accuracy. A ~$100 device like the eTrex 10 is going to be just as accurate as the ~$700 Monterra. Spending more money just gets you more "bells and whistles".

    To get the best accuracy with your 64, use the "waypoint averaging" feature. If you stand stationary, the device takes a position fix every second and averages the results. This usually gives better accuracy, but there are a lot of variables such as the position of the satellites at the time you take the measurements.
  • g_org 0 Points
    Thank you, Boyd, for the comment!
    Would someone recommend an external antenna, just for the purpose of having the receiving 2m higher, than you usually hold your device (so it is less likely to be covered by myself or surrounding objects)?
  • sussamb 655 Points
    IMO that really would be a waste of time/expense. If you're outside use the waypoint averaging feature as explained by Boyd.
  • DaveM 104 Points
    I agree with Boyd and sussamb. The only way I thank an external antenna would help is if the GPS was inside something blocking the signal and the antenna was outside. If the signal is being blocked by heavy cover or the boulders one option to improve accuracy would be to turn on GPS + GLONASS. I have played with it and I don't see any difference in most cases but if I'm someplace where the signal is so blocked I'm getting a bad signal it will help. In most cases if a satellite is blocked by something say one of the boulders there is enough other satellites it doesn't matter. However turning GLONASS on won't cost you anything other then shorter battery life so why not. As far as the 2 m higher with the antenna I don't think it will help but if you think it would in the place you are at you could hold the GPS over your head when you mark the waypoint and get around 1/2 the extra height for free
  • Tim 1457 Points
    I'm going to disagree a little bit with what has been said. While the antennas (perhaps more accurately called receivers) inside a GPS are very sensitive, they are not very large. They don't have a very big net to capture the signals. While they might be sensitive to detecting weaker signals they don't necessary do a great job of capturing weak signals.

    GPS is essentially a radio broadcast your GPS receiver listens to, very similar to FM or AM radio. FM radio operates at 88 - 108 MHz while GPS operates up near 1100 - 1600 Mhz. Recall old radios with telescoping antennas and how much of a difference extending the antenna can make.

    An external antenna will help in a similar fashion. It isn't that the external antenna in a different location than the GPS (with exception for the scenario noted above where the GPS is blocked inside something like a car) but that the antenna is longer and larger and thus is able to cast a bigger net to catch more signals.

    The big, blocky antennas on older GPS devices helped cast a bigger net when the receiver chip wasn't as good at picking out weaker signals. As the chips became more sensitive the bulky antennas were no longer necessary and wen't out of style. But they are still useful in casting a bigger net.

    Don't get me wrong, I don't think it will make a huge difference, but an external antenna does provide more of an advantage of simply putting the receiver in a different location.
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