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GPS Problems

I've been running my KiwiSDR now for about a week, but it never has gotten a GPS fix. I assumed it was the antenna location and kept on repositioning the GPS antenna - even though any other GPS device gets a fix in no time on my desk, away from the window. While experimenting, I tried to add an extension coax between the KiwiSDR and the antenna, and when I unscrewed the SMA connector, all of a sudden, I saw more and stronger GPS signals on the GPS status page.

I just connected a different GPS antenna I had, and it seems to also show better signals, but I am still not getting a fix. Here is what I am seeing:

I have "Enable GPS" and "Always acquire" set to "Yes", "Include alerted" is set to "No" (but I've also tried "Yes"), "Kalman filter" is set to "Yes" (but I've also tried "No"). With the new antenna (not the one that came with the Kiwi), I see between two and eight satellites in the list. They show up, and it seems that when the SNR is greater than 15 or so, I get information in the gain, wdog, status (yellow "U") and RSSI columns. Sometimes I see a hold value. The wdog timer sees to count up to 30, and then the entry goes away. Sometimes the entry goes away before it hits 30. This has been going on for a while, and even though I see e.g. 8 tracked satellites for a second or two, I usually end up with two or three tracked satellites within a few seconds.

What am I doing wrong? And, is there a way to get a replacement antenna for the one that obviously works worse than an open SMA connector?

Thanks and 73,
Karl Heinz - K5KHK

Comments

  • where on your property to you have the puck located
  • It's in a window, facing West. It's close to where I have a GPS that I use for a GDO, which has excellent reception.
  • You could try a reflector behind it. I don't think I'd get much here facing west but there could be some other interference desensing it.
    I'd also carefully check the voltage at the SMA pins 3.3V.

    Is there any way you could actually put the puck outside (away from other kit) even if you have to remove the HF antenna?
    To me it would be good to test it where it should work before assuming there is some fault.
    k5khk
  • Short answer: The Kiwi GPS is not as good/sensitive as other commercial GPS devices you may have. You cannot place the antennas of both at an inside window and expect similar results. Why this is so is a complicated story. Before you decide your Kiwi GPS puck antenna is broken it really needs to be tested while outside with a clear, unobstructed view of the sky (and without any cable extensions). Or with a longer length antenna/amplifier/low-loss cable combination that delivers an appropriate signal level to the Kiwi GPS input.

    What you observed while unscrewing the SMA connector is likely just the elevation of the noise floor causing spurious sat acquisitions. The Kiwi GPS acquisition threshold is set very low to try and compensate for the low system gain at the expense of more false acquisitions. You are not receiving legitimate sat signals until the numbered "subframe" blocks start turning colors. Whenever the acquisition SNR exceeds 16 an attempt is made to track the signal (real or not). At that point you'll see gain, watchdog, status and RSSI numbers as you mentioned until the watchdog goes off after there have been no valid subframes detected and the channel goes back to being idle.

    As to why the Kiwi isn't as good as a commercial GPS I'll save that for another post.
    k5khk
  • Does the KiWI GPS antenna work OK with your other GPS receiver ?

    Is the KiWi GPS antenna placed on a metal surface ? This helps because the antenna was designed to sit on a motor vehicle roof which acts as a ground plane / reflector. It doesn't work as well if there is no metal surface below it.

    When mounted outside in the clear and on a metal surface the GPS antenna and KiWi receiver can work quite well.

    Regards,

    Martin - G8JNJ
    k5khk
  • I also have my GPS antenna inside, facing a south window. I found that placing it on a sheet of aluminum foil as a reflector helped quite a bit. Also the placement of the antenna is critical, moving it just an inch appears to cause a significant change in the number of satellites tracked. Unfortunately with three cats in the house, it doesn't tend to stay put. I may need to duct tape it down. :)

    Relocating it outside is a project for once winter is over, but for now it seems to work "good enough".
    k5khk
  • Thanks everybody. Lots of good information. I have a few tings to try.
  • The lid from a tin can as small as this makes a big difference in GPS reception - some channels even dial in -2 for gain at times now.
    KA7UWA2ZKD
  • The stock GPS antenna is on top of a second floor bookshelf on a small square of aluminum foil.

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