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Possible problem with GPS on one of my KiWi's

edited January 2020 in Problems Now Fixed
I moved one of my KiWi's to a new site and initially everything was working OK.

http://kiwi.farnham-sdr.com:8073/

However I now find that GPS reception has become erratic.

The antenna seems to be OK (edit maybe not after all, as the RSSI values are only around a few hundred) and I get good results some of the time with multiple satellites, but then they suddenly drop and only one or two satellites are listed.

The Az/El plot doesn't show any stats and the position map doesn't update.

Is there a command I can use to log the GPS stats in order to try and figure out what's going on ?

Regards,

Martin - G8JNJ

Comments

  • I have found GPS quite intolerant of reflections, move the antenna a little?
    Coax cover (at least the small puck antennas) - one small nick and the "copper" disappears into white dust very efficiently, the only outward sign being a less flexible coax (which can be tested for by running a small curve along it's length).

    Here I have a £13 car GPS repeater with the RX antenna on a ~100mm dia disk at gutter level with the 5m of tiny coax through the loft and down the wall to the active repeater puck in a coffee can screwed to the wall (the lower limit of the coax). I throw as many other RX antennas as I need in there.
    With that I have to make sure the re-radiater is not too close to the end of the can or everything gets flaky.

    Probably completely unrelated but hey no other replies yet.

    Regards
    Stu
  • edited January 2020
    many GPS receivers do not do well with multipath, but that does not extend to all. Repeaters can create multipath.
  • I second the multipath suggestion, when I had the hockey puck antenna in the shack, I found moving it by an inch produced substantial differences in GPS performance. With three cats, this was a problem.

    I finally got a "Lucent GPS 40dB High Gain Timing Antenna" from ebay for $46 and mounted it outside and never looked back. 11 or 12 satellites all the time now.
    WA2ZKD
  • Hi,

    New Kiwisdr user here.

    I am not doing too well on the GPS front. Firstly I don't know what all the terms /symbols on the GPS page of the admin web page mean, and secondly, I live in a cottage with two foot thick, solid stone walls. The antenna puck is on the window sill and I know not to expect miracles.

    My main concern is how much of a GPS signal the sdr needs to correct time and frequency calibration. I suspect I don't have lock on any sats. The image below represents my best situation.

    What do the status letters U, P & S mean. Sometimes the U turns amber and that's about it.

    I know to mess about with the puck position, and I have tried sitting it on a 6 inch square sheet of alu foil. That has worked for me with my QRP Labs U3 gps rx, increasing the signal strength significantly and enabling lock. It didn't do much on this set up.

    This doesn't seem to be a big issue - the Kiwi is working really well with excellent performance in comparison to my Funcube Pro Plus dongle. Interesting to get down to vlf..... :)

  • The qrp-labs GPS receiver module has really an exceptional performance. It even gets a lock behind 90 cm solid stone walls at my QTH. It's a miracle to me, how this is possible.

    Your screen indicates that the GNSS signal is heavily attenuated. With an unobstructed view, galileo satellites (PRN Exx) have a RSSI of 2000+.
    What happens if you stick the puck out of the window?
  • which side of your house is the window on?
  • Thanks for the info.

    My situation is going to make the GPS tricky. As well as having thick stone walls, the house is in a place with a cliff rising immediately behind, a small river in front and rising ground in front. My phone gets a 3d fix in three minutes, so it isn't impossible, but I am thinking the site imposes difficult conditions . The window faces south but the sky horizon is about thirty degrees from horizontal and about sixty at the back.

    Thanks for the link John.That is exactly what I needed. I did search but didn't find that.

    Cheers.
  • try mounting the puck on a vertical metal plate and point it south.
  • Thanks Jim.
  • Another option.
    Get a car GPS repeater, put that RX antenna as high and clear as possible.
    Angle it as Jim suggests but either way get the first RX antenna a clear view into as much "sky" as possible, then put the Kiwi puck facing the repeater puck in a metal mesh or enclosure.

    (I'm at work so don't have time to check what I bought but "Car GPS Signal Amplifier Aerial Antenna Auto Navigation Receiver ANT-1573 " looks like it)

    Once you separate the RX location from the Kiwi there are more ways to increase the signal.
  • Thanks, I can look at the idea of a gps repeater, but having messed about a bit with the calibration based on the 10mhz wwv signal, once corrected and running 24/7 it seems quite stable moving perhaps 2 hz in 24 hours. I'm now thinking, if the clock doesn't wander about wildly, and the kiwi is in a stable temperature environment, it isn't a big issue to calibrate it once in a while.

    I am going to monitor the stability over a few days and think about whether I need to do anything more.

    Thanks for the input.
  • >
    >I moved one of my KiWi's to a new site and initially everything was working OK.
    >http://kiwi.farnham-sdr.com:8073/
    >However I now find that GPS reception has become erratic.
    >The antenna seems to be OK (edit maybe not after all, as the RSSI values are only around a few hundred) and I get good results some of the time with multiple satellites, but then they suddenly drop and only one or >two satellites are listed.
    >The Az/El plot doesn't show any stats and the position map doesn't update.
    >Is there a command I can use to log the GPS stats in order to try and figure out what's going on ?
    >

    A couple of my friends were able to vist the site today and swapped the GPS antenna over to a GPS-TMG-26N for me.

    The antenna feeds an external GPS splitter which provides power to the antenna.

    The new antenna gives good results on another GPS receiver and the cable feeding the KiWi GPS port tested good, but the KiWi still only has RSSI values in the region of 200 which is not good.

    So it looks like the KiWi GPS receiver has developed a fault.

    Are there any commands that can be used to remotely check the GPS perfromance ?

    If not, I guess I'll have to swap the KiWi out at some time in the future when I've sorted out a repalcement and I'm able to visit the site.

    Regards,

    Martin - G8JNJ
  • Ha,

    Ignore my last posting.

    It just took a bit longer than expected to sort itself out after having lost lock, and it's now working again.

    Pressing the 'Post comment' button must have fixed it :-)

    Regards,

    Martin - G8JNJ
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