GPS Prerequisites

Hi all:

I have run my Kiwi for quite a while without a GPS antenna, so I'm new to the party so to speak. I purchased a generic automotive "puck" style antenna with an SMA connector, with ~28-30 dB of gain.

After several hours of operations and a couple of reboots/restarts, I see that it never appears to "lock" to any of the GPS satellites, despite typically showing 3-4 in view and as many as 8 at once, but apparently at signal levels too low to decode.

After trolling through the archives, I gather that the GPS receiver is less than ideal and needs good conditions to work. I never see a SNR value more than about 20 dB. My antenna is on a metal plate near an east-facing window.

Are there any diagnostics to determine whether the system is working correctly, or do I need a 40 dB gain antenna or a northerly exposure?

Thanks es 73,

Larry NM7A

Comments

  • @jks would be the one to comment on diagnostics but do be aware that there is significant radiation from the SMA connectors/end of a KiwiSDR that can pretty easily QRM the rx.

    If you haven't already, move the antenna at least 15' away from the SDR and see if anything changes.

    Glenn n6gn

  • jksjks
    edited February 24

    If the RSSI graph entries are red there really isn't any GPS signal. The acquisition process is just trying to lock to the noise floor.

    Temporarily extend your power and Ethernet so the entire Kiwi can be placed closer to an outside door or window such that the GPS antenna can be placed outside in the clear with a reasonable view of the sky. Then you should get some sats to lock. Don't try and extend the antenna cable of those puck antennas. Especially not with more RG-174. http://kiwisdr.com/quickstart/index.html#id-faq-gps-sens

    If things are working then you can try bringing the antenna closer to an inside window and see what happens. A north-facing window is actually worse because of the (so-called) polar hole effect.

  • There is the car repeater trick too.

    Cheap car repeater with its GPS antenna exposed to as much open sky as possible, as jks says better towards the equator than poles. That can give an extra 3/5M then stick the Kiwi GPS antenna and the repeater TX puck in a metal container.

    You would need a 5V source to power the repeater but that is USB and probably will allow a bit voltage of drop.

  • At the urging of Tom, WA2TP, I created a GPSDistribution PCB which isolates both the center and shield of the coaxes of up to 8 Kiwi's from one another at all but GPS frequencies:

    It has an additional,narrower? GPS filter and 8-10 dB gain to each output at 1575 MHz along with a lot of isolation at HF on both differential and common modes.

    add 30 dB to S21 to account for attenuation that was not included in the calibration.

    This will likely give some additional immunity to both self-QRM and insufficient levels.

    Rob has reported that it is providing better performance at KPH with the bank of KiwiSDRs there.

    Write me if you want a .brd design and Material list (on the order of US$25 with PCB, I think, maybe a little more) if you want to have one fabricated and assemble it yourself. It does have very small pitch SMD parts so consider this first.

    Glenn n6gn

  • I have lots of tall trees in my yard, great for hanging HF antennas, not so great for good GPS reception. I could never get decent GPS signals with the included hockey puck antenna, whether it was inside the house, or outside.

    I ended up getting a NOS high gain active GPS antenna off eBay, mounted it on the deck up in the air. Plenty of signal, even with a pretty crude 8 port home brew splitter that feeds the KiwiSDRs and a Thunderbolt 10 MHz reference.

  • This leads to a related question: If the "acquire" checkbox is disabled for Navstar, QZSS, and Galileo, does that effectively disable the GPS subsystem?

  • edited February 24

    Larry asks..... "or a northerly exposure?"

    those N of the equator should never point a puck north..... there's the potential for poor coverage if you point towards the pole.


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