A tip for the KiwiSDR GPS antenna installation.

cathalferriscathalferris
in Installation, Antennas and Interference
One item that I found to be really useful, that I have not seen in the installation or use documentation, is to ensure that the GPS puck is on a decent ground plane.

The only area where I can put my GPS puck antenna is on a ground floor windowsill between my house and the house next door. There is ~4 meters to the opposite wall, and 9 meters of wall height on each wall. This means that I do not have good visibility to the sky, and I had issues with GPS reception. Often I would only have one or two satellites in view. I decided to go and do something about that, and try a few things before investing in another GPS antenna or having to make an extension coax cable. I located a spare steel biscuit tin left over from last Christmas, and I stuck the GPS antenna via the inbuilt magnet onto the top of the tin lid, and taped the lid to the windowsill.

This addition of this ground plane was enough to give me enough SNR to consistently see and track three to six satellites, and ensuring a consistent clock and location, even with my limited sky visibility.

It might be worth adding to the installation webpage to try putting the GPS antenna on top of a metal plate, if GPS signals are proving hard to receive.

Comments

  • WA2ZKDWA2ZKD
    see section 5.1 https://www.u-blox.com/sites/default/files/products/documents/GPS-Antenna_AppNote_(GPS-X-08014).pdf?utm_source=en/images/downloads/Product_Docs/GPS_Antennas_ApplicationNote(GPS-X-08014).pdf
  • WA2ZKDWA2ZKD
    I use TTFF on the admin GPS page as a quick metric on how GS in working. A range of 42-48 is what I typically see. Its use is mentioned in the doc I linked above.
  • cathalferriscathalferris
    Cheers for the reminder about that doc. It makes for interesting reading. I had last looked at that, when setting up my GPS-led network time server and researching which cheap serial-accessible GPS kit was going to be most fun to play with.

    My time to first fix on the Kiwi is always abysmal, but that is entirely due to poor antenna location. If I extend the coax to put the antenna another ~5m away in the general vicinity of my NTP server's antenna, then I'll have most of a sky to play with. Plans for later in the week I think.
  • WA2ZKDWA2ZKD
    Keep in mind that a look to the North, or if you are in the South Hemisphere, to the south, can be fruitless for sat views
  • jksjks
    It's also abysmal because the Kiwi GPS code does no processing of the GPS almanac data to give it a hint to the current positions/doppler of the sats thus speeding TTFF. A good project for someone..
  • WA2ZKDWA2ZKD
    beyond my coding skill... but here's where to get almanacs https://www.navcen.uscg.gov/?pageName=gpsAlmanacs
  • jksjks
    You'd go to the considerable effort of dealing with the almanac if you had a device that needed quick GPS TTFF e.g. cellphones, car nav, ... This is totally unimportant for the Kiwi.
  • WA2ZKDWA2ZKD
    I thought your previous post suggested it might be a thing to do (?)
  • cathalferriscathalferris
    I added a 10m rg8 extension (with f-connectors and adapters to SMA) to the GPS coax cable, that brought the antenna from the gap between the houses to the top of the garden shed with a much better view of the sky..

    I've now got a solid and consistent track 12 / use 12 instead of the 5/4 I was getting before tin lid addition, and better than the 3/1 I was getting at the beginning. Now also getting a consistent 30 fix/second.
  • markmark
    Hi - I understand it's not the standard use case, but I've noticed that the kiwi's GPS reception appears to degrade when it is moving e.g. in a car (but recovers when stationary). Is there anything inherent in the kiwi's GPS receiver / processing chain that would make receiving GPS whilst in motion more problematic?
  • ChristophChristoph
    Hi - I understand it's not the standard use case, but I've noticed that the kiwi's GPS reception appears to degrade when it is moving e.g. in a car (but recovers when stationary). Is there anything inherent in the kiwi's GPS receiver / processing chain that would make receiving GPS whilst in motion more problematic?

    The Kalman filter assumes that the KiwiSDR GPS antenna does not move. Turning off the Kalman filter while moving in a car should help.
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