GPS Antenna aiming

WA2ZKDWA2ZKD
edited March 2018 in Installation, Antennas and Interference
There been some discussion about antenna gain and coax loss but perhaps more important is can your antenna "see" the birds. In exploring different antenna types, I learned a few things:
  • many people think GPS sats are geostationary which they are not (I never thought that BTW!)
  • many people think that the constellation of sats in uniform around the Earth, it is not
  • depending on where you live there may be few usable sats to your north or south, in my case the former.
  • if your sat view is like mine or similar, avoid aiming into the "hole", see the graphic below from my kiwi
  • you can preview your sat coverage on a cellphone using something like GPS Status (Android)
  • there are sites that show the constellations such as https://in-the-sky.org/satmap_radar.php
image
M0TAZ

Comments

  • jksjks
    I've been using this site to see when Galileo sats will appear in my antennas limited field-of-view: https://www.n2yo.com/whats-up/?c=22
    You can see from the orbital tracks why the shadow hole exists if you're located close enough to the poles. From the north pole all GPS sats would appear biased toward the south.

    Here's a great animation of various satellite orbital heights: https://upload.wikimedia.org/wikipedia/commons/b/b4/Comparison_satellite_navigation_orbits.svg

    Due to QZSS I learned there is a difference between the terms geosynchronous and geostationary. 3 of the 4 QZSS sats trace the same figure-8 pattern on the Earth because they are geosynchronous (and have a high inclination). QZS-3 is truly geostationary (appears at a single point on the Earth) like the majority of the communications satellites.

    WA2ZKD
  • WA2ZKDWA2ZKD
    Knowing that north is a "lost cause" for me worked out. I was able to side-mount my GPS antenna higher on an existing structure and now have a low elevation view on other birds with no degradation on the north to worry about.
  • SM2BYCSM2BYC
    Well, for some of us, it might be difficult to avoid aiming into the hole with the GPS antenna...

    Regards

    Mauritz / SM2BYC


    image

    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/eb/7aa81ce9e6e85d171bd32a221d2f8a.png
    M0TAZ
  • WA2ZKDWA2ZKD
    Thanks so much for posting that!! I wondered what happened when a station was further north. What type of GPS antenna do you use?
  • SM2BYCSM2BYC
    Just a "normal" type of GPS antenna similar to those in the Kiwi kits. I found mine in my junk box. It is at least 15 years old but seems to work OK. Had to extend the feed line with about 3 meters of RG58 so I could place the antenna at a good location outdoors.
  • G8JNJG8JNJ
    Wow, that's an impressive 'hole' :-)

    I must admit that I hadn't realised that there were fixed 'holes' in GPS coverage, as I'd always assumed it was a whole earth constellation.

    That explains why I've always had a slight null to the north, which I previously assumed had always been due to the position of the antenna relative to the mounting pole. 

    Maybe (when the weather improves) I'll tilt it and see what difference that makes to the low angle coverage.

    My thanks to James for highlighting the issue. 

    Regards,

    Martin - G8JNJ


  • WA2ZKDWA2ZKD
    edited March 2018
    Martin

    If your antenna is a helix type, tilting it may make no difference.  Here is some related info


    -Jim
  • jksjks
    Obligatory: https://xkcd.com/1356/

  • WA2ZKDWA2ZKD
    edited March 2018
    the "aiming" may only be a matter of staying off the north wall of a structure (or south in the that hemisphere). If your view is omnidirectional, that's as good as you'll get. My previous experience was that my antenna that was 1M off the ground, on the north wall, performed better at the NW corner. Now, I see why 
  • WA2ZKDWA2ZKD

    GPS access recently mentioned so this old post may be relevant

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