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I've recently bought a USB-connected multi-constellation GNSS-receiver with a U-blox 8 chip in it. I've downloaded the U-center software that can display a lot of details about the received signals. One of those is the 'Sky View' window which logs the trajectories of the satellites the receiver is aware of. I've let it run in my windowsill for more than 12 hours, logging trajectories for the GPS, Glonass and Galileo satellites. I'm living in the Netherlands. This is the resulting image:

Sky View

The green lines show the satellite trajectories used for calculating my position, the red ones are the known parts of their trajectories where they were not used. This lines up as expected with the view direction from my window.

My question is about the white area directly to the north where no satellite seems to cross. Is this a bug or effect of my setup or is there really such an area which GNSS-satellites don't cross? If the latter, what is the reason they don't?

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  • $\begingroup$ Don't worry, it's real! space.stackexchange.com/q/28017/12102 $\endgroup$
    – uhoh
    Nov 5, 2018 at 16:27
  • $\begingroup$ 12 hours is too short. For GPS, you should wait 24 hours, for GNSS even longer. But what about the windowsill and the house? The roof of the house will block the view to the satellites overhead and north. The window looks to the south. $\endgroup$
    – Uwe
    Nov 7, 2018 at 9:34

2 Answers 2

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GPS satellite orbits go up to only 55 degrees inclination, so there are regions over the poles that they do not fly directly over (they are high up enough that they give coverage in the polar regions). If you were sitting up at the North Pole, you would never see a GPS satellite climb higher than 55 degrees from the horizon, whereas if you were on the equator, you would see them pass overhead.

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    $\begingroup$ Thank you. I see now that I'd been looking at this plot wrong. I was confused by the trajectories at the top of the image near N, but of course with me being at 52 degrees N, that is already all the way across the North Pole over the Pacific. Makes sense now. I presume the simple reason for the 55 degree limit is that there is less demand for satellite navigation at high latitudes so it's better to concentrate the capacity elsewhere? $\endgroup$
    – Bart Noordervliet
    Nov 5, 2018 at 17:20
  • $\begingroup$ You need to see four satellites to get a unique position, and I believe the constellation picked the number of satellites and those orbits because you have a high probability anywhere on the Earth to see at least four at any time. Galileo is using a similar design. There's probably a nice RAND corporation paper weighing the pros and cons on it from the 60's or 70's, but I couldn't find anything on or Galileo with a quick search. $\endgroup$
    – Dave
    Nov 5, 2018 at 17:38
  • $\begingroup$ @BartNoordervliet, it's basic orbital dynamics. High inclinations are harder to reach than low inclinations, so if you can get 100% coverage without putting any satellites above a 55-degree orbit, you do so -- even if it does mean Amundsen-Scott Station never gets GPS satellites much above the horizon. $\endgroup$
    – Mark
    Nov 5, 2018 at 23:22
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Bart's correct. To elaborate, I was part the USAF team that tested the first DoD acquired GPS receivers.

From what I remember, the GPS and common to all positioning systems, the orbital geometry was mostly limited by economic practicality. + and - 55 degrees, as stated in the first answer, covered the planet's highest population density. The higher latitudes would require a much larger constellation of satellites to keep 4 in view at all times; 3 low angle (the nearer 5 degrees above horizon the better) and 1 high angle (the nearer to overhead the better) to provide optimum Horizontal Dilution of Precision (HDOP) and Vertical Dilution of Precision (VDOP). The 5 degree above horizon is the limit the GPS signal can reliable account for atmospheric propagation delay in it's position calculations.

The software recognizes each vehicle by number and their current position based on the ephemeris table and at any given time. All that to say, the higher the latitude, the less likely you'll have favorable satellite geometry for a usable HDOP and even worse VDOP to the point that you'll have little H and no V over the poles. The physics for curing that would require polar orbits with vehicles crossing often enough to provide position fixes for a relatively small area with relatively little usage. I'm sure the aircraft serving the polar stations would disagree, but that's why they still have inertial-integrated and stellar navigation systems.

Edit added: The US designed, deployed and continues to mantaine and manage the Global Positioning System as it was always intended to be a "global" system but as such would obviously be a two edged sword, so signal characteristics were added to provide US and allied forces position certainty while operating in hostile EM environments. I signed my NDA when I left the service, but the best explanation of the Anti-Jam/Anti-Spoof that I found in the clear are at https://www.novatel.com/tech-talk/velocity/velocity-2013/understanding-the-difference-between-anti-spoofing-and-anti-jamming/

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  • $\begingroup$ Great answer, welcome to space! I'd always thought that the original motivation for the GPS system was military, and so covering "the planet's highest population density" would not have been the primary consideration during planning of the original system. Certainly the northern polar areas had military significance? $\endgroup$
    – uhoh
    Nov 6, 2018 at 3:39
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    $\begingroup$ Selective Availability (SA) and other signal parameters were incorporated for military considerations to prevent enemy use of, jamming or spoofing of the signal. Clinton turned off SA while he was president and 3 or 4 new block series sats have been launched since I was involved. It's use has become way too much a part of the global trade, navigation, even banking infrastructure to tamper with so the military's had to focus on "user equipment" side, making smarter receivers. $\endgroup$
    – JD Clayton
    Nov 6, 2018 at 3:55
  • $\begingroup$ But I'm talking about when the GPS orbits were originally chosen, not about what happened later once the system was in place and had been used for decades. The system was designed and those birds were launched an orbited for their military utility. Maybe this is a different question in the making, or just an article that needs reading (by me) rather than something to discuss in comments here though. $\endgroup$
    – uhoh
    Nov 6, 2018 at 4:08
  • $\begingroup$ @JDClayton To prevent spoofing yes. To prevent jamming, though? How does an encoding prevent jamming? $\endgroup$
    – Lightness Races in Orbit
    Nov 6, 2018 at 10:36
  • $\begingroup$ If you want to see a great bit of information on the genesis of the GPS system, there is a nice multi-part piece on it. $\endgroup$
    – Dave
    Nov 6, 2018 at 14:16

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