What's the minimum sky clearance when installing GNSS antenna?

Question

For mobile cellsite clock synchronization, we need to install GNSS antenna. Reading some online guidance for GPS installation, a sky clearance (free of any obstacle 10 degree above the horizon) is generally expected to ensure smooth operation. But my vendor advised that for a sky clearance (45 degree above the horizon) we could track for at least four satellites on clock syn. Is that true? Any general practice?

mapj1 · Accepted Answer

From Hong Kong you wont have anything like as much problem achieving that as you would here. you just need to get a decent cone of sky in view - in the UK that becomes a cone of sky laying on its side and pointing south - a harder target to meet. 
 If you want to calculate exactly how bad you can get away with you will have to look in detail at the constellation patterns and consider how often do you do clock sync, and how long is at acceptable to wait, if you try and do it at a time when the stars are not in alignment as it were. For GPS the orbital period is just under 12 hours, so the same satellite will be in view at at any given place not quite the same times twice per day. Glonass is a bit different, and the orbital period is 8/17 of a day, and each disk holds 8 satellites. For completeness the Beidou inclined disc (IGSO orbit) period is 7/13 of a day. but there are also some geostationary over China that provide infill. I'd expect that to be pretty solid over HK too. 
 I must admit that the 3d calcs are beyond my mental maths, and I tend to rely on published computer models if I really need to know how many will be in view from any given place at a given time. 
 
 Mike

What's the minimum sky clearance when installing GNSS antenna?

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