[time] Is there a best orientation for a GPS receiver?

[Tapio Sokura]

Philip M. White wrote:
> The system administrator whom I mentioned in my previous email (who will
> allow me to run a stratum-1 server at my university) asked me: if
> placing the GPS on the roof won't work for some reason and I have to
> simply set it next to the window, which direction is ideal?

It depends on where you are located in latitude, longitude doesn't 
matter. The GPS satellites have an orbital inclination of about 55 
degrees, this means that GPS satellites never fly further north than 55 
N latitude and never further south than 55 S. In practice this means 
that for any given location considerably off the equator in latitude, 
there is a circular empty spot in the skyview, where GPS satellites 
never go. North of the equator, this void is in the north and south of 
the equator in the south. From here on I'll just talk about the case 
where you are north of the equator, but the same is true in reverse in 
the south as well.

At latitude 55 degrees, the edge of the void is just overhead, the other 
edge is (in elevation) around 20 degrees from north. You could picture 
the void as a frisbee held up to the sky so that one edge is directly 
overhead and the opposite dege is 20 degrees up from north. Between 0 
and 20 degrees elevation north you'd see some satellites, because they 
are seen from the other side of the globe (if you were at 55 N 30 W, 
"the other side" I'm talking about would be 55 N 150 E). When you get 
further north, the void gradually moves overhead so that at north pole, 
you'd never see GPS satellites in over 35 degrees elevation (90 - 55 = 
35), but you'd see them equally in all directions.

So in summary, if you are further than say 20 degrees in latitude away 
from the equator, best side for GPS receiption is the side facing the 
equator, if the local environment is ignored. If you are closer to the 
equator or very close (within 10 degrees) to the poles, the side doesn't 
matter much. Of course you have to take into account local surroundings. 
If there is a big building right next to where you are blocking the view 
to the equator, you might get better reception in the east or west sides 
if they have a better skyview. Even the side facing the (closer) pole 
can be best if other directions are badly blocked.

In any case if you are using the receiver for stationary timing purposes 
only and the receiver is so equipped, you might want to turn on the 
position hold mode. This way the receiver only needs to receive one 
satellite to achieve accurate timing, because location isn't a variable 
in the GPS calculations anymore.

   Tapio