GPS Satellites in View at High Latitude
Posted: Tue Dec 05, 2017 4:14 pm
In the USAF NavStar global positioning system, commonly referred to as GPS, the satellites are in medium earth orbits with an inclination of 55-degrees. This means that the highest latitude the satellite orbit reaches is 55-degrees, North or South. If you are a user of GPS in high latitude, say at 55-degrees, you might think that all GPS satellites will appear to your point of view as being in the southern sky or at best overhead; I made that assumption myself, but discovered it was not correct.
If you are located at 55-degrees North latitude, then occasionally a GPS satellite should pass just about overhead, but never to your North--except that on the other side of the world those same satellite may also cross your meridian at 55-North latitude on that hemisphere. Using some simple geometry, we can see that a satellite that is on your meridian but anti-polar and reaches 55-N latitude will actually be visible to you with an elevation of roughly 6-degrees, and will appear to you to be due North. As a user moves to higher latitudes, more satellites over the North Pole will be visible to them low in their northern sky.
A recently published article in GPS World magazine included some azimuth/elevation plots of satellites in view and their tracks as seen from high latitudes in Finland. I reproduce one of them below to illustrate how GPS satellites from the other side of the earth show up in the northern sky of users in high latitudes:
As can be seen in the polar plot of GPS satellite orbit tracks, many GPS satellites will be in view to the North to a user in high latitudes.
If you are located at 55-degrees North latitude, then occasionally a GPS satellite should pass just about overhead, but never to your North--except that on the other side of the world those same satellite may also cross your meridian at 55-North latitude on that hemisphere. Using some simple geometry, we can see that a satellite that is on your meridian but anti-polar and reaches 55-N latitude will actually be visible to you with an elevation of roughly 6-degrees, and will appear to you to be due North. As a user moves to higher latitudes, more satellites over the North Pole will be visible to them low in their northern sky.
A recently published article in GPS World magazine included some azimuth/elevation plots of satellites in view and their tracks as seen from high latitudes in Finland. I reproduce one of them below to illustrate how GPS satellites from the other side of the earth show up in the northern sky of users in high latitudes:
As can be seen in the polar plot of GPS satellite orbit tracks, many GPS satellites will be in view to the North to a user in high latitudes.