31 GPS Satellites in Orbit and More Are Being Launched: WHY?
The U.S Air Force launched a new GPS satellite on August 1 and, after the usual orbit insertion and check-out period, it was declared operational on September 17. Another satellite is scheduled for launch on October 29 and four more are scheduled for 2015 through early 2016. Following that, the third generation of GPS satellites will start to be launched. Spaceflight Now has a good article here. However, there is a question: Why do we need so many?
Two answers: an aging constellation and the deployment of new capabilities. The average age of the satellites in orbit is 12 years. That is beyond the average ‘life expectancy’ of the GPS satellites (the technical term is ‘mean mission duration’) which is in the 7-10 year range depending on the ‘model’ of GPS satellite. Suffice it to say that new satellites are needed to replace old ones before they become unusable. This is an important point. As the recent deployment shows, getting a new satellite launched and operational, even if you have one available, is not a quick process. The Air Force has been launching new satellites and positioning them near old ones so that there will not be a loss of ‘coverage’ when the old ones are shut down.
I will leave the discussion of new capabilities for another time and will focus now on coverage. A GPS receiver needs to process signals from at least four satellites in order to determine position. Four are needed because the receiver simultaneously solves for three dimensions of position (e.g., latitude, longitude and altitude) as well as the offset of the receiver’s clock from so-called GPS time (system time). The time synchronization is a necessary part of the positioning process but, again, I will discuss that another time.
It turns out that not only are four satellites needed, their relative locations in the sky influence the accuracy of the position. If two satellites are close to the East direction and two satellites are close to the West direction (with respect to the user), then longitude accuracy will be relatively good but latitude accuracy (i.e., north-south position) will be relatively poor. GPS thus needs a constellation such that the average user not only can receive signals from four or more satellites but also that they are widely dispersed throughout the sky.
The SatNav Toolbox for MATLAB produced by GPSoft can be used to illustrate how the GPS satellites move through the sky. Examples are shown on the website. Users near the equator have excellent coverage. At northern latitudes there is a hole in coverage in the north direction. Up at the north pole, there are no satellites overhead. This results in good horizontal positioning accuracy but relatively poor vertical positioning accuracy.
