One of the most frequently asked questions I hear is “Why does my GPS take so long to get a position”? Anyone who has used a GPS receiver knows that when it is first turned on, it could take anywhere from 30-60 seconds to calculate a position. In this “instant everything” world of ours, who has the patience to wait this long? In this blog post, I will explain the reasons why and what you can do about it.
Why does GPS take so long to get a position?
In order for a GPS receiver to calculate a position, it needs to know exactly where the satellites are in space. The way a GPS receiver knows where the satellites are is by downloading a fairly complicated message broadcast from each satellite in view. Each of these messages contain something called an “Ephemeris” which is a fancy way to say “I am satellite A, here is where I am at time X”. Unfortunately, these messages are broadcast at a very slow rate of 50 bps (bit per second), this translates to about 30 seconds or more for the GPS receiver to receive.
How Extended Ephemeris can help?
If downloading broadcast ephemeris takes so long, is there a way to get around it? Luckily, the answer is “yes”.
Extended Ephemeris technology is a way to predict where the satellites are for the near future, hence, the word “extended”. By comparison, broadcast ephemeris is typically valid for 2-4 hours, whereas the state of the art Extended Ephemeris technology can be used for up to 28 days. By knowing where the satellites are for the next 28 days, a GPS receiver can totally skip downloading the broadcast ephemeris and process directly to calculate a position as soon as the receiver is turned on. The net result of using Extended Ephemeris is that instead of taking 30-60 seconds to get a position, a typical receiver will now take 1-2 seconds to do the job.
