Why does GPS take so long to get a position and How Extended Ephemeris can help?

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.

To make it worse, for a GPS receiver to calculate a position, it needs ephemeris data from 4 different satellites. Depending on the environment, the total time to receive all 4 ephemeris can take much longer than 30 seconds, if at all.

 

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.

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