What is Nikon's A-GPS .ee update file, and why does it expire after about a week?

Question

I discovered Nikon's A-GPS `.ee` update file, which is supposed to speed up GPS position acquisition. What information does this file contain? Is it something that can be read on a PC? Also, why does the file expire after roughly one week?

user2321 · Accepted Answer

In a nutshell¹ AGPS (Assisted/Augmented GPS) is a method to help a GPS device determine the position of the GPS satellites by supplying pre-computed data. The data itself changes in time due to the fact that the satellites themselves change in position over time in a manner that can't be expressed by a simple formula directly implemented by the GPS device itself. Which explains why the data expires over time.

In lieu of using the pre-computed data, the GPS device can receive the equivalent data from the satellites themselves. However this data is only transmitted at about 50 bits/second, or about 6 characters per second. Thus with no AGPS data, the GPS device needs to accumulate enough data from the visible satellites to "fill in the blanks" when calculating a GPS position - which takes time.

If you shut off a GPS device and then turn it on again after moving it it a significant distance (or keep it in the same spot but turned off for a significant time), the required data needs to be refreshed. Providing AGPS data minimizes the time needed to acquire a GPS fix.

I just downloaded an ee file (130k in size) from Nikon and took a look at it. It appears to be binary data. Given the size and format I doubt there is any practical use for looking at it on a computer.

I'm not a GPS engineer nor did I play one on TV or stay in a specific brand of hotel last night. This is just my general understanding on how things work.

What is Nikon's A-GPS .ee update file, and why does it expire after about a week?

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