Gnss: xtra: XTRA file validity duration implications


the XTRA feature on e.g. the BG95 allows to use XTRA-files with different validity duration. I could imagine that the contained data (e.g. satellite ephemerides) gets inaccurate over time.

So my question is, if using an XTRA-file with short validity duration can result in reduced time-to-first-fix or improved positioning accuracy?


There is not a simple answer.

The XTRA file helps predict the surrounding satellites and saves the receiver searching from scratch. It makes the fix more like a hot fix every time. Receiver has different sensitivity for acquisition and tracking. Effectively XTRA always puts receiver in tracking mode of higher sensitivity.

The precision of the positioning is determined by the chip i.e. up to 50m and is not related to the XTRA file.

However, environmental factors can dissolve the advantage brought by XTRA. For example:

  1. A lot of satellites seen by receiver but their CNo values are all weak
  2. Satellites are not widely spread over the sky but clustered over a small region
  3. Mountains or buildings nearby creating reflections and propagation delays

So if you place the receiver under open sky and the satellites have good signal strength, XTRA makes a big difference in the fix time. If receiver is placed under unfavorable environment the fix time and positioning confidence could still be unsatisfactory.

I agree with you. XTRA improves things a lot, but it always depends on the conditions.

The question was more in the direction if using a XTRA file with short validity duration (e.g. one day) can have advantages compared to a XTRA-file with longer validity duration (e.g. seven days).

Although Quectel documents say the prediction is becoming less accurate when getting close to the expiry, I do not see a difference in the fix time when the file is fresh or reaching the end of validity.

I highly encourage the use of XTRA, because BG95’s acquisition and fix time performance without XTRA is far far worse than other standalone GPS receivers (e.g. uBlox) without AGPS. BG95 GPS also consumes much higher power than other standalone receivers. XTRA can help save power by reducing the fix time .

Would you be able to point out where i can find that statement? (Just want to make sure i got all available docs at hand on that topic).

Sorry I actually read it somewhere else. Different suppliers using the same chipset may provide different details in their documents.
SIMCOM GNSS Application