Re: EGNOS working at last!

In article <qs93g.7710$Vn.26@xxxxxxxxxxxxxxxxxxxx>, Jim Ford <jaford@xxxxxxxxxxxxxxxxxxxxxxxxx> writes

Still having no luck picking up any EGNOS satellites on my Geko 201.

I've tried a hard reset, which seems to work OK, and went into the park with clear(ish) horizons to the North and South.

The satellites are to the south and quite low in the sky in the UK. The further north you go the harder it is to receiver them as they are lower in the sky.

From my observations, I have the following questions.

1) When a bar comes up and is black, I assume it means that a satellite signal has been acquired with adequate strength. Is the height of the bar related to signal strength

Yes.
and if so, what does a high empty bar mean?

Empty means it is not being used in the position solution. Once a normal GPS satellite has been tracked for long enough (roughly 30 seconds without any interruptions) the receiver will have the required data to use it in the solution and it will turn black.

(I've seen empty bars on satellites 33 and 39,

As they are not transmitting ranging data they cannot be used in the position solution. They probably will transmit ranging data in the future and then will be able to be used as additional GPS satellites in the position solution.

They may or may not be transmitting valid correction data for the normal GPS satellites.

but as I've never seen a 'D' on the other bars, I guess the data hasn't been acquired?)

When the receiver has enough correction data for a satellite's signals to be corrected the D will appear for that satellite indicating that it is Differentially corrected.


2) How can I tell when 'the almanac' has been acquired (whatever that means in the context of a GPS!)?

You can't! The almanac is the rough position data for all the satellites in the GPS constellation. It takes approximately 12.5 minutes of uninterrupted reception to receive the whole almanac.

However the almanac is only used when the receiver is trying to obtain its first fix from cold. It uses it to have a reasonable guess as to where the satellites are in the sky. This speeds up the acquisition process. Almanac data can still be useful even if it many months old because the orbits are mostly predictable.

On the other hand the epehmeris data is much more precise data which allows the receiver to accurately compute the position each satellite. It is transmitted by each satellite itself and takes ~ 30 seconds to be fully received. Usually when a bar is hollow the receiver is still trying to get a complete set of ephemeris data from the satellite.

3) Does the GPS give up looking for a WAAS/EGNOS satellite after a time, or does it keep searching whenever the GPS is on?

If 'WAAS' is turned on it keeps looking for the WAAS/EGNOS etc. satellites.

4) From the walkers point of view, is EGNOS a washout and our('the taxpayer') money been wasted?

I'm not convinced it would ever be much use to mainstream walkers even if it was easily used. For most people accuracy being improved from ~7m to ~2m really won't be much on an issue.

--

Dominic Sexton
.

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