r3 - 20 May 2009 - 20:55:39 - JosephGauthierYou are here: TWiki >  Documentation Web  >  AskedQuestions > NavigationFiles > MeteorologicalFilesAndNavigationFiles
-- SergioCoelho - 08 Apr 2009

Hi, I'm a student of electrical engineering from Portugal.

I have some question about different files that I see is used to estimate atmospheric errors, and the questions are: he meteorological files, they give data like temperature and what more?? Where I can find the meteorological files (sites)? And I see something about almanac what is that?


Answer:
It really depends on what meteorological files you are talking about. As an example, I'll talk about the RINEX Meteorological files. If you take a look at one of them, you'll notice it has a header, and then a bunch of data listed below. For example, one line of data could be:

08 2 7 8 15 0 984.7 27.0 63.1

The data above follows this format:

Year : Month : Day : Hour : Minute : Second : Pressure[mbar] : Dry Temperature[degC] : Relative Humidity[%]

So the data point above corresponds to... February 7, 2008, 8th hour, 15th minute, and 0 seconds, with a pressure of 984.7mbar, 27degC, and a relative humidity of 63.1%.

About the almanac... The satellites broadcast two types of data, ephemeris and almanac. The Almanac data is essentially orbital parameters for each satellite. Each satellite broadcasts the almanac data for all of the satellites. However, the almanac data is not very precise. On the other hand, the ephemeris data is very precise and is necessary for precise positioning. Each satellite broadcasts its own ephemeris only (this happens once ever 30 seconds).

One might ask what use the almanac is if its data cannot be used for precise positioning. The short answer to that question is that when the GPS unit is turned on after being off for more than 30 minutes (this is how long the ephemeris data is good for), it finds the available satellites based on the parameters in the almanac. This enables the satellites to be located faster than they would have been otherwise for the initial search.

I hope that helps.

-- JosephGauthier - 20 May 2009

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