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Re: [gpsd-dev] AIS NMEA TAG Blocks

From: Iván Sánchez Ortega
Subject: Re: [gpsd-dev] AIS NMEA TAG Blocks
Date: Wed, 27 Aug 2014 10:09:54 +0200
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El Miércoles, 27 de agosto de 2014 00:42:16 Ed W escribió:
> Feature request: I would like to see an option to get the NMEA data out
> of gpsd, but augmented with a timestamp on each sentence and details of
> which input fed in the data. TAG blocks appear to solve that problem
> perfectly.

There is one *big* problem with using TAG blocks to timestamp the sentences. 
The standards (as far as I/we know) say that the timestamp is **either** the 
number of seconds **or** milliseconds since the UNIX epoch, but we do **not** 
know if the initial state should be secs, msecs, or nothing. We do not know 
how clients reading that TAGblocked NMEA stream should ask GPSD to switch the 
timestamp units (or enable/disable them).

There are a few sentences introduced in NMEA 4.00 that supposedly allow that - 
the client sends a sentence to the server, and then the server starts 
TAGblocking sentences. But, alas, there are no examples anywhere.

>From my personal experience, the way to know if a TAGblocked timestamp is in 
seconds or milliseconds is ask the people in charge of setting up the 
device/service. Perhaps some devices output only secs/msecs, and some other 
accept the NMEA 4.00 TAGblock configuration sentences.

I, for one, would like to see the secs/msecs problem solved before GPSD 
embarks on the enterprise of writing TAGblocks.

If anyone has (live) examples of the behaviour for TAGblock timestamps, please 
do share.

> I think all this is effectively emitted with the JSON output (?),

Nope. The JSON output does not include timestamps for each sentence.

Iván Sánchez Ortega <address@hidden> <address@hidden>

Web & geospatial developer at Aptomar AS

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