[Thumbgps-devel] Fwd: Long term SiRF data

[Dave Taht]

I can't remember if I had shared this already.


---------- Forwarded message ----------
From: Hal Murray <hmurray at megapathdsl.net>
Date: Tue, Oct 25, 2011 at 2:58 AM
Subject: Long term SiRF data
To: Eric Raymond <esr at thyrsus.com>
Cc: Hal Murray <hmurray at megapathdsl.net>, Dave Taht
<dave.taht at gmail.com>, Jim Getty <jg at freedesktop.org>, Gary Miller
<gem at rellim.com>



I've been collecting data from 2 SiRF units.  I'm up to about 12 days now.

Quick summary: both suck.

Both are located inside my house, poor conditions.


The first is a Holux GR-213.  It's setup to only send GPRMC sentences.
That's what I would use with ntpd.

Here is the startup:
 http://www.megapathdsl.net/~hmurray/bb/gps/Holux-1.png

The green marks are "good" sentences.  The Y offset is the difference between
the actual arrival time and the time stamp in the sentence.  The blue marks
are the fraction part of the time stamp in the sentence.  The red marks are
invalid sentences.

At about -2.94 (hours) the reported time jumped by 1 second.  My guess is
that it learned about the latest leap second or something like that.

At about -2.82 hours, the fractional part of the report switched to 0.  I
have no idea what caused that.  It doesn't really matter much.  It wasn't
useful anyway.

Here is the big picture:
 http://www.megapathdsl.net/~hmurray/bb/gps/Holux-2.png
There is a mode shift every 1-3 days.  What's the right term?

For reference, here is an old graph with the mode shift every 8-12 hours.
 http://www.megapathdsl.net/~hmurray/bb/gps/SiRF-GPRMC-4800.png

This is the previous 2 pictures glued together:
 http://www.megapathdsl.net/~hmurray/bb/gps/Holux-3.png

Here is one day:
 http://www.megapathdsl.net/~hmurray/bb/gps/Holux-4.png

-------------

The second unit is a Global Sat BU-353.  It ignored my attempts to change the
configuration, so I let it run in the default setup.  Normally it sends
GPGGA, GPGSA, and GPRMC.  Every 5 seconds it includes 3 GPGSV sentences
before the GPRMC.  I think that fits in 1 second at 4800 baud, but the GPRMC
gets pushed over to the next second.

Here is the graph for the GPGGA sentences:
 http://www.megapathdsl.net/~hmurray/bb/gps/BU-353-gpgga.png
The long term cycle time is 8-10 days.

Here is the graph for the GPRMC sentences:
 http://www.megapathdsl.net/~hmurray/bb/gps/BU-353-gprmc.png
The top band of green is the data that gets pushed over to the next second.
The blue and purple are the number of satellites.  (They are scaled up by
100.)  I don't see any pattern.

This unit doesn't always return 000 for the fraction part of the time stamp.
Sometimes it's 998 or 999 with the previous second.
 http://www.megapathdsl.net/~hmurray/bb/gps/BU-353-gpgga-off.png



--
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-- 
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
http://www.bufferbloat.net