[ntp:questions] Re: Oncore driver in 4.2.0

Wolfgang S. Rupprecht wolfgang+gnus20031113T091857 at dailyplanet.dontspam.wsrcc.com
Thu Nov 13 17:45:33 UTC 2003


hmurray at suespammers.org (Hal Murray) writes:
>>Right. According to my Oncore User's Guide (June 1998), the
>>position-hold mode command (@@At), which can also initiate a
>>site survey, is in the UT Oncore models but not in GT Oncore.
>
>>Sorry I don't have any information for the M12 Oncore.
>
> I have some info on the M12.  It has a list of commands which
> doesn't include the @@At command.  There is an @@Gd command
> to set the position/altitude hold mode and other commands to
> set the position/altitude.

The @@Gd command exists in the version 5 oncore documentation (now
freely ftp-able from Motorola).  The tricky part is the @@Gd3 mode
pretends to be starting a site-survey bey ack-ing the command but
doing nothing.

> I'm pretty sure you could write your own version of a site
> survey routine.  Just let it run for a while and collect a
> lot of data.  You might have to hack the driver to get it.

The oncore refclock does do its own averaging if the hardware survey
command comes back with an error code.  When I hacked the code to
never trust the m12oncore to do a HW survey it worked a bit better.
(The software site survey is just a normal average with the samples
added and then divided by the number of samples taken.)

I also added the lat/lon/alt to the clockstats file so in theory I can
now do a 30-day site survey like the big boys when they set up the
coast-guard reference stations.

> Anybody know how to analyze that sort of data without getting
> tricked by things like asymmetries in the reception due to
> nearby buildings on one side and things like that?

Thats called multipath and the first line of defense is to have an
antenna shield that stops low-angle signals.  The second line of
defence is to add a "mask angle" to the gps software and ignore any
GPS's under a certain angle (say 5-degrees off of the horizon).  

I did do a bit of experimenting with detecting ground interference
(and using it to generate 1.5GHz back-lit pictures of my neighborhood.

     http://www.wsrcc.com/wolfgang/gps/panorama/

In theory I could use this as a gating signal to reject any satellite
that got too close to ground clutter.  The m12oncore does let you tell
the gps which satellites to ignore.  I could tell it to ignore any
satellite that ventured into a bad part of the sky.

I am told one way to see if you have a multipath problem is to
correlate the data from a large number of days together and see if
there is a periodic daily twitch.  Looking at my panorama picture I
can see why that would be.  One I get enough data taken on adjacent
days I'll have a go at doing this.

-wolfgang
-- 
Wolfgang S. Rupprecht 		     http://www.wsrcc.com/wolfgang/
           The From: address is valid.  Don't mess with it.



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