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Re: [gpsd-users] Get fix mode with disconnect antenna

From: Yakov Shmulevich
Subject: Re: [gpsd-users] Get fix mode with disconnect antenna
Date: Wed, 20 Nov 2013 09:23:39 +0200

Hi Gary,

On Tue, Nov 19, 2013 at 8:11 PM, Gary E. Miller <address@hidden> wrote:
Yo Yakov!

On Tue, 19 Nov 2013 09:58:42 +0200
Yakov Shmulevich <address@hidden> wrote:

> > > You don't understand me, I disconnect the antenna, so at this time
> > > the PVT data by sure changed.
> >
> > Are you sure?  Do you see that in the debug output?  Most GPS will
> > just stop sending anything when they lose the antenna.
> >
> I run gpspipe at this  time and see all periodical NMEA messages come.
> And in the GGA message I see GPS Quality Indicator=0 (Fix not
> available or invalid)

Can you make a copy of the gpspipe output and annotate it with the
remove event?

Follow gpspipe output with antenna connected: 
                                                                                        -- GPS Quality Indicator(1=GPS SPS Mode, fix valid)  
                      -- Mode(3=3D)
                                       |                                                                                                    |
                                       -- Status(A=Data Valid)                                                            -- Mode Indicator(A=Autonomous mode)

Follow gpspipe outout with antenna disconnected: 
                                                                                        -- GPS Quality Indicator(0=Fix not available or invalid)  
                      -- Mode(1=Fix not available)
                                       |                                                                                                    |
                                       -- Status(V=Navigation receiver warning)                             -- Mode Indicator(N=Data not valid)

> More then, if I reconnect my client by gps_close() and gps_open() I
> got gpsdata.fix.mode = MODE_NOT_SEEN.


> But if I not reconnect my client I continue to get MODE_3D

Expected, that means you client has cached the last state.

Why it is expected? 
If so, I am not understand how the libgps works in case of  shared memory export.
As it is write in your documentation: "Whenever the daemon recognizes a packet from any attached device, it writes the accumulated state from that device to a shared memory segment."  So, I expect that every time than I call gps_read(), it brings me the last updated data from shared memory. What means "client has cached the last state"?
I would to return to my question again: Why then I re-open my session by gps_close() and gps_open() I start to get the right PVT data?

> > > But I got the data as before disconnecting antenna, include
> > > gpsdata.fix.mod=MODE_3D.
> >
> > Is so, we need to see some debug output to see what is happening.
> >
> What debug print you need? I can run the gpsd with -D 7 and send you ?

gpspipe will do to start.

> > > I found the workaround, but it seems me very ugly.
> > > Now every time that I want to get PVT data, I open the session by
> > > gps_open(), and after I get the PVT data I close the session by
> > > gps_close(). Also this solution have some strange behavior - I
> > > found that after gps_open() I get right PVT data only after 3-5
> > > gps_read().
> >
> > Sounds like you are not starting gpsd with -n.  Can you confirm how
> > you are starting gpsd?
> >
> gpsd -b -n /dev/ttyUSB1

That is good, except we discourage the use of -b.  That is intended mostly
for broken bluetooth GPS.  Why are you using that?

I am working with GPS module through FT2232H with 1PPS on DCD pin. So, the GSP receiver is connected via USB.
For -b is was mentioned also about USB, therefore I add this options.
Is it wrong or can be used?

Not knowing what you are doing in the gps_read() it is hard for us to
comment.  Can you see this behavior in cpgs or gpspipe?  Something we
can duplicate?

For cgps:  then I disconnect the antenna the window with satellites information is cleared, 
but in window with PVT data remained the same information as before antenna disconnecting.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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