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Re: SiRF module and bluetooth

From: Dave Riesz
Subject: Re: SiRF module and bluetooth
Date: Fri, 26 Jun 2020 19:37:11 -0700

Well, I think I may bail on the bluetooth link since it's adding complication without giving me much in return.

Part of your response gets to what I'm working on that doesn't deal directly with gpsd: signals and noise in the receiver.

A while ago I wanted to try to use a pair of GPS receivers to collect some reasonably accurate (or at least consistent) position information for my property using one unit as a stationary reference and the other to collect locations.  The units are identical.  Same receiver module, same antenna, same RPi hardware, same software.  Yet, even with the antennas sitting side-by-side the units would report positions that not only were quite different, but also jumped around with apparent randomness.  Thus my attention turned to noise in the system.  I'd read that the 3.3v pins on the RPi (that I was using to power the GPS modules) weren't very clean and that the local oscillator(s) on a GPS unit could be sensitive to voltage fluctuations.  So I tried an RF choke on the power leads and then a separate power supply.  The bluetooth idea was to totally isolate the receiver from the RPi.

Anyway, if noise from the computer really isn't an issue, what might be causing the discrepancy between the receivers?  The location and setup of my test units is not ideal and checks all of the boxes you mentioned:

- limited skyview: right next to the exterior house wall
- multipath reflection: house is wrapped in a steel stucco lath
- cable loss: cheap mouse antenna probably has a lossy cable

Not a great setup, but again both units are identical and are seeing very different things.

Dave Riesz

On Fri, Jun 26, 2020 at 6:25 PM Gary E. Miller <> wrote:
Yo Dave!

On Fri, 26 Jun 2020 18:01:40 -0700
Dave Riesz <> wrote:

> The GPS module (EM410) works just fine.  I can connect it directly to
> the UART pins on a Raspberry Pi, start the gpsd service, and
> everything works.

yes, I got that.

> I wanted to try to isolate it from the RPi to see if I could reduce
> the noise reaching the GPS receiver,

No worries, the Raspberry Pi emits no noise that would affect any
GNSS receiver.  The SiRF III is itself the noise source.

With GNSS receivers there are only three signals things to worry about:

        1. Skyview.  The antenna wants to see as much unobstructed
           sky as possible.
        2. Reduce multipath signals.  Stay away from things that
           refelect 1.5GHz.
        3. Cable loss.  Keep the loss from antenna to receiver low by
           using low loss cable, proper 50 Ohm practice, as short as

> so I got the bluetooth serial
> module (HC-06).  I expected it to work something like a "wireless
> serial cable," but it's not so straightforward.

Yup.  Better off with USB over CAT-5 or wireless USB.  Bluetooth by
being energy efficient is slow.

> The GPS module connects to the bluetooth serial module.  I connect
> wirelessly to the bluetooth module with the builtin bluetooth adapter
> on the RPi board.  So on the RPi, the configured link shows up as a
> the serial device "/dev/rfcomm0".

Yup. just like in the doc.

> Using stty, I can set the speed of
> /dev/rfcomm0, but the remote bluetooth module will still be running
> at its preset baud rate (in this case 9600bps).  Since I can't set
> the wireless link to 4800, I can't make the initial connection to the
> GPS module.

I'm don't understand the BT protocol well, but some gpsd users have
managed to change the serial port speed of their BT.

One page on the HC-06 says:

   "In programming you need to set default baud rate of UART serial
   communication to 9600. The value is default setting of module and can
   be change in program."

But otherwise the doc was unclear to me exactly how to do that.

> Now, I can take everything apart, physically connect to the bluetooth
> module, and tell it to communicate at 4800bps.  When I put everything
> back together, I can then stty the serial device to 4800 and speak to
> the GPS module.


> But then I'll be stuck at 4800bps forever.

Maybe.  And 4800 bps not so bad if you just want lat/lon/time.
Worts case you need a different BT slave that is 9600.

> Again, the cheapo GPS modules work great with gpsd already.  I'm
> getting a little too ambitious with them...

Not seen anything special yet...

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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