From ca4c48548e28c411805ea1a11c5fff488815924e Mon Sep 17 00:00:00 2001 From: "Eric S. Raymond" Date: Fri, 2 Jul 2010 03:16:37 -0400 Subject: Added a FAQ entry on coping with fixed-baud-rate devices. --- www/faq.html | 44 ++++++++++++++++++++++++++++++++++++-------- 1 file changed, 36 insertions(+), 8 deletions(-) (limited to 'www') diff --git a/www/faq.html b/www/faq.html index e92b66d1..27815070 100644 --- a/www/faq.html +++ b/www/faq.html @@ -76,11 +76,11 @@ GPSD Frequently Asked Questions
  • My gpsd sometimes stops responding overnight
  • Why use the gpsd protocol rather than parsing raw NMEA?
  • How should I interface my application with gpsd?
    -
  • Can GPSD work use Assisted GPS data from cellphone networks??
    +
  • Can GPSD use Assisted GPS data from cellphone networks??
  • How can I improve fix accuracy from my GPS?
  • How can I improve time reference accuracy from my GPS?
  • Why does my GPS get lost when I sleep/wake my laptop?
    -
  • How do I get gpsd data into a web page?
    +
  • Why is there no option to fix baud rate?

    How do I report bugs in GPSD?

    @@ -517,10 +517,10 @@ effects due to signal bounce from the ground or water, which can cause the GPS to mistake its position and the time signal. The correct location for a boat GPS antenna is on the gunwale rail or pushpit rail, close to the water and as far from the mast as possible -(to reduce signal bounce from the mast). If -you're outside or in a fixed location, put the GPS antenna as far from -buildings as possible, and on the ground. If you're in a car, don't -put the GPS antenna on the roof, put it on the towbar or similar.

    +(to reduce signal bounce from the mast). If you're outside or in a +fixed location, put the GPS antenna as far from buildings as possible, +and on the ground. If you're in a car, don't put the GPS antenna on +the roof, put it on the towbar or some similar location.

    If you're driving in a heavily built up area, you're going to get signal bounce off buildings and and reduced accuracy. That's just how @@ -554,7 +554,7 @@ will have to wait for the computer to poll it. This polling happens half-millisecond. Furthermore, the USB chip might run at anything close to 1000 Hz. If it runs at 1000.1 Hz, every 10th second the next time-report will come 1 whole millisecond earlier. This will have NTP -observing something like a 1 ms high sawtooth in the time-reports.

    +observing something like a 1ms-high sawtooth in the time-reports.

    For accurate time reference, use a PPS line over RS232 triggering an interrupt. Serial bus interrupt latencies on modern hardware on the @@ -579,7 +579,7 @@ USB/serial adaptor, but on what order you plug devices in: 1st device gets /dev/ttyUSB0, 2nd gets /dev/ttyUSB1, etc....

    This collides with what happens during a suspend/resume. If you -suspensd while gpsd has a device active, it will hold the +suspends while gpsd has a device active, it will hold the device open while your laptop is asleep - but, meanwhile, the suspend logic is shutting down hotpluggable devices to be recreated at resume time. On resume, Linux will see that the old device is open @@ -649,6 +649,34 @@ srsName="urn:ogc:def:crs:EPSG:6.6:4326"> </Report> +

    Why is there no option to fix baud rate?

    + +

    There is no option to fix baud rate because gpsd is +designed to (a) be autoconfiguring, and (b) handle multiple devices. +There are some more philosophical reasons as well; read the +Hacker's Guide for discussion.

    + +

    Unfortunately, this causes problems with some devices (notably +Bluetooth GPSes) that are designed to operate at fixed baud rates. +Some of these go catatonic if you try to set the baud rate, which is +why we have a -b option that prevents gpsd from trying to +configure the GPSes it talks to.

    + +

    On Linux systems, there's trick you can play to simulate fixing the +baud rate. It utilizes the fact that under Linux, setting baud rate 0 +is interpreted as "use the hardware port's existing baud rate without +change". The autoconfiguring hunt loop in gpsd always starts +with this behavior.

    + +

    Accordingly, you can short-circuit the autobaud if you use stty(1) to set +the bit rate just before starting gpsd. For example, suppose you know that +your GPS is on serial port 0 and operates at a fixed bps of 54600. You +can set that up like this:

    + +
    +        stty speed 54600 
 
 
    
 
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