On Mon, 05 May 2014 15:51:08 -0700, Bruce Hoult wrote:

On Tuesday, May 6, 2014 9:16:17 AM UTC+12, Bastoune wrote:
Hi all,



I have a GPS puck that acquires/computes a GPS fix every second (1Hz).
This GPS then transmits this data to a Vario (V7) and then PDA.



The default baud rate of the GPS is 4800 baud but I have the ability to
increase it to 9600 or 19200 bauds. Is there a benefit of a higher
transmission baud rate for the GPS? Does a 4800 baud rate allow the
full transmission of a GPS NMEA 0183 sentence every second? I am not
sure how much data is contained in a NMEA 0183 sentence, and if it can
"all go through" every seconds at 4800 bauds. (Note: The vario and PNA
can receive a 19200 bauds). What are the drawbacks of a baud rate set
too high? Transmission errors?



Given the GPS limitation (1Hz fix) I understand that, best case, my
vario (V7) and PDA cannot get a fix more frequent than one per second.
I want to set the baud rate high enough where the GPS 1Hz fix is the
limiting factor, not the speed of communication between the GPS and the
Vario/PDA.

Thanks all for your inputs.

B.

You've got it right.

GPSs always started with 4800 BPS because there's too much data for
2400. If you run it at 19200 then you'll be sure to get the up to date
data with less than 1/4 of a second delay.

The chances of error at 19200 are higher, but still minuscule with a
short, well shielded, cable in an electrically quiet environment. You
could probably run at 1 Mbps like ancient 3rd party AppleTalk stuff did
on RS422 (if your gear supported it) and seldom see errors. And anyway,
the NMEA sentences have a checksum which will pick up most errors.

I think a basic GPS which doesn't have a route set in it just sends a
GPRMC message every second. This says where the receiver is and contains
65 characters. So, a GPS running at the default 4800 baud with 8 bits + 1
stop bit per character, which can send just over 533 chars/sec can easily
send one every second. This is exactly what a simple, 'blind' GPS such as
the Garmin GPS35, sends and all it can send.

A GPS that will allow you to set a route into it, such as most of the
hiking GPS units, will send a few more message types so the instrument
receiving the messages can know where its going as well as where it is.
The route is one or more waypoints, so in our terms that can be anything
from the next turnpoint to a whole predeclared task. This adds three more
message types:

GPRMB - 66 characters, says where you're coming from and going to,
distance, bearing and speed toward
destination, cross-track error
GPROO - 24 characters + 9 per turnpoint in the route, lists the
turnpoints that define the route
GPWPL - 33 characters, waypoint details

Each second the GPS sends the GPRMC and GPRMB messages together with
*one* of the other messages, which cycle so that a complete route
containing 'n' waypoints repeats every n+1 seconds.

In other words, the most characters that would be sent in a second would
be 171 for a route with four waypoints and this would take about 320 mS
to transmit, so even this still uses only 1/3 of the available time at
4800 baud.

Just run it at the higher speed :-)

Indeed, though in practise 4800 baud will still handle almost any task
we'd want to fly.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |