GPS - GPS Programming?

Author

GPS Programming?

philsonj@yahoo.com

2005-11-15, 5:48 pm

Can I program a handheld GPS unit to output specific data to a computer
in realtime that I specify? For example, I want to have a handheld
device that, on "initialization", will output stream the following data
to a computer:

1. On initialization, send current altitude, lat/lon, compass reading,
and time.
2. Every second after initialization, send current compass reading and
time.

Basically, this would allow me to know where someone "stopped" and then
when they were stopped, I could recreate the directions they were
looking in as a function of time.

Can I do this, or do handheld GPS units already allow me to do this
without any special programming of my own? If anyone might recommend a
particular unit that would be suited to an application like this, I
would be very greatful.

Thanks!

Sam Wormley

2005-11-15, 5:48 pm

philsonj@yahoo.com wrote:
> Can I program a handheld GPS unit to output specific data to a computer
> in realtime that I specify? For example, I want to have a handheld
> device that, on "initialization", will output stream the following data
> to a computer:
>
> 1. On initialization, send current altitude, lat/lon, compass reading,
> and time.
> 2. Every second after initialization, send current compass reading and
> time.
>
> Basically, this would allow me to know where someone "stopped" and then
> when they were stopped, I could recreate the directions they were
> looking in as a function of time.
>
> Can I do this, or do handheld GPS units already allow me to do this
> without any special programming of my own? If anyone might recommend a
> particular unit that would be suited to an application like this, I
> would be very greatful.
>
> Thanks!
>

Many GPS receivers allow user selectable NMEA sentence output which
can be captured and processed by computer software.

See: http://www.ucalgary.ca/~clarko/mapsoft/nmea.htm

philsonj@yahoo.com

2005-11-15, 5:48 pm

> Many GPS receivers allow user selectable NMEA sentence output which

> can be captured and processed by computer software.
>
> See: http://www.ucalgary.ca/~clarko/mapsoft/nmea.htm

Sounds like that is exactly what I need, however after reading your
reply, I went and read the specs and manuals for about 12 handheld
units and didn't get very far. While they all claim to support NMEA
output, none of them specifically say that the user can specificy the
NMEA output sentence and the frequency at which it is recorded or
dumped to a PC.

When you state that "many" GPS receivers allow user selectable NMEA
sentence output, are you referring to handhelds, or higher end units?
I have only been looking at $200 - $600 handhelds. Might anyone here
know of a handheld that has this feature?

Nospam@nowhere.com

2005-11-16, 5:48 pm

Apparently on date 15 Nov 2005 12:59:16 -0800, philsonj@yahoo.com said:

>
>Sounds like that is exactly what I need, however after reading your
>reply, I went and read the specs and manuals for about 12 handheld
>units and didn't get very far. While they all claim to support NMEA
>output, none of them specifically say that the user can specificy the
>NMEA output sentence and the frequency at which it is recorded or
>dumped to a PC.
>
>When you state that "many" GPS receivers allow user selectable NMEA
>sentence output, are you referring to handhelds, or higher end units?
>I have only been looking at $200 - $600 handhelds. Might anyone here
>know of a handheld that has this feature?

GPS NMEA output is fairly fixed, and is related directly to the NMEA0183
standard that came out in 1983, as opposed to the 0180 or 0182 standard. It's a
marine navigation aids thing rather than actually related to GPS.

So for instance, while a GPS unit puts out NMEA sentences that start "$GP..."
a Loran-C unit will work just the same but will have sentences that start
"$LC..."

To be compliant with the standard and therefore meet various criteria to be
certified for use with boats, etc, the device is supposed to send out NMEA
sentences in standard ascii over what will be to a PC an RS232 output (but is
actually a bit more sophisticated than that) at 4800 baud, 8, n, 1.

NMEA devices vary their rate of update to some extent, but this is rarely
faster than one update per second and the actual update rate of the satellites
in the GPS constellation also derives a one second update rate so there is no
real value in sending NMEA updates more often than one hertz, not to miss the
fact that sending out all the sentences per update at 4800 bits per second
means you wouldn't be able to fit two updates into the 600 bytes of data
available.

So it is, that pretty much all GPS that have an NMEA output, will be sending
out some set of NMEA sentences once a second, regardless of what else may be
happening. Most units that I've messed with can also let you select which of
the messages it has available, are actually sent.

You can sometimes vary the interface as well, e.g. NMEA requires 4800,8,n,1
serial data but a given unit may well allow you to up this to 9600 or maybe
even 57,600 baud. The unit would then be non-NMEA0183 compliant but since most
units are now used by non-sailors and the unit will revert back to compliance
on a full reset this isn't an issue.

You could then, in theory, update more often than once a second as you'd have
the bandwidth, but GPS doesn't give you an update any quicker so you're out of
luck there. I dunno what happens in Sirf, it seems to work more smoothly, but
hey.

The place to do your part of the filtering is the other end of this interface.
Dataloggers exist, these can be programmed in various ways, e.g. to store
messages less often, or discard certain types of message. Or just filter on the
laptop itself, e.g. when position is the same as last time, +/- value, ignore
the message, otherwise write it to the output file.

Each message has time and date stamp, so you can reconstruct things later on.

If you want some sentence descriptions, plenty on the web. Here's one
http://tinyurl.com/atqcc

Note that you can easily fabricate NMEA sentences. I made up a log showing some
outlandish speeds, just to wind someone up. It's only text, and can be edited
to show what you like. It'll fool most "experts" so long as you keep the
timebase the same as a real track and merely move the positions along
gradually. To avoid prosecution, invert the altitude so that hills become
valleys.

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