Re: Garmin GPS 76 questions


JF Mezei <jfmezei.spamnot@xxxxxxxxxxxx> writes:
I have been away for a few years, but my trusty GPS II+ has become

Welcome back. Just upgraded form my emap and popped in to find
recommendations. Luckily the choice was easy - garmin started selling
sirf-III based units so the choice boiled down to "do I want a big
flat version or a smaller rounded version of the same design."

However, while I have downloaded an updated copy of the garmin protocol
specification,

Good man.

You get to update the tracklog-point and waypoint structs in your
program. The pvt stays the same as its been since the 12xl days. The
usb version of the protocol is similar to the serial but adds a
different, longer preamble to the header. BTW. The usb transfer is
nice and fast. You'll like it. Instead of minutes of tracklog
download its only 15 seconds or so.

I have no idea of what the 76's capabilities are. I called Garmin,
but the drone only had access to the owners' manual so he couldn't
answer this. Is there any place on the net that lists the
capabilities (from the garmin protocol point of view) of various
models ? Seems to me that this is a very important metric when
choosing a GPS.

tracklog D302
waypoint D110
satellite_status 0x72 (documented only in the gps18 manual -
download that one too along with the usb specs)

Second question: the GPS II+ was built prior the the end of SA and
supposedly had some filtering. I assume that post SA units no loger have
such filtering ? I notice that the model 76 has a "Speed Filter" setting
that is used to average speed/direction of travel. Is it correct to
state that setting a value of 1 second would mean no filtering is being done?

I don't think you can (or would want to) ever turn all filtering off.
Even without SA the signal is darn noisy. My motorola oncore has a
raw-ish lat/lon/elevation data out and a filtered one. The unfiltered
one bounces around a lot.

(for instance, on a bike, if I want to calibratre my real bike computer,
I don't want the GPS to filter speeds out).

Speed and position calculations come from totally different sources.
Your speed comes from doppler (and a rough position solution). The
position calculation comes from working out the distances to the
satellites. The position solution is heavily affected by multipath
and bounces around as multipath sources come and go. If you want to
log speed, log the doppler speed that the gps provides in the pvt.

I am extremely disapointed to learn in the manual that the publicized
autonomy of 16 hours is only when the unit operates in battery saver
mode. (they don't say that in the store web sites). The manual talks of
a "significant" increase in battery life. What wouyld be the real life
battery life on the unit in normal mode with current alkaline batteries ?

The GPS II+ had somewhere between 20 and 24 hours of autonomy on a
single set of 4 betteries. If the new GPS can't take 12 hours, it would
mean having to change batteries in the middle of a long bike ride, which
is not as good as the same number of batteries giving to more autonomy
without having to turn off the unit to replace batteries.

The new gps has only 2 AA batteries. Scale up for the extra batteries
and you have the same number.

Since I am not a slave to Microsoft, I will not be able to perform
software upgrades on the Unit. Would a unit purchased today have a more
of less current software revision in it, or will it have the original
software rev from 2001 and expect me to upgrade it right away ?

I'm not a ms slave either, but I'm not about to run old buggy firmware
simply because some company isn't about to release the full interface
specs so I can download the firmware from a more secure OS.

I normally keep an old 2.5" laptop disk marked with a toxic warning
sticker and some version of bill's os on hand for just such an
emergency. I pull out the bsd disk, unplug the ethernet and stick the
toxic-OS disk in.

The unit you get will not have the correct firmware on it. You really
want to upgrade to 2.90 which allows 1GB memory cards.

This unit is WAAS capable. This is new stuff for me. Since WAAS isn't
transmitted in the almanach, how does the unit find out about which WAAS
satellites are available ? (sorry if this is in some FAQ, but I didn't
locate it easily).

It appears to do a sky search for any waas dsss polynomials. It took
a few hours for my gps to initially find the waas birds when I first
got it.

Trackpoints: the documentation states the unit can store 10 tracks. But
there is no mention of the max number of points in each track. (or is
it a total number of points for all tracks (eg: one very short and one
very long) ? Or is this dependant on the amount of memory left in the
unit ?

10,000 I think. At least its is over 8,000 which my unit just passed.

POI: I read in this group that some people may have been succesful in
uploading a different POI databased that came from a different unit
(being in canada and landbased, I have no use for marine POI in the
USA). I take it that such transfers are highly proprietary and
undocumented ?

I think POI's are mutually exclusive. The gps doesnt' really do
layers, it just has one layer of each type.

If you find anything out about POI's please post. I'd really like to
merge the Garmin ones with quite a few of my own. I've collected a
few thousand waypoints over the years since my first garmin g38 and it
would be nice to have access to all of them.

Since I'm in the SF Bay Area it would be great to put earthquake
epicenters and fault lines in. You can see many of the faults with
the naked eye if you know what to look for and where. Sort of like
geocaching, only using real geological features.

The Antenna on the GPS II+ was powered (aka: voltage applied to the
leads), and whenever one went thourgh a sunden thunderstorm with sharp
drop in termeparture, salty water from a road would be sucked into the
antenna due to air inside contracting due to cooling. That salty water
would conduct electricity and not only reduce antenna's efficeint, but
also rust/brun though the thin film of conducting metal and make the
antenna "broken" permanently. (fixed by resoldering a connection to
bridge the gap i the film and resealing the antenna).

The internal antenna on the unit is sealed. The optional external
antenna is mcx and is only sealed with a rubber cap when not in use.
When in use the connector must be protected from salt-water in some
other way. (Clear baggy with an RTV "grommet" sealing the hole the
external antenna goes out of?) In any case I haven't needed the
external antenna on my 60csx. You probably won't need to use it
unless you ride under very heavy tree cover.

-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/
.

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