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Old March 19th 17, 06:29 PM posted to rec.bicycles.tech
Jeff Liebermann
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On Sun, 19 Mar 2017 08:34:51 -0700, Joerg
wrote:
Miles is bad. That means a position indication is useless for any
serious trail riding.


Not exactly. It depends on what you're doing. If you try to plot
your ride, you might get some screwed up data points mixed in with the
data. For example, my last hike into the bottom of a local canyon
yielded a maximum altitude of 3,000 feet higher than the ground. There
was only one or two bad data points, but it was enough to screw up all
the statistics. Same with maximum speeds traveled, where the distance
covered between a real position indication and a bogus point or two is
high enough for me to claim breaking the sound barrier.

Mapping software authors know about all this and do their best to
compensate. The most common and best method is to do a sanity check
on all positions. If the GPS suddenly claims you've instantly moved
many miles, that point gets dropped. You probably won't see garbage
data on your smartphone or mapping GPS because of this feature. You
will see garbage if you use raw NMEA-183 data in some application. If
you want to see if you have a potential problem, just connect a data
logger to the GPS and collect some $GPGLL sentences. Write a program
that looks for large changes in adjacent sentences. The glitches, if
present, should be drastic and obvious.

Somewhere in my mess is a Windoze program that takes this data and
provide both graphical and tabular accuracy statistics. I used to use
it when we had to deal with selective availability. It's kinda neat.
You park yourself in a highly reflective location (bottom of a rock
canyon) and record about 30 mins of position data. Position
excursions on the display are obvious. I use it for averaging
readings over a long period in order to obtain better accuracy.

Are at least the maps and the satellite view as
good as on a PC? As long as it buffers enough before going off-grid that
would help because I can fix my position pretty well via the use of
landmarks. Good old triangulation.


I'm not sure. Everything depends on the antenna sensitivity and
bandwidth. There's a huge difference in performance between an
antenna that uses a choke ring to reduce ground reflections as on
survey receivers:
https://en.wikipedia.org/wiki/Choke_ring_antenna
and a smartphone that uses a tiny ceramic patch antenna. These
articles cover the problem quite nicely:
https://www.u-blox.com/sites/default/files/products/documents/GPS-Antenna_AppNote_%28GPS-X-08014%29.pdf?utm_source=en%2Fimages%2Fdownloads%2F Product_Docs%2FGPS_Antennas_ApplicationNote%28GPS-X-08014%29.pdf
http://www.taoglas.com/wp-content/uploads/2015/07/Internal-GPS-Active-Patch-AntennaAPN-13-8-002.B.pdf
Hmmm... probably more than you want to know. Suffice to say that the
smaller the antenna, the narrower the usable bandwidth. This is
important because small antennas work very badly with WAAS and barely
can handle the L1/L2 bands. Gain also suffers. From the Taoglas
article:
Typical peak gain for GPS patch antennas on standardized
ground planes are as follows:
25mm Patch 5 dBi
18mm Patch 2 dBi
15mm Patch 1 dBi
12mm Patch 0.5 dBi
10mm Patch -2 dBi
By comparison to what's found in a smartphone, these patch antennas
are HUGE. I can't seem to find the smartphone GPS antenna vendor, but
as I vaguely recall, the typical gain was about -8dBi with a rather
narrow view of the sky.

Anyway, back to your question... If you look at the antenna, and
assume that the receivers are all rather similar, your performance
will be almost totally dependent on the GPS antenna.

With me that's a problem because my favorite routes are off-road. I try
to avoid raods whenever possible for many reasons. Like the one
yesterday where a driver came very close and leaned on the horn in an
attempt to push me from the lane to the side. No danger because he had
slowed down to my pace but such low-lifes with a drivers license are
annoying. And dangerous, especially when they are soused or nowadays
high on whatever.


Well, you could weaponize your bicycle to act as a deterrent.
https://www.google.com/search?q=bicycle+gun&tbm=isch

I wore out the BB on my road bike. Again :-(
But it was 40 miles of fun (except for the road part)


Sigh. At least you wore it out and didn't break it. Out of
curiosity, what wore out? Bearings? Raceway? Seals? Mechanical
doping motor?


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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