<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>Anyone ever built a gps logger ? </div><div><br></div><div>I am writing to an sd card every 5 mins</div><div><br></div><div>Having some read write issues</div><div><br></div><div>Went up to a class 10 micro sd</div><div><br></div><div><br><br>Sent from my iPhone<div>Andrew Rich</div></div><div><br>On 09/12/2012, at 1:45 PM, Andrew Rich <<a href="mailto:vk4tec@tech-software.net">vk4tec@tech-software.net</a>> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="content-type" content="text/html; charset=utf-8"><div>I have a survey gps on loan will try <br><br>Sent from my iPhone<div>Andrew Rich</div></div><div><br>On 09/12/2012, at 12:55 PM, Greg D <<a href="mailto:ko6th.greg@gmail.com">ko6th.greg@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div>
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Hi Steven,<br>
<br>
Stephen H. Smith wrote:
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<div class="moz-cite-prefix">On 12/8/2012 4:26 PM, Andrew Rich
wrote:<br>
</div>
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<div><font face="Arial" size="2">I am not sure I have asked this
before but I will ask it again.</font></div>
<div> </div>
<div><font face="Arial" size="2">I have a GPS on the side of a
building and it can only see half the sky - to the north</font></div>
<div> </div>
<div><font face="Arial" size="2">What is curious - is that when
plotted on google earth - the postion is north of where it
should be </font></div>
<div> </div>
<div><font face="Arial" size="2">I will move it into full view
of the sky and see what that does </font></div>
<div> </div>
<div><font face="Arial" size="2">- Andrew - </font></div>
<br>
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<br>
A gps receiver should be able to function normally WITHOUT seeing
the entire sky; they do this all the time on car dashboards. <br>
<br>
Most likely you either have the unit set for a datum other than
WGS84 (the GLOBAL grid system created by and for GPS). <br>
<br>
Or the device is being confused by reflections from nearby
structures. Since GPS works by timing the difference in time of
arrival of signals from multiple satellites, multipath reflections
that add to path length (and thereby delay the time of arrival of
signals) will translate directly into a shifted position. In
downtown large-city "urban canyons" of glass-faced high-rise
buildings that bounce the 1575 MHz GPS signals every which way,
it's not uncommon to see GPS "go nuts" and jump back and forth
several blocks. <br>
<br>
</blockquote>
<br>
While I agree with everything you said, one experience I had
suggests that the particular constellation of satellites in view
will make a difference in your apparent position... <br>
<br>
I was a passenger on a small corporate plane, with my Garmin GPS-III
sitting in hand by a side window (half of the sky visible). As we
made a 180 degree turn to line up with the runway, my apparent
position suddenly shifted some distance directly sideways - maybe a
quarter of a mile or so. I chalked it up to the GPS seeing a
different set of satellites, and coming to a slightly different
conclusion at where I was. <br>
<br>
There's always some uncertainty in your reported position, and in
theory the real position should be inside of that uncertainty
circle. What I don't know is if there is a <i>bias</i> as to where
in the uncertainty circle you will be reported to be, given a
particular view of the sky. This is two data points now that
perhaps there is.<br>
<br>
Greg KO6TH<br>
<br>
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