[aprssig] GPS question

[Dave B]

Andrew (and others)

There are some very good generic GPS surveying programs on t'interweb.  
"Visual GPS" being one of the better ones, and free too.

It's able to build you a display of your sky visibility, also showing the 
satellite tracks that pass by.   Leave it running for a few hours to 
collect data.

73.

Dave TR/G0WBX.


On 9 Dec 2012 at 13:45, Andrew Rich wrote:

> 
> I have a survey gps on loan will try
> 
> Sent from my iPhone
> Andrew Rich
> 
> On 09/12/2012, at 12:55 PM, Greg D <ko6th.greg at gmail.com> wrote:
> 
>     Hi Steven,
> 
>     Stephen H. Smith wrote: 
>     On 12/8/2012 4:26 PM, Andrew Rich wrote:
>     I am not sure I have asked this before but I will ask it again.
> 
>     I have a GPS on the side of a building and it can only see half
>     the sky - to the north
> 
>     What is curious - is that when plotted on google earth - the
>     postion is north of where it should be 
> 
>     I will move it into full view of the sky and see what that does 
> 
>     - Andrew - 
> 
> 
>     A gps receiver should be able to function normally WITHOUT 
>     seeing the entire sky; they do this all the time on car 
>     dashboards. 
> 
>     Most likely you either have the unit set for a datum other 
>     than WGS84 (the GLOBAL grid system created by and for GPS). 
> 
>     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. 
> 
> 
>     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... 
> 
>     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. 
> 
>     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 bias 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.
> 
>     Greg KO6TH
>