William asks…
What comes out of a GPS receiver?
I’m interested in what information is produced by a standard GPS receiver (and possibly what NMEA standards are and how are they interpretted?). I’d like to create my own GPS software and wondered if the purpose of a GPS receiver is to tell a computer (with software that interprets this info) where the GPS unit is (possibly Longtitude and Latitude). Can anyone help or answer my question? I’m looking more toward gettin an answer on ‘How do I get a location from a standard GPS module?’ rather than how GPS works (triangulation, etc I understand). Cheers!
admin answers:
If you have a module that connects to a PC, it will probably send a continuous stream of more or less incomprehensible “stuff”. With luck it will be in the NMEA format, considerable information is available on the web, a few sites I had bookmarked are in “sources” below. I haven’t actually done much with this since I use the Microsoft software, but i was curious about it, so i did a little looking.
I have a Pharos unit that came with Microsoft Streets and Trips™ that sends something like this:
first four lines are information about the Satellites that are in view
$GPGSA,A,3,21,22,18,03,26,,,,,,,,2.9,1.8,2.2*3D
$GPGSV,3,1,10,22,74,262,41,18,63,058,38,03,46,269,33,21,36,114,37*7A
$GPGSV,3,2,10,19,32,307,17,14,22,189,27,09,15,090,,15,11,111,*73
$GPGSV,3,3,10,26,07,034,32,24,03,114,*7C
next is the “Recommended Minimum Specific GPS/TRANSIT Data”
$GPRMC,115913.000,A,4744.0527,N,12438.1005,W,0.03,140.67,040807,,,A*74
and last is “Global Positioning System Fix Data”
$GPGGA,115914.000,4744.0527,N,12438.1005,W,1,05,1.8,16.3,M,-17.5,M,,0000*5B
note that Yahoo clips long lines so there is stuff missing, but this is just for example.
Steven asks…
How can we perform Differential GPS technique in simple way?
Differential GPS is useful to reduce errors when taking coordinates readings of a point on earth. in fact, and due to many reasons when we take latitude and logitude of a point using our standard GPS receiver , the readings will be shown somewhere els !
There is so many ways to reduce errors
one is and less coastly , using two receivers and set one as a base station … the question is HOW ??
Appreciate everyone who tried and simplified it here for me
Thanks in advance
admin answers:
You have received two excellent answers so far. But I think you are still wondering how to do a differential land survey, right? Can you get hold of two GPS and two laptop PCs? It is then just a matter of registering the two positions and time sets and compare them later. I don’t know your budget for this project but you could e.g. Buy cheap PDA with GPS incorportated. You’ll find cheap ones like Mio and Acer. You’ll most probably find also freeware or shareware that reads the NMEA port of the GPS and creates a file with position and time. Then you might want to write your own application that reads both files, compare position for the same time and calculate the offset in whatever unit you want. Most probably when working at this accuracy, your geographical position is not important, only the relative one from one GPS to the other. But if you have a map reference, you can trace it from that. As a warning, don’t use GWS84 references on an old map. I know from experience that e.g. Nautical charts may be very relatively accurate but inaccurate to a common world reference such as GWS84.
Linda asks…
Do Standard GPS Navigations show latitude and longitude coordinates?
I work for a RA, and sometimes customer have no idea where they are located, even with a GPS.
Example, if a customer is on the interstate, broken down (they have GPS) and they can’t pinpoint where they are at (ie; mile marker, exit numbers, etc) I was wondering if there were latitude & longitude coordinates on their GPS navigation tool.
admin answers:
On my Garmin Nuvi (and on most Garmin units) the Satellite information shows latitude and longitude along with satellite signal strength and altitude.
When the unit is turned on and tracking, there are signal strength bars on the top left (Nuvi units, at least). Touching the bars gets you to the Satellite Information Page.
I would suspect most GPS units have some similar way to display coordinates.
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