Glenn Baddeley - My Experiences With GPS

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Introduction

I've been interested in GPS technology and its practical uses for more than ten years. I first saw a GPS in action at Buchan in 1992. It was a Trimble Transpack II, one of the original portable receivers, and was the size of a large hard-back book. It depleted a set of four alkaline D-cells with just a few hours of use.

I also witnessed a Garmin GPS45 being used on a Nullarbor Plain caving expedition in 1994, where we recorded the coordinates of landscape features and used it to navigate directly across arid and desert country to isolated and hard to find locations.

It wasn't until September 1998 that I actually purchased a second hand Garmin GPS38. I gained practical experience with that for a couple of years, mainly for recording cave entrance location coordinates and navigating through the dense Australian bush. The GPS38 was first released onto the market in February 1996, so it is quite an old unit. It only has a single channel receiver so it is very slow (3 - 5 minutes) to get a 3D position fix after turning it on.

Interface cables and NMEA data analysis

I have electronics as a hobby and I quickly obtained the plug connector for the power / data output / data input port on the back of the GPS38 and constructed two sets of PC serial interface cables and regulated power supplies for both the car and a mains plug-pack.

I am also a computer programmer. The first effort at writing a GPS program was a versatile NMEA sentence data logging and graphical display program for MS-DOS which was run on an old Toshiba laptop PC. A data analysis program was also written in Visual Basic for Windows 95. I used this to do some investigation of position averaging under varying conditions, and "poor mans" Differential GPS.

'Exploring GPS' book

I purchased the book Exploring GPS - A GPS Users Guide, published by GPSCO in February 1998. It is a very good introduction to all aspects of GPS and its practical use and is especially easy to read. Many illustrative cartoons are included in the 106 glossy B&W pages, which makes it even more appealing. GPSCO appears to be part of the Surveyor General's Department of New South Wales, in Australia. Contact me for details.

Updating from GPS 38 to GPS etrex yellow

In February 2001 I sold the GPS38 to someone who was just starting to learn about GPS, and purchased a second hand Garmin etrex. It was the model with the Yellow case, which has the most basic features of the etrex range of receivers. I didn't really need the fancy barometer, flux-gate compass and enhanced mapping which are in the other models in the etrex range. It was nice to now have a 12 channel receiver.

I updated the firmware from version 2.8 to 2.10 by downloading an IBM-PC program from the Garmin web site and running it over the serial interface cable to the receiver. The main new feature in version 2.10 is the satellite geometry and signal strength page. This is invaluable for ensuring the receiver is in a position to pick up the best signal and obtain the best position accuracy. Knowing where the satellites are in the sky means that obstructions like trees and buildings can be avoided by moving just a couple of metres.

Feel free to discuss GPS with me if you wish.

Glenn
Melbourne, Australia.

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Copyright © Glenn Baddeley 2005
http://home.pacific.net.au/~gnb/gps/mystuff.html was last updated 4 September 2005.
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