[Cdt-l] GPS 101

bcss at bresnan.net bcss at bresnan.net
Sun Apr 14 11:19:59 CDT 2013

GPS satellites send out radio signals which travel at speeds approaching the speed of light. The distance from the GPS to a satellite is calculated by measuring the amount of  time it takes for a signal to travel the 12,000+ miles to earth and multiplying that times the speed of light. By determining distances to multiple satellites in view,  the relative position of the GPS on the ground can thereby be determined.   The accuracy of that position is affected by atmospheric drag, weather events, etc. which all tend to slow down the signals.  The atmospheric effects are overcome in the professional GPS world by having another GPS running nearby and stationary at a known location.   The positional errors caused by the atmosphere can be determined in this manner and applied to the GPS which is roving around, making that position more accurate.  


The quality of the GPS hardware affects the results.  Industry grade units have more precise clocks and bigger, better antennas which improve accuracy. The units used in the Mapping project are rated sub-meter, meaning that in a perfect world you can turn one on and locate yourself to that precision. 


Most of the newer GPS receivers have a feature called WAAS. (Wide Area Augmentation System)  This was developed for air traffic, but has great value for all users.  WAAS enabled GPS receivers are receiving correction data from a nationwide network of continuously operating ground stations via broadcasts from other satellites.  It works very well.    


GPS receivers (like the Etrex 20 and 30) also can be configured to use GLONASS.  GLONASS is the Russian GPS system.  By using both US and Russian satellites simultaneously it is possible to get results in places where GPS didn’t used to work at all.  (Like rough terrain and under tree cover)  Unfortunately, the Etrex units have a tiny antenna and can’t be hooked to a external antenna so they aren’t as good as they could be.  


Sage - Recreational GPS receivers are generally not of high enough quality to define better than a 5-10 meter sized spot.  Getting a 5 meter disagreement between two such receivers is not unusual, especially if they are not the same exact make and model.  Differences in the clocks and antennas might explain the discrepancy.   Having WAAS turned on for one and not the other could account for differences.  An external antenna which is mounted above head height is preferable – the head and body of the user can block multiple satellites.  

I am with you in preferring UTM to geographic (lat/long) coordinates because of the ease of locating a point or finding yourself on a map.  The native language of GPS is WGS84 geographic, however.  When a GPS displays a UTM position it is converting it via software. NAD83 is the United State’s localized implementation of WGS84 and the two currently differ by about a meter.  Mixing those up would not be as critical as if you did it with NAD27.  Most of the USGS topo maps are NAD27 because they were produced prior to 1984.      


best wishes,


Jerry Brown

mailto:bcss at bresnan.net



From: cdt-l-bounces at backcountry.net [mailto:cdt-l-bounces at backcountry.net] On Behalf Of Sage Clegg
Sent: Saturday, April 13, 2013 8:15 PM
To: cdt-l at backcountry.net
Subject: Re: [Cdt-l] Cdt-l Digest, Vol 67, Issue 15


Howdy folks!

     I just wanted to throw out a big thanks to all of you GPS Geeks (I mean that in the most endearing way possible) for finally explaining why a datum setting in my GPS might matter. I never hike with a GPS (I will be this summer on the Oregon Desert Trail though..), but I take data on my unit everyday as a tortoise biologist. I always wondered what the consequence was if I was using a different datum than someone else, now I know! 100' is a big deal when I'm collecting data!

      Another GPS issue I have been encountering lately is that my gps is consistently about 5 meters off of my co-worker's. We are both using the same datum (NAD83). We have changed batteries, turned them off and on, I even cursed mine and then ignored it (it sometimes works, I swear!) Any thoughts on how to "re-calibrate" a GPS? Might it be broken?

     Another question I have is about UTM's vs Lat/Long. As a biologist I have always used UTM's, but the current project I am on is using Lat/Long. Maybe it's just because I don't like dealing with all the extra digits of lat/long, but I haven't warmed up to using them. Do any of you use lat/long? Why? Does anyone have any tips for a simple way to make the switch from my UTM focused universe to a jumble of degrees, minutes, seconds, and decimal places?

       Thanks, and I hope the wind storms that keep spitting dust in my eyes here in the Mojave aren't slamming those NOBO hikers right now. If they are I highly recommend getting some ski goggles!!!

         Happy trails!




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