Performance Testing of the Garmin eTrex, Garmin GPSIII Plus, Magellan GPS 2000XL, and Magellan Blazer Recreation Type Global Positioning System Receivers
By Santiago Mancebo and Ken Chamberlain
These tests were made to evaluate the accuracy of low-cost recreational GPS receiver performance under forest canopy at the Clackamas Test site and at the open sky control station Estacada. The following receivers were tested:
Tests were made to examine the following:
1) The accuracy of various receivers at the manufacturers defined waypoint logging rate. All receivers logged 1 position per waypoint, except Garmin GPS III Plus which allowed user defined waypoint logging times. Waypoints were logged at 1 position and 60 positions for this unit.
2) The efficiency of receiver performance. Since these receivers do not record data to memory efficiency was observed non-quantitatively by simply observing the receiver performance under the canopy for number of positions determined during the work.
No attempt was made to evaluate the ease of use, menu systems, or other user user-interface features.
These tests were made between June 21 and 22, 2000.
All tests were made with Selective Availability (SA) off.
All the receivers tested have 12 parallel channels.
· Magellan GPS 2000XL: This is a slightly larger unit which is waterproof, field ruggedized with wraparound rubber armoring, and scratch-proof display. The GPS 2000 XL receiver stores 200 waypoints. This unit offers NMEA data output for PC communication. The battery life is about 24 hours for 4 AA batteries. Weight is 10 ounces. Cost is approximately $150.
No testing of the NMEA 0183 output to personal computers was made.
Test Network and Survey Station Data:
Forested Site: The Clackamas Test Network is located in Western Oregon on the Mt. Hood National Forest. Clackamas GPS Test Network is a site for testing P and C/A Code (resource grade) GPS receiver performance under moderate to heavy western Oregon timber canopy. The vegetation at the site consists of Douglas Fir and Western Hemlock over story, these trees are approximately 24-40" d.b.h., with a Vine Maple and Red Alder under story. The terrain at the site is nearly flat with no terrain obstructions above 20 degrees. The test network is composed of twelve points with known geographic positions. These twelve points were established by a conventional Total Station closed traverse survey which was based on two GPS points adjacent to the site which had reasonable satellite horizon. These two points were established by static carrier phase GPS survey connected to the Oregon High Accuracy Reference Network (HARN).
Open Site: The station “Estacada” was used as a control site due to its clear-sky nature with no obstructions. Station “Estacada” is an Oregon High Accuracy Reference Network (HARN) Order A survey mark established by the National Geodetic Survey in 1998, PID-AI1994. The station is 487 KM, N 47 W of the Clackamas Test Network site.
Field Data Logging Procedures:
All GPS observations were made at approximate antenna height of 1.5 meters.
Estacada control point:
Office Data Processing Procedures:
The data analysis was made using Microsoft Office 2000 Excel spreadsheet application.
All accuracies report at 2DRMS (see appendix 1 for formulas). The 2DRMS value closely approximates the 95% error for a position. In other words, 1 out of 20 measurements will have errors larger than those published in this report.
All data was analyzed in the North American Datum of 1983 (NAD 83).
These tests were made under dry conditions.
Conclusions on Recreation GPS receivers:
The National Map Accuracy Standard (NMAS 1974) for point positions on 1:24,000 Scale Quadrangle maps is 12.2 meters at 90% or to put it in the same confidence interval as our study 14.8 meters at 95%. Therefore these receivers when used in open sites meet national map accuracy standards. When operating under tree canopy these receivers do not meet national map accuracy standards.
Appendix A: Accuracy computation formulas used in this paper.
Appendix B : Receiver photos: (not to scale)