Meridian GPS vs Garmin Etrex
Comparison of GPS receivers is a very difficult task. Specification shown by manufacturers reflects performance at some ideal not specified conditions. Often customers are screwed up by advertising campaign and some "reviews" of one selected receiver. On this page I show some results on real experiments with two different receivers taken at the SAME conditions.
Table 1 shows basic technical specification of the receivers under study: Magellan Meridian GPS (Merigreen) (mapping receiver) and Garmin Etrex (non mapping receiver).
. Both receivers were set to 4800 8N1 NMEA output (Garmin Etrix has only this setting).
3 types of test were performed:
1) position is fixed, relatively weak signal (indoor)
In all tests receivers were put close to each other, so the conditions could be considered as the same. Receivers were connected to 2 USB ports of Laptop PC computer with RS232-USB converters. Raw logs of NMEA messages were recorded SIMULTANIOUSLY for both receivers and then analyzed with specially designed software. $GPGGA messages were used for analysis. Format of the $GPGGA message:
$GPGGA,hhmmss.ss,llll.ll,a,yyyyy.yy,a,x,xx,x.x,x.x,M,x.x,M,x.x,xxxx*hh 1 = UTC of Position 2 = Latitude 3 = N or S 4 = Longitude 5 = E or W 6 = GPS quality indicator (0=invalid; 1=GPS fix; 2=Diff. GPS fix) 7 = Number of satellites in use [not those in view] 8 = Horizontal dilution of position 9 = Antenna altitude above/below mean sea level (geoid) 10 = Meters (Antenna height unit) 11 = Geoidal separation (Diff. between WGS-84 earth ellipsoid and mean sea level. -=geoid is below WGS-84 ellipsoid) 12 = Meters (Units of geoidal separation) 13 = Age in seconds since last update from diff. reference station 14 = Diff. reference station ID# 15 = Checksum
In the experiment I used the following parameters: a) time 2) latitude 3 )longitude 4) GPS quality (0-invalid, 1-valid, 2- waas corrected 5) number of satellites in use. 11,12,13,14 are always blank for Magellan GPS.
Magellan Meridian GPS receiver sometimes generates the same messages with the same time fix (5-6 messages every 60-120 sec while some idle processes). These messages were eliminated from consideration. Messages with GPS quality=0 (invalid fixes) were eliminated (generated by Garmin Etrex, Magellan Meridian GPS does not produce $GPGGA messages with quality=0). Messages with number of satellites =0 were eliminated (Garmin Etrex, Magellan Meridian GPS never produces such messages). Both receivers do not generate messages with valid fixes if number of satellites <3.
Weak signal test
For deviation measurements WGS84 Lat/Long data were reprojected to UTM metric scale, the deviation was calculated as sqrt((X-Xm)^2+(Y-Ym)^2), where X and Y are UTM coordinates (in meters) and Xm and Ym are the mean values of coordinates.
Fig 1 shows deviation versus time together with number of satellites in use (red color is for Garnin Etrex and blue color is for Magellan Meridian GPS). Both receivers showed not WAAS corrected data in this test (Garmin Etrex does not have WAAS support).
Fig. 1 Deviation of coordinate versus time and number of satellites in use. Red line and circles are for Garmin Etrex and blue line and circles are for Meridian GPS.
Deviation of coordinate is about the same for both receivers, time to get the first fix is slightly shorter for Garmin Etrex (~40 sec and ~1:45, respectively). Fig. 2 shows the same data on XY plot.
Fig 2. XY plot of deviation for Garmin Etrex (left image) and Magellan Meridian GPS (right image)
Strong signal test
The test was done outdoor at relatively clear sky view to the South. Fig. 3 shows deviation of coordinate and number of satellites in use for Garmin Etrex and Meridian GPS, the top plot and the bottom plot, respectively. Garmin Etrex indicated WAAS off all the time, Meridian GPS indicated no WAAS for the first 2 min, then WAAS was on. From ~15:00 to ~18:00 receivers were shielded to weaken the signal.
Fig 3. Deviation of coordinate and number of satellites in use. Top image is for Garmin Etrex and the bottom one is for Magellan Meridian GPS.
When signal is strong time to get the first fix is about the same for both receivers (~46 sec for Garmin Etrex and ~16 sec for Meridian GPS)). Similar to the weak signal test, the coordinate deviation is about the same for both receivers, Meridian GPS indicated big deviation when shielded, after the shield was removed the coordinate returned to the previous values. Fig 4 shows the same data plotted as XY:
Fig 4. XY plot of deviation of coordinate (left plot is for Pharos GPS-360 and the right plot is for Meridian GPS.
This test indicates the real performance of both receivers as a navigation devices for the car moving in the urban area. Coordinates from the log were converted to Microsoft Streets&Trips data format; and are shown by red (Garmin Etrex) and blue (Meridian GPS) circles on the map. Actual path was similar to shown by Meridian GPS. The result is shown in Fig. 5.
Fig. 5 Map with Garmin Etrex (red circles) , and Meridian GPS fixes (blue circles). A indicates area of weak signal shown in detail in the next figure.
Meridian GPS shows almost perfect correspondence between coordinates and the actual path. Garmin Etrex shows much worse results: for a long period of time there were no valid fixes (~12 min) (but NMEA messages were genereated, so connection with PC was not lost). Accuracy of coordinates is also not good. Similar to Pharos GPS-360, firmware of Garmin Etrex produces approximated by streight line data. To my opinion better do not have fixes at all, than some imaginated by firmware data. See Fig. 6 for details.
Fig. 6 Zoomed A area. When signal is weak Garmin Etrex is aproximating data by streight line. Also less sensitivity of Garmin Etrex compared to Meridian GPS is clearly seen in this figure.
Fig 7 shows quality of fixes (if valid) and number of satellites during the trip
Fig. 7. Valid fixes quality and number of satellites during the trip (Note, Garmin Etrex does not have WAAS correction).
Table 2 summarize some statistics during the trip:
1) Meridian GPS and Garmin Etrex show similar time to get the first fix (in some experiments Etrex is faster, in the others Meridian is faster)
My choice (of tested here receivers) without any doubts is Magellan Meridian GPS.