Meridian GPS vs Pharos GPS-360

Comparison of GPS receivers is a 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 report some results on real experiments with two different receivers taken at the SAME conditions.

The GPSRs

 
Fig.1a MeridianGPS and Explorist500
picture taken with flash (bright illumination similar to sunny day conditions); backlight of both units at maximum.
 
Fig.1b MeridianGPS and Explorist500
picture taken with the exposure set to the Explorist500 screen (similar to dark conditions); backlight of both units at maximum.

Fig. 1a and Fig. 1b show Meridian GPS and Explorist 500 receivers.It is difficult to compare screens of both unit, to have an idea I took 2 pictures, with flash illumination and without flash illumination. Backlight of both units was set to maximum, without flash we can see that brightness of the Explorist500's screen much higher than Merigreen's screen. Right (dark) picture was taken under 100 Wt lamp illumination (ceiling lamp). Screen of Explorist 500 is similar to vivid screens of cell phones or PDAs.

Table 1 shows basic technical specification of the receivers under study: Magellan Meridian GPS (Merigreen) (b/w mapping receiver) and Magellan Explorist 500 (color mapping receiver)

Parameter
Magellan Meridian GPS (Merigreen)
Explorist 500
Number of satellites
12
14
WAAS enabled
yes
yes
Antenna
Helix
Patch
Interface (note 1)
RS232 serial

a) USB 1.0
b) virtual modem (virtual serial)
c) true RS232 serial

Periodicity of NMEA
1 sec
1 sec
Screen
4 gray levels (120x160)
16 colors (176x220)
Battery
2xAA
LiIon rechargable, 1300 mAh @ 3.7V
What is inside

Note 1. a) USB mode for file transfer to/from the internal memory or SD card, appears as an USB mass storage device when this mode is selected from the unit; b) when NMEA mode is selected, a virtual modem appears, standard serial output (NMEA) can be grabbed via associated serial port (as I did in these test with data logger designed by myself). Here are screenshoots of the modes a) and b). Mode c) is unspecified by manufacturer; it can be called from the hidden menu and is needed for system restore/analysis (the mode c) information is courtesy of Rhamphorinkx (http://rhamphorinkx.newmail.ru), see his messages on 07/15/05 in the mobilemapper2 yahoo group).

.

Experimental

2 types of test were performed:

1) position is fixed, relatively strong signal (outdoor)
2) road test in the urban area (bridges, overpasses, 4-20 store buildings), receivers were placed at the center of passenger seat.

In all tests receivers were put close to each other, so the conditions could be considered as the same. Receivers were connected to Laptop PC computer (Merigreen with RS232-USB converter) and Explorist 500 with USB connector (virtual modem b) mode, see Table 1). Raw logs of NMEA messages were recorded SIMULTANIOUSLY for both receivers and then analyzed with specially designed by myself 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 of the NMEA: 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.

Both Magellan Meridian GPS and Explorist 500 receivers sometimes generate the same messages with the same time fix (5-6 messages every 60-120 sec while some idle processes). These messages were eliminated from the analysis. All generated by Merigreen and Explorist 500 NMEA messages are valid (no messages appear if number of satellites<3); no NMEA messages with fixes indicated as non valid).

Strong signal test

For deviation measurements WGS84 Lat/Long data were reprojected to the 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 the coordinates.

The test was done outdoor at the relatively clear sky view to the South. Fig. 2 shows 40 minute plot of the deviation and number of satellites in use. Explorist 500 data are shown in red; Meridian GPS data are shown in blue.

Fig 2. Deviation of coordinate and number of satellites in use.

Time to get the first fix is about the same for both receivers. Fig 3 shows the same data plotted as XY:

Fig 3. XY plot of deviation of coordinate (left plot is for Explorist 500 and the right plot is for Meridian GPS. Big deviation on Meridian plot (seen as a dotted curve) is a start-up process and must be excluded from our consideration.

Both receivers show the same average coordinate . Interesting, that accuracy of both GPSRs is the same in Y-direction, while Meridian GPS shows better accuracy in X-direction. I read several times that patch antenna has different sensitivity when horizontally or vertically oriented. I did not notice this effect in my measurements, number of satellites in use was always the same in horizontal and vertical positions.

Road test

This test indicates the real performance of both receivers for the car moving in the urban area (35 min trip time). Coordinates from the log were converted to Microsoft Streets&Trips data format and are shown by red (Explorist 500) and blue (Meridian GPS) circles on the map.

Fig. 4 Map with Explorist 500 fixes (red circles), and Meridian GPS fixes (blue circles). A,B,C and D indicate areas discussed below.

Meridian GPS and Explorist 500 show reasonable behavior, if signal is lost there were no valid fixes indication (in contrast to Etrex and Pharos, see reports on this site), The fixes from both receivers more less correspond to the actual path.

 
Fig. 5a Magnified A area   Fig. 5b Magnified B area

Fig. 5a and Fig 5b show magnified areas indicated as A and B in Fig.4. Actual path corresponds to the fixes of Meridian GPS (blue circles). Explorist 500 sometimes (when signal is weak and there are fast turns) shows shifted from the actual path fixes. The maximum shift observed (area A) was about 40m. Interesting that as it was in the previous test, displacement was observed in only X (East-West) direction (to make it clear notes in Fig 5b show fixes taken at the same moment).

Fig 5c. Area C. Relatively weak signal (sky is blocked by buildings). Magellan GPS shows slightly better performance. Here we can see also that both receivers show real fixes without any interpolation (some other receivers (at least Garmin Etrex and Pharos-360 tested by me) do some interpolation when signal is lost; this leads to uncertain real position. Better do not have fixes at all than some "generated" fixes.

Fig 5d. This D area shows that sensitivity of Magellan GPS and Explorist 500 is about the same order. Movement is from bottom to top. We can see that Explorist (red circles) loses signal for a while but Magellan GPS (blue circles) does not; in some time the situation is opposite: Explorist show fixes, but Meridian GPS does not.

Table 2 summarize some statistics during the trip:

 

Parameter Magellan Meridian GPS (Merigreen) Explorist 500
Satellites (% of time)
WAAS corrected (as indicated in the NMEA), % of totall
100%
0%
Valid fixes (as in theNMEA), % of the trip time
86%
80%

 

The WAAS

In contrast to Meridain GPS Explorist includes WAAS satellite(s?) to the total number of tracking sattelites seen on the first screen (and even shows green bar for it (them?)); I dont know if they are included to the number of sattelites reported by NMEA. WAAS correction reported in NMEA is not clear to me. Meridian GPS always indicates WAAS correction in all fixes; Explorist 500 show no WAAS corrected fixes at all (all the trip test). On the other hand WAAS satellites were indicated on the Explorist 500 screen (with green "W"). In the strong signal test (no movement) Meridian Green indicated 100% WAAS, Explorist 500 had no WAAS for the first ~20 minutes (half of the test) and then WAAS indication for the rest of this test (another 20 min). This probably was a reason of the shift of the average coordinate at the half of the strong signal test (see Fig. 3). May be relatively weak WAAS sensitivity of the Explorist 500 is also the reason of the shifts in the road test. I am not familiar with WAAS principles, but it seems WAAS corrects relatively long-term deviations of satellites orbits, so it should affect average values (tens of seconds averaging) but not the instant ones. Exactly what did we see in our experiments. (This is not for sure, more experiments is needed to make clear Magellan/Explorist WAAS correction).

Summary

1) Both receivers show about the same time to get the first fix.
2) Both Magellan Meridian GPS and Explorist 500 show quite reasonable data during navigation in the urban area. No indications of any software interpolation was found (like it was for Garmin Etrex and Pharos-360)
3) WAAS correction is ambiguous (not clear to me yet). On the one hand, 100% WAAS correction is very doubtful in the case of Meridian GPS. On the other hand weak WAAS sensitivity (as indicated by Explorist 500 while all test) can explain shifts in coordinates shown by Explorist 500 in some areas.
4) Explorist 500 (patch antenna) shows about the same sensitivity in either horizontal or vertical positions.
4) Sensitivity of both receivers is about the same for both receivers. Magellan GPS overperform Explorst 500 but not significantly.
5) Accuracy of measurements of coordinates is better for Meridian GPS in all tests. The long-term difference appears as a shift in horizontal (East-West) direction for Explorist 500. Up to 40 m shift from the actual path was found for Explorist 500 (in "bad" areas only, most of the time Explorist 500 is quite accurate).
6) Size, weight, screen, connection speed is much better for Explorist 500.

In spite of slightly better performance of the old Meridian GPS, Explorist 500 is outstanding device compatible (by accuracy and sensitivity) with the Meridian GPS. I do belive that the above results can be extended to all Meridian and Explorist mapping series, because the receiving electronics stays the same within each of the series.