Meridian GPS vs Pharos GPS-360

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.

The GPSRs

Table 1 shows basic technical specification of the receivers under study: Magellan Meridian GPS (Merigreen) (mapping receiver) and Microsoft branded Pharos GPS-360 sensor which is the part of Microsoft Streets&Trips 2005 GPS bundle. The Pharos GPS-360 is SIRFII powered, this chipset is one of the most popular chipsets for sensors, smartphones, PDAs, GPS mapping receiver etc. This chipset is positioned as one of the most sensitive and fast chipsets

Parameter Magellan Meridian GPS (Merigreen) Pharos-360 (Microsoft branded)
Number of satellites
12
12
WAAS enabled
yes
yes
Antenna
Helix
Patch
Interface
serial
serial (with USB converter)
Periodicity of NMEA
1 sec
1 sec

. Both receivers were set to 4800 8N1 NMEA output.

Experimental

3 types of test were performed:

1) position is fixed, relatively weak signal (indoor)
2) position is fixed, relatively strong signal (outdoor, clear view to the South)
3) road test in 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 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 Pharos GPS-360 only, Magellan Meridian GPS does not produce $GPGGA messages with quality=0). Messages with number of satellites =0 were eliminated (Pharos GPS-360, Magellan Meridian GPS never produces such messages). Note, Meridian GPS does not generate messages if number of satellites <3, while GPS-360 does.

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 Pharos GPS-360 and blue color is for Magellan Meridian GPS). Both receivers showed not WAAS corrected data.

Fig. 1 Deviation of coordinate versus time and number of satellites in use. Red line and circles are used for Pharos GPS-360 and blue line and circles are for Meridian GPS.

Deviation of coordinate is much smaller for Meridian GPS, but time to get the first fix is significantly shorter for Pharos GPS-360 (~1:50 and ~5:40 respectively). Fig. 2 shows the same data on XY plot.

Fig 2. XY plot of deviation for Pharos GPS-360 (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 Pharos GPS-360 and Meridian GPS, the top plot and the bottom plot, respectively. Green color indicates WAAS off, magenta color indicates WAAS on (as given by quality parameter of $GPGGA message). From ~12:00 to ~15:00 receivers were shielded to weaken the signal.

Fig 3. Deviation of coordinate and number of satellites in use. Top image is for Pharos GPS-360 and the bottom one is for Magellan Meridian GPS. WAAS off- and on- periods are shown by green and magenta. From ~12 to ~15 min the receivers were shielded.

When signal is strong time to get the first fix is shorter compared to weak signal at test 1 (~15 sec for GPS-360 and ~40 sec for Meridian GPS). Similar to the weak signal experiment deviation of coordinate is larger for Pharos GPS-360. 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.

Road test

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 written in Microsoft Streets&Trips data format and are shown by red (Pharos GPS-360) and blue (Meridian GPS) circles on the map. Actual path is shown by brown line. This map is shown in Fig. 5.

Fig. 5 Map with Pharos GPS-360 fixes (red circles), Meridian GPS fixes (blue circles) and the actual path (brown line). A,B and C indicates underpasses, where signal is weak.

Meridian GPS shows reasonable behavior, if signal is lost there were no valid fixes indication, while behavior of Pharos GPS-360 is absolutely unacceptable: instead of showing real lost of fixes it approximates the way by straight line for a long period of time and after about 0.5-1-2 minutes tries to adjust the fixes to the real position. I think this is done to screw up the customer with "unbelievable" sensitivity of the device. Accuracy of Meridian GPS is good enough to show real position on the road. Fig. 6 shows magnified C area to demonstrate the above mentioned difference between meridian GPS and Pharos GPS-360 behaviors.

Fig. 6 When signal is lost (underpass) Meridian GPS shows no valid fixes, while Pharos GPS-360 is trying to be "smart" (to my opinion this is absolutely unacceptable for navigation).

Fig 7 shows quality of fixes (if valid) and number of satellites during the trip

Fig. 8. Valid fixes quality and number of satellites during the trip.

Table 2 summarize some statisticsduring the trip:

Parameter Magellan Meridian GPS (Merigreen) Pharos-360 (Microsoft branded)
Satellites (% of time)
WAAS corrected (% of all)
100%
4%
Valid fixes (as in NMEA), % of trip time
73.77%
73.47%

 

Summary

1) Pharos GPS-360 takes fixes faster than Meridian GPS. The only question are they real or just generated "to be the best".
2) Magellan Meridian GPS shows quite reasonable data during navigation in urban area. Accuracy of Meridian GPS is significantly better than Pharos GPS-360 in all tests.
3) I doubt on indication of WAAS by Meridian, probably sometimes it really does correction, but I can not believe in 100% of WAAS while road test.
4) "Signal-to-noise" ratio for Meridian GPS is much better than for Pharos GPS-360, I think this is achieved by averaging of data (taken many times within 1 sec interval), Pharos GPS-360 signal looks like one fix a second without any averaging.
5) Pharos GPS-360 is unacceptable for car navigation because of tricks in the firmware. On the other hand it works well when signal is strong and movement is not fast (tested while skiing, see GBA GPS page for description of apparatus).
6) Pharos GPS-360 generates messages with crappy data (zero satelites in use, shows valid fixes when they are not exist); Magellan Meridian GPS generate messages only for trusted data.

My choice (of tested here receivers) without any doubts is Magellan Meridian GPS.