Garmin's DGPS-53 All-in-One Differential Marine Receiver
(29 Nov. 2000)


The G-53 ($395 US) is the first DGPS system we have reviewed, because the US Coast Guard DGPS signals have just recently been available in Atlanta, GA.  The closest station is Macon, GA, some 60 miles SE with an ID of #48.  The unit is unique in containing a GPS and DGPS receiver, GPS and DGPS antennas.  The unit is completely automatic in selecting the strongest DGPS signal and needs no user input.  The unit has also detected signals from Savannah, GA (ID #36) and Hartsville, TN (ID #144).  (SA Watch will identify the DGPS station being received.) The ruler in the picture above is six inches (15cm) long, and the unit weighs 1 1/2lb (774gm).
Garmin quotes an accuracy of 1-5m, 2DRMS.  The unit draws 170ma.  One must obtain a standard 1"x14 threads-per-inch marine mount for installation.  A single 30-foot cable is attached to the unit with a twist-lock connector.  (A 60-foot cable is also available).  Five wires must be connected by the user to 12vdc power, ground, NMEA 4800 baud serial to chart plotter or computer input/output, and ON/OFF switch.  GPS status and one pulse per second (PPS with an accuracy of one microsecond) optional connections are also available.  The GPS doesn't have a data-storage capacity; this must be provided by a computer and map program.
We soon discovered that surrounding trees prevented getting useful data from an initial roof-mounted installation.  An SA Watch plot of the problem is shown (HERE).  This should also alert any who use GPS or DGPS, that a clear horizon is essential to realizing the capability of any GPS.

We don't have a (floating) boat to test this GPS rig on, so we equipped one with four wheels!

The first tests were to drive in a relatively clear area around a clockwise course (FIVE times) in order to detect any variation in accuracy from one loop to another.  A Garmin G-III+ with a GA-26 mag mount antenna was also used to record  five tracks at the same time as the G-53 tracks were being recorded by Ozi Explorer on a USGS DRG map at a 2x zoom level (above).  Later the G-III+ track was downloaded to Ozi, so they could be compared on the same map.

The blue line is the G-III+ track which overlies the red line of the G-53.  It's interesting that on this  map scale (1:12,000) there is no discernible spread of the tracks of either GPSs due to errors!  Also there is no difference in the apparent accuracy of either GPS except perhaps the western leg where the G-III+ seems to be farther from the road line than the G-53.  Driving on the RIGHT side of the road is clearly evident.  Thus the difference in GPS and DGPS cannot be seen reliably on a map of this scale.

Next it was decided to move the test station to a large open parking lot.  A DRG map of the test location shows the station to be sitting on the 1,020-foot contour line, plus 6 feet to the antenna (1,026 feet), or 313m antenna altitude.  First an SA Watch HDOP-weighted mean was obtained by three hours recording of the G-53 position to establish a reference point.

The next day at the same time, another 3 hr. plot (above) was made to determine repeatability (if not absolute accuracy).  As indicated by the above SA Watch plot, the mean altitude was 316m (a 3m error, data line 2), the mean position was 0.3m@222° from the reference point (data line 4), and 95% of the data points fell within 2.7m of the reference point (data line 9).

The next day at approximately the same time, another  3-hr. plot (above) was recorded with a G-III+.  In this plot the mean altitude was 338m (a 25m error), the mean position was 1.0m@063° from the reference point,  and 95% of the data points fell within 5.4m of the reference point.  These GPS figures naturally aren't as good as the DGPS figures.  Note a few wild points to the north.  The DGPS system apparently is able to eliminate these types of points, and in a safety situation could be very important.

The basic purpose of this GPS is to provide an uncomplicated and accurate source of marine GPS navigation -which it should do quite well.


  NMEA Output Sentences       NMEA Input Sentences
            $GPALM                  $PGALM
            $GPGGA                  $PGRMI
            $GPGSA                  $PGRMC
            $GPGSV                  $PFRMO
            $GPRMC                  $PSLIB

Jack Yeazel