Recently, thanks to the courtesy of the people from Lowrance Electronics and Lowrance Canada, I had a chance to play with the Lowrance DGPS Beacon Receiver.

The basic description of the unit can be found on Lowrance website (see links at the end of the article), so I’m going to limit this article to my personal findings.

The receiver came packaged in an oversized box. It’s beyond me why a spindle shaped unit, under 3” in diameter and 11” long, would require a 1 ft cube box? When I opened the box, it looked empty! Well, luckily it wasn’t ;-)

Inside, I found the receiver with a 24 ft. long cable, manual, list of US Coast Guard DGPS beacons and a UPS label.  The antenna or GPS receiver connectors are not included, which is understandable, since the receiver can be interfaced with a number of various GPSRs. What’s less understandable is the fact that the US residents get a “preferential treatment”. The prepaid shipping (in case of necessary warranty repairs) is not valid outside of the USA. Also, even though the receiver was picked up directly from Lowrance Canada, there was no list of the Canadian Coast Guard beacons.

 The manual is very straightforward. It describes the necessary connections to a GPS receiver. Lowrance (Eagle / SEA Electronics) cables are color coded, so following the instructions is easy. Within 15 minutes I had the receiver hooked up to my GM100. For power, I just soldered in a typical DC adapter socket. It allowed me to use just about any power source within the voltage spec. Since I was only going to have the receiver for a limited time, I didn’t bother with a switch on the power line. Turning the receiver OFF could simply be accomplished by unplugging the power supply. The manual does not clearly describe how to connect the DGPS receiver and GPS receiver and a computer (or other NMEA device) at the same time. It’s not hard to figure it out, but IMHO it should be addressed in the manual.

 After connecting everything up, I took the set to the backyard, screwed in an 8ft. whip antenna and used a 12V gel cell to supply power. The receiver is "manually tuned". It means that the frequency and the baud rate of the reference station has to be set by the GPS. The closest station to me is in Youngstown N.Y.. It's where the Niagara River empties into Lake Ontario. It's 70 kilometres away from my home. The range of that station is listed as 150 NM, so I set the frequency and the baud rate to that station. Mere seconds later, a DGPS correction acquired message appeared on the screen. The signal from that station was nearly perfect all the time. The DGPS status screen on my GM100 indicated that the data were never older than 3 seconds, signal strength was 100 and the SNR ratio was 30. As suggested by Ron Wilson, I decided to try a shorter antenna. 8 and 4ft are the recommended antenna lengths, but besides the proper 8 ft whip, all I had was an old, 3.5 ft. long telescopic one. Changing the antenna to the short one had no impact. All the numbers stayed as they were.

The next test was to try to get corrections from a bit more remote beacon. The next closest station is in Wiarton ON, on the shore of Lake Huron's Georgian Bay, with an advertised range of 250 km. I'm 186 km away from it and all of the distance is over dry land. When I tuned to that station, the short antenna did not provide sufficient.reception.  The 8 ft. antenna gave barely enough signal to give some corrections. The signal to noise indicator was jumping between 0 and 25. Corrections would sometimes be renewed at 5 -10 seconds and sometimes get old (over a minute) and a DGPS lost alarm would be displayed. That was without any ground plane attached. OK, so I figured I needed a ground plane. The closest suitable object was... myself.  I just held the receiver in my hand in such a way as to touch the grounding screw.  The SNR started to oscillate between 25 and 30. The corrections became timely, never exceeding 5 seconds. When I tried the short antenna in the same manner, the result was virtually identical to using a long antenna without any ground plane. Keep in mind that this was a "far away" station and the antenna was 6" short of the spec.

 For collecting of the actual position data, I settled on the 8ft whip antenna without a ground plane.

 I collected a number of hours of position data. Using the calculated average as a benchmark, the accuracy showed to be between 7 and 11 meters 95% (2 RMS error.) Note that from the position in my backyard, approximately 60deg of the horizon is masked to approximately 30 - 50 deg elevation by my house and surrounding trees (VERY DENSE). In other directions, there are spot trees, but generally the mask angle is under 10 degrees.

 It was time for the next test. This time it was done over an actual benchmark. Thanks to the Kimber Gray and staff at, Provincial Georeferencing, Office of the Surveyor General, Ministry of Natural Resources, I was provided with some benchmark locations. Unfortunately, the information about the date of the latest survey was not available.

Nevertheless, I decided to try it. One of the locations was almost perfect for the exercise. The mask angle was under 10 degrees all around and I was the only obstruction in the vicinity.

 The set up consisted of the Eagle Expedition 2 and Magellan GPS315 as well as the GlobalMap100 with the DGPS receiver connected to it. The GM100 was placed right on the monument, EEx2 and the 315 were placed roughly 2 meters away from it. During the test that lasted several hours, I took some readings of the positions averaged by the receivers. Note that the readings of the position errors are not based on a continuous series and were taken from different receivers at different times. The error shown is as the distance in meters to the “official” benchmark location.

 The most interesting (and scientifically valid) came from Ron Wilson who analyzed the plot recorded by the GM100.

 The RMS error was calculated to be 3.738842 meters. It indicates 95% accuracy to be within 7.477683 meters (2 RMS)

 In reality, the maximum deviation recorded over that benchmark was just under 10 meters.

 The graph below might give a bit more visual representation of the position error.

As suggested by Ron, I also took the opportunity to assess the suitability of the Lowrance DGPS receiver to a portable use.

The receiver’s tubular shape with the antenna coupling at the top of it makes it almost ideal for strapping to the side of a backpack. The receiver is also very light. The receiver itself tipped my kitchen scale at 14 oz (400 gram). The 24 feet of the cable weights about the same, but obviously can be trimmed to a desired length.

Power requirements are also very reasonable.  I connected the receiver to a battery pack of 11 AA Duracell batteries. An odd number, but it gave an initial voltage of 17.7V, which was just below the 18V maximum specified by Lowrance.

After 14.5 hrs of continuous use, the voltage was down to 10.85V and everything was working just fine. When I checked fifteen minutes later, the DGPS was not giving corrections and the voltage was down to 6V. Earlier, I took only three other measurements (all of them under the load)

Initial - 17.7V

12 hrs. - 13.4V

13 hrs. - 12.7V

 In summary, the receiver proved itself to be a very compact and capable unit, that makes Selective Availability just a bad memory.  It is light enough, small enough, and energy efficient enough to be considered a mobile unit.  Prospective buyers, however,  should remember that it will work only with GPS receivers capable of outputting proper controlling sentences.  It will not work, for example, with a Magellan 315 which requires an automatic DGPS receiver.

Internet links: 

Lowrance DGPS Beacon Receiver is shown at: http://www.lowrance.com/accessories/acc_dgps.htm

To try the DGPS corrections over the Internet visit:   http://www.wsrcc.com/wolfgang/gps/dgps-ip.html

For the locations of the Canadian Coast Guard operated DGPS reference stations see: http://www.ccg-gcc.ca/tosd-dsto/awtj/dgps/table.htm

 For the locations of the US Coast Guard operated DGPS reference stations see: http://www.navcen.uscg.gov/dgps/coverage/Default.htm