Article 50408 of sci.geo.satellite-nav: Path: matra.meer.net!news1.best.com!news.maxwell.syr.edu!news-peer.sprintlink.net!news.sprintlink.net!Sprint!cpk-news-hub1.bbnplanet.com!news.bbnplanet.com!newsfeed.internetmci.com!164.67.42.145!awabi.library.ucla.edu!137.82.194.1!unixg.ubc.ca!cs.ubc.ca!not-for-mail From: davem@cs.ubc.ca (Dave Martindale) Newsgroups: sci.geo.satellite-nav Subject: Lowrance differential beacon receiver with Garmin GPS Date: 9 Nov 1997 18:00:55 -0800 Organization: Computer Science, University of B.C., Vancouver, B.C., Canada Lines: 161 Message-ID: <645psn$3q8@redgreen.cs.ubc.ca> NNTP-Posting-Host: redgreen.cs.ubc.ca Summary: it works. Xref: matra.meer.net sci.geo.satellite-nav:50408 I recently acquired the Lowrance differential beacon receiver (DBR). To the best of my knowledge, it's the cheapest commercial receiver for the Coast Guard low-frequency DGPS beacons. The unit is somewhat unusual in construction, which may account for its low cost. Some discussion of this can be found at the bottom of this article. Anyway, the Lowrance receiver lists at $300 US, while the Garmin two-box equivalent is about $500, and receivers that have controls and a display are even more expensive. This makes the Lowrance unit attractive if it fits your needs. Of course, the Lowrance receiver will work with Eagle/Lowrance GPSes, but I have Garmin hardware. After reading some reports here that the two were compatible, I got one. As far as I can tell, the Lowrance DBR and the Garmin GPS receivers are completely compatible with one another. The Garmin successfully controls the receive frequency and bit rate of the DBR, and the Lowrance DBR successfully sends differential corrections to the Garmin GPS. The Garmin interface configuration page (where the DBR tuning controls are located) also receives regular reports of the signal-to-noise ratio from the DBR and displays them (30 dB seems to be the maximum value reported). I tried using a 45XL and a 12XL with the Lowrance DBR, and both functioned perfectly with it. The 45XL actually displays a little "D" at the bottom of the signal strength bar for each satellite that is being differentially corrected. The 12XL does not do this - the bars are narrower to fit 12 in and I guess there was no room for the D. Too bad. There doesn't seem to be any way to tell with the 12 which satellites are being corrected. In addition, the Canadian Coast Guard transmitters send a message twice per hour that gives their own location. The Garmin receivers intercept this message and display the distance from the GPS to the differential beacon transmitter. The GPS must actually save this location along with the beacon frequency somewhere, too. When I operated the 45XL and the 12XL with differential corrections for the first time, they had no idea where the beacon transmitter is, and left the field blank. Once they had been "listening" long enough to hear the beacon's location, they started displaying distance to the beacon. Now, they display the distance immediately when the beacon is tuned in, without waiting for the beacon to transmit its location. A nice touch, Garmin! In short, everything seems to work. I don't know if the Garmin DBR would give any additional features when used with a Garmin receiver, but I suspect not - all of the things in the Garmin GPS interface seemed to work properly. I also don't know if a Lowrance receiver would work better than a Garmin one for controlling the DBR, but again I suspect not - the DBR manual does not hint at any features that the Garmin GPS can't control. The Lowrance receiver is physically designed for mounting on a boat. The base of it has a female 1 inch thread, for mounting on a standard marine pole mount or ratchet mount. I bought an inexpensive nylon deck mount and screwed it into the bottom of the DBR, which allows the DBR to stand upright on the floor. Without something like this, the DBR will not stand on its bottom. The Lowrance receiver does not include an antenna. There is a 3/8-24 female thread on the top of it to attach an antenna. You are expected to buy and install a "LORAN" antenna, which is just a whip antenna with the correct threads. There are two common constructions, either a fibreglass whip with the antenna element buried inside, or a thin stainless metal whip. The fibreglass seems more common (perhaps it stands up to boat use better) but the stainless version is a lot lighter. You can also get either 4 or 8 foot length whips in either construction. 8 feet would give the greatest reception range, but is extremely awkward to deal with if the receiver is not in fact fastened to a boat. I found a used 4 foot metal whip at a marine supply store. There is a Canadian Coast Guard differential beacon transmitter only about 5 km from here. The signal is strong enough, and the Lowrance receiver works well enough, that I get maximum reported signal to noise ratio (30 dB) using a 1-foot-long screwdriver as an antenna. Inside the house! Outside, I can also receive the Whidbey Island US Coast Guard transmitter with the 4 foot antenna, but no other beacons. The DBR has a ground screw for connecting an external ground to improve reception of weak signals; I haven't tried it yet. The Lowrance receiver requires a source of external 12 V power, at about 120 mA. On a boat, you'd just connect it to the electrical system, and make the differential signal wiring part of the boat's permanent wiring harness. I wanted the system to be portable, so I can install it in a car or even carry it, so all my wiring needed to be portable as well. Connecting all of this equipment for portable use could be somewhat of a nightmare. You really need four cables coming together in one spot: 12 V power in, a cable to the DBR, a cable to the GPS, and a NMEA output cable to a computer. I decided to put all the interconnections in a small metal box to protect them. There are also two fuses in the box (one for the DBR, one for the GPS). I was originally going to permanently wire the cables to the box, but later decided to install Molex connectors on all of the cables with mating connectors on the box. This makes everything come apart for storage. It also lets me use different power input wires (cigarette lighter plug or permanent connection) and different computer interface cables in different circumstances. For truly portable use, you do need a 12 V battery of some sort - the Lowrance DBR has no internal power source. Since I have to bring along a heavy battery of some sort anyway, the GPS is powered from the 12 V supply as well. This means that my setup won't work with a GPS 38 or 12 because of the voltage limitations of those unit's power supplies; not a problem for me. I've tried using the system in my car. If the DBR and antenna are completely inside the body of the car, the DBR receives nothing. I wouldn't expect it to; the car body is a pretty good Faraday cage at these wavelengths. But standing the DBR up vertically and sticking a few feet of the antenna out a partially-open window, I once again get a full-scale signal-to-noise ratio. The ideal would be to mount the whole antenna and receiver on the roof of the car. The receiver is completely waterproof, but it likely isn't designed to withstand the stress of having the whip hit low-ceilinged parking structures. I've done some tests on the accuracy of the 45XL and 12XL while receiving differential corrections. But that will have to wait for another article. Dave Some background info: The frequency of the Coast Guard beacons is about 300 kHz, so the wavelength of the signal is about 1 km. Thus, there is no hope of using a resonant antenna for this band - a quarter-wave antenna would be 250 m high! Instead, the usual low-frequency antenna is a vertical whip (the longer the better) combined with a good ground. The signal picked up by this antenna is at at high impedance, and couldn't be connected to standard low-impedance coax cable without losing most of the signal. At least a matching transformer would be necessary. Most LORAN receivers (LORAN is even lower frequency than the DGPS beacons) just put a preamp right at the base of the antenna, and then feed the amplified signal via standard coax back to the LORAN receiver located in the cabin or cockpit of the boat or plane. It seems that most DGPS DBRs have been built the same way, with an antenna preamp at the antenna plus the receiving electronics somewhere else. Unlike a LORAN, though, the DGPS receiver doesn't display any navigational information - it simply passes data on to the GPS receiver. Some DBRs have front-panel controls for tuning, so it makes sense to put them somewhere near the GPS operator's location. But some DBRs have no controls at all - they take all of their tuning commands from an NMEA data stream, and the GPS provides the user interface for tuning the DBR. In this case, the DBR is just a box. This seems to be how the Garmin DBR is set up - the antenna and preamp mounts in one place, while the DBR box mounts somewhere else, and the GPS may be in yet another place. Lowrance has combined the preamp and DBR electronics into a single cylindrical box that sits at the base of the antenna, where usually only the preamp would be. There is no second box, nor coax connecting the antenna to the receiver. The cable coming out of the Lowrance DBR has just four wires: ground, power, serial data in (NMEA), and serial data out (RTCM). In a sense, Lowrance has done exactly the same thing with the beacon receiver that several GPS manufacturers have done in producing "smart antenna" GPS units - the antenna and electronics are all in one, and you only need serial connections and power to use it. There is one restriction to this design: you can't use an alternate type of antenna. Some DGPS receiver manufacturers (e.g. CSI) offer a loop antenna as well as a whip, for circumstances where a whip isn't practical. With a two-box unit, you just replace the antenna and preamp with the other design. With the Lowrance, you have to use a whip. Given the price difference between the Lowrance receiver and one that can use a loop antenna (e.g. CSI), I'll live with the restriction. Dave