In an earlier blog, we discussed how you could feed positions in from a 3rd party GPS or GNSS antenna into Lowrance HDS and thus into BioBase if you already had another higher end antennae or if the survey required a certain guaranteed level of accuracy. We recently connected with Geologist and BioBase super user Rob Baker CPG, PG from RMBAKER LLC and he shared with us his experiences networking positions via an older, yet more widespread protocol called NMEA0183. Mr. Baker shares some useful insights and justification for why networking a third pary antennae through NMEA0183 may be a good way to go for certain bathymetric survey projects.
The case for a 3rd party GPS/GNSS
Many practitioners of BioBase use autonomous drones, or unmanned surface vessels (USV), to map waterbodies (Figure 1). These USV’s are commonly equipped with GNSS systems intended to support the autonavigation components, with an onboard Lowrance or Simrad chartplotters and transducers that separately record position and sonar data in a sonar log file (.sl2 or .sl3 format). On occasion, the USV operator might want to replace the Lowrance GPS positioning with data from their more advanced autonav GNSS system. In this post we will describe our own USV set up and how we were able to make a successful connection via the NMEA0183 port of our Lowrance HDS-7 Live. We will also provide some insight into why we like this capability for our own USV mapping projects.
Making the connection
Data collection on our USV can happen using either the onboard PC and Hydromagic software or by recording a sonar log file directly to a microSD card in the HDS-7 (see Figure 2). We generally use both methods simultaneously, however there are some limitations. The Hydromagic software does not receive the full sonar dataset, but only sonar related NMEA sentences. Combined with the position and motion NMEA sentences provided by our onboard Emlid M2 GNSS, Hydromagic provides a powerful tool for mapping bathymetry and sidescan imagery, but does not include other very useful components of the HDS-7 dataset. In contrast, the sonar log file (.sl3) from the Lowrance HDS-7 includes the entire 200Khz (primary) profile, depth picks, downscan profile, sidescan images and “multiple” echoes needed for composition determinations. Because our workflow relies heavily on the HDS-7 sonar log files for processing in Biobase or other software, we also wanted the very best GPS positioning linked to that sonar data within the HDS-7 itself.
After some trials we were finally able to establish a stable network connection between the M2 and the HDS-7. The key to the connection was an independently powered serial converter (Figure 3) that linked the RS-232 port of the M2 to the RS-422 port of the HDS-7. This converter was independently connected to the 12.8VDC main battery of the USV, with a small fuse on the power line near the converter. When a networked GPS signal becomes available the connection will be listed as a possible GPS “source” within the network menu options in the Lowrance data sources menu under the settings directory.
In some cases, the connection of RS-232 to RS-422 can occur by wiring the RS-422 negative receive wire to ground, but this did NOT work in this case. Ultimately the converter provided the necessary wiring and needed voltages to establish a connection that was otherwise not possible. Other equipment set ups may give different results when connecting to an HDS unit using this port, but for a high end GNSS connection expect to use a converter like the one we have chosen.
For any equipment to be connected to a Lowrance HDS unit, be sure to check with the manufacturer for pin-out diagrams so that you don’t have to diagnose which wires are important. If a pinout is not available, there should be a voltage present between the ground wire and the transmit wire, so use a volt meter to look for it while the device is actively streaming positions. In any NMEA0183 setup, there are “talkers” and “listeners.” Talkers transmit data, and listeners receive data. Talker transmit wires need to then be connected to listener receive wires. In my case the M2 was the talker and the HDS-7 was the listener. The converter I used simply acted as an extension of the talker wiring.
We were also able to connect our M2 RS-232 port to a NMEA2000 network and stream position data to the HDS-7 using the newer NMEA2000 protocol. We used a Yacht Devices NMEA0183 to NMEA2000 Gateway connector (YDNG-03N). The NMEA2000 network was powered from the 12.8VDC main battery. However, on the advice from a colleague, we abandoned this approach in favor of the NMEA0183 connection. A NMEA2000 network can be electrically “noisy” and cause latency, . Some users may find that they prefer a NMEA2000 network and should explore that option. NMEA0183 ports may not be hard-wired within future chartplotters where the preference leans toward NMEA2000 connectivity.
To be fair, the Navico Group (the marine electronics division of Brunswick and owners of Lowrance® and Simrad® products) manufactures low-cost recreational fish finding and navigational equipment, not pricey high-end GNSS and hydrographic mapping gear. A huge advantage of using Navico equipment is that professional mapping practitioners, as well as amateurs, can keep costs down for mapping projects, do not need any formal training in surveying and hydrography, and can easily record high quality data sets just by relying upon the on-board sonar and GPS. However, when projects call for more positional accuracy and/or are GPS-challenged, the practitioner can and should choose to make improvements in their kit. If one is mapping with a USV like ours, the higher quality GNSS system like my Emlid M2 has likely already been purchased. Installing the serial converter (Figure 3) costs less than $150 in parts, and so the upgrade to improve GPS positioning quality can be technically easy and affordable.
Test Case using NMEA0183 Streaming Capability
Recorded Lowrance or Simrad sonar data files (.sl2 or .sl3) only store positional data in whole number Lowrance Mercator values in units of meters. Unfortunately, this means that even when improved positions are streamed via the NMEA0183 protocol, most but not all that positional excellence makes it into the recorded file. Now factor in that recent BioBase database updates include storage of nine decimal places for latitude and longitude positions (which means there is storage space for centimeter level precisions within Biobase). What does this all mean?
A test dataset illustrating these data differences is shown in Figure 4. We tasked the USV with a small survey covering several acres, and uploaded a highly irregular track into the autonav system. RTK corrections were broadcasted to the USV from a base station positioned onshore. We chose to perform this test using streamed RTK-corrected positions because we wanted to see how well extremely precise positions would be handled. For this test we did not compare the WAAS-corrected positions of our M2 relative to those of the HDS-7.
At smaller scales the streamed RTK positions recorded within the HDS-7 .sl3 file (green dots) appeared to map along the RTK track (red line) very precisely, and in fact the positioning relationship seemed pretty good. However, when examined at larger scales, the whole number truncation of position data within the .sl3 file created clusters of points with identical X or Y values (green dots). The clustering means that the extreme precision of the RTK-corrected positions was left out of the recorded .sl3 file.
In the Figure 4 test case where we collected M2 RTK fixed positions and simultaneously streamed those positions into the .sl3 format, 99.3% of the HDS-7 .sl3 positions still plotted within 2 feet of the RTK connector polyline. That’s pretty good despite the obvious clustering and data being left out of the .sl3 format. Contrast that scenario with normal WAAS-corrected HDS-7 positions where 97.9% of the positions extended out to 7 feet from the same RTK connector polyline. Comparatively, a position scatter buffer of 4 feet around the actual path using the downgraded .sl3 format is still a lot better than the 14 foot spread produced by the normal WAAS-corrected positions.
The best part of the test case came when examining the positional output of Biobase. The online app is able to resample the slightly scattered .sl3 points using all nine decimal places and using heading information as a guide to produce a resultant track that very closely follows the RTK polyline. In fact, 70% of Biobase points plotted with 0.5 feet of the RTK polyline, and 100% within 2 feet. Processing sonar data through Biobase actually enhances the positional accuracy of the original .sl3 file, something that other processing applications do not do.
Conclusion
Although we would prefer to see greater capabilities to record precise locations in future .sl3 file structures, there are clear advantages to streaming high quality positions regardless. Certainly, the positional error imposed by the .sl3 file structure would seem to undo some of the benefits of streaming superior GNSS positions via a NMEA0183 port, but not entirely so. There are clear advantages for professional mappers to recording the best possible position data into the .sl3 file because of the rich supply of sonar imagery available that can greatly enhance site interpretations. Processing data using Biobase also presents advantages, where the .sl3 position data is resampled something much more tightly constrained to the RTK positions originally streamed via the NMEA0183 connection. The capability of Biobase to do this conversion to a large degree balances out the data losses built into the .sl3 format. Streaming high quality positions via the NMEA0183 port to an HDS chartplotter will improve the quality and reproducibility of any maps produced by Biobase or other softwares, and this capability will only improve in the future.
