Swathe Services is a UK firm providing hydrographic surveying expertise. Swathe does not actually perform hydrographic surveying; rather, the firm focuses on training and assembling qualified teams, and providing and packaging advanced equipment—multi-beam sonar systems, unmanned survey vessels, laser scanners, etc.—as a subcontractor on challenging bathymetric projects worldwide. Swathe is particularly interested in cutting-edge instrumentation and sensors that are more accurate and productive than standard solutions. “70% of what we do is selling other people’s products, and the rest is assembling and training personnel for projects,” says managing director James Williams. “So, in a sense we are not using equipment and instruments to do survey work, but we are constantly creating combinations of cutting-edge instruments, and we regularly demonstrate these packages to prospects and end users. We are always looking for ways to get the most out of the latest hydrographic technology available.”

Most hydrographic surveying projects make use of inertial navigation systems that use GPS receivers and various sensors, such as gyroscopes and accelerometers, that extend ‘dead reckoning’ navigation systems into the modern age. But on nearly all projects, accurate geolocation is also needed, to precisely locate project features at the correct global XYZ coordinates. As such, Swathe Services absolutely requires highly accurate global navigation satellite system (GNSS) corrections for real-time kinematic (RTK) location that is constantly available in remote marine areas. This last requirement tended to rule out the conventional RTK networks that are now widely available in most countries—though highly accurate and excellent for terrestrial surveying applications, these networks tend to be unavailable or inaccurate offshore. Marine surveying also tends to rule out the use of GNSS base stations to provide local corrections, as stable platforms for receivers and base stations are scarce on marine job sites. “Since modern multi-beam sonar equipment is now so accurate, we really needed a location service that was similarly precise, to demonstrate what is really possible now,” Williams says.

Since early 2016, Swathe has been using the Atlas GNSS Global Correction Service, developed and supported by Hemisphere GNSS (based in Scottsdale, AZ, USA), when packaging subcontracted hydrographic solutions or when demonstrating their solutions to prospective clients. To access Atlas GNSS corrections, Swathe now includes Hemisphere’s AtlasLink® GNSS Smart Antenna with their hydrographic packages, and when demonstrating solutions to prospective customers.

More Accurate, More Widely Available

Rather than relying primarily on ground-based continually operating reference stations (CORS) to transmit RTK corrections, Atlas delivers correction signals via L-band satellites. This makes Atlas the most flexible correction service available, especially for marine work, providing many advantages over terrestrial RTK networks.

L-band is a frequency band, from 1-2 GHz, allocated to satellite navigation and telecommunications. Like most correction services, Atlas ultimately relies on the use of multiple established global navigation satellite systems like GPS, GLONASS, and BeiDou to determine the XYZ positions relative to Earth, plus more than 200 land-based reference stations worldwide. But unlike most systems, L-band satellites are used to transmit positional information. This factor alone makes Atlas more useful for maritime operations, since L-band signal distribution, via strategically placed geostationary satellites, is continuously available and, globally, verges on ubiquitous in terms of spatial coverage. L-band distribution covers from 75°N to 75°S, excluding only the extreme polar regions.

Hemisphere’s AtlasLink GNSS receiver, using boards developed and manufactured by Hemisphere, is optimized for the reception of L-band signals, including automatic tuning to the best frequencies available in project locations. The AtlasLink receiver also supports GNSS receivers by other manufacturers and integrates well into existing GNSS operations. Swathe Services builds the mounting brackets and other hardware necessary for vessel-mounted use of GNSS receivers in hydrographic work and has been very pleased with the build quality of AtlasLink receivers—none have failed physically, even those deployed in harsh conditions.

In addition to near-constant and near-ubiquitous positioning, the L-band signal also enables fast convergence and high accuracy. Depending on subscription terms, Atlas accuracies range from sub-meter to sub-decimeter levels, and correction signals are attained with convergence times between 10 and 40 minutes.

Exceeding Expectations

Swathe Services first tried out the sub-decimeter version of Atlas on a 30-day trial subscription, and quickly committed to an annual subscription based on results observed. Williams says that actual accuracy observed was much better than decimeter levels, usually in the 3-4 cm range.

Atlas use during the trial period was typically for equipment demos, and, prior to acquiring the Atlas subscription, Swathe used standard RTK networks for location correction. These were potentially more accurate, down to sub-centimeter, but Williams found that these networks were generally unavailable when offshore, due to the distance from land-based reference stations. In the year since Swathe acquired the annual subscription and began to package AtlasLink receivers and the Atlas GNSS correction service into bespoke solutions for hydrographic clients, Williams has consistently observed positional accuracies in the 3-4 cm range. “That was the key thing for us, it was amazingly accurate wherever and whenever we were working,” he says. “In theory, an RTK network supplying corrections with cellular links can be even more accurate, but that just is not true for us when we are offshore. Nothing we looked at was comparable in terms of accuracy and availability. With Atlas, we could just turn it on and go to work.”

This accuracy range has been particularly important given Swathe’s emphasis on cutting-edge use of multi-beam sonar equipment. Depending on ping rate, depth of work, sound speed profiles, and many other factors, hydrographic mapping performed with advanced sonar sensors can approach the resolution of laser scanning and other advanced surveying techniques. But real-time positioning is obviously a critical limiting factor—if the position of the vessel transporting the sensor is not accurately and continuously known during survey work, the resulting survey runs the risk of being precise (mapped points located accurately relative to each other to within a few centimeters) but not accurately geolocated (the very precise survey could be positioned many feet from its actual location on Earth). “We monitor accuracy real-time while working, and we have never observed a product failure,” Williams says. “Accuracy is always within specifications, and almost always better than that.”

This is not a theoretical concern for Swathe Services—in many cases, for example, their clients are trying to register (accurately overlap) hydrographic maps of marine facilities with 3D topographic mapping (obtained with laser scanning and total stations, and accurate within a few millimeters) of above-surface infrastructure, such as keywalls. Swathe is very good at assembling sensor unit packages with custom brackets, power supplies, computing applications, etc. so that their clients are able to gather very precise 3D information about subsurface conditions. “Laser scanning tends to produce survey work with tight footprints, and matching hydrographic work is important,” Williams explains. By adding Atlas into the mix, Swathe can be confident that their precise sonar work is also accurately geolocated, and on the same basis as terrestrial work.

In addition to sonar work, Swathe Services has also extended Atlas GNSS corrections to diver navigation systems. “When divers go down, they have an underwater positioning system that is actually tied into the AtlasLink receivers,” Williams says. “So, they can navigate underwater very accurately. We are seeing some huge contracts coming up for this technology.”

Cost-Effective and Easy-To-Use

Williams says that Swathe’s Atlas subscription costs are substantially less than similar services providing less accuracy and less reliability. This makes it easy to extend the use of the service to their clients who are involved in non-commercial ventures. In April 2017, for example, Swathe Services supported other agencies like the Cornish IFCA, Historic England, MSDS Marine, and DARKwright Archaeology to successfully complete several multi-beam bathymetric and backscatter surveys of shipwrecks within the Isles of Scilly European Marine Site. These surveys were positioned accurately using Atlas, with correction signals received by AtlasLink GNSS smart antennas, sourced and mounted by Swathe Services. “What they were able to accomplish in a few months was awesome,” Williams says. “The RTK corrections provided by Atlas were more robust, more accurate, and cheaper than anything else available, and combining that with all the latest sonar equipment and unmanned vessels, and other cutting-edge solutions, made this an amazing project.”

In the UK, Atlas is represented by Saderet, Ltd., Hemisphere’s European distributor—Swathe Services already knew Saderet, having worked with the firm for twelve years before subscribing to Atlas. Williams reports that service and support provided by Saderet has been uniformly excellent, and that Saderet has even supplied necessary equipment on a rush basis from its own stock.

Swathe Services also conducts a great deal of training in hydrographic surveying, in an annual course and on behalf of the clients with whom they subcontract. In this regard, another advantage of Atlas is its relative ease of use. If the service was difficult to access or use, or if complex post-processing was required, it could make the correction service an untenable option for clients. But instead, Williams reports that Atlas software and firmware are supported with clear interfaces, and that customers find them easy to learn and use.

All in all, the addition of the Atlas GNSS global correction service and AtlasLink GNSS smart antennas was positive for Swathe Services—adding accurate, cost-effective, and truly global positioning to their high-precision sensor platforms provided immediate value for the firm and its customers. “Atlas has made our lives a lot easier,” Williams says. “It gave us an excellent solution for our work and for our clients, and one that we can actually use all the time because it is cost-effective.”