A 1.289Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE
It’s a given that Global Navigation Satellite Systems (GNSS) don’t work everywhere, for example if the signals are sufficiently blocked by such things as tree canopy, or urban canyons. Because of the need to locate things everywhere, a great deal of research is going on in this field. Some of the research involves the use of radio frequencies and a form of triangulation, but the most promising involves the use of inertial systems. These systems work with gyroscopes to obtain orientation and accelerometers to detect movement. We all know gyros from early childhood, and can remember our amazement at watching a device that, while maintaining its orientation, resisted a change in the direction it was pointed and seemingly defied gravity.
One company, Toronto-based Applanix, has been at the forefront of inertial guidance research since the early 90s, and with the explosion in the demand for positioning devices that work where GNSS alone doesn’t, I recently paid a visit to the company to check out the state-of-the-art.
Applanix started out in 1991 as Applied Analytics Corporation, serving the aerospace and defense markets. From the beginning it has focused its efforts on shrinking not only the size of its sensors, but constantly upping the performance as well. Its Positioning and Orientation Systems (POS) are now the industry-standard for three types of moving platforms: airborne, marine and land. In particular, its POS AV georeferencing system for airborne sensor data is in use in most of the aerial imagery acquisition aircraft around the world. In 2003 the company became a wholly owned subsidiary of Trimble.
Applanix systems integrate precision GNSS with advanced inertial technology to provide uninterrupted measurements of the position, roll, pitch and true heading of moving platforms. By combining GNSS and inertial technologies, the company offers high-productivity in-motion positioning, direct data georeferencing, and robust mobile mapping.
Key to the many Applanix technological innovations is POS (hardware) and POSPac (software). POSPac includes patent-pending technology called Applanix IN-FusionTM and Applanix SmartBaseTM, which integrates Post-processed GNSS Virtual Reference Station (PPVRS) with Inertial Navigation Systems (INS). IN-Fusion and SmartBase are useful across all of the Applanix air (AV), land (LV) and marine (MV) platforms. Many surveyors today take advantage of RTNs that allows them to do RTK surveying without the need to set up a dedicated base station. The SmartBase software uses the same approach to allow very accurate and efficient mobile positioning within a network of GNSS stations.
Something surveyors can relate to is the loss of signal due to buildings and foliage. Whenever this occurs, the ambiguity resolution process must begin anew. Because the INS measures changes in velocity and orientation up to a thousand times a second, IN-Fusion minimizes the problems associated with, for example, going under bridges, banking in turns during an aerial photo mission, or dealing with the heave of waves.
Key to IN-Fusion is the GNSS Azimuth Measurement Subsystem or GAMS. The GAMS uses a two-receiver, short-baseline GNSS configuration to maintain heading. For land applications, also aiding in distance measurement is the Distance Measurement Indicator Subsystem. This wheel-mounted device provides redundancy which, when coupled with all the other sensors, allows very precise positioning information.
I met with several people during my visit, including Dr. Steven Woolven, President; Louis Nastro, who heads up the land products division; Peter Canter, who heads up the marine products; Joe Hutton, who heads up the airborne products; and Eric Liberty, who, at the time, was the product manager for the Digital Sensor System (DSS), which is the Applanix medium-format airborne digital camera system. Liberty is now the Director of Sales. These folks are all highly educated and have many years of industry experience. And in fact, out of the nearly 100 employees around the globe, 10 have PhDs and 21 have masters degrees.
Of particular interest while I was there was their newly-released LANDMark mobile mapping solution. LANDMark is primarily a mobile geospatial data acquisition system with several different lidar and camera options that can be configured for various applications (such as dynamic 3D models, road planning, as-builts, and asset inventory). Differing from the Lynx mobile mapping system from Optech we recently featured in the magazine (which only uses one type of lidar scanner), LANDMark is available with most commercial lidar systems. It also offers a range of accuracy levels depending on customer requirements, as well as a camera system for acquiring georeferenced imagery. As many as six cameras can be mounted to provide a 360º view around the vehicle. As proof of Applanix’s penetration into the market, the Lynx uses a POS LV 420 sensor for positioning and POSPac Land software for dynamic navigation solutions. According to Optech, tightly coupled processing combining GNSS and INS measurements maximize the accuracy of the navigational solution used for lidar and image geo-registration.
Nastro explained, "Mobile mapping is the single most cost-efficient way to obtain quality civil infrastructure GIS data." LANDMark encompasses acquiring 3D models, gathering video log data for right-of-way and pavement condition analysis, electric pole inventories, street/ trackside asset management projects for government regulations, and more.
He went on to say. "The Applanix LANDMark is the industry’s first commercial off-the-shelf mobile mapping solution which can be customized to increase productivity for both data acquisition companies and end users in the collection of quality infrastructure data. It virtually eliminates expensive data re-acquisition costs."
One cool aspect of LANDMark is its sign recognition module which uses artificial intelligence to eliminate operator time in identifying assets. For building 3D city models, airborne and LANDMark data can be combined to create complex models.
Marine Applications
Another interesting area of focus for Applanix is marine applications. Today, more than 500 Applanix POS MV systems are in use worldwide, mostly aboard hydrographic research ships, gathering data from more than 350 ports. In years past marine applications have focused on surveying below the waterline; today there is a trend for mapping near shore surface features and attributes as well. Canter discussed the need to move from traditional local charting datums to hydrographic mapping on the ellipsoid, adding that Applanix is following NOAA’s VDatum initiative. At the time of my visit, Applanix’s POSPac MMS was being used by the NOAA ship Rude in the Chesapeake Bay where it has proved it is possible to acquire better than ±5cm heights . (Note: Rude is pronounced "Rudy"; the ship is named for Captain Gilbert T. Rude who was one time Chief of the Division of Coastal Surveys, U.S. Coast and Geodetic Survey.)
Perhaps the most exciting marine application coming from Applanix is its fusion of an echo sounder and shoreline surface image data. The surface data is now, for the first time, being obtained from lidar and camera sensors located on the vessel. This has considerable safety advantages. Notwithstanding the dangers associated with having a boat close to shore, shoreline verification addresses safety concerns associated with the need to map obstructions near the surf zone or in rocky areas. Marine imaging from a vessel allows the map maker to obtain data on obstructions wi
thout having to leave the vessel.
Another new marine mapping application is found in the use of videogrammety to detect change along the coastline and in port and harbors. Change detection can sound an alert if, say, an unexpected feature suddenly appears in the landscape.
Applanix marine gear is being used in a wide variety of applications including the inspection of levees. Canter showed me a fascinating application that, because the equipment is so sensitive, can detect a boat’s wake long after the ship has left the dock by sensing bubbles left in the water. As he explained, "The Holy Grail has been the fusion of aerial/lidar imagery, terrestrial scanning, bathymetry and videogrammetry, and we’ve achieved this."
Aerial Applications
The next portion of my visit was the aerial division. Arguably the area for which Applanix is most famous, as stated above, POS AV is the most widely used in-the-plane hardware and software system for georeferencing sensor data. POS AV is used for aerial cameras, scanning lasers, imaging sensors, synthetic aperture radar, and other lidar technologies. Additionally, Applanix offers the POSTrack system, which includes a flight management system for mission planning and operations. According to Hutton, unlike other systems that consist of separate components, POSTrack is a single, easy-to-operate solution and covers all aspects of flying an airborne mission, from preparation, to image acquisition, to data archiving.
Applanix also manufactures a turn-key medium format aerial imaging system, the Digital Sensor System (DSS). More than 100 have been sold, evenly split between standalone cameras and those integrated with lidar sensors. DSS is designed for the airborne environment, and combines POS AV with a precision digital metric camera, in a small, ruggedized, and easy to install package. Designed to capture color and color infrared digital imagery, the DSS produces an accurate product with 0.05m to 1m GSD (ground sample distance, or size of each pixel on the ground). Liberty discussed how DSS has been used in Afghanistan to produce maps within hours of the photo mission.
After our sit-down meetings, I was fortunate to take a tour of the facilities. As with all of the electronic equipment we surveyors use, each unit that is manufactured is unique and calibration constants are developed that go with each piece of equipment. Applanix is proud of the fact that the DSS is the only medium-format camera system certified by the USGS to be of mapping-grade quality. This is a result of exacting manufacturing processes. As an example, I was shown an apparatus that is used to machine the mounting plates for the camera backs. The plates are then rigorously examined and the results are fed to a CAD model which is then used to drive the CNC machine. Adding another complication is the fact that medium-format lenses are subject to more distortion than larger lenses. World-renowned Carl Zeiss makes the lenses for the DSS.
As part of the calibration/QC process, each DSS system is flown. Applanix has a dedicated plane and pilot for this purpose, made easier by the fact that the company is located right next to a small airport. The same rigorous QC/QA approach is used for the LANDMark products: each is driven over a known course.
According to Nastro, Trimble’s acquisition of Applanix brought an important worldwide sales and distribution network. Applanix has also shifted from only manufacturing equipment to providing solutions, support and service. He continued, "This is all made possible by knowing within a few centimeters where you are. It’s not just about positioning, it’s about integration."
Marc Cheves is Editor of the magazine.
Sidebar:
INS-101
Newton’s First Law of Motion, also known as the Law of Inertia, states that an object in motion will not change velocity until acted upon by a force. This resistance to a change in direction makes gyroscopes extremely important in everything from bicycles and motorcycles, to yo-yos and Frisbees, to the advanced navigation system on the space shuttle. A typical commercial airliner uses about a dozen gyroscopes in everything from its compass to its autopilot. Every satellite that depends on solar power needs to maintain a certain orientation to the sun. Satellites such as GPS contain redundant gyros called reaction wheels. Gyroscopes, along with accelerometers, are the basis of inertial navigation systems (INS).
At the ACSM/ASPRS conference in Seattle in 1997, I had the good fortune to tour a Trident nuclear submarine that was undergoing re-supply and maintenance. In the center of the command center was a large (as I recall, at least three feet in diameter) cylindrical gyro for the sub’s inertial guidance system. My guide explained how, with a system of angle prisms and mirrors, a beam of light was transmitted from the gyro through the sub back to the nuclear missiles. This short baseline gave the missiles an initial orientation. Once out of the water, the missiles would rotate until a sensor behind a glass window in the side of the missile could acquire the stars and correct the azimuth to its target. A GPS antenna on the top of the sub could be used if the vessel broke the surface, but the inertial system provided all the necessary positioning and orientation information.
My first exposure to gyros for positioning was back in the 1980s as I learned about inertial guidance systems mounted on helicopters. The main problem at the time was that the systems massively drifted over time, and the helicopter had to frequently hover over a known position to correct-up for the drift. Early photogrammetry systems had GPS antennas festooned all over the aircraft, but over time it was realized that small, very accurate INS systems would provide the necessary precision and update rate (a photo-mission plane typically travels at 160 feet per second) and reduce the need to one GPS antenna.
Inertial systems have come a long way since then, both in performance and size. One of the largest sources of error in an INS is the heading. The INS used by the Applanix LandMark system is good for one kilometer or one minute, after which it needs an update from the GNSS.
The gravity-defying part of a gyroscope is due to precession. An example of precession is the earth itself: the earth’s axis wobbles in space over a period of 23,000 years. This is why, when we observe Polaris, its apparent motion takes the form of a racetrack. In general, precession works like this: if you have a spinning gyroscope and you try to rotate its spin axis, the gyroscope will instead try to rotate about an axis at right angles to the force axis.
Using a spinning bicycle wheel to demonstrate Newton’s Law of Inertia, if a force is applied to the top of the wheel, it moves in that direction, but because it is spinning it keeps going in the same direction as it was before the force was applied. The precession takes place around the axle of the wheel.
Therefore, once a gyroscope starts to spin, its axle will keep pointing in the same direction. If the gyroscope is mounted in a set of gimbals so that it can continue pointing in the same direction, it will. This is the basis of a gyrocompass. If two gyroscopes are mounted with their axles at right angles to one another on a platform, and the platform is placed inside a set of gimbals, the platform will remain completely rigid regardless of how the gimbals rotate. Image stabilization in aerial cameras is provided by gimbaled gyros.
In an INS, sensors on the gimbals’ axles detect when the platform rotates. The INS uses those signals to understand the platform’s rotations relative to its initial orientation. If a set of three sensitive accelerometers is added to the platform, it is possible to tell exactly how the platform is moving and how it was initially orientated with resp
ect to the earth (by sensing gravity).
Modern non-mechanical laser gyroscopes are the cost-effective state-of-the-art today. Two laser beams move in opposite directions around a ring, creating a standing wave. If the housing is rotated, the frequency of the two rings are doppler-shifted in two different directions, so the standing wave is no longer stable. This makes the standing wave build and reduce a number of times that relates to how far the device has been rotated. Keeping count of these shows how far it has rotated. This same technique is used with fiber optic gyros that use a coil of fiber instead of a ring.
Another way of doing it is with a microelectromechanical systems (MEMS) gyroscope, which is basically a tiny machine made of silicon. Some of these work on the same principle as Foucault’s pendulum (the one that rotates with the earth). A small resonator is created that stays still as the housing rotates and the forces upon it are measured. They are not as good but much smaller and cheaper than real gyro or a laser gyro. For applications that don’t need high accuracy, a MEMS gyroscope will have a great deal of use.
The other half of an INS is an accelerometer, which measures acceleration or acceleration due to gravity (inclination). An accelerometer is capable of detecting even the slightest movements, from the tilting of a building to smallest vibration caused by a musical instrument. Inside the accelerometer sensor, minute structures are present that produce electrical charges if the sensor experiences any movement. Accelerometers are used in automobile air bags to detect the sudden drop in the speed of the vehicle and to trigger the air bag release. Even laptops are now being equipped with accelerometers in order to protect the hard drive against any physical dangers caused by accidental drops.
Accelerometers are used in two ways in an INS: first, they detect how the platform (i.e., vehicle) is initially tilted with respect to the earth; second, they measure acceleration that is then summed twice to compute change in position as the vehicle moves. Determining initial tilt is simple: if the three accelerometers are mounted at right-angles to each other and the vehicle is not moving, the tilt is directly proportional to the amount of gravity sensed by the horizontal accelerometers. Once the initial tilt is known, the gyros can then measure the change in orientation with respect to the initial platform tilt. Together, this gives the orientation of the platform at any point in time. The INS uses the orientation to remove the sensed gravity from the accelerometers, leaving only the acceleration due to vehicle motion. This is then summed twice to measure the change in position of the platform.
We have come a long way from the days of mechanical gyros and accelerometers. Today, the marriage of physics and microelectronics has resulted in super-accurate devices that will fit in the palm of your hand.
A 1.289Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE