The Navy is making a concerted effort to find alternative navigation systems to mitigate the risk of relying solely on GPS. The surface fleet already should be prepared to safely navigate in the event of a total loss of GPS. Sadly, it is not.
A system currently deployed and ready to provide accurate navigational data is being overlooked—the Inertial Navigation System (INS) AN/WSN-7 Ring Laser Gyro Navigator (RLGN). Training for bridge navigation teams, RLGN technicians, and deck officers should be updated so ships can optimize all available positioning, navigation, and timing (PNT) sources. Operating procedures also should be updated to emulate the submarine force’s methodology for extended navigation in a GPS-denied environment. These changes can be implemented immediately. They do not require the extensive cost and time associated with installing GPS-alternative hardware systems.
Navigation Fundamentals
With modern technology, even the least-experienced sailor can accurately navigate to destinations far and wide. However, blind reliance on GPS is a danger to any blue-water sailor who lacks a fundamental understanding of the history and art of navigation, familiarity with navigation tools old and new, and the operational experience to blend this knowledge with modern technology.
Standing night watch in the charthouse on board the USS Independence (CV-62) in 1982, I found the American Practical Navigator (Bowditch) nestled tightly in the bookshelf above the chart table. I was instantly hooked on the immense volume of knowledge and wisdom it contained. I reflect fondly on those lonely North Atlantic night watches during which I began my awakening to the age-old art and science of navigation. As I plan my upcoming retirement—and now live on board my world-cruising sailing catamaran Ohana—there is a special place at my chart table for Bowditch.
Chapter One, “Introduction to Marine Navigation,” illuminates the fundamental principles of navigation succinctly:
In practice, a navigator synthesizes different methodologies into a single integrated system. He or she should never feel comfortable utilizing one method when others are available. Since each method has advantages and disadvantages, the navigator must choose methods appropriate to each situation, and never rely completely on only one system.
The introduction to the Surface Ship Navigation Department Organization and Regulations Manual includes an equally salient observation:
Technological advances have changed the way we display charts, plot ship’s position, control the ship, and maintain situational awareness, but the fundamentals of safe navigation remain unchanged [emphasis added]. As such, no specific technological advances or set of rules can be created that can keep the ship safe in all possible conditions. However, thorough operational knowledge of the installed ship control and navigation systems, coupled with good judgment applied to the specifics of each situation, is critical to ship safety.
Studies show several factors can reduce PNT accuracy, including a navigator’s lack of understanding of navigation system performance and lack of user knowledge regarding the capabilities and limitations of GPS data and the installed INS.
Unprepared for a GPS Outage
Analysis of the manual’s requirements and guidance yields two serious concerns. First, it defines just three categories of GPS accessibility: unrestricted access and accuracy, limited (jammed) access, and spoofed. The manual does not consider a complete loss of GPS, be it brief or extended. Second, though INS is mentioned in certain areas, there is a lack of understanding about the high degree of positional accuracy the RLGN INS can provide the surface fleet.
The submarine warfare community already employs the methodology, guidance, and procedures necessary to fully use INS-derived positions to accurately navigate for extended durations in a GPS-denied environment. Submarine navigation teams are better trained in using the tools to continually evaluate INS performance with respect to master gyro compass–derived dead reckoning (DR) positions, all while operating for extended periods with no GPS data. Only when the DR versus INS positions diverge will the boat obtain a GPS fix, evaluate that fix for accuracy, reset the INS, and then return to navigating without GPS. The surface fleet should adopt these navigation procedures to reduce its reliance on GPS.
Maximizing INS Usage
Inertial navigation systems are glorified dead reckoning systems. Accelerometers sense vessel movement, integration with time yields velocity, and a second integration yields displacement. Since accelerometers cannot discern the difference between vessel acceleration and gravitational acceleration, gyroscopes isolate the horizontal accelerometers from the effect of gravity, orient the system north, and yield both the vessel’s change in position and its attitude (heading, roll, pitch). Modern inertial navigation systems, such as the AN/WSN-7 RLGN or the future AN/WSN-12 RLGN, can produce highly accurate navigation solutions for extended periods of time without GPS input. Because these systems are Earth-oriented and Schuler-tuned to account for the Earth’s curvature, they do require an external source of ship speed such as the electromagnetic log to dampen Earth loops and reduce oscillations.
Unfortunately, surface fleet procedures configure the RLGNs to slave to GPS for both damping and fix/reset. This is completely antithetical to the proper functioning of an INS. If GPS becomes degraded, the inertial systems will follow the GPS error for a time. Should GPS then be lost, the INS fix/reset filter will already have bad data and thus may not perform at expected levels. A properly operated and functioning inertial system must not be allowed to propagate errors over time. RLGN operator and maintenance training should be updated to include theoretical inertial training to support monitoring and understanding of normal error propagation and enable operators to take appropriate actions to ensure maximum accuracy. GPS damping should be the secondary and not the normal mode.
Two basic errors can degrade INS long-term accuracy—internal errors related to sensor quality and stability (gyroscopes and accelerometers) and external errors related to Earth functions. Since the systems are tied to the center of the Earth and operate with a mathematical model of the Earth (a reference ellipsoid), they are subject to periodic oscillations. The primary oscillation has a period of 24 hours as the Earth loops are affected by the rotation of the Earth. The secondary oscillation has a period of 84.4 minutes—the period of a pendulum with a length equal to the radius of the Earth.
Whenever the vessel operates in an area where the true local gravity vertical diverges from the reference ellipsoid vertical, a Schuler oscillation occurs. Technically, all Earth-oriented inertial systems are always experiencing a Schuler oscillation of some magnitude, but in areas of large gravitational anomalies (mountain, trench, or continental shelf), the magnitude of oscillation may become unmanageable and make an accurate reading impossible unless the system is provided an external source of ship speed.
Close coordination between the bridge navigation team and the RLGN operators is mandatory when approaching areas of vertical deflection. The INS must be closely monitored, and the damping mode changed to reduce the long-term effect of large vertical deflections. Current surface navy training fails to address this critical coordination. It no longer teaches the in-depth theoretical inertial principles necessary for operators to understand how the system is operating correctly and how to recognize when it is not.
As the aggregate compendium of navigation principles and knowledge since 1800, Bowditch should be mandatory reading for all surface ship navigation team members. The surface navy should regularly train without GPS, ensuring these teams know how to use all the other tools available to aid in navigating. Sailors will revel in their newfound knowledge and ability to navigate without GPS.