Before satellite-based navigation came along, lead, log and lookout were the keys to marine navigation. While we still advocate for the 3 Ls, the advent of GPS changed the marine navigation seascape and opened the door to marine autonomous surface ships (MASS).
With MASS hull-up on the horizon, GPS – or any other satellite-based position fixing system – isn’t enough. The Maritime and Coastguard Agency’s MGN 379 reminds human navigators to be aware of the dangers of over-reliance on the output from, and accuracy of, a single navigational aid. The same warning applies to autonomous ships.
According to a definition from an otherwise-forgotten textbook, GPS is a satellite-based radio aid to navigation which operates on the basis of range determination by time-difference measurements. A GPS receiver calculates the range from at least three satellites to generate spheres of position. The GPS antenna must be wherever those spheres intersect.
While it’s a fantastic system, it’s not perfect. It’s subject to a range of errors including errors in time measurement, atmospheric errors, radio signals bouncing around before getting to the receiver and, more recently, GPS jamming and spoofing. Add the fact that continued use relies on the goodwill of a foreign government, and we have to question how much we should rely on it.
It’s the threat of these errors that prompts warnings against reliance on a single navigational aid. This is even more important for autonomous ships. Because of this, governments and shipping companies are considering alternatives.
Doppler shift
In 1957, William Guier and George Weiffenbach discovered that they could not only detect the signal from the Russian satellite Sputnik I but could use the signal to track the satellite. Because they knew both their position and the frequency of the signal transmitted by the satellite, they could use the signal’s doppler shift to calculate Sputnik’s orbit.
About a year later, they realised they could reverse the process: if they already knew a satellite’s orbit, they could use the doppler shift to calculate the receiver’s position. The US Navy’s Navigation Satellite System, Transit, was born. It remained in use until 1996, when it was superseded by more modern global navigation satellite systems (GNSS).
Iridium is a satellite communication company. Their Next constellation of low-earth-orbit satellites can provide Satelles Location and Timing (STL), a doppler-based solution for position determination. In tests, STL showed promising results, and could be a viable alternative to GPS.
Inertial navigation
In many ways, inertial navigation is very similar to dead reckoning. Simply, dead reckoning is a way to estimate your position based on the distance and direction you’ve travelled from your starting position. In the good old days, we used a magnetic compass and the ship’s log to estimate the course and speed; more recently inertial navigation systems use accelerometers and gyroscopes to determine the direction and speed of motion. The problem is, with no further information the accuracy of the dead reckoning position decreases over time.
Before the Cold War, missiles used to navigate solely on inertial navigation systems (INS); many military aircraft and ships still use them.
Automated celestial navigation
In many ways, celestial navigation is very similar to GPS. At its most basic, a navigator measures the angle of a celestial body above the horizon, known as taking a sight, then calculates the location of the spot on earth directly below that body.
The navigator uses the measured angle to work out how far they are from that spot, giving a line-of-position. A bearing, known as an azimuth, gives a second line-of-position; where those two lines intersect is the navigator’s position. Taking more sights gives more lines-of-position, and accuracy improves.
Even for an experienced navigator, it’s a time-consuming process; in daylight or when it’s overcast, it’s difficult or impossible. And that’s where technology comes in. It doesn’t even have to be new technology.
Automated astro-inertial navigation (ANS) was introduced in 1958 to improve the navigation of the SM-62 Snark cruise missile. The automated system automatically tracked celestial bodies and used the calculated position to correct the INS drift errors; nowadays, INS systems use GPS to do the same thing.
In 2014, Northrop Grumman and Trex Enterprises announced an agreement to collaborate to bring celestial navigation technology into the 21st Century. Trex Enterprises’ multi-aperture Daytime Stellar Imager can detect 6.3 magnitude stars during the hours of daylight. Combined with their automated star detection and pattern recognition algorithm, it makes automated celestial navigation a viable alternative – or supplement – to GPS and inertial navigation for myriad applications.
Hyperbolic navigation
When it comes to hyperbolic navigation, Wikipedia is straight to the point:
“Hyperbolic navigation is a class of obsolete radio navigation systems in which a navigation receiver instrument on a ship or aircraft is used to determine location based on the difference in timing of radio waves received from fixed land-based radio navigation beacon transmitters.”
There have been several hyperbolic navigation systems, including Omega, Decca and Loran. As Wikipedia correctly points out, they’re no longer in use; however, over the last few years various governments have considered implementing eLoran as a resilient land-based alternative to GPS.
eLoran is a more advanced version of Loran-C, and would use repurposed Loran towers. Various governments, including the USA, UK and EU have wavered over eLoran implementation. Currently, none of them plans to bring an eLoran system into operation; despite that, eLoran remains a feasible component of a resilient position-fixing system.
Integrated position, navigation and time systems
Like so many things in life, there are pros and cons to every navigation system. Fortunately, we don’t need to rely on a single system. An integrated navigation system can consolidate multiple inputs, compare the positions, remove outliers, and output a single position.
Whether it’s on the bridge of a standard manned ship or a MASS, an integrated system with multiple independent sources of position information would provide a resilient and user-friendly basis for electronic navigation.
Original Article
