1. How do ships find their position when only water surrounds them?
When a ship is surrounded entirely by water, there are no buildings or mountains to use as reference points. Despite this, a ship can determine its location with an accuracy of approximately 10 meters [2]. The key tool is radio waves arriving from far above.
GPS satellites are said to orbit at an altitude of about 20,200 km [4]. Each satellite continuously sends its own position and time information via radio signals. A receiver on the ship measures how long it takes for the signal to travel from the satellite to itself and converts that time into a distance [2].
Knowing the distance to just one satellite only tells you that you are somewhere on the surface of a sphere with that radius. According to the Geospatial Information Authority of Japan, knowing the distances to four or more satellites simultaneously allows the observer’s position to be fixed [2]. This is similar to estimating the distance to fireworks by measuring the time difference between seeing the light and hearing the sound. However, GPS uses radio waves for both signals rather than light and sound, and it reads time information to calculate the time difference.
The error in this method (standalone positioning) is said to be about 10 meters due to factors like satellite position and signal delays [2]. While conditions vary, this size of error is sufficiently useful for the open ocean. However, since radio waves travel 1 meter in approximately one-third of a billionth of a second, even a slight clock error causes large distance shifts. The next section discusses these clocks.
2. Why do tiny clock errors cause big GPS position mistakes?
The time it takes for radio waves to travel 1 meter is approximately 3.3 nanoseconds (calculated using the speed of light at about 300 million m/s). Because this time is so short, even a tiny clock error results in significant distance discrepancies.
Clocks on satellites flying high above Earth do not tick at the same rate as clocks on the ground because relativistic effects are at work. According to Ashby’s explanation, the frequency correction for satellite clocks is approximately 4.4645 × 10⁻¹⁰, which corresponds to about 38.6 microseconds per day. Without this correction, navigation errors would accumulate to more than 11 km in one day [1].
Multiplying 38.6 microseconds by the speed of light gives approximately 11.6 km. This calculation aligns with the source’s claim of "more than 11 km." Compared to the roughly 3.3 nanoseconds mentioned earlier, 38.6 microseconds is more than 10,000 times larger.
Therefore, satellite clocks are adjusted in advance to account for differences caused by relativity. The efforts to synchronize these satellite clocks support the ability to measure position accurately. Now that we have a position point, which map is it placed on?
3. Where does an electronic chart place a ship's position point?
Even if a ship knows its position, it needs a map to view it. Sea maps are called nautical charts and include information such as water depth, lighthouse positions, and the speed and direction of ocean currents and tidal streams [5]. With exceptions for some small vessels, all ships are required to carry nautical charts.
Electronic Navigational Charts (ENCs) are digital versions of these paper charts, standardized by the International Hydrographic Organization (IHO) and stored on electronic media. On board, they are displayed on the screen of an Electronic Chart Display and Information System (ECDIS). The ship’s position information from GPS and radar images are overlaid onto this display [5]. In other words, the point derived from GPS is placed directly onto the chart.
It is possible to set alerts for when the point approaches shallow areas or rocks. However, a warning has been issued by UK shipowners’ protection and indemnity associations that isolated shoals might not be displayed depending on alarm settings or screen display modes. Electronic charts are tools that require understanding their settings.
In Japan, ENCs are published by the Hydrographic Department of the Japan Coast Guard and distributed in encrypted form by the Japan Hydrographic Association. The data is divided into "cells" for each sea area, and users select types according to their voyage needs [6].
International rules have also been established. In 2000, the International Maritime Organization (IMO) amended SOLAS Convention regulations to recognize ECDIS as a means of satisfying chart carriage requirements. Subsequent amendments made it mandatory for new international passenger ships (500 gross tons or more), effective January 1, 2011 [7].
Safety Note: Nautical charts include water depth, lighthouse positions, and current speeds/directions. Carriage is mandatory for all ships except some small ones. ECDIS allows setting safety contours and depths to trigger alarms when approaching shoals or entering dangerous depths [5].
4. How often are electronic charts updated?
The ocean changes. New lighthouses appear, and shallow spots are discovered, so chart contents must be corrected. In Japan, ENC update data is issued every Friday [6]. However, if the expiration date passes, updates can no longer be applied.
If an up-to-date ENC is used with an appropriate backup device, it is considered equivalent to carrying paper charts [6]. This means electronic charts are not a "buy once" item; they serve as chart replacements only when updates are continuously incorporated. Even if a ship’s position is known to within a few meters, if the map is old, the point is placed on an outdated map.
So how does this compare to the location points we use daily on smartphones?
5. How does your smartphone's location compare with ship navigation?
Try it outside: With a parent or guardian, observe the blue dot on your smartphone map at two locations: one in a wide-open plaza and another right next to a tall building. Stand still for about 30 seconds in each spot; the dot may move slightly.
According to GPS.gov, GPS-enabled smartphones generally have an accuracy within a radius of about 4.9 meters in open sky areas, but this worsens near buildings, bridges, or trees [3]. Observe if the difference between the plaza and the building side matches this explanation.
When ships place points on electronic charts, they often travel through wide-open skies at sea, using these points to make decisions about avoiding rocks and shoals. Ship navigation involves knowing sky and position conditions, managing settings, and handling updates.
Look up resources: The Japan Coast Guard’s chart explanation pages and the Japan Hydrographic Association’s ENC pages are available in Japanese. The Geospatial Information Authority of Japan’s page on electronic reference points and GNSS allows you to verify the concept of standalone positioning [2].