The Global Positioning System (GPS) is indeed a cornerstone of modern navigation, widely regarded for its user-friendliness, accuracy, and reliability. However, it’s crucial to recognize that GPS signals are susceptible to degradation or complete blockage from both natural phenomena and human-made interference. These disruptions can lead to either inaccurate positional data or a complete loss of GPS signal, posing significant risks in various applications, particularly in safety-critical domains like maritime navigation, aviation, and autonomous vehicles, reports NorthStandard.
Common Problems Faced
GPS (Global Positioning System) is a vital navigational tool, but its accuracy and reliability can be affected by several factors:
Incorrect Installation: If the GPS equipment and antenna are not installed correctly onboard the vessel, the reported position may be inaccurate, or the GPS signal might not be received properly. It is crucial to conduct thorough checks during the installation process to ensure that all equipment is fitted and commissioned according to the manufacturer’s specifications.
User Error: Mistakes made by the operator can also lead to GPS inaccuracies. For example, leaving the GPS in dead reckoning (DR) mode, which relies on the vessel’s last known position and movement sensors rather than satellite signals, can result in positional drift over time. Implementing a routine of checking the navigational equipment before the start of each watch is good practice. These checks not only familiarize the user with the current navigational data but also enhance their understanding of the equipment’s displays, menus, and alarm systems.
Atmospherics: Irregularities in the Earth’s atmosphere can interfere with signals from GPS satellites. A common example is refraction, where the signal’s path is lengthened as it passes through the atmosphere, causing positional errors. Users should learn how to manually select satellites to optimize the satellite geometry and minimize this effect. Additionally, solar storms can generate electromagnetic interference that can overwhelm the satellite signal, leading to significant positioning errors.
Local Issues: In certain geographical locations, such as polar regions, the availability and quality of GPS signals can be problematic. The distribution (‘spread’) and the number of visible satellites can impact the accuracy of the provided information. Multipath error can also occur when satellite signals reflect off nearby objects like structures or mountains before reaching the vessel’s antenna, leading to inaccurate position readings.
Deliberate Acts: Malicious actions such as jamming, spoofing, and hacking can intentionally disrupt a vessel’s GPS signal. Jamming involves locally generated interference that effectively drowns out the legitimate GPS signal. A notable incident in San Diego harbor in 2007 demonstrated the widespread impact of jamming, causing disruptions not only to shipping but also to critical services like the naval medical center, emergency pagers, and the harbor’s vessel traffic services.
Regular Cross-Checking
It’s crucial to avoid becoming overly dependent on GPS and to maintain proficiency in alternative methods of determining a vessel’s position. Regular cross-checking is essential, and navigators should be comfortable using traditional position-fixing techniques, even when utilizing Electronic Chart Display and Information Systems (ECDIS).
Here are some simple yet vital measures to ensure accurate position monitoring:
- Plot the position: Take multiple ranges and bearings from clearly identifiable landmarks or navigational features. Relying on a single range or bearing should be avoided. Even when using ECDIS, crew members should still manually plot ranges and bearings on the electronic chart to independently verify the vessel’s GPS-derived position.
- Increase the frequency: Plot the vessel’s position at appropriate intervals to ensure that any potential deviation or danger can be identified and addressed promptly between fixes. When navigating close to the coastline or in areas with hazards, position fixing should be performed more frequently.
- Parallel indexing: This is a straightforward yet highly effective technique for continuously monitoring the vessel’s track relative to planned routes or hazards. Parallel index lines should be incorporated into the voyage plan and regularly checked.
- Use the echo sounder: The echo sounder provides another valuable means of confirming the accuracy of the vessel’s plotted position. For instance, if the expected under-keel clearance at a plotted position is known (e.g., 14 meters), a quick check of the echo sounder reading can provide immediate confirmation.
- Beam bearings: Utilizing beam bearings (bearings taken at right angles to the vessel’s heading) is a highly effective visual method for confirming the precise moment to execute a course alteration based on a planned passing distance from a navigational aid or landmark.
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Source: NorthStandard
