Global Navigation Satellite Systems (GNSS): satellite systems considered to have global satellite coverage. There are 4 main systems active as of 2024; GPS (USA), GLONASS (Russia), BDS (China), and Galileo (Europe), and each system has a different number and configuration of satellites globally. The satellite 'waves' which transmit info to ground-based stations work via line of sight; so a clearer line of sight with a satellite will mean more accurate information. Satellite transmissions are subject to many errors, such as distortion due to the troposphere, reflection off of targets, and most importantly a difference in time (time=lag). Multiple satellites will work together to draw 'lines of position' onto a target- the accuracy of which is measured through
Dilution of Position (DOP). The higher the DOP number, the less accurate the position fixing is due to greater errors affecting the signal. The GNSS System has land-based systems (GBAS), such as radio towers, which either transmit or recieve information from satellites or other ground stations in order to augment the satellite (SBAS) and air-based (ABAS) systems within GNSS.
Automatic Identification System (AIS): obtains vessel position info based off of information fed to it via GPS, rather than the vessels actual position in the world. AIS transmits different classes of information about a vessel; static and dynamic (and voyage information in class A models). The static information transmitted are things such as a vessels name, its characteristics (length and width), and what kind of ship it is. Dynamic information includes the position and speed. While only class A transponders (fitted on passenger and large ships) will transmit voyage information, that will include information like the destination headed, the number of people onboard, or the cargo. Most vessels will carry a class A transponder- the class B type are often carried by yachts and pleasurecraft. AIS is highly useful for collision avoidance (so long as the vulnerability of gps-based collision avoidance is not overlooked), search and rescue and vessel tracking. It is also integrated into the Aids to Navigaton system and there are AIS-specific navigation aids which exist digitally but not physically. You can view
AIS online to see the extent of the system and how useful it is for keeping track of vessels. (
wikipedia)
Magnetic Compass: During the process of construction, ships are made to have magnetic properties through external forces (magnetism
flowing from one pole to the other, welding in the vicinity, etc.) as they are built primarily with
ferromagnetic materials, like steel. A ship during
construction will obtain permanant, temporary, and induced magnetic characteristics, all of which will contribute
to
magnetic deviation effecting the magnetic compass
(causing it to point other directions than north). Deviation is one of many compass errors that the magnetic
compass may be subject to, along with magnetic variation, which is the annual shifting of
magnetic north away from 'true north' (geographic
north). While the magnetic compass is typically not the primary navigational compass used in the modern age because
of the errors it is subject to, it is still an effective piece of equipment as it does not require any power to
function. Modern ships will normally use a
gyroscopic compass or
fluxgate compass for navigation, but
must carry a magnetic compass as backup.
Nauticalsite.in
has a really fantastic article about magnetism, compasses, and compass work/course conversions used in chartwork. To minimize the effects of magnetic interference, the magnetic compass is stored in and viewed through a
binnacle. In cases where the binnacle is stored outside of the wheelhouse, it can often still be read through a periscope.
a free gyroscope can spin any direction
Gyro Compass: a non-magnetic compass which operates under the forces of
gyroscopic precession and gravity, allowing it to be made to point towards true north with a high degree of accuracy, and to continue to hold that position despite both vessel movement and rotation of the earth. A gyrocompass will take a few hours to spin up and point north but this process can be sped up via damping, which is the provision of precession to the gyrocompass to nudge it towards being north-facing. There are a lot of complicated principles that allow the gyrocompass to function, and the
nauticalsite article on gyro compasswork does a great job of explaining some of it. There is an archived service manual for the 1994
sperry mark 14 gyro-compass, which explains how the compass works. The modern gyrocompass is much more compact than this model. The gyrocompass is typically hooked up to compass repeaters, which allow the heading to be read off of the repeater and feed heading information into the bridge systems.
Radar: one of the most important modern devices onboard the ship. The radar works by sending out
RF energy waves, which reflect off of targets and return to the transponder; indicating the position of whatever they reflect off of. Certain materials are more reflective than others- water and steel are highly reflective; whereas wood tends to absorb the signal more. There are different types of radar antennae that serve different purposes; an 'open-array' style radar is not compatible with ships that have extensive rigging overhead- where a compact 'radome' style antennae is a far better choice.
In operation of the radar it is crucial that the pulee length allows time for the signal to return- otherwise the signals will intercept one another and produce a false reading. There are two types of radar bands,
X-Band ('extra-close') and
S-Band. The 'X-Band' uses a lower frequency signal, and is better for collision avoidance, and close range coastal navigation. The 'S-Band' signal travels further and is better used for long range detection, for weak signals, and in the rain. It is important to scrutinize radar information and in order to detect and account for errors like 'ghost echoes', gyro misalignment and yawing. What the radar excels at is range detection- its bearing detection may not be considered accurate because multiple targets can 'merge together', but the range is subject to little error and is highly accurate. In collision avoidance radar is a crucual aide by providing distance and approximate position, in addition to giving speed information which allows the navigator to plot a relative bearing and closest point of approach using the ARPA system.
Autopilot: the autopilot system works by maintaining a set heading, as opposed to manual or hand-steering where a dedicated quartermaster is necessary to achieve this purpose. It operates either in heading (holds set heading) or track mode (follows a pre set route and alters at waypoints), and in either follow-up (rudder will automatically move to hold course) or non follow-up mode (rudder will move only under control). The autopilot is not to be considered as a replacement for the quartermaster and it must be possible to switch from auto to manual steering 'immediately', which is technically defined as 3 seconds. In close quarters and collision avoidance scenarios it is unacceptible to use to autopilot system. It is important to note that the autopilot system is considered one of the various steering systems onboard, and in the event that control is lost elsewhere it can always be tested as a back up method (as in, in addition to the emergency hydraulic controls from the steering flats). In rough weather the autopilot may not be the best choice as the yawing motion can allow it to swing between courses, causing a greater rolling motion.
Echo Sounder: an underwater device which sends out soundwaves, which are then reflected back to provide a depth reading. It works by using the difference in time between when the signal was transmitted and when it was recieved, if at all, to measure the depth travelled. As with all radar devices it is important that the pulse repetition rate and
pulse repetition frequency are balanced in order for the signal to travel uninterrupted to minimize error. It is of utmost importance to always be aware of the depth of water underkeel and the echo sounder is an important device that changed
depth sounding forever and phased out the
lead line. If placed in an unstable position vulnerable to vessel fore-and-aft pitching, the echo sounder can also give false readings because it is moving from the position where it originally sent out its signals. The sounder should be placed perferably midships (depending on characteristics of course) and with as close to a 90 degree vertical beam projection as possible. The salinity of the water will have an effect on signal strength; meaning that in low salinity a depth reading may present as greater because the signal moves faster through the water, and in high-salinity may present as shallower beacuse the signal moves slower. The echo sounder is not a single unit; it is a system made up of different units to generate, transmit, and recieve the signals it deals with. The
pulse generator generates the signals, the
transducer recieves and transmits signals, the
amplifier enhances the signals recieved, which is then
recorded to produce a reading.
Doppler Log: a device for measuring the vessels speed by using the
doppler effect. The log will transmit a signal to the seafloor, and the time difference between the transmission and retrieval is used to measure speed. A doppler log consists of multiple systems like the echo sounder. Most importantly to the navigator, the doppler log will provide a reading of
speed through the water as opposed to the speed over the ground reading that GPS provides.