Radar and sonar — vital elements of ship navigation

Malcolm Latarche
Malcolm Latarche

12 April 2017


Predating the latest generation of electronic navigation technology Radar and SONAR are still vital elements of navigation and their role has not yet been surpassed by any suitable alternative. While both can still be used as stand-alone systems they can also be integrated with more modern technologies.

Radar stands for RAdio Detection And Ranging an acronym composed by US military during WWII but the development dates back to the same era as gyro compasses. A radar set simply transmits a radio signal and receives its echo bounced off ships and other structures both natural and man-made. From the transmitted and received signals it is possible to determine the position and distance of the targets.

The transmitted pulse can be produced in two ways. In early radar sets and still the most common method in use today is the magnetron. This is a vacuum tube that emits an extremely brief, powerful pulse of microwave radiation. It is energy intensive and has an unstable pattern that can result in ‘noise’ as returning signals are overwhelmed. Modern pulse radars may use digital processing to clean up the signal. Pulse radars use time to measure distance, and the direction the radar array is aimed at the time of transmission to measure bearing.

The technology moves on

In recent years a solid state alternative to the magnetron has appeared and is now offered by several system makers. Instead of the magnetron a solid-state, broadband transmitter that outputs a clean, frequency-stable signal is employed. The transmitter is coupled to a receiver dialled into a more precise band of frequencies and so eliminates much of the background noise that would mask the faint echoes from a target. These systems are less power hungry and produce much sharper signal returns.

Instead of transmitting a single output pulse, solid state radar transmits a continuous wave of ascending frequency known as a Frequency Modulated Continuous Wave (FMCW). The wave retains its frequency as it travels out and reflects back from any objects. The pioneering IMO standard solid state radar system was Kelvin Hughes SharpEye introduced in 2007. It had a peak output power of just 170W compared to the typical 30kW of a magnetron system.

Marine radar systems operate on one of two different bands X and S. The X-Band radar which makes use of a small light antenna operates on the 9,000 MHz band and has a wavelength of 3cm. The short wavelength allows better directivity but signal loss in rain is higher than on S-Band which operates 3,000 MHz band and has a wavelength 10 cm. S-Band is used for longer range detection and is less affected by poor weather.

Radar was first used some 70 years ago on merchant ships in 1946 and initially mostly by ferries allowing them to continue to operate safely in fog and at night. In 1960 recommendations for the use of radar were formulated for inclusion in the collision regulations. Later in 1974 with the new SOLAS convention, radar was made mandatory on ships in a rollout programme starting in 1980 and completing as surprisingly recently as 2002.

Is it obligatory to have a radar system?

All passenger ships and other ships above 300gt are obliged to carry at least one radar system operating on the X-band and ships over 3,000gt are required also to carry a second radar operating in the S-band. The radar systems installed can be stand-alone systems or they can be systems that connect to other navigation systems in an integrated system.

Considering the length of time that radar has been used on merchant ships and that use of radar forms part of the competencies required of navigators under STCW, its improper use has been cited in many official investigations as the root cause of collisions and groundings. Most criticisms centre on mistakes in plotting and identification of targets which carry no identifying features.

It was to help with this that AIS was made mandatory and as part of the introduction of AIS and the move to increasing use of electronic navigation, electronic plotting was also mandated.

Ships built before 1 July 2002 are subject to slightly different regulations with regard to how the movement of targets on the radar display can be plotted. For system fitted the since 1 July 2002 radars must be equipped with plotting aids, the type of which depends upon the size of ship:-

Electronic Plotting Aid (EPA)
EPA equipment enables electronic plotting of at least 10 targets, but without automatic tracking (Ships between 300 and 500gt)

Automatic Tracking Aid (ATA)
ATA equipment enables manual acquisition and automatic tracking and display of at least 10 targets (Ships over 500gt). On ships of 3,000gt and over the second radar must also be equipped with an ATA, the two ATAs must be functionally independent of each other.

Automatic Radar Plotting Aid (ARPA)
ARPA equipment provides for manual or automatic acquisition of targets and the automatic tracking and display of all relevant target information for at least 20 targets for anti-collision decision making. It also enables trial manoeuvre to be executed (Ships of 10,000gt and over). The second radar must incorporate ATA if not ARPA.

To estimate risk of collision with another vessel the closest point of approach (CPA) must be established. Choice of appropriate avoiding action is facilitated by the knowledge of the other vessel’s track using the manual or automatic plotting methods. The accuracy of the plot, however obtained, depends upon accurate measurement of own ship’s track during the plotting interval.
An inaccurate compass heading or speed input will reduce the accuracy of true vectors when using ARPA or ATA. This is particularly important with targets on near-reciprocal courses where a slight error in ownship’s data may lead to a dangerous interpretation of the target vessel’s true track. The apparent precision of digital read-outs should be treated with caution.

Electronic plotting will not detect any alteration of a target’s course or speed immediately and therefore should also be monitored constantly using all methods available especially visual contact through the bridge window.

If two radars are fitted (mandatory for ships of 3,000gt and over) it is good practice, especially in restricted visibility or in congested waters, for one to be designated for anti-collision work, while the other is used to assist navigation. If only one of the radars is fitted with ARPA then this should be the one used for anti-collision work and the other for navigation.

Because the radar display is produced electronically from the signals generated by the system, it was always going to be an essential component of e-Navigation and integrated systems. Radar can be integrated with ECDIS or be used in a less sophisticated chart radar configuration. The chart overlays of a chart radar may have a limited amount of data and are not the equivalent to an Electronic Navigational Chart (ENC) used in the ECDIS or paper charts. Unlike the ECDIS, they should not therefore be used as the primary basis for navigation.

Wide Screen Radar Displays

No one would argue that the migration to wide screen displays as used in integrated systems has taken radar to a new level compared with the original cathode ray tubes but future improvements and enhancements promise to add more value and functionality.
Moving away from the monochrome cathode ray tube display to modern screen displays also allows the use of a wider colour palette to differentiate between systems and targets. As an example, Kelvin Hughes introduced a feature called Enhanced Target Detection (ETD) mode, which it added to its Manta Digital range.

ETD enhances the display of slow-moving or stationary targets without interfering with the normal radar appearance or controls by treating static returns in a different way from moving returns and displays the moving ones in a different colour. Other added value systems include oil spill radar and small target radar that can identify very small targets in rough seas and have radar that can accurately determine dominant wave length, direction and period, significant wave height and superficial currents and send the data to systems for use in navigation support and safety measures.

Oil spill radar is not a navigational requirement but its use in offshore oil production and anti-pollution activities means it is carried on many ships working in those sectors.

Sonar

Radar may give ships the ability to ‘see’ other ships and fixed and floating objects at and above sea level but it is equally important for vessels to stay in water deep enough to keep afloat and avoid running aground.

All passenger vessels and other ships above 300gt are required to be fitted with an echo sounder which uses sonar to measure the depth beneath the ship. The IMO performance standards require a range from 2m – 200m and the ability to use two different scales; one for shallow waters to 20m and a second for deeper waters.

The echo sounder must be capable of giving an alarm when a pre-determined minimum depth is encountered. Data must be recorded and at least the previous 12 hours of information recorded and be available. Older echo sounders recorded the information on paper rolls and this is still a permitted method. Newer more advanced models make use of electronic recording and also take positional input from a GPS. The display on a newer model will generally be in colour on a small LCD screen. In common with other modern equipment, echo sounders often have features that exceed the IMO requirements.

There is a potential conflict between sonar and ECDIS where the data included in ENCs covers depth information. Using ECDIS for passage planning involves entering a ships draught and setting alarm parameters. Just as with paper charts there is potential for the depths recorded in ENCs to be inaccurate and it may be necessary to deviate from the passage plan if the sonar reports a difference in the ENC data and the actual under keel depth.

As mentioned in the section on e-navigation, water depths measured in real time by buoy-mounted sensors may be a complement to Sonar at some time in the future.