ARPA and RADAR
A maritime radar with Automatic Radar Plotting Aid (ARPA) capability can create tracks using radar contacts. The system can calculate the tracked object’s course, speed and closest point of approach (CPA), thereby knowing if there is a danger of collision with the other ship or landmass.
Development of ARPA started after the accident when the Italian liner SS Andrea Doria collided in dense fog and sank off the east coast of the United States. ARPA radars started to emerge in the 1960s and, with the development of microelectronics. The first commercially available ARPA was delivered to the cargo liner MV Taimyr in 1969 and was manufactured by Norcontrol, now a part of Kongsberg Maritime. ARPA-enabled radars are now available even for small yachts.
Radar and ARPA (Automatic Radar Plotting Aids) are standard systems on all commercial vessels and are widely used in the leisure maritime sector. This fully revised new edition covers the complete radar/ARPA installation, including AIS (Automatic Identification System) and ECDIS (Electronic Chart Display & Information Systems). It serves as the most comprehensive and up-to-date reference on equipment and techniques for radar observers using older and newer systems alike. Suitable for use both as a professional user’s reference and as a training text, it covers all aspects of radar and ARPA technology, its use and its role in shipboard operations. Reference is made throughout to IMO (International Maritime Organisation) Performance Standards, the role of radar in navigation and in collision avoidance, and to international professional and amateur marine operations qualifications.
The use of radar as primary navigational aid as well as safety tool is still an essential part of safe watchkeeping. Understanding the basic radar theory with plotting techniques will lead to a proper use of ARPA and other functions. The fast development of integrating radar, ECDIS and further navigational aids requires permanent training.
The availability of low cost microprocessors and the development of advanced computer technology during the 1970s and 1980s have made it possible to apply computer techniques to improve commercial marine RADAR systems. Radar manufactures used this technology to create the Automatic Radar Plotting Aids. ARPAs are computer assisted radar data processing systems which generate predictive vectors and other ship movement information.
The International Maritime Organization (IMO) has set out certain standards amending the International Convention for the Safety of Life at Sea requirements regarding the carrying of suitable automated radar plotting aids. The primary function of ARPAs can be summarized in the statement found under the IMO Performance Standards. It states a requirement of ARPAs….”in order to improve the standard of collision avoidance at sea: Reduce the workload of observers by enabling them to automatically obtain information so that they can perform as well with multiple targets as they can by manually plotting a single target”. As we can see from this statement the principal advantages of ARPA are a reduction in the workload of bridge personnel and fuller and quicker information on selected targets.
A typical ARPA gives a presentation of the current situation and uses computer technology to predict future situations. An ARPA assesses the risk of collision, and enables operator to see proposed maneuvers by own ship.
While many different models of ARPAs are available on the market, the following functions are usually provided:
1. True or relative motion radar presentation.
2. Automatic acquisition of targets plus manual acquisition.
3. Digital read-out of acquired targets which provides course, speed, range, bearing, closest point of approach (CPA, and time to CPA (TCPA).
4. The ability to display collision assessment information directly on the PPI, using vectors (true or relative) or a graphical Predicted Area of Danger (PAD) display.
5. The ability to perform trial maneuvers, including course changes, speed changes, and combined course/speed changes.
6. Automatic ground stabilization for navigation purposes. ARPA processes radar information much more rapidly than conventional radar but is still subject to the same limitations. ARPA data is only as accurate as the data that comes from inputs such as the gyro and speed log.
The radar picture of a raster-scan synthetic display is produced on a television screen and is made up of a large number of horizontal lines which form a pattern known as a raster. This type of display is much more complex than the radial-scan synthetic display and requires a large amount of memory. There are a number of advantages for the operator of a raster-scan display and concurrently there are some deficiencies too. The most obvious advantage of a raster-scan display is the brightness of the picture. This allows the observer to view the screen in almost all conditions of ambient light. Out of all the benefits offered by a raster-scan radar it is this ability which has assured its success. Another difference between the radial-scan and raster-scan displays is that the latter has a rectangular screen. The screen size is specified by the length of the diagonal and the width and height of the screen with an approximate ratio of 4:3. The raster-scan television tubes have a much longer life than a traditional radar cathode ray tube (CRT). Although the tubes are cheaper over their counterpart, the complexity of the signal processing makes it more expensive overall.