Characteristics Common to All MNRs
The radiating characteristics of marine navigation radars are extensively regulated, principally by mandates from the IEC and ITU, through publications such as ITU-R.M.1313, Technical Characteristics of Marine Navigation Radars, and the IEC-60936 series which define performance requirements, methods of testing and required results for marine navigation and communications equipment. Manufacturers are usually required to obtain type-approval certification for their systems from a nationally-designated authority before they can be marketed or used operationally. Specifications cover many aspects, from range resolution to the use of colors and the labeling and operation of controls. The following paragraphs cover only the principal observable characteristics:
• Modern systems are required to conform to slightly different standards than older systems. In particular, there is growing pressure to minimize spectral interference; in consequence, modern magnetrons will have increasingly "pure" spectral behavior with progressively-reducing out-of-band radiation.
• Nominal radio frequencies for virtually all X-band MNRs are specified as 9410 MHz ± 30 MHz, although some MNRs -- older systems; and modern systems with a peak power less than 4 kW -- may be specified with a nominal center RF of 9445 MHz.
• In S band, the norm is specified as 3050 MHz, with documented deviations ranging between ± 20 and ± 30 MHz; some magnetrons claim a more tightly-constrained operating range, of better than ± 10 MHz; and a very few systems are specified with a nominal RF of 2975 MHz.
• It is very rare for an MNR magnetron to be tunable. Only one radar manufacturer (Kelvin Hughes) advertises a tuning capability, and the RF of their designs is normally set during assembly.
• In case studies described elsewhere in the handbook, there was strong evidence of two general trends of 3-cm magnetrons: a progressive reduction in nominal RF, typically of 1.0 to 1.5 MHz per month of continual operation; and a cyclic seasonal variation, where winter-time RFs were higher than summer-time RFs by as much as 5 MHz.
Pulse Repetition Frequency:
• Advertised pulse repetition frequencies may be little better than a guide to the likely nominal transmission rate, and reveal nothing of the complexity of behavior that is necessary to satisfy present regulations on interference reduction (complex inter-pulse timings are invaluable in satisfying these).
• In some instances of physical access it has been possible to identify the mechanisms by which inter-pulse intervals are defined in the radar. Where they have been identified, a very high frequency - typically 20 to 90 MHz - stable oscillator is used in conjunction with countdown circuitry. Only one manufacturer (Kelvin Hughes) is positively known to diverge from this philosophy, using a tunable pulse-frequency oscillator in conjunction with a noise-generating circuit. Assuming that the same stable oscillator drives the data-clocking rate described below, and that this rate will allow for at least two samples within the duration of the shortest pulse, it may be inferred that the lowest possible frequency of the oscillator is around 20 MHz.
Pulse Duration, also known as Pulse Length and Pulse Width:
• Magnetron manufacturers provide data sheets defining typical operating conditions, with minimum and maximum ratings beyond which damage may occur. Most X band magnetrons used in MNRs are tested with a 1.0 µS pulse; typically, when data-sheets for these devices show a maximum PD, they give values ranging between 1.0 and 2.0 µS, with a clear correlation between maximum pulse duration and peak power output. In S band, the longest pulse used in normal test conditions is also 1.0 µS, and the maximum safe PDs are usually documented as 1.2 and 2.0 µS (30 and 60 kW peak power output) respectively. The bottom line is that, with modern MNRs, there will be few instances where a PD greater than 1.0 µS (X band) or 1.2 µS (S band) is intentionally radiated.
• At the other end of the scale, present regulations require that modern MNRs have a minimum detection range no worse than 30 meters when operating in short range-scales, i.e. typically a 3 nautical-mile range-scale or smaller. Given this, the shortest PD likely to be radiated by a magnetron is rarely longer than 0.10 µS, and most magnetron manufacturers test their magnetrons with 0.07 µS pulses. These very short-duration pulses set upper bounds on data-clocking rates within the processor: it is likely that a modern MNR will digitize the output of its receiver system into range-bins that are equivalent to not more than half of its shortest PD.
• Typically, the magnetron drive pulse is provided by a solid-state electronic circuit, and the duration of this very precisely-shaped pulse defines the intended PD out of the magnetron. In reality, the magnetron continues to resonate for a short period after the drive-pulse terminates; in consequence, the radiated RF pulse has a more sharply defined leading edge than trailing edge. This, when coupled with other resonances and propagation effects, may cause the observed RF pulse to have a perceived duration greater than the magnetron design might allow.
• Duty cycle, also known as duty ratio (the ratio of the average PD to the average inter-pulse interval), is another of the key characteristics identified by the ITU as an interference-mitigation issue: MNRs are required to maintain a very low duty cycle (0.005%), so as to limit their effect on adjacent systems.
• Regulations governing High Speed Craft (HSC) radar performance mandate a higher antenna rotation rate - typically 41 rpm or more. These antennas may be in X band or S band, and their rotation speed may be fixed.
• Pleasure craft may be intermingled with high-speed marine traffic as well as being themselves high-speed craft. There is a growing trend in radar manufacture that caters to this market by the provision of multi-speed rotation - typically 24, 31-32 and 42 rpm, and the choice of drive speed may coupled either to range-scale selection or to the vessel's speed through the water.
• It is quite normal for X-band antenna systems to be DC-powered, whereas it is quite normal for S-band antennas to be driven by a 3-phase power supply. The antenna turn-rate for the latter is dependent on the frequency of the ship's power supply, usually either 60 Hz or 50 Hz. For example, the same S-band radar antenna model may rotate at 21 rpm in one ship and at 26 rpm in another.
• As with all publicized values, there is likely to be substantial variation between radars, but somewhat less likely to be significant variation in the rotation rate of a single installation. In fact, some modern designs seek to damp variations, even those caused by platform maneuvers, by comparing the intervals between successive azimuth-marker "pulses" and varying the motor speed accordingly. The result is a fairly constant rate independent of craft maneuvers, intended to simplify the process of target tracking required in larger vessels to conform to IMO regulations regarding ARPA, ATA and EPA tracking facilities. However, it should be noted that in case studies described elsewhere in the handbook, there was strong evidence of cyclic seasonal variability, with antennas employing slower scans in cooler months and faster scans in warmer months.
• As well as coupling antenna rotation rate to platform speed, some designs increase rotation rate when a very short range-scale is selected by the user. There are also systems capable of operating two displays, with very different range-scales selected - for example, to support navigation in confined waters while also monitoring horizon-range shipping. When operated in this way, these systems may switch between pulse rates scan-by-scan; to satisfy IMO regulations on display updating, they will typically operate with a very fast scan rate - typically 1.2 to 1.5 seconds per rotation, or 40-50 rpm.
• Antenna Leveling: A common problem with small craft, and especially sail-craft, is that the vessel may only rarely be upright, and the orientation of radar main-beam may thus rarely be aligned parallel to the sea surface. A small number of accessory-manufacturers and installers offer gimbaled self-leveling antenna platforms to overcome this; present designs are suitable only for radome-style antenna systems.
• "Sleep" Mode: In small-craft systems, especially battery-powered systems, it is growing design practice to include a "sleep" mode (sometimes referred to as "Watchman"), in which the radar is activated automatically for one minute every n minutes (a user-selectable value). Such systems require automatic target-detection facilities and hence EMI suppression capability, and audibly-insistent alarm systems. The automated tracking capability is sometimes called MARPA, an abbreviation for Mini-Automated Radar Plotting Aid; this designation is not associated with or regulated by the IMO.
• Antennae must be impedance-matched with the transceiver system if self-inflicted damage is to be avoided. Thus, antenna size alone does not indicate suitability for a system; for that, power rating must also be considered.
Antenna Radiation Pattern:
• Present IMO/ITU regulations mandate that the detectability of a reflective object (a shoreline) must remain constant, whether the radar platform is upright or heeling by up to 10° in any direction, and that the center of the radiated beam is to be parallel to the surface when the vessel is upright. These same regulations also prescribe the angular discrimination that radars for different classes must have - a value ranging downwards from 2.4°. From these, it is possible to define with confidence the radiation pattern: a fan-shaped beam not less than 20° wide in the vertical plane and not more than 2.4° in the horizontal plane. Many radar designs create a much narrower horizontal beam (around 0.75° is the minimum known, although this would be too narrow to conform to IMO regulations); and the vertical beam is typically 22-27° in width.
• The effective radiation pattern has been described as "an heroic compromise" between the intent of the designer and the real world in which the design functions. Clearly, a dominant external influence is the character of the immediate surroundings of the antenna, which may vary widely from ship to ship. In some installations, moreover, it may vary widely from day to day, or even hour to hour. Many inland water-way craft have their radars mounted on telescopic masts, allowing them to be lowered to the height of the ship's bridge so that the vessel may pass under low-clearance obstructions. (Alternatively, some of these craft have a hinged mast-structure that may be struck down for the same purpose; any radar mounted on one of these would clearly need to cease operation!) The changes in vessel interaction caused by raising and lowering the radar scanner will clearly impinge on the system's radiation pattern.
• Civil vessels not governed by IMO regulations, principally recreational craft, must comply with other technical characteristics of their radars, since they may impinge on other SOLAS devices (as well as providing SOLAS services in their own right), but their radiation patterns are less restrictive: wide vertical beamwidths are even more important, because of these vessels' more lively behavior, and wider (up to 7°) horizontal beamwidths are considered an acceptable compromise in order to allow radars to operate in the more confined upper works. These vessels' radar characteristics are usually regulated by national authorities such as the Federal Communications Commission (FCC) and other such regulatory bodies.