S-Band and X-Band Radar
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The S-band portion of the microwave band of the electromagnetic spectrum ranges from 2.0 to 4.0 GHz., crossing the boundary between UHF and SHF at 3.0 GHz. The S-band wavelength is around 10 cm. The maritime industry shares S-band radar with other entities permitted by the Federal Communications Commission (FCC) in this country and by the International Telecommunication Union (ITU) worldwide; and this permission is granted to such entities as weather services, police and fire departments. Communications are operated by such as Digital Audio Radio Satellite (DARS) are also users of a portion of the S-band (and broadcasts in the S-band around 2.3 GHz), and is currently used by Sirius Satellite Radio and XM Satellite Radio. More recently, the FCC has approved for portions of the S-band between 2.0 and 2.2 GHz the creation of Mobile Satellite Services (MSS) networks in connection with Ancillary Terrestrial Components (ACN).
The maritime industry uses a pulse-type system for maximum transmission of a signal since a steady transmission such as that used by communication sis not necessary. This allows more efficient use of the power needs for a signal to go out and return to the unit. The advantages of S-band for the maritime industry include the ability to send a signal as far away as 48 nautical miles and receive it with dependable return. This wavelength is also optimum at these ranges when taking in consideration weather and solar disturbances. At ranges shorter than a few miles however, S-band radar’s usefulness is hindered by its own characteristics required of it for the greater distances.
A disadvantage of S-band, albeit a small one is the size of the slotted waveguide array antenna that is associated with that system which can measure 3.6 meters across its face.
X-band is a radio frequency range designation that denotes the operational frequency of a specific radar system. X-band radar, itself has a variety of types. Among the list the following is included:
* Phased array.
As with the above-mentioned S-band system, in the maritime service, the X-band radar also uses pulse-type transmission. X-band radar has various uses in civil, military and government institutions. X-band radar is used in some systems for: weather monitoring, air traffic control, defense tracking, and vehicle speed detection for law enforcement.
X-band radar systems have been of great interest in the last few decades. The relative short wavelength at X-band frequencies makes possible high-resolution imaging radars for target identification and target discrimination. This is especially true for shipboard use.
Minimum Detection Range of Targets:
The maritime needs of radar as a tool for target detection dictate a number of parameters necessary to make efficient use of the system. Parameters can include but are not limited to:
* Radar Pulse length (in microseconds)
* Pulse repetition rate (PRR)
* Beam height and width
* Radar power
Pulse length has to be of a limited length to allow it to travel out and back the minimum distance without it overlapping the tail end of the transmitted pulse. This shorter pulse length of around .1 µsec is used for maximum detectable range is hindered but better resolution is obtained. Atmospheric conditions can play a role in determining how long the pulse length needs to be to satisfy the needs of the operator. Some of the adverse effect can be offset by additional power, if available. However, most radar systems are preset with power requirements for normal use.
Another method of getting around this limitation can be through the characteristic of the beam emission itself. The beam can be narrowed both horizontally as well as vertically somewhat so as to successfully receive the returned signal. However, limiting the vertical width of the beam is itself limited due to the need to attempt reception of the emitted signal while the ship rolls and pitches in a seaway. Horizontal beam width can be manipulated within the selectivity that is pre-programmed by the manufacturer. Most of these methods are usually not necessary since short range target acquisition does not need power to locate.
Other factors that may effect minimum range is sea return or sea clutter. A filter can be turned on most systems but in doing so, the filter may also have the effect of “masking” a close-in target. Side lobes of the transmitted pulse can also have an adverse effect on close-in target acquisition.
Finally, the characteristic of the radar beam, its location above the sea surface may create a “blind” spot just ahead of the vessel where smaller targets could actually hide in the shadow of the vessel and not be detected by radar at all.
Electronic plotting is based on data that constantly changes. Basis radar limitations must allow for this unsteady input of data. As the information goes into the processor for use with ARPA, it’s digitizing incorporate errors since it is accepting constantly changing analog data. ARPA smoothing can be accomplished by taking this inconsistent data and electronically “plotting” a course and speed of the target in question and in doing so creating a “reasonable” vector for that target. This vector can be reflected on a display in ether an actual vector or a relative vector (relative to our vessel). This vector can greatly assist the operator in determining his/her own course to take for collision avoidance.
ARPA smoothing can develop vectors to better determine course and speed of targets with input of data from various sources such as GPS, ship’s compass and log, gyro, and the radar itself. When working properly and with accurate data input, this system allows the operator to use his/her time in other activities for safe operation of his/her vessel. Instead of having to go through the arduous task of hand plotting a track with pencil and plotting board, he or she is free to run the ship.
These course/speed vectors created by automatic electronic plotting can be a result of bad or scanty information received by all of the input devices and can show not only the wrong course but also the wrong speed. If allowed to continue with a check, these errors can multiply and can cause false sense of security in collision avoidance.