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Height-finding Radar
A height-finding radar is one "whose function is to measure the range and elevation angle to a target,thus permitting computation of altitude or height;such a radar usually accompanies a surveillance radar which determines other target parameters." Thus it is a 2D radar that scans in the elevation plane rather than in azimuth. Because of their cyclical up-and-down scanning motion,height-finding radar antennas are sometimes referred to as "nodding" antennas.
A 2D search radar maintains surveillance over the volume of interest, detecting targets and measuring their ranges and azimuth angles. Upon detection and establishment of a track on a new target, a request is sent to the height finder for elevation measurement. The height finder slews its antenna to the azimuth designated by the search radar and performs a scan over an elevation sector appropriate to the range designated by the 2D radar. Target echoes are displayed on a range-height indicator (RHI), and elevation angle θt is measured by an operator or an angle-gate circuit. Range R is also measured to an accuracy better than that provided by the search radar. The target height above the horizontal plane at the radar site is then calculated as
ht = R sin qt
This height is corrected as necessary for site altitude, atmospheric refraction, and curvature of Earth to give target altitude above sea level, as needed for ground-controlled
intercept of the target by a fighter aircraft. A typical nodding-beam height finder is shown in Fig. H1. The antenna is elongated in the vertical direction to provide a narrow elevation beam for accurate measurement, while the azimuth beamwidth is wide enough to accommodate errors in designation from the search radar.
Height-finders are gradually becoming obsolete as more three-dimensional, stacked-beam or phased-array search radars are being deployed.
Height finders are radars designed to measure the elevation angle of targets in a surveillance system,permitting target altitude to be calculated from measured range. The methods by which elevation angle and hence altitude is determined include:
(1) Assignment of a specialized radar that performs sector scan in elevation for measurement in that coordinate, on targets designated by a 2D search radar.
(2) Search with a scanning-beam 3D radar, in which a narrow beam is scanned over a raster covering both azimuth and elevation and providing measurement of both angles,along with range, on detected targets.
(3) Search with a stacked-beam 3D radar, in which multiple beams cover the elevation sector as the antenna scans in azimuth, providing monopulse measurement in elevation.
(4) Measurement of multipath time delay on targets detected in a 2D search radar, such that target altitude may be calculated from known target range, radar antenna altitude,and multipath delay.
(5) .Measurement of the ranges at which a target passes through multipath lobes of a 2D search radar antenna pattern,from which a constant target altitude may be calculated.
(6) Measurement of relative amplitude or phase of target echoes in two antennas displaced in altitude, leading to a monopulse estimate of elevation angle.
Reliability
The general definition of reliability is “the ability of an item to perform a required function under stated conditions for a stated period of time.” For radar it can be defined as the ability to perform assigned functions while retaining values of specified performance figures within assigned limits for each mode and condition of service, maintenance, repair, storage, and transportation. Typically, reliability is described by three components:
(1) Failure-free performance.
(2) Maintainability.
(3) Storability.
Failure-free performance is the ability to operate without failures for a specified operational time . Maintainability is “the ability of an item, under stated conditions of use, to be retained or restored to a state in which it can perform its required functions, when maintenance is performed under stated conditions and using prescribed procedures and resources.” Storability is the property of a radar to retain its operational conditions during storage and transportation. Reliability is an important radar performance characteristic,since it has significant effect on the effectiveness of radar operation and cost.
The main approaches to achieve reliability include
(1) The proper choice of technology and design efforts to avoid failures.
(2) Using a proper troubleshooting system (e.g., built-intest equipment) to determine and localize a failure as soon as possible after it happens.
The former approach includes the proper choice of reliable radar components and subsystems, and also the incorporation of the necessary redundancy in radar subsystems. The reliability of solid-state devices is usually much higher than that of vacuum-tube devices. As a result, a tube transmitter is usually one of the least reliable radar subsystems, and use of solid-state transmitters gives considerable improvement in radar reliability, permitting the manufacture of maintenancefree radars (at least in the sense that the permanent presence of maintenance personnel at the radar site is not required.).
Multichannel solid-state transmitters and phased arrays offer fail-soft operation in case of failure of one or more modules (e.g., several amplifiers the transmitter or array can fail without degrading radar performance significantly).
Another common approach to reliability is to use redundancy. This approach is often used in radars where even a short-duration failure is critical (e.g., military or civil ATC radars). A common technique is to use two adequate receiver-signal-processor channels with automatic reconfiguration to place the standby channel in operation in case of failure. An example of high-reliability, solid-state radar with redundant receiver channels and built-in test equipment is the family of ATC radars .
Two-Dimensional Radar
A two-dimensional (2D) radar mechanically scans a fixed beam, either in the azimuth plane (the conventional air search radar) or in the elevation plane (the nodding height-finder radar). A fundamental property of 2D air search radar is that the height of the antenna aperture must not be greater than that required to produce an elevation beamwidth that is matched to the required vertical coverage sector. Smaller values may, for mechanical reasons, be used, but at the expense of wastefully spreading energy above the required elevation coverage sector. The width of the antenna is determined by the azimuth resolution required and the requirement for obtaining a given minimum observation time (or time on target) while scanning. As in the case of the 3D search radar, this minimum observation time is largely determined by the doppler processing requirements, which in turn, are a function the clutter environment, and the diversity gain needed on the target.
For air search operations over broad elevation sectors, the minimum power-aperture product strongly favors use of the lower radar frequencies in 2D radar, although higher frequencies are often used for surface search, where the target is a land or sea-vehicle, a surface (for navigation), or a fixed structure. Operation at microwave frequencies is feasible here because, for surface-based radars, the required elevation sector is generally small, while for airborne radars the required elevation search sector is either small enough to match the
beamwidth of the available antenna aperture height, or such as to require only a few overlapping “bar” scans in elevation.