EW Simulation, Part 10 -- Threat Antenna-Pattern Emulation.
The four figures show various types of scans. Each scan type is described in terms of what the antenna is doing and the time history of the threat antenna-gain pattern as it would be seen by a receiver in a fixed location.
The circularly scanned antenna rotates in a full circle, as shown in Figure 1. The received pattern is characterized by even time intervals between observations of the main lobe.
As shown in Figure 1, the sector scan differs from the circular scan in that the antenna moves back and forth across a segment of angle. The time interval between main lobes has two values, except in the case in which the receiver is at the center of the scan segment.
The helical scan covers 360 [degrees] of azimuth and changes its elevation from scan to scan, as shown in Figure 1. It is observed with constant main-lobe time intervals, but the amplitude of the main lobe decreases as the threat antenna elevation moves away from the elevation of the receiver location.
The raster scan covers an angular area in parallel lines, as shown in Figure 1. It is observed as a sector scan, but with the amplitudes of the main-lobe intercepts reduced as the threat antenna covers raster "lines" that do not pass through the receiver's location.
The conical scan is observed as a sinusoidally varying waveform, as shown in Figure 2. As the receiver location (T) moves toward the center of the cone formed by the scanning antenna, the amplitude of the sine wave decreases. When the receiver is centered in the cone, there is no variation in the signal amplitude, since the antenna remains equally offset from the receiver.
The spiral scan is like a conical scan, except that the angle of the cone increases or decreases, as shown in Figure 2. The observed pattern looks like a conical scan for the rotation which passes through the receiver's location. The antenna gain diminishes in amplitude as the spiral path moves away from the receiver location. The irregularity of this pattern comes from the time history of the angle between the antenna beam and the receiver location.
The Palmer scan is a circular scan that is moved linearly, as shown in Figure 3. If the receiver were right in the middle of one of the circles, the amplitude would be constant for that rotation. In the figure, it is assumed that the receiver is close to the center, but not exactly centered. Therefore, the third cycle shown is a low-amplitude sine wave. As the cone moves away from the receiver location, the sine wave becomes full size, but the amplitude of the signal diminishes.
If the conical scan is moved in a raster pattern, as shown in Figure 3, the received-threat gain history will look like the Palmer scan for the line of the raster which moves through the receiver location. Otherwise, the pattern becomes almost sinusoidal, with diminishing amplitude as the raster lines move farther from the angle of the receiver location.
The antenna snaps between four pointing angles, forming a square, as shown in Figure 3, to provide the requisite tracking information. Like the other patterns, the received threat antenna-gain history is a function of the angle between the threat antenna and the receiver's location.
LOBE-ON RECEIVE ONLY
In this case, as shown in Figure 4, the threat radar tracks the target (the receiver location) and keeps its transmitting antenna pointed at the target. The receiving antenna has lobe switching to provide. tracking information. The receiver sees a constant signal level, since the transmit antenna is always pointed at it.
Since a phased array is electronically steered, as shown in Figure 4, it can randomly move from any pointing angle to any other pointing angle instantly. Thus, there will be no logical amplitude history observed by the receiver. The received gain depends on the angle between the instantaneous threat-antenna pointing angle and the receiver location.
ELECTRONIC ELEVATION SCAN WITH MECHANICAL AZIMUTH SCAN
In this case, as shown in Figure 4, the threat antenna is assumed to have a circular scan with the elevation arbitrarily moved by a vertical phased array, furnishing a constant time interval between main lobes, but their amplitude can vary without any logical sequence. The azimuth scan can also be a sector scan or can be commanded to fixed azimuths.
Next month, we'll complete the discussion of threat emulation by consideration of the control mechanisms and practical limitations of emulators. For your comments and suggestions, Dave Adamy can be reached at email@example.com.
|Printer friendly Cite/link Email Feedback|
|Comment:||EW Simulation, Part 10 -- Threat Antenna-Pattern Emulation.|
|Publication:||Journal of Electronic Defense|
|Article Type:||Brief Article|
|Date:||Jan 1, 2000|
|Previous Article:||A Sampling of Night-Vision Systems.|
|Next Article:||Ballistic Missile Database.|