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Radar A- Scope

Figure 1: View of an A-scope

Figure 1: Principle of operation of an A-scope

Radar A- Scope

The A-scope or A-display is the simplest and oldest form of display in a radar. Comparable to the display of an oscilloscope, it uses a so-called deflection modulation. An A-scope can display only the target distance and the relative size of the echo signals (in exceptional cases, an additional identification). It shows only the echo signals received from the direction in which the antenna is currently pointing. The bearing or even the elevation angle must be presented as dial or digital readouts that correspond to the actual physical position of the antenna.

The “main bang”
next
pulse period
echo signals
fixed clutter
range marks
noise

Figure 2: Basic representation of the contents of an A-scope

The “main bang”
echo signals
fixed clutter
range marks
noise

Figure 2: Basic representation of the contents of an A-scope (interactive picture)

Historical A-scopes use a cathode ray tube with electrostatic deflection plates. The horizontal deflection is caused by a sawtooth-shaped pulse at the horizontal deflection plates, the ramp of which should be as linear as possible. The length of the pulse (i.e., the duration of the deflection) determines the scale for the distance (see interactive block diagram). If the duration of the deflection is equal to the duration of the maximum travel time of the electromagnetic wave then the maximum range will also be displayed on the A-scope. Shortening the time for the deflection causes a change in scale, which means here that the distance is only shown in sections. The true-to-scale display is achieved by keeping the ratio of the propagation speed of the electromagnetic wave in space (in this case, approximately the speed of light c0) to half the horizontal speed of the deflection beam constant.

When no video signal is applied, the A-scope displays a horizontal straight line, which is therefore also called the zero line. The video signal is applied to the vertical deflection plates and causes a small portion of this zero line to being deflected up or down. The A-scope thus displays the current amplitude of the video signal as a function of time..

The start of the deflection (and thus the start of the time measurement) is always the rising edge of the transmitter's pulse. The transmitter's pulse can also be seen on the A-scope because the duplexer is not an ideal switch and thus a small, albeit heavily attenuated portion of the high transmitting energy still reaches the highly sensitive receiver input. This pulse can also be used for tuning purposes at the receiver. Often a part of this pulse is decoupled by a measuring attachment. It is additionally delayed and fed back in at the receiver input to obtain a special control pulse for tuning in a distance range. The delay causes that for this control pulse the time-dependent automatic gain control of the receiver is not active.

The A- scope display is using in older radar sets only as monitoring oscilloscope. In modern digital radar sets don't exist a similar video signal of the backscatter. The target messages are transmitted to the displays as a digital word. There isn't any possibility to get a synchronizing signal for these asynchronous serial digital signals. Well, the oscilloscope can get an internal trigger only. Therefore it is impossible to analyze the bit sequence with a simple oscilloscope. The one and only statement is possible seeing this picture: a digital word exists on this line, which means, obviously the driver module for this line works.

In exceptional cases, however, the radar system software can generate an A-scope-like display on a computer screen from the digital radar data. This is especially useful for training purposes or as an aid for the radar mechanic who can see the performance of the radar set at a glance based on this image.

Picture gallery of A- Scopes
A-Scope

Figure 3: A control-pulse shown at an A-scope of the russian VHF-radar „Spoon Rest”

Figure 4: the attempt to see digital serial signals with an A-scope

Figure 5: back from digital data converted analog signals, such as those are displayed in the didactic primary radar DPR886.