PPI Display in Radar | Plan Position Indicator

The PPI display shown in the figure is by far the most used radar display. It is a polar coordinate display of the area surrounding the radar platform. 

Own position is represented as the origin of the sweep, which is normally located in the center of the scope but maybe from the center on some sets.

The PPI display uses a radial sweep pivoting about the center of the presentation. This results in a map-like picture of the area covered by the radar beam.

A long persistence screen is used so that the display remains visible until the sweep passes again.

Bearing to the target is indicated by the target’s angular position in relation to an imaginary line extending vertically from the sweep origin to the top of the scope.

The top of the scope is either true north or ships heading.

PPI Display

The basic PPI display presentation results from raw(unprocessed) video. The raw video provides only a blip on the indicator screen, leaving the target interpretation entirely to the operator. 

PPI display is an intensity-modulated circulator representation or display in which echoes are in two coordinates i.e. Range and Azimuth in polar coordinates.

Functional Block Diagram of PPI Display

A brief explanation of the functional block diagram of PP displays s being discussed here. The function of each block is explained in brief. The figure Functional block diagram of the PPI display is shown in the figure below:  

1. Gate Circuit

• The gate circuit develops pulses, which synchronize the indicator with the transmitter.
• Trigger pulses from the synchronizer synchronize the gate circuit itself. It then provides timing for the intensity gate generator, sweeps generator, and sweep control circuit.
• While the PPI display at the older radar triggers pulses directly from the modulator synchronize units.

2. Sawtooth Generator

• The sweep generator circuits produce currents, which deflect an electron beam across the CRT.
• Varying voltages from the sweep control circuits are applied to the deflection coils.
• Gate voltages determine sweep rate, and therefore the effective distance covered by a each sweep.
• The amplitudes of the sawtooth sweep currents are varied simultaneously, corresponding to the rotation of the antenna.
• Notice that there is a 90-degree phase difference between the amplitude variations of the horizontal and vertical waveforms.

3. Intensity Gate Generator

• The intensity gate generator provides a gate, which unblanks the CRT during sweep periods. 
• The intensity of the trace appearing on the CRT is determined by the DC level of this gate.
• This circuit is also synchronized by the gate circuit.

4. Deflection Current Amplifier

• Electron deflection in the electromagnetic CRT is proportional to the strength of the magnetic fields.
• Magnetic field strength depends on the current in the coil. The sweep circuits associated with electromagnetically deflected cathode-ray tubes must provide currents, rather than voltage, to produce the desired beam deflection.
• A sawtooth current s required to produce a linear trace.
• Because of the inductance of the coil, a trapezoidal voltage must be applied across the coil to produce a sawtooth through it.

5. Video Amplifier

• The video amplifier circuits amplify the video signal from the receiver and apply it to the CRT intensity-modulated element.
• At this, a balanced relationship is necessary between brightness and contrast.
• The brightness of the grid of the azimuth and range marks should be adjusted very weakly.

6. Sweep Control Circuit

• To synchronize sweep rotation with antenna rotation, you must convert antenna azimuth information into an electrical signal.
• these signals usually provided by synchros, control the amplitudes and polarities of the sawtooth sweep currents applied to the deflection coils.
• The amplitude of the sawtooth sweep currents is varied sinusoidally, corresponding to the rotation of the antenna.

7. Cathode Ray Tube (CRT)

• There are two possibilities for the deflection of the electron beam:

• The Electrostatic deflection and focusing
• The electromagnetically controlled deflection and electrostatic of the electron beam

Frequently Asked Question on PPI Display

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Read More

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2. Radar Basics,  types, and application of Radar
3. Derivation of Radar Range Equation
4. What is Doppler Effect in Radar