Radar Principles — Page 180, Lesson 174

I want to walk you through radar principles, and I'll start by showing you just how many jobs radar does in the aviation world. Radar isn't one single system — it's a family of techniques, and the book groups them into three big application areas. First, Air Traffic Control uses radar to monitor aircraft in relation to each other while they fly on airways, in control zones, or in the airfield vicinity, and to vector the aircraft if necessary. Vectoring means giving the pilot headings to steer. ATC also uses radar to provide a radar talk-down to a given runway — that's the Surveillance Radar Approach, or SRA, or the military Precision Approach Radar, which is PAR. They control and monitor aircraft on ILS let-downs, or during airfield instrument approaches. And they use radar to provide information regarding weather, for example storm clouds. Second, air/ground navigational systems use radar. Secondary Surveillance Radar provides ATC with information regarding an aircraft's call sign, altitude, speed, track history, destination, and type of emergency when appropriate. Distance Measuring Equipment, or DME, provides a pilot with very accurate slant ranges from a ground-based receiver/transmitter known as a transponder. Slant range is the straight-line distance from the aircraft to the ground station — not the horizontal distance. Third, the Airborne Weather Radar, or AWR, is used to depict the range and bearing of clouds, to indicate areas of the heaviest precipitation and associated turbulence, to calculate the height of cloud, and to do ground mapping. Now, why do radar systems operate in the VHF band and above? The book gives four reasons. These frequencies are free from external noise, static, and ionospheric scatter. The shorter wavelengths produce narrow, efficient beams for target discrimination and bearing measurement. The shorter wavelengths can produce shorter pulses. And efficient reflection from an object depends upon its size in relation to the wavelength — shorter wavelengths are reflected more efficiently. That last point is crucial. A radar works by bouncing energy off a target. If the target is small compared to the wavelength, the wave just flows around it and you get almost no reflection. So shorter wavelengths let you detect smaller objects. Now let's look at the pulse technique, because both primary and secondary radar systems use it. The pulse technique is the transmission of radio energy in very short bursts. Each burst of energy is in a pulse form of a predetermined shape. The duration of the pulse is equal to the pulse length or width. Although a pulse is of short width in time, it can contain many cycles — meaning many complete oscillations of the radio wave. Look at the figure — you'll see the pulse width, and then the time between pulses. That time between two pulses is called the Pulse Recurrence Interval, or PRI, also called the Pulse Recurrence Period, or PRP. And the Pulse Recurrence Frequency, or PRF, is the number of pulses transmitted in one second, measured in pulses per second, or pps. Here's the worked example from the book. If the PRF is 250 pps, what is the PRI? PRI equals 1 divided by 250 seconds. That's 1,000,000 divided by 250 microseconds, which gives you 4,000 microseconds. So one pulse every 4,000 microseconds, or 4 milliseconds. The relationship is simple: PRI is the reciprocal of PRF.