CH 3: Radar Signal Characteristics

Question 1

Radar Transmitter Characteristics (determined by transmitter)

Answer 1

1. Pulse width (pulse duration)
2. Pulse recurrence time (pulse repetition interval)
3. ,Pulse repetition frequency
4. Power

Question 2

Pulse width (PW) definition

PW unit of measurement

Answer 2

Pulse width, sometimes called pulse duration, is the time that the transmitter is sending out RF energy.

PW is measured in microseconds. It has an impact on range resolution capability, that is, how accurately the radar can discriminate between two targets based on range.

Question 3

Pulse Recurrence Time (PRT) definition

PRT unit of measurement

Answer 3

Pulse recurrence time is also know as pulse repetition time. PRT is the time required for a complete transmission cycle. This is the time from the beginning of one pulse of RF energy to the beginning of the next.

PRT is measured in microseconds.

PRT is the same as pulse repetition interval (PRI) which is used in RWRs and other systems to discriminate between radar systems.

Question 4

Pulse Repetition Frequency (PRF) definition

PRF unit of measurement

PRF and PRI relationship

Answer 4

PRF is the rate at which pulses or pulse groups are transmitted. PRF is the number of pulses generated per second and is expressed in hertz (Hz).

PRI is inverse of PRF

Warning:the operating freq of the radar (also measures in Hz) is not the same as PRF.

Question 5

Constant PRF radar, PRF stagger, PRF jitter definition

Answer 5

A pulse radar operating at an unvarying PRF is called a constant PRF Radar. The time between each pulse is the pulse repetition interval (PRI).

PRF stagger is accomplished when no adjacent PRIs are equal. The number of different PRIs generated is called the ‘position’ of the stagger. Ex: 2 position stagger would have 2 PRI values.

PRF jitter is considered a random stagger. It has no repeating pattern of PRI values.

PRF stagger and jitter are employed as electronic protection (EP) techniques against repeater or synchronous jammers.

Question 6

Radar Receiver Characteristics (determined by receiver)

Answer 6

1. Rest time
2. Recovery time (RT)
3. Listening time (LT)

Question 7

Rest Time Definition

Rest time unit of measurement

Answer 7

Rest time is the time between the end of one transmitter pulse and the beginning of the next. It represents the total time that the radar is not transmitting.

Rest time is measured in microseconds.

Question 8

Recovery Time (RT) definition

Answer 8

Recovery Time (RT) is the time immediately following transmission time during which the receiver is unable to process returning radar energy.

RT is determined by the amount of isolation between the transmitter and receiver and the efficiency of the duplexer. A part of the high power transmitter output spills over into the receiver and saturates this system. The time required for the receiver to recover from this condition is RT.

Question 9

Listening Time (LT) definition

LT unit of measurement

Answer 9

Listening time (LT) is the time the receiver can process target returns. It is measured from the end of the recovery time to the beginning of the next pulse.
LT=PRT-(PW+RT)

LT is measured in microseconds

Question 10

Duty Cycle definition

Answer 10

Duty cycle is the ration of time the transmitter operates to the time it could operate during a given transmission cycle.

Duty cycle has no units. CW radars have a duty cycle of 100%, while EW radars May have a duty cycle of around 1%.

Duty Cycle=PW/PRT or Duty Cycle=PWxPRF

Question 11

Peak Power definition

Answer 11

Peak power is the amplitude, or power, of an individual radar pulse. It is simply the power, measures in watts or megawatts, that is radiated when the transmitter is on.

The energy in a radar pulse determines maximum radar detection range. Power is the rate of flow of energy, energy in a radar pulse is equal to the peak power multiplied by the time the radar is transmitting or pulse width.

Energy per Pulse= Peak Power x PW

Question 12

Average Power definiton

Answer 12

Average power is the power distributed over the pulse recurrence time. The energy transmitted by average power can be computer by multiplying average power by PRT.

Average power or energy provides a better measure of the detection range of a radar than does peak power.

Average power can be increased by increasing the PRF, PW, or by increasing peak power.

Average Power= Peak Power x (PW/PRT)
Energy = Average power x PRT

Question 13

Modulation definition

Answer 13

Modulation is the process of changing an RF signal in order to transmit information on the signal. Modulation is accomplished by combing a basic RF signal, called a carrier wave, with a modulating signal that contains the desired information. The resulting waveform is then used to transmit the desired information.

Question 14

Amplitude Modulation (AM) definition

Answer 14

Amplitude modulation (AM) is when the carrier wave is combined with a modulating signal containing info of varying amplitude. Waveforms have the same frequency as the carrier wave but with a varying amplitude based on the info from the modulating signal.

Question 15

Frequency modulation (FM) definition

Answer 15

Frequency modulation (FM) is the combination of the carrier wave with a modulating signal containing information of varying frequency. The waveform has the same amplitude as carrier wave but different frequency based on the info from the modulating signal.

FM is also used with CS radars to make them more resistant to jamming and to add range determination capability.

Question 16

Pulse modulation (PM) definition

Answer 16

A type of amplitude modulation used in pulse radars to produce the short, powerful bursts of RF energy. PM combines the carrier wave with a rectangular pulse that acts like a switch. PM turns the transmitter on, leaves it on for a predetermined times, and then turns it off. The result is a waveform that produces radar pulses that can be used to measure range and angle to the target.