Every radar produces a radio frequency (RF) signal with specific characteristics that differentiate it from all other signals and define its capabilities and limitations. Pulse width (pulse duration), pulse recurrence time (pulse repetition interval), pulse repetition frequency, and power are all radar signal characteristics determined by the radar transmitter. Listening time, rest time, and recovery time are radar receiver characteristics. An understanding of the terms used to describe these characteristics is critical to understanding radar operation.
PULSE WIDTH (PW)
PW, sometimes called pulse duration (PD), 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.
The pulse width of the transmitted signal is to ensure that the radar emits sufficient energy to allow that the reflected pulse is detectable by its receiver. The amount of energy that can be delivered to a distant target is the product of two things; the output power of the transmitter, and the duration of the transmission. Therefore, pulse width constrains the maximum detection range of a target.
Weapons-control radar, which requires great precision, should be able to distinguish between targets that are only yards apart. Search radar is usually less precise and only distinguishes between targets that are hundreds of yards or even miles apart. Resolution is usually divided into two categories; range resolution and bearing resolution.
Range resolution is the ability of a radar system to distinguish between two or more targets on the same bearing but at different ranges. The degree of range resolution depends on the width of the transmitted pulse, the types and sizes of targets, and the efficiency of the receiver and indicator. Pulse width is the primary factor in range resolution. A well-designed radar system, with all other factors at maximum efficiency, should be able to distinguish targets separated by one-half the pulse width time.
PULSE RECURRENCE TIME (PRT)
Pulse recurrence time is also known 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 radar warning receivers and other electronic warfare support (ES) assets to discriminate between radar systems. It also affects maximum radar range.
PULSE REPETITION FREQUENCY (PRF)
One of the most important characteristics of a pulse radar signal is pulse repetition frequency. PRF is the rate at which pulses or pulse groups are transmitted. Generally, PRF is the number of pulses generated per second and is expressed in hertz (Hz). PRF and PRI are related in that PRI is the inverse of PRF.
A word of caution does not confuse the operating frequency of the radar, which is measured in Hz, with the pulse repetition frequency, which is also measured in Hz. They are entirely different characteristics of a pulsed radar signal.
In order to build up a discernible echo, most radar systems emit pulses continuously and the repetition rate of these pulses is determined by the role of the system. An echo from a target will therefore be 'painted' on the display or integrated within the signal processor every time a new pulse is transmitted, reinforcing the return and making detection easier. The higher the PRF that is used, then the more the target is painted. However, with the higher PRF the range that the radar can "see" is reduced. Radar designers try to use the highest PRF possible commensurate with the other factors that constrain it.
There are two other facets related to PRF that the designer must weigh very carefully; the beamwidth characteristics of the antenna, and the required periodicity with which the radar must sweep the field of view. Radar with a 1° horizontal beamwidth that sweeps the entire 360° horizon every 2 seconds with a PRF of 1080 Hz will radiate 6 pulses over each 1-degree arc. If the receiver needs at least 12 reflected pulses of similar amplitudes to achieve an acceptable probability of detection, then there are three choices for the designer: double the PRF, halve the sweep speed, or double the beamwidth. In reality, all three choices are used, to varying extents; radar design is all about compromises between conflicting pressures.
Pulse radar operating at an unvarying PRF is called constant PRF radar. Pulse radar systems can employ PRF stagger or PRF jitter as an electronic protection (EP) technique against repeater or synchronous jammers. The time between each pulse is the PRI. PRF stagger is accomplished by assuring that no adjacent PRIs are equal. The number of different PRIs generated is called the “position” of the stagger. Two-position stagger would have two PRI values, for example, 300 microseconds and 500 microseconds. PRF jitter may be considered a random stagger. It is also an EP technique to counter synchronous jammers. PRF jitter has no repeating pattern of PRI values.
RADAR RECEIVER CHARACTERISTICS
Pulse repetition frequency, pulse recurrence time, and pulse width are determined by the transmitter. The pulse radar signal characteristics that relate to receiver operation are rest time, recovery time (RT), and listening time (LT).
Rest time is the time between the end of one transmitted pulse and the beginning of the next. It represents the total time that the radar is not transmitting. Rest time is measured in microseconds.
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.
Listening time (LT) is the time the receiver can process target returns. Listening time is measured from the end of the recovery time to the beginning of the next pulse, or PRT minus (PW + RT). Listening time is measured in microseconds.
Duty cycle is the ratio of the time the transmitter operates to the time it could operate during a given transmission cycle. The duty cycle of radar can be computed by dividing the PW by the PRT, or by multiplying the PW times the
PRF. Duty cycle has no units. Continuous Wave radars have a duty cycle of
100%, while early warning radars may have a duty cycle of around 1%.
Duty cycle is the fraction of time that a system is in an “active” state. In particular, it is used in the following contexts: Duty cycle is the proportion of time during which a component, device, or system is operated. Suppose a transmitter operates for 1 microsecond, and is shut off for 99 microseconds, then is run for 1 microsecond again, and so on. The transmitter runs for one out of 100 microseconds, or 1/100 of the time, and its duty cycle is therefore 1/100, or 1 percent. The duty cycle is used to calculate both the peak power and average power of a radar system.
Basic Radar Frequency
The power output of radar is normally expressed in terms of peak power or average power. Peak power is the amplitude, or power, of an individual radar pulse. It is simply the power, measured in watts or megawatts that are radiated when the transmitter is on. The power, radar transmits is normally used to determine the maximum detection range of that radar. However, it is the energy in a radar pulse that determines maximum radar detection range. Since power is the rate of flow of energy, the energy in a radar pulse is equal to the peak power multiplied by the time the radar is transmitting, or pulse width.
Average power is the power distributed over the pulse recurrence time. It can be computed using the formula
Average Power = Peak Power *(PW/PRT)
The energy transmitted by average power can be computed by multiplying average power by PRT. Since the energy in a set of pulses determines detection range, average power or energy provides a better measure of the detection range of radar than does peak power. Average power can be increased by increasing the PRF, by increasing the pulse width, or by increasing peak power.
The characteristics of an RF signal must be changed in order to transmit information on the signal. This process is called modulation. Modulation is accomplished by combining 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.
One basic modulation technique is amplitude modulation (AM). The carrier wave is combined with a modulating signal containing information of varying amplitude. Waveforms produced have the same frequency as the carrier wave but with varying amplitude based on the information from the modulating signal. AM is used extensively in communications and broadcast radio transmissions.
Frequency modulation (FM) is another means of impressing information on a carrier wave. Frequency modulation is accomplished by combining the carrier wave with a modulating signal containing information of varying frequency. The waveform produced has the same amplitude as the carrier wave, but the frequency varies based on the information from the modulating signal. FM is used extensively in communications and commercial radio. FM is also used with continuous wave (CW) radars to make them more resistant to jamming and to add range determination capability.
A type of amplitude modulation known as pulse modulation (PM) is 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 time, 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
The radar signal characteristics of PW, PRI, PRF, and power determine the maximum range and the range resolution capability of specific radar. When combined with the frequency of the carrier wave of the radar signal, these parameters provide a unique signature to identify a specific radar signal.
Modulation is the method used to put information on an RF carrier wave. The primary modulation techniques used in radar signal generation include amplitude, frequency, and pulse modulation. The radar signal characteristics of PRF, PRI, power, and modulation are the keys to understanding radar operation and jamming techniques.