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Radar Basics Radar Radar Basics WHAT IS RADAR? ........................................................................ 3 RADAR APPLICATIONS ................................................................ 3 FREQUENCY BANDS .................................................................... 3 CONTINUOUS WAVE AND PULSED RADAR .................................. 4 PULSED RADAR ........................................................................... 4 PULSE POWER ................................................................................ 4 RADAR EQUATION ........................................................................... 5 PULSE WIDTH ................................................................................ 5 PULSE MODULATION ....................................................................... 6 Linear FM Chirps ................................................................... 6 Phase Modulation .................................................................. 6 Frequency Hopping ................................................................ 6 Digital Modulation ................................................................. 6 PULSE COMPRESSION ....................................................................... 6 LIFECYCLE OF RADAR MEASUREMENT TASKS .............................. 7 CHALLENGES OF RADAR DESIGN & VERIFICATION ............................... 7 CHALLENGES OF PRODUCTION TESTING............................................. 7 SIGNAL MONITORING ..................................................................... 8 BASIC RF PULSED RADAR SIGNALS ..................................................... 8 TRANSMITTER TESTS....................................................................... 9 RECEIVER TESTS ............................................................................. 9 Modern radars produce complicated pulses that can present significant measurement challenges. Improvements to range, resolution, and immunity to interference have motivated numerous coding schemes, frequency and phase modulated pulses, frequency chirped pulses, and narrow pulses with high overall bandwidth. Frequency Bands Radar frequency bands are classified into letter designations defined by the IEEE (see Table 1). The most commonly used bands used today for radar are the L-band through the KU- band or 1 – 18 GHz. Short-range automotive radar systems use the very high W-band frequencies in the 75+ GHz range. Figure 1: A Weather Radar Display Table 1 Common Radar Frequency Bands What is Radar? Band Use Range Radar (RAdio Detection And Ranging) is actually a fairly HF Long-range, over the horizon 3-30 MHz simple process of bouncing radio waves off objects and (OTH) looking at the reflections to determining presence, size, VHF Long-range, over the horizon 30-300 MHz distance, position and speed. (OTH) Typically, radars illuminate their targets with an RF pulse and UHF Long-range, over the horizon 300 MHz-3 then listen for the return echo. Since the RF pulse propagates (OTH) GHz at the speed of light, the time it takes the echo to return is proportional to the distance from the target. This, of course, L Long range Air Traffic Control 1-2 GHz applies to a primary-radar, one that relies on reflected energy (ATC), surveillance bouncing back off the target. Secondary-radars, that re- S Medium Range, Airport 2-4 GHz transmit the signal back from a transponder, have additional Surveillance Radar (ASR), delays. long range weather, marine radar Transmitted RF signals are “bounced” off a target and a C Weather, snow/ice mapping, 4-8 GHz receiver measures the characteristics of the echo. Most often, precipitation detection low duty cycle pulses are used. As the technology has evolved, the basic concept of radar has X Missile guidance, marine radar, 8-12 GHz not changed. At the same time, the accuracy, resolution and weather, ground surveillance, number of radar applications has increased dramatically over short-range airport tracking radar time. Ku High-resolution radar, satellite 12-18 GHz transponders Radar Applications K Cloud detection, police radar 18-27 GHz Radar technology is used heavily in military applications. Ka Short range, airport surveillance, 27-40 GHz Ground-based radar is used for long-range threat-detection red-light traffic camera trigger and air traffic control. Ship-based radar provide surface-to- surface and surface-to-air observation. Airborne radar is V Very short range, space 40-75 GHz utilized for threat detection, surveillance, mapping, and applications altitude determination. Finally, missile radars are used for W Automotive/vehicle 75-110GHz tracking and guidance. parking/warning radar systems, There are many commercial aviation applications of radar imaging such as air traffic control (ATC) long range surveillance, terminal air traffic monitoring, surface movement tracking, and weather surveillance. Additionally, short-range radar is increasingly being used in automotive applications for collision avoidance, driver assistance, and autonomous driving. Specialized radars can also be used to provide imaging through fog, through walls, and even underground. www.tektronix.com/radar 3 better resolution. This relationship constitutes one of the Continuous Wave and Pulsed fundamental trade-offs in radar engineering. Pulse compression with a modulated carrier is often used to Radar enhance resolution while maintaining a narrow PW allowing Radar systems can use continuous wave (CW) signals or, for higher power and longer range. more commonly, low duty-cycle pulsed signals. CW radar The Pulse Repetition Interval (PRI) is the time the pulse cycle applications can be simple unmodulated Doppler speed takes before repeating. It is equal to the reciprocal of the PRF sensing systems such as those used by police and sports or Pulse Repetition Rate (PRR), the number of transmitted related radars, or may employ modulation in order to sense pulses per second. PRI is important because it determines the range as well as speed. Modulated CW applications have maximum unambiguous range or distance of the radar. In fact, many specialized and military applications such as maritime / pulse-off time may actually be a better indication of the radar naval applications, missile homing, and radar altimeters. The system's maximum design range. detection range of CW radar systems is relatively short, due to the constraints of continuous RF power. There is no minimum Traditional radar systems employ a Transmit/Receive (T/R) range, however, which makes CW radar particularly useful for switch to allow the transmitter and receiver to share a single close-in applications. antenna. The transmitter and receiver take turns using the antenna. The transmitter sends out pulses and during the off- Pulsed Radar time, the receiver listens for the return echo. The pulse-off time is the period the receiver can listen for the reflected echo. The radar measurements discussed in this document are all The longer the off-time, the further away the target can be pulse measurements. Although there are several continuous without the return delay putting the received pulse after the transmission types of radar, primarily Doppler, the great next transmitted pulse. This would incorrectly make the target majority of radars are pulsed. There are two general appear to be reflected from a nearby object. To avoid this categories of pulsed radar, Moving Target Indicator (MTI) and ambiguity, most radars simply use a pulse-off time that is long Pulsed Doppler. MTI radar is a long-range, low pulse enough to make echo returns from very distant objects so repetition frequency (PRF) radar used to detect and track weak in power, they are unlikely to be erroneously detected in 2 small (~2m ) moving targets at long distances (up to ~30km) the subsequent pulse's off-time. by eliminating ground clutter (aka chaff). MTI is useful when velocity is not a big concern (i.e. “just tell me if something is Figure 3 illustrates the need for pulse compression to obtain moving”). Pulsed Doppler radar, in contrast, utilizes a high good range and resolution. Wider PWs have higher average PRF to avoid “blind speeds” and has a shorter “unambiguous” power, which increases range capability. However, wide PWs range (~15km), high resolution, and provides detailed velocity may cause echoes from closely spaced targets to overlap or data. It is used for airborne missile approach tracking, air run together in the receiver, appearing as a single target. traffic control, and medical applications (e.g. blood flow Modulated pulses mitigate these issues, providing higher monitoring). power and finer resolution to separate closely spaced targets. The RF pulse characteristics reveal a great deal about a radar's capability. Electronic Warfare (EW) and ELectronic lNTelligence (ELINT) experts specialize in the study of these pulsed signals. Pulse characteristics provide valuable information about the type of radar producing a signal and what its source might be - sailboat, battleship, passenger plane, bomber, missile, etc. Figure 3. Pulse Width versus Resolution Pulse Power Figure 2. Common Pulse Characteristics Another consideration for the maximum range of a radar is the transmitted power. Peak power is a measure of the maximum Pulsed radar typically utilizes very low duty cycle RF pulses (< instantaneous power level in the pulse. Power droop, pulse 10%). The range and resolution is determined by the pulse top amplitude, and overshoot are also of interest. ELINT repetition frequency (PRF), pulse width (PW),
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