Radar applications, configurations, and pulse measurements based on scattering parameters

The demands placed on radars by the modern world present many challenges to radar designers and test engineers. To meet the emerging demands, multi-purpose/functional/mode adaptive radars that can meet different applications have emerged. Advanced radar systems must have higher accuracy to measure narrower pulse widths and achieve higher resolution to detect intra-pulse behavior (including rising/falling edge effects or waveforms within a pulse compression signal). To fully understand the complex design of modern radars, it is best to review the basic principles of radar systems and pulse measurements.

This white paper introduces common applications and types of radar systems, key elements and typical test parameters of radar systems, and three commonly used pulse measurement methods based on scattering parameters.

Radar Formula

Radar is the abbreviation of radio detection and ranging. Its basic principle is as follows: when electromagnetic waves propagate in a specific direction at a known power and frequency, they will be reflected when they encounter a target, that is, part of the electromagnetic wave signal is reflected back by the target, and the reflected electromagnetic wave signal can be measured by the receiving device. The power of the received signal (Pr) or the distance between the radar transmitter and the target (R) can be calculated by the following radar equation:

in:

Pr = received signal power;

R = distance between radar transmitter and target;

Pt = power of the transmitted signal;

G = radar antenna gain;

σ = radar cross section of the target;

Ae = effective antenna aperture;

λ = wavelength of the transmitted signal.

In addition to measuring distance, other information about the target can be measured by changing the radar system parameters, such as speed and direction. For example, scanning an area with a highly directional antenna can measure the azimuth and altitude of the target, and these two parameters can be used to determine the direction of the target, while the speed of the target can be determined by measuring the frequency shift of the received signal.

Radar classification based on purpose

Measuring the distance to a target is one of the basic functions of most radar systems. However, the technology of radar systems has improved significantly, including the manufacturing process, the signals used, the range of information obtained, and the use of the obtained information in various applications. Radar is widely used in military and civilian fields, and its specific uses include (Figure 1):

Figure 1: Radar has a wide range of uses (Courtesy of Raytheon).

· Surveillance (e.g. threat identification, moving object detection or proximity fuze generation);

· Detection and tracking (e.g. target identification and tracking or maritime rescue);

Navigation (e.g., automotive collision avoidance or air traffic control);

High-definition imaging (e.g. terrain mapping or landing navigation);

· Weather tracking (e.g. storm shelter or wind profile data acquisition).

Here are some common radar systems that use different types of signals (Figure 2):

Figure 2: Radar uses different signals depending on its application.

CW (Doppler) radar: This radar system transmits a continuous wave signal at a constant frequency. The received signal undergoes a Doppler shift , which can be used to determine the speed of the target. Police often use this radar system to monitor traffic conditions.

· FMCW radar: This radar system modulates the frequency of a CW signal to generate a timing reference. The user can determine the range and speed of a target from the timing reference. A significant advantage of CW-based radars (compared to pulsed radar systems) is that they provide continuous detection results. Aircraft are often equipped with this radar system to accurately measure altitude during landing. [page]

Pulse radar: This is a basic (non-coherent) pulse radar system that calculates the distance and direction of a target by measuring the time interval between the transmitted pulse and the received pulse. Since the phase between the pulses is random, pulse radar systems are non-coherent radar systems. This radar system is mostly used for long-range air surveillance.

Pulse Doppler radar: This is a coherent radar system that measures the range, direction, and velocity of a target based on the phase differences between received pulses. Pulse Doppler radar systems typically use a high pulse repetition rate (PRR), which allows for more accurate radial velocity measurements, but is less accurate when measuring range. This radar system is often used to detect moving targets while suppressing stationary clutter, and is very effective in weather monitoring applications.

Moving Target Indicator (MTI) radars: These radars also use Doppler frequency to distinguish the echoes of moving targets from stationary objects and clutter. The MTI radar waveform is a series of low PRR pulses, which can avoid range ambiguity, but cannot accurately estimate the speed of the target. These radars are commonly used for surface-to-air search and surveillance.

Pulse compression radar: Narrow pulse signals can achieve better range resolution, but the range is limited. Wide pulse signals contain more energy and can measure longer distances, but the resolution is poor. Pulse compression combines the power advantage of wide pulses with the resolution advantage of short pulses. By adjusting the frequency (e.g., linear frequency modulation) or phase (e.g., using Barker codes) of the transmitted signal, the wide pulse can be compressed at the receiver by the inverse of the adjusted signal width (Figure 3). Many weather monitoring systems already use pulse compression radar.

Figure 3: Pulse compression combines the power advantages of wide pulses with the resolution advantages of short pulses.

Elements of radar systems and their impact on system performance

When introducing radar equations, signal types, and antenna configurations, we mentioned many elements in a radar system. Table 1 summarizes some of the elements of a radar system and their impact on system performance.

Testing radar

Given the role they play, radar systems must operate as intended, or there could be serious consequences. Therefore, radar systems must be rigorously tested. Verifying the performance of the RF chain (Figure 5) is an important part of radar testing. Testing can be done on subsystems (such as the transceiver components of an active phased array radar) or specific components in the RF chain (such as the power amplifier of a transmitter or the low-noise amplifier of a receiver).

Figure 5: Simplified block diagram of an active phased array radar system.

The measurements included in a typical test are as follows:

Pulse measurement type

There are many common types of scattering parameter based and pulse shaping measurement techniques used in radar applications. This section discusses three of the most common measurement methods.

In-pulse point measurement

Intra-pulse point measurements quantify the scattering parameter data at any point in time within the pulse. These measurements use frequency or power sweep techniques and are plotted on demand after the measurement. This measurement method is useful when it is necessary to avoid possible pulse edge effects. For example, amplifiers are often stabilized at the beginning of a pulse. [page]

For point-in-pulse measurements, a long measurement period is required to obtain data, and the user needs to specify the corresponding interval for the synchronization pulse, namely T0 (Figure 6). This time interval is generally quantified by the delay (T1) and the required measurement time window width. By adjusting the delay (T1) of the measurement time window start time, initial effects such as amplifier settling time can be avoided. The following equation can be used to determine the minimum measurement time window:

TMW≥1/IFBW

Figure 6: Intra-pulse point-in-time measurements quantify the scattering parameter data at any point in time within the pulse.

If you use Anritsu (Anritsu) MS4640B vector network analyzer options 035 and 042 (PulseView), its 200MHz intermediate frequency bandwidth (IFBW) can form a minimum measurement time window (TMW) of 5ns.

If additional dynamic range is required, an average level can be specified for this purpose, so that the same time interval is analyzed over multiple pulses, the results are sampled from the same coherent clock, and the phase information is maintained.

This measurement method is suitable when the internal structure of the pulse and the differences between pulses do not need to be analyzed, but only the pulses need to be measured as a whole.

Pulse shaping measurements

Pulse shaping measurements focus on the data structure within the pulse (Figure 7). Pulse shaping measurements are performed in the time domain while the frequency and power are held constant. This measurement method is often used to determine pulse characteristics such as overshoot/undershoot, waveform droop, and edge response (e.g., rise/fall times).

Figure 7: Pulse shaping measurements focus on data structures within the pulse, such as overshoot/undershoot, waveform droop, and edge response.

To characterize the shaped pulse, the start time (Tstart), the end time (Tstop) and multiple time points relative to the synchronization pulse, T0, are set (Figure 8). If necessary, the measurement can start before the physical pulse and end after the DUT during the difference between UT and UTC. The measurement time window width is specified and it is also possible to average over multiple pulses. For point-in-pulse measurements, a wide range of measurement time window widths is allowed.

Figure 8: To specify the characteristics of a shaped pulse, it is necessary to set the start time, end time, and multiple time points relative to the synchronization pulse.

In pulse shaping measurements, the differences between pulses are often not noticeable, so the measurement may be an average of multiple pulses. However, the measurement can be designed to observe the behavior of the pulse from an absolute start time without averaging, thereby observing the entire evolution of the pulse behavior. Once the measurement data is obtained, it is usually plotted against time, so more complex measurements can be made using multiple channels or setups.

Pulse to Pulse Measurement

Pulse-to-pulse measurement is the process of quantifying the differences between pulses within a pulse stream. This measurement is also made in the time domain, where frequency and power are held constant. This measurement is often used to determine if pulse characteristics are changing over time. For example, a high power amplifier may experience thermal effects that can cause gain differences or phase differences.

Figure 9 shows a pulse-to-pulse measurement of three pulses. During the measurement, the corresponding delay (T1) is set by the synchronization pulse (T0) and each pulse is processed individually.

Figure 9: Pulse-to-pulse measurement is the process of quantifying the differences between pulses within a pulse stream to understand the thermal effects of high-power amplifiers.

In the measurement methods mentioned above, the available measurement time window width and pulse width range are wide, as long as the maximum width recorded is not exceeded. Among them, the maximum measurement time window width recorded on the MS4640B vector network analyzer is 0.5s. Therefore, even wider or lower repetition rate pulses can be measured. In addition, people can use multiple channels or settings to measure by cycling through various frequencies/powers.

Summarize

Modern radar systems require increasingly accurate measurements. Modern test solutions must eliminate and overcome the trade-offs and limitations commonly seen in pulse measurements. The Anritsu MS4640B vector network analyzer uses a high-speed digitizer architecture that delivers the highest resolution and timing accuracy in the industry. For more information on eliminating trade-offs in pulse measurement testing, see Anritsu white paper (11410-00709) - Eliminating and Overcoming Trade-Offs and Limitations in Pulse Measurement Test Solutions. For more information on the high-speed architecture of the MS4640B, see Anritsu white paper (11410-00711A) - VNA High-Speed ​​Architecture Improves Timing Resolution and Accuracy for Radar Pulse Measurements.

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The Anritsu MS4640B vector network analyzer with options 035 and 042 (PulseView) has the ability to generate and measure pulse signals. The instrument's four internal signal generators can generate singlet, doublet, triplet, quadruple/burst pulse signals. The Anritsu MS4640B vector network analyzer supports a variety of pulse measurement methods, such as pulse shaping measurement, fixed-point measurement within a pulse, and pulse-to-pulse measurement.