In this article, we are going to learn about the most powerful Radar which is called MTI radar. We will discuss the MTI radar in every detail. We will discuss the MTI radar block diagram, limitations of MTI radar, MTI radar applications, advantages of MTI radar, and the difference between MTI and pulse Doppler radar in detail. So let’s start with the introduction from basic knowledge of radar.
Introduction to Radar
The Radar system till now was required to detect targets in the presence of noise. But in practice, radars have to deal with more than receiver noise when detecting targets while they can also receive echoes from the natural environment such as land, sea, and weather. These echoes are called clutter since they tend to clutter the radar display with unwanted information.
The clutter echoes signal has a greater magnitude than the echo signal received from the aircraft. When an aircraft echo and a clutter echo appear in the same radar resolution cell, the aircraft might not be detected. But the Doppler effect permits the radar to distinguish moving targets in the presence of fixed targets even though the echo signal from fixed targets has a comparatively greater magnitude than the moving targets such as aircraft.
Echo signals from fixed targets are not shifted in frequency, but the echo from a moving target with relative velocity Vr will be shifted in frequency by giving the Doppler formula.
The MTI radar has a PRF low enough to not have any range ambiguities Run=c/fp (such as multiple around echo). It does however have many ambiguities in the Doppler domain (such as Blind Speed).
The pulsed Doppler radar on the other hand has a PRF large enough to avoid Doppler ambiguities but it can have numerous range ambiguities. There is also a medium PRF pulsed Doppler radar that accepts both range and Doppler ambiguities.
There is a type of MTU radar that does use the information directly from the Doppler but it observing the area of the map scan to scan. Only moving objects which change their position from scan to scan from mapping operations are displayed. This type of MTI radar is known as Area MTI radar.
MTI Radar Block Diagram
A schematic block diagram of MTI Radar is shown in the figure below. This block diagram has got two channels, one is the receiver channel, and the other is a coherent oscillator (COHO channel).
A receiver channel is identical to the standard radar receiver except for the local oscillator which is designed to have good frequency stability and a limiter is included in the IF stage so the amplitude of the echo signal fed to the phase detector should have the same amplitude.
The reference signal is provided by the COHO oscillator and is a stable1 oscillator (STALO) operating at a frequency that is near the difference between the transmitter and Stalo frequencies.
The phase of each transmitter pulse is randomly related to the phase of the last pulse in the radar using magnetrons as a transmitter. It is essential that the phase of the echo oscillator be matched with the phase of the transmitter pulse is injecting into the COHO channel. If the frequency of the COHO oscillator is stable, its phase in the time interval between pulses is then related to the phase of the oscillations of the last outgoing pulse in exactly the same way for successive pulses.
The phase detector used in MTI radar compares the phase of the COHO oscillator and the amplitude-limited intermediate frequency echo signal. The phase detector output for successive pulses may be subtracted from each other by employing a single delay line. A delay line provides the delay of one PRP, it may employ a mercury delay line or quartz transmission line, upon which are impressed mechanical vibrations lying in the frequency range of 10 to 20 Mhz, modulated by the video signals.
The phase comparison is carried out at the IF stage instead of at a much higher transmitted frequency. The output of the phase detector is fed as the input to the delay line canceller, which is acting as a high-pass filter. It will separate the moving target from the echo received from the stationary target.
The fixed targets with the constant amplitude from pulse to pulse are cancelled on subtraction. But the moving target has varying amplitude from pulse to pulse which is not cancelled on subtraction. The output from the subtractor unit is bipolar in nature and it is to be converted into unipolar video by using a full wave rectifier before presenting on displaying unit.
By practical MTI system is capable of canceling echoes from fixed targets having an amplitude of the order of 40 dB or greater above visibility. At the same time, it is possible to observe moving targets superimposed over the fixed targets and which are weaker than the fixed targets by as much as 25 dB.
MTI Radar using Power Oscillator as Transmitter
A block diagram of MTI radar using a power oscillator is shown in the figure below. A portion of the transmitted signal is mixed with the STALO output to produce an IF beat signal whose phase is directly related to the phase of the transmitter.
This IF pulse is applied to the coherent oscillator (COHO) and causes the phase of the COHO CW oscillation to “Lock” in step with the phase of the IF reference pulse. The phase of the COHO is then related to the phase of the transmitted pulse and may be used as the reference signal for echoes received from the particular transmitted pulse.
Upon the next transmission, another IF locking pulse is generated relocking the phase of the CW COHO until the next locking pulse comes along. This type of MTI radar has wide applications.
Non-Coeherent MTI Radar
The echo signal received from a moving target or from clutter fluctuates both in amplitude and phase. Where the MTI makes use of the phase fluctuation then it is called coherent MTI Radar and where the amplitude fluctuation is considered then it is called non-coherent MTI radar, the amplitude fluctuation is used to recognize the Doppler components produced by a moving target. It is also called externally coherent.
The block diagram of the non-coherent MTI radar is shown in the figure below.
In non-coherent MTI amplitude limiters can not be used otherwise desired amplitude fluctuation would be lost. Instead of using a phase detector, we are using an amplitude detector. Therefore, the IF amplifier should be linear and should have a large dynamic range.
A logarithm amplifier may be used as an IF amplifier to have logarithm gain characteristics, such as protection from saturation, and uniform output with variation in the clutter input amplitude.
The output of the IF amplifier is to be detected through the amplitude detector. The output of the amplitude detector may be further processed wither using a delay line canceller or it may be detected over the A-scope. A butterfly effect can be observed on the A-scope due to Doppler in amplitude fluctuation.
The transmitter should be stable over the pulse duration to prevent beat from the overlapping ground clutter.
The main advantage of non-coherent MTI radar is that it is very simple and may be used where space and weight are limited.
Advantages of MTI Radar
- Moving Target Tracking: MTI radar is designed to detect and track moving targets accurately.
- Clutter Rejection: It effectively filters out unwanted radar returns from stationary objects or clutter, improving target detection.
- Reduced False Alarms: By suppressing clutter, MTI radar reduces false alarms, allowing for more reliable target identification.
- Improved Situational Awareness: MTI radar enhances situational awareness by focusing on moving targets, providing a clearer picture of the dynamic environment.
- Versatility: MTI radar can be used in various applications such as military surveillance, air traffic control, maritime navigation, and ground-based radar systems.
- Enhanced Target Discrimination: MTI radar distinguishes between moving targets and background noise, enabling better target discrimination and classification.
Limitations of MTI Radar
- Blind Speed Zones: MTI radar may have blind speed zones where certain target speeds can go undetected or produce ambiguous results.
- Doppler Ambiguity: MTI radar can encounter issues in distinguishing between targets moving at different speeds but with similar Doppler frequencies, leading to target confusion.
- Directional Ambiguity: In some cases, MTI radar may struggle to determine the direction of moving targets accurately.
- Sensitivity to Weather Conditions: Adverse weather conditions, such as heavy rain or snow, can affect the performance of MTI radar by degrading target detection and tracking capabilities.
- Limited Range Resolution: MTI radar may have reduced range resolution due to the processing techniques employed to suppress clutter, resulting in limited precision in target location determination.
- False Target Generation: Certain environmental factors or interference sources can create false targets or cause clutter that appears as moving targets on MTI radar displays.
MTI Radar Applications
- Military Surveillance: MTI radar is extensively used for military surveillance purposes, including target detection, tracking, and identification in both land and maritime environments.
- Air Traffic Control: MTI radar aids in air traffic control by detecting and tracking aircraft movements, providing valuable information for safe and efficient air traffic management.
- Border Security: MTI radar plays a crucial role in border security operations, assisting in the detection and tracking of unauthorized crossings, smuggling activities, and suspicious movements.
- Maritime Navigation: MTI radar is utilized in maritime environments to detect and track vessels, monitor shipping traffic, and enhance maritime situational awareness for effective navigation and collision avoidance.
- Weather Monitoring: MTI radar can be employed for weather monitoring applications, including the detection and tracking of precipitation patterns, severe weather phenomena, and monitoring atmospheric conditions.
- Search and Rescue: MTI radar assists in search and rescue operations by detecting and tracking moving targets, such as lost or distressed individuals or vessels, in order to facilitate timely rescue efforts.
- Ground-Based Radar Systems: MTI radar is integrated into ground-based radar systems for applications such as perimeter surveillance, asset protection, and monitoring of critical infrastructure.
Difference between MTI and Pulse Doppler Radar
|Pulse Doppler Radar
|Moving Target Detection
|Detects and tracks moving targets while filtering out clutter.
|Detects and tracks moving targets while mitigating range-velocity ambiguity.
|Filters out clutter and stationary objects for improved target detection.
|Utilizes Doppler filtering techniques to suppress clutter.
|May have blind speed zones and difficulty distinguishing similar Doppler frequencies.
|Mitigates range-velocity ambiguity for accurate velocity determination.
|May have limitations in accurately determining target direction.
|Capable of determining target direction based on Doppler shifts.
|Performance may be affected by adverse weather conditions.
|Generally less affected by adverse weather conditions.
|Range resolution may be reduced due to clutter suppression techniques.
|Provides improved range resolution.
|Military surveillance, air defense systems, weather monitoring, and target tracking in cluttered environments.
|Military surveillance, air defense systems, weather monitoring, target tracking in cluttered environments.
Frequently Asked Questions(FAQs)
What is the MTI radar?
The moving target indicator (MTI) radar is a pulsed radar that uses the Doppler frequency shift as a means for discriminating moving targets from stationary clutter.
Where is MTI radar used?
The systems are used to locate aerial and ground-based targets of interest as well as vessels. MTI radar that is specifically used to detect and track ground vehicles is known as GMTI (Ground Moving Target Indicator).
What is MTI radar used to remove?
A typical MTI radar uses a high-pass filter to remove energy at low Doppler frequencies. Since the frequency response of an FIR high-pass filter is periodic, some energy at high Doppler frequencies is also removed.
What is MTI blind speed?
Read from Thi slink about Blind Speed in radar: https://easyelectronics.co.in/blind-speed-in-radar/
What is the disadvantage of MTI radar?
Transmitter frequency instabilities.
Pulse repetition interval (PRI) jitter.
Pulse width jitter.
Uncompensated motion either in the radar platform or the clutter.
What are the advantages of non-coherent MTI radar?
The major advantage of a non-coherent MTI is that it is simple and it is mostly used in applications where space and weight are limited.