Moving Target Indication
MTI is an upcoming from the English language and the world anchored term used in radar target acquisition. "Moving Target Indicator " means an assembly of " clutter suppression " in radar devices. Since coherent signals are processed in this module, it is also called coherent channel
Slow moving or stationary objects, mostly the landscape or stones, rocks are, in this MTI method from the presentation on the " PPI -scope ", ie the radar display screen, hidden or " suppressed ", it only remains to relative to the radar moving objects in the image. This filters out disturbances such as land vehicles, railways, other metallic reflective objects on demand and allows, for example, the presentation only of objects that are faster than a front - adjustable speed relative to the observer radar.
It Doppler frequency methods are usually applied. There are two ways to distinguish by means of the Doppler frequency of a moving object to a stationary:
- Direct measurement of the Doppler frequency (fast Fourier analysis)
- Indirect measurement of the Doppler frequency by measuring and comparing the phase position.
Doppler frequency in radar
The Doppler effect occurs ( the "Target " ) even in a moving and observed by a radar device object. The frequency shift of the transmitting frequency by the Doppler effect is a measure of the radial velocity. However, in a radar unit of the Doppler effect occurs twice. Once on the way from the radar antenna to the moving object. When a radar warning receiver in the aircraft would receive this signal, it would here the transmit frequency plus a Doppler frequency measured, because the aircraft is moving, for example, the radar apparatus. The signal with this frequency is then reflected at the plane surface. This reflected energy ( transmit frequency plus the doppler frequency of the forward path ) is provided on a return path for a second time, a Doppler shift. On the radar receiver so come on twice the Doppler frequency.
As an approach to derive the formula, the phase shift of the high frequency signal as the ratio of the wavelength of the radar transmitter to the distance traveled (2 · r) may be made of the electromagnetic wave.
Pulse Pair Processing
When the aircraft is moving towards the radar, it is also of transmitting pulse to transmission pulse, the path length r changed. Therefore, when a moving target with a radial velocity of each echo signal is received with a different phase.
The module Moving Target Indicator now must be able to detect this phase change. Here, two or three pulse periods are compared and evaluated phase differences of the echo signal. The process is also called " pulse -pair processing" ( in German: Intermediate Period compensation) called. If the phase of the echo signal from the pulse period to pulse period constant, there is a fixed target. If the phase position variable, then it is a moving object.
Has virtually an entire pulse period, that is, a deflection on the display device are stored in an appropriate assembly, to compare them with the following pulse period. The comparison consists of two pulse periods from a simple subtraction and amplification of the difference. Target characters having no phase shift will thus be extinguished. Target character moving, have a phase shift, and thus a difference in amplitude at the output of the phase detector and displayed on the display device.
In the history of the first radar apparatus, the memory is implemented as an analog potential storage tube, which has a total period spirally written on a cesium layer. Later, small capacitors were used as memory: the distance here in small digital steps ( Range Cells ) and the amplitude still analogue as capacitor charge (example: PRW -13). With the use of digital storage of the data processing, this module has been built completely digitally. Today, the entire assembly is often only a software module in Radar Data Processor, but which have to work in the program run exactly as their hardware predecessors save a complete pulse period and comparing the amplitudes of the current pulse period.
A separation of the echo signals from clutter and goals to the frequencies in analog systems is difficult because the Doppler frequencies occurring are very small relative to the transmitting frequency. First digital signal processing systems, the Doppler frequencies that occur can be separated and detected. Here both the low-pass circuits ( " zero- Doppler filter " and " clutter Doppler filter " ) as well as filters with predetermined frequencies ( " Doppler filter") used to represent a certain radial velocity of the radar system.
The " Doppler filter" may be connected to, for example, eight different Doppler frequencies to form a block in parallel, and giving an output signal when the Doppler frequency is detected, the filter fitted in the scheme. Modern digital Doppler filter banks achieve a number of more than 1000 different parallel filters. In this way, particular targets such as helicopters can be recognized in the state of limbo and identified by their Doppler spectrum, reflect a mixture of different Doppler frequencies.
Disadvantages of the process
- The method reduces the sensitivity of the receiving tract significantly and should be turned on only in the areas in which clutter also occur, for example in the vicinity or at the lower elevation angles ( for low beam ).
- From the observed radar distant objects that pass tangentially on the radar, have too little radial velocity and thus also suppressed.
- By the periodicity of the oscillations, it is possible that the phase shift of the echo signal having a multiple of 360 °, and thus the same value, such as 0 °. Here is then spoken by a blind speed, which must be suppressed by additional circuit measures the pulse -pair processing.
- Large volume targets (clouds) can also be obtained by the wind a Doppler frequency and are then shown on the display. A wind compensation can resolve this problem partially.
- Wind turbines in the vicinity of radar devices disturbs the radar image because the movement of the windmill blades a strong Doppler frequency is generated.
- The need to send multiple pulse periods in the same direction, causing problems with the time balance of speed cameras. That's why often greatly shortened pulse periods are used in modern radars for the MTI method which restricts the application back to the close range.