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Digital MTI Implementation using LabVIEW: Single Delay Line Canceller - Prof. Erik Goodman, Papers of Principles of Theater Design

Michigan State University (MSU)Principles of Theater Design
Prof. Erik Goodman

The implementation of a simple moving target indicator (mti) filter using labview and a single delay line canceller. The paper explains the concept of mti filters, the use of delay line cancellers, and provides an example labview program for demonstration. The objective is to facilitate the use of this filter in a common embedded system environment.

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IMPLEMENTATION OF A SIMPLE MOVING TARGET INDICATOR USING
LABVIEW
GARRETT WARNELL
ECE 480, FALL 2008
TEAM 4
ABSTRACT
This paper discusses the digital implementation of a simple moving
target indicator (MTI) using LabVIEW. A single delay line canceller is
chosen for study due to its simplicity in both concept and
implementation. A modification is made to facilitate implementation
in a common embedded systems environment, and an example
LabVIEW program is developed for demonstration.
KEYWORDS: moving target indicator, MTI, filter, radar, digital signal
processing, DSP, LabVIEW, delay line canceller
INTRODUCTION
For radar and other similar systems, it is often desirable to be able to distinguish
between moving targets and those that are stationary. Such information can
greatly aid operators when the surveillance targets of interest, such as vehicles
or people, are located in environments with a high amount of clutter (undesirable
return signals from the surroundings). Fortunately, through the use of digital
signal processing, it is possible to extract this information from the system in real
time using a moving target indicator (MTI) filter.
OBJECTIVE
This paper will focus on the development of a simple MTI filter using LabVIEW.
A modification will be made to a classical filter in order to facilitate use in a non-
ideal embedded system. The exact system and the method of signal acquisition
will be considered arbitrary, though it is assumed that data will arrive in
successive blocks, each representing some sort of pulse. Further, it is not the
objective of this note to introduce the user to LabVIEW, and basic knowledge of
the software will be assumed throughout.
BACKGROUND
One of the simplest methods of moving target information extraction is the use of
a single delay line canceller as an MTI filter. A block diagram of the system is
shown in Figure 1 below.
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IMPLEMENTATION OF A SIMPLE MOVING TARGET INDICATOR USING

LABVIEW

GARRETT WARNELL

ECE 480, FALL 2008

TEAM 4

ABSTRACT This paper discusses the digital implementation of a simple moving target indicator (MTI) using LabVIEW. A single delay line canceller is chosen for study due to its simplicity in both concept and implementation. A modification is made to facilitate implementation in a common embedded systems environment, and an example LabVIEW program is developed for demonstration. KEYWORDS: moving target indicator, MTI, filter, radar, digital signal processing, DSP, LabVIEW, delay line canceller INTRODUCTION For radar and other similar systems, it is often desirable to be able to distinguish between moving targets and those that are stationary. Such information can greatly aid operators when the surveillance targets of interest, such as vehicles or people, are located in environments with a high amount of clutter (undesirable return signals from the surroundings). Fortunately, through the use of digital signal processing, it is possible to extract this information from the system in real time using a moving target indicator (MTI) filter. OBJECTIVE This paper will focus on the development of a simple MTI filter using LabVIEW. A modification will be made to a classical filter in order to facilitate use in a non- ideal embedded system. The exact system and the method of signal acquisition will be considered arbitrary, though it is assumed that data will arrive in successive blocks, each representing some sort of pulse. Further, it is not the objective of this note to introduce the user to LabVIEW, and basic knowledge of the software will be assumed throughout. BACKGROUND One of the simplest methods of moving target information extraction is the use of a single delay line canceller as an MTI filter. A block diagram of the system is shown in Figure 1 below.

Delay L System Σ

- x(t) + y(t) Figure 1 Assuming a pulsed radar system is used (i.e. the RF energy is transmitted periodically for discrete amounts of time), if the delay L corresponds to the time between pulses, then the output y(t) appears as the error between consecutive pulses. Therefore, the output only contains information pertaining to changes in the surroundings that have occurred since the last pulse, i.e. moving targets. The single delay line canceller shown above can also be described by the discrete time equation

y [ n ]= x [ n ]− x [ n − L ]

Equation 1 While other more advanced MTI filters exist, the filter described by Equation 1 will be the main focus of this paper. SINGLE DELAY LINE CANCELLER MODIFICATION The concept of a single delay line canceller MTI filter can be extended in order to facilitate its use in a more realistic embedded system. That is, this filter can be modified to work in a system in which pulse data arrives in discrete blocks with an undetermined amount of time between successive pulses. Since the purpose of the filter is to subtract the previous pulse observation from the current one, it will be sufficient to simply store pulse data in memory and use this data in the modified implementation of the MTI filter. A block diagram of the modified filter is shown below. Data Acquisition Device Memory Σ

- x (^) i [n] + x (^) i-1 [n] yi [n] Figure 2 The signal xi[n] represents the finite duration digital signal containing observation data from the current pulse, while xi-1[n] is the same signal from the previous pulse.

To allocate memory for the system, an initial value must be chosen. It is best to choose a value of all zeros for the first run, as subtracting this is equivalent to the system output without the filter. For successive pulses, the memory will be updated and the filter will perform as expected. For efficiency, memory should only be allocated during the first loop iteration, and then overwritten during successive iterations. The allocated memory must be of the same length as the expected pulse data. Here, it is assumed that each pulse will be represented by 1024 samples. The memory is also assigned to an indicator so that it may be referred to as a variable for the remainder of the program. Figure 4 shows one method by which to achieve this memory allocation. Figure 4 Now that the memory for the system has been allocated and initialized, there needs to be functionality by which to subtract the values in memory from the current pulse data. This is done by referencing the variable pertaining to the system memory and utilizing the LabVIEW subtraction block. The output is then passed to the filter output block. This implementation is shown in Figure 5.

Figure 5 Finally, at the end of each iteration, the values in the system memory need to be updated to reflect the data from the current pulse. These values will be used during the next iteration as the “pulse history.” This update is done through a reference to the system memory. It is also necessary to place the subtraction and update functions in a sequence structure to guarantee the sequential order of “subtract, then update.” This is shown in Figure 6. Figure 6

REFERENCES

  1. Levanon, N. and Mozeson, E. Radar Signals. Wiley-IEEE, 2004.
  2. Zverev, A (1968). Digital MTI Radar Filters. IEEE Transactions on Audio and Electroacoustics. 16(3), pp. 422-432.
  3. Mark, J (1972). A Recursive Digital MTI Radar Filter. Proceedings of the IEEE. June 1972.