Radar Waveforms

In practice, instead of pulsing the transmitter, one usually codes a cw signal with a sequence of 180° phase reversals and cross-correlates the echo with a representation of the code (e.g., using the decoder in Fig. 4), thereby synthesizing a pulse train with the desired values of Ar and fpRp. With this approach, one optimizes SNR because it is much cheaper to transmit the same average power continuously than by pulsing the transmitter. Most modern ground-based radar astronomy observations employ cw or repetitive, phase-coded cw waveforms. 

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Bell, M., Information theory and radar waveform design , IEEE Trans. Information Theory, Vol.39, No.5, pp 1578-1597, 1993. 

Figure 7. FMCW radar waveform (red) and the corresponding receive signal (blue) for a single target situation.

The basic unit of the radar waveform is the impulse response h(n) of the transmit filter. It is expedient to view this response in the time domain as an FM chirp signal with envelope a(n) and phase ty(n)... 

Waveform diversity can be used on the sequence of transmitted radar pulses to realize the above goal by suppressing the range foldover returns. In ordinary practice, a set of identical pulses are transmitted. To suppress returns due to range foldover, for example, individual pulses... 

S. U. Pillai, B. Himed, K. Y. Li, Waveform Diversity for Space Based Radar, Proc. of Waveform Diversity and Design, Edinburgh, Scotland, Nov 8-10, 2004. 

M. A. Ringer and G. J. Frazer, Waveform analysis of transmissions of opportunity for passive radar, in Proc. ISSPA, Brisbane, Australia, Aug. 1999, pp. 511-514. 

Keywords Radar sensor scheduling waveform beam-shape control detection tracking revisit time myopic non-myopic. 

Illumination of the scene is provided by a signal that is emitted from the radar system. This signal is usually a waveform that is relatively slowly varying superimposed on a rapidly oscillating sinusoidal carrier. Thus it can be expressed as... 

Now we consider the possibility that the target is moving relative to the radar. The scattered waveform is modified by the Doppler effect. If this is done correctly it results in a time dilation of the return signal, so that, if the target has a radial velocity v, the return signal su(t) becomes... 

By varying the waveform, we are able to vary the shape of the ambiguity and thereby the kind of blurring that the radar process does to... 

With the advent of radars capable of waveform agility, the design of optimal waveform libraries comes into question. The purpose of this section is to consider the design of such waveform libraries for radar tracking applications, from an information theoretic point of view. We note that waveform libraries will depend in general on the specific applications in which the systems are to be used. Airborne radars will require different libraries from ship-borne ones. Radars used in a tracking mode will require different optimal libraries than radars in a surveillance mode. 

In the context of our discussion in this chapter, we represent the measurement obtained using the waveform 4> as a Gaussian measurement with covariance Rfy The current state of the system is represented by the state covariance matrix P. Of course, the estimated position and velocity of the target is also important for the tracking function of the radar, but in this context they play no role in the choice of waveforms. In a clutter rich (and varying) scenario, the estimate of the target parameters will clearly play a more important role. The expected information obtained from a measurement with such a waveform, given the current state of... 

This is the mutual information between the target variable (range and Doppler) X and the processed (with a matched filter) radar return Y, resulting from the use of the waveform identity matrix. We use this expected information as the MoE of the waveform more information we extract from the situation the better. 

To meet all these system requirements specific waveform design techniques must be considered. For ACC systems both radar types of classical pulse waveform with ultra short pulse length (10 ns) or alternatively continuous wave (CW) transmit signal with a bandwidth of 150 MHz are considered. The main advantage of CW systems in comparison with classical pulse waveforms is the low measurement time and low computational complexity. 

Each sensor of the radar network has an individual position behind the front bumper. Therefore, each sensor will calculate individual values for target range and velocity based on the four measured beat frequencies, equation 8, inside the FMCW waveform. The measurement result is described by an eight-element parameter vector. 

SC.2.1.1 The design of the Mode S waveforms used by TCAS must provide compatibility with Modes A and C of the ground-based secondary surveillance radar system (l2.6). 

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