Introduction to Radar Basics - MATLAB Simulations for Radar Systems Design - page 29

Other factors that may introduce further variation to the amplitude of the
returned pulses include target RCS and propagation path fluctuations. Addi-
tionally, when the radar employs a very fast scan rate, an additional loss term is
introduced due to the motion of the beam between transmission and reception.
This is referred to as scan loss. A distinction should be made between scan loss
due to a rotating antenna (which is described here) and the term scan loss that
is normally associated with phased array antennas (which takes on a different
meaning in that context). These topics will be discussed in more detail in other
chapters.

Finally, since coherent integration utilizes the phase information from all
integrated pulses, it is critical that any phase variation between all integrated
pulses be known with a great level of confidence. Consequently, target dynam-
ics (such as target range, range rate, tumble rate, RCS fluctuation, etc.) must be
estimated or computed accurately so that coherent integration can be meaning-
ful. In fact, if a radar coherently integrates pulses from targets without proper
knowledge of the target dynamics it suffers a loss in SNR rather than the
expected SNR build up. Knowledge of target dynamics is not as critical when
employing non-coherent integration; nonetheless, target range rate must be
estimated so that only the returns from a given target within a specific range
bin are integrated. In other words, one must avoid range walk (i.e., avoid hav-
ing a target cross between adjacent range bins during a single scan).

A comprehensive analysis of pulse integration should take into account
issues such as the probability of detection , probability of false alarm ,
the target statistical fluctuation model, and the noise or interference statistical
models. These topics will be discussed in Chapter 2. However, in this section
an overview of pulse integration is introduced in the context of radar measure-
ments as it applies to the radar equation. The basic conclusions presented in
this chapter concerning pulse integration will still be valid, in the general
sense, when a more comprehensive analysis of pulse integration is presented;
however, the exact implementation, the mathematical formulation, and /or the
numerical values used will vary.

P D P fa

1.7.1. Coherent Integration
In coherent integration, when a perfect integrator is used (100% efficiency),
to integrate pulses the SNR is improved by the same factor. Otherwise,
integration loss occurs, which is always the case for non-coherent integration.
Coherent integration loss occurs when the integration process is not optimum.
This could be due to target fluctuation, instability in the radar local oscillator,
or propagation path changes.

n P

Denote the single pulse SNR required to produce a given probability of
detection as . The SNR resulting from coherently integrating pulses ( SNR ) 1 n P

is then given by