Introduction to Radar Basics - MATLAB Simulations for Radar Systems Design - page 40

d. Range Gate Straddle:

The radar receiver is normally mechanized as a series of contiguous range
gates (bins). Each range bin is implemented as an integrator matched to the
transmitted pulsewidth. Since the radar receiver acts as a filter that smears
(smooths), the received target echoes. The smoothed target return envelope is
normally straddled to cover more than one range gate.

Typically, three gates are affected; they are called the early, on, and late
gates. If a point target is located exactly at the center of a range gate, then the
early and late samples are equal. However, as the target starts to move into the
next gate, the late sample becomes larger while the early sample gets smaller.
In any case, the amplitudes of all three samples should always roughly add up
to the same value. Fig. 1.25 illustrates the concept of range straddling. The
envelope of the smoothed target echo is likely to be Gaussian shaped. In prac-
tice, triangular shaped envelopes may be easier and faster to implement. Since
the target is likely to fall anywhere between two adjacent range bins, a loss in
the SNR occurs (per range gate). More specifically, a targetÓs returned energy
is split between three range bins. Typically, straddle loss of about 2 to 3 dB is
not unusual.

Example:

Consider the smoothed target echo voltage shown below. Assume resis-
tance. Find the power loss due to range gate straddling over the interval
.

1Ω

,{} 0 τ

v(t)

K

time

– τ τ 0

-1

Solution:
The smoothed voltage can be written as

 

K +

τ

1

  t

-------------

 K + ; t < 0 

 

 

v () =

 

K +

τ

1

 

  t

K – ------------- ; t ≥ 0

 

 

 

The power loss due to straddle over the interval is

,{} 0 τ