Radar Equation for Distributed Targets

Recall the radar equation for a single target:


For multiple targets, (1) can be written as:


where the sum is over all targets within the pulse volume. 

If we assume that h/2 << ri,

then (2) can be written as:


It is advantageous to sum the backscattering cross sections over a unit volume of the total pulse volume.

Hence the sum in (3) can be written as:


where the total volume is the volume of the pulse.

Thus, (4) can be written as:


Substituting (5) into (3) gives:



Note that:

 Pr is proportional to R-2 for distributed targets

Pr is proportional to R-4 for point targets.

WHY?????  Answer


The sum of all backscattering cross sections (per unit volume) is referred to as the radar reflectivity (h).  In other words,


Q:  What are the units of h Answer

In terms of the radar reflectivity, the radar equation for distributed targets (6) can be written as:



all variables in (8), except h are either known or measured.


Now, we need to add a fudge factor due to the fact that the beam shape is gaussian (we could derive this, but it's not worth the trouble.  Check out the reference books if you are interested).

Hence, (8) becomes;



The backscattering cross section (si) can be written as:



For water, = 0.93

For ice, = 0.197.

Notice that the value for water is much larger than for ice.  All other factors the same, this creates a 5 dB difference in returned power.

So, let's incorporate this information into the radar equation.

Recall from (7) that .  Using (10), h can be written as:


Taking the constants out of the sum;


Remember that the sum is for a unit volume.  Substituting (12) into (9) gives:


Simplifying terms gives:



Note the Di6 dependence on the average received power.



In Equation (14), all variables except the summation term, are either known or measured.

We will now defined the radar reflectivity factor, Z, as:


Q:  What are the units of Z?  Answer

 Substituting (15) into (14) gives the radar equation for distributed targets: