ISAR#2
Define, in general terms, what is meant by Synthetic Aperture Radar.
ISAR#3
Describe the various components in the diagram below, which represents
the key elements involved ISAR:
What do the quantities y, z,, , ,
and B represent in terms of the process for interferometric synthetic aperture
radar?
ISAR#4
Explain the relation, .
Why does y not appear on the right hand side if z is a function
of y?
ISAR#5
Justify the sequence of equations in detail:
and show how these lead to
ISAR#6
The phase difference f between the two antennas is directly
proportional to ,
with constant of proportionality .
Explain how this relation leads to the equation,
, where
. What does represent
in this equation?
ISAR#7
Relate f and
in an equation involving a whole number n. What do
and n represent?
At this stage, a technique called "phase unwrapping," is needed to solve for the integer number of wavelengths, n, and obtain the absolute range. Height estimates are averaged over a surface resolution element of the radar image (often termed a picture element or "pixel"), typically tens of meters in diameter.
The goal is get reasonably accurate height estimates. To get a
handle on how accurate these are, it helps to relate the uncertainty in
height, ,
to the uncertainty in the orientation (altitude) angle, .
For a spacecraft system where
is large (at least several hundred kilometers), this last equation shows
that the altitude error
must be very small to achieve acceptable height accuracy.
Now consider a single-antenna SAR system that revisits the same position and images the same area on the ground after several days or weeks--the so-called "repeat-pass approach." Assuming there has been no significant change in the surface between acquisition of the two images, we can perform an analysis similar to the dual-antenna case and recover an estimate of topography with essentially the same accuracy. Again, the key is knowledge of the baseline length and orientation (vector B) between the two antenna positions; only in this case, it is the same physical antenna in a different position at a later time.
Estimating topography from repeat-pass observations is obviously more
challenging than from simultaneous observations. However, if the aircraft
or spacecraft is accurately tracked so that its position and orientation
are known as function of Time, the problem is tractable.