Synthetic aperture radar(SAR) - point target Doppler history
Target position
Range index
Azimuth index
Animation control
Azimuth speed
Set azimuth idx
Graphs of results
UI Color code
BLUE rectangle like shape on the ground indicates a positive Doppler
effect, up to +0.5 PRF - a blue shift
RED rectangle like shape on the ground indicates a negative Doppler
effect, down to -0.5 PRF - a red shift
WHITE lines indicate the near and far range of the zero Doppler line
ORANGE line indicates the time when the point target Doppler effect is
between +- 0.5 PRF
ORANGE point is the geolocated point target
Information
The animation simulates and visuzalized the core the principles of SAR -
the azimuth FM modulation
A freely chosen place on the ground is chosen as a reference point
target, the satellite orbit state vectors are used to calculate the
geometry of the overflight
The BLUE/RED visualization on the ground only shows areas of +-0.5 PRF
Doppler frequency on the ground. Ideally, the SAR antenna beam center is
directed at zero Doppler line and every point on the ground only
produces azimuth Doppler data within +- 0.5 PRF, however real life is
much more complicated.
The Doppler frequency changes close to linearly in respect to azimuth
index, thus this called an azimuth chirp and this used to compress the
Doppler history in azimuth compression
RCM - Range Cell Migration - as the target is in the +- 0.5 PRF area,
the range index of the target changes as in azimuth direction. This is a
major issue in azimuth compression
Vr can be deduced from the R graph, via fitting data to
R2(n) = R02 + Vr2(n)2
Metadata input:
The default example is extracted metadata from an Envisat IM0 dataset. The
webpage does not do any error checking on the input and incorrect data can
likely completely break the animation.
TODO/Ideas
Envisat model orientiation
BLUE/RED polygon gradients, to visualize increasing/decreasing Doppler
shifts