Doppler Resistant Waveforms for Spaceborne Imaging Radar

D. Mark Haines
Center for Atmospheric Research
University of Massachusetts
600 Suffolk Street, Lowell, MA, 01854, USA


Remote Sensing from space, or in space, is made more difficult by the high speed of the
space vehicle, which may cause a large Doppler shift of the echo signal. The rapidly
changing signal phase complicates the task of coherent signal processing, and since only
coherent signals can be detected under conditions of negative signal-to-noise ratio, it is
of great interest to design signals which do not lose their coherent properties under
conditions of high Doppler shifts. The paper presents methods for determining the
coherence intervals (spacial and temporal), as well as deriving the limits of range and
Doppler ambiguity thresholds. It then moves on to describe two waveforms which allow a
maximum signal processing gain while resolving Doppler ambiguity out to very high
velocities. 

The two waveforms described are Chirp pulses, and what we call a Staggered Pulse sequence,
which is also known as a Minimum Redundancy sequence. The new name is meant to
differentiate the new sequences from previous practise, in that they are much longer than
previously reported designs. The Chirp pulses are very resistant to high Doppler shifts,
but will eventually produce a range error when the Doppler shift is a significant
percentage of the signal bandwidth. However, a pulse stream of Doppler shifted Chirp
pulses can be integrated with significant advantage over a period approaching the
coherence interval as defined by the medium. 

The implementation of these two waveforms on the IMAGE satellite will be described, along
with four other waveforms built into the Radio Plasma Imager instrument. This is NASA's
Explorer mission for the year 2000, which is to be launched in January 2000, and operate
for at least two years.