IEEE Geoscience and Remote Sensing Magazine - June 2015 - 19

V. OBSERVATION CONCEPT
Tandem-L combines unprecedented imaging capabilities
with equally ambitious observation requirements. This section provides an outline of how the different science objectives can be optimally served with the mission. The overall
mission concept includes two alternating phases: a bistatic
phase with both spacecraft flying in close formation in order to allow single-pass interferometric acquisitions, and
a monostatic phase, with both spacecraft flying independently in order to halve the revisit time. This section will
focus on the bistatic phase, since it is the most challenging
from a mission design point of view.
Aside from the nominal imaging capabilities of the SAR
system, the observation concept is mainly constrained by
the scarcity of two resources:
1) The total available delta-v budget, or, in physical terms,
the amount of propellant available for maneuvers. This
sets a constraint on what is possible in terms of formation flying and, consequently, the temporal evolution
of the cross-track baseline (or, equivalently, the vertical
wavenumber, k z ). For example, large vertical baselines,
achieved through a relative eccentricity vector, would
imply costly maneuvers to maintain the relative argument of perigee.

Staggered SAR

Transmitted
Pulses

Received
Echoes

Raw
Data

Azimuth

Transmitted
Pulses

Range

Focused
Data

Azimuth

PRI

SAR with Constant PRI

I
PR1
I2
PR
I0
PR
I
PR1
I2

C. Staggered Sar OperatiOn and perfOrmanCe
Staggered SAR is an innovative concept, based on the continuous variation of the pulse repetition interval (PRI) [25],
[26]. DBF on receive allows simultaneous imaging of multiple sub-swaths through multiple elevation beams, but
"blind ranges" are present between adjacent sub-swaths,
as the radar cannot receive, while it is transmitting (left
panel of Figure 12) [27]. If the PRI is continuously varied,
the position of the blind ranges will be different for each
transmitted pulse (right panel of Figure 12). A method has
been developed to select the PRIs of the sequence such that
two consecutive samples in the azimuth direction are never
missed [25]. In this way, if the signal is averagely oversampled in azimuth, it is possible to accurately interpolate the
data on a uniform grid and obtain a high resolution SAR
image over a wide continuous swath. The oversampling in
azimuth determines an increase of the data volume to be
downlinked, which can, however, be significantly reduced
by filtering and decimating the data on-board [28]. If such
sequences of PRIs are used, and data are resampled using
best linear unbiased (BLU) interpolation, it can be shown
that the aforementioned system is able to image a 350 km
continuous swath with noise equivalent sigma zero and
ambiguity performance shown in Figure 13.

Focused
Data

Blind Ranges

Wide Continuous Swath

FIguRE 12. Top: Transmitted pulses and corresponding received echoes (same colors as the transmitted pulses) with blind ranges (samples

in black, where the echo cannot be recorded, as the radar is transmitting). Middle: Raw data obtained by arranging side by side the received
echoes. Bottom: Focused data for a SAR with constant Pulse Repetition Interval - PRI (left) and for a staggered SAR (right).
june 2015

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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - June 2015