IEEE Geoscience and Remote Sensing Magazine - June 2015 - 15

yaw maneuvers for one satellite. This enables, for most
areas on the Earth's surface, the timely acquisition of a
SAR image within one or two days. An additional 90 degree rotation of the satellite along its radar line-of-sight
axis can moreover be used to acquire fully polarimetric
SAR images with an azimuth resolution below 1 m over
a swath width of 50 km.
The innovative imaging modes and techniques of Tandem-L provide the opportunity for unique large-scale
measurements with frequent revisits, as required for the
global monitoring of dynamic processes on the Earth's
surface. Above and beyond the primary mission goals, the
data set generated by Tandem-L has immense potential for
developing new scientific and commercial applications.

lines to optimize forest height and vegetation profile measurements in the 3-D structure mode. To provide accurate
tree heights and vertical structure profiles in the tropical
forests, up to six acquisitions with vertical wavenumbers
k z ranging from 0.05 rad/m up to 0.6 rad/m are planned
for each season. Assuming an orbital altitude of 745 km
and incident angles ranging from 30° to 40° in the bistatic
quad-pol mode, the required perpendicular baselines vary
between 800 m and 10.7 km. At the equator, this corresponds to horizontal baselines between 1.1 km and 16.6
km in case of using a helix formation [32] with no radial
orbit separation at zero latitude. An elegant technique to
provide this wide range of cross-track baselines exploits
the naturally occurring differential secular variations of
the right ascension of the ascending nodes in response to
slightly different inclinations. Figure 8 illustrates the evolution of the horizontal baselines at the equator for different
inclination offsets (expressed as horizontal baselines at the
northern and southern orbit turns).
A further challenge for Tandem-L is the adjustment of large
cross-track baselines at higher latitudes for the observation of

B. Space Segment
For the space segment it is assumed that two identical satellites are developed using a 2-ton class satellite bus. A sunsynchronous orbit with a height of 745 km and a repeat
cycle of 16 days has been selected. This orbit ensures full
global coverage in the quad-polarized mode which can
provide a swath width of 175 km. Since the
single- and dual-polarized modes provide a
doubled swath width of 350 km, each point
table 1. Key Parameters and PerformanCe figures
for the tandem-l/alos-next satellites.
on the Earth can be mapped with at least two
incident angles from each, ascending and deParameter
Value
Comments
scending passes. This supports, for example,
Orbit height
745 km
231 cycles/16 days
the derivation of 3-D displacement vectors
Orbital tube
Refers to master satellite
500 m ^3vh
in the deformation mode. The satellites will
Horizontal baselines 1 km ... 18 km
Variable horizontal baselines for bistatic mode
be designed for a lifetime of 10 years, with
in close formation
all consumables supporting a mission exRadial baselines
0 m ... 400 m
Radial baselines are mainly for passive safety
tension to more than 12 years. Both, bistatic
in close formation (Helix concept)
and independent monostatic operation will
Local time
6 h / 18 h
Dawn/dusk
be adopted in dedicated mission phases [2].
Inclination
98.4°
Sun synchronous orbit
For bistatic measurements, a bidirectional RF
Revisit time
16 days
The 350 km wide swath mode enables up to 4
link is foreseen that can provide a posteriori
global data acquisitions from different viewing
directions every 16 days
(i.e. on ground) a highly accurate mutual
Frequency
L-band
Available frequency band: 1215 ... 1300 MHz
time and phase referencing with accuracies
Range bandwidth
Up to 85 MHz
Split frequency modes for ionospheric correcin the order of a few picoseconds and a few
tions (for reduced bandwidth modes)
degrees in L-band, respectively (heritage
Azimuth resolution
7 ... 10 m
For swath width up to 350 km (single/dual
from TanDEM-X, [32]). The relative position
pol.), for swath width up to 175 km (quad pol.)
between the satellites will moreover be de3m
For swath width up to 175 km (quad polarizatermined by double differential GPS meation), for swath width up to 350 km (single/
dual pol.)
surements in two frequency bands. This
1m
For 50 km wide swath (all polarizations), swath
approach, which already has been used operaccess by mechanical steering of satellite
ationally during the TanDEM-X mission, can
Downlink capacity
` 8 terabytes/day Based on Ka-band downlink to a ground
provide precise estimates of the 3-D baseline
station network, extension with X-band and/
vector between the two satellites with accuor Laser Communication Terminal (LCT) to be
investigated in phase A
racies in the order of a few millimeters [32].
Look direction
Right & left
Nominal: right-looking,
Table 1 provides a summary of some key
left-looking by horizontal satellite rotation
parameters and performance figures of the
Reflector diameter
15 m
Deployable reflector with 10/15 m boom
Tandem-L space segment.
c. Formation Flight
The Tandem-L acquisition plan foresees a
systematic variation of the cross-track basejune 2015

Mission lifetime

10 years

Resources available for at least 12 years

Polarization

Single/dual/quad

Optional use of hybrid and compact pol
modes shall be investigated in conceptual
design/phase-A study

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