IEEE Geoscience and Remote Sensing Magazine - June 2014 - 10

FIgurE 2. Orbit illustration of Helix satellite formation. Red: TSX,
Green: TDX; Typical baseline values vary from 120 to 500 m.

FIgurE 1. Artist's view of TerraSAR-X and TanDEM-X flying in
close formation.

acquisition strategy. A description of the interferometric calibration and various steps of the DEM processing
chain in Sections III and IV is complemented by a summary of the DEM performance and global DEM status in
Sections V and VI. This paper ends with an outlook to the
planned Science Phase (Section VII) and conclusions.
II. MISSION CONCEPT
The TanDEM-X mission is an extension of the TerraSAR-X
radar mission [4], co-flying a second satellite of nearly identical capability in a close formation. The TerraSAR-X satellite
(TSX), as basis for TanDEM-X, was successfully launched
into a sun-synchronous dusk-dawn orbit with 97.44° inclination on June 15, 2007. The nominal orbit height is 514.8
km and the orbit repeat cycle is 11 days. TSX is not only a
10

high performance SAR system, but it has already built in
all necessary features required for the implementation of
the TanDEM-X mission. Examples are additional X-band
horn antennas for inter-satellite phase synchronization,
the availability of a dual-frequency GPS receiver for precise orbit determination, excellent RF phase stability of the
SAR instrument, and PRF synchronization based on GPS
as a common time reference. The second satellite (TDX,
launched June 21, 2010) is as much as possible a rebuild
of TSX with only minor modifications like an additional
cold gas propulsion system for formation fine tuning, double-sized on-board solid-state memory for increased data
recording capacity, and an additional S-band receiver to
enable the reception of telemetry and GPS position information broadcast by TSX. The instruments on both satellites
are advanced high-resolution X-band (center frequency 9.65
GHz, chirp bandwidth up to 300 MHz) synthetic aperture
radars, which can be operated in Spotlight, Stripmap, and
ScanSAR mode with full polarization capability [5]. The
active phased array antenna enables agile beam pointing
and flexible beam shaping as required for the acquisition
of a wide range of image products with varying resolutions
and scene sizes. The TSX and TDX satellites were designed
for a nominal lifetime of 5.5 years. Predictions for TSX based
on the current status of system resources indicate up to five
extra years (until the end of 2017) of lifetime, providing at
least five years of joint operation with TDX.
A. Orbit COnfigurAtiOn And fOrmAtiOn flying
The TanDEM-X operational scenario requires the coordinated operation of two satellites flying in close formation.
ieee Geoscience and remote sensing magazine

june 2014



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