IEEE Geoscience and Remote Sensing Magazine - June 2014 - 11

200
180
160

HOA_AVERAGE
Horizontal [m]
Phase of Libration [°]
Vertical [m]

500

Height of Ambiguity [m]

120

300

100
200

80
60

Formation Parameters [m]
Phase of Libration [°]

400
140

100

40
20

15
5-0 6
14 -2
20 -03 4
14 -0
20 -02 6
14 -1
20 -12 7
13 -2
20 -10 7
13 -0
20 -09 9
13 -1
20 -07 0
13 -3
20 -05 0
13 -1
20 -04 9
13 -1
20 -02 1
13 -3
20 -12
12 -11
20 -11 2
12 -2
20 -09
3
12 -0
20 -08 4
12 -1
20 -06 5
12 -2
20 -04 6
12 -0
20 -03 6
12 -1
20 -01
12 -27
20 -11
11 08
20 -1011 -19
20 -08
11 -30
20 -06
11 -11
20 -05
11 -22
20 -03
11 -31
20 -01
11 -12
20 -12
10
20
Date
1st Coverage

2nd Coverage

Antarctica Difficult Terrain Antarctica
1st Coverage
2nd Coverage

Figure 3. TanDEM-X formation parameters for global DEM acquisitions between December 12, 2010 and May 15, 2014: horizontal (blue) and
vertical (red) distance between the satellites, phase of libration (black), and resulting average height of ambiguity (HOA) per acquisition (grey).

The adjustment parameters for the formation are the orbits
ascending nodes, the angle between the perigees, the orbit
eccentricities and the phasing between the satellites. With
these parameters, several options have been investigated
during the phase A study, and the Helix satellite formation
shown in Fig. 2 has finally been selected for operational
DEM generation. This formation combines an out-of-plane
(horizontal) orbital displacement by different ascending
nodes with a radial (vertical) separation by different eccentricity vectors resulting in a helix-like relative movement of
the satellites along the orbit. Since the two orbits never cross,
arbitrary shifts and drifts of the satellites along their orbits
are allowed. This enables a safe spacecraft operation without
the necessity for autonomous control. It is furthermore possible to optimize the along-track displacement at predefined
latitudes for different applications: cross-track interferometry aims at along-track baselines which are as short as possible to ensure an optimum overlap of the Doppler spectra
and to avoid temporal decorrelation in vegetated areas,
while other applications like along-track interferometry or
superresolution require selectable along-track baselines in
the range from hundred meters up to several kilometers.
For a complete mapping of the Earth with dedicated
heights of ambiguity (see II.E below) the Helix formation
june 2014

ieee Geoscience and remote sensing magazine

parameters, namely horizontal and vertical distances
between the satellites and the so-called phase of libration
[3], are constantly adjusted. The term motion of libration
denotes the natural rotation
of the eccentricity vectors
due to secular disturbances.
Being at least 30 times
The phases of this libration
can either be kept in a fixed
more accurate than the
relative position with small
presently availaBle
maneuvers using the cold
gloBal data sets the
gas thrusters on a daily basis
tandem-X dem is eXpected
[6] or let drift for fine tuning
to Become a new reFerence
of the cross-track baselines.
For commercial and
Southern and northern latiscientiFic applications.
tudes can be mapped with
the same formation by using
ascending orbits for one and
descending orbits for the
other hemisphere. Fig. 3 shows the as flown formation
parameters since the beginning of the global DEM acquisitions. One can e.g. clearly see the increase in the vertical
and horizontal distance during the second global acquisition compared to the first global coverage resulting in
heights of ambiguity around 35 m compared to 45 m.
11

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - June 2014