SAR INTERFEROMETRY WITH TERRASAR-X
M. Eineder, H. Runge, E. Boerner, R. Bamler, N. Adam, B. Schättler, H. Breit, S. Suchandt
German Aerospace Center (DLR), Oberpfaffenhofen, D-82234 Wessling, Germany, Email: [email protected]
ABSTRACT
The TerraSAR-X project is a public private partnership between Astrium GmbH and the German Aerospace Center DLR.
Astrium will launch the satellite in late 2005 and holds the rights of commercial data exploitation. DLR is currently developing
the ground segment and is responsible for the scientific exploitation of the data. Even if the mission goal is not primarily SAR
interferometry, TerraSAR-X offers a number of new perspectives to SAR interferometry when compared to ERS and also
ENVISAT: a) High resolution of 3 meters and better in stripmap and spotlight mode. b) The option for a burst synchronized
ScanSAR mode. c) The high range bandwidth will allow large baselines and the option for highly precise DEM generation. d) XBand will show new scattering properties. e) High observation frequency due to the short repeat cycle and variable incidence
angles. f) An along track interferometric mode.
The available products relevant for interferometry are presented and other relevant topics like orbit control and delta-k
interferometry are discussed.
1.
INTRODUCTION
The German X-Band radar satellite TerraSAR-X will be launched in 2006. Because of the high resolution and the X-band
frequency the primary application areas are regional mapping and security. However, realizing the immensly growing importance
of SAR interferometry in the recent years, care is taken at DLR and Astrium that the TerraSAR-X sensor will be not only suitable
for SAR interferometry but even specifically improved for this application. Compared to present sensors in space, the instrument
is highly flexible and even has some special interferometric features.
2.
SATELLITE AND INSTRUMENT CHARCTERISTICS
The characteristics of the satellite are briefly listed in Table 1. An artists view shows the satellite in Fig. 1. More information on
the satellite and the project can be found in [3] and [10].
Satellite
Wet mass
1023 kg
Power
800 W average
Orbit
514 km altitude, 11 days repeat
Solar Generator
Instrument
Antenna
Bandwidth
Imaging modes
Data
Subsystem
Memory
Downlink
4.8 meter active array, multi-pol, steerable
in elevation and azimuth
150 MHz (300 MHz experimental)
Stripmap, Spotlight, ScanSAR, ATI
X-Band Downlink
Antenna
X-Band Active Array Antenna
256 Gbit (end of life)
300 Mbit/s
Table 1: Characteristics of the TerraSAR-X satellite
Fig. 1: Artist view of TerraSAR-X
____________________________________________________________
Proc. of FRINGE 2003 Workshop, Frascati, Italy,
1 – 5 December 2003 (ESA SP-550, June 2004) 52_eineder
3.
STANDARD IMAGING MODES AND PRODUCTS
The operational imaging modes stripmap, spotlight and ScanSAR are depicted in Fig. 2 and their parameters are listed in Table 2.
In order to achieve an azimuth resolution similar to the range resolution on the order of 1 to 3 meters, the antenna length in
azimuth has to be short. This in turn requires a high pulse repetition frequency (PRF) of typically 6 kHz that limits the range
extension of the swath to about 30 kilometers. Using a 4 beam ScanSAR mode a swath width of 100 kilometers can still be
achieved. Complex products suitable for interferometry will be available from stripmap and spotlight mode while complex
ScanSAR products are foreseen as experimental. The satellite is able to trigger an acquisition by real-time GPS position and is
thus not only able to precisely locate a spotlight scene but also to acquire repeat pass interferometric ScanSAR data with
synchronized burst patterns.
S tripMap
S potL ight
S c anS AR
Θ 1=20
o
>3
Θ 2=45
0
o
km
10
10
km
0
km
Fig. 2: TerraSAR-X standard imaging modes
Swath width
Acquisition length
Full performance incidence angle
range
Azimuth resolution
Ground range resolution
Stripmap
30 km
(15 – 30 km polarimetric)
Max. 1650 km
20°-45°
Spotlight
10 km
ScanSAR
100 km total
5 km
20°-55°
Max. 1650 km
20°-55°
3m
1.7 - 3.5 m
1m
1.5 - 3.5 m
16 m
1.7 - 3.5 m
Table 2: TerraSAR-X imaging mode parameters
A spotlight image derived from airborne SAR data with simulated TerraSAR-X performance is shown in Fig. 4. The higher
resolution becomes visibile when compared with an ENVISAT/ASAR image of mode IS2 shown in Fig. 3. More on the scientific
potential of the products has been published in [9].
Fig. 3: ENVISAT / ASAR image of airport area at DLR /
Oberpfaffenhofen (© ESA)
4.
Fig. 4: Simulated TerraSAR-X image from airborne ESAR
data of Microwaves and Radar Institute, DLR
SHORT ACCESS TIME AND FREQUENT COVERAGE
The rather short orbit repeat cycle of 11 days and the electronically steerable antenna allow fast and frequent imaging of a certain
site. At average latitudes an interferometric pair can be acquired within only 12 days. And within 22 days 12 interferometric pairs
with different observation geometries can theoretically be acquired as listed in Table 3. Note that the nominal antenna look
direction is to the right. Left looking mode is possible but has some operational deficiencies so that it will only be used in high
priority situations.
Frequent interferometric coverage helps especially monitoring events in shorter time scales, e.g. subsidence in urban areas.
Because a large number of interferograms can be acquired within a short time, fast events can be detected and atmospheric delay
errors can be reduced by averaging many interferograms.
No
Orbit Time [UTC]
1
2
3
4
5
6
7
8
9
10
11
12
1 day, 16:19:54
2 day, 05:19:32
2 day, 16:01:01
5 day, 05:56:28
6 day, 05:37:35
6 day, 16:56:53
7 day, 16:00:02
7 day, 16:37:54
8 day, 04:59:42
8 day, 15:41:08
9 day, 04:40:42
11 day, 05:36:36
Ascending /
Descending
A
D
A
D
D
A
A
A
D
A
D
D
Look angle [º]
Look Direction
30.9
37.4
47.1
42.7
23.0
47.4
25.2
30.6
32.5
43.9
48.4
46.0
left
right
left
Left
Left
Right
Left
Right
Right
Left
Right
Left
Table 3: TerraSAR-X coverage simulation for Oberpfaffenhofen / Germany at 49° latitude within one orbital cycle
5.
LARGE BASELINES
Due to the high bandwidth of 150 MHz (or even 300 MHz) interferometric baseline decorrelation becomes relevant only for
effective baselines larger than several kilometers, depending on the incidence angle. Fig. 5 shows that baselines of 10 kilometers
and more are possible forming an interferometer that is extremely sensitive to topography. Volume scattering in vegetation may
decorrelate already at smaller baselines, but solid terrain may be imaged with e.g. 1000 meter baseline leading to a height
sensitivity of 4 meters per fringe at 35° incidence angle as shown in Fig. 6.
Fig. 5: Critical baselines as a function of incidence angle
for flat terrain (black) and for terrain of ±20 ° slope (green)
at 150 Mhz bandwidth
6.
Fig. 6: At a baseline of 1000 meters the height of ambiguity is
only 5 meters – a very sensitive interferometer for topography
ORBIT AND ATTITUDE
Due to the X-band frequency TerraSAR-X interferometry will be more sensitive to orbit errors than current SAR sensors that
operate in C-band or L-band. A single frequency GPS receiver plus an additional dual-frequency GPS flown as an experimental
payload will deliver an orbit accuracy in the order of centimeters. The high sensitivity to topography shown in Fig. 6 becomes a
strong error term if temporal changes shall be measured by interferometry and if the terrain is not precisely known. This
sensitivity calls for small baselines. Investigations are performed to keep the orbit in a ±250 meter tube with respect to a reference
orbit all over the mission and within a ±10 meter tube during selected mission phases [1].
Another parameter important for interferometry is the Doppler centroid. For interferometric coherence a common Doppler
bandwidth is required which in turn calls for tight attitude control. Also, the interferometric phase errors due to residual
misregistration of the two complex images are lower if the absolute Doppler centroid is low. A new improved zero Doppler
steering law has been proposed in the framework of the project [11] and will be implemented in the satellite attitude control
system.
The satellite attitude is measured by star trackers with an accuracy of 0.01°. It will be used in the SAR processor for precise
geometric corrections like antenna pattern correction and for Doppler frequency estimation.
7.
HIGH GEOMETRIC SENSITIVITY
Temporal decorrelation will limit the interferometric capabilities in X-band and it is expected that application will focus on stable
scatterers in urbanized areas. Here, TerraSAR-X may have some advantages compared to ERS. Assuming corner scatterers of a
given size, the smaller resolution cell size and the higher frequency of TerraSAR-X will raise the signal to clutter ratio so that the
interferometric phase error is reduced by a factor of 6.4. Due to the smaller X-band wavelength this smaller phase error results in
a subsidence estimation error reduction by a factor of 10. The scattering mechanism of stable points in urban and in natural areas
will be studied in more detail.
8.
DUAL RECEIVE ANTENNA MODE
The receive part of the antenna can be electrically split into two halfes along track and fed into two parallel receiving chains
allowing new operation modes. Firstly, an experimental quad-polarization mode is possible by using different polarization in the
two channels. Secondly, an along track interferometry (ATI) mode with an along track antenna separation of 2.4 meters is
possible. Of course, temporal decorrelation is not a problem during the delay of 0.34 milliseconds between the imaging time of
the two antennas. DLR is investigating the exploitation potential of this experimental ATI mode in the field of traffic monitoring.
Further information on the TerraSAR-X ATI mode can be found in [5]. First demonstration examples are shown in Fig. 7 and
Fig. 8. They could be generated by processing selected SRTM/X-band data where the primary and secondary antenna were
separated 7 meters along track. See also [2], [6], [7].
Fig. 7: Vehicle velocities estimated from SRTM/X-SAR ATI
data on highway A9 north of Munich / Germany
9.
Fig. 8: Tidal current velocity estimated from SRTM/XSAR ATI data take 127.060 on Orkney Islands / Scotland.
One color cycle corresponds to 4 m/s on ground
DELTA-K INTERFEROMETRY
The large bandwidth of up to 300 MHz will allow to exploit split bandwidth techniques that were so far not relevant for space
borne SAR systems because of their small bandwidths. However, if two interferograms of e.g. 50 MHz bandwidths are generated
from sub-bands that are separated by 250 MHz, the ratio between carrier frequency (9.6 Ghz) and sub-band separation is only 38
compared to 800 for ERS. The cross interferogram of these sub-band interferograms corresponds to an interferogram with a
synthesized wavelength of 1.2 meters. This synthesized dual frequency may significantly support phase unwrapping. Preliminary
results are shown in [4].
10. SUMMARY
SAR interferometry with TerrSAR-X will be difficult over vegetated areas but it has good potential over bare soil and in urban
areas. The small wavelength generally requires higher orbit accuracies and smaller baselines than in C-band. Care is taken in the
instrument and in the ground segment to satisfy these requirements and to profit from the experiences from the ERS mission. The
large bandwidth and the flexible instrument will allow new applications out of the reach of exisiting systems.
11. REFERENCES
[1]
[2]
[3]
[4]
[5]
C. Arbinger et. Al., DLR internal project communication.
H. Breit, M. Eineder, J. Holzner, H. Runge, R. Bamler, „Traffic Monitoring using SRTM Along-Track Interferometry”,
IGARSS 2003, Toulouse.
S. Buckreuß, R. Werninghaus, W. Pitz, “The TerraSAR-X Satellite Project”, IGARSS 2003, Toulouse.
M. Eineder, “Problems and Solutions for InSAR DEM Generation of Mountainous Terrain”, ESA FRINGE Workshop 2003.
J. Mittermayer and H. Runge, “Conceptual Studies for exploiting the TerraSAR-X dual receive antenna”, IGARSS 2003,
Toulouse.
[6]
[7]
[8]
[9]
[10]
[11]
R. Romeiser, H. Breit, M. Eineder, H. Runge, "Demonstration of current measurements from space by along-track SAR
interferometry with SRTM data", IGARSS 2002, Toronto.
S.N. Madsen, „On absolute phase determination techniques in SAR interferometry“, SPIE Conference on Radar Sensor
Technology, 1995, p. 393-401
Romeiser, R., Breit, H., Eineder, M., Runge, H., Flament, P., et al.: On the Suitability of TerraSAR-X Split Antenna Mode
for Current Measurements by Along-Track Interferometry. IGARSS 2003, Toulouse, IEEE, Proceedings of IGARSS'03,
(2003)
A. Roth, “TerraSAR-X: A new perspective for scientific use of high resolution spacebome SAR data”, Remote Sensing and
Data Fusion over Urban Areas, 2003. 2nd GRSS/ISPRS Joint Workshop on , 22-23 May 2003
M. Suess, S. Riegger, W. Pitz, R. Werninghaus, “TerraSAR-X-design and performance”, in Proc. 4th European Conference
on Synthetic Aperture Radar, Eusar2002, Cologne, pp. 49-52,2002
H. Fiedler, E. Börner, J. Mittermayer, DLR internal project communication.