IEEE Geoscience and Remote Sensing Magazine - June 2014 - 14

Abs. Height Error - SRTM/ICESat / m

Absolute Height Error

-20

-40

100
200
Argument of Latitude / [deg]

300

(a)

+10 m

-10 m

TanDEM-X-ICESat/SRTM [m]
(b)

FIGuRE 9. Absolute height error of TanDEM-X Raw DEMs compared to SRTM/ICESat. (a) Error over argument of latitude for first (blue) and

second (red) coverage. (b) Projection of the error on the globe; larger deviations over Greenland and boreal forests can be attributed to differences in signal penetration.
and against ICESat [12] reference heights in order to further improve their absolute height accuracy and to allow
a seamless mosaicking of adjacent scenes without tilts or
displacements (compare Fig. 7).
This section describes the calibration of the bistatic
SAR interferometer in order to deliver precise and well
geocoded Raw DEMs. The final DEM calibration is
explained later on in Section IV.B.
The interferometric calibration includes three different parts: The calibration of the baseline between both
satellites, the adjustment of internal delays to allow a
radargrammetric determination of the coarse absolute
height of the Raw DEM, and finally the correction of
phase offsets to tune the precise absolute height location
of the DEM in the order of a few meters.
Precise baseline determination is performed by a double differential evaluation of GPS carrier phase measurements [13], [14], [15]. However, primarily due to uncompensated offsets from the SAR antenna phase centers, the
relative satellite positions derived from GPS measurements are biased. The accuracy with which the baseline
needs to be known is in the order of 1 mm, which corresponds to a height error of roughly 1 m for a typical
height of ambiguity. For the baseline calibration globally
distributed flat test sites with known height are repeatedly acquired. By this, initial offsets in the baseline can
be derived and corrected and a monitoring of the baseline stability is established. This is shown in Fig. 8. The
measurements show a standard deviation of 1.31 mm [16],
[17]. The remaining offsets are applied in the calibrated
baseline, which is used for Raw DEM processing.
The interferometric measurement utilizes the phase
difference of the acquired SAR data between both satellites. This phase is ambiguous by the wavelength, and
thus the correct ambiguity band needs to be resolved.
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This can be achieved by radargrammetric height determination, which exploits the unambiguous radar signal
travel times. For this, a highly precise delay calibration
is required to compensate different delays in the instruments that can vary depending on radar parameters.
Detailed investigations revealed that the delays change
with the used synchronization horn antenna, the receiver
gain setting, the employed bandwidth, the transmitting
satellite, and the position in the orbit [17]. After the determination of appropriate calibration parameters, a correction of the delays is performed in the interferometric processor (see Section IV.A).
The final adjustment is performed by calibrating the
phase offset between both acquisitions based on a comparison with reference data (SRTM and ICESat).1 This
phase offset depends on the transmitting satellite, on
precise synchronization and start-stop time determination, and on the inclusion of tropospheric path delays
[18], [19]. Finally, it has been adjusted such that 90% of
all Raw DEMs initially are within ! 10 m versus SRTM/
ICESat, as can be seen in Fig. 9 on the left. The residual
10% are mainly caused by an unresolvable ambiguity in
the synchronization link (the so-called r-ambiguity).
These ambiguities are largely resolved by reprocessing
with a measured phase cycle offset as input which is compensated for in the phase-to-height conversion.
IV. DEM PROCESSING CHAIN
The processing from instrument raw data to the final
DEM products is performed by two consequent processors: the Integrated TanDEM-X Processor (ITP) [20] and
the Mosaicking and Calibration Processor (MCP) [21].
1SRTM is used for absolute height comparison in the latitude regions between +/-60°. ICESat points are used for regions outside the SRTM coverage.

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june 2014

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