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TerraSAR-X In-Orbit Antenna Model Verification Results
Abstract
TerraSAR-X is a highly flexible X-band radar satellite. Its primary objective is the acquisition of high quality SAR images in a multitude of possible acquisition modes. The great amount of antenna patterns needed for image acquisition requires an antenna model accurately describing all beams. To guarantee the required image quality, the model is verified in-orbit. As TerraSAR-X is scheduled for launch in June 2007, this paper will present the necessity of the antenna model and the methods to verify it.
... An antenna model was established to simulate the patterns before launch validated by a dedicated near-field measurement campaign with a high measurement accuracy of 0.2 dB. Thus, the in-orbit verification can be reduced to a small number of beams [12], [13]. The antenna model calculates the beam shapes, considering different input parameters like the following: @BULLET geometry of the antenna; @BULLET beam excitation coefficients of all 384 array modules; @BULLET an antenna performance matrix; @BULLET embedded radiation patterns measured on ground on single subarrays of the antenna. ...
Modern synthetic aperture radars (SARs) are equipped with active phased array antennas to electronically generate various antenna beams. The TerraSAR-X satellite is a high resolution SAR system launched in June 2007. Its active phased array X-band antenna hosts 384 transmit/receive modules (TRMs) for controlling the electronic beam steering in azimuth and elevation direction. The precise modeling of the antenna performance is only possible if the actual characteristics of each individual TRM are monitored. TerraSAR-X has been equipped with an innovative characterization mode based on a coding technique, which is the so-called pseudonoise gating method. The individual and simultaneous characterization of all TRMs is realized under most realistic conditions with power supply loads like in nominal radar operation. For the first time, this novel technique has been applied on a spaceborne SAR system.
Advanced method of inner calibration of individual receive modules of active phased antenna arrays (APAA) in synthetic aperture radar (SAR) is discussed. This method allows fast and accuracy amplitude and phase characterization of every receive modules and uses pseudo-noise sequences as core of calibrating sequences system. Decorrelation method of cancelling mutual interference using simple matrix algebra is introduced. Its limitations due to strict timing synchronization requirements is described too. Some computing considerations of this method are discussed. Characterizations results are developed through simulation in the case of different types of receive modules failures.
Two important aspects of internal calibration of polarimetric synthetic aperture radar are discussed, i.e., system gain calibration and individual transmit/receive module (TRM) calibration. System gain variation is measured, utilizing the internal calibration loop. A dual-channel TRM gain and phase calibration is carried out using orthogonal phase coding, where a signal of individual TRM is phase encoded according to a set of orthogonal codes in order to be separated from the composite calibration signal of all TRMs. Calibration results are developed both theoretically and through simulation in the case of TRM failure. Error introduced by phase-shifter toggling inaccuracy is discussed. A cross talk model is used to investigate the effect of imperfect isolation between the two polarization channels of each TRM on the calibration error.
Two important aspects of internal calibration of polarimetric synthetic aperture radar (SAR) are discussed, i.e., individual transmit/receive module (TRM) calibration and system gain calibration. The system has a general structure utilizing a phased array antenna composed of dual-channel TRMs. TRM gain and phase calibration is carried out using orthogonal phase coding (OPC). The signal of the individual TRM is phase-encoded according to a set of orthogonal codes to be separated from the composite calibration signal. OPC uses 1 bit of a digital phase-shifter for encoding, without the need for additional encoding hardware. Performance of the method is examined. Calibration results are developed both theoretically and through simulation in case of TRM amplifier or phase-shifter failure. Zero-padding is used to eliminate calibration error of the first TRM. A crosstalk model is proposed to investigate the effect of imperfect isolation between the two polarization channels of each TRM, and a way to reduce this error is also given. At last, system path gain variation is measured utilizing the internal calibration loop. The OPC method has an accuracy of 0.2 dB for gain and better than 2deg for phase, with 10-dB signal-to-noise ratio and perfect isolation between the two polarization channels. The error due to imperfect isolation is usually small and can be ignored. The simple way to detect TRM malfunction is verified through simulation, and it is also in accordance with TerraSAR-X in-orbit calibration outcomes. The proposed OPC method is shown to be an effective way of internally calibrating TRMs of a phased array antenna.
TerraSAR-X is a versatile X-Band SAR satellite operating in Stripmap, Spotlight and ScanSAR modes with selectable or
dual polarisation. Additionally, experimental modes are possible, like wide bandwidth operation providing even higher
resolution, or a left-looking mode. Consequently, cost and time effective concepts for external calibration are mandatory
because of the large number of modes and possible antenna patterns.
External calibration meets the challenge of calibrating in a short time frame during commissioning. The paper describes
the developed strategy planned for the TerraSAR-X commissioning phase and discusses its implementation. External
calibration is presented as a part of the IOCS Calibration Section in the TerraSAR-X Ground Segment.
TerraSAR-X is a high resolution synthetic aperture radar (SAR) satellite due for launch in 2007. Its active phased array X-Band antenna hosts 384 transmit/receive modules controlling the beam steering in azimuth and elevation direction. Precise modelling of the antenna is only possible if the actual characteristics of each individual transmit/receive module are known. TerraSAR-X has been equipped with an innovative characterisation mode based on the so-called PN Gating method. Individual and simultaneous characterisation of all transmit/receive modules is realised under most realistic conditions with the same power loads like in the nominal mode. This paper shows the results of PN Gating measurements on a satellite SAR system.
TerraSAR-X is Germany's first national remote sensing satellite being implemented in a public-private partnership between the German Aerospace Centre (DLR) and EADS Astrium GmbH, with a significant financial contribution from the industrial partner. This radar satellite, which is to be launched in June 2007 will supply high-quality radar data for purposes of scientific observation of the Earth for a period of at least five years. At the same time it is designed to satisfy the steadily growing demand of the private sector for remote sensing data in the commercial market. This contribution will describe first the public-private partnership scheme, the roles and responsibilities of the partners as well as the overall project organization. The mission and system design will then be described, followed by a brief overview of the satellite, the related Ground Segment and the applied data policy. The contribution will then focus on the actual mission status. Finally a brief outlook will be given on the activities to come.
Antenna elevation pattern estimation is based on acquisitions over a large homogeneous rain forest area in the Amazon basin. The estimation procedure accepts IMP and WSM products as input and generates a combined estimate from as many input products as are available for a certain beam. For IS3-IS6 IM and WS products can be combined as well. Beam patterns for vertical polarization have been generated. Comparison with the pre-launch patterns allows to verify the compensation of the receiver gain droop. Pattern corrected range profiles have been analysed with respect to beam- to-beam radiometric normalization.
Abstract—In this paper an elaborated development over the
usual way of estimating a SAR antenna pattern with measurements
over the rain forest is presented. The current standard
procedure is to fit these data to a polynomial curve or sometimes
to a power-cosine law. This antenna model is not a physical one
but a convenient mathematical simplification to eliminate noise.
This work’s contribution is the implementation of a physically
meaningful model, i.e. a superposition of far-field wave fronts
from each of the array elements, together with the Singular
Value Decomposition (SVD) technique. In addition to eliminate
noise, this method allows us to obtain an estimate of the in-flight
corrected excitation laws of the antenna array.
The motivation of the study is the characterisation of the
German TerraSAR-X satellite antenna, a phased array consisting
of 384 subarrays, each of which contains two slotted waveguides,
one for horizontal and one for vertical polarisation. The utility of
the technique is tested on data acquired by the ASAR instrument
onboard the ESA ENVISAT satellite, more specifically from
both image (IMP) and wide swath mode (WSM) products.
TerraSAR-X is the first German Radar satellite for scientific and commercial applications. The project is a public-private partnership between DLR and EADS Astrium GmbH. TerraSAR-X consists of a high resolution Synthetic Aperture Radar at X-Band. The radar antenna is based on active phased array technology that allows the control of many different instrument parameters and operational modes (Stripmap, ScanSAR and Spotlight) with various polarizations.
In preparation for the TS-X Launch, scheduled for June 2006, a comprehensive SAR performance estimation update is presented in this paper. The paper covers all available TerraSAR-X operational modes and provides the performance estimation based on the latest instrument measurements and parameter optimizations. This overall SAR performance estimation provides the best possible pre-launch information to the final user.
The strategies for the instrument parameter settings are discussed, ranging from the acquisition parameters like PRF, transmitted bandwidth or receiving window length over antenna excitation coefficients to the BAQ-settings. Processing parameters like processed bandwidth and applied weighting functions are included as well as calibration errors. The performance estimation also includes realistic error budgets.
The following standard performance parameter classes are included in the estimation: geometric and radiometric resolution, noise, scene coverage and product location accuracy, as well as ambiguity analysis.
The first section of the paper introduces the parameters that define the SAR performance of TerraSAR-X. The second one summarizes all operational modes by assigning the specified values for the performance parameters. In the third section, the strategies for the performance optimizations are presented. Finally, the latest performance estimation results are given in section four.
TerraSAR-X is a scientific and commercial SAR mission to be launched in June 2007. As it provides high-resolution products for a multitude of operation modes and incidence angles, the radiometric and geometric calibration of TerraSAR-X has to ensure the product quality and the correct in-orbit operation of the entire SAR system. This paper describes the calibration activities for TerraSAR-X and the dedicated tasks to be performed during the five months commissioning phase. Results from on-ground tests are discussed with respect to geometric and radiometric calibration of the TerraSAR-X system.
TerraSAR-X is a highly flexible X-band radar satellite. Its primary objective is the acquisition of high quality SAR images in a multitude of possible acquisition modes. The great amount of antenna patterns needed for image acquistion requires an antenna model accurately describing all beams. To guarantee the required image quality, the model has to be verified on-ground and validated in-orbit. The results of the verification will be described here as well as the validation approach.
TerraSAR-X is a space borne high resolution imaging synthetic aperture system, based on advanced active phased array technology. The paper presents an overview of the TerraSAR-X frontend and discusses the strategy for antenna testing, verification and in-orbit pattern prediction. As an integral part of this approach, an antenna model has been developed which allows, on the basis of embedded subarray measurements, the calculation of the overall antenna pattern for a given gain/phase setting of the T/R-modules.
Rosich: Antenna Elevation Pattern Estimation from Rain Forest Acquisitions, ENVISAT/ASAR Calibration Review(ECR) of ESTEC
- M Zink
M. Zink, B. Rosich: Antenna Elevation Pattern Estimation from Rain
Forest Acquisitions, ENVISAT/ASAR Calibration Review(ECR) of
ESTEC European Space Agency (ESA), Noordwijk, Netherland, 2002
The TerraSAR-X Antenna System; Radar ConferenceActive Antenna Module Characterisation by Pseudo-Noise Gating
- B Grafmüller
- A Herschlein
- C Fischer
- M Zink
B. Grafmüller, A. Herschlein, C. Fischer: The TerraSAR-X Antenna
System; Radar Conference, 2005 International, May 2005
[3]
D. Hounam, M. Schwerdt, and M. Zink, "Active Antenna Module
Characterisation by Pseudo-Noise Gating," in 25th ESA Antenna
Workshop on Satellite Antenna Technology, Noordwijk, Netherl., 2002.
[4]
The External Calibration of TerraSAR-X, a Multiple Mode SAR-System TerraSAR-X Performance Update
- B Bräutigam
- M Schwerdt
- M Bachmannmartinez
- C Gonzalez
- J Mittermayer
B. Bräutigam, M. Schwerdt, M. Bachmann: The External Calibration
of TerraSAR-X, a Multiple Mode SAR-System; Proceedings of
European Conference on Synthetic Aperture Radar (EUSAR), Dresden,
Germany, May 2006
[9]
J. Márquez-Martinez, C. Gonzalez, J. Mittermayer: TerraSAR-X
Performance Update; Proceedings of European Conference on
Synthetic Aperture Radar (EUSAR), Dresden, May 2006