TD-PD-PL 0032TanDEM-X Science Phase 19052014

Doc.: TD-PD-PL-0032 Issue: 1.0 Date: 19.05.2014 Page: 2 of 27

Microwaves and Radar Institute TanDEM-X Ground Segment

Announcement of Opportunity: TanDEM-X Science Phase – Public –

DOCUMENT PREPARATION

The document on hand was prepared with contributions from the personnel listed below: Name (name, surname) Organizational Unit Microwaves and Radar Institute (DLR-HR) Hajnsek, Irena IHR, Radar Concepts Department Busche, Thomas IHR, Radar Concepts Department Krieger, Gerhard IHR, Radar Concepts Department Zink, Manfred Schulze, Daniel

IHR, Satellite SAR Systems Department IHR, Satellite SAR Systems Department

Moreira, Alberto IHR




DOCUMENT CHANGE CONTROL

This document is under configuration control. Latest changes to the document are listed first.

Issue Date Chapter Changes

1.0 01.05.2014 All First issue




TABLE OF CONTENTS

1 Introduction ................................................................................................................... 5 1.1 Purpose ...................................................................................................................... 5 1.2 Scope ......................................................................................................................... 5 1.3 Position within Project Framework .............................................................................. 5 1.4 Executive TanDEM-X Science Phase Summary ............................................................. 5

2 References ..................................................................................................................... 6 2.1 Applicable References ................................................................................................. 6 2.2 Normative References ................................................................................................. 6 2.3 Informative References ............................................................................................... 6

3 Terms, Definitions and Abbreviations ......................................................................... 8 3.1 Terms and Definitions ................................................................................................. 8 3.2 Abbreviations ............................................................................................................. 8

4 The TanDEM-X Science Phase ....................................................................................... 9 4.1 Introduction ............................................................................................................... 9 4.2 The TanDEM-X Mission Objectives .............................................................................. 9

4.2.1 Orbital Configuration ...................................................................................................... 10 4.3 Observation Modes .................................................................................................. 11

4.3.1 Interferometric Modes .................................................................................................... 11 4.3.2 Dual Receive Antenna Mode ........................................................................................... 13 4.3.3 Combination of Interferometric and Imaging Modes ....................................................... 15

4.4 Science Phase Mission Timeline ................................................................................. 16 4.4.1 Baseline Variation as a Function of Latitude ..................................................................... 17

5 The TanDEM-X Science Phase Experiments ............................................................... 21

6 Recommendations ....................................................................................................... 24 6.1.1 Alternating Bistatic Mode ................................................................................................ 24 6.1.2 Bistatic Mode .................................................................................................................. 24 6.1.3 Short Across-Track Baseline Phase ................................................................................... 24 6.1.4 Polarimetric and Stripmap-ATI Mode ............................................................................... 25

ANNEX A. Perpendicular Baseline for different Phases of Libration (PM Mode) .......... 26




1 Introduction

1.1 Purpose The purpose of this document is to provide background about the specific Science Phase of the TanDEM-X mission to scientists who are interested to use operational and experimental TanDEM-X data exclusively for scientific purpose. It describes the Science Phase with its interferometric and imaging modes, as well as the time-line and the application experiments.

1.2 Scope This document is dedicated to all science users of the TanDEM-X mission and as such it is accessible to the public.

1.3 Position within Project Framework The TanDEM-X science coordination belongs to the supporting team of the TanDEM-X project management. The science coordination activities are part of the Instrument Operation & Calibration Segment (IOCS) within the TanDEM-X Ground Segment and the Science Service Segment. The Science Service Segment consists of the Science Service System, the Online Order & Delivery via EOWEB and the Science Coordinator & Order Desk, which interacts with the Ground Segment and the science user.

1.4 Executive TanDEM-X Science Phase Summary TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) opened up a new era in spaceborne radar remote sensing. The single pass interferometer collected data for a global high resolution digital elevation model and will now start to acquire data for the Science Phase.

The TanDEM-X Science Phase will focus on the secondary objectives of the TanDEM-X mission. The main goal of the Science Phase is the demonstration of new SAR techniques that have yet not or only partially been demonstrated on ground using airborne SAR sensors. The importance of the demonstration lies in the development of new scientific application products and/or new technologies. For example, beyond the generation of a global TanDEM-X DEM as the primary mission goal, local DEMs of even higher accuracy level (spatial sampling of 6 m and relative vertical accuracy of 0.8 m) and applications based on Along-Track Interferometry (ATI) like measurements of ocean currents are important secondary mission objectives. Along-track interferometry will also allow for innovative applications to be explored and can be performed by the so-called dual-receive antenna mode on each of the two satellites and/or by adjusting the along-track distance between TSX and TDX to the desired value. Combining both modes will provide a highly capable along-track interferometer with four phase centers. The different ATI modes will e.g. be used for improved detection, localisation and ambiguity resolution in ground moving target indication and traffic monitoring applications. Furthermore, TanDEM-X supports the demonstration and application of new SAR techniques, with focus on multi-static SAR, polarimetric SAR interferometry, digital beamforming and super resolution.

The Science Phase will last 15 months with the defined operation schedule. Its starts in October 2014 and ends in December 2015 and will operate in two main interferometric modes. The science proposal submission for data requests is available over the TanDEM-X Science Service System.




2 References

2.1 Applicable References

The following documents are fully applicable together with this document. Document ID Document Title Issue

[A1] TDX-RD-DLR-1201 I. Hajnsek, M. Weber, “TanDEM-X User Requirements Document (URD)”, 20.06.2005 (project internal)

1.2

[A2] TDX-PD-RS-0001 M. Bartusch, et al. “TanDEM-X Mission Requirements Document (MRD)”, 07.06.2011 (project internal)

4.0

[A3] TD-PD-PL-0068 I. Hajnsek, T. Busche, ‘’TanDEM-X Science Plan”, 30.06.2010 (public) 1.0

[A4] TD-GS-PS-3028 T. Fritz,” TanDEM-X Experimental Product Description” 27.01.2012 (public) 1.2

[A5] TD-GS-UM-0115 T. Busche,”TanDEM-X Science Service Manual”06.07.2010 (public) 1.0

2.2 Normative References

The following standards have been used for preparing the plan on hand. Document ID Document Title Issue

None None

2.3 Informative References

The following documents, though not formally part of this document, amplify or clarify its content. Document ID Document Title Issue

[R1] Krieger, Gerhard; Zink, Manfred; Bachmann, Markus; Bräutigam, Benjamin; Schulze, Daniel; Martone, Michele; Rizzoli, Paola; Steinbrecher, Ulrich; Walter Anthony, John; De Zan, Francesco; Hajnsek, Irena; Papathanassiou, Kostas; Kugler, Florian; Rodriguez Cassola, Marc; Younis, Marwan; Baumgartner, Stefan; Lopez Dekker, Paco; Prats, Pau and Moreira, Alberto (2013) TanDEM-X: A Radar Interferometer with Two Formation Flying Satellites. Acta Astronautica, Elsevier, vol. 89, pp 83-98, 2013

[R2] Krieger, Gerhard; Moreira, Alberto; Fiedler, Hauke; Hajnsek, Irena; Werner, Marian; Younis, Marwan; Zink, Manfred, TanDEM-X: A Satellite Formation for High Resolution SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, vol. 45, no.11, pp. 3317–3341, 2010

[R3] Pitz, W. ; Miller, D., The TerraSAR-X Satellite, IEEE Transactions on Geoscience and Remote Sensing, IEEE Transactions on, vol. 48 , issue 2,p 615-622, 2010

[R4] R. Werninghaus, S. Buckreuss, TerraSAR-X System and Mission Design, IEEE Transactions on Geoscience and Remote Sensing, vol 48, no 2, pp. 606-614, 2010.

[R5] TD-PD-RP-0012 G. Krieger, H. Fiedler, “TanDEM-X Mission Analysis Report” (project internal)

1.1

[R6] TX-PGS-PL-4001 A. Roth, “TerraSAR-X Science Plan” 1.0

[R7] TD-GS-PS-0021 B. Wessel, “DEM Products Specification Document” 3.0

[R8] J. Mittermayer, H. Runge, “Conceptual studies for exploiting the TerraSAR-X dual receive antenna” Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. vol. 3, pp. 2140 – 2142, 2003

[R9] S. Suchandt, H. Runge, H. Breit, U. Steinbrecher, A. Kotenkov, U. Balss,




“Automatic Extraction of Traffic Flows Using TerraSAR-X Along-Track Interferometry”, IEEE Transaction on Geoscience and Remote Sensing, vol. 48, no. 2, pp. 807-819, 2010

[R10] M. Gabele, B. Brautigam, D. Schulze, U. Steinbrecher, N. Tous-Ramon, M. Younis, „Fore and Aft Channel Reconstruction in the TerraSAR-X Dual Receive Antenna Mode”, IEEE Transactions on Geoscience and Remote Sensing, vol. 48 , Issue 2, pp. 795-806, 2010




3 Terms, Definitions and Abbreviations

3.1 Terms and Definitions Term Definition

Perpendicular Baseline Geometric Baseline independent of the Satellite formation

Effective Baseline Baseline resulting from the Satellite formation

3.2 Abbreviations Abbreviation Meaning

ATI Along-track SAR Interferometry

COFUR Cost of fulfilling user requests

DEM Digital Elevation Model

CoSSC Coregistered single look slant range complex

DRA Dual Receive Antenna

DTED Digital Terrain Elevation Data

EOWEB Earth Observation WEB Interface

IOCS Instrument Operation & Calibration Segment

MOS Mission Operation Segment

PGS Payload Ground Segment

PI Principle Investigator

PM Pursuit Monostatic

PRF Pulse Repetition Frequency

RF Radio Frequency

SRTM Shuttle Radar Topography Mission

TanDEM-X TerraSAR-X add-on for Digital Elevation Measurements

TDX TanDEM-X Satellite

TD-X TanDEM-X Mission

TSX TerraSAR-X Satellite

TS-X TerraSAR-X Mission

XTI Across-track SAR Interferometry




4 The TanDEM-X Science Phase

4.1 Introduction The TanDEM-X Science Phase is dedicated to the demonstration of innovative techniques and experiments that have special orbital and imaging requirements in order to foster the development of new methods and applications. In this phase the secondary science requirements of the TanDEM-X mission are fulfilled and adapted to the science needs [A3],[A5].

The science needs have been collected during an extensive survey in the beginning of the mission and during the splinter meetings of the TanDEM-X science meetings. As a result a predefined system configuration and time-line for the Science Phase as well as potential experiments that can be conducted are described in this document.

4.2 The TanDEM-X Mission Objectives TanDEM-X stands for TerraSAR-X add-on for Digital Elevation Measurements and its mission concept is essentially based on an extension of the TerraSAR-X mission by a second TerraSAR-X like satellite [R2], [R6], [R7], [R1]. The mission is realized in the framework of a Public Private Partnership (PPP) between the German Aerospace Center (DLR) and Airbus Defence and Space, as for TerraSAR-X.

The TerraSAR-X satellite (TSX), as basis for TanDEM-X, is not only a high performance SAR system with respect to SAR image and operational features, but it has already built in all necessary features required for the implementation of the TanDEM-X mission [R3], [R4]. Examples are additional X-band horn antennas for inter-satellite phase synchronization, the availability of a dual-frequency GPS receiver for precise orbit determination, the excellent RF phase stability of the SAR instrument, and PRF synchronization based on GPS as a common time reference.

The second satellite (TDX) is a rebuild of TSX with only minor modifications like an additional cold gas propulsion system for constellation fine tuning and an additional S-band receiver to enable a reception of status and GPS position information broadcast by TSX. This guarantees a low development risk and offers the possibility for a flexible share of operational functions among the two satellites. Both systems can also be employed for monostatic data takes which is necessary to fulfil the data requirements of the TerraSAR-X mission together with the TanDEM-X mission goals.

Both 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.

The instruments on both satellites are advanced high resolution X-band synthetic aperture radars based on active phased array technology which can be operated in different imaging modes like Spotlight, Stripmap, and ScanSAR with multi-polarization capability, respectively [R3], [R4]. The centre frequency of the instruments is 9.65 GHz with a selectable SAR chirp bandwidth of up to 300 MHz. The active phased array antenna, which has an overall aperture size of 4.8 m x 0.7 m, is fixed mounted to the spacecraft body and incorporates 12 panels with 32 waveguide sub-arrays for both H and V polarization. This enables agile beam pointing and flexible beam shaping.

The primary objective of the TanDEM-X mission is the generation of a worldwide, consistent, timely, and high precision Digital Elevation Model (DEM) as the basis for a wide range of scientific research, as well as for operational and commercial DEM generation. Further, TanDEM-X will also exploit and demonstrate a wide range of advanced and innovative SAR techniques for the first time in space. These experiments, assigned as secondary mission objectives, are regarded as an integral part of the mission [A1], [A2].




4.2.1 Orbital Configuration

The TanDEM-X operational scenario requires a coordinated operation of two satellites flying in close formation. The adjustment parameters for the formation are the node line angle, 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 illustrated in Figure 4.1 has finally been selected for operational DEM generation [R2]. 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 there exists no crossing of the satellite orbits, one may now allow for arbitrary shifts of the satellites along their orbits. 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 will aim 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 super resolution require selectable along-track baselines in the range from hundred meters up to several kilometers.

The Helix formation enables a complete mapping of the Earth with a stable height of ambiguity by using a small number of formation settings [R2]. Southern and northern latitudes can be mapped with the same formation by using ascending orbits for one and descending orbits for the other hemisphere, as illustrated in Figure 4.1 on the right. A fine-tuning of the cross-track baselines can be achieved by taking advantage of the natural rotation of the eccentricity vectors due to secular disturbances, also called motion of libration. The phases of this libration can be kept in a fixed relative position with small maneuvers using the cold gas thrusters on a daily basis, while major formation changes as well as a duplication of the orbit keeping maneuvers required by TSX will be performed by the hot gas thrusters. On the other side the natural rotation, if not corrected, can be used for the collection of different baselines. In this case the arguments of perigree between TDX and TSX are drifting apart from each other, which lead to a natural drift of the geographic position where the maximum and minimum vertical baselines are obtained. During the Pursuit Monostatic Phase the satellite drift is used to obtain the diversity of baselines within a short time period.

horizontalbaseline

verticalbaseline

SH(desc.)

NH(asc.)

effectivebaselines

horizontalbaseline

verticalbaseline

SH(desc.)

NH(asc.)

effectivebaselines

Figure 4.1: Helix satellite formation for TanDEM-X. Left: orbital arrangement. Right: cross-track baselines as function of the orbit position. The positions correspond to one complete orbit cycle where NH and SH mean northern and southern hemisphere, respectively. The Helix formation enables an interferometric mapping of the complete Earth surface with a stable height of ambiguity using a small number of formation settings. Southern and northern latitudes can be mapped with the same formation by using ascending orbits for one and descending orbits for the other hemisphere, as illustrated on the right.




4.3 Observation Modes Two major interferometric operation modes are foreseen, the pursuit monostatic mode and the bistatic mode, but also the alternating bistatic mode can be partly switched on during the bistatic operation. For both interferometric modes, an operation either in single/dual or in quad polarization using the dual receive antenna technique will be possible. The pursuit monostatic phase starts in autumn 2014 and lasts approx. 5 months. It is characterized by a set of drifting across-track baselines that range between 0 m and 750 m. Since these baselines are available at all latitudes and within short time periods, they are well suited for SAR tomography as well as large baseline investigations in the Polar Regions. The large along-track separation of 76 km (10 seconds) between the satellites enables, moreover, the observation of slow drifts and movements. The bistatic phase will then begin in early spring 2015 using a formation with short along-track but large across-track baselines. These baselines are well suited for the monitoring of crop heights during the growing season, but they can also serve many other applications that demand repeated acquisitions with large but temporally constant baselines. In autumn 2015, the bistatic baselines will be reduced to 0-250 m as required for forest applications and along-track interferometry. While science acquisitions will have priority during this 15-month period, it is nevertheless planned to acquire additional data to fill gaps and further improve the quality of the TanDEM-X DEM and for the higher resolution DEM.

For completeness and a better understanding of the parameter that a Principle Investigator can select for a desirable observation scenario a brief summary of the orbital configuration, data acquisition modes and the science mission timeline is given.

4.3.1 Interferometric Modes

Interferometric data acquisitions with the TanDEM-X satellite formation can be achieved from three interferometric modes: Bistatic, Pursuit Monostatic, and Alternating Bistatic. The three cooperative modes may further be combined with different TSX and TDX SAR imaging modes like Stripmap, ScanSAR, and Spotlight, the last mode being in sliding spotlight acquisition geometry [R5]. Also the new imaging modes like Staring Spotlight and Wide Swath (wide Beam ScanSAR) mode will be available. Further the polarimetric capability and short along-track baseline interferometry using the Dual Receive Antenna Mode can be explored. Selected data acquisition modes are highlighted in the following.

4.3.1.1 Pursuit Monostatic Mode

In the pursuit monostatic InSAR mode, the two satellites are operated independently from each other (Figure 4.2). The along-track distance will be 76 km to avoid RF interference between the radar signals. Temporal decorrelation is still small for most terrain types except vegetation at moderate to high wind speeds as well as for water. The interferometric height sensitivity is doubled with respect to the bistatic operation and this requires higher baseline determination accuracy. Neither pulse nor phase synchronization is required in the pursuit monostatic mode. For ScanSAR acquisitions an sufficient burst synchronization is ensured by an appropriate satellite commanding.




Figure 4.2: Pursuit monostatic mode for TanDEM-X data acquisition.

4.3.1.2 Bistatic Mode

This mode uses either TSX or TDX as a transmitter to illuminate a common radar footprint on the Earth’s surface. The scattered signal is then recorded by both satellites simultaneously (Figure 4.3). This simultaneous data acquisition makes dual use of the available transmit power and is mandatory to minimize possible errors from temporal decorrelation and atmospheric disturbances. A prerequisite for bistatic InSAR operation is PRF synchronization between the two satellites. Accurate interferometric measurements require moreover relative phase referencing to compensate the mutually uncorrelated phase noise from the two local oscillators. The bistatic mode will be the standard mode during the data acquisition for the TanDEM-X digital elevation model.

Figure 4.3: Bistatic mode for TanDEM-X data acquisition.




4.3.1.3 Alternating Bistatic Mode

A third operational mode is the alternating bistatic mode, where the transmitter is switched on a pulse-to-pulse basis. The scattered signal from the ground is then recorded by both receivers simultaneously as shown in Figure 4.4.

Figure 4.4: Alternating bistatic mode for TanDEM-X data acquisition.

The alternating bistatic mode acquires two monostatic and two bistatic SAR images during a single pass of the satellite formation. A comparison of the bistatic and monostatic images is hence well suited for the measurement of oscillator induced phase errors, thereby enabling an accurate phase calibration of the bistatic SAR interferometer. After phase calibration, the two bistatic images can be combined into a single bistatic SAR image with double PRF. For cross-track interferometry, two interferograms with different phase-to-height sensitivities can be derived:

The combination of one monostatic and the bistatic image yields a cross-track interferogram with a height of ambiguity of hamb = ( r sin(i)) / B, where is the wavelength, r the slant range, i the incident angle, and B the baseline perpendicular to the line of sight. Either the first or the second monostatic image can be selected, and a combination of both interferograms can be used to improve both the phase calibration and the phase stability.

The combination of the two monostatic SAR images yields a second interferogram with double phase-to-height sensitivity resulting in a height of ambiguity of hamb = ( r sin(i)) / (2 B).

The use of alternating transmitters in the bistatic mode allows hence for the simultaneous acquisition of two cross-track interferograms with phase-to-height sensitivities differing by a factor of two which will facilitate the process of interferometric phase unwrapping if the monostatic baseline is adjusted to fulfill the height determination accuracy requirements, i.e. half of the bistatic baseline is selected. Data takes in the alternating bistatic mode are used for system verification and calibration purposes.

4.3.2 Dual Receive Antenna Mode

The Dual Receive Antenna Mode enables the acquisition of a full polarimetric coherent scattering matrix or alternatively the operation with four phase centers, e.g. for tracking fast moving objects. The whole radar antenna can be electronically split into two parts in reception in along-track. The signals of both antenna parts are received and recorded separately. As a drawback the received gain of a half antenna is reduced by 3 dB and the overall swath width is reduced by half [R8], [R9], [R10]. In addition in the




Dual Receive Antenna mode the azimuth ambiguities are high. Both, the SNR and ambiguities, can be improved or reduced under certain constrains.

In the single polarimetry case the whole antenna is transmitting and receiving the signal in one polarisation. Dual polarimetry uses also the full antenna for the transmission and reception of the radar pulses. The two polarisations are required by toggling the polarisation from pulse to pulse. The fully polarimetric acquisition can only be done by the electronically split antenna (Figure 4.5). In this case the whole antenna is transmitting one polarisation and in reception the split into two separate parts enables the recording of two polarisations. To obtain the whole scattering matrix the transmitted polarisation is toggled pulse by pulse. Fully polarimetric observations are important to distinguish different scattering mechanisms occurring within a resolution cell. It is recommended to choose for the investigation stronger backscattered regions and steeper incidence angle as the noise component of the SNR and the ambiguities are severely reducing the performance of the SAR image.

Figure 4.5: Sketch of the operation of the Dual Receive Antenna Mode for polarimetric operation.

The split radar antenna has two physical phase centers separated by 2.4 m resulting in a theoretical effective along-track baseline of 1.2 m. The distance can be used to measure velocities of moving objects like for ocean currents and traffic observations. The distance within one satellite is providing a fixed short baseline, whereas the distance between the satellites provides longer variable baselines. At the same time with the two satellites, both operated in the Dual Receive Antenna mode, 4 phase centers can be obtained and is named as Stripmap-ATI mode.

Figure 4.6: Sketch of the operation of the Dual Receive Antenna Mode for along-track interferometry




4.3.3 Combination of Interferometric and Imaging Modes

For the interferometric modes only a selection of imaging modes can be operationally processed by DLR’s processing facility. The selection of imaging modes is different between the single receive antenna mode and the dual receive antenna mode (see Table 4.1 and 4.2).

In the case of the single receive antenna mode 12 combinations, depending on the interferometric and imaging mode, can be operationally processed by the DLR processor, they are collared in Table 4.1. All other combinations cannot be processed and are marked with crosses. The delivered product is the co-registered single-look slant range complex (CoSSC) product with the exception of the red marked combination: For the imaging modes ScanSAR and Wide ScanSAR, acquired only during the Pursuit Monostatic phase, there will be two TerraSAR-X like L1B products available [A4].

In the case of the dual receive antenna mode three combinations between the imaging modes and the interferometric modes are available. Only in the Stripmap mode fully polarimetric CoSSC products for the bistatic and pursuit monostatic mode will be available. The single polarisation DRA mode represents the 4 phase center mode for along-track interferometric acquisitions.

SRA bistatic alternating bistatic pursuit monostatic

Pol S D Q S D Q S D Q

Stripmap x x x x

Spotlight/High Resolution Spotlight x x x x

Starring Spotlight x x x x x x x x

ScanSAR/Wide ScanSAR x x x x x x x x

Table 4.1: Possible combination between interferometric modes and imaging modes for the Single Receive Antenna Mode (SRA) (S: single, D: dual, Q: quad polarisation). Blue: operational processing available and product delivery as CoSSC, Red: available as two separate TerraSAR-X Mission like products (no CoSSCs), Crosses: not available.

DRA Bistatic alternating bistatic pursuit monostatic

Pol S D Q S D Q S D Q

Stripmap x x x x x

Spotlight/High Resolution Spotlight x x X x x x x x x

Starring Spotlight x x X x x x x x x

ScanSAR/Wide ScanSAR x x X x x x x x x

Table 4.2: Possible combination between interferometric modes and imaging modes for the Dual Receive Antenna Mode (DRA) (S: single, D: dual, Q: quad polarisation). Blue: operational processing available and product delivery as CoSSC, Orange: ATI and 4 phase center experiments possible, Crosses: not available.




4.4 Science Phase Mission Timeline

The Science Phase is planned for 15 months starting in October 2014 and ending in December 2015 (Table 4.3). Within this time two main operation modes will be executed, the pursuit monostatic and the bistatic mode with varying perpendicular and along-track baselines. In addition, the Dual Receive Antenna mode will be available for both operation modes enabling the acquisition of polarimetric and 4 phase center data. Further, all imaging modes, from Stripmap, Spotlight, Staring Spotlight, ScanSAR to Wide ScanSAR mode are available to be selected for data acquisition. In the following, the different operation phases are summarised:

Pursuit Monostatic Phase: The pursuit monostatic phase is characterized by a set of drifting across-track baselines at all latitudes and within short time periods ranging between 0 – 750 m. After two months of operation the Dual Receive Antenna mode is switched on and will be operated until the end of this phase. The available variety of baselines is ideal for SAR tomography studies and for Polar Regions applications. The along-track separation between the satellites will be large enabling the observation of small velocities, as they can be detected from ships or drifting sea ice. In addition to the Bistatic TanDEM-X imaging modes the new modes Staring Spotlight and Wide ScanSAR are available during this phase.

Bistatic Phase: The bistatic phase is characterised by three elements, the operation of the Dual Receive Antenna mode, the stable huge (3-4 km) and the short (0-250 m) across-track interferometric baselines. This Phase starts in March 2015 with the Commissioning Phase for the verification, validation and tests for the Dual Receive Antenna mode in bistatic operation and will be in use until the end of the Science Phase, being suitable for fully polarimetric and 4 phase center experiments. The across-track baseline variation is split into two sub-phases, the large baseline and the short baseline phase.

o Large Baseline Phase: During 5 months large to huge baselines are operated for example for growth observation of short vegetation growth in the Northern hemisphere.

o Short Baseline Phase: During one month the baselines are reduced back to the initial distance operated during the TanDEM-X DEM phase and are kept in the short baseline configuration until the end of the Science Phase suitable for forest height estimation experiments or for ocean applications with short along-track baselines on the Southern hemisphere.

While science acquisitions will have priority during the 15-month Science Phase period, it is nevertheless planned to acquire additional data to fill gaps and further improve the quality of the TanDEM-X DEM and to acquire data for high resolution DEMs




Table 4.3 Science Phase Timeline (CP: Commissioning Phase, PC: Phase Center)

Months 2014/15

1

Oct

2

Nov

3

Dec

4

Jan

5

Feb

6

Mar

7

Apr

8

May

9

Jun

10

Jul

11

Aug

12

Sep

13

Oct

14

Nov

15

Dec

Operation Mode

Pursuit Monostatic Bistatic

Polarimetry Single/Dual Polarimetry

Single/Dual/Quad Polarimetry

CP Single/Dual/Quad Polarimetry

Phase Center (PC)

2 PC 2/4 PC CP 2/4 PC

Baseline (values for all latitudes)

0-750 m (perp. baseline) (slow drift over 5 months)

3-4 km

at Equator

3-4 km (horizontal baseline at Equator) smaller at higher latitudes

(stable baselines over the whole period)

Fast drift back

to 300 m

0-250 m (stable short perp.

baselines)

Along-track baselines

76 km 0-500 m (both hemispheres) 300-850 m (ascending)

~ 0-300 m (descending)

The Science Phase timelline represents a guideline for the proposal submission for data request and is a major outcome of the splinter meetings at the 4th TanDEM-X Science Meeting in June 2013. The science users should plan their data take requests during proposal submission according to this table. An overview of achievable baselines during the different sub-phases is given in the following section.

For an overview of the different across-track baselines the following categories are established:

small: 0-200 m

normal: 200-500 m

large: 500-1000 m

very large: 1000-3000 m

huge: < 3000 m

4.4.1 Baseline Variation as a Function of Latitude

For those interested into more details on the variation of the baselines for given geographical latitudes the following section is provided. The plots show the across-track and along-track baselines for the specific Science Phases as a function of latitude.

Pursuit Monostatic Phase

As already pointed out the highlight of this phase is the baseline variation in short time. This is achieved by the natural drifting of the satellite that is kept over a time period. The satellite drift is controlled by the libration phase. As a result a different baseline plot is obtained for each phase of libration. The phase of libration is varying between 0-360 degrees within 104 days. In the following only two examples are given for a specific configuration, which illustrate that nearly any across track baseline can be achieved for a given latitude during this phase. The whole set of baseline diversity for each libration phase is provided in Annex A.




Figure 4.6: TanDEM-X perpendicular baselines for different latitudes for a given phase of libration (left: 0 degree and right: 90 degrees). In each figure the baseline variation is plotted as a function of look angles (colors) and for ascending and descending passes (continuous and dotted lines). The Northern hemisphere is represented by positive and the Southern hemisphere by negative latitude values.

In summary the pursuit monostatic phase is characterised by the following features:

Collection of a variety of across-track baselines within a short time obtained by means of the satellite drift.

A fine tuning of the cross-track baseline (or matching height of ambiguity) can be performed by a careful selection of the incidence angle, as the difference can be large between different beams. In general steep incidence angels deliver shorter across-track baselines.

Similar cross-track baselines can be achieved in both ascending and descending orbits, and for both hemispheres.

The along-track baselines are close to stable over the latitudes for the whole Pursuit Monostatic Phase period and in the order of 76 km.




Bistatic Phase

The main characteristics of the bistatic phase are the stable long and short across-track baselines and short along-track baseline. In the bistatic phase the natural drift of the satellite is corrected on a daily basis to avoid baseline variations. In summary the bistatic long and short baseline periods are characterised by the following features:

Large Across-Track Baseline Phase (Figure 4.7):

o The perpendicular baseline has a strong dependence on the latitude. At the Equator the largest across-track baselines are obtained and they become shorter towards the poles.

o In the Southern hemisphere the descending orbits show slightly larger across-track baselines, compared to ascending orbits, and the opposite is true for the Northern hemisphere. This means, both orbit directions can be utilized for both hemispheres as the differences are small.

o In general steeper incidence angles show larger across-track baselines.

Figure 4.7: TanDEM-X perpendicular (left) and along-track (right) baselines for different latitudes for the bistatic large baseline phase. In each figure the baseline variation is plotted as a function of incidence angles (colors) and for ascending and descending passes (continuous and dotted lines). The Northern hemisphere is presented by positive and the Southern hemisphere by negative latitude values.

o The along-track baselines are short close to the Poles at around 70-80 degree North and South and are independent of incidence angle.

o Only in the Southern hemisphere at a small range of latitudes (70-80 degree) both short along and across-track baselines are obtained as desired for Ocean current experiments.

Short Across-Track Baseline Phase (Figure 4.8):

o In the Southern hemisphere normal across-track baselines are obtained in the descending orbits. Small across-track baselines are obtained in the ascending orbits.

o In contrary, on the Northern hemisphere large across-track baselines are obtained in the ascending orbit and small baselines in the descending orbits.

o Short along-track baselines are available on the Northern and Southern hemisphere in the ascending orbit with a minimum close to ±45 degree latitude.




o However, both together, short across and along-track baselines (less than 100 m) are only available in the ascending orbit on the Southern hemisphere.

Figure 4.8: TanDEM-X perpendicular (left) and along-track (right) baseline for different latitudes for the bistatic short baseline phase. In each figure the baseline variation is plotted as a function of incidence angles (colors) and for ascending and descending passes (continuous and dotted line). The Northern hemisphere is presented by positive and the Southern hemisphere by negative latitude values.




5 The TanDEM-X Science Phase Experiments

During the TanDEM-X Science Meeting in June 2013 several experiments covering a wide range of applications have been proposed for the TanDEM-X Science Phase. On the basis of this first exchange with the Principle Investigators the Science Phase timeline has been established. Even though most of the requirements of the different scientific experiments could be accommodated in the time line not all could be included, as seen from the tables in this section. However, the timeline is now the guideline for all scientists requesting data for their application experiments.

The application experiments listed in the following tables are only examples and should trigger the proposition of further application experiments not listed here. Some of the suggestions made by the scientist are not supported by the operational processor (please see Table 4.1 and 4.2).

Vegetation Experiments

Application InSAR Mode

Imaging Mode

DRA Baseline Coverage Time Comments

Forest height

Bistatic SM

SL

single/dual/quad

single/dual/quad

set of short across-track baselines (100-300 m)

Northern/Southern hemisphere

boreal forest

Amazon basin

All year around

Leaf on/off

Need access to global DEM data

Forest structure

Bistatic or PM

SM

SL

single/dual/quad

single/dual/quad

set of short across-track baselines (100-300 m) for Tomography

Northern/Southern hemisphere

boreal forest

All year around

Leaf on/off

Need access to global DEM data

Forest mapping

Bistatic and AB

SM dual Large to very large baselines (100-1000 m)

Amazon Basin 2 times acquisition: wet/dry season

AB with 20 m HoA

Crop height Bistatic SM

SL

dual/quad

Very long to huge baselines (1-4 km)

selected sites (Northern/Southern hemisphere)

Europe/Asia/USA/Australia

3-4 months growth period

Noise need to be reduced in HV (e.g. reduction of bandwith)

Figure 5.1: Vegetation Experiments (SM: Stripmap, SL: Spotlight, PM: Pursuit Monostatic Mode, AB: Alternating Bistatic, HoA: Height of Ambiguity)




Cryosphere Experiments


Imaging Mode


Sea Ice Depth PM SM dual/quad

Very long/huge across-track baselines (1-4 m)

Selected sea ice regions

Polar winter (4-5 cycles)

Sea Ice Topography

Bistatic SM

single/dual

Huge baselines (3-4 km)

Super test sites

Selected site at the Northern hemisphere

North Greenland/Svalbard

Polar winter (4-5 cycles)

Deci-meters to centimeters height resolution; combination with ground campaign

Sea Ice Classification

Bistatic SM

dual/quad

Normal baselines (200-500 m)

Super test sites

Selected site

Late polar winter/early spring

Combination with ground campaign; airborne/ satellite

Surface Elevation Change/Mass Balance

Bistatic or PM

SM Single/dual Normal baselines (200-500 m)

Super test sites

Selected sites

Spring/ summer; (3-4 cycles)

Combination with ground/ airborne/ satellite and coordinated with Ice Bridge, Polar5/6 campaigns

Snow/Ice structure

Bistatic or PM

SM Dual/quad set of across-track baselines (100-500 m) for Tomography

Super test sites

Selected sites

Spring/autumn (3-4 cycles)

Combination with ground/ airborne/ satellite

Snow Depth Bistatic or PM

SM Dual/quad Large/huge baselines (500 m-4 km)

Super test sites

Selected sites

Jan-April (1-3 cycles)

Snow Accumulation

Bistatic or PM

Single/dual Large/huge baselines (500 m-4 km)

Inner Antarctica Polar summer/winter (5-6 cycles)

Figure 5.2: Cryosphere Experiments (SM: Stripmap, SL: Spotlight, PM: Pursuit Monostatic Mode)

Geosphere Experiments


Imaging Mode


Volcano Monitoring

Bistatic or PM

SM

SL

single/dual

set of across-track baselines (100-500 m)

Super test sites

All volcanos on the Northern hemisphere

Assal/El Hiero/Merapi/Colima

All year as many as possible

Tectonic Monitoring

Bistatic or PM

SM

single/dual

set of across-track baselines (100-500 m)

Wenchuan (China)

Three Gorge (China) All year as many as possible

High resolution DEM required

Figure 5.3: Geosphere Experiments (SM: Stripmap, SL: Spotlight, PM: Pursuit Monostatic Mode)

Urban Experiments





Imaging Mode


City Monitoring

Bistatic or PM

SM

SL

single/dual

set of across-track baselines (100-1000 m) for tomography

Super test site

Megacities (London, Paris, Beijing,LA, Mexico)

All year as many as possible

Figure 5.4: Urban Experiments (SM: Stripmap, SL: Spotlight, PM: Pursuit Monostatic Mode)

Ocean Experiments


Imaging Mode


Ocean Currents

Bistatic SM

ScanSAR

single/dual

single

small along and across-track baselines (0-100 m)

Selected areas All year (3-4 cycles)

3D Wave Estimation

Bistatic and PM

SM

SM

ATI

single/dual/quad

small along-track baselines (50-100 m)

Selected areas All year One Satellite should be physically squinted

Figure 5.5: Ocean Experiments (SM: Stripmap, SL: Spotlight, PM: Pursuit Monostatic Mode)

New Technique Demonstration


Imaging Mode


Global fully polarimetric

Bistatic SM

quad any global All year Background mission

Bistatic Angle Diversity

Bistatic SM

quad Long/huge along and across baselines

Selected areas All year Variation of incidence angles

MTI Bistatic and PM

SM Dual/quad Small across and along-track baselines (0-100 m)

Selected areas over land All year

High Resolution Wide Swath

Bistatic or PM

SM 4 Phase Centers

Small across and along-track baselines (0-100 m)

Selected area over land All year

Figure 5.6: New Techniques Demonstration (SM: Stripmap, SL: Spotlight, PM: Pursuit Monostatic Mode)




6 Recommendations

This section is created to provide some recommendations to the scientist on the particularities of the different interferometric and imaging modes. They should present a guideline for the planning of the individual experiments that the scientist likes to propose.

6.1.1 Alternating Bistatic Mode

Please consider the restrictions for the Alternating Bistatic Mode. The Alternating Bistatic Mode is only available during the Bistatic Phase. Furthermore, due to exclusion zones (for the transmit capabilities of the one satellite) in one pass direction for this interferometric mode it is recommended to enter the data take request in the following way:

Use ascending orbits for the Northern hemisphere.

Use descending orbits for the Southern hemisphere. Only for a small latitude region close to the equator (between ~ 25 deg North and -25 deg South latitude) the exclusion zones are not applied. However, as the exclusion zones will slightly vary with the different helix settings over time it is not recommended to use this regions. The Alternating Bistatic Mode is NOT suited for Ocean Current Measurements (or any other ATI technique, which needs both very short perpendicular across-track AND very short along-track baselines below 100m) during the short across-track baseline within the Bistatic phase, as such an area would fall inside the exclusion zone of the Southern hemisphere; this means that the use of ascending orbits in the Southern hemisphere, which would serve such a configuration, is not allowed. Only if you can compensate the topographic phase induced by larger across-track baselines by external references (e.g a high resolution DEM) ATI measurements using the Alternating Bistatic mode might be feasible during the bistatic phase. Otherwise it is recommended to use the Stripmap-ATI mode.

6.1.2 Bistatic Mode

For Ocean Current Measurements, both, very short perpendicular across-track and very short along-track baselines are needed (in order to minimize topographical effects and due to the short decorrelation time of water). For this application the following strategy is recommended:

Check the temporal constraints for those applications given by the time table and the helix plots. Only in the short baseline bistatic mission phase (October-December 2015) the helix will be set-up in such a way that even for middle latitudes the mentioned baseline requirement can be met, but on the Southern hemisphere only! The area where the across-track and the along-track baselines are both below 100 m is located between -60 degree and -30 degree latitude in ascending orbits only. For entering data takes in this latitude corridor please use only ascending orbits for the Southern hemisphere to minimize the across- and along-track baseline.

6.1.3 Short Across-Track Baseline Phase

Whenever acquisitions stacks with different baselines (normal to large across-track baselines AND short across-track baselines for forest height experiments) are needed:

Use the ascending orbits for the Northern hemisphere.

Use the descending orbits for the Southern hemisphere. Only for areas close to the equator (-10 degree to 10 degree) there is not much difference

between ascending and descending orbits.




6.1.4 Polarimetric and Stripmap-ATI Mode

The polarimetric and the 4 phase center mode (Stripmap-ATI) require the operation of the Dual Receive Antenna mode on both satellites. Due to the system reconfiguration the SNR is reduced by 3 dB and the azimuth ambiguity ratio is increased. In order to retrieve data with a good performance it is recommended to use steep incidence angles. Another system modification is the duplication of the data amount on the satellites. Please try to set your data take length to the minimal extend of your test area.




ANNEX A. Perpendicular Baseline for different Phases of Libration (PM Mode)




Documents

TD-PD-PL 0032TanDEM-X Science Phase 19052014