Monitoring Dynamic Processes on the Earth’s Surface Using Synthetic Aperture Radar Michelangelo Villano, Gerhard Krieger, Konstantinos P. Papathanassiou, and Alberto Moreira Microwaves and Radar Institute German Aerospace Center (DLR) Wessling, Germany
[email protected] Abstract—Synthetic aperture radar (SAR) has proven to be a key remote sensing technique for Earth observation. However, conventional SAR systems are limited in that a wide coverage can only be achieved at the expense of a degraded resolution. Staggered SAR overcomes this limitation by means of digital beamforming and continuous variation of the pulse repetition interval (PRI). Staggered SAR is currently being considered as the baseline acquisition mode of the Tandem-L mission, whose powerful instrument will deliver weekly high-resolution global images of our planet, thereby allowing quantification of a number of essential climate variables. Keywords—synthetic aperture radar, Earth observation, highresolution wide-swath imaging, Tandem-L, staggered SAR, frequent monitoring.
I.
INTRODUCTION
Synthetic aperture radar (SAR) is a remote sensing technique that exploits the Doppler shift arising from the sensor movement relative to the ground to improve the resolution in the flight direction well beyond the diffraction limit of the radar antenna [1]. SAR therefore achieves high-resolution imaging, while keeping an important feature of active microwave instruments, namely the ability to operate independently of weather conditions and sunlight illumination. Furthermore, the joint exploitation of multiple SAR images, acquired in different polarizations (polarimetric SAR), from slightly different observation angles (SAR interferometry, polarimetric SAR interferometry, and SAR tomography) and/or at different times (differential and permanent scatterer interferometry), allows retrieving a huge amount of additional information. SAR is nowadays an established tool for Earth observation: several satellites have been launched and operated as of 1978, and many airborne SAR systems have allowed early demonstrations of novel techniques, which have later been implemented in spaceborne missions [2]. II.
STATE OF THE ART
More than 15 spaceborne SAR sensors are currently in operation, all launched within the last decade and characterized by a spatial resolution at least one order of magnitude higher than the sensors of the previous generation. The DLR Microwaves and Radar Institute has made a significant contribution to the field with TerraSAR-X and TanDEM-X, the first satellites flown in a closely controlled formation to
generate a seamless global digital elevation model with unprecedented accuracy and resolution [3]. State-of-the-art sensors also offer a much higher flexibility in that several acquisition modes can be selected for different trade-offs between resolution and coverage thanks to the use of phased array antennas with electronic beam steering. TerraSAR-X, for instance, can operationally achieve a resolution of 1 m in (sliding) spotlight mode or a 100 km swath width in ScanSAR mode, while its experimental modes allow an even higher resolution (0.2 m in staring spotlight mode), a wider swath (up to 260 km in wide ScanSAR mode), or dualand fully-polarimetric acquisitions. TerraSAR-X data have been used to demonstrate several applications of SAR to environmental monitoring, such as the observation of the very fast movement of the Drygalski glacier, Antarctica, through thirty images acquired by TerraSAR-X during one year [Fig. 1 (a)], the mapping of a flood of the Mississippi River, USA [Fig. 1 (b)], the measurement of ground subsidence (up to 10 cm in four months) as a result of the water extraction in Mexico City, Mexico [Fig. 1 (c)], and the assessment of extensive logging in Mato Grosso, Brazil [Fig. 1 (d)]. In the latter case, clearings appear in the radar image as rectangular, relatively dark zones within the otherwise homogeneous surface of the forest; whereas covering large areas with optical cameras mounted on satellites is problematic in tropical regions due to the dense cloud layers. While very powerful and flexible, TerraSAR-X can map in stripmap mode only 2% of the Earth’s landmass during its 11day repeat cycle, due to its relatively small orbit duty cycle (the satellite can only operate 3 minutes per orbit) and its 30-km swath width. Sensors launched more recently, such as Sentinel1 and ALOS-2, are still characterized by comparable mapping capabilities at that resolution. This limitation is not due to technology development, but is inherent in the SAR acquisition principle. A brute force solution to this problem consists of flying a constellation of satellites on the same orbit, as done for Cosmo-SkyMed and Sentinel-1. This solution affords an increase in mapping capability by a factor equal to the number of satellites of the constellation, but becomes costly or even unfeasible, if the mapping capability has to be boosted by one or even two orders of magnitude.
III.
TANDEM-L
In recent years there has been increased interest in the scientific community in understanding and quantifying dynamic processes within the Earth system occurring at different spatial and temporal scales, as well as their interdependency and interaction. Many of these processes are currently inadequately researched and understood. An important reason for this is the lack of suitable observation data for analyzing such interactions. Fig. 2 summarizes the requested observation intervals for the systematic monitoring of some exemplary dynamic processes on the Earth surface. (a)
(b)
(c)
The imaging performance and/or measurement resolution and accuracy of existing remote sensing configurations are often inadequate to draw reliable conclusions as to the dynamics of large-scale processes. The measurement of dynamic processes requires a continuous, extended and systematically planned observation strategy in order to detect changes and quantify them with sufficient accuracy. Depending on the processes to be observed, changes have to be measured on variable spatial and temporal scales and then related to one another. The combination of short revisit times and extended acquisitions over several years is required when it comes to accurate and high-resolution monitoring of fast developing, highly dynamic processes, such as the relaxation following an earthquake, as well as slowly developing processes, such as the inter-annual variation of forest biomass. SAR represents the ideal candidate to provide answers to these questions, but spaceborne SAR sensors currently in operation do not have the resolution and mapping capability needed to meet these scientific requirements. In particular, a SAR sensor is required, capable of mapping the entire Earth surface twice a week in fully-polarimetric mode and with a spatial resolution below 10 m (this corresponds to a mapping capability two orders of magnitude better than that of TerraSAR-X). In response to these needs, a proposal for a highly innovative L-band SAR mission, Tandem-L, was started at DLR with a joint pre-phase A study with the National Aeronautics and Space Administration (NASA) in 2008 and is currently undergoing a phase B1 study [4]-[5]. Fig. 3 depicts an artist’s view of Tandem-L. Tandem-L uses a deployable reflector antenna in combination with innovative digital beamforming (DBF) techniques. This increases the sensitivity and leads to a considerable reduction in transmit power. Because of this, the SAR instrument can be operated virtually continuously. DBF allows forming multiple elevation beams, which simultaneously map multiple subswaths [6]. In this way, a resolution higher than 10 m over a 350 km wide ground swath can be achieved, as required for the aforementioned mission aimed at monitoring dynamic processes on Earth’s surface.
(d) Fig. 1 Examples of applications of TerraSAR-X to environmental monitoring. (a) Observation of the Drygalski glacier, Antarctica. (b) Mapping of a flood of the Mississippi river, USA. (c) Measurement of ground subsidence in Mexico City, Mexico. (d) Assessment of extensive logging in Mato Grosso, Brazil.
Fig. 2 Requested observation intervals for the systematic monitoring of some exemplary dynamic processes on the Earth surface.
Fig. 3 Artist’s view of Tandem-L.
IV.
STAGGERED SAR
A key element of the Tandem-L instrument is staggered SAR, a novel concept based on the continuous variation of the pulse repetition interval (PRI) and investigated at DLR. The architecture with multiple elevation beams, in fact, yields “blind ranges” between the different subswaths, as the radar cannot receive, while it is transmitting (left panel of Fig. 4). If the PRI is continuously varied, however, the ranges, from which the echoes are not received, because the radar is transmitting, will be different for each transmitted pulse (right panel of Fig. 4). A proper selection of the PRIs together with an average oversampling of the signal in azimuth allows imaging a wide continuous swath with high resolution [7]-[12]. The performance of a staggered SAR system depends on the adopted sequence of PRIs and the method used to resample the non-uniform signal to a uniform grid. If sequences of PRIs are used, where two consecutive azimuth samples are never missed and data are resampled using best linear unbiased (BLU) interpolation, it can be shown that a system with a 15 m reflector antenna with multiple feeds in the elevation direction is able to image a 350 km continuous swath with 7 m azimuth resolution and outstanding imaging performance. This means an improvement of the mapping capability by almost one order of magnitude if compared to existing SAR systems. As part of the study, the impact of staggered SAR operation on image quality has also been assessed with experiments using real data. As a first step, highly oversampled F-SAR airborne data have been used to generate equivalent staggered SAR data sets and evaluate the performance for different sequences of PRIs and interpolation methods. Secondly, the German satellite TerraSAR-X was commanded to acquire data
over the Lake Constance in staggered SAR mode. Measurements on these data show very good agreement with predictions from simulations [13]. Staggered SAR is currently being considered as the baseline acquisition mode of the Tandem-L mission, where a strategy for onboard data volume reduction able to cope with the increased amount of data has also been devised [8]. Analyses carried out recently have shown that staggered SAR also represents an appealing option for the NASA-ISRO Synthetic Aperture Radar (NISAR) mission, a mission with similar scientific objectives as Tandem-L, but where the system’s size is constrained by available resources, and it is planned to operate with a lower pulse rate than would be optimum. V.
FUTURE DEVELOPMENTS
While Tandem-L is a huge step forward, some important applications require an even more frequent, ideally daily temporal sampling. An example is soil moisture retrieval for water cycle research, which could also help make weather predictions more accurate. However, a mission which meets this requirement with affordable costs cannot simply be based on dimensioning a classical SAR system, but calls for radically new concepts and a system design driven by the applications. Frequent Earth monitoring with SAR can be accomplished by satellites in higher orbits or myriads of small satellites, exploiting disruptive technologies and techniques, such as inflatable antennas, joint radar-communication systems, distributed and fractionated SAR, as well as waveform diversity. The development of these ideas will further boost the imaging capability and provide easy and affordable access to space [14]-[16].
Fig. 4 Top: Transmitted pulses and corresponding received echoes (same colors as the transmitted pulses) with blind ranges (samples in black, where the echo cannot be recorded, as the radar is transmitting). Bottom: Raw data obtained by arranging side by side the received echoes for a SAR with constant Pulse Repetition Interval – PRI (left) and for a staggered SAR (right).
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