Maurice Rüegg

Optimal bandwidth reservation in hose-model VPNs with multi-path routing

A virtual private network (VPN) provides private network connections over a publicly accessible shared network. Bandwidth provisioning for VPNs leads to challenging optimization problems. In the hose model proposed by Duffield et al., each VPN endpoint specifies bounds on the total amount of traffic that it will send or receive at any time. The network provider must provision the VPN so that there is sufficient bandwidth for any traffic matrix that is consistent with these bounds. While previous work has considered tree routing and single-path routing between the VPN endpoints, we demonstrate that the use of multipath routing offers significant advantages. On the one band, we present an optimal polynomial-time algorithm that computes a bandwidth reservation of minimum cost using multi-path routing. This is in contrast to tree routing and single-path routing, where the problem is computationally hard. On the other hand, we present experimental results showing that the reservation cost using multi-path routing can indeed be significantly smaller than with tree or single-path routing.

Focusing of Airborne Synthetic Aperture Radar Data From Highly Nonlinear Flight Tracks

Standard focusing of data from synthetic aperture radar (SAR) assumes a straight recording track of the sensor platform. Small nonlinearities of airborne platform tracks are corrected for during a motion-compensation step while maintaining the assumption of a linear flight path. This paper describes the processing of SAR data acquired from nonlinear tracks, typical of sensors mounted on small aircraft or drones flying at low altitude. Such aircraft do not fly along straight tracks, but the trajectory depends on topography, influences of weather and wind, or the shape of areas of interest such as rivers or traffic routes. Two potential approaches for processing SAR data from such highly nonlinear flight tracks are proposed, namely, a patchwise frequency-domain processing and mosaicking technique and a time-domain back-projection-based technique. Both are evaluated with the help of experimental data featuring tracks with altitude changes, a double bend, a 90deg curve, and a linear flight track. In order to assess the quality of the focused data, close-ups of amplitude images are compared, impulse response functions of a point target are analyzed, and the coherence is evaluated. The experimental data were acquired by the German Aerospace Centers E-SAR L-band system.

Vibration and Rotation in Millimeter-Wave SAR

Synthetic aperture radar (SAR) provides high-resolution images of static ground scenes, whereas processing of data containing ground object motion results in varying focusing effects. Special cases of such motion are vibration and rotation, which are closely related to each other. Their patterns may be distinctly recognizable in focused SAR intensity images as well as in a time-frequency analysis. Millimeter-wave (mmW) SAR is well suited to image vibration because its wavelength is close to typical vibration amplitudes. Through a thorough motion analysis in a standard SAR system model, we show the effects of rotation and vibration in mmW SAR theoretically and in simulated and real data

Capabilities of Dual-Frequency Millimeter Wave SAR With Monopulse Processing for Ground Moving Target Indication

Ground moving target indication (GMTI) for synthetic aperture radar (SAR) provides information on nonstatic objects in radar imagery of a static ground scene. An efficient approach for GMTI is the use of multichannel SAR systems for a space- and time-variant analysis of moving targets. This allows the indication, correction of position displacement, and estimation of radial velocity components of moving targets in a SAR image. All three steps are possible due to a determinable Doppler frequency shift in the radar signal caused by radial target movement. This paper focuses on the millimeter wave (mmW) SAR system MEMPHIS with multichannel amplitude-comparison monopulse data acquisition and the ability to use carrier frequencies of 35 and 94 GHz simultaneously, making it a dual-frequency SAR. This paper includes mmW-specific SAR GMTI considerations, an adaptive algorithm to collect velocity and position information on moving targets with mmW monopulse radar, and a discussion on GMTI blind speed elimination and target velocity ambiguity resolving by dual-frequency SAR. To determine the capabilities of both, system and algorithm, three large-scale experiments with MEMPHIS in different environments are presented

Measurement of Ionospheric Faraday Rotation in Simulated and Real Spaceborne SAR Data

The influence of the atmosphere on a frequency-modulated electromagnetic wave traversing the ionosphere is becoming increasingly important for recent and upcoming low-frequency and wide-bandwidth spaceborne synthetic aperture radar (SAR) systems. The ionized ionosphere induces Faraday rotation (FR) at these frequencies that affects radar polarimetry and causes signal path delays resulting in a reduced range resolution. The work at hand introduces a simulation model of SAR signals passing through the atmosphere, including both frequency-dependent FR and path delays. Based on simulation results from this model [proven with real Advanced Land Observing Satellite Phased Array L-band Synthetic Aperture Radar (PALSAR) data], estimation of FR in quad-polarized SAR data using the given approach is shown for raw, range-compressed, and focused radar images. Path delays and signal chirp bandwidth effects are considered. Investigations discuss the suitability of raw and compressed data versus combination of total electron content maps with the Earths magnetic field for FR estimation and deduced from a large number of analyzed PALSAR data sets.

High resolution millimeter wave SAR interferometry

High resolution millimeter wave synthetic aperture radar (SAR) interferometry is presented using the MEMPHIS multi-baseline InSAR system. A complete processing chain is used to generate digital elevation models starting from the radar raw data. A deeper focus is laid on the phase unwrapping step, which is achieved using the multi-baseline properties of the system. In November 2006, an experiment was realized including two test sites in Switzerland; the actual results are presented and discussed.

Moving target indication with dual frequency millimeter wave SAR

Ground moving target indication (GMTI) for synthetic aperture radar (SAR) provides information on non-static objects in a static ground scene. An efficient approach for GMTI is the use of multi-channel SAR systems for a space- and time-variant analysis of moving targets. This allows the indication, correction of position errors, and estimation of radial velocity components for moving targets in the SAR image. All three steps are possible because of the Doppler frequency shift in the radar signal caused by the radial target movement. Our work focuses on the millimeter wave (mmW) SAR system MEMPHIS with multi-channel amplitude-comparison monopulse data recording and the ability to use carrier frequencies of 35 and 94 GHz simultaneously, making it a dual frequency multi-channel SAR. Our discussions include mmW specific SAR GMTI considerations and an adaptive algorithm to collect information on moving targets with a mmW monopulse radar, and GMTI blind speed elimination and target velocity ambiguity resolving by dual frequency SAR. For an experiment with MEMPHIS, frequency spectra, processed SAR images with position corrected moving targets, and accurate target velocities and positions are presented to verify the developed algorithm.

Constant Motion, Acceleration, Vibration, and Rotation of Objects in SAR Data

Synthetic aperture radar (SAR) provides high resolution images of static ground scenes, but processing of data containing moving objects results in varying phase and amplitude effects. The work at hand illustrates via theoretical considerations and concrete simulations what happens to SAR imagery when parts of a scene are not static. We differentiate between four types of motion. Objects moving with a constant velocity cause position errors in azimuth as well as target defocusing and smearing in azimuth and range. Accelerating objects are responsible for even stronger shift and defocusing effects since the position errors are now a function of time. Closely related are vibrations of an object. They may be interpreted as a regular and continuous de- and acceleration whose range component results in so-called paired echoes on each side of an object in azimuth. Finally, rotation as an extreme example of constant radial acceleration may disturb a SAR image over a wide area. Through a thorough motion analysis, we developed a flexible SAR raw data simulator. Our simulations of point scatterers in raw data are based on the radar radiation pattern as a function of the system carrier frequency and the relative positions between the radar and each scatterer. All four types of movement described above may be expressed as varying relative positions and Doppler frequency shifts due to instantaneous phase variations. The standard SAR processing steps of range and azimuth compression for the simulated data provide impressive results for freely adaptable system parameters of the movement and of the SAR system.

Focusing SAR Data Acquired from Non-Linear Sensor Trajectories

Standard focusing of SAR data assumes a straight recording track of the sensor platform. Small non-linearities of airborne platform tracks are corrected for during a motion compensation step while keeping the assumption of a linear flight path. In the following, the processing of SAR data from non-linear tracks is discussed as may originate from small aircraft or drones flying at low altitude. They fly not a straight track but one dependent on topography, influences of weather and wind, or dependent on the shape of dedicated areas of interest such as rivers or traffic routes. A time-domain back-projection based technique, is proposed and evaluated with the help of experimental data featuring a drop in height, a double bend, a 90-degree curve and a linear flight track. In order to assess the quality of the focused data, close-ups of amplitude images are compared and the coherence is evaluated. The experimental data was acquired by the German Aerospace Centers E-SAR L-band system.

Estimation of ionospheric TEC and Faraday rotation for L-band SAR

Spaceborne synthetic aperture radar (SAR) systems are used to measure geo- and biophysical parameters of the Earths surface, e.g. for agriculture, forestry and land subsidence investigations. Upcoming SAR sensors such as the Japanese Phased Array L-band Synthetic Aperture Radar (PALSAR) onboard the Advanced Land Observing Satellite (ALOS) exemplify a trend towards lower frequencies and higher range chirp bandwidth in order to obtain additional information with higher geometric resolution. However, the use of large bandwidths causes signal degradation within a dispersive medium such as the ionosphere. Under high solar activity conditions at L-band frequencies, ionosphere-induced path delays and Faraday rotation become significant for SAR applications. Due to ionospheric effects, blind use of a generic matched filter causes inaccuracy when correlating the transmitted with the received signal. Maximum correlation occurs where the length of the matched filter, based on a synthetic chirp model of the transmitted signal, is adjusted to correspond to that of the received signal. By searching for the proper adjustment necessary to reach this maximum, the change in length can be estimated and used to derive variations in the total electron content (TEC) and degree of Faraday rotation within the ionosphere from all range lines in a SAR image.