Boresight Axis Research Articles

LIDAR adaptive parameters selection and target pointing control for close-proximity space operations

This paper deals with advanced Guidance, Navigation and Control (GNC) functions required to enable autonomous and safe operations of a chaser spacecraft in close-proximity to an uncooperative space target, as in Active Debris Removal or On-Orbit servicing scenarios. Specifically, it presents an original approach to autonomously and adaptively select the field of view and resolution of a scanning LIDAR to improve both state estimation accuracy and computational efficiency of a LIDAR-based relative navigation system. In general, the correct operation of such system is also determined by the capability to keep the boresight axis of the relative navigation sensor aligned with the target geometric center. To address this task, an original control technique, based on the sliding-mode formulation and relying on a reduced attitude representation, is proposed. This control scheme is also compared to state-of-the-art PD and PID approaches in terms of pointing accuracy and control effort. The proposed techniques have been numerically validated in a simulation environment integrating the chaser attitude control and the LIDAR-based relative navigation functions in a closed-loop architecture. The simulation environment realistically reproduces the generation of LIDAR-based point clouds, and the spacecraft relative dynamics in close proximity. Results show that the adaptive selection of the LIDAR operational parameters improves the relative navigation performance, while the proposed sliding-mode control guarantees higher pointing accuracy than PD and PID control approaches.

Robust spacecraft relative pose estimation via CNN-aided line segments detection in monocular images

Autonomous spacecraft relative navigation via monocular images became a hot topic in the past few years and, recently, received a further push thanks to the constantly growing field of artificial neural networks and the publication of several spaceborne image datasets. Despite the proliferation of spacecraft relative-state initialization algorithms developed, most architectures adopt computationally expensive solutions relying on convolutional neural networks (CNNs) that provide accurate output at the cost of a high computational burden that seems unfeasible for current spaceborne hardware. The paper addresses this issue by proposing a novel pose initialization algorithm based on lightweight CNNs. Inspired by previous state-of-the-art algorithms, the developed architecture leverages a fast and accurate target detection CNN followed by a line segment detection CNN capable of running with low inference time on mobile devices. The line segments and their junctions are grouped into complex geometrical groups, reducing the solution search space, and subsequently, they are adopted to extract the final pose estimate. As a main outcome, the analyses demonstrate that the lightweight architecture developed scores high accuracy in the pose estimation task, with a mean estimation error of less than 10 cm in translation and 2.5°in rotation. The baseline algorithm scores a mean SLAB error of 0.04552 with a standard deviation of 0.22972 in the test dataset. Detailed analyses demonstrate that the uncertainties on the overall pose score are driven mainly by errors in the relative attitude, which gives the highest contribution to the pose error metric adopted. The analyses on the error distributions point out that the uncertainties on the estimated relative position are higher in the camera boresight axis direction. Concerning the relative attitude, the algorithm proposed has higher uncertainties in estimating directions of the target x and y axes due to ambiguities related to the target geometry. Notably, the target detection CNN trained in this work outperforms the previous top scores in the benchmark dataset. The performances of the proposed algorithm have been investigated further by analyzing the effects on the accuracy due to the relative distance and the presence of background in the images. Lastly, the paper delves into the possibility of adopting a sub-portion of the 2D-to-3D match matrix made by the most complex perceptual groups identified that positively affects the overall run-time, pointing out the performances in terms of accuracy of the estimates and providing a comparison of both the baseline and the reduced match matrix versions against state-of-the-art algorithms concerning relative position and attitude errors and solution availability, highlighting the high accuracy and solution availability of the proposed architectures.

Orientation-Independent Human Activity Recognition Using Complementary Radio Frequency Sensing.

RF sensing offers an unobtrusive, user-friendly, and privacy-preserving method for detecting accidental falls and recognizing human activities. Contemporary RF-based HAR systems generally employ a single monostatic radar to recognize human activities. However, a single monostatic radar cannot detect the motion of a target, e.g., a moving person, orthogonal to the boresight axis of the radar. Owing to this inherent physical limitation, a single monostatic radar fails to efficiently recognize orientation-independent human activities. In this work, we present a complementary RF sensing approach that overcomes the limitation of existing single monostatic radar-based HAR systems to robustly recognize orientation-independent human activities and falls. Our approach used a distributed mmWave MIMO radar system that was set up as two separate monostatic radars placed orthogonal to each other in an indoor environment. These two radars illuminated the moving person from two different aspect angles and consequently produced two time-variant micro-Doppler signatures. We first computed the mean Doppler shifts (MDSs) from the micro-Doppler signatures and then extracted statistical and time- and frequency-domain features. We adopted feature-level fusion techniques to fuse the extracted features and a support vector machine to classify orientation-independent human activities. To evaluate our approach, we used an orientation-independent human activity dataset, which was collected from six volunteers. The dataset consisted of more than 1350 activity trials of five different activities that were performed in different orientations. The proposed complementary RF sensing approach achieved an overall classification accuracy ranging from 98.31 to 98.54%. It overcame the inherent limitations of a conventional single monostatic radar-based HAR and outperformed it by 6%.

Attitude Guidance Algorithms for Agile Satellite Dynamic Imaging

Dynamic imaging modes are increasingly crucial for agile satellites to perform complicated Earth observation tasks. In this study, a direct guidance algorithm is developed to calculate the reference attitude and velocity for dynamic imaging mode from target geolocation information while considering the constraints on both the satellite camera boresight axis and the image motion vector. The two slew angles are determined directly, and no rotation around the boresight or reference vector is required. The proposed approach employs a direct solution instead of the iterative process to obtain the reference attitude, which releases the onboard control system from the intensive computational load. To illustrate the performance of the proposed guidance algorithms, numerical simulation results are presented.

On the relation of GNSS phase center offsets and the terrestrial reference frame scale: a semi-analytical analysis

Phase center offsets (PCOs) of global navigation satellites systems (GNSS) transmit antennas along the boresight axis introduce line-of-sight-dependent range changes in the modeling of GNSS observations that are strongly correlated with the estimated station heights. As a consequence, changes in the adopted PCOs impact the scale of GNSS-based realizations of the terrestrial reference frame (TRF). Vice versa, changes in the adopted TRF scale require corrections to the GNSS transmit antenna PCOs for consistent observation modeling. Early studies have determined an approximate value of alpha =-0.050 for the ratio of station height changes and satellite PCO changes in GPS orbit determination and phase center adjustment. However, this is mainly an empirical value and limited information is available on the actual PCO-scale relation and how it is influenced by other factors. In view of the recurring need to adjust the IGS antenna models to new ITRF scales, a semi-analytical model is developed to determine values of alpha for the four current GNSSs from first principles without a need for actual network data processing. Given the close coupling of satellite boresight angle and station zenith angle, satellite PCO changes are essentially compensated by a combination of station height, zenith troposphere delay, and receiver clock offset. As such, the value of alpha depends not only on the orbital altitude of the considered GNSS but also on the elevation-dependent distribution of GNSS observations and their weighting, as well as the elevation mask angle and the tropospheric mapping function. Based on the model, representative values of alpha _text {GPS}=-0.051, alpha _text {GLO}=-0.055, alpha _text {GAL}=-0.041, and alpha _text {BDS-3}=-0.046 are derived for GPS, GLONASS, Galileo, and BeiDou-3 at a 10^circ elevation cutoff angle. These values may vary by varDelta alpha approx 0.003 depending on the specific model assumptions and data processing parameters in a precise orbit determination or precise point positioning. Likewise changes of about pm 0.003 can be observed when varying the cutoff angle between 5^circ and 15^circ .

Path loss compensated millimeter wave antenna module integrated with 3D ‐printed radome

Millimeter wave antennas designed at 28 GHz is essential for future 5G base stations or access points and mobile terminals. In this paper, a compact pattern diversity module for millimeter wave 5G base stations is proposed by using 3D-printing for radome design. In order to achieve path loss compensation in an indoor base station context, the gains of the antennas radiating at ±45° must be 3 dB higher than the antenna radiating in the boresight axis. First, an inset fed patch antenna integrated with a low-cost industry standard 3D-printed superstrate with Polylactic acid (PLA) is investigated to study its radiation characteristics. The radome is designed in such a way for optimal gain enhancement with minimal physical footprint. The height of the 2 mm thick superstrate is optimized for a boresight gain of 8 dBi. Second, the 3D-printed superstrate is optimized for a boresight gain of 11 dBi, which satisfies the criterion of path loss compensation, in this case the antenna achieves an aperture efficiency of close to 72% at 28 GHz. A compact pattern diversity module with customized 3D-printed radome is also presented to achieve path loss compensation and wide angular coverage of ±70° with associated isolation of less than 35 dB across the ports and the band. Detailed simulation and measurement results are presented.

Star sensor mounting for highly agile Earth imaging satellite

Star sensor (SS) is one of the most critical instrument on-board a spacecraft when it comes to attitude determination and control. Capability of providing precise attitude information makes it even more important for high resolution imaging satellite with stringent pointing requirements. At the same time, these instruments are highly sensitive to bright objects like Sun and Earth Albedo. In this paper, selection of star sensor mounting has been elaborately studied and presented for missions primarily performing highly agile Earth imaging. This paper addresses selection factors like Sun and Earth avoidance from bore-sight axis, orbit altitude and local time, angular separation between SS, sensitivity of SS on body axis and functional availability during imaging. Results are presented in the form of available mounting options and the way they satisfy geometrical constraints over a year.

Circularly polarized microstrip Yagi antenna array with tilted beam for improved monopulse characteristics

AbstractAn antenna array composed of four inclined microstrip Yagi antennas is proposed for a direction finding (DF) system operated in L band. Each element antenna is designed to have a tilted beam toward a central array axis in order to have high gain in the boresight axis of the antenna array. In comparison to a patch antenna array, having the same structure, antenna array gain is improved by a 2.3 dB maximum around the array axis, monopulse slope is increased by 72% and the linearity of the slope is improved significantly. Also, axial ratio is improved by 3 dB max within ±25° from boresight.

Statistical model and analysis of two-scale modification of conical scanning direction finding method

Statistical analysis method and its results are presented for a two-scale modification of the conical scanning direction finding method generalizing it for a case of elliptical cross-section of antenna beam. In order to utilize additional positional information that is excluded in a typical implementation of conical scan method, a two-scale procedure is proposed for estimating the direction to object. It is shown, that application of this procedure does not lead to worsening of the accuracy of object’s coordinates estimate near boresight axis and also attains a significant increase of angular operating area of direction finding due to the increase in number of harmonics of conical scanning frequency included in processing. Analytical expressions are obtained and discriminator and fluctuation curves are calculated, revealing the possibilities to increase the angular operating area of direction finding depending on signal-to-noise ratio and ellipticity of antenna beam’s cross-section.

Bicep2. III. INSTRUMENTAL SYSTEMATICS

In a companion paper we have reported a $>5\sigma$ detection of degree scale $B $-mode polarization at 150 GHz by the BICEP2 experiment. Here we provide a detailed study of potential instrumental systematic contamination to that measurement. We focus extensively on spurious polarization that can potentially arise from beam imperfections. We present a heuristic classification of beam imperfections according to their symmetries and uniformities, and discuss how resulting contamination adds or cancels in maps that combine observations made at multiple orientations of the telescope about its boresight axis. We introduce a technique, which we call "deprojection", for filtering the leading order beam-induced contamination from time ordered data, and show that it removes power from BICEP2's $BB$ spectrum consistent with predictions using high signal-to-noise beam shape measurements. We detail the simulation pipeline that we use to directly simulate instrumental systematics and the calibration data used as input to that pipeline. Finally, we present the constraints on $BB$ contamination from individual sources of potential systematics. We find that systematics contribute $BB$ power that is a factor $\sim10\times$ below BICEP2's 3-year statistical uncertainty, and negligible compared to the observed $BB$ signal. The contribution to the best-fit tensor/scalar ratio is at a level equivalent to $r=(3-6)\times10^{-3}$.

Reduced-Attitude Boresight Guidance and Control on Spacecraft for Pointing, Tracking, and Searching

A guidance and a control law are developed to maneuver the boresight axis of a spacecraft relative to a reference vector, using knowledge of the reference vector and the angular rates of the spacecraft. Therefore, this can be used in cases when the full attitude is not known or when the angle of the spacecraft around the boresight or reference vector does not need to be controlled. A Lyapunov-based analysis is performed to prove asymptotic stability of the system to a reference trajectory. Even in the presence of inertia uncertainty, it is shown that asymptotic stability to a stationary attitude can be guaranteed. To assist with practical gain selection, a linear analysis is performed. Finally, the boresight guidance and control laws are applied to four different scenarios to demonstrate their utility. The first two show examples of pointing in an inertially-fixed direction and tracking a time-varying target. The third and fourth demonstrate searching tasks. A novel example of searching for the Sun by reducing the search space from a full -steradian sphere to a circle of a sphere is presented. If the full sphere must be searched, a spherical spiral maneuver is presented to perform this task.

Using near‐field coupled circularly polarised antennas as frequency‐independent variable phase shifters

A novel concept of frequency-independent variable phase shifters is introduced. It is based on the use of circularly polarised (CP) antennas coupled in the near-field. For transmission between two CP antennas in the far-field, a rotation around the boresight axis produces a phase change in the transfer function. This concept is exploited and validated also for near-field coupling between two CP antennas. Therefore, two close-coupled CP antennas can act as an adjustable phase shifter with the phase shift controlled by the angle between the structures. The bandwidth depends on the CP bandwidth of the antenna. This concept is valuable for future phase shifters in millimetre-wave applications where rotation can be facilitated by MEMS elements.

Covariance Analysis of Maximum Likelihood Attitude Estimation

An attitude determination covariance measurement model for unit vector sensors with a wide field-of-view is analyzed and compared to the classic QUEST covariance model. The wide field-of-view model has been previously proposed as a more realistic alternative for sensors where measurement accuracy depends on angular distance from the boresight axis. Both QUEST and the wide field-of-view models are evaluated relative to a measurement model that uses the two-dimensional sensor focal plane measurements directly, rather than first converting them to unit vectors. The Fisher information matrix is derived for attitude determination based on such direct sensor measurements, and the wide field-of-view measurement model is shown to have the same Fisher information matrix. Numerical simulations confirm that an extended Kalman filter based on the wide field-of-view model outperforms a filter based on the QUEST measurement model, and also that the wide field-of-view 3σ bounds are effectively identical to those of a filter based on the direct two-dimensional sensor measurements.

Autocorrelation Function of Return Waveforms in High Precision Spaceborne Radar Altimeters Employing Chirp Transmit Pulses

This paper analyses the autocorrelation function of return waveforms in high precision radar altimeters employing chirp-pulse transmit signal under the condition of near-nadir deviations of the antenna boresight axis. It is shown that in case of ultra wideband transmit signals providing very high time resolution the correlation function can be approximated by a product of two separate functions of time.

Spin rate estimation of sounding rockets using GPS wind-up

Carrier phase wind-up is a well-known effect that arises from the relative rotation between a transmitting and receiving antenna. In GPS measurements at L1 frequency, this effect translates into an error of 19.029 cm per full relative rotation of antennas. Since this effect is independent of the satellite elevation for pure rotation about the antenna boresight axis, it is usually absorbed by the clock estimation in navigation algorithms. Therefore, the impact of wind-up is usually neglected for applications that do not require accuracies to the cm level like RTK. However, in receiving platforms with high rotation rate, the accumulated wind-up value can be important and actually be larger than receiver noise or even ionospheric variations. Therefore, in such scenarios, the wind-up contribution can be isolated and used as a source of information to compute the spin rate of such platforms using an appropriate combination of GPS observables. This work shows some results of a coarse, yet simple, approach to monitor the rotation angle and spin-rate of spin stabilized sounding rockets flown by DLR.

Eye array dereverberation by corner placement

A new signal processing algorithm accompanied with a novel array structure for sound source localization (SSL) in three-dimensional spaces was presented [Hedayat Alghassi et al., J. Acoust. Soc. Am. 120(5) (2006)]. This methodology has some analogy to the eye. The microphone array’s proper placement is particularly important in enclosed areas, where reverberation is the dominant problem. It will be demonstrated that by placing the eye array’s symmetry axis along the boresight axis of an upper orthogonal trihedral corner in an enclosed area, the adverse effect of early reverberation can be substantially reduced. This enhancement is the consequence of both the retroreflection property of the orthogonal trihedral corners plus the eye array’s insensitivity to back reflections. There is a pseudo-coincidence between the estimation cells and the source direction in the eye array SSL method. Therefore, the majority of the early reflections from the trihedral corner strike the estimation cells in the direction opposite to the sound source, where the sensitivity is lowest. This approach not only reduces the adverse effect of reverberation on the SSL accuracy, but also creates an approximately flat error for all source directions. Experiments confirmed that this method achieves a reasonable accuracy improvement.

Brightness-independent start-up routine for star trackers

Initial attitude acquisition by a modern star tracker is investigated here. Criteria for efficient organization of the on-board database are discussed with reference to a brightness-independent initial acquisition algorithm. Star catalog generation preprocessing is described, with emphasis on the identification of minimum star brightness for detection by a sensor based on a charge coupled device (CCD) photodetector. This is a crucial step for proper evaluation of the attainable sky coverage when selecting the stars to be included in the on-board catalog. Test results are also reported, both for reliability and accuracy, even if the former is considered to be the primary target. Probability of erroneous solution is 0.2% in the case of single runs of the procedure, while attitude determination accuracy is in the order of 0.02/spl deg/ in the average for the computation of the inertial pointing of the boresight axis.

Satellite Attitude Control and Power Tracking with Energy/Momentum Wheels

A control law for an integrated power/attitude control system (IPACS) for a satellite is presented. Four or more energy/momentum wheels in an arbitrary noncoplanar cone guration and a set of three thrusters are used to implement the torque inputs. The energy/momentum wheels are used as attitude-control actuators, as well as an energy storage mechanism, providing power to the spacecraft. In that respect, they can replace the currently used heavy chemical batteries. The thrusters are used to implement the torques for large and fast (slew) maneuvers during the attitude-initialization and target-acquisition phases and to implement the momentum management strategies. The energy/momentum wheels are used to provide the reference-tracking torques and the torques for spinningupor down thewheels for storing or releasing kineticenergy. The controller publishedina previous work by the authors is adopted here for the attitude-tracking function of the wheels. Power tracking for charging and discharging the wheels is added to complete the IPACS framework. The torques applied by the energy/momentum wheels are decomposed into two spaces that are orthogonal to each other, with the attitude-control torques and power-tracking torques in each space. This control law can be easily incorporated in an IPACS system onboard a satellite. The possibility of the occurrence of singularities, in which no arbitrary energy proe le can be tracked, is studied for a generic wheel cluster cone guration. A standard momentum management scheme is considered to nullthe total angular momentumof the wheels so as tominimizethe gyroscopic effects and prevent the singularity from occurring. A numerical example for asatellite inalow Earth near-polar orbit is provided totest the proposed IPACS algorithm. The satellite’s boresight axis is required to track a ground station, and the satellite is required to rotate about its boresight axis so that the solar panel axis is perpendicular to the satellite‐sun vector.

Dual-mode antenna design for microwave heating and noninvasive thermometry of superficial tissue disease.

Hyperthermia therapy of superficial skin disease has proven clinically useful, but current heating equipment is somewhat clumsy and technically inadequate for many patients. The present effort describes a dual-purpose, conformal microwave applicator that is fabricated from thin, flexible, multilayer printed circuit board (PCB) material to facilitate heating of surface areas overlaying contoured anatomy. Preliminary studies document the feasibility of combining Archimedean spiral microstrip antennas, located concentrically within the central region of square dual concentric conductor (DCC) annular slot antennas. The motivation is to achieve homogeneous tissue heating simultaneously with noninvasive thermometry by radiometric sensing of blackbody radiation from the target tissue under the applicator. Results demonstrate that the two antennas have complimentary regions of influence. The DCC ring antenna structure produces a peripherally enhanced power deposition pattern with peaks in the outer corners of the aperture and a broad minimum around 50% of maximum centrally. In contrast, the Archimedean spiral radiates (or receives) energy predominantly along the boresight axis of the spiral, thus confining the region of influence to tissue located within the central broad minimum of the DCC pattern. Analysis of the temperature-dependent radiometer signal (brightness temperature) showed linear correlation of radiometer output with test load temperature using either the spiral or DCC structure as the receive antenna. The radiometric performance of the broadband Archimedean antenna was superior compared to the DCC, providing improved temperature resolution (0.1 degree C-0.2 degree C) and signal sensitivity (0.3 degree C-0.8 degree C/degree C) at all four 500 MHz integration bandwidths tested within the frequency range from 1.2 to 3.0 GHz.

A Multiple-Beam Architecture for Sidelobe Cancellers

Using a computer simulation, we investigate the use of a small bundle of beams centered on the boresight axis to provide adaptive sidelobe cancelling. Advantages of this approach include the fact that the number of jammers suppressed is not limited to the number of adaptive loops provided. Also, the technique adapts well to the suppression of jammers over a wide frequency band.