An Enhanced FGI-GSRx Software-Defined Receiver for the Execution of Long Datasets.

The Global Navigation Satellite System (GNSS) software-defined receivers offer greater flexibility, cost-effectiveness, customization, and integration capabilities compared to traditional hardware-based receivers, making them essential for a wide range of applications. The continuous evolution of GNSS research and the availability of new features require these software-defined receivers to upgrade continuously to facilitate the latest requirements. The Finnish Geospatial Research Institute (FGI) has been supporting the GNSS research community with its open-source implementations, such as a MATLAB-based GNSS software-defined receiver `FGI-GSRx' and a Python-based implementation `FGI-OSNMA' for utilizing Galileo's Open Service Navigation Message Authentication (OSNMA). In this context, longer datasets are crucial for GNSS software-defined receivers to support adaptation, optimization, and facilitate testing to investigate and develop future-proof receiver capabilities. In this paper, we present an updated version of FGI-GSRx, namely, FGI-GSRx-v2.0.0, which is also available as an open-source resource for the research community. FGI-GSRx-v2.0.0 offers improved performance as compared to its previous version, especially for the execution of long datasets. This is carried out by optimizing the receiver's functionality and offering a newly added parallel processing feature to ensure faster capabilities to process the raw GNSS data. This paper also presents an analysis of some key design aspects of previous and current versions of FGI-GSRx for a better insight into the receiver's functionalities. The results show that FGI-GSRx-v2.0.0 offers about a 40% run time execution improvement over FGI-GSRx-v1.0.0 in the case of the sequential processing mode and about a 59% improvement in the case of the parallel processing mode, with 17 GNSS satellites from GPS and Galileo. In addition, an attempt is made to execute v2.0.0 with MATLAB's own parallel computing toolbox. A detailed performance comparison reveals an improvement of about 43% in execution time over the v2.0.0 parallel processing mode for the same GNSS scenario.

Combating Single-Frequency Jamming through a Multi-Frequency, Multi-Constellation Software Receiver: A Case Study for Maritime Navigation in the Gulf of Finland.

Today, a substantial portion of global trade is carried by sea. Consequently, the reliance on Global Navigation Satellite System (GNSS)-based navigation in the oceans and inland waterways has been rapidly growing. GNSS is vulnerable to various radio frequency interference. The objective of this research is to propose a resilient Multi-Frequency, Multi-Constellation (MFMC) receiver in the context of maritime navigation to identify any GNSS signal jamming incident and switch to a jamming-free signal immediately. With that goal in mind, the authors implemented a jamming event detector that can identify the start, end, and total duration of the detected jamming event on any of the impacted GNSS signal(s). By utilizing a jamming event detector, the proposed resilient MFMC receiver indeed provides a seamless positioning solution in the event of single-frequency jamming on either the lower or upper L-band. In addition, this manuscript also contains positioning performance analysis of GPS-L5-only, Galileo-E5a-only, and Galileo-E5b-only signals and their multi-GNSS combinations in a maritime operational environment in the Gulf of Finland. The positioning performance of lower L-band GNSS signals in a maritime environment has not been thoroughly investigated as per the authors' knowledge.

Potential of active multispectral lidar for detecting low reflectance targets.

The calibration and sampling of the multispectral Light Detection and Ranging (lidar) intensity is still challenging because the data acquisition has to be optimized for simultaneous 3D measurement, and the intensity retrieval methods need to be fast to enable real-time detection. We have studied the spectral measurement of low reflectance targets with an 8-channel hyperspectral lidar with improved waveform sampling and sensitivity, which now allow the detection of spectral differences even at low reflectance values. Our initial analysis resulted in a classification accuracy greater than 80%, which indicates that the multispectral lidar is able to detect the small differences in target spectral properties when reflectance at two or more channels is compared at the same time.

Portable hyperspectral lidar utilizing 5 GHz multichannel full waveform digitization.

The Finnish Geospatial Research Institute hyperspectral LiDAR (FGI HSL) was one of the first multichannel terrestrial LiDARs capable of producing simultaneous 3-dimensional topography with spectral data. Supercontinuum-based HSL instruments developed so far have suffered from portability and sensitivity issues, severely restricting potential applications. Recently, we have implemented a new robust field design of the FGI HSL together with an improved pulse digitizing scheme. Small size and significantly improved measuring accuracy of this new system enable a range of novel applications that so far have been impractical for multichannel terrestrial LiDARs. Particularly, this new design has enabled us to perform measurements in underground mines and detect minute spectral differences in various rock types. In this paper, we present the design of our new LiDAR and preliminary algorithms together with a brief performance assessment of the device. In addition, we provide example measurements of typical rock samples found in a ferrochrome mine.

Improved waveform reconstruction and parameter accuracy retrieval for hyperspectral lidar data.

In this paper, we aim to provide a solution for choosing an optimized digitization frequency for full-waveform reconstruction of narrow FWHM laser pulses to improve the performance accuracy required for recognition and characterization of a wide range of materials with distinctive reflectance features. This type of analysis, for the first time, to the best of our knowledge, gives an assessment of the absolute accuracy with which waveform peak intensity and spatiotemporal peak location can be detected using multi- or hyperspectral lidar instrumentation. Different full-waveform reconstruction algorithms with varying characteristics are implemented on simulated Gaussian laser pulse data sets obtained with varying sampling frequencies ranging between 1 GHz and 5 GHz. The data sets are analyzed, and an optimized digitization frequency is found based on observation of algorithmic parameter retrieval accuracy. The accuracy of the full-waveform retrieval in relation to the type of algorithm used and its robustness related to the digitization frequency are also discussed. We find that optimizing the algorithmic processing of multi-wavelength sampled radiance data recorded by multispectral and hyperspectral lidar instruments significantly improves the accuracy of reflectance retrieval and target material characterization capabilities of the instrument.

Full waveform hyperspectral LiDAR for terrestrial laser scanning.

We present the design of a full waveform hyperspectral light detection and ranging (LiDAR) and the first demonstrations of its applications in remote sensing. The novel instrument produces a 3D point cloud with spectral backscattered reflectance data. This concept has a significant impact on remote sensing and other fields where target 3D detection and identification is crucial, such as civil engineering, cultural heritage, material processing, or geomorphological studies. As both the geometry and spectral information on the target are available from a single measurement, this technology will extend the scope of imaging spectroscopy into spectral 3D sensing. To demonstrate the potential of the instrument in the remote sensing of vegetation, 3D point clouds with backscattered reflectance and spectral indices are presented for a specimen of Norway spruce.

Absolute radiometric calibration of Als intensity data: effects on accuracy and target classification.

Radiometric calibration of airborne laser scanning (ALS) intensity data aims at retrieving a value related to the target scattering properties, which is independent on the instrument or flight parameters. The aim of a calibration procedure is also to be able to compare results from different flights and instruments, but practical applications are sparsely available, and the performance of calibration methods for this purpose needs to be further assessed. We have studied the radiometric calibration with data from three separate flights and two different instruments using external calibration targets. We find that the intensity data from different flights and instruments can be compared to each other only after a radiometric calibration process using separate calibration targets carefully selected for each flight. The calibration is also necessary for target classification purposes, such as separating vegetation from sand using intensity data from different flights. The classification results are meaningful only for calibrated intensity data.

Two-channel hyperspectral LiDAR with a supercontinuum laser source.

Recent advances in nonlinear fiber optics and compact pulsed lasers have resulted in creation of broadband directional light sources. These supercontinuum laser sources produce directional broadband light using cascaded nonlinear optical interactions in an optical fibre framework. This system is used to simultaneously measure distance and reflectance to demonstrate a technique capable of distinguishing between a vegetation target and inorganic material using the Normalized Difference Vegetation Index (NDVI) parameters, while the range can be obtained from the waveform of the echoes. A two-channel, spectral range-finding system based on a supercontinuum laser source was used to determine its potential application of distinguishing the NDVI for Norway spruce, a coniferous tree, and its three-dimensional parameters at 600 nm and 800 nm. A prototype system was built using commercial components.

Use of naturally available reference targets to calibrate airborne laser scanning intensity data.

We have studied the possibility of calibrating airborne laser scanning (ALS) intensity data, using land targets typically available in urban areas. For this purpose, a test area around Espoonlahti Harbor, Espoo, Finland, for which a long time series of ALS campaigns is available, was selected. Different target samples (beach sand, concrete, asphalt, different types of gravel) were collected and measured in the laboratory. Using tarps, which have certain backscattering properties, the natural samples were calibrated and studied, taking into account the atmospheric effect, incidence angle and flying height. Using data from different flights and altitudes, a time series for the natural samples was generated. Studying the stability of the samples, we could obtain information on the most ideal types of natural targets for ALS radiometric calibration. Using the selected natural samples as reference, the ALS points of typical land targets were calibrated again and examined. Results showed the need for more accurate ground reference data, before using natural samples in ALS intensity data calibration. Also, the NIR camera-based field system was used for collecting ground reference data. This system proved to be a good means for collecting in situ reference data, especially for targets with inhomogeneous surface reflection properties.

Uncertainty in multispectral lidar signals caused by incidence angle effects.

Multispectral terrestrial laser scanning (TLS) is an emerging technology. Several manufacturers already offer commercial dual or three wavelength airborne laser scanners, while multispectral TLS is still carried out mainly with research instruments. Many of these research efforts have focused on the study of vegetation. The aim of this paper is to study the uncertainty of the measurement of spectral indices of vegetation with multispectral lidar. Using two spectral indices as examples, we find that the uncertainty is due to systematic errors caused by the wavelength dependency of laser incidence angle effects. This finding is empirical, and the error cannot be removed by modelling or instrument modification. The discovery and study of these effects has been enabled by hyperspectral and multispectral TLS, and it has become a subject of active research within the past few years. We summarize the most recent studies on multi-wavelength incidence angle effects and present new results on the effect of specular reflection from the leaf surface, and the surface structure, which have been suggested to play a key role. We also discuss the consequences to the measurement of spectral indices with multispectral TLS, and a possible correction scheme using a synthetic laser footprint.

Aperture size effects on backscatter intensity measurements in Earth and space remote sensing.

Most materials show a peaked intensity versus phase (light-source-target-detector angle) curve. For nonnegligible angular apertures of the source and/or the detector, the measured intensity at and near zero phase (backscatter) is lower than the real one. We derive an averaging aperture integral that represents this effect, and with it we invert measured intensity values to obtain the actual intensity curve. We also give a practical formula for estimating the magnitude of the aperture effect in zero-phase intensity measurements and show that only two such measurements made at different apertures are sufficient for deriving the real intensity. These corrections are needed in the comparison of measured reflectances in an increasing number of validation efforts for remote sensing applications requiring ground truth measurements.

Effect of incidence angle on laser scanner intensity and surface data.

We present a comprehensive experimental set of data on the dependence of the laser intensity on the angle of incidence to the target surface. The measurements have been performed in the laboratory for samples with a Nd:YAG laser and terrestrial laser scanner. The brightness scale data were also compared with data acquired by airborne laser scanning (ALS). The incidence angle effect is evident for all the targets. The effect is significant for incidence angles >20 degrees, and stronger for bright targets. However, effects due to some of the other surface properties, such as roughness, were also detected. We also found a set of gravel samples for which the incidence angle effect was minor even up to 40 degrees . The data provide an important reference for the interpretation and applications, e.g., full-waveform data processing of a laser scanner and ALS intensity calibration.

Backscattering measurements from individual Scots pine needles.

We present ground reference measurements of the directional scattering properties of conifer needles. As the development of multiangular remote sensing instruments sets a growing need for reliable ground reference measurement techniques and databases, there is an increasing demand for data on the spectral properties of conifer needles in forest reflectance modeling and the inversion of physically based models. These data are scarce due to technical and conceptual problems related to measuring thin needles, and the needle directional spectral properties are currently nonexistent even for single wavelengths. We present results from measuring the monochromatic backscattering of Scots pine needles in a controlled laboratory environment; we feel these results of the hot spot signatures of individual conifer needles are unique. The experiment was conducted at 1,064 nm with an instrument constructed specifically for backscatter measurement, based on techniques commonly used for laser backscattering measurements and CCD photometry. Strong backscattering peaks near 0 degrees were observed for the needles, the amplitude of the brightening being up to approximately 40%.

Small-angle goniometry for backscattering measurements in the broadband spectrum.

We present experiments on spectral bidirectional reflectance distribution function (BRDF) effects at backscatter and discuss the feasibility of new methods for laboratory and field simulations of remote sensing of land surfaces. The extreme sharpness of the intensity peak allows both directional and comparative experimental spectral studies of hot spots. We demonstrate wavelength-dependent features in the hot-spot reflectance signatures that facilitate extension of spectral and directional BRDF measurements of natural targets (such as forest understories and ice surfaces) into retroreflection to exploit their hot-spot characteristics in the interpretation of spaceborne and airborne data.

Laboratory experiments on backscattering from regolith samples.

The investigation of the backscattering peak has applications in the surface texture characterization of asteroids and planetary surfaces. Laboratory experiments are important because they give an opportunity for systematic variation and comparison of samples. A backscattering experiment from regolith samples, which uses a laser light source and a beam splitter to reach the smallest phase angles, is presented. Measurements at zero and small phase angles for Sahara sand and meteorite rocks are made, and the preliminary results are presented in comparison with the phase curve observed for asteroid 69 Hesperia. The results are applicable to the further interpretation of the coherent backscattering opposition effect.