Feasibility of GNSS-R ice sheet altimetry in Greenland using TDS-1

Radar altimetry provides valuable measurements to characterize the state and the evolution of the ice sheet cover of Antartica and Greenland. Global Navigation Satellite System Reflectometry (GNSS-R) has the potential to complement the dedicated radar altimeters, increasing the temporal and spatial resolution of the measurements. Here we perform a study of the Greenland ice sheet using data obtained by the GNSS-R instrument aboard the British TechDemoSat-1 (TDS-1) satellite mission. TDS-1 was primarily designed to provide sea state information such as sea surface roughness or wind, but not altimetric products. The data have been analyzed with altimetric methodologies, already tested in aircraft based experiments, to extract signal delay observables to be used to infer properties of the Greenland ice sheet cover. The penetration depth of the GNSS signals into ice has also been considered. The large scale topographic signal obtained is consistent with the one obtained with ICEsat GLAS sensor, with differences likely to be related to L-band signal penetration into the ice and the along-track variations in structure and morphology of the firn and ice volumes The main conclusion derived from this work is that GNSS-R also provides potentially valuable measurements of the ice sheet cover. Thus, this methodology has the potential to complement our understanding of the ice firn and its evolution.Peer ReviewedPostprint (published version

Sea-Ice remote sensing with GNSS reflections

Peer Reviewe

The Impact of Inter-Modulation Components on Interferometric GNSS-Reflectometry

The interferometric Global Navigation Satellite System Reflectometry (iGNSS-R) exploits the full spectrum of the transmitted GNSS signal to improve the ranging performance for sea surface height applications. The Inter-Modulation (IM) component of the GNSS signals is an additional component that keeps the power envelope of the composite signals constant. This extra component has been neglected in previous studies on iGNSS-R, in both modelling and instrumentation. This letter takes the GPS L1 signal as an example to analyse the impact of the IM component on iGNSS-R ocean altimetry, including signal-to-noise ratio, the altimetric sensitivity and the final altimetric precision. Analytical results show that previous estimates of the final altimetric precision were underestimated by a factor of 1 . 5 ∼ 1 . 7 due to the negligence of the IM component, which should be taken into account in proper design of the future spaceborne iGNSS-R altimetry missions.This work was supported in part by the European Space Agency (ESTEC RFP/IPL- PTE/FE/yc/1157/2015) and in part by the Spanish Ministry of Economy and Competitiveness (ESP2015-70014-C2-2-R). We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)

City branding as economic necessity

Kvalitetno brendiranje grada je preduvjet za njihovu prepoznatljivost, kvalitetno pozicioniranje i stvaranje dodatne vrijednosti. Praksa i mnogobrojni primjeri potvrđuju ispravnost ove teze. Brendiranje gradova je nužno kako bi se pojačala konkurentnost, ostvarila veća dobit i osigurao razvoj mjesta. No ne radi se samo o ekonomskim kategorijama jer se pod razvojem mjesta podrazumijevaju i pozitivna demografska kretanja, obogaćivanje kulturnih sadržaja kao i drugih činitelja koji podižu ukupnu kvalitetu života. Izazov je to i nužnost i za gradove u Hrvatskoj kako bi bili konkurentni u oštroj tržišnoj konkurenciji.Quality city branding is a precondition for their recognazibility, quality positionig and creating of added value. Practice and numerous examples confirm correction of this theses. City branding is necessary to enhance concurence, gain bigger profit and ensure place development. But this is not only about economic categories because under place development it is understandable alsto positive demographic movement, enrichment of cultural contens as well as other factors which raise total quality of life. This is as well a challenge as it is a necessity for cities in Croatia so they could be concurente in harsh economy concurence

Analysis of Baseband Algorithms for LEO PNT †

This paper provides an analysis of the impact of transmitter dynamics on baseband algorithms for position, navigation and timing applications using a low Earth orbit constellation. In particular, the acquisition and tracking of signals with high dynamics have larger Doppler and Doppler drift values that might impair the use of standard methods. Our analysis combines a theoretical assessment of acquisition and tracking thresholds with a performance evaluation by means of a simulation. Three frequency bands are tested (UHF, S and Ka) and an open-source software receiver has been adapted for such a purpose. The results obtained show the potential feasibility of LEO-PNT at baseband level under minimal considerations

Analysis of baseband algorithms for LEO PNT

This article belongs to the Proceedings of European Navigation Conference ENC 2023.This paper provides an analysis of the impact of transmitter dynamics on baseband algorithms for position, navigation and timing applications using a low Earth orbit constellation. In particular, the acquisition and tracking of signals with high dynamics have larger Doppler and Doppler drift values that might impair the use of standard methods. Our analysis combines a theoretical assessment of acquisition and tracking thresholds with a performance evaluation by means of a simulation. Three frequency bands are tested (UHF, S and Ka) and an open-source software receiver has been adapted for such a purpose. The results obtained show the potential feasibility of LEO-PNT at baseband level under minimal considerations.This research was carried out under a programme funded by the European Space Agency (ESA): project ESA AO/1-10372/20/NL/CRS “LEO Positioning Navigation and Timing from User Equipment”.Peer reviewe

Measuring Greenland Ice Sheet Melt Using Spaceborne GNSS Reflectometry From TechDemoSat-1

The capability of spaceborne Global Navigation Satellite System-reflectometry (GNSS-R) for Greenland ice sheet melt detection is investigated using the TechDemoSat-1 satellite (TDS-1) data. The melt detection is based on the sensitivity of GNSS-R signal to the presence of liquid water in snow pack. Statistical analysis during the 2018 melt season shows melt detection using GNSS-R is possible with an agreement of 90% comparing to the microwave radiometer (MWR) data. In 37% of the cases GNSS-R detects melting when MWR does not. The inconsistency observed between GNSS-R and MWR is mainly due to the different features of the two observation concepts and the limitations of currently available GNSS-R data. Furthermore, the large penetration depth of the GNSS signal can potentially provide complementary information on melt occurrence in ice sheet subsurface. These results show the potential of future GNSS-R missions for ice sheet melt detection.This work was supported in part by theSpanish Ministry of Economy and Competitiveness under Grant ESP2015-70014-C2-2-R and Grant RTI2018-099008-B-C22(MCIU/AEI/FEDER, UE

Assessment of Spaceborne GNSS-R Ocean Altimetry Performance Using CYGNSS Mission Raw Data

This article assesses the ocean altimetry performance of spaceborne Global Navigation Satellite Systems reflectometry (GNSS-R) by processing the raw data sets collected by the Cyclone GNSS (CYGNSS) constellation. These raw data sets, i.e., the intermediate frequency signal streams before any receiver processing, are processed on the ground with a software receiver, from which the reflected waveforms of GPS L1, Galileo E1, and BeiDou-3 B1 band open service (OS) signals are generated following the conventional GNSS-R approach. By using different retracking algorithms, the bistatic delays of the reflected signals are derived from these waveforms, in which the retracking biases are removed with the specular point (SP) delay and power information computed from the corresponding waveform model. After applying a set of standard delay corrections, the bistatic delay observations are converted into sea surface height (SSH) measurements and compared with the mean SSH model. Both the random error (precision) and systematic effects (accuracy) are characterized with intratrack and intertrack analyses of the bistatic delay measurements. The two-way ranging precision can reach up to 3.9 and 2.5 m with 1-s GPS and Galileo group delay measurement (a factor of ~2 better for altimetry solution), and its evolution with the signal-to-noise ratio shows good consistency with the theoretical model. A significant delay dispersion of 3.0 m between different tracks is found, which is mainly attributed to the receiver orbit error and ionospheric correction residuals. These results can provide useful inputs for the development of future GNSS-R missions dedicated to ocean altimetry applications

Effects of Arctic Wetland Dynamics on Tower-Based GNSS Reflectometry Observations

A tower-based global navigation satellite system reflectometry (GNSS-R) experiment is set up in an Arctic wetland environment for investigating the possibility of monitoring wetland inundation and freeze/thaw (FT) dynamics which are additionally impacted by snow on the ground. Effects of inundation, snow cover, and soil FT state on observed GNSS-R signal-to-noise ratios (SNRs) are analyzed for horizontal (H) and vertical (V) polarizations. A simple classification approach is suggested to detect the inundated, frozen, or thawed soil state. A simple forward reflectivity model is formulated to evaluate the influence of snow cover, overlying frozen, or thawed soil, on the reflected GNSS signals. Reflectivity time series are simulated in H- and V-polarizations using in situ observations of the Arctic wetland site. The simulations are used to verify the tower-based observations, which show a significant impact of wet snow on reflectivity during melting conditions in spring. The observed SNR is strongly correlated with the Sentinel-1 backscatter coefficient. Generally, soil states detected by GNSS-R are in high agreement with ground truth soil states, especially for inundated and frozen soils. Wet snow conditions, however, complicate the correct timing estimation of soil thawing by inducing reflectivities of a similar order as thawing soil. It is recommended that GNSS-R land application models and retrieval algorithms consider snow cover effects to reduce false classification, especially in FT detection. Overall, the outcome of this study is relevant to the upcoming ESA HydroGNSS mission.This work was supported in part by the European Space Agency (ESA) Scout Mission under Grant ESA CN: 4000129140/19/NL/CT and in part by the Academy of Finland under Grant 325397. The GNSS-R installation was funded by the Spanish Grant RTI2018-099008-B-C22/AEI/10.13039/50110001103