Weiqiang Li

Partial Interferometric Processing of Reflected GNSS Signals for Ocean Altimetry

The bistatically reflected global navigation satellite systems (GNSS) signals have become an attractive tool for spaceborne ocean altimetry. The interferometric processing that can exploit the full bandwidth of the available GNSS signals without the knowledge of the actual ranging codes was proposed for the PARIS IoD mission to improve the ranging precision. This letter presents a novel on-board processing method which utilizes the interferometry of partial GNSS signal components to explore a further improvement of the altimetry precision. The scheme of extracting partial signal components and interferometry procedure, for GPS L1 band as an example, are illustrated. The assessment and comparison of achievable altimetric performance of the proposed method including the altimetric sensitivity, the resolution per pulse, the signal-to-noise ratio, and the overall altimetric precision are also introduced. The extension of the proposed method to other existing and planned GNSS systems and signals would guarantee an improvement in the overall performance of the PARIS concept.

Typhoon Wind Speed Observation Utilizing Reflected Signals from BeiDou GEO Satellites

Typhoon monitoring utilizing reflected GNSS signals is a new application of GNSS-R technique. Coastal observations are an efficient way for the model of geophysical parameters retrieval in Typhoon and identified as a promising complementary technique with respect to the satellite instruments. However, the relationship between GNSS-R observables and the sea surface wind speed in Typhoon could not be fully described through theoretical models for the coastal regions. Meanwhile the instability of the coastal GNSS-R geometry makes it difficult to optimize an empirically calibrated model. The BeiDou GEO satellites could provide stable geometry and better coverage capability in mid- and low-latitude region where most of the typhoons occur. Based on this consideration, ocean reflected signals from BeiDou GEO satellites are exploited for coastal Typhoon observation in this paper. The relationship between reflected waveform parameters, such as coherent time, and the ocean geophysics parameters, such as wind speed is analysed. Preliminary analysis of the BeiDou reflected signal collected during the TIGRIS experiment shows good agreement between the GNSS-R measured wind speeds and the in situ measurements, the average deviation is 1.6 m/s with the root-mean-square error of 2.4 m/s.

Initial Results of Typhoon Wind Speed Observation Using Coastal GNSS-R of BeiDou GEO Satellite

Sea surface-reflected signals of global navigation satellite system (GNSS) were collected during a coastal experiment to evaluate the potential use of these signals on typhoon investigation. This work focuses on processing the signals from BeiDou geostationary Earth orbit (GEO) satellites and assessing the sensitivities of waveform observables to the wind speed evolution during the typhoons. After the processing of the raw samples, both the delay- and spectral-related observables are obtained from the complex waveforms and then compared with in situ wind measurement collected during two tropical cyclones. Results from the data analysis are presented, confirming that the proposed observables are well correlated with the wind speed evolution and suitable for coastal wind speed retrieval.

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.

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.

Sea-State Observation Using Reflected BeiDou GEO Signals in Frequency Domain

This letter focuses on exploiting parameters, including peak power spectral density (PSD), integrated power, mean frequency, and spectrum width of reflected BeiDou signals in the frequency domain, to retrieve wind speed. The PSD of reflected signals is estimated after choosing proper parameters of the Welch method. Then, the features of PSD are illustrated, and a method is proposed based on fitting estimated PSD with a Gaussian function to evaluate the aforementioned PSD parameters. The estimated parameters of collected BeiDou medium earth orbit (MEO)/inclined geosynchronous satellite orbit (IGSO) and geostationary orbit (GEO) data are fitted with in situ wind speed using polynomial. Fitting results show that the peak PSD, mean frequency, and spectrum width of reflected signals from GEO have more evident dependence on wind speed, compared with MEO/IGSO. To obtain the most accurate results, the impact of delay used in lagging direct replica to align reflected signals is analyzed. Results show that regardless of delay locating within the interval of [τ₀ - τ_c, τ₀ + τ_c], the better root-mean-square error (rmse) of less than 1.7 m/s and the larger coefficient of determination over 0.8 can be obtained by retrieving from the spectrum width, compared with peak PSD and mean frequency, whose optimal results, with rmse values of 2.03 and 1.70 m/s and coefficient of determination values of 0.73 and 0.81, are obtained as delay is τ₀ - 0.85τ_c and τ₀ + 0.9τ_c, respectively, where τ₀ is the delay of specular reflection, and τ_c is the length of the B1 code.

Ocean surface target detection and positioning using the spaceborne GNSS-R Delay-Doppler maps

A number of applications have been proposed by using the technique of global navigation satellite system reflectometry (GNSS-R) such as ice monitoring and sea surface wind retrieval. Using GNSS-R for target detection has also been raised for its advantages of all-weather capabilities, worldwide coverage and the ability of counter the attack of anti-radiation missiles (ARM). In this work, ocean surface target detection and positioning by this technique is discussed. The analysis mainly focuses on two aspects: 1) the detecting principle and the ability of this approach, and 2) the target positioning method based on the corresponding relation between Delay-Doppler points and the spatial points of the observing area. The positioning ambiguity is also taken into account. The performance of the proposed detection and positioning methods are evaluated by some simulations in a realistic scenario.

“Cookie”: A Satellite Concept for GNSS Remote Sensing Constellations

This paper presents a concept, “Cookie,” for a satellite particularly suited for dense spatial sampling by future Global Navigation Satellite Systems (GNSS) remote sensing constellations. Solely based on the reception of the direct and reflected signals transmitted by the GNSS, the satellite can provide observations of the Earth useful for a wide range of scientific and operational applications. The Cookie is capable of receiving direct and reflected signals, in both right- and left-hand circular polarizations, from any of the GNSS systems, and from virtually any arrival direction in both the upper and lower field-of-view hemispheres, i.e., providing nearly full 4π spherical coverage toward any navigation satellite not eclipsed by the Earth. The on-board remote sensing payload produces interferometric observables, e.g., the auto- and cross-correlation of several adequate combinations of the received signals. The interferometric processing is general, the same, and transparent, with respect to any of the signals transmitted by the current or planned GNSS systems. The instrument can implement any other suitable processing schemes too. Such payload can provide, in parallel, accurate GNSS Radio-Occultation (GNSS-RO) observations of the atmosphere and forward and backward GNSS Reflectometry (GNSS-R) measurements from the Earth surface. Several Cookies can be piled up inside the fairing of a rocket minimizing the launch cost of a constellation. A constellation of three Cookies has been simulated and its sampling performance characterized. The key concepts of the Cookie payload could be demonstrated through ESAs GNSS Reflectometry, Radio-Occultation, and Scatterometry experiment on board the International Space Station (GEROS-ISS).

Wind Speed Retrieval Using Coastal Ocean-Scattered GNSS Signals

This paper aims to propose a new coastal observable based on the computation of the ratio between the coherent and incoherent averaging as an alternative to the coherent time and the effectiveness of incoherent averaging proposed by previous works. Experimental data have been processed to develop the relationship between wind speed and the proposed observable. The influence on fitting results of elevation angle, averaging samples, and the delay lag to align the local replica to the scattered signals are analyzed to obtain an optimal retrieval. When the number of averaged samples is less than 150, and the delay range relative to the specular reflection is from -0.4 to 0.4, fitting results with a root mean square error (RMSE) less than 2.0 m/s and a correction coefficient larger than 0.8 for signals from BeiDou GEO and GPS implemented compensation of an elevation angle are obtained. Finally, the feasibility of multisatellite observation using multiple regression and neural networks is demonstrated. Neural networks can get better fitting results than multiple regression for both BeiDou GEO and GPS. In addition, for GPS satellites, when the elevation angle of GPS is considered as an input of a neural network, the influence of the elevation angle is greatly weakened, so that the RMSE of 1.03 m/s and the correlation coefficient of 0.96 can be obtained.

Typhoon observations using the interferometric GNSS-R technique

Typhoon motoring using Global Navigation Satellite System (GNSS) signals is a new application of the GNSS-R technique, with increasing interest in the last years. Examples of that can be the CYGNSS (Cyclone Global Navigation Satellite System) spaceborne mission proposed by NASA, or the TIGRIS (Typhoon Investigation using GNSS-R Interferometric Signals) experiment, conducted in the framework of ESA-China cooperation. This paper focuses on the TIGRIS experiment, presenting the preliminary results obtained using the interferometric GNSS-R (iGNSS-R) technique.