Ground-based Global Navigation Satellite System Reflectometry (GNSS-R) is quickly maturing toward the objective of becoming a viable alternative for operational coastal sea-level (SL) altimetry in a geocentric reference frame. SL has immense societal implications related to climate change. Of particular interest is the exploitation of existing coastal GNSS sites for reflectometry by means of signal-to-noise ratio (SNR) observables. We report results from the first inter-comparison campaign on SNR-based GNSS-R. The goal was to cross-validate retrieval solutions from independent research groups under comparable conditions. This action was an initiative of the International Association of Geodesy working group 4.3.9 (2015–2019 term). Data collected at the Onsala Space Observatory for a 1-year period (2015–2016) were compared to a co-located tide gauge (TG). SNR data for the GPS L1-C/A signal were processed by four groups, in Sweden, Luxembourg/Brazil, Germany, and the UK. Semidiurnal tidal constituents showed good agreement between TG and all GNSS-R groups. SL variations at diurnal and longer periods were also well captured by all series. Most GNSS-R solutions exhibited spurious tones at integer fractions of one sidereal day, the satellite revisit time of the particular GNSS constellation employed (GPS). Band-pass filtering between 3 h and 30 h confirmed that the dominant tidal components were well captured by most GNSS-R solutions. Higher-frequency SL variations (periods < 3 h) are poorly represented by GNSS-R as a consequence of its low temporal resolution. The solution with the worst agreement neglects a correction associated with the rate of change in sea level and uses narrower satellite elevation ranges per retrieval. Overall, there was excellent agreement, with correlation coefficients exceeding 0.9 and RMSE smaller than 5 cm.

SNR-based GNSS reflectometry for coastal sea-level altimetry: results from the first IAG inter-comparison campaign

Monitoring sea level is critical due to climate change observed over the years. Global Navigation Satellite System Reflectometry (GNSS-R) has been widely demonstrated for coastal sea-level monitoring. The use of signal-to-noise ratio (SNR) observations from ground-based stations has been especially productive for altimetry applications. SNR records an interference pattern whose oscillation frequency allows retrieving the unknown reflector height. Here we report the development and validation of a complete hardware and software system for SNR-based GNSS-R. We make it available as open source based on the Arduino platform. It costs about US$200 (including solar power supply) and requires minimal assembly of commercial off-the-shelf components. As an initial validation towards applications in coastal regions, we have evaluated the system over approximately 1 year by the Guaiba Lake in Brazil. We have compared water-level altimetry retrievals with independent measurements from a co-located radar tide gauge (within 10 m). The GNSS-R device ran practically uninterruptedly, while the reference radar gauge suffered two malfunctioning periods, resulting in gaps lasting for 44 and 38 days. The stability of GNSS-R altimetry results enabled the detection of miscalibration steps (10 cm and 15 cm) inadvertently introduced in the radar gauge after it underwent maintenance. Excluding the radar gaps and its malfunctioning periods (reducing the time series duration from 317 to 147 days), we have found a correlation of 0.989 and RMSE of 2.9 cm in daily means. To foster open science and lower the barriers for entry in SNR-based GNSS-R research and applications, we make a complete bill of materials and build tutorials freely available on the Internet so that interested researchers can  replicate the system.

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An open-source low-cost sensor for SNR-based GNSS reflectometry: design and long-term validation towards sea-level altimetry

For designing maritime wireless transmission system, the radio propagation over sea surface needs to be known first. One of the distinct characteristics of maritime radio propagation is the impact of ship motions due to the fluctuation of sea waves. This paper establishes a radio propagation model with the integration of the effects of ship motions. Using such an integrated radio propagation model, the maritime radio propagation characteristics are analyzed and discussed under different transmission distances, different carrier frequencies, and different motion types. Index Terms—Radio propagation, Maritime communications, Channel modeling

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Maritime Radio Propagation with the Effects of Ship Motions

Evaluation of multipath signal loss for AIS signals transmitted on the sea surface

Dear editor, Increase in maritime activities with exclusive economic zones leads to an increase in demands for information security in maritime communication networks. In maritime, all ships must install an automatic identification system (AIS) to ensure navigational safety, including eavesdropper ships (ESs). Therefore, the navigation information of the ships is easy for an operator to obtain via the AIS. In the study of [1], the author proposed a new AIS-based ad hoc network model and an AISbased ad hoc routing protocol for ship-to-ship and ship-to-shore data communications. To enhance the secrecy of wireless communications, physical layer security (PLS) uses the characteristics of wireless channels [2]. A relay-aided PLS is suited for maritime networks. In [3], fountain code and relay-aided PLS techniques were found to be suitable for wireless networks. In [4], the PLS in opportunistic relay selection networks over independent and identically distributed Rayleigh fading channels was studied. We propose a location-information-assisted secure transmission scheme in the space-ocean integrated network. The maritime network ensures security by means of routing and the satellite provides maritime broadcast services. However, with the aid of navigation information, we can use routing to avoid eavesdroppers. If interception by an eavesdropper is unavoidable, the proposed scheme can also limit the received information of eavesdroppers by using power optimization, which reduces the amount of the received encoded packets of eavesdroppers. In addition to power optimization, using multiple transmission links, the scheme can also split the raw data into pieces and transmit it. If the eavesdropper is not able to intercept all the transmission links, it cannot receive enough encoded packets to recover the message. Model. In this study, we consider a multi-user maritime communication scenario. If two ships are close enough, then device-to-device (D2D) communications are enabled. Under the proposed scenario, one high-tower base station (HBS), multiple relay ships (RSs), and a satellite (SAT) cooperatively provide the services for maritime user ships. In particular, the SAT provides broadcast services for user ships. However, the eavesdroppers can also receive fountain packets from the SAT. Thus, the network control center (NCC) executes the transmission scheme and manages the control signals. The NCC can obtain the AIS information and decoding situation of the user ship from the AIS control center. According to the transmission scheme, the NCC selects a secure relay link to transmit signals and plans the routing of D2D communication. Afterward, the control signal of the D2D network and the AIS information can also broadcast together via SAT. More details of the system model are referenced in Appendix A. Algorithm development. The transmission scheme can be divided into two phases: the re-

Secure routing and transmission scheme for space-ocean broadband wireless network

Radio wave propagation test is introduced in this paper. It was held in the sea area near Qingdao city, and involved different band including L band, S band, C band and X band. Using the path loss model given in the International Telecommunication Union (ITU) recommendation propagation 1411, we derived the characteristic parameters such as path loss exponent and shadow fading for different frequencies. The statistic propagation model can be used for providing a reference for ship information communication.

Radio wave propagation characteristics measurement and modeling over the sea

Determination of tides using reflected signals is an important aspect of GNSS application technique at the present For the problem of remote sensing the tide heights, a method of retrieving the height of sea surface is proposed from the power of the interference signals obtained by a coastal GPS receiver with singular antenna An experiment was carried out at the coast of Qingdao Results of the receiving antenna height over the sea surface from observed interference signals are presented Compared to tide gauges, the results show its validity of the method based on GPS interference measurements using right-hand circular polarization antenna

http://ieeexplore.ieee.org/iel5/6391570/6408685/06408848.pdf

Wang Hongguang

Papers

Radio wave propagation characteristics measurement and modeling over the sea

A model for remote sensing sea level with GPS interferometric signals using RHCP antenna