Global Navigation Satellite System Radio Research Articles

Precise Orbit Determination for Climate Applications of GNSS Radio Occultation including Uncertainty Estimation

Global Navigation Satellite System (GNSS) Radio Occultation (RO) is a highly valuable remote sensing technique for probing the Earth’s atmosphere, due to its global coverage, high accuracy, long-term stability, and essentially all-weather capability. In order to ensure the highest quality of essential climate variables (ECVs), derived from GNSS signal tracking by RO satellites in low Earth orbit (LEO), the orbit positions and velocities of the GNSS transmitter and LEO receiver satellites need to be determined with high and proven accuracy and reliability. Wegener Center’s new Reference Occultation Processing System (rOPS) hence aims to integrate uncertainty estimation at all stages of the processing. Here we present a novel setup for precise orbit determination (POD) within the rOPS, which routinely and in parallel performs the LEO POD with the two independent software packages Bernese GNSS software (v5.2) and NAPEOS (v3.3.1), employing two different GNSS orbit data products. This POD setup enables mutual consistency checks of the calculated orbit solutions and is used for position and velocity uncertainty estimation, including estimated systematic and random uncertainties. For LEOs enabling laser tracking we involve position uncertainty estimates from satellite laser ranging. Furthermore, we intercompare the LEO orbit solutions with solutions from other leading orbit processing centers for cross-validation. We carefully analyze multi-month, multi-satellite POD result statistics and find a strong overall consistency of estimates within LEO orbit uncertainty target specifications of 5 cm in position and 0.05 mm/s in velocity for the CHAMP, GRACE-A, and Metop-A/B missions. In 92% of the days investigated over two representative 3-month periods (July to September in 2008 and 2013) these POD uncertainty targets, which enable highly accurate climate-quality RO processing, are satisfied. The moderately higher uncertainty estimates found for the remaining 8% of days (∼5–15 cm) result in increased uncertainties of RO-retrieved ECVs. This allows identification of RO profiles of somewhat reduced quality, a potential benefit for adequate further use in climate monitoring and research.

A New Algorithm for the Retrieval of Atmospheric Profiles from GNSS Radio Occultation Data in Moist Air and Comparison to 1DVar Retrievals

The Global Navigation Satellite System (GNSS) Radio Occultation (RO) is a key technique for obtaining thermodynamic profiles of temperature, humidity, pressure, and density in the Earth’s troposphere. However, due to refraction effects of both the dry air and water vapor at low altitudes, retrieval of accurate profiles is challenging. Here we introduce a new moist air retrieval algorithm aiming to improve the quality of RO-retrieved profiles in moist air and including uncertainty estimation in a clear sequence of steps. The algorithm first uses RO dry temperature and pressure and background temperature/humidity and their uncertainties to retrieve humidity/temperature and their uncertainties. These temperature and humidity profiles are then combined with their corresponding background profiles by optimal estimation employing inverse-variance weighting. Finally, based on the optimally estimated temperature and humidity profiles, pressure and density profiles are computed using hydrostatic and equation-of-state formulas. The input observation and background uncertainties are dynamically estimated, accounting for spatial and temporal variations. We show results from applying the algorithm on test datasets, deriving insights from both individual profiles and statistical ensembles, and from comparison to independent 1D-Variational (1DVar) algorithm-derived moist air retrieval results from Radio Occultation Meteorology Satellite Application Facility Copenhagen (ROM-SAF) and University Corporation for Atmospheric Research (UCAR) Boulder RO processing centers. We find that the new scheme is comparable in its retrieval performance and features advantages in the integrated uncertainty estimation that includes both estimated random and systematic uncertainties and background bias correction. The new algorithm can therefore be used to obtain high-quality tropospheric climate data records including uncertainty estimation.

An OSSE evaluation of the GNSS-R altimetry data for the GEROS-ISS mission as a complement to the existing observational networks

Simulated signals from Global Navigation Satellite Systems (GNSS), reflected off the sea surface and received aboard low Earth orbiting satellites, have been used to derive sea surface height (SSH) and assimilated into an ocean model in an Observing System Simulation Experiment (OSSE). The experimental approach is named GNSS Reflectometry (GNSS-R), which was proposed for the International Space Station (ISS). This scientific experiment was conducted in the frame of the ESA mission called “GNSS REflectometry, Radio Occultation and Scatterometry aboard the International Space Station” (GEROS-ISS). In this study, three sources of uncertainties of the planned GNSS-R altimeter are considered by the GNSS-R simulator: the troposphere, the ionosphere, and a noise term. An ensemble optimal interpolation (EnOI) data assimilation system is set up for an eddy-resolving HYbrid Coordinate Ocean Model (HYCOM) of the South China Sea (SCS), and two data assimilation runs are performed from the 18th June to the 31st July 2014 with and without GNSS-R. In the run assimilating GNSS-R, the measurements come in addition to traditional Sea Level Anomalies (SLA) from present-day altimeters. In spite of the lower precision of individual GNSS-R retrievals, the results obtained in July show an overall improvement of the Root Mean Squared Difference (RMSD) by 14%, compared to traditional altimeter data only. Considering the crossing of Typhoon Rammasun through the SCS, the GNSS-R data improve the realism of the three largest eddies. The temperature sections along the typhoon track show large differences in the upper 200 m depths in excess of 1 °C near the shelf break. Finally, diagnostics of Degree of Freedom for Signal (DFS) provide a quantitative Impact Factor (IF) of the GNSS-R altimetry data over the conventional altimeter data. On average in July, the IF is low (<5%), but for the period of the typhoon it reaches values over 20%. This indicates the complementary of the GNSS-R altimetry data to the present observing system, especially in filling the gaps of the traditional altimeters.

Topside Electron Density Representations for Middle and High Latitudes: A Topside Parameterization for E‐CHAIM Based On the NeQuick

AbstractIn this study, we present a topside model representation to be used by the Empirical Canadian High Arctic Ionospheric Model (E‐CHAIM). In the process of this, we also present a comprehensive evaluation of the NeQuick's, and by extension the International Reference Ionosphere's, topside electron density model for middle and high latitudes in the Northern Hemisphere. Using data gathered from all available incoherent scatter radars, topside sounders, and Global Navigation Satellite System Radio Occultation satellites, we show that the current NeQuick parameterization suboptimally represents the shape of the topside electron density profile at these latitudes and performs poorly in the representation of seasonal and solar cycle variations of the topside scale thickness. Despite this, the simple, one variable, NeQuick model is a powerful tool for modeling the topside ionosphere. By refitting the parameters that define the maximum topside scale thickness and the rate of increase of the scale height within the NeQuick topside model function, r and g, respectively, and refitting the model's parameterization of the scale height at the F region peak, H0, we find considerable improvement in the NeQuick's ability to represent the topside shape and behavior. Building on these results, we present a new topside model extension of the E‐CHAIM based on the revised NeQuick function. Overall, root‐mean‐square errors in topside electron density are improved over the traditional International Reference Ionosphere/NeQuick topside by 31% for a new NeQuick parameterization and by 36% for a newly proposed topside for E‐CHAIM.

Vulnerabilities in Localization With Regard to GNSS and Harmful Radio Interference: International and EU Law Aspects

Harmful radio interference contravenes many national, regional, and international laws. Global navigation satellite system (GNSS) services have global coverage, therefore development of international law is necessary. European Union (EU) also requires harmonized law for its member states. This paper focuses on the current state of international and EU laws to tackle GNSS, radio spectrum and radio equipment usage, and probable interference with regard to location services and privacy. From a legal point of view, this paper seeks to include mitigation of unlawful interference in radio communication in order to ensure a secured location estimation and positioning system.

Characterizing Rigging Crew Proximity to Hazards on Cable Logging Operations Using GNSS-RF: Effect of GNSS Positioning Error on Worker Safety Status.

Logging continues to rank among the most lethal occupations in the United States. Though the hazards associated with fatalities are well-documented and safe distances from hazards is a common theme in safety education, positional relationships between workers and hazards have not been quantified previously. Using GNSS-RF (Global Navigation Satellite System-Radio Frequency) transponders that allow real-time monitoring of personnel, we collected positioning data for rigging crew workers and three common cable logging hazards: a log loader, skyline carriage, and snag. We summarized distances between all ground workers and each hazard on three active operations and estimated the proportion of time crew occupied higher-risk areas, as represented by geofences. We then assessed the extent to which positioning error associated with different stand conditions affected perceived worker safety status by applying error sampled in a separate, controlled field experiment to the operational data. Root mean squared error was estimated at 11.08 m in mature stands and 3.37 m in clearcuts. Simulated error expected for mature stands altered safety status in six of nine treatment combinations, whereas error expected for clearcuts affected only one. Our results show that canopy-associated GNSS error affects real-time geofence safety applications when using single-constellation American Global Positioning System transponders.

Supporting the detection and monitoring of volcanic clouds: A promising new application of Global Navigation Satellite System radio occultation

The altitude of volcanic clouds and the atmospheric thermal structure after volcanic eruptions are studied using Global Navigation Satellite System (GNSS) Radio Occultation (RO) profiles co-located with independent radiometer images of ash and sulfur dioxide plumes. We use geographically co-located RO profiles to detect the top altitude of volcanic clouds and to analyze their impact in terms of temperature change signatures. We obtained about 1300 RO profiles co-located with two representative eruptions (Puyehue 2011, Nabro 2011) and found that an anomaly technique recently developed for detecting convective cloud tops and studying the vertical thermal structure of deep convective systems can also be applied to volcanic clouds. Analyzing the atmospheric thermal structure after the eruptions, we found clear cooling signatures induced by volcanic cloud tops in the upper troposphere for the Puyehue case. For the Nabro case we detected a significant warming in the stratosphere which lasted for several months, indicating that the cloud reached the stratosphere. The results are encouraging for future large-scale use of RO data for supporting the monitoring of volcanic clouds and their impacts on weather and climate.

SNR and Standard Deviation of cGNSS-R and iGNSS-R Scatterometric Measurements

This work addresses the accuracy of the Global Navigation Satellite Systems (GNSS)-Reflectometry (GNSS-R) scatterometric measurements considering the presence of both coherent and incoherent scattered components, for both conventional GNSS-R (cGNSS-R) and interferometric GNSS-R (iGNSS-R) techniques. The coherent component is present for some type of surfaces, and it has been neglected until now because it vanishes for the sea surface scattering case. Taking into account the presence of both scattering components, the estimated Signal-to-Noise Ratio (SNR) for both techniques is computed based on the detectability criterion, as it is done in conventional GNSS applications. The non-coherent averaging operation is considered from a general point of view, taking into account that thermal noise contributions can be reduced by an extra factor of 0.88 dB when using partially overlapped or partially correlated samples. After the SNRs are derived, the received waveform’s peak variability is computed, which determines the system’s capability to measure geophysical parameters. This theoretical derivations are applied to the United Kingdom (UK) TechDemoSat-1 (UK TDS-1) and to the future GNSS REflectometry, Radio Occultation and Scatterometry on board the International Space Station (ISS) (GEROS-ISS) scenarios, in order to estimate the expected scatterometric performance of both missions.

Is It Time for Commercial Weather Satellites? Analyzing the Case of Global Navigation Satellite System Radio Occultation

This article argues that while the creation of a commercial weather satellite sector is neither easy nor assured, the future prospects for commercial weather satellites are good. Global Navigation Satellite System Radio Occultation (GNSS-RO) satellite commercial data buys are poised to be the vanguard in the opening of this new sector. The article discusses the current state of the U.S. weather satellite system and the trends providing the impetus for commercial activity in this area. It identifies the most likely government and commercial actors for GNSS-RO data provision and analyzes the legal, technical, economic, and international relations issues facing these entities. Based on this analysis, the article concludes that the commercial weather satellite sector is poised to begin, with government purchases of commercial GNSS-RO data leading the way. It concludes by discussing critical policy decisions that will affect the success of GNSS-RO companies as well as the implications of success for other areas of the commercial weather satellite sector.

Atmospheric polarimetric effects on GNSS radio occultations: the ROHP-PAZ field campaign

Abstract. This study describes the first experimental observations showing that hydrometeors induce polarimetric signatures in global navigation satellite system (GNSS) signals. This evidence is relevant to the PAZ low Earth orbiter, which will test the concept and applications of polarimetric GNSS radio occultation (RO) (i.e. ROs obtained with a dual-polarization antenna). A ground field campaign was carried out in preparation for PAZ to verify the theoretical sensitivity studies on this concept (Cardellach et al., 2015). The main aim of the campaign is to identify and understand the factors that might affect the polarimetric GNSS observables. Studied for the first time, GNSS signals measured with two polarimetric antennas (H, horizontal, and V, vertical) are shown to discriminate between heavy rain events by comparing the measured phase difference between the H and V phase delays (ΔΦ) in different weather scenarios. The measured phase difference indicates higher dispersion under rain conditions. When individual events are examined, significant increases in ΔΦ occur when the radio signals cross rain cells. Moreover, the amplitude of such a signal is much higher than the theoretical prediction for precipitation; thus, other sources of polarimetric signatures have been explored and identified. Modelling of other hydrometeors, such as melting particles and ice crystals, have been proposed to explain the obtained measurements, with good agreement in more than 90 % of the cases.

Ionosphere probing with simultaneous GNSS radio occultations

Radio occultation (RO) is a powerful technique for providing vertical profiles of refractivity, temperature, pressure, and water vapor of the neutral atmosphere and electron density of the ionosphere. The Abel inversion method which is based on the spherical symmetry assumption has been widely utilized to retrieve electron density profiles (EDPs) from RO measurements, which are available by observing Global Navigation Satellite System (GNSS) satellites from low-earth-orbit satellites. It is well known that the Abel inversion is subject to errors in the presence of ionospheric horizontal gradients. With the arrival of new navigation systems, the opportunities of establishing simultaneous GNSS RO events are increasing. We develop an improved Abel inversion technique that accounts for pairs of simultaneous RO events to relax the spherical symmetry assumption. Through the use of Tikhonov regularization, the problem is formulated so that numerical conditioning is improved and a priori information such as expected electron density, asymmetric factor, and vertical total electron content can be incorporated. Appropriate weighting can be determined to reflect the availability and quality of information. By balancing the reference data and measurements, the method thus paves a way for ionospheric probing in challenging geomagnetic conditions as both the EDP at the intersection and the horizontal gradients are retrieved. Simulation and experimental results are provided to show the effectiveness of the proposed method. The robustness and sensitivity of the proposed method are also assessed.

Recent global warming hiatus dominated by low‐latitude temperature trends in surface and troposphere data

AbstractOver the last 15 years, global mean surface temperatures exhibit only weak trends. Recent studies have attempted to attribute this so called temperature hiatus to several causes, amongst them incomplete sampling of the rapidly warming Arctic region. We here examine zonal mean temperature trends in satellite‐based tropospheric data sets (based on data from (Advanced) Microwave Sounding Unit and Global Navigation Satellite System Radio Occultation instruments) and in global surface temperatures (HadCRUT4). Omission of successively larger polar regions from the global mean temperature calculations, in both tropospheric and surface data sets, shows that data gaps at high latitudes cannot explain the observed differences between the hiatus and the prehiatus period. Instead, the dominating causes of the global temperature hiatus are found at low latitudes. The combined use of several independent data sets, representing completely different measurement techniques and sampling characteristics, strengthens the conclusions.

CHAMP climate data based on the inversion of monthly average bending angles

Abstract. Global Navigation Satellite System Radio Occultation (GNSS-RO) refractivity climatologies for the stratosphere can be obtained from the Abel inversion of monthly average bending-angle profiles. The averaging of large numbers of profiles suppresses random noise and this, in combination with simple exponential extrapolation above an altitude of 80 km, circumvents the need for a "statistical optimization" step in the processing. Using data from the US–Taiwanese COSMIC mission, which provides ~1500–2000 occultations per day, it has been shown that this average-profile inversion (API) technique provides a robust method for generating stratospheric refractivity climatologies. Prior to the launch of COSMIC in mid-2006, the data records rely on data from the CHAMP (CHAllenging Mini-satellite Payload) mission. In order to exploit the full range of available RO data, the usage of CHAMP data is also required. CHAMP only provided ~200 profiles per day, and the measurements were noisier than COSMIC. As a consequence, the main research question in this study was to see if the average bending-angle approach is also applicable to CHAMP data. Different methods for the suppression of random noise – statistical and through data quality prescreening – were tested. The API retrievals were compared with the more conventional approach of averaging individual refractivity profiles, produced with the implementation of statistical optimization used in the EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) Radio Occultation Meteorology Satellite Application Facility (ROM SAF) operational processing. In this study it is demonstrated that the API retrieval technique works well for CHAMP data, enabling the generation of long-term stratospheric RO climate data records from August 2001 and onward. The resulting CHAMP refractivity climatologies are found to be practically identical to the standard retrieval at the DMI (Danish Meteorological Institute) below altitudes of 35 km. Between 35 and 50 km, the differences between the two retrieval methods started to increase, showing largest differences at high latitudes and high altitudes. Furthermore, in the winter hemisphere high-latitude region, the biases relative to ECMWF (European Centre for Medium-range Weather Forecasts) were generally smaller for the new approach than for the standard retrieval.

MSK-BCS Modulation for GNSS Radio Frequency Compatibility in C Band

Due to insufficient Radio Navigation Satellite Service (RNSS) portion of Radio Frequency (RF) spectrum for many services that are being planned for the future on L band where has come up a frequency overlapping situation, C band as an alternative spectral allocation has already been proposed in the previous years. The future C band signals could make use of the frequency band between 5010MHz and 5030MHz, offering a narrow bandwidth of 20 MHz. The main goal of C band signal design aims at having band limited signals. Consequently, it should be designed with a view to compatibility in band and the power emissions out of band. The Minimum Shift Keying(MSK) modulation is investigated to provide constant envelope and a good spectrum confinement, furthermore, a Binary Coded Symbol (BCS) modulation based on MSK pulse that has sharper autocorrelation, larger Gabor bandwidth, smaller CramerRao Lower Bound and less multipath error has been presented by Liu. Suppose spread interoperability signals employing the BCS modulation based on MSK pulse for Global Navigation Satellite System (GNSS) in C band with only a narrow bandwidth of 20MHz. Simulation results reveal the interference for the receiver prompt correlator channel processing and code tracking induced by navigation satellite system to each other. The result can be significant for the future signal design in C band

Scaling of GNSS Radio Occultation Impact with Observation Number Using an Ensemble of Data Assimilations

Abstract An ensemble of data assimilations (EDA) approach is used to estimate how the impact of Global Navigation Satellite System (GNSS) radio occultation (RO) measurements scales as a function of observation number in the ECMWF numerical weather prediction system. The EDA provides an estimate of the theoretical analysis and short-range forecast error statistics, based on the ensemble “spread,” which is the standard deviation of the ensemble members about the ensemble mean. This study is based on computing how the ensemble spread of various parameters changes as a function of the number of simulated GNSS RO observations. The impact from 2000 up to 128 000 globally distributed simulated GNSS RO profiles per day is investigated. It is shown that 2000 simulated GNSS RO measurements have an impact similar to real measurements in the EDA and that the EDA-based impact of real data can be related to the impact in observing system experiments. The dependence of the ensemble statistics on observation error statistics assumed when assimilating the data, rather than the actual observation errors, is emphasized. There is no evidence of “saturation” of forecast impact even with 128 000 GNSS RO profiles per day. However, this result is a well-known consequence of always improving the theoretical analysis and short-range forecast error statistics when adding new observations that are assumed to have uncorrelated observation errors. In general, it is found that 16 000 GNSS RO profiles per day have around half the impact of 128 000 profiles, based on the reduction of ensemble spread values where the GNSS RO measurements have the largest impact.

Degradation of Effective Carrier-Power-to-Noise Density ratio based on Spectral Separation Coefficient for GNSS Radio Frequency Compatibility in C Band

As a result of the congestion of current navigation services in the L band, the frequency band between 5010MHz and 5030MHz allocated as C band has returned as a candidate global navigation satellite systems (GNSS). The radio frequency compatibility in C band must be assessed carefully before any new signal or service of GNSS is deployed. Due to the narrow bandwidth, the signals transmitted in C band may be limited. This paper only analyzes the compatibility of the signals as the interoperability or classical signals in L band between BeiDou III (BD III) B1C and GPS L1C/A, L1C, Galileo E1OS in the C band to see whether they are appropriate to transmit in C band. The paper uses degradation of effective carrier-power-to-noise density ratio based on spectral separation coefficient (SSC) as the compatibility assessment methodology. Simulation results reveal the interference between BD III B1C and GPS L1C/A, L1C, Galileo E1OS.

Parametric Study of Calibration Blackbody Uncertainty Using Design of Experiments

NASA is developing the Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission to provide accurate measurements to substantially improve understanding of climate change. CLARREO will include a Reflected Solar (RS) Suite, an Infrared (IR) Suite, and a Global Navigation Satellite System-Radio Occultation (GNSS-RO). The IR Suite consists of a Fourier Transform Spectrometer (FTS) covering 5 to 50 micrometers (2000-200 cm-1 wavenumbers) and on-orbit calibration and verification systems. The IR instrument will use a cavity blackbody view and a deep space view for on-orbit calibration. The calibration blackbody and the verification system blackbody will both have Phase Change Cells (PCCs) to accurately provide a SI reference to absolute temperature. One of the most critical parts of obtaining accurate CLARREO IR scene measurements relies on knowing the spectral radiance output from the blackbody calibration source. The blackbody spectral radiance must be known with a low uncertainty, and the magnitude of the uncertainty itself must be reliably quantified. This study focuses on determining which parameters in the spectral radiance equation of the calibration blackbody are critical to the blackbody accuracy. Fourteen parameters are identified and explored. Design of Experiments (DOE) is applied to systematically set up an experiment (i.e., parameter settings and number of runs) to explore the effects of these 14 parameters. The experiment is done by computer simulation to estimate uncertainty of the calibration blackbody spectral radiance. Within the explored ranges, only 4 out of 14 parameters were discovered to be critical to the total uncertainty in blackbody radiance, and should be designed, manufactured, and/or controlled carefully. The uncertainties obtained by computer simulation are also compared to those obtained using the “Law of Propagation of Uncertainty”. The two methods produce statistically different uncertainties. Nevertheless, the differences are small and are not considered to be important. A follow-up study has been planned to examine the total combined uncertainty of the CLARREO IR Suite, with a total of 47 contributing parameters. The DOE method will help in identifying critical parameters that need to be effectively and efficiently designed to meet the stringent IR measurement accuracy requirements within the limited resources.

An Evaluation of GNSS Radio Occultation Technology for Australian Meteorology

Earth atmospheric information has been primarily observed by a global network of radiosonde weather observation stations for global weather forecasting and climatologic studies for many years. However, the main disadvantage of this method is that it can not sufficiently capture the complex dynamics of the Earth’s atmosphere since its limited and heterogeneous geographic distribution of launching stations. Since the first low earth orbit (LEO) satellite equipped with a GPS receiver was launched in early 1990s, there are more than a dozen of GPS receivers onboard LEO satellites used for Earth atmospheric observation. Recent research has shown that the Global Navigation Satellite System (GNSS) radio occultation (RO) derived atmospheric profiles have great potentials to overcome many limitations of existing atmospheric observation methods. Constellation Observing Systems for Meteorology, Ionosphere, and Climate (COSMIC) retrieved atmospheric profiles are investigated using radiosonde measurements at 42 collocated stations in the Australian region. Statistical results show that the difference in average temperature is about 0.05 ?C with a standard deviation of 1.52?C and the difference in average pressure is -1.06 hPa with a standard deviation of 0.91 hPa. This research has also demonstrated that the GNSS RO derived atmospheric profiles have good agreement with the radiosonde observations.