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The precision of sea surface altimetry using bistatically reflected signals of the Global Navigation Satellite System (GNSS) is typically one to two orders of magnitude worse than dedicated radar altimeters. However, when the scattering is coherent, the electromagnetic phase of the carrier signal can be tracked, providing precise ranging measurements. Under grazing angle (GA) geometries, the conditions for coherent scattering are maximized, enabling carrier phase-delay altimetric techniques over sea waters. This work presents the first implementation of GA carrier phase sea surface altimetry using data acquired from a spaceborne platform (NASA Cyclone GNSS mission) and transmitted from both GPS and Galileo constellations. The altimetric results show that the measurement system precision is 3/4.1 cm (median/mean) at 20 Hz sampling, cm level at 1 Hz, comparable to dedicated radar altimeters. The combined precision, including systematic errors, is 16/20 cm (median/mean) precision at 50 ms integration (a few cm level at 1 Hz). The wind and wave requirements to enable coherent scattering at GA geometries appear to be below 6 m/s wind and 1.5 m significant wave height, although only 33% of tracks under these conditions present sufficient coherence. Given that this technique could be implemented by firmware updates of existing GNSS radio occultation missions, and given the large number of such missions, the study indicates that the resulting precision and spatio¿temporal resolution would contribute to resolving some submesoscale ocean signals., The work conducted at ICE-CSIC/IEEC was supported by Spanish Grant RTI2018-099008-B-C22 (MCIU/AEI/FEDER, UE). (Corresponding author: Estel Cardellach.)

In global navigation satellite systems reflectometry (GNSS-R), the autocorrelation property of the transmitted GNSS signal is an important factor in the delay-Doppler map of the scattered power. The autocorrelation functions (ACFs) of the GPS Level 1 (L1) coarse/acquisition signals exhibit sidelobes. For some of the pseudorandom noise (PRN) codes, the high-amplitude sidelobes are located within one chip from the main lobe and, thus, broaden or narrow its width. These PRN-dependent ACF deviations can introduce significant biases into the GNSS-R scatterometric observables and the wind speed measurements. Simulation results for Cyclone GNSS (CYGNSS) mission show that the ACF deviation induced wind speed error can be larger than 10% (e.g., 2–3 m/s for 20 m/s and 7–8 m/s for 40 m/s wind speeds). These effects can be corrected efficiently by applying empirical factors on the CYGNSS L1 observables.

The full constellation of Chinese Global Navigation Satellite System (GNSS) BeiDou-3 has been deployed completely and started fully operational service. In addition to providing global Positioning, Navigation and Timing (PNT) services, the BeiDou-3 satellites transmissions can also be used as the sources of illumination for Earth Observation (EO) with a bistatic radar configuration. This innovative EO concept, known as GNSS reflectometry (GNSS-R), allows to measure the Earth surface characteristics at high resolution via the reflected L-band radar signals collected by a constellation of small, low cost and low Earth orbiting satellites. For the first time in orbit, earth reflected BeiDou-3 signal has been detected from the limited sets of raw data collected by the NASA¿s Cyclone GNSS (CYGNSS) constellation. The feasibility of spaceborne BeiDou-3 reflections on two typical applications, including sea surface wind and flooding inundation detection, has been demonstrated. The methodology and results give new strength to the prospect of new spaceborne GNSS-R instruments and missions, which can make multi-GNSS reflectometry observations available to better capture rapidly changing weather systems at better spatio-temporal scales., This work was supported in part by the Spanish Ministry ofEconomy and Competitiveness and EU/FEDER (ESP2015-70014-C2-2-R), the International Science and TechnologyCooperation Projects of Shanghai (No. 17220730600), andthe ESA-MOST China Dragon5 Program (ID. 58070). Wei-qiang Li and Estel Cardellach are the members of the CycloneGlobal Navigation Satellite System mission’s extended scienceteam. The authors are grateful to the CYGNSS team for pro-viding the CYGNSS L0 raw data, and making the L1 datapublicly available through the NASA EOSDIS PhysicalOceanography Distributed Active Archive Center (PO.DAAC)(http://dx.doi.org/10.5067/CYGNS-L1X21

This paper analyzes the ocean altimetry precision of global navigation satellite systems’ reflectometry (GNSS-R) by characterizing the noise statistics of the measured waveform. For this purpose, we first investigate the dependence of the altimetry precision on the statistics of the observed waveform and then derive an analytical model of the waveform statistics after incoherent averaging. The main data set used for altimetry analysis and model validation is the GPS L5 signals obtained from an airborne experiment. Two different delay estimators, based on the leading-edge derivative (DER) and the waveform fitting (FIT) over the leading edge, have been implemented. Later, the relationship between the statistics of the observed waveform and the altimetry precision is derived and validated for both the estimators with the airborne data. Then, the analytical model of the statistics of the incoherently averaged waveform is built from the correlation properties of the complex waveform, and also validated with both airborne and the TechDemoSat-1 spaceborne data. After that, the proposed waveform statistics and altimetry precision models are applied to spaceborne cases with different orbit altitudes. The altimetry performance of different onboard processing methods, i.e., conventional GNSS-R and interferometric GNSS-R, and delay estimators, i.e., DER and FIT, are preliminarily assessed. Finally, the dependence of altimetry precision on different parameters is analyzed and a simplified model of altimetry precision is proposed for the spaceborne case.

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.

This paper evaluates the capability of interferometric global navigation satellite system reflectometry (GNSS-R) to perform sea surface altimetry in a synoptic scenario. Such purpose, which requires the combination of the results from different GNSS signals, constitutes a unique characteristic of this approach. Interferometric GNSS-R group delay altimetry has been proven to be more precise than conventional GNSS-R. However, the self-consistency and accuracy of their synoptic solutions (simultaneous multi-static results) have never been proved before. In our work, we analyze a dataset of GNSS signals reflected off the Baltic Sea acquired during an airborne campaign using a receiver that was developed for such a purpose. Among other features, it enables beamformer capability in post-processing to get multiple and simultaneous GNSS signals under the interferometric approach’s restrictions. In particular, the signals from two GPS and two Galileo satellites, at two frequency bands (L1 and L5), covering an elevation range between 28° and 83°, are processed to retrieve sea surface height estimations. The results obtained are self-consistent among the different GNSS signals and data tracks, with discrepancies between 0.01 and 0.26 m. Overall, they agree with ancillary information at 0.40 m level, following a characteristic height gradient present at the experimental site., This work has been carried out with the financial support of the following research projects and contracts: Spanish research grant ESP2015-70014-C2-2-R (MINECO/FEDER) and ESA Contract: RFQ/3-12747/09/NL/JD-CCN4., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)

We present a new passive bi-static radar measurement concept that employs digital satellite TV signals transmitted from geostationary orbit as signals of opportunity to measure precipitation. We also present experimental results using artificial targets, and provide results on measured Doppler frequencies as well.

In Global Navigation Satellite System Reflectometry (GNSS-R), power waveforms or Delay-Doppler Maps (DDM) are incoherently averaged to reduce the standard deviation of the fluctuations caused by the speckle and thermal noise. This letter proposes a simple and innovative processing concept based on the computation of the variance of the complex waveforms. By this approach, the coherent part of the signal is subtracted from the incoherent averaged waveform. It can be useful, for example, in scenarios where reflected signal is contaminated by the direct signal, as ground-based scenarios or in future experiments such as GEROS-ISS due to the nearby multipath of direct signals.

The Software PARIS Interferometric Receiver (SPIR) was conceived as a multiple beam-forming GNSS-Reflectometry receiver capable of implementing different signal processing strategies (clean-replica, interferometric, etc.) in order to demonstrate the synoptic capabilities for sea-surface altimetry using the interferometric GNSS-R technique. The instrument has been designed as a 16-channel very-high speed (320 MB/s) hardware recording receiver followed by a software post-processor, implementing the beam-forming and signal processing. The presence of 16 channels together with the beam-forming capabilities requires to characterize the quantization offsets, instrument time-keeping and relative instrumental delays between the different channels in order to have a well-functioning instrument. In this paper we present several calibration aspects of SPIR.

Due to the launch of a number of dedicated GNSS-R satellite missions during the last years, there is a potential raise of research interest in this field. This paper presents an analysis and simulation tool for the GNSS-R community: wavpy. More than just a simple waveform simulator, this library provides a set of object-oriented classes dedicated to each of the different elements that characterize a GNSS-R scenario. Then, the user can focus on just a particular piece of analysis or, by exploiting their combined synergies, to perform a more comprehensive simulation exercise.

This work is to analyse the ocean altimetry precision of Global Navigation Satellite Systems Reflectometry (GNSS-R) based on the statistics of observed waveform. The GPS L5 signals obtained from an airborne experiment are used for this study. The altimetry precision is analysed for two specific algorithms, based on the leading edge derivative (DER) and the leading edge fitting (FIT), in which the relationship between the altimetry precision and the waveform statistics is established and validated. Then, we introduce an analytical model of the statistics of the incoherently averaged waveform, which has been validated with both airborne and spaceborne data. Finally, as an example, the altimetry precision is predicted for the GEROS-ISS mission, and preliminary results are presented.

In this paper, we present the Software PARIS Interferometric Receiver (SPIR), a high-speed GNSS reflectometry recording receiver which has been designed and implemented with the primary goal of demonstrating the synoptic capabilities of the interferometric technique in GNSS Reflectrometry. Thanks to the use of large bandwidth GNSS signals, this technique is advantageous in comparison to the so-called clean-replica processing, when sea surface altimetric applications are pursued. The SPIR receiver down-converts, samples, and records the GNSS signals acquired by the sixteen elements of two antenna arrays. It can operate at any of the common GNSS L1, L2, or L5 bands. Digital beam-forming and signal processing is performed off-line by its dedicated signal processor, so that the GNSS reflectometry can be applied to different transmitting satellites using the same set of recorded signals. Alternatively, different processing techniques can be compared by applying them to exactly the same signals. This article focuses on the SPIR instrument hardware and software, as well as the remote sensing observables that can be obtained using this equipment.

Since the nadir 1-second altimetry precision was first estimated to be of 56 cm when the PARIS concept was developed back in 1993 (it was assumed the interferometric processing of GPS P-code signals and a 4×4 m×m receiving antenna at 700 km orbital altitude) [1], many detailed analyses and experiments have been conducted leading to far more optimistic values, as low as some 15 cm (using the interferometric processing and a 1 m2antenna from 400 km altitude). This paper presents the instrument and concept advances that have allowed such improved expectations.

Tracking, in radar altimetry, is the positioning of the waveforms in the correlation window. This letter presents a tracking strategy in spaceborne altimetry using global navigation satellite system reflectometry. First, the tracking procedure is illustrated, and the tracking parameters are discussed one by one: the determination of the correlation window, the accuracy of specular delay guess, and the tracking-refresh period. Based on the results, the proposed European Space Agency Passive Reflectometry and Interferometry System In-Orbit Demonstrator tracking case study is examined.

The Software PARIS Interferometric Receiver (SPIR) is a multichannel GNSS-R recording receiver. It is capable to record the GNSS signals at L1, L2 or L5 bands from eight up-looking and eight down-looking antenna elements simultaneously at a rate of 80 MHz. A software post-processor processes the recorded data off-line to obtain altimetric estimates. Its development has been carried out in order to validate experimentally the synoptic capabilities of GNSS-R using the PARIS Interferometric Technique and to allow the comparison of different GNSS-R processing techniques on exactly the same set of recorded data. The paper presents the instrument's main characteristics and preliminary results.

The GNSS rEflectometry, Radio Occultation and Scatterometry onboard the International Space Station (GEROS-ISS) is an innovative experiment for climate research, proposed in 2011 within a call of the European Space Agency (ESA) for installation at the ISS. This international proposal was the only one selected for further studies by ESA out of ∼25 submitted ones. In this work, an updated assessment of the instrument performance for the near-nadir altimetry (GNSS-R) mode and scatterometry is assessed, including the effect of the inter-modulation signals, and the expected degradation due to presence of radio-frequency interference coming from DME/TACAN systems. These results correspond to the study lead by Airbus Defence and Space-Spain, one of the two parallel Phase A studies conducted by ESA.

We describe the development and implementation of a method for extracting altimetric information using the Passive Reflectometry and Interferometry System (PARIS), i.e., from GPS signals after their reflection off the sea surface. We have formalized one idea laid out in the description of a bistatic system for ocean altimetry using the GPS signal, by Hajj and Zuffada (Jet Propulsion Laboratory), and have extended it to real situations encountered in PARIS aircraft experiments. Second, we have developed the corresponding algorithms to produce real-time altimetric observables to be used in dedicated digital signal processors. Finally, we have applied this method to estimate sea-surface height from one flight experiment in the North Sea off the coast of Norway.

This paper describes a new instrument that was specially designed and developed to gather Global Positioning System (GPS) signals after they have been reflected from suitable surfaces (sea, ice, and ground), for Earth remote sensing. The device has been called the GPS open-loop differential real-time receiver (GOLD-RTR). Its main and most innovative feature is its computation and storage, in real time, of complex-valued (I and Q) cross correlations (waveforms) between GPS L1-C/A signals - received directly and after reflection - and the corresponding models of these signals. Particularly, the GOLD-RTR schedules consecutive coherent integration time slots of 1 ms over which ten parallel correlation channels, with 64 lags each, work simultaneously and continuously with the input raw data sampled at 40 MHz. The total throughput is 10 000 waveforms per second, each waveform being 64 lags long. These real-time correlation resources can be flexibly distributed in several configurations according to the observational requirements, for instance: Doppler/delay maps or up to ten simultaneous reflected waveforms for ten different GPS satellites are examples of what can be done. The further processing of the real-time computed 1-ms waveforms in a flight campaign over the ocean, ice, or ground can be used to obtain geophysical parameters such as sea level and tides, sea surface mean-square slopes, ice roughness and thickness, soil moisture and biomass, or future applications. This paper covers the GOLD-RTR architecture and hardware, signal processing and data storage issues, machine-user interface, laboratory readiness tests, and waveform data samples from the first two jet aircraft campaigns at 9300 m over the sea

The impact of GPS zenith total delay (ZTD) measurements on mesoscale weather forecasts is studied. GPS observations from a permanent European network are assimilated into the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) using its three-dimensional variational data assimilation (3DVAR) system. The case study focuses on a snow storm that occurred during the period of 14–15 December 2001 over the western Mediterranean Sea. The experiments show that the most significant improvement in forecast is obtained when GPS ZTD data are assimilated together with local surface meteorological observations into the model within a cycling assimilation framework. In this case, the root-mean-square (rms) differences between forecasted and observed values are reduced by 1.7% in the wind component, 4.1% in the temperature variable, and 17.8% in the specific humidity field. This suggests the deployment of GPS receivers at surface stations to better initialize numerical weather prediction mo...

We present an inversion procedure for the extraction of atmospheric parameters from travel-time data of signals emitted by Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS). The procedure uses a tomographic approach, using raytracing to obtain the propagation trajectories of the electromagnetic waves. In order to allow the application of this procedure in systems processing large amounts of data, as in operational weather forecasting, we have paid attention to balance the computing time needed, the robustness of the algorithm and the accuracy of the result. We describe the inversion procedure used to extract the refractivity field. In cases cases where moderate amounts of data are available we have aimed to extract the vertical profile (i.e. we assumed that the refractivity depends only on height). We have compared this procedure with the Abel transform inversion approach, the baseline algorithm currently used for radio occultation observations, which requires the assumption that the observed field is spherically symmetric. The raytracing procedure, however, is in principle able to extract three-dimensional information (i.e. does not require the assumption that the horizontal gradients are zero), and can be used to resolve this horizontal structure from the data. However, although it is algorithmically possible, a full three-dimensional map of the field of refractive index requires massive amounts of data, which generally will not be available. In most practical operational cases a smaller, limited amount of data will be available, leading to a non-unique, and highly correlated, solution. If we used GNSS as the only source of data the result would be very poor, but in general more information is available. Instead of forcing some symmetry, we can opt to keep the correlation information and use it in the framework of an assimilation procedure. As can be seen, these are the ingredients needed to construct the “observation part” of the assimilation cost function. We can thus leave to the assimilation procedure the task of defining the symmetry properties of the refractivity field.

The MEditerranean Balloon EXperiment (MEBEX), conducted in August 99 from the middle–up stratosphere, was designed to assess the wind retrieval sensitivity of Global Navigation Satellite Systems Reflections (GNSSR) technology from high altitudes. Global Positioning System reflected signals (GPSR) collected at altitudes around 37 km with a dedicated receiver have been inverted to mean square slopes (MSS) of the sea surface and wind speeds. The theoretical tool to interpret the geophysical parameters was a bistatic model, which also depends on geometrical parameters. The results have been analyzed in terms of internal consistency, repeatability and geometry-dependent performance. In addition, wind velocities have been compared to independent measurements by QuikSCAT, TOPEX, ERS/RA and a Radio Sonde, with an agreement better than 2 m/s. A Numerical Weather Prediction Model (NWPM, the MM5 mesoscale forecast model) has also been used for comparison with varying results during the experiment. The conclusion of this study confirms the capability of high altitude GPSR/Delay-map receivers with low gain antennas to infer surface winds.

The processing of the Global Positioning System (GPS) observables is regularly performed using some “a priori” standard meteorological values. These can be very unrealistic if only few observations are available since the actual atmospheric conditions over the geodetic network are neglected. This seems to be the case of randomly moving GPS receivers far from continuously observing reference stations, as geodetic coordinates and tropospheric parameters cannot be accurately estimated. More realistic initial values for the troposphere may be obtained from a Numerical Weather Prediction (NWP) model. Thus, the impact of the ingestion of NWP forecasts into the GPS data processing is analyzed for short time series in terms of the geodetic vertical coordinate. The non-hydrostatic MM5 model with boundary and initial conditions given by the ECMWF analysis is used to compute estimates of the zenith wet and dry delays. These modeling simulations are then ingested into the GPS data analysis of the GIPSY software package. First, this approach is applied to GPS data gathered at the UK permanent stations of the COST-716 first benchmark campaign where it is possible to estimate a stable 24 h reference solution. Here, an improvement of about 60% in the vertical coordinate bias is found. Still, the absolute accuracy cannot expected to be better than 1 dm due to the remaining mismodeling of the troposphere. Then, the modeling approach is applied to a kinematic analysis of GPS buoy data. In this case also, a qualitative improvement is noticeable as demonstrated by a comparison with independent satellite radar altimeter data.

Modelled values of zenith wet delay (ZWD) from the non-hydrostatic numerical weather prediction (NWP) model MM5 are compared to estimated values retrieved from observations by geodetic very long baseline interferometry (VLBI), global positioning system (GPS) receivers, and water vapour radiometers (WVRs). In addition, sparse radiosonde (RS) data are used to augment the available data sets. The comparison is done for three stations of the European geodetic VLBI network for six observing sessions during the year 1999. The stations (Madrid, Onsala, and Wettzell) were primarily chosen to have the maximum number of collocated measuring techniques. In general, the time series for the different techniques show a good agreement. The correlation values between the techniques amount to 75–95%. The RMS differences of MM5 with respect to the other techniques obtain values of ±1.3–1.6 cm. The bias between MM5 and VLBI lies at about 1.0 cm, the bias between MM5 and GPS varies in the range of 0.0–0.6 cm and appears to be station dependent.

Reflectometry using Global Navigation Satellite System’s signals (GNSS-R) of opportunity was originally conceived in the early 1990 s for mesoscale altimetry, and since then, many studies have shown its applicability to other remote sensing applications such as sea state determination, soil moisture, vegetation, snow monitoring, etc. In December 2012, the Phase A studies of ESA’s PAssive Reflectometry and Interferometry System In-orbit Demonstration (PARIS IoD) mission concluded. In conventional GNSS-R (cGNSS-R), the satellite navigation signals scattered over the Earth’s surface are cross-correlated with a locally generated replica of the transmitted ones shifted in frequency ( ${\Deltab}{{\bf f}_{\bf d}}$ ), and in delay ( ${\Deltab \taub}$ ). However, in PARIS, a different technique called interferometric GNSS-R (iGNSS-R) is used, which allows the use of the whole signal’s bandwidth, and improve the altimetry precision, despite the large bandwidth signals’ codes being not publically available. This is achieved by using the direct signal collected by a directive antenna, instead of the locally generated replica. This study presents a methodology to optimize the configuration of a generic iGNSS-R altimeter, and evaluate its performance. The methodology presented is then particularized to a PARIS IoD-like case.

Data from the Turbo-Rogue GPS receiver onboard the Danish Orsted Satellite are used to derive satellite-to-satellite Total Electron Content (TEC). We present preliminary results obtaining Electronic Density fields from ionospheric tomography using TEC Orsted data. TEC is based on single frequency measurements, since only the L1 signal is of good quality. The C/A pseudo range (C1) and the L1 phase are used to obtain the TEC measurement. Electron density profiles from individual occultations are derived, based on the Abel transform, and compared with the tomographical solution. The assumption of spherical symmetry in the Abel transform limits the accuracy of the profiles and often results in a bias. We select occultations observed along the orbital plane to minimize the spherical assymetry effects. The tomographic grid is also confined to a narrow torus close to the orbital plane. The TEC calculations are based on data from 6 hour (3.6 orbits) periods. Two days have been selected for this study, December 5, 1999 and February 12, 1999.

To deliver tropospheric water vapor contents derived from GPS ground networks for the assimilation into numerical weather prediction models the GPS data have to be analyzed within near real-time with a delay of about one hour. Two near real-time processors for the trophospheric water vapor monitoring developed at IEEC Barcelona and GFZ Potsdam are compared using one week of data acquired in the German near real-time network. The resulting small differences in the integrated water vapor being on the level of 1–2 mm precipitable water vapor reflect that both processors fulfill the requirements for numerical weather predictions. Therefore the standardization for processors should not be too strict not to obstruct a fruitful competition in the software developments and improvements.

Tropospheric tomography using data from local networks of Global Positioning System (GPS) receivers is producing encouraging spatio-temporal representations of the wet refractivity field. In this work we present the results from a small-scale geodetic experiment that we carried out at the Onsala Space Observatory. Seven GPS receivers distributed within a radius of 3 km from the center, were deployed during 21 days in the summer 1998. The limited number of sites and their spatial configuration present a challenge for tropospheric tomography. Using novel GPS techniques to determine the vertical structure of the atmosphere, we observed, for one session, a strong horizontal water-vapor gradient with a leading edge at higher altitude than the trailing edge, entering from the north. The vertical structure obtained independently using tomographic techniques matched such situation. These results suggest tomography is a promising technique for the determination of the spatio-temporal structure of the atmosphere. We will present preliminary results of the tropospheric tomography, using simulations and experimental data, together with some comparisons with radiosonde data.

The availability of real-time tropospheric data is an important question for atmospheric parameters to be ingested in weather prediction models or in monitoring studies. In the frame of the EU funded project CLIMAP we have developed an on-line system that acquires GPS data from a number of IGS stations distributed around Europe providing hourly batches of GPS observables. An off-line system processing data with a one-day delay was run during one year to tune the characterisitics of the present on-line system. The hourly data are processed using GIPSY/OASIS II software, to obtain the Zenith Total Delay in a near-real-time basis. Every hour, the new batch is merged with the observations of the previous 23 hours to allow the system to process enough data to yield robust results. The question of what prediction for orbits to use has also been addressed, designing the system to always use the best orbits available from the Center for Orbit Determination in Europe, using the quality codes for the prediction of the orbits to remove “bad quality” satellites from the solution. To validate the results, the ZTD time series for each station have been compared against the solutions obtained through a Precise Point Positioning using final precise orbits and clock corrections. The estimation processing has been tuned to account for the particularities of a near-real-time scenario such as the satellite clock correction and the day boundary issue.

We describe two ways in which horizontal atmospheric structure affects GPS observations. For a single site, such structure results in azimuthal variations in the atmospheric propagation delay. For a network of sites, the structure will induce an intersite variability in the atmospheric propagation delays. The former effect is more sensitive to high- than low-altitude atmospheric gradients above the site, whereas the latter is insensitive to the altitude of the gradient above the network. This difference in sensitivity can be utilized to probe the local vertical structure of the atmosphere. We demonstrate this technique using GPS data from a small network on the Swedish west coast. We infer for one observing session the presence of a strong horizontal gradient which varies with height.

We have analyzed data from an experiment over the Madrid (Spain) area obtained from 5 GPS receivers and 3 Water Vapor Radiometers (WVR), in order to compare their retrievals of Tropospheric Slant Delays. For this purpose we have fitted a simple gradient model to both types of data, using a Kalman filter to account for the temporal variability of the zenith and gradient parameters. We show that the retrieved gradients with the two instruments are compatible, thus suggesting that the derived slant delays can be useful for tomographic analysis. We compare the estimates of ZWD obtained with the GIPSY and the Bernese software packages. Finally, we compare the estimated gradients with those obtained with HIRLAM, a Numerical Weather Prediction model.

We develop the formalism for a PIM-based functional for stochastic tomography with a Kalman filter, in which the inversion problem associated with four-dimensional ionospheric stochastic tomography is regularized. For consistency, GPS data is used to select dynamically the best PIM parameters, in a 3DVAR fashion. We demonstrate the ingestion of GPS (IGS and GPS/MET) data into a parameterized ionospheric model, used to select the set of parameters that minimize a suitable cost functional. The resulting PIM-fitted model is compared to direct 3D voxel tomography. We demonstrate the value of this method analyzing IGS and GPS/MET GPS data, and present our results in terms of a 4D model of the ionospheric electronic density., 8 pages, 2 figures

We compare TEC measurements from the NASA Radar Altimeter and DORIS instrument on board TOPEX/POSEIDON with GPS TEC estimates, and evaluate different GPS data analysis strategies. We verify that global tomographic GPS analysis using a voxel grid is well suited for ionospheric calibration of altimeters. We show that a 1-day fit of 20-second-averaged NRA ionospheric correction data versus GPS tomographic TEC data has a bias of 3.4 TECU and a root mean square deviation of 3.2 TECU. Tomographic inversion using simulated data from the Parametrized Ionospheric Model highlights the strong correlation between GPS bias constants, electronic densities at the highest layer, and unmodeled protonospheric TEC. This suggests that GPS TEC estimates at the TOPEX/POSEIDON altitude are more accurate if the bias constants are estimated and if a layer above TOPEX/POSEIDON is added to the grid.

The authors present results from analysis of GPS/MET occultation data obtained from the University Corporation for Atmospheric Research database. They have used high-rate occultation data to study the performance of an earlier 4D ionospheric electron content model-obtained tomographically with the use of the GPS constellation-with satisfactory results. In addition, they have processed UCAR level-one medium-rate occultation data by using the GIPSY package to obtain aligned phases. Under the assumption of local spherical symmetry, this phase information has been processed to yield ray-path bending angles through Doppler-shift analysis, which have then been used to yield profiles of electronic density via the Abel transform. These profiles give important information for preparing future tomographic work, although the limitations imposed by the working assumptions in this approach cannot be ignored.

The authors develop a minimization functional in order to regularize the inverse problem associated with three-dimensional (3D) ionospheric stochastic tomography. This functional is designed to yield, upon minimization, a solution which maximizes the frequency content of the solution below a certain cutoff, while keeping /spl chi//sup 2/ constant. The authors show how this functional can be rewritten in terms of the correlation function of the image, thereby facilitating the algorithmic implementation of the method. They then implement this functional in a Kalman filter and obtain a smoothing algorithm that acts in both space and time. Finally, they use this technique to perform global scale Global Positioning System (GPS) tomography of the ionospheric electron content.

The Spanish participation as an Associate Member in the Millimeter Array (MMA) project - led by the U.S. National Radio Astronomy Observatory (NRAO) - has been recently proposed to the Spanish Science and Technology Bureau (OCYT). This request to the Spanish Government is based on two key factors: (1) The high scientific interest and support of the Spanish scientific community, for whom major new fields of research--inaccessible with other instruments--will become available. (2) An industrial return that will clearly benefit Spanish companies developing cutting-edge technology and providing them with a high international profile.

This work describes a novel dual-band Global Navigation Satellite Systems Reflectometer (GNSS-R) that uses the P(Y) and C/A signals scattered over the sea surface to perform highly precise altimetric measurements. The results derived from two different ground-based field experiments over a dam and over the sea under different surface's roughness conditions are presented. The analysis of the altimetric performance shows that the results obtained using the P(Y) code improve by a factor between 1.4 and 2.4 as compared to the results obtained using the C/A code, respectively, for high and mid-low satellite's elevation angles.

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.

We combine GPS/MET data from 29 occultations and IGS ground data collected from 160 stations around the world to perform stochastic tomography of the ionosphere with a 4×20×20 global grid of voxels extending from 200 to 650 km above the mean surface of the Earth. A correlation functional approach that limits the spatial high frequency content of the images is used, and a Kalman filter is applied in the time direction. The combination of ground and occultation data and the use of smoothing techniques is robust enough for vertical resolution in this four-layer model analysis. We discuss the role of noise on the choice of the correct range of eigenvalues in the inversion problem, and the impact of occultation data, showing that ground data alone is insufficient for vertical resolution even in a three-layer, noise-less simulation.

We present a new approach to model the Ionosphere based on GPS data. Previous authors have used models with an unique shell. In this case we have included a second shell to account for the distribution of the electrons in the outer part of the Ionosphere. We have analyzed the ionospheric electron content of a region above 30 degrees in declination in different conditions of ionospheric activity using the Kalman filter. The data used has been obtained from the International GPS Service for Geodynamics (IGS) network. Simultaneously we have studied the receiver and transmitter differential biases showing the effects of neglecting the outer part of the Ionosphere in the model. It appears a systematic variations for the receivers—depending on its latitude—not for the satellites.

The Global Positioning System (GPS) was first conceived for positioning and navigation purposes, but it has also been used for Earth observation. Later, it has also been used for Earth observation as a bistatic radar for altimetry and scatterometric applications. At present, the ultimate influence of the different Global Navigation Satellite System Reflectometry (GNSS-R) parameters in the precision and accuracy of the altimetric products is still being analyzing. The impact of the different error sources as well as the expected theoretical height precision and the correction of the different bias terms must be compared with experimental results obtained in a real scenario. The results of a ground-based field experiment performed to test the precision and the accuracy of a new P(Y) & C/A ReflectOmeter (PYCARO) are presented. The experiment demonstrates a 1 cm precision from a fixed location in a reservoir.

We report preliminary results of an aircraft experiment aimed to proof the PARIS Interferometric Technique. The experiment was performed in the Gulf of Finland during a two hours flight.We installed a PARIS Interferometric Receiver together with a GOLD-RTR instrument to collect reflected C/A, P(Y) and M-code GPS signals. The collected data has been analyzed to produce altimetric observables with both techniques.

The ionospheric effect is one of the main sources of error in Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS) high precision geodesy. Although the use of two frequencies allows the estimation of this effect, in some cases dual observations are not possible due to the available equipment or the type of observation. This paper presents the ionospheric calibration of single frequency VLBI and GPS observations based on the ionospheric electron content estimated from dual frequency GPS data. The ionospheric delays obtained with this procedure and the VLBI baseline length results have been compared with those obtained with dual frequency data. For the European geodetic VLBI baselines, both solutions agree at the 3–5 parts in 10−9 level. The noise introduced by the GPS-based calibration is in the order of 3 cm for the VLBI observables and of 10 cm for the GPS observables.

It can be concluded that an upper bound has been found for the SNR of the signals at the PARIS-IOD if one-bit quantization is used and a linear cross-correlation with the same shape as the analog correlation is desired. That is, it is not possible to increase the signal SNR at any desired level without distorting the cross-correlation waveform if one-bit digitalization is used. Simulation results have been presented confirming the upper bound for the SNR of the signals.

Reflectometry using Global Navigation Satellite System's (GNSS) opportunity signals was originally conceived for altimetry [1], but it has been demonstrated today that it can be applied to sea state determination, soil moisture, vegetation, snow monitoring... So far, GPS signals scattered over the Earth's surface have been cross-correlated with a local replica of the transmitted signal (typically the only GPS available: the C/A code) shifted in frequency (Δfd) and in delay (τ). However, to achieve the full potential (bandwidth) of these signals the direct and reflected signals can be directly cross-correlated, but in this case, high gain antennas are required to increase the signal-to-noise ratio. Therefore these antennas must be steerable so as to track the direct satellite and the specular reflection point. This is the concept of the European Space Agency PARIS In Orbit Demonstrator (IoD) mission [2], a new concept that was demonstrated experimentally for the first time in 2010 [3]. This work deals with the analysis of the preliminary instrument error budget of this mission to address the critical design parameters.

The "Cooperative Airborne Radiometer for Ocean and Land Studies" (CAROLS) L-Band radiometer was designed and built as a copy of the EMIRAD II radiometer constructed by the Technical University of Denmark team. It is a fully polarimetric and direct sampling correlation radiometer. It is installed on board a dedicated French ATR42 research aircraft, in conjunction with other airborne instruments (C-Band scatterometer--STORM, the GOLD-RTR GPS system, the infrared CIMEL radiometer and a visible wavelength camera). Following initial laboratory qualifications, three airborne campaigns involving 21 flights were carried out over South West France, the Valencia site and the Bay of Biscay (Atlantic Ocean) in 2007, 2008 and 2009, in coordination with in situ field campaigns. In order to validate the CAROLS data, various aircraft flight patterns and maneuvers were implemented, including straight horizontal flights, circular flights, wing and nose wags over the ocean. Analysis of the first two campaigns in 2007 and 2008 leads us to improve the CAROLS radiometer regarding isolation between channels and filter bandwidth. After implementation of these improvements, results show that the instrument is conforming to specification and is a useful tool for Soil Moisture and Ocean Salinity (SMOS) satellite validation as well as for specific studies on surface soil moisture or ocean salinity.

© 2010 IEEE. Reprinted, with permission, from Nogués, O., Ribó, S., Arco, J. C., Cardellach, E., Rius, A., València, E., A. Camps, van der Marel, H., Martín-Neira, M., The proof of concept for 3-cm altimetry using the PARIS interferometric technique, Proceedings of IEEE International Geoscience and Remote Sensing Symposium (IEEE IGARSS), and july/2010. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of CSIC products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions@ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.

© 2010 IEEE. Reprinted, with permission, from Cardellach, E., Rius, A., Cerezo, F., Garcia-Primo, M.A., de la Torre-Juarez, M., Cucurull, L., Ector, D., Polarimetric GNSS Radio-Occultations for heavy rain detection, Proceedings of IEEE International Geoscience and Remote Sensing Symposium (IEEE IGARSS), and july/2010. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of CSIC products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions@ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it., A Global Navigation Satellite System (GNSS) Radio-Occultation (RO) experiment is being accommodated in the Spanish Low Earth Orbiter (LEO) for Earth Observation PAZ. The RO-payload will provide globally distributed vertical thermodynamic profiles of the Atmosphere in Near Real-Time (NRT) suitable to be assimilated into Weather Numerical Prediction Models. The NRT operability will be dispensed by the Ground Segment services of the U.S.A. National Oceanographic and Atmospheric Administration (NOAA). Moreover, the mission will run, for the first time from Space, a double-polarization GNSS experiment, to assess the capabilities of polarimetric GNSS-RO for detection of heavy rain events.

Global navigation satellite system reflectometry (GNSS-R) remote sensing is a new remote sensing technique of satellite navigation application. Essentially, it entails a method of remote sensing that receives and processes microwave signals reflected from various surfaces. The GPS open-loop differential real-time receiver (GOLD-RTR) instrument have been proposed as GNSS-R instrument, to gather global positioning satellite system signals after they have been reflected from suitable surfaces (e.g. sea, ice and ground); and extract useful information about those surfaces. However to fully benefit from real-time characteristic, the overhead between the stringent real-time parallel processing and the storage of amount of multi-channel cross-correlation waveforms(CC-WAVs) prior to downlink issues have been addressed. Over last years, several embedded solutions for parallel processing are ready available: Symmetric Multiprocessing (SMP), Network-On-Chip (NOC). Indeed, higher performance is achieved, but bus congestion and memory allocation issues have increased with these new embedded solutions. This paper presents a novel architecture, namely, the Heterogeneous Transmission and Parallel Computing Platform (HTPCP); And the mandates imposed by the stringent timing constraints by a Dual-Port RAM (DPRAM). The pros and cons of different approaches are discussed, range from providing parallel computing to analyzing bus busy ratio and memory access time. The numerical results show that HTPCP reaches a speed-up of 8.17x in comparison to the SMP architecture, which enables the highest throughput in the real-time system design of GNSS-R.

© 2010 IEEE. Reprinted, with permission, from Fabra , F., Cardellach, E., Nogués, O., Oliveras, S., Ribó, S., Rius, A., Belmonte, M., Semmling, M., Macelloni, G., Pettinato, S., Zasso, R., D Addio, S., Monitoring sea ice and dry snow with GNSS reflections, Proceedings of IEEE International Geoscience and Remote Sensing Symposium (IEEE IGARSS), and july/2010. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of CSIC products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions@ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it., GPS reflected signals have become a source of opportunity for remote sensing of the Earth's suface. In this work, we present several capabilities of this technique in two different polar environments: Greenland and Antarctica. The first part is dedicated to the retrieval of sea-ice properties, giving emphasis to the study of the coherent phase for altimetric and roughness estimations, and polarimetric measurements for the determination of the ice salinity variation. The results show good agreement with a tide model and daily ice charts. On the second part, some preliminary results and analysis strategies to retrieve dry snow signatures are presented.

In the last three years, the European Geodetic Very Long Baseline Interferometry (VLBI) Network has grown to a total of six fixed antennas placed in Germany, Italy, Spain and Sweden, all equipped with the standard geodetic VLBI instrumentation and data recording systems. During this period of time, several experiments have been carried out using this interferometer providing data of very high quality due to the excellent sensitivity and performance of the European stations. The purpose of this paper is to study the consistency of the VLBI geodetic results on the European baselines with respect to the different degrees of freedom in the analysis procedure. Used to complete this study were both real and simulated data sets, two different software packages (OCCAM 3.0 and CALC 7.4/SOLVE), and a variety of data analysis strategies.