The major objective of the GRACE Follow On (GFO) mission with its two satellites GF1 and GF2 is to obtain precise global and high-resolution models for both the static and the time variable components of the Earth's gravity field. Additional goal is the continuation of the GPS radio occultation (RO) measurements from the predecessor GRACE, successfully performed between 2006 and 2017. The GRACE/GRACE FO data contribute to the global RO dataset consisting of several missions provided by different centres since the pioneering GPS/MET mission in 1995/97. Beside climate applications, GFO data are used for the assimilation in numerical weather forecast models by the leading weather service centres. After several on-board software updates and raw data reader improvements since March 2020 rising occultations from GF1 and since September 2021 setting occultations from GF2 are continuously available. Both satellites provide about 400 atmospheric profiles daily. The RO data are processed based on different measured observables: For different GPS satellites combinations of L1CA/L2P, L1CA/L2C, or L1CA/L5 amplitude and phase measurements are available. In this study results of GFO processing and validation are presented. Bending angle, refractivity, and temperature data are compared with ECMWF operational analyses. The quality of the different measured variables is evaluated for different geographical regions. In addition, GFO data are compared with co-located COSMIC-2 and Spire radio occultations.

We have performed an analysis of reprocessed GPS/MET data spanning 1995–1997 generated by the COSMIC Data Analysis and Archive Center (CDAAC) in 2007. CDAAC developed modified dual-frequency processing methods for the encrypted data (anti-spoofing (AS)-on) during 1995–1997. We compared the CDAAC data set to the Modern-Era Retrospective Analysis for Research and Applications-2 (MERRA-2) reanalysis, separately for AS-on and AS-off, focusing on the altitude range 10–30 km. MERRA-2 did not assimilate GPS/MET data in the period 1995–1997. To gain insight into the CDAAC data set, we developed a single-frequency data set for GPS/MET, which is unaffected by the presence of encryption. We find excellent agreement between the more limited single-frequency data set and the CDAAC data set: the bias between these two data sets is consistently less than 0.25 % in refractivity, whether or not AS is on. Given the different techniques applied between the CDAAC and the new data set presented here (designated JPL), agreement suggests that the CDAAC AS-on processing and the single-frequency processing are not biased in an aggregate sense greater than 0.25 % in refractivity, which corresponds approximately to a temperature bias less than 0.5 K. Since the profiles contained in the new single-frequency data set are not a subset of the CDAAC profiles, the combination of the CDAAC data set, consisting of 9579 profiles, and the new single-frequency data set, consisting of 4729 profiles, yields a total number of 11 531 unique profiles from combining the JPL and CDAAC data sets. All numbers are after quality control has been applied by the respective processing activities.

This paper presents the calibration and validation studies for the Radio Occultation and Heavy Precipitation experiment aboard the PAZ satellite. These studies, necessary to assess and characterize the noise level and robustness of the differential phase shift (ΔΦ) observable of polarimetric radio occultations (PROs), confirm the good performance of the experiment and the capability of this technique in sensing precipitation. It is shown how all the predicted effects that could have an impact into the PRO observables (e.g., effect of metallic structures nearby the antenna, the Faraday rotation at the ionosphere, signal impurities in the transmission, and altered cross-polarization isolation) are effectively calibrated and corrected, and they have a negligible effect on the final observable. The on-orbit calibration, performed using an extensive dataset of free-of-rain and low-ionospheric activity observations, is successfully used to correct all the collected observations, which are further validated against independent precipitation observations confirming the sensitivity of the observables to the presence of hydrometeors. The validation results also show how vertically averaged ΔΦ can be used as a proxy for precipitation.

An objective of the GRACE-FO mission is the continuation of GRACE radio occultation measurements successfully performed between 2006 and 2017. GRACE and GRACE-FO radio occultations contribute to the overall radio occultation dataset used in weather and climate applications. Since mid-2019 rising occultations from GF1 are available while setting radio occultations from GF2 are still disabled. After several on-board software updates and raw data reader improvements about 280 daily GF1 radio occultations are available since March 2020. Currently GF1 radio occultation data are processed on the basis of different measured variables: For different GPS satellites a combination of L1CA/L2P, L1CA/L2C, or L1CA/L5 is available. In this study first results of GF1 processing are presented. Refractivity and temperature data up to an altitude of 60 km will be compared with ECMWF operational analyses and the quality of the different measured variables will be evaluated.

A low cost, low power, and low mass GNSS receiver (called Cion) has been developed and is currently flying on the CICERO cubesats. The receiver was designed in less than a year by JPL for Tyvak and GeoOptics for use in the GeoOptics CICERO constellation and leverages 25 years of JPL GNSS reciever design experience. Cion uses a commercial off-the-shelf (COTS) computer along with existing space qualified RF down-converters, software, and firmware to produce atmospheric Radio Occultation (RO) data. By combining a FPGA with dual core ARM processor and an embedded system controller, the Xilinx Zynq processor is an enabling technology that provides a customizable digital signal processing platform integrated into the computer (System on a chip) and enables off-the-shelf hardware to become the main engine behind this software defined radio. Using Linux for the on-board computer allows for fast development times and liberal use of existing open source software libraries. The parts of the receiver that require real-time implementation are performed in the Field Programmable Gate Array (FPGA), which can also be reprogrammed in flight. While the Zynq is not rad hard, the silicon on insulator (SOI) technology is rad tolerant 'by accident', allowing for its use in many space-based applications. Early results show that the Cion is working as designed, has demonstrated the first known GLONASS occultations, and obtains high quality atmospheric profiles with excellent lower troposphere penetration (near Earth's surface).

We construct a 9-year data record (2007-2015) of the tropospheric specific humidity using Global Positioning System radio occultation (GPS RO) observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission. This record covers the +/- 40 degrees latitude belt and includes estimates of the zonally averaged monthly mean specific humidity from 700 up to 400 hPa. It includes three major climate zones: (a) the deep tropics (+/- 15 degrees), (b) the trade winds belts (+/- 15-30 degrees), and (c) the subtropics (+/- 30-40 degrees). We find that the RO observations agree very well with the European Centre for Medium-Range Weather Forecasts Re-Analysis Interim (ERA-Interim), the Modern-Era Retrospective Analysis for Research and Applications (MERRA), and the Atmospheric Infrared Sounder (AIRS) by capturing similar magnitudes and patterns of variability in the monthly zonal mean specific humidity and interannual anomaly over annual and interannual timescales. The JPL and UCAR specific humidity climatologies differ by less than 15% (depending on location and pressure level), primarily due to differences in the retrieved refractivity. In the middle-to-upper troposphere, in all climate zones, JPL is the wettest of all data sets, AIRS is the driest of all data sets, and UCAR, ERA-Interim, and MERRA are in very good agreement, lying between the JPL and AIRS climatologies. In the lower-to-middle troposphere, we present a complex behavior of discrepancies, and we speculate that this might be due to convection and entrainment. Conclusively, the RO observations could potentially be used as a climate variable, but more thorough analysis is required to assess the structural uncertainty between centers and its origin.

This paper presents a characterization of L-band ionospheric scintillation observed at a single site in the Arctic auroral zone, and an analysis of GPS-based point positioning error caused by carrier phase data degradation during scintillation. Observed amplitude and phase scintillations of L1CA, L2C, and L5 signals during auroral electrojet activities driven by space weather disturbances show the following three types: continuous, intermittent, and spike (sudden intensity drop). The relations of scintillation strength between different signals are derived for carrier phase, signal intensity, and code-carrier divergence fluctuations, showing precise linear relation, de-correlation, and different linear fitting slope, respectively. These relations can help predict phase scintillation behavior of one signal from measurements of another, except for amplitude scintillation. Experiments of precise point positioning show that phase data degradation due to scintillation can cause significant positioning error increase if the scintillation effects are not considered carefully in GNSS data processing and design of positioning approaches. Copyright © 2017 Institute of Navigation

The University of Southern California (USC) and the Jet Propulsion Laboratory (JPL) have jointly developed the Global Assimilative Ionospheric Model (GAIM) to monitor space weather, study storm effects, and provide ionospheric calibration for space weather applications. JPL/USC GAIM is a physics‐based 3‐D data assimilation model that uses both four‐dimensional variational analysis and Kalman‐filter techniques to solve for the ion and electron density state and key drivers such as equatorial electrodynamics, neutral winds, and production terms. Here we report on GAIM Kalman filter‐based assimilation results using ground‐based GPS and COSMIC‐derived total electron count (TEC) measurements. We find that assimilating COSMIC measurements into GAIM improves critical ionospheric parameters such as NmF2 and HmF2. Assimilating COSMIC data produces higher‐accuracy vertical electron density profile “shapes,” as verified by comparisons to independent electron density profiles measured at Arecibo, Jicamarca, and Millstone Hill incoherent scatter radar (ISR). We also find significant improvement in global vertical TEC (VTEC) maps when assimilating COSMIC measurements, verified by comparing GAIM output with VTEC measurements from the Jason ocean altimeter. For 3 days in June 2006, improvement in accuracy compared to ground‐data‐only assimilation is found to be 30%, 28%, and 44%, respectively.

In this paper, we study the local time response of the low-latitude ionosphere during the sequence of geomagnetic storms occurring on 7, 9, and 10 November 2004. Three distinct storm main phases leading to peak negative Dst index of at least -250 nT occur during this period. We analyze total electron content (TEC) measurements from the global network of GPS receivers available from the International Global Navigation Satellite Systems (GNSS) Service. Average TEC within the geomagnetic latitude band +/- 25 degrees is computed as a function of local time for several hours following storm commencement. We find that significant TEC increases are observed on 7 November (increase of 100%) and 9 November (50%). During 7 November, the increases tend to occur during solar local times 1200-1600 LT. During 9 November, the largest TEC increases tend to occur at earlier local times (similar to 1000-1200 LT). No daytime TEC increase is observed on 10 November, although the K-p index suggests that this storm is comparable in intensity to the 7 November storm. Vertical drift measurements from the Jicamarca radar (Peruvian sector) for the 7 and 9 November storm periods suggest large (2-3 mV/m) low-latitude "prompt penetration" eastward electric fields (PPEF), consistent with observed increases in TEC. For the 10 November storm, published estimates of electric fields using the dual-magnetometer technique near the Japanese sector, which is near similar to 1400 LT at storm onset, suggest the absence of eastward directed electric fields during daytime. A time series analysis of TEC in the Japanese sector throughout the 7-11 November period reveals significant daytime TEC reduction of 33% relative to quiet conditions. Reduced TEC occurs coincident with the onset of the 10 November storm period, persisting into the 11th. Global Ultraviolet Imager (GUVI) retrievals of thermospheric atomic oxygen to nitrogen ratio suggest depleted oxygen in the southern hemisphere that may contribute to the TEC reduction on 10 November. Solar rotation reduces solar X-ray and EUV fluxes from 10 through 12 November, which may be a contributing factor to reduced TEC on 10 and 11 November. In conclusion, we postulate that an additional source of electric field is present on 10 November during daytime that counteracts the PPEF of magnetospheric origin.

The four-dimensional Global Assimilative Ionospheric Model (GAIM) is applied to a study of ionospheric disturbances. The investigation is focused on disturbance features, particularly in the altitude and latitude dimensions, at low latitudes during a geomagnetic storm on 7 August 2006, under solar minimum conditions. The modeling of storm-time ionospheric state (electron density) is conducted by assimilating an unprecedented volume of line-of-sight TEC data collected by the Global Positioning System (GPS) occultation receivers on board six FORMOSAT-3/COSMIC satellites and geodetic-quality GPS receivers at two hundred globally-distributed ground tracking stations. With a band-limited Kalman filter technique to update the ionospheric state, the assimilative modeling reveals a pronounced enhancement in the equatorial anomaly in the East Asia sector during dusk and evening hours. The disturbance characteristics, obtained by comparing with the quiet conditions prior to the storm also modeled in this study through data assimilation, include lifted F layer and reduced electron density in the equatorial region, enhanced density at the magnetically conjugate anomaly latitudes, and tilted feature of density increase towards higher altitudes at lower latitudes. The characteristics are attributed to the enhanced plasma fountain effect driven by an enhanced eastward zonal electric field. These results enable us to distinguish the storm-time electric field perturbations clearly from other sources during the storm. The possible origins of electric field perturbations are also discussed, including penetration of the magnetospheric electric field and wind dynamo disturbances.

To examine the suitability of GPS radio occultation (RO) observations as a climate benchmark data set, this study aims at quantifying the structural uncertainty in GPS RO‐derived vertical profiles of refractivity and measured refractivity trends obtained from atmospheric excess phase processing and inversion procedures. Five years (2002–2006) of monthly mean climatologies (MMC) of retrieved refractivity from the experiment aboard the German satellite CHAMP generated by four RO operational centers were compared. Results show that the absolute values of fractional refractivity anomalies among the centers are, in general, ≤0.2% from 8 to 25 km altitude. The median absolute deviations among the centers are less than 0.2% globally. Because the differences in fractional refractivity produced by the four centers are, in general, unchanging with time, the uncertainty of the trend for fractional refractivity anomalies among centers is ±0.04% per 5 years globally. The primary cause of the trend uncertainty is due to different quality control methods used by the four centers, which yield different sampling errors for different centers. We used the National Centers for Environmental Prediction reanalysis in the same period to estimate sampling errors. After removing the sampling errors, the uncertainty of the trend for fractional refractivity anomalies among centers is between −0.03 and 0.01% per 5 years. Thus 0.03% per 5 years can be considered an upper bound in the processing scheme–induced uncertainty for global refractivity trend monitoring. Systematic errors common to all centers are not discussed in this article but are generally believed to be small.

Each of the six COSMIC satellites carries one GPS receiver with two antennas for orbit determination (OD) and another two antennas for radio occultation. In this paper we describe the features of the GPS data associated with COSMIC satellite attitude. Method and results of the precise orbit determination for the COSMIC satellites using the tracking data from both OD GPS antennas are presented. Special issues in satellite orbit determination with multiple GPS antennas are discussed. Orbit precision is evaluated and the effects of multipath errors are investigated.

Radio occultation measurements of the atmosphere using transmissions of the Global Positioning System (GPS) are discussed in the Decadal Survey for Earth Science released in 2007. Several successful examples of RO missions are currently in orbit: CHAMP, SAC-C, COSMIC and GRACE. RO retrievals have the fortunate characteristic of being based on time delay measurements, whose fundamental unitiquestthe secondiquestis absolutely calibrated using atomic clocks. Due to the absence of bias or long-term drift, multi-decadal time series of GPS RO retrievals are natural to develop for climate monitoring of atmospheric properties from the troposphere to the stratosphere. Highly accurate temperature profiles with high vertical resolution (50 m-200 m) are retrieved from the stratosphere to the mid-troposphere. Water vapor profiles are available from approximately 5 km altitude to the surface. The technique has sufficient vertical resolution to resolve the planetary boundary layer over much of the globe. Understanding the spatial sampling properties of GPS RO is important when bringing these data into the broader Earth observing context. In this paper, we discuss the unique relationship between vertical and horizontal resolution and describe the technology development needed to achieve maximum benefit for climate and weather applications.