On the accuracy of integrated water vapor observations and the potential for mitigating electromagnetic path delay error in InSAR

Abstract. A field campaign was carried out in the framework of the Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water Vapour Effects (METAWAVE) project sponsored by the European Space Agency (ESA) to investigate the accuracy of currently available sources of atmospheric columnar integrated water vapor measurements. The METAWAVE campaign took place in Rome, Italy, for the 2-week period from 19 September to 4 October 2008. The collected dataset includes observations from ground-based microwave radiometers and Global Positioning System (GPS) receivers, from meteorological numerical model analysis and predictions, from balloon-borne in-situ radiosoundings, as well as from spaceborne infrared radiometers. These different sources of integrated water vapor (IWV) observations have been analyzed and compared to quantify the accuracy and investigate the potential for mitigating IWV-related electromagnetic path delay errors in Interferometric Synthetic Aperture Radar (InSAR) imaging. The results, which include a triple collocation analysis accounting for errors inherently present in every IWV measurements, are valid not only to InSAR but also to any other application involving water vapor sensing. The present analysis concludes that the requirements for mitigating the effects of turbulent water vapor component into InSAR are significantly higher than the accuracy of the instruments analyzed here. Nonetheless, information on the IWV vertical stratification from satellite observations, numerical models, and GPS receivers may provide valuable aid to suppress the long spatial wavelength (>20 km) component of the atmospheric delay, and thus significantly improve the performances of InSAR phase unwrapping techniques

Long-term monitoring of geodynamic surface deformation using SAR interferometry

Thesis (Ph.D.) University of Alaska Fairbanks, 2014Synthetic Aperture Radar Interferometry (InSAR) is a powerful tool to measure surface deformation and is well suited for surveying active volcanoes using historical and existing satellites. However, the value and applicability of InSAR for geodynamic monitoring problems is limited by the influence of temporal decorrelation and electromagnetic path delay variations in the atmosphere, both of which reduce the sensitivity and accuracy of the technique. The aim of this PhD thesis research is: how to optimize the quantity and quality of deformation signals extracted from InSAR stacks that contain only a low number of images in order to facilitate volcano monitoring and the study of their geophysical signatures. In particular, the focus is on methods of mitigating atmospheric artifacts in interferograms by combining time-series InSAR techniques and external atmospheric delay maps derived by Numerical Weather Prediction (NWP) models. In the first chapter of the thesis, the potential of the NWP Weather Research & Forecasting (WRF) model for InSAR data correction has been studied extensively. Forecasted atmospheric delays derived from operational High Resolution Rapid Refresh for the Alaska region (HRRRAK) products have been compared to radiosonding measurements in the first chapter. The result suggests that the HRRR-AK operational products are a good data source for correcting atmospheric delays in spaceborne geodetic radar observations, if the geophysical signal to be observed is larger than 20 mm. In the second chapter, an advanced method for integrating NWP products into the time series InSAR workflow is developed. The efficiency of the algorithm is tested via simulated data experiments, which demonstrate the method outperforms other more conventional methods. In Chapter 3, a geophysical case study is performed by applying the developed algorithm to the active volcanoes of Unimak Island Alaska (Westdahl, Fisher and Shishaldin) for long term volcano deformation monitoring. The volcano source location at Westdahl is determined to be approx. 7 km below sea level and approx. 3.5 km north of the Westdahl peak. This study demonstrates that Fisher caldera has had continuous subsidence over more than 10 years and there is no evident deformation signal around Shishaldin peak.Chapter 1. Performance of the High Resolution Atmospheric Model HRRR-AK for Correcting Geodetic Observations from Spaceborne Radars -- Chapter 2. Robust atmospheric filtering of InSAR data based on numerical weather prediction models -- Chapter 3. Subtle motion long term monitoring of Unimak Island from 2003 to 2010 by advanced time series SAR interferometry -- Chapter 4. Conclusion and future work

Wet Path Delay Corrections from Line-of-Sight Observations of Effelsberg’s Water Vapour Radiometer for Geodetic VLBI Sessions

Water vapour induced excess path lengths in electromagnetic waves have been one of the most unmanageable errors in space geodesy, such as GPS and VLBI. The difficulty mainly comes from the highly variable distribution of atmospheric water vapour both in time and space. In general, these wet path delays cannot be estimated accurately by atmospheric models that are conventionally used in space geodetic applications. In the last few decades, water vapour radiometry has shown great potential for measuring atmospheric water vapour content. However, the wet path delay retrieval processes are strongly dependent on radiosonde data, although periodic radiosonde observations are rarely available in the vicinity of water vapour radiometers (WVRs). Radiosonde observations are weather profiles from balloon starts which are transmitted by radio signals. On the other hand, the possibility of using a numerical weather model (NWM) instead of a radiosonde has been on the increase in recent years. NWM can provide meteorological profiles for those places where radiosonde data is not available. The focus of this thesis is mainly on the improvement of the wet path delay corrections in geodetic VLBI sessions using the WVR observations at the 100m Effelsberg radio telescope. Compared to other WVRs, the Effelsberg one has a great advantage in terms of observation. It always points at the same direction as the VLBI antenna because it has been installed on the prime focus cabin of the telescope. However the Effelsberg station does not make periodic radiosonde observations. To overcome this weakness, the numerical weather model of the European Centre of Medium Range Weather Forecasts (ECMWF) was introduced. It provides meteorological profiles over Effelsberg such as atmospheric pressures, temperatures, and water vapour pressures. Those profiles were processed by a radiative transfer model, which calculates theoretical measurements of brightness temperature and converts them into wet path delays. These two models were combined to be compared with WVRobserved wet path delays. For a better comparison between wet path delays from the WVR and the models, zenith wet delays (ZWDs) were used. As the results of the comparison illustrate, ZWDs from the models showed higher values than the WVR-measured ones by roughly 30 mm. For comparison with GPS-derived values, average offsets and standard deviations of the models and the WVR were -4.3±11.0 mm and -44.8±24.0 mm, respectively. From these ZWD comparisons it was found that further corrections to the WVR ZWDs are necessary. In addition, the noisy behaviour of the raw WVR ZWD measurements should be smoothed by a running mean method before application. In addition, averaged offsets between the models and the WVR measurements should be determined for the correction of individual sessions. However, already at this step it became obvious that the instrumental calibrations of the radiometer are far from being mature resulting in erroneous absorption profiles. ZWDs from the WVR measurements with different levels of corrections were applied as corrections to the wet components of the atmospheric refraction in the five geodetic VLBI sessions. Impacts on baseline repeatability and height precision by these were investigated. As the results show, the baseline repeatability was improved in terms of Root Mean Squared Error (RMS) when the offset correction was applied. However, the improvement was less than one percent. Although the repeatability of the height component was improved in terms of Weighted RMS (WRMS) with respect to the short term mean height by a factor of 2, the height component itself showed a larger deviation from the original value than that expected from the ZWD corrections. A possible reason is that the estimation of the many parameters in the least squares adjustment can easily affected the height parameter. The conclusion of this study is that the Effelsberg WVR observations are not perfectly suited for wet path delay corrections yet. This is mainly due to the imperfectness of instrumental calibration. Further studies based on an increased number of WVR data with better internal calibrations seems to be necessary to make a final judgment regarding the usefulness of the WVR for wet path delay corrections in geodetic VLBI.Zur Korrektur von feuchtebedingten Laufzeitverzögerungen mit dem co-linearen Wasserdampfradiometer in Effelsberg für geodätische VLBI-Messungen Wasserdampfinduzierte Refraktionseffekte der elektromagnetischen Wellen stellen die zurzeit größte Fehlerquelle bei Messverfahren der Satellitengeodäsie, wie z.B. GPS und VLBI, dar. Die Problematik rührt hauptsächlich her von der stark variierenden Verteilung von atmosphärischem Wasserdampf sowohl in der Zeit als auch im Raum. Im Allgemeinen können diese Laufzeitverzögerungen durch den feuchten Anteil der Atmosphäre nicht exakt genug durch atmosphärische Modelle berechnet werden, die herkömmlich in Satellitengeodäsieanwendungen genutzt werden. In den vergangenen Jahrzehnten hat die Wasserdampfradiometrie ein großes Potential entwickelt, um den atmosphärischen Wasserdampfbestandteil zu messen. Allerdings ist der Prozess der Umrechnung von gemessenen Helligkeitstemperaturen in Laufzeitverzögerungen stark von gleichzeitig durchgeführten Radiosondenmessungen abhängig. Dabei werden die Messergebnisse von an aufsteigenden Ballons befestigten Wettersensoren für verschiedene Druckstufen per Radiosignal ausgesendet. Leider werden periodische Radiosondenbeobachtungen aber nur selten in der Nähe des Wasserdampfradiometers (WVR) durchgeführt. Dem gegenüber besteht seit einigen Jahren die Möglichkeit, ein numerisches Wettermodell anstelle der Radiosondenergebnisse zu nutzen. Ein numerisches Wettermodell kann meteorologische Profile für solche Orte liefern, wo eine Radiosonde nicht verfügbar ist. Der Schwerpunkt dieser Dissertation liegt hauptsächlich auf der verbesserten Bestimmung der Laufzeitverzögerungen durch den feuchten Anteil der Atmosphäre in der geodätischen VLBI, wobei die Wasserdampfradiometerbeobachtungen am Radioteleskop in Effelsberg genutzt werden. Verglichen mit anderen Wasserdampfradiometern hat dieses Instrument große Vorteile hinsichtlich der Messwertgewinnung. Es zeigt immer in dieselbe Richtung wie die VLBI-Antenne, weil es im Primärfokus des Teleskopes installiert ist. In oder in der Nähe von Effelsberg werden jedoch keine Radiosondenbeobachtungen durchgeführt. Um diese Schwäche zu beheben, wurde ein numerisches Wettermodell des European Centre for Medium Range Weather Forecasts (ECMWF) für die Bestimmung von Kalibrierwerten herangezogen. Es liefert für das Radioteleskop in Effelsberg meteorologische Daten wie z.B. Druck, Temperatur und Wasserdampfdruck. Solche Profile wurden in einem Strahlungsübertragungsmodell verarbeitet, welches theoretische Messungen der Helligkeitstemperatur ermittelt und diese in Laufzeitverzögerungen durch den feuchten Anteil der Atmosphäre umwandelt. Um die Laufzeitverzögerungen durch den feuchten Anteil der Atmosphäre aus Wasserdampfradiometermessungen und die Modelle besser vergleichen zu können, wurden alle Laufzeitverzögerungen durch den feuchten Anteil der Atmosphäre auf die Zenitrichtung (Zenith Wet Delays, ZWD) bezogen. Der Vergleich hatte zum Ergebnis, dass die ZWDs der Modelle einen um ca. 30 mm höheren Wert zeigten als jene, die mit einem Wasserdampfradiometer gemessen wurden. Im Vergleich zu GPS-abgeleiteten ZWDs betrugen die durchschnittlichen Offsets der Modelle und des Wasserdampfradiometers -4.3±11.0 mm beziehungsweise -44.8±24.0 mm. Diese ZWDVergleiche haben gezeigt, dass eine Korrektur der WVR ZWDs erforderlich ist. Außerdem hatte es den Anschein, dass die rohen WVR-ZWD-Messungen geglättet werden sollten, um das Rauschen des Instruments zu reduzieren. Für die Fehlerkorrektur wurden außerdem in jeder einzelnen Session durchschnittliche Offsets zwischen den Modellen und den Wasserdampfradiometern berechnet und angesetzt. Allerdings zeigte sich schon hier, dass die interne Kalibrierung des Instruments einige Defizite aufwies und die Ergebnisse dadurch in ihrer Genauigkeit eingeschränkt waren. Die Korrekturen an den Laufzeitverzögerungen in Zenitrichtung aus verschiedenen Ansätzen wurden in fünf geodätischen VLBI-Sessionen verwendet und die Auswirkungen auf die Basislinienwiederholbarkeit und Höhengenauigkeit untersucht. Es stellte sich heraus, dass die Basislinienwiederholbarkeit bei manchen Basislinien verbessert werden konnte, wenn Offsets an den gemessenen WVR-Ergebnissen angebracht wurden. Die Verbesserung war jedoch kleiner als 1 Prozent. Obwohl die Höhengenauigkeit, ausgedrückt als Root Mean Squared Error (RMS) und Weighted RMS (WRMS), um den Faktor 2 verbessert werden konnte, zeigte die Höhenkomponente selbst eine größere Ablage von den Ursprungswerten als erwartet. Als Ursache dafür wurde die Vielzahl der zu schätzenden Parameter und ihre zum Teil hohen Korrelationen identifiziert. Die Schlussfolgerung dieser Untersuchung ist somit, dass die Waserdampfradiometerbeobachtungen in Effelsberg noch nicht gänzlich für die Fehlerbehebung der Laufzeitverzögerungen durch den feuchten Anteil der Atmosphäre geeignet sind, was hauptsächlich auf die Unvollkommenheit einer instrumentellen Kalibrierung zurückzuführen ist. Es werden weitere Studien mit einer größeren Zahl von WVR- Messwerten mit einer verbesserten Kalibrierung des WVR notwendig sein, um die Zweckmäßigkeit des Wasserdampfradiometers für die Fehlerbehebung der Laufzeitverzögerungen durch den feuchten Anteil der Atmosphäre in der geodätischen VLBI abschließend nachweisen zu können

Quality assessment of integrated water vapour measurements at the St. Petersburg site, Russia: FTIR vs. MW and GPS techniques

The cross-comparison of different techniques for atmospheric integrated water vapour (IWV) measurements is the essential part of their quality assessment protocol. We inter-compare the synchronised data sets of IWV values measured by the Bruker 125 HR Fourier-transform infrared spectrometer (FTIR), RPG-HATPRO microwave radiometer (MW), and Novatel ProPak-V3 global navigation satellite system receiver (GPS) at the St. Petersburg site between August 2014 and October 2016. As the result of accurate spatial and temporal matching of different IWV measurements, all three techniques agree well with each other except for small IWV values. We show that GPS and MW data quality depends on the atmospheric conditions; in dry atmosphere (IWV smaller than 6mm), these techniques are less reliable at the St. Petersburg site than the FTIR method. We evaluate the upper bound of statistical measurement errors for clear-sky conditions as 0.29±0.02mm (1.6±0.3%), 0.55±0.02mm (4.7±0.4%), and 0.76±0.04mm (6.3±0.8%) for FTIR, GPS, and MW methods, respectively. We propose the use of FTIR as a reference method under clear-sky conditions since it is reliable on all scales of IWV variability

Tropospheric water vapor: a comprehensive high-resolution data collection for the transnational Upper Rhine Graben region

Tropospheric water vapor is one of the most important trace gases of the Earth's climate system, and its temporal and spatial distribution is critical for the genesis of clouds and precipitation. Due to the pronounced dynamics of the atmosphere and the nonlinear relation of air temperature and saturated vapor pressure, it is highly variable, which hampers the development of high-resolution and three-dimensional maps of regional extent. With their complementary high temporal and spatial resolutions, Global Navigation Satellite Systems (GNSS) meteorology and Interferometric Synthetic Aperture Radar (InSAR) satellite remote sensing represent a significant alternative to generally sparsely distributed radio sounding observations. In addition, data fusion with collocation and tomographical methods enables the construction of detailed maps in either two or three dimensions. Finally, by assimilation of these observation-derived datasets with dynamical regional atmospheric models, tropospheric water vapor fields can be determined with high spatial and continuous temporal resolution. In the following, a collection of basic and processed datasets, obtained with the above-listed methods, is presented that describes the state and course of atmospheric water vapor for the extent of the GNSS Upper Rhine Graben Network (GURN) region. The dataset contains hourly 2D fields of integrated water vapor (IWV) and 3D fields of water vapor density (WVD) for four multi-week, variable season periods between April 2016 and October 2018 at a spatial resolution of (2.1 km)2. Zenith total delay (ZTD) from GNSS and collocation and refractivities are provided as intermediate products. InSAR (Sentinel-1A/B)-derived double differential slant total delay phases (ddSTDPs) and GNSS-based ZTDs are available for March 2015 to July 2019. The validation of data assimilation with five independent GNSS stations for IWV shows improving Kling–Gupta efficiency (KGE) scores for all seasons, most notably for summer, with collocation data assimilation (KGE = 0.92) versus the open-cycle simulation (KGE = 0.69). The full dataset can be obtained from https://doi.org/10.1594/PANGAEA.936447 (Fersch et al., 2021)

Tropospheric water vapor: a comprehensive high-resolution data collection for the transnational Upper Rhine Graben region

Tropospheric water vapor is one of the most important trace gases of the Earth\u27s climate system, and its temporal and spatial distribution is critical for the genesis of clouds and precipitation. Due to the pronounced dynamics of the atmosphere and the nonlinear relation of air temperature and saturated vapor pressure, it is highly variable, which hampers the development of high-resolution and three-dimensional maps of regional extent. With their complementary high temporal and spatial resolutions, Global Navigation Satellite Systems (GNSS) meteorology and Interferometric Synthetic Aperture Radar (InSAR) satellite remote sensing represent a significant alternative to generally sparsely distributed radio sounding observations. In addition, data fusion with collocation and tomographical methods enables the construction of detailed maps in either two or three dimensions. Finally, by assimilation of these observation-derived datasets with dynamical regional atmospheric models, tropospheric water vapor fields can be determined with high spatial and continuous temporal resolution. In the following, a collection of basic and processed datasets, obtained with the above-listed methods, is presented that describes the state and course of atmospheric water vapor for the extent of the GNSS Upper Rhine Graben Network (GURN) region. The dataset contains hourly 2D fields of integrated water vapor (IWV) and 3D fields of water vapor density (WVD) for four multi-week, variable season periods between April 2016 and October 2018 at a spatial resolution of (2.1 km)$^2$. Zenith total delay (ZTD) from GNSS and collocation and refractivities are provided as intermediate products. InSAR (Sentinel-1A/B)-derived double differential slant total delay phases (ddSTDPs) and GNSS-based ZTDs are available for March 2015 to July 2019. The validation of data assimilation with five independent GNSS stations for IWV shows improving Kling–Gupta efficiency (KGE) scores for all seasons, most notably for summer, with collocation data assimilation (KGE = 0.92) versus the open-cycle simulation (KGE = 0.69). The full dataset can be obtained from https://doi.org/10.1594/PANGAEA.936447 (Fersch et al., 2021)

GPS-Derived Precipitable Water Compared with the Air Force Weather Agency\u27s MM5 Model Output

Lightning poses a threat to aircraft in flight. To mitigate this threat, the U.S. Air Force requested a study of lightning distances. Three-Dimensional lightning data were examined for this study, spanning 1 March 1997 to 31 May 2001 and obtained from the Lightning Detection and Ranging System (LDAR) at the Kennedy Space Center, FL. The LDAR data points were first grouped into lightning flashes and branches using spatial and temporal criteria. Rawinsonde data were vertically interpolated to determine the temperature at the flash source point and each branch end point. The horizontal distance from flash sources to branch end was calculated. Percentiles of branch distance were examined as a function of altitude and temperature of the flash source and branch end points. The longest 99th percentile of branch distance (35 to 40 km) was found at 2 to 7 km altitude and between 10 and -20C. The altitude range of the longest branches remained similar by season, but the longest branches were found in the winter and spring months, with summer and autumn distances shorter by 5 to 10 km. Summer results showed longer branch distances to the south and the winter data showed a significant elongation to the north

Modelling atmospheric wet refractivity profile using ground and space-based global positioning system

Precise measurement of atmospheric water vapour has been very challenging due to some limitations of the conventional meteorological systems. Hence, there is a need for Global Positioning System (GPS) for meteorology or GPS meteorology. Therefore, the ground-based GPS meteorology and the space-based GPS Radio Occultation (GPS RO) techniques have been used. The major challenges of groundbased GPS meteorology approach include the lack of surface meteorological data collocating with the location of the ground-based GPS receivers as well as its inability to profile the atmosphere. Whereas the GPS RO technique has a problem of generating profile for the lower tropospheric region which holds the largest amount of water vapour. This research investigates an approach for estimating wet refractivity profile using GPS data. Three specific objectives were set for the study which was conducted in three phases. The first objective assessed GPS Integrated Water Vapour (GPS IWV) in which GPS IWV from interpolated meteorological data and the applicability of Global Pressure and Temperature (GPT2w) model for GPS meteorology was evaluated. The results revealed that the GPS IWV from Automatic Weather Station (AWS) presents good correlation with the radiosonde IWV, the standard deviation of the biases vary spatially from 3.162kg/m2 to 3.878 kg/m2. The actual influence of the errors of GPT2w meteorological parameters on GPT2w-based GPS IWV lies between 2kg/m2 and 3kg/m2, translating to an average relative accuracy of 1.2%. Meanwhile, the sensitivity of the GPS RO data to equatorial water vapour trend was evaluated to achieve second objective. It was found that the GPS RO IWV is highly comparable with the ground-based GPS IWV, having average bias of 1.8kg/m2. Finally, a methodology for GPS wet refractivity retrieval was developed towards achieving the third objective of this research. The Modified Single Exponential Function (MSEF) model for retrieving wet refractivity profile from ground-based GPS Zenith Wet Delay (ZWD) was realised. The output validation using profile from radiosonde and GPS RO observations showed high correlation in each case. In order to improve the performance of the MSEF model, an approach for integrating the ground-based and the space-based GPS data (GIWRef) was formulated. The GIWRef profile is highly correlated with the GPS RO profile, which showed an average improvement of 41% over the initial MSEF method with average correlation coefficient of 0.99. It can be concluded from the foregoing results of the study that the MSEF and GIWREF concepts developed in this work, presents a potential for augmenting weather forecasting and monitoring water vapour system

Excess path delays from sentinel interferometry to improve weather forecasts

A synthetic aperture radar can offer not only an accurate monitoring of the earth surface deformation, but also information on the troposphere, such as the total path delay or the columnar water vapor at high horizontal resolution. This can be achieved by proper interferometric processing and postprocessing of the radar interferograms. The fine and unprecedented horizontal resolution of the tropospheric products can offer otherwise unattainable information to be assimilated into numerical weather prediction models, which are progressively increasing their resolving capabilities. A number of tricks on the most effective processing approaches, as well as a novel method to pass from multipass differential interferometry products to absolute tropospheric columnar quantities are discussed. The proposed products and methods are assessed using real Sentinel-1 data. The experiment aims at evaluating the accuracy of the derived information and its impact on the weather prediction skill for two meteorological events in Italy. The main perspective of the study is linked to the possibility of exploiting interferometric products from a geosynchronous platform, thus complementing the inherent high resolution of SAR sensors with the required frequent revisit needed for meteorological applications