Assessment and reduction of zenith path delay biases due to day boundary effect

The troposphere consists of dry air and water vapor, delaying the GNSS signal by about 2.4 m in the zenith direction. The water vapor only causes an error of about 0.2 m in distance measurement, but it is challenging to model and overcome. From 2003 the International GNSS Service (IGS) started to provide the new product of zenith path delay (ZPD) with an accuracy of 1.5-5 mm. However, we found an error in these products up to 30 mm at epochs between 2 days due to the day boundary effect and an average of 16mm RMS for nine days. Our research shows that for reducing the impact, the most critical factor is selecting the initial value for the ZPD, followed by satellite orbit/clock and, finally, the station coordinate values. By choosing an appropriate initial value for ZPD and employing a 3 days orbit/clock, the ZPD error due to the day boundary effect can be reduced to negligible. Meanwhile, the change in the station coordinate value in cm level does not impact the effect.

A ship-based network for GNSS-meteorology over the northwestern Mediterranean Sea

<p>Atmospheric events are driven by surface sea physical parameters, including the exchanges of water vapor with the overlying atmosphere. Oceans cover around 70 percent of the Earth's surface and influence the atmospheric circulation, causing some of the main weather events. The lack of surface observations over the vast ocean areas is a critical problem to be addressed for improving the performance of weather forecasting.</p><p>Even if weather observations over sea from ships have been collected for over 200 years and used for meteorological research and climate applications, only recently the availability of different telecommunication solutions make real time access to measurements possible, even from remote areas. This is consequently opening new opportunities to use data from marine areas in operational weather applications.</p><p>Ground based GNSS receivers has been used for many years to determine a quantity that is of major interest for meteorologists and climatologists, the water vapor content, derived from the Zenith Path Delay. GNSS meteorology has been also tested over ships during some measurement campaigns in the past.</p><p>This work presents the implementation of the first GNSS meteo infrastructure on ships operating on the northwestern Mediterranean Sea, involving 9 commercial vessels, real-time collecting a list of GNSS meteo parameters: the signals from Galileo, GPS, GLONASS and Beidou constellations, measurements of pressure, temperature, humidity, wind and precipitation. These 9 moving platforms are complemented by a number of fixed ground platforms, used as a reference.</p><p>The difficulties in ship based GNSS meteorology, with respect to the classical approaches from fixed stations, lie both in the exposure of the hardware instruments to challenging environmental conditions as in the open sea and in the computation algorithms, which must be applied to kinematic conditions and continuously solve the receiver position with very high accuracy.</p><p>Two different processing schemes have been applied to the dataset (i.e. few months): the first one is based on differential GNSS using the TRACK suite of GAMIT software, and the second one is based on precise point positioning using the GLAB software. As it is well known, if network solutions are adopted (as in the first case), the satellites and receivers clock errors can be eliminated with very high accuracy, while PPP-based methods (as in the second case) require ultrafast precise satellite ephemeris products, but they give the possibility to implement standalone instruments, so not to send large amounts of full RINEX files to a ground processing centre.</p><p>The ZPD quantities retrieved from the first period of observations aboard ships are shown, using both the techniques. The comparison shows some discrepancies both in the absolute quantity and in the short-term trends. Even if preliminary, the comprehension of the quality of such an unprecedent source of information is of great interest, because the perspectives of this infrastructure are both scientific and operational, thinking for example to the data assimilation into numerical weather prediction models.</p>

An Accurate Method to Correct Atmospheric Phase Delay for InSAR with the ERA5 Global Atmospheric Model

Differential SAR Interferometry (DInSAR) has proven its unprecedented ability and merits of monitoring ground deformation on a large scale with centimeter to millimeter accuracy. However, atmospheric artifacts due to spatial and temporal variations of the atmospheric state often affect the reliability and accuracy of its results. The commonly-known Atmospheric Phase Screen (APS) appears in the interferograms as ghost fringes not related to either topography or deformation. Atmospheric artifact mitigation remains one of the biggest challenges to be addressed within the DInSAR community. State-of-the-art research works have revealed that atmospheric artifacts can be partially compensated with empirical models, point-wise GPS zenith path delay, and numerical weather prediction models. In this study, we implement an accurate and realistic computing strategy using atmospheric reanalysis ERA5 data to estimate atmospheric artifacts. With this approach, the Line-of-Sight (LOS) path along the satellite trajectory and the monitored points is considered, rather than estimating it from the zenith path delay. Compared with the zenith delay-based method, the key advantage is that it can avoid errors caused by any anisotropic atmospheric phenomena. The accurate method is validated with Sentinel-1 data in three different test sites: Tenerife island (Spain), Almería (Spain), and Crete island (Greece). The effectiveness and performance of the method to remove APS from interferograms is evaluated in the three test sites showing a great improvement with respect to the zenith-based approach.

Mitigation of Tropospheric Delay in SAR and InSAR Using NWP Data: Its Validation and Application Examples

The neutral atmospheric delay has a great impact on synthetic aperture radar (SAR) absolute ranging and on differential interferometry. In this paper, we demonstrate its effective mitigation by means of the direction integration method using two products from the European Centre for Medium-Range Weather Forecast: ERA-Interim and operational data. Firstly, we shortly review the modeling of the neutral atmospheric delay for the direct integration method, focusing on the different refractivity models and constant coefficients available. Secondly, a thorough validation of the method is performed using two approaches. In the first approach, numerical weather prediction (NWP) derived zenith path delay (ZPD) is validated against ZPD from permanent GNSS (global navigation satellite system) stations on a global scale, demonstrating a mean accuracy of 14.5 mm for ERA-Interim. Local analysis shows a 1 mm improvement using operational data. In the second approach, NWP derived slant path delay (SPD) is validated against SAR SPD measured on corner reflectors in more than 300 TerraSAR-X High Resolution SpotLight acquisitions, demonstrating an accuracy in the centimeter range for both ERA-Interim and operational data. Finally, the application of this accurate delay estimate for the mitigation of the impact of the neutral atmosphere on SAR absolute ranging and on differential interferometry, both for individual interferograms and multi-temporal processing, is demonstrated.

SPATIO-TEMPORAL ESTIMATION OF INTEGRATED WATER VAPOUR OVER THE MALAYSIAN PENINSULA DURING MONSOON SEASON

This paper provides the precise information on spatial-temporal distribution of water vapour that was retrieved from Zenith Path Delay (ZPD) which was estimated by Global Positioning System (GPS) processing over the Malaysian Peninsular. A time series analysis of these ZPD and Integrated Water Vapor (IWV) values was done to capture the characteristic on their seasonal variation during monsoon seasons. This study was found that the pattern and distribution of atmospheric water vapour over Malaysian Peninsular in whole four years periods were influenced by two inter-monsoon and two monsoon seasons which are First Inter-monsoon, Second Inter-monsoon, Southwest monsoon and Northeast monsoon.

Benchmark campaign and case study episode in Central Europe for development and assessment of advanced GNSS tropospheric models and products

Initial objectives and design of the Benchmark campaign organized within the European COST Action ES1206 (2013-2017) are described in the paper. This campaign has aimed at supporting the development and validation of advanced GNSS tropospheric products, in particular high-resolution and ultra-fast zenith total delays (ZTD) and tropospheric gradients derived from a dense permanent network. A complex dataset was collected for the 8-week period when several extreme heavy precipitation episodes occurred in central Europe which caused severe river floods in this area. An initial processing of data sets from Global Navigation Satellite System (GNSS) and numerical weather models (NWM) provided independently estimated reference parameters – zenith tropospheric delays and tropospheric horizontal gradients. Their provision gave an overview about the product similarities and complementarities and thus a potential for improving a synergy in their optimal exploitations in future. Reference GNSS and NWM results were inter-compared and visually analysed using animated maps. ZTDs from two reference GNSS solutions compared to global ERA-Interim re-analysis resulted in the accuracy at the 10-millimeter level in terms of RMS (with a negligible overall bias), comparisons to global GFS forecast showed accuracy at the 12-millimeter level with the overall bias of -5 mm and, finally, comparisons to mesoscale ALADIN-CZ forecast resulted in the accuracy at the 8-milllimetre level with a negligible total bias. The comparison of horizontal tropospheric gradients from GNSS and NWM data demonstrated a very good agreement among independent solutions with negligible biases and the accuracy of about 0.5 mm. Visual comparisons of maps of zenith wet delays and tropospheric horizontal gradients showed very promising results for future exploitations of advanced GNSS tropospheric products in meteorological applications such as severe weather event monitoring and weather nowcasting. The GNSS products revealed a capability of providing more detailed structures in atmosphere than the state-of-the-art numerical weather models are able to capture. Initial study on contribution of hydrometeors (e.g. cloud water, ice or snow) to GNSS signal delays during severe weather reached up to 17 mm in zenith path delay and suggested to carefully account them within the functional model. The reference products will be further exploited in various specific studies using the Benchmark dataset. It is thus going to play a key role in these highly inter-disciplinary developments towards better mutual benefits from advanced GNSS and meteorological products.