Low-cost dual-frequency receivers and antennas have created opportunities for a wide range of new applications, in regions and disciplines where traditional GNSS equipment is unaffordable. However, the major drawback of using low-cost antenna equipment is that antenna phase patterns are typically poorly defined. Therefore, the noise in tropospheric zenith delay and coordinate time series is increased and systematic errors may occur. Here, we present a field calibration method that fully relies on low-cost solutions. It does not require costly software, uses low-cost equipment (~500 Euros), requires limited specialist expertise, and takes complex processing steps into the cloud. The application is more than just a relative antenna calibration: it is also a means to assess the quality and performance of the antenna, whether this is at a calibration site or directly in the field. We cover PCV calibrations, important for deformation monitoring, GNSS meteorology and positioning, and the computation of PCOs when the absolute position is of interest. The method is made available as an online web service. The performance of the calibration method is presented for a range of antennas of different quality and price in combination with a low-cost dual-frequency receiver. Carrier phase residuals of the low-cost antennas are reduced by 11–34% on L1 and 19–39% on L2, depending on the antenna type and ground plane used. For the cheapest antenna, when using a circular ground plane, the L1 residual is reduced from 3.85 mm before to 3.41 mm after calibration, and for L2 from 5.34 mm to 4.3 mm. The calibration reduces the Median Absolute Deviations (MADs) of the low-cost antennas in the vertical direction using Post Processed Kinematic (PPK) by 20–24%. For the cheapest antenna, the MAD is reduced from 5.6 to 3.8 mm, comparable to a geodetic-grade antenna (3.5 mm MAD). The calibration also has a positive impact on the Precise Point Positioning (PPP) results, delivering more precise results and reducing height biases.@en

Whether in cars, smartphones, watches or fitness-trackers - the use of Global Navigation Satellite Systems (GNSS) has become a part of our daily life. Currently there are more than 100 GNSS satellites in orbit. They are routinely utilized for positioning and timing purposes, but their signals can also be used to monitor our environment. The basic principle GNSS measurements rely on is measuring the time difference between the transmitted signal of the satellite antenna and the receiving antenna (typically on the ground). While propagating through the atmosphere, the signal is delayed by the physical properties of the particles in its various layers. This delay is traditionally seen as undesired noise that should be eliminated from the data. This noise however also includes information about the state of the atmosphere which can be described by various parameters. One of such parameters is the delay caused by the 'wet' particles (predominantly water vapor) in the troposphere (lower 20km of the atmosphere). Weather models can use this information to correct the amount and location of atmospheric humidity which has proved to be beneficial for rainfall forecasts. To extract this information from the total signal delay, the delay caused by the ionosphere (upper part of the atmosphere, up to about 1000km) must be eliminated. A standard method is to make use of the dispersive character of the ionized particles in this layer and to eliminate the majority of this error by forming a so-called ionosphere-free linear combination. This requires signals on at least two different frequencies. Traditionally, only geodetic instruments e.g. utilized as permanent ground receivers operated by (inter-) national organizations use hardware that track GNSS signals on at least two frequencies. Such receivers are expensive (in the order of several thousand Euros) and as a result many GNSS networks outside developed areas lack the station density that is needed to capture the complex distribution of atmospheric water vapor. A densification for meteorological purposes with geodetic-grade GNSS receivers and antennas is economically not feasible. Similarly, local precision positioning equipment is not accessible for many regions, foremost situated in the Global South, due to the coarse distribution of static GNSS ground stations and expensive equipment to perform surveying tasks. Technological advances in recent years enabled the release of cost-efficient single- and dual-frequency GNSS receivers and antennas which may offer an alternative to the high-grade technology. However, the use of consumer-grade hardware is associated with challenges that need to be overcome. In this thesis, the performance of low-cost GNSS receivers in combination with antennas of a range of different type and qualities for high-precision applications was analyzed. In particular, the efficiency of using this equipment for meteorological and positioning applications was experimentally quantified and methods to enhance their performance were developed and implemented.@en

The recent release of consumer-grade dual-frequency receivers sparked scientific interest into use of these cost-efficient devices for high precision positioning and tropospheric delay estimations. Previous analyses with low-cost single-frequency receivers showed promising results for the estimation of Zenith Tropospheric Delays (ZTDs). However, their application is limited by the need to account for the ionospheric delay. In this paper we investigate the potential of a low-cost dual-frequency receiver (U-blox ZED-F9P) in combination with a range of different quality antennas. We show that the receiver itself is very well capable of achieving high-quality ZTD estimations. The limiting factor is the quality of the receiving antenna. To improve the applicability of mass-market antennas, a relative antenna calibration is performed, and new absolute Antenna Exchange Format (ANTEX) entries are created using a geodetic antenna as base. The performance of ZTD estimation with the tested antennas is evaluated, with and without antenna Phase Center Variation (PCV) corrections, using Precise Point Positioning (PPP). Without applying PCVs for the low-cost antennas, the Root Mean Square Errors (RMSE) of the estimated ZTDs are between 15 mm and 24 mm. Using the newly generated PCVs, the RMSE is reduced significantly to about 4 mm, a level that is excellent for meteorological applications. The standard U-blox ANN-MB-00 patch antenna, with a circular ground plane, after correcting the phase pattern yields comparable results (0.47 mm bias and 4.02 mm RMSE) to those from geodetic quality antennas, providing an all-round low-cost solution. The relative antenna calibration method presented in this paper opens the way for wide-spread application of low-cost receiver and antennas.

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Dual-frequency Global Navigation Satellite Systems (GNSSs) enable the estimation of Zenith Tropospheric Delay (ZTD) which can be converted to PrecipitableWater Vapor (PWV). The density of existing GNSS monitoring networks is insufficient to capture small-scale water vapor variations that are especially important for extreme weather forecasting. A densification with geodetic-grade dual-frequency receivers is not economically feasible. Cost-efficient single-frequency receivers offer a possible alternative. This paper studies the feasibility of using low-cost receivers to increase the density of GNSS networks for retrieval of PWV. We processed one year of GNSS data from an IGS station and two co-located single-frequency stations. Additionally, in another experiment, the Radio Frequency (RF) signal from a geodetic-grade dual-frequency antenna was split to a geodetic receiver and two low-cost receivers. To process the single-frequency observations in Precise Point Positioning (PPP) mode, we apply the Satellite-specific Epoch-differenced IonosphericDelay (SEID)model using two different reference network configurations of 50-80 km and 200-300 km mean station distances, respectively. Our research setup can distinguish between the antenna, ionospheric interpolation, and software-related impacts on the quality of PWV retrievals. The study shows that single-frequency GNSS receivers can achieve a quality similar to that of geodetic receivers in terms of RMSE for ZTD estimations. We demonstrate that modeling of the ionosphere and the antenna type are the main sources influencing the ZTD precision.

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Accurate sensing of water vapor is crucial for the now- and forecasting of rainfall in numerical weather prediction (NWP) models. Especially severe weather events are characterized by their small spatial scale. Improved forecasting of such events is essential for flood-prone areas and urban stormwater systems which rely on precise rainfall forecasts. Current operational meteorological systems undersample the amount of precipitable water vapor (PWV) which has proven to be an important indicator for rainfall forecasts. There are several possibilities to detect water vapor in the atmosphere. Traditional radiosonde launches offer precise data but are limited in their spatial extent and their trajectory is limited by the wind. Satellite-image-based measurements offer a good spatial resolution but are limited in temporal extent. One possibility to overcome these limitations is to expand existing ground-based GNSS networks. European nationwide GNSS monitoring systems are characterized by inter-station distances typically in the order of tens of kilometers. Due to economic reasons, the densification cannot be achieved with expensive geodetic-grade dual-frequency receivers. Instead, low-cost single-frequency sensors can be used to achieve higher receiver network density. In this work we present first results and an analysis of the error budget from the newly installed regional continuous monitoring network in the urban testbed of Rotterdam. In order to process the data in Precise Point Positioning (PPP) mode, the existing dual-frequency receiver network was used to account for the ionospheric error. The newly installed network consisted of four additional low-cost single-frequency GNSS receivers and was characterized by inter-station distances between 4-5 kilometers. We present comparisons with radiosonde, co-aligned dual-frequency PPP results and existing PWV monitoring campaign reference datasets. This experiment aims to address the feasibility of small-scale water vapor monitoring using low-cost devices. It focuses on the improvement possibilities of such a densification for numerical weather modeling and may help to improve extreme rainfall forecasting. @en

Earlier this year, the European Commission started funding the project “Transforming Water, weather, and climate information through In situ observations for Geo-services in Africa”, or TWIGA. ‘Twiga’ is the Swahili word for giraffe, an animal that derives a competitive advantage from carefully observing its environment. The project aims to develop new geo-services for the water and climate sectors through development of new sensors and associated value chains. Sensors at different Technology Readiness Levels are foreseen to be developed, tested, and embedded in actionable information services. Special emphasis is paid to ground-based sensors that enhance satellite observations. The consortium has eighteen partners from Europe and Africa, including SMEs, universities, and government organizations. Using the network of meteorological stations built in the framework of the TAHMO (Trans-African Hydro- Meteorological Observatory, see www.tahmo.org), new sensors can be rapidly deployed and tested at hundreds of sites in Africa. The present list of experimental sensors to be built and tested include: - 100 Euro neutron counter - Laser micro scintillometer - Evaporometer (developed at Oregon State University) - Intervalometer rain gauge* - Lightning tracking* - GNSS water vapor* - GNSS soil moisture status* The starred (*) sensors are sensors for which some early activities have been undertaken and for which results will be presented. For all sensors, the general idea and usefulness will be explained. Finally, TWIGA is an open project and we would like to extend an invitation to other research groups to use our network to test new sensors in our African network. This project is sponsored by the European Union through Project #776691 “TWIGA”. @en

Exploiting GNSS signal delays is one possibility to obtain PrecipitableWater Vapor (PWV) estimates in the atmosphere. The technique is well known since the early 1990s and by now an established method in the meteorological community. The data is crucial for weather forecasting and its assimilation into numerical weather forecasting models is a topic of ongoing research. However, the spatial resolution of ground based GNSS receivers is usually low, in the order of tens of kilometres. Since severe weather events such as convective storms can be concentrated in spatial extent, existing GNSS networks are often not sufficient to retrieve small scale PWV fluctuations and need to be densified. For economic reasons, the use of low-cost single-frequency receivers is a promising solution. In this study, we will deploy a network of single-frequency receivers to densify an existing dual-frequency network in order to investigate the spatial and temporal PWV variations. We demonstrate a test network consisting of four single-frequency receivers in the Rotterdam area (Netherlands). In order to eliminate the delay caused by the ionosphere, the Satellite-specific Epoch-differenced Ionospheric Delay model (SEID) is applied, using a surrounding dual-frequency network distributed over a radius of approximately 25 km. With the synthesized L2 frequency, the tropospheric delays are estimated using the Precise Point Positioning (PPP) strategy and International GNSS Service (IGS) final orbits. The PWV time series are validated by a comparison of a collocated single-frequency and a dual-frequency receiver. The time series themselves form the basis for potential further studies like data assimilation into numerical weather models and GNSS tomography to study the impact of the increased spatial resolution on local heavy rain forecast. @en

Recent research has shown that assimilation of Precipitable Water Vapor (PWV) measurements into numerical weather predictions models improve the quality of rainfall now- and forecasting. Local PWV fluctuations may be related with water vapor increases in the lower troposphere which lead to deep convection. Prior studies show that about 20 minutes before rain occurs, the amount of water vapor in the atmosphere at 1 km height increases. Monitoring the small-scale temporal and spatial variability of PWV is therefore crucial to improve the weather nowand forecasting for convective storms, that are typically critical for urban stormwater systems. One established technique to obtain PWV measurements in the atmosphere is to exploit signal delays from GNSS satellites to dualfrequency receivers on the ground. Existing dual-frequency receiver networks typically have inter-station distances in the order of tens of kilometers, which is not sufficiently dense to capture the small-scale PWV variations. In this study, we will add low-cost, single-frequency GNSS receivers to an existing dual-frequency receiver network to obtain an inter-station distance of about 1 km in the Rotterdam area (Netherlands). The aim is to investigate the spatial variability of PWV in the atmosphere at this scale. We use the surrounding dual-frequency network (distributed over a radius of approximately 25 km) to apply an ionospheric delay model that accounts for the delay in the ionosphere (50-1000 km altitude) that cannot be eliminated by single-frequency receivers. The results are validated by co-aligning a single-frequency receiver to a dual-frequency receiver. In the next steps, we will investigate how the high temporal and increased spatial resolution network can help to improve high-resolution rainfall forecasts. Their supposed improved forecasting results will be evaluated based on high-resolution rainfall estimates from a polarimetric X-band rainfall radar installed in the city of Rotterdam.@en