Investigation into the effect of atmospheric particulate matter (PM2.5 and PM10) concentrations on GPS signals

The Global Positioning System (GPS) has been widely used in navigation, surveying, geophysical and geodynamic studies, machine guidance, etc. High-precision GPS applications such as geodetic surveying need millimeter and centimeter level accuracy. Since GPS signals are affected by atmospheric effects, methods of correcting or eliminating ionospheric and tropospheric bias are needed in GPS data processing. Relative positioning can be used to mitigate the atmospheric effect, but its efficiency depends on the baseline lengths. Air pollution is a serious problem globally, especially in developing countries that causes health problems to humans and damage to the ecosystem. Respirable suspended particles are coarse particles with a diameter of 10 micrometers or less, also known as PM10. Moreover, fine particles with a diameter of 2.5 micrometers or less are known as PM2.5. GPS signals travel through the atmosphere before arriving at receivers on the Earth’s surface, and the research question posed in this paper is: are GPS signals affected by the increased concentration of the PM2.5/PM10 particles? There is no standard model of the effect of PM2.5/PM10 particles on GPS signals in GPS data processing, although an approximate generic model of non-gaseous atmospheric constituents (<1 mm) can be found in the literature. This paper investigates the effect of the concentration of PM2.5/PM10 particles on GPS signals and validates the aforementioned approximate model with a carrier-to-noise ratio (CNR)-based empirical method. Both the approximate model and the empirical results show that the atmospheric PM2.5/PM10 particles and their concentrations have a negligible effect on GPS signals and the effect is comparable with the noise level of GPS measurements

Estimating Zenith Tropospheric Delays from BeiDou Navigation Satellite System Observations

The GNSS derived Zenith Tropospheric Delay (ZTD) plays today a very critical role in meteorological study and weather forecasts, as ZTDs of thousands of GNSS stations are operationally assimilated into numerical weather prediction models. Recently, the Chinese BeiDou Navigation Satellite System (BDS) was officially announced to provide operational services around China and its neighborhood and it was demonstrated to be very promising for precise navigation and positioning. In this contribution, we concentrate on estimating ZTD using BDS observations to assess its capacity for troposphere remote sensing. A local network which is about 250 km from Beijing and comprised of six stations equipped with GPS- and BDS-capable receivers is utilized. Data from 5 to 8 November 2012 collected on the network is processed in network mode using precise orbits and in Precise Point Positioning mode using precise orbits and clocks. The precise orbits and clocks are generated from a tracking network with most of the stations in China and several stations around the world. The derived ZTDs are compared with that estimated from GPS data using the final products of the International GNSS Service (IGS). The comparison shows that the bias and the standard deviation of the ZTD differences are about 2 mm and 5 mm, respectively, which are very close to the differences of GPS ZTD estimated using different software packages

Estimating Zenith Tropospheric Delays from BeiDou Navigation Satellite System Observations

The GNSS derived Zenith Tropospheric Delay (ZTD) plays today a very critical role in meteorological study and weather forecasts, as ZTDs of thousands of GNSS stations are operationally assimilated into numerical weather prediction models. Recently, the Chinese BeiDou Navigation Satellite System (BDS) was officially announced to provide operational services around China and its neighborhood and it was demonstrated to be very promising for precise navigation and positioning. In this contribution, we concentrate on estimating ZTD using BDS observations to assess its capacity for troposphere remote sensing. A local network which is about 250 km from Beijing and comprised of six stations equipped with GPS- and BDS-capable receivers is utilized. Data from 5 to 8 November 2012 collected on the network is processed in network mode using precise orbits and in Precise Point Positioning mode using precise orbits and clocks. The precise orbits and clocks are generated from a tracking network with most of the stations in China and several stations around the world. The derived ZTDs are compared with that estimated from GPS data using the final products of the International GNSS Service (IGS). The comparison shows that the bias and the standard deviation of the ZTD differences are about 2 mm and 5 mm, respectively, which are very close to the differences of GPS ZTD estimated using different software packages

GPS Studies of Subtle Deformation Signals in the Western United States

In the past fifteen years, the network of Global Positioning System (GPS) stations in the western United States has dramatically expanded, greatly improving the spatial resolution at which we can resolve geophysical signals. This is particularly important for areas such as the Basin and Range, where data limitations prevented substantial analysis in the past. In addition to improved network geometries, many robust data analysis techniques have been produced, and revised reference frames and data processing strategies have greatly improved data quality. While these advancements have expanded our understanding of long term tectonics in the western United States, they also provide the opportunity to robustly investigate temporally variable, subtle deformation signals. Many of these signals were previously below uncertainty levels of the data, or station coverage was too sparse. The research presented in this dissertation takes advantage of this progress, to advance our understanding of the interaction of subtle deformation signals within the western United States, across a range of spatio-temporal scales. The first study investigates drought induced deformation observed at GPS stations near the Great Salt Lake (GSL), in Utah, between 2012 and 2016. During this time, GPS timeseries show a subtle, but distinct, three-dimensional change in trend, with horizontal motion away from the lake and vertical uplift centered upon it. Concurrently, GSL lost a total of 1.89 m of surface elevation. Previous hydrologic studies have typically only used vertical GPS displacements to quantify load variation over broad, regional scales. Here, we find that at small spatial scales, three-dimensional GPS is sensitive to not just the unloading of the lake, but the nearby groundwater as well. In our preferred model, the volume lost by GSL is equivalent to that observed, at 5.5 ± 1.0 km3, and the inferred groundwater is substantial at 10.9 ± 2.8 km3. Seismicity is modulated by the hydrologic cycle within the inferred load region, revealing increased earthquake rates during drier periods as stresses on faults under the loads are reduced. This study highlights the impact of subtle, multi-year, drought signals on GPS time series, and indicates, that for robust regional analyses, small scale hydrologic loading must be accounted for. In the second study, we focus on correcting subtle deformation signals within the central Basin and Range, and produce the most robust interseismic velocity field of the region to date. Since deformation rates are low, the combined corrections produced in this study for postseismic deformation, hydrologic loading, and regional common mode error, substantially alter the velocity field and resulting strain rates. Station uncertainties reduce by 62.1% and 53.8% in the east and north components, compared to the original velocity field. The Pahranagat Shear Zone is strongly affected by postseismic relaxation, which accounts for as much as half the shear along its western extent. We find that east–west extension across the Las Vegas Valley is substantially larger than previously estimated at 0.5 – 0.6 mm/yr and our preferred strain rates within the Las Vegas Valley are 8.5 ± 2.4 x10−9yr−1, supporting that crustal deformation is active within the urban area of Las Vegas. These results show in detail the significant impact that subtle deformation signals impart on regional analyses and their interpretation. Positioning errors present in five-minute GPS time series propagate subtly into the daily position of the station. In the final study, we produce a sensitivity analysis of the zenith tropospheric delay (ZTD) random walk constraint, and show that station vertical scatter can be greatly improved by loosening its value. We find that large wavelike displacements of ~100 mm, which occurred along the coast of California during Winter Storm Ezekiel in 2019, are suppressed when using a random walk constraint of 24 mm/√(hr) (i.e., eight times looser than the default value). Global station RMS and repeatability shows improvements of 4% – 9% and 10% – 21% respectively, when using uniform random walk constraints of 6 – 12 mm/√(hr). Further improvement is attained when defining characteristic random walk constraints to the stations, with a 10% improvement in repeatability globally. A daily optimal random walk approach reveals 24% improvement in global station repeatability. These findings reveal an opportunity to greatly improve five-minute vertical positioning, not just for stations in the western United States during storms, but for the global GPS network as a whole, by loosening the ZTD random walk constraint at least to a value of 6 mm/√(hr)

Real-time retrieval of precipitable water vapour from GNSS precise point positioning

Global Positioning System (GPS) meteorology (GPS-MET) as a novel approach for precipitable water vapour (PWV) sounding using ground-based GPS receivers has been conducted since earlier 1990s. Further research to date is based on post-processing or near-real-time processing using differenced GPS observations. It still remains a challenging task at high temporal resolutions and in real time. In addition, new Global Navigation Satellite Systems (GNSS) are under development quickly. This has the potential to improve the retrieval of PWV, leading GPS-MET research to a new stage of GNSS-MET. This study aims to take these aspects into account and investigates the retrieval of zenith total delay (ZTD) and PWV using real-time precise point positioning (PPP) approach. The PPP processing in this study is conducted using the BKG (the Federal Agency for Cartography and Geodesy) NTRIP Client (BNC) software platform which is substantially modified. The modifications include the modelling of tropospheric delay in which GPT2 is implemented and the corrections of error sources such as solid Earth tides, ocean tide loading and the antenna-related. The retrieved ZTD is then converted into PWV by multiplying a dimensionless proportionality which is derived from the Forecast Vienna Mapping Functions 1 (VMF1-FC) model. The retrievals of ZTD and PWV are validated using GPS observations in a one-month period at 20 globally distributed stations. The derived real-time ZTDs at most stations agree well with the tropospheric products from the International GNSS Service (IGS) and the root mean square (RMS) errors are &amp;lt;12 mm. The RMS errors of the PWVs in comparison with the radiosonde data are &amp;le;3 mm. Note that 15 mm accuracy is the threshold if ZTDs are input to Numerical Weather Prediction (NWP) models and 3 mm accuracy is the threshold if PWVs are inputs to weather nowcasting according to the document by World Meteorological Organization (WMO). Furthermore, the theoretical accuracy of PWVs in various conditions is analysed. The RMS error of PWV is proved to be a strictly increasing function of zenith wet delay (ZWD) and weighted mean temperature. Hence the retrieval of PWV is more challenging in higher temperature and humidity conditions. This research proves that even in poor retrieval conditions, i.e., high humidity and temperature, an accuracy of PWV at 3 mm level is still achievable using the real-time ZTD from PPP and the empirical models for the determination of weighted mean temperature. A preliminary study of the ZTD retrieval using multi-GNSS data is also conducted in this study. The addition of GLONASS (GLObal NAvigation Satellite System) observations will significantly increase the number of visible satellites and improve the Dilution of Precision (DOP) indices like Positional DOP (PDOP) and Geometric DOP (GDOP). However, a test of ZTD retrieval at 12 global IGS stations shows that adding GLONASS data degrades the accuracy of ZTD. A further analysis implies that the multi-GNSS processing can be improved by the refinement of functional model and real-time GLONASS orbits and clocks. This research realised the real-time, high-accuracy, high temporal- and spatial-resolution retrievals of ZTD and PWV in a context of multiple GNSS constellations. The implemented PPP approach demonstrates its high accuracy in those retrievals. The retrieved real-time ZTD and PWV potentially have a wide range of applications in meteorology such as improving the NWP models and weather nowcasting

Methods for Improving Performance in Consumer Grade GNSS Receivers

Viimeisten kolmen vuosikymmenen aikana satelliittinavigointi on kehittynyt ammatti ja sotilaskäyttäjien tekniikasta kaikkien saatavilla olevaksi tekniikaksi. Varsinkin viimeisen 15 vuoden aikana, kun vastaanottimet alkoivat pienentyä ja halpenivat, on lisääntynyt määrä yrityksiä, jotka toimittavat GPS-laitteita satoihin erilaisiin sovelluksiin. Kaikille moderneille tekniikoille on myös tyypillistä, että tutkimukseen ja siihen liittyvään vastaanottimien kehittämiseen on käytetty valtavasti rahaa, mikä on johtanut huomattavaan parantumiseen vastaanottimen suorituskyvyssä. GPS-vastaanottimien kehitystyön lisäksi uusien maailmanlaajuisten satelliittinavigointijärjestelmien, kuten venäläisen GLONASS, kiinalaisen BeiDou- ja eurooppalaisen Galileo-järjestelmien käyttöönotto tarjoaa entistä enemmän mahdollisuuksia suorituskyvyn parantamiseen. Sekä GPS että nämä uudet järjestelmät ovat myös ottaneet käyttöön uudentyyppisiä signaalirakenteita, jotka voivat tarjota parempilaatuisia havaintoja ja siten parantaa kaikkien vastaanottimien suorituskykyä. Lopuksi menetelmät, kuten PPP ja RTK, jotka aiemmin olivat varattu ammattikäyttäjille, ovat tulleet kuluttajamarkkinoille mahdollistaen ennennäkemättömän suorituskyvyn jokaiselle satelliittinavigointivastaanottimien käyttäjälle. Tässä opinnäytetyössä arvioidaan tämän kehityksen vaikutusta sekä suorituskykyyn että vastaanottimen arkkitehtuuriin. Työssä esitellään yksityiskohtaisesti FGI:ssä kehitetyn ohjelmistopohjaisen vastaanottimen, FGI-GSRx:n. Tämän vastaanottimen avulla on työssä arvioitu miten sekä uudet konstellaatiot että uudet nykyaikaiset signaalit ja niitten seurantamenetelmät vaikuttavat suorituskykyyn ja vastaanotin arkkitehtuuriin. Tämän lisäksi on arvioitu PPP- ja RTK-tarkkuuspaikannusmenetelmien vaikutus FinnRefCORS-verkkoa käyttäen useiden erityyppisten vastaanottimien kanssa, mukaan lukien kuluttajalaatuiset vastaanottimet. Tulokset osoittavat, että enemmän konstellaatioita ja signaaleja käytettäessä paikannusratkaisun tarkkuus paranee 3 metristä 1,4 metriin hyvissä olosuhteissa ja yli 10-kertaiseksi tiheästi rakennetuissa kaupungeissa, jossa käytettävissä olevien signaalien määrä kasvaa kertoimella 2 käytettäessä kolmea konstellaatiota. Uusia moderneja modulaatiotekniikoita, kuten BOC-modulaatiota, käytettäessä tulokset osoittavat Galileo-ratkaisun tarkkuuden paranevan lähes 25%:lla ja esitelty uusi signaalinkäsittelymenetelmä lisää tällaisen tarkkuuden saatavuutta 50%:sta lähes 100%:iin. Lopuksi tarkkuuspaikannusmenetelmien tulokset osoittavat, että 15 cm:n tarkkuus on saavutettavissa, mikä on merkittävä parannus verrattuna 1,4 metrin tarkkuuteen. Näiden parannusten saavuttamiseksi on olennaista, että itse vastaanotin on mukautettu hyödyntämään näitä uusia signaaleja ja konstellaatioita. Tämä tarkoittaa, että nykyaikaisten kuluttajamarkkinoiden vastaanottimien suunnittelu on haastavaa ja monissa tapauksissa ohjelmistopohjainen vastaanotin olisi parempi ja halvempi valinta kuin uusien mikropiirien kehittäminen.For the last three decades, satellite navigation has evolved from being a technology for professional and military users to a technology available for everyone. Especially during the last 15 years, since the receivers started getting smaller and cheaper, there has been an increasing number of companies delivering Global Positioning System (GPS) enabled devices for hundreds of different kind of applications. Typical for any modern technology, there has also been an enormous amount of money spent on research and accompanied receiver development resulting in an immense increase in receiver performance. In addition to the development efforts on GPS receivers the introduction of new global navigation satellite systems such as the Russian Globalnaja Navigatsionnaja Sputnikovaja Sistema (GLONASS), the Chinese BeiDou, and the European Galileo systems offers even more opportunities for improved performance. Both GPS and these new systems have also introduced new types of signal structures that can provide better quality observations and even further improve the performance of all receivers. Finally, methods like Precise Point Positioning (PPP) and Real Time Kinematic (RTK) that earlier were reserved for professional users have entered into the consumer market enabling never before seen performance for every user of satellite navigation receivers. This thesis will assess the impact of this development on both performance as well as on receiver architecture. The design of the software defined receiver developed at FGI, the FGI-GSRx, is presented in detail in this thesis. This receiver has then been used to assess the impact of using multiple constellations as well as new novel signal processing methods for modern signals. To evaluate the impact of PPP and RTK methods the FinnRef Continuously Operating Reference Station (CORS) network has been used together with several different types of receivers including consumer grade off the shelf receivers. The results show that when using more constellations and signals the accuracy of the positioning solution improves from3 meters to 1.4 meters in open sky conditions and by more than a factor 10 in severe urban canyons. For severe urban canyons the available also increases by a factor 2 when using three constellations. When using new modern modulation techniques like high order BOC results show an accuracy improvement for a Galileo solution of almost 25 % and the presented new signal processing method increase the availability of such an accuracy from 50 % to almost 100 %. Finally, results from precise point positioning methods show that an accuracy of 15 cm is achievable, which is a significant improvement compared to an accuracy of 1.4 m for a standalone multi constellation solution. To achieve these improvements, it is essential that the receiver itself is adapted to make use of these new signals and constellations. This means that the design of modern consumer market receivers is challenging and in many cases a software define receiver would be a better and cheaper choice than developing new Application Specific Integrated Circuit (ASIC)’s