GNSS is indispensable to self-driving vehicles by delivering decimeter-level or better absolute positioning solutions. Such a high precision normally requires a convergence time spanning seconds to ...minutes, which is, however, unrealistic in extremely difficult driving conditions where GNSS signals are obstructed frequently. Such convergences, no matter how short, will greatly risk and discredit autonomous driving in satisfying stringent life safety standards. In this study, we therefore developed an extendable GNSS precise point positioning (PPP) model to exploit the advanced Galileo/BeiDou-3 more-than-three-frequency signals with the goal of achieving instant or single-epoch 10–30 cm positioning accuracy and over 99% availability for the horizontal components over wide areas. In particular, uncombined Galileo/BeiDou-3 signals on all available frequencies were injected simultaneously into PPP to perform single-epoch wide-lane ambiguity resolution (PPP-WAR) after phase bias calibrations on raw observations. Experimenting on the Galileo five-frequency data from 36 stations in Australia, we found that instant PPP-WAR was accomplished at more than 99.5% of all epochs; we achieved an instant positioning accuracy of 0.10 and 0.11 m (1
σ
) for the east and north components, respectively, using Galileo E1/E5a/E5/E5b/E6 signals from less than 10 satellites, while 0.16 and 0.23 m using BeiDou-3 B1C/B1I/B2a/B2b/B3I signals from only 5–6 satellites per epoch observed by 10 stations within China. Moreover, we carried out vehicle-borne experiments collecting multi-frequency Galileo/BeiDou-3 signals in the case of overpass and tunnel adversities. With 7 Galileo/BeiDou-3 satellites per epoch on average, instant PPP-WAR reached a mean positioning accuracy of 0.23 and 0.24 m for the horizontal components, which can be further improved to 0.14 and 0.12 m when multi-epoch filtering is preferably enabled. More encouragingly, though this positioning accuracy can also be ensured with triple-frequency data, the data redundancy favored by even more frequencies can reduce the high-precision recovery time from up to 4 s to 2 s in the case of total signal blockages. With the rapidly ongoing deployment of Galileo, BeiDou-3 and other GNSS constellations, we can envision an instant global positioning service characterized by around 20 cm horizontal accuracy and over 99% availability for self-driving vehicles.
High-precision positioning is essential for emerging mass-market applications such as autonomous driving, unmanned aerial vehicles and intelligent transportation in complex urban canyons. Without ...global navigation satellite system (GNSS) reference stations, the real-time kinematic precise point positioning (PPP-RTK) technique is widely used in these fields because of its fast and accurate absolute positioning capability. However, in complex urban canyons, the performance of PPP-RTK deteriorates dramatically due to the satellite signal obstruction and interference. In contrast, inertial navigation system (INS) and visual navigation can provide precise relative pose estimation, but they suffer from serious error accumulation. Therefore, there is a strong complementary advantage between GNSS and INS/Vision navigation and positioning. In this study, a tightly coupled PPP-RTK/INS/Vision integration model is developed, aiming at improving the performance of PPP-RTK in terms of precision and availability to achieve robust positioning in complex urban canyons. Vehicle-borne experiments with different typical urban scenarios were carried out. We found that the positioning precision, availability and the ambiguity fixing rate of PPP-RTK can be significantly improved by tightly coupling with INS and Vision. The positioning precision of tightly coupled micro-electro-mechanical system (MEMS)-based PPP-RTK/INS/Vision is 6 cm, 5 cm and 10 cm in the east, north and up components, respectively, with an ambiguity fixing rate of 83.6% in typical urban environments, which are comparable to the statistics of a tactical-based PPP-RTK/INS tight integration. Even in complex urban canyons, the positioning precision of tightly coupled MEMS-based PPP-RTK/INS/Vision solution can still reach 11 cm, 7 cm and 13 cm in the east, north and up components, respectively, exhibiting improvements of over 90% compared to GNSS-only PPP-RTK. These results indicate that PPP-RTK tightly coupled with low-cost MEMS-IMU and monocular vision can provide high-precision and high-reliability navigation and positioning solutions even in complex urban environments.
Precise point positioning (PPP) has been suffering from slow convergences to ambiguity-fixed solutions. It is expected that this situation can be relieved or even resolved using triple-frequency GNSS ...data. We therefore attempt an approach where uncombined triple-frequency GPS/BeiDou/Galileo/QZSS (Quasi-zenith satellite system) data are injected into PPP, whereas their raw ambiguities are mapped into the extra-wide-lane, wide-lane and narrow-lane combinations for integer-cycle resolution at a single station (i.e., PPP-AR). Once both extra-wide-lane and wide-lane ambiguities are fixed to integers, the resulting unambiguous (extra-)wide-lane carrier-phase can usually outweigh the raw pseudorange to improve the convergence of positions and narrow-lane ambiguities. We used 31 days of triple-frequency multi-GNSS data from 76 stations over the Asia Oceania regions and divided them into hourly pieces for real-time PPP-AR. We found that the positioning accuracy for the first 10 min of epochs could be improved by about 50% from 0.23, 0.18 and 0.43 m to 0.12, 0.08 and 0.27 m for the east, north and up components, respectively, once wide-lane ambiguity fixing was achieved for triple-frequency PPP. Consequently, 48% of PPP solutions could be initialized successfully with narrow-lane ambiguities resolved within 2 min, in contrast to only 26% for dual-frequency PPP. On average, 6 min of epochs were required to achieve triple-frequency PPP-AR, whereas 9 min for its dual-frequency counterpart. Of particular note, the more satellites contribute to triple-frequency PPP-AR, the faster the initializations will be; as a typical example, the mean initialization time declined to 3 min in case of 20–21 satellites. We therefore envision that only a few minutes of epochs will suffice to reliably initialize real-time PPP once all GPS, BeiDou, Galileo and QZSS constellations emitting triple-frequency signals are complete in the near future.
Techniques enabling precise point positioning with ambiguity resolution (PPP-AR) were developed over a decade ago. Several analysis centers of the International GNSS Service (IGS) have implemented ...such strategies into their software packages and are generating (experimental) PPP-AR products including satellite clock and bias corrections. While the IGS combines individual orbit and clock products as standard to provide a more reliable solution, interoperability of these new PPP-AR products must be confirmed before they can be combined. As a first step, all products are transformed into a common observable-specific representation of biases. It is then confirmed that consistency is only ensured by considering both clock and bias products simultaneously. As a consequence, the satellite clock combination process currently used by the IGS must be revisited to consider not only clocks but also biases. A combination of PPP-AR products from six analysis centers over a one-week period is successfully achieved, showing that alignment of phase clocks can be achieved with millimeter precision thanks to the integer properties of the clocks. In the positioning domain, PPP-AR solutions for all products show improved longitude estimates of daily static positions by nearly 60% over float solutions. The combined products generally provide equivalent or better results than individual analysis center contributions, for both static and kinematic solutions.
The availability of GNSS raw measurements and the improved performance of mass-market GNSS chipsets for tracking multi-constellation dual-frequency signals have facilitated the development of ...smartphone high-precision GNSS positioning. However, the channel-dependent carrier phase biases within typical smartphone GNSS chipset prevent Android multi-GNSS ambiguity resolution. In this study, the channel-dependent biases were investigated for the Android GLONASS G1, BDS B1I, Galileo E5a and QZSS L1 carrier phase observations. They destroy the integer property of GNSS ambiguities and result in varying GLONASS inter-frequency bias (IFB) rates. As a result, Android multi-GNSS double-difference ambiguity resolution is hindered and the traditional method of correcting for a constant GLONASS IFB rate is no longer applicable. We propose an on-the-fly phase biases correction method, which introduces reliability verification by resolving only the bias-free ambiguities and estimating the phase bias corrections on-the-fly by gain filtering. In this way, GPS/GLONASS/Galileo/BDS/QZSS dual-frequency ambiguities are resolved for a representative Xiaomi Mi 8 smartphone. For a ~ 13 km short baseline, when the smartphone is connected to an external survey-grade antenna, the time to first fix was 50 s and the fixing rate was 99.21% with the root mean square (RMS) of positioning errors of 1.32, 1.48 and 1.92 cm for the east, north and up components, respectively. Compared with dual-frequency GPS and five-constellation GNSS without phase biases correction, the ambiguity-fixing rate was improved by 30.4% and 99.2%, and the positioning accuracy was improved by 93.4% and 41.8%, respectively. In the case of the smartphone’s embedded antenna, the ambiguity-fixing rate of five-constellation GNSS dropped to about 60% due to multipath, but the positioning precision of its ambiguity-fixed solutions was still at centimeter level, which was less than one-tenth of its float solutions. Therefore, the implementation of multi-GNSS ambiguity resolution will further boost the potential of high-precision GNSS positioning for smartphones.
Frequent earthquakes have brought enormous life and property losses to Taiwan Island, and thus reliable earthquake forecasts are valuable. We form 20 tectonics-based shallow earthquake forecasts of ...Taiwan Island for the years 2019 and after, each with a different threshold moment magnitude, ranging from 5.45 in steps of 0.05 up to 6.40. Our results suggest that Taiwan Island may experience about 25 Mw ≥ 6.25, 40 Mw ≥ 6.00, 65 Mw ≥ 5.75 and 106 Mw ≥ 5.50 shallow earthquakes every 100 years. We demonstrate the consistency between the forecast results and the seismic catalog used for the forecasts and carry out retrospective tests against the seismic catalogs of Taiwan Island not used in the forecasts, which show higher forecast success relative to previous studies of global earthquake forecasts. We identify three gaps of Mw ≥ 6.40 shallow earthquakes, located in Yilan County, Hualien County and Taitung County. We find a general negative correlation between our Mw ≥ 6.40 shallow earthquake forecast and the population density distribution in Taiwan Island. However, we emphasize that the regions with relatively low forecast values need special attention, because these regions generally have relatively high population densities but may also experience strong earthquakes.
Multipath remains one of the major challenges in high-precision GNSS positioning. The multipath hemispherical map (MHM) based on satellites’ location repeatability in space is a popular method to ...mitigate GNSS multipath effects, but its performance depends on the availability of sufficient satellite orbital tracks in the skyplot. For instance, for BDS-3 medium Earth orbiters and Galileo satellites with 7-day and 10-day orbital repeat times, respectively, the skyplot of their orbital tracks will be too sparse to cover the shifting orbital tracks on the succeeding days, if only a few days of observations are used to construct MHMs. In this study, we establish an interoperable MHM using the overlap frequency signals of GPS, Galileo and BDS-3 (i.e
.
, GPS L1/L5, Galileo E1/E5a and BDS-3 B1C/B2a). We compared the performance of GPS/Galileo/BDS-3 MHM (i.e
.
, MP_GEC) and single-constellation MHMs (i.e
.
, MP_G, MP_E and MP_C). The mean reduction rates of the L1/E1/B1C and L5/E5a/B2a carrier-phase residuals for the MP_GEC applied to GPS, Galileo and BDS-3 are 36% and 48%, respectively, which are 10–30% points larger compared to the MP_G, MP_E and MP_C. The MP_GEC constructed using 4 days of observations reduced the Galileo RMS positioning errors by 26%, 31% and 29% for the east, north, and up components, respectively, showing improvements of about 16, 18 and 17% points compared to the MP_E, and even approaching the RMS errors of the MP_E constructed using 10 days of observations. The results show that the interoperable GPS/Galileo/BDS-3 MHM is able to improve the spatial resolution, modeling efficiency and correction performance in mitigating multipath effects for high-precision GNSS positioning.
High‐rate GPS is a precious seismogeodetic tool to capture coseismic displacements unambiguously and usually improved by sidereal filtering to mitigate multipath effects dominating the periods of ...tens of seconds to minutes. We further introduced GLONASS (Globalnaya navigatsionnaya sputnikovaya sistema) data into high‐rate GPS to deliver over 2000 24 h displacements at 99 stations in Europe. We find that the major displacement errors induced by orbits and atmosphere on the low‐frequency band that are not characterized by sidereal repeatabilities can be amplified markedly by up to 40% after GPS sidereal filtering. In contrast, integration with GLONASS can reduce the noise of high‐rate GPS significantly and near uniformly over the entire frequency band, especially for the north components by up to 40%, suggesting that this integration is able to mitigate more errors than only multipath within high‐rate GPS. Integrating GPS with GLONASS outperforms GPS sidereal filtering substantially in ameliorating displacement noise by up to 60% over a wide frequency band (e.g., 2 s–0.5 days) except a minor portion between 100 and 1000 s. High‐rate multi‐GNSS (Global Navigation Satellite System) can be enhanced further by sidereal filtering, which should however be carefully implemented to avoid adverse complications of the noise spectrum of displacements.
Key Points
GPS sidereal filtering can potentially amplify errors on the lowest frequency band of a high‐rate displacement time series
Integration with GLONASS reduces the noise of high‐rate GPS by up to 40% over the entire frequency band of a displacement time series
High‐rate multi‐GNSS can be enhanced by sidereal filtering which should be carefully implemented to avoid complicating the noise spectrum
The dearth of frequency-specific satellite antenna phase centers (APCs), such as GPS Block IIF L5 phase center offsets (PCOs) and BeiDou System (BDS) phase variations (PVs), inconveniences ...multi-frequency precise point positioning (PPP). We find that the GNSS observation biases caused by incorrect frequency-specific APCs are both spatially incoherent and time variable. This spatiotemporal incoherency will be magnified by over a hundred times in the case of multi-frequency PPP wide-lane ambiguity resolution (PPP-WAR) and is likely to defer PPP convergences. We hence first impose deliberate errors on the Galileo frequency-specific APCs to mimic the common faulty operations of equating the GPS L5 with the L1/L2 ionosphere-free PCOs and ignoring the BDS PVs in typical high-precision GNSS. We then investigate how such APC errors can harm multi-frequency PPP using one month of E1/E5a/E5b data from 43 globally distributed stations. Although the APC errors tested in this study have minimal impact on dual-frequency PPP, a 5-mm horizontal PCO error does prolong multi-frequency PPP convergences by 15%; a 200-mm vertical PCO error or a PV error of up to 10 mm can even grow the convergence times by 60%. The vertical positioning precision of single-epoch PPP-WAR is deteriorated on average by 15 cm under a 200-mm vertical PCO error. Therefore, accurate frequency-specific GPS/BDS satellite APCs should be determined for multi-frequency PPP to maximize its convergence advantages over dual-frequency PPP.
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