Onboard Global Positioning System Research Articles

Real‐time precise orbit determination for FY‐3C and FY‐3D based on BDS and GPS onboard observation

AbstractThe code multipath error associated with elevation in BDS‐2 is one of the main factors whose impact on the precision of real‐time reduced‐dynamic precise orbit determination (POD) using spaceborne multi‐global navigation satellite systems (GNSS) observation data for FengYun‐3C (FY‐3C) and FengYun‐3D (FY‐3D) satellites. The aim is to construct a code multipath piece‐wise linear error model and analyse the contribution of BDS‐2 to the performance of low earth orbit POD. Conclusions are drawn out from the experimental results: (1) the real‐time POD average precision of FY‐3C and FY‐3D is improved by about 11% and 12% after the correction of BDS‐2 code multipath error; (2) Due to the small number of available BDS‐2 satellites and the limited accuracy of geostationary earth orbit (GEO) satellite real‐time orbit products, the precision of the real‐time overlapping comparison for FY‐3C/FY‐3D by using the BDS‐2 onboard observation data can only reach the decimetre level. However, with the BDS‐2 signal capture capability of the FY‐3D onboard GNSS Occultation Sounder upgraded, the average precision of the BDS‐2‐based POD for FY‐3D is better than that of FY‐3C in each direction and with 10% improvement in the average 3 dimensional‐root mean square (3D‐RMS); (3) Cases of onboard global positioning system (GPS), GPS + BDS‐2 (with GEO), and GPS + BDS‐2 (without GEO) for real‐time POD can meet 5–8 cm precision requirement. The average 3D‐RMS of the real‐time POD for FY‐3C/FY‐3D, which utilises onboard GPS + BDS‐2 data is inferior only to GPS data, owing to the influence of the orbit accuracy of GEO satellites. However, after excluding the influence of GEO satellites, the average result (3D‐RMS) of the real‐time POD for FY‐3C (6.11 cm) and FY‐3D (5.95 cm) is better than those of other cases.

Freight Operational Characteristics Mined from Anonymous Mobile Sensor Data

Effective transportation performance measurement (TPM) benefits from ubiquitous transportation system monitoring both spatially and temporally. In the context of freight-oriented TPM, traditional devices such as inductive loops, cameras, manual counts, and so forth, may fail to provide comprehensive and high-resolution coverage, providing, for example, only volume counts for a small subset of links across a large network with no indication of trip linkages. New sources of big data from mobile sensors including on-board global positioning system (GPS) devices allow more comprehensive network coverage and insights into trip chaining behaviors. However, to gain actionable insights into system performance from large and noisy streams of mobile sensor data, it is necessary to mine it for relevant operational characteristics of the trucks it represents. Such characteristics include stop locations, stop duration, stop time of day, trip length, and trip duration. To address this methodological need, this paper presents three heuristic algorithms: “stop identification,”“path identification,” and “trip identification.” To address the issue of determining relevant operational characteristics, a multinomial logit (MNL) model approach is applied to determine the commodity carried based on the outputs of the heuristic algorithms. The MNL model is novel in that it relates operational characteristics to commodity carried thus filling a critical data gap that currently limits the development of advanced freight forecasting models. The set of models developed in this paper allow large-scale GPS data to be used for freight planning while maintaining levels of data anonymity that allow such data to be shared with public agencies.

Performance assessment of real-time orbit determination for the Haiyang-2D using onboard BDS-3/GPS observations

The Global Navigation Satellite System (GNSS) receivers equipped on the Haiyang-2D (HY-2D) satellite is capable of tracking the signals of both the third generation of BeiDou satellite navigation System (BDS-3) and the Global Positioning System (GPS), which make it feasible to assess the performance of real-time orbit determination (RTOD) for the HY-2D using onboard GNSS observations. In this study, the achievable accuracy and convergence time of RTOD for the HY-2D using onboard BDS-3 and GPS observations are analyzed. Benefiting from the binary-offset-carrier (BOC) modulation, the BDS-3 C1X signal includes less noise than the GPS C1C signal, which has the same signal frequency and chipping rate. The root mean squares (RMS) of the noises of C1X and C1C code measurements are 0.579 m and 1.636 m, respectively. Thanks to a ten-times higher chipping rate, the code measurements of BDS-3 C5P, GPS C1W and C2W are less noisy. The RMS of code noises of BDS-3 C5P, GPS C1W, and C2W are 0.044 m, 0.386 m, and 0.272 m, respectively. For the HY-2D orbit, the three-dimensional (3D) and radial accuracies can reach 31.8 cm and 7.5 cm with only BDS-3 observations, around 50 % better than the corresponding accuracies with GPS. Better performance of the BDS-3 in RTOD for the HY-2D is attributed to the high quality of its broadcast ephemeris. When random parameters are used to absorb ephemeris errors, substantial improvement is seen in the accuracy of HY-2D orbit with either BDS-3 or GPS. The 3D RMS of HY-2D orbit errors with BDS-3 and GPS are enhanced to 23.1 cm and 33.6 cm, and the RMS of the radial components are improved to 6.1 cm and 13.3 cm, respectively. The convergence time is 41.6 and 75.5 min for the RTOD with BDS-3 and GPS, while it is reduced to 39.2 and 27.4 min after the broadcast ephemeris errors are absorbed by random parameters. Overall, the achievable accuracy of RTOD with BDS-3 reaches decimeter level, which is even better than that with GPS, making real-time navigation using onboard BDS-3 observations a feasible choice for future remote sensing missions.

Inferring the purposes of passenger trips from GPS records

A technological revolution has emerged in the context of mobility surveys with the widescale usability of wearable and onboard global positioning system (GPS) devices. Sri Lanka is also at the edge of utilizing these, in replacing the traditional methods. The earnestness' for this transition is supported by the deficiencies such as higher cost, higher nonresponses rates, over and under-reporting, and small sample sizes of traditional surveys. The activity that a passenger performs after a trip, or the purpose of a trip is a vital concern in transportation research as it is the reason behind the generation of travel demands. Hence, trip purpose inference from GPS data has become an important study area in this context [1]. Gong et al. [2] reviewing the existing trip purpose inference studies categorized the methodologies that had been used into three as rule-based, probabilistic, and machine learning-based. In this study, we utilized the GPS records of taxi trips from a popular service provider in the country as shown in Figure 1 in developing a suitable trip purpose inference approach.

Localization in Unprecedentedly Crowded Airspace for UAVs and SUAVs

The unprecedented proliferation of Unmanned Aerial Vehicles (UAVs), and Swarm Unmanned Aerial Vehicles (SUAVs) has garnered considerable attention from industry and academia owing to their extensive landscape of applications from disaster relief towards smart agriculture. However, flying several UAVs at once poses many challenges to safely and efficiently localize and monitor them. Further, they need to maintain their formation distance to avoid collision between team members and any environmental obstacles. Besides, SUAVs are mainly equipped with an on-board Global Positioning System (GPS) receivers to obtain their positions, but they are not accurate enough and suffer from several vulnerabilities that restrict their applications. Thus, in GPS-denied situations, the acquisition of the positions of UAVs can be assisted by alternative technologies and solutions. This paper is one of the foremost in-depth work that presents, the topic of localization of SUAVs from various perspectives including current research challenges on positioning systems, telecommunication, path planning, along with future opportunities on automated delivery services such as medicine, a remote inspection of industrial sites, and precision agriculture.

A Scoring Framework and Apparatus for Epilepsy Seizure Detection Using a Wearable Belt.

To develop a wearable device that can detect epilepsy seizures. In particular, due to their prevalence, attention is focused on detecting the generalized tonic-clonic seizure (GTCS) type. When a seizure is detected, an alert phone call is initiated and an alarm SMS sent to the nearest health-care provider (and/or a predesignated family member), including the patient's location as global positioning system (GPS) coordinates. A wearable belt is developed including an Arduino processor that constantly acquires data from four different sensing modalities and monitors the acquired signal patterns for abnormalities. The sensors are a heart rate sensor, electromyography sensor, blood oxygen level (oxygen saturation) sensor, and an accelerometer to detect sudden falls. Higher-than-normal threshold levels are established for each sensor's signal. If two or more signal measurements exceed the corresponding threshold value for a predetermined time interval, then the seizure alarm is triggered. Clinical trials were not pursued in this study as this is the initial phase of system development (phase 0). Instead, the instrumented belt seizure detection prototype was tested on nine healthy individuals mimicking, to some degree, seizure symptoms. A total of eighteen trials took place of which half had <2 sensor thresholds exceeded and no alarm, whereas the other half resulted in activating the alarm when two or more sensor thresholds were exceeded for at least the predetermined time interval corresponding to each of the higher-than-normal sensor readings. For each trial that triggered the alarm when a seizure was detected, the on-board GPS and global system for mobile communication (GSM) units successfully initiated an alert phone call to a predesignated number in addition to sending an SMS message, including GPS location coordinates. Continuous real-time monitoring of signals from the four different sensors allows the developed wearable belt to detect GTCS while reducing false alarms. The proposed device produces an important alarm that may save a patient's life.

Precise Relative Orbit Determination for Chinese TH-2 Satellite Formation Using Onboard GPS and BDS2 Observations

TH-2 is China’s first short-range satellite formation system used to realize interferometric synthetic aperture radar (InSAR) technology. In order to achieve the mission goal of InSAR processing, the relative orbit must be determined with high accuracy. In this study, the precise relative orbit determination (PROD) for TH-2 based on global positioning system (GPS), second-generation BeiDou navagation satellite system (BDS2), and GPS + BDS2 observations was performed. First, the performance of onboard GPS and BDS2 measurements were assessed by analyzing the available data, code multipath errors and noise levels of carrier phase observations. The differences between the National University of Defense Technology (NDT) and the Xi’an Research Institute of Surveying and Mapping (CHS) baseline solutions exhibited an RMS of 1.48 mm outside maneuver periods. The GPS-based orbit was used as a reference orbit to evaluate the BDS2-based orbit and the GPS + BDS2-based orbit. It is the first time BDS2 has been applied to the PROD of low Earth orbit (LEO) satellite formation. The results showed that the root mean square (RMS) of difference between the PROD results using GPS and BDS2 measurements in 3D components was 2.89 mm in the Asia-Pacific region. We assigned different weights to geostationary Earth orbit (GEO) satellites to illustrate the impact of GEO satellites on PROD, and the accuracy of PROD was improved to 7.08 mm with the GEO weighting strategy. Finally, relative orbits were derived from the combined GPS and BDS2 data. When BDS2 was added on the basis of GPS, the average number of visible navigation satellites from TH-2A and TH-2B improved from 7.5 to 9.5. The RMS of the difference between the GPS + BDS2-based orbit and the GPS-based orbit was about 1.2 mm in 3D. The overlap comparison results showed that the combined orbit consistencies were below 1 mm in the radial (R), along-track (T), and cross-track (N) directions. Furthermore, when BDS2 co-worked with GPS, the average of the ambiguity dilution of precision (ADOP) reduced from 0.160 cycle to 0.153 cycle, which was about a 4.4% reduction. The experimental results indicate that millimeter-level PROD results for TH-2 satellite formation can be obtained by using onboard GPS and BDS2 observations, and multi-GNSS can further improve the accuracy and reliability of PROD.

Reduced-Dynamic Precise Orbit Determination of Haiyang-2B Altimetry Satellite Using a Refined Empirical Acceleration Model

The Haiyang 2B (HY-2B) satellite requires precise orbit determination (POD) products for geodetic remote sensing techniques. An improved set of reduced-dynamic (RD) orbit solutions was generated from the onboard Global Positioning System (GPS) measurements over a 14-month period using refined strategies and processing techniques. The key POD strategies include a refined empirical acceleration model, in-flight calibration of the GPS antenna, and the resolution of single-receiver carrier-phase ambiguities. In this study, the potential periodicity of empirical acceleration in the HY-2B POD was identified by spectral analysis. In the along-track direction, a noticeable signal with four cycles per revolution (CPR) was significant. A mixed spectrum was observed for the cross-track direction. To better understand the real in-flight environment, a refined empirical acceleration model was used to cope with the time variability of empirical accelerations in HY-2B POD. Three POD strategies were used for the reprocessing for superior orbit quality. Validation using over one year of satellite laser ranging (SLR) measurements demonstrated a 5.2% improvement in the orbit solution of the refined model. Reliable correction for the GPS antenna phase center was obtained from an over-420-day dataset, and a trend in radial offset change was observed. After application of the in-flight calibration of the GPS antenna, a 26% reduction in the RMS SLR residuals was achieved for the RD orbit solution, and the carrier phase residuals were clearly reduced. The integer ambiguity resolution of HY-2B led to strong geometric constraints for the estimated parameters, and a 15% improvement in the SLR residuals could be inferred compared with the float solution.

Airborne LiDAR and Photogrammetric Point Cloud Fusion for Extraction of Urban Tree Metrics According to Street Network Segmentation

This paper provides a practical procedure for fusing LiDAR and photogrammetric point clouds for the extraction of tree metrics. Aerial photogrammetric point clouds are first generated using the structure-from-motion and dense-matching methods. Registration of the LiDAR and photogrammetric point clouds is then performed using an onboard global positioning system and inertial measurement unit. However, due to systematic deviations, it is impossible to directly merge the two types of point cloud. Therefore, an urban street network obtained from the OpenStreetMap digital mapping system is utilized for point cloud segmentation. After segmentation, each chunk is finely registered and merged based on the iterative closest point algorithm, allowing the two types of data to be accurately co-registered and a fused point cloud obtained. Finally, we conducted experiments to extract stand and individual tree metrics from fused point clouds created for two study plots. The height distributions of the fused point clouds were highly consistent with LiDAR data, with the 5%, 10%, 25%, 50%, 75%, 90%, and 95% height percentiles showing acceptable similarities. The height distribution of individual trees was also consistent with that of field measurements. Furthermore, the fused point clouds contain a high point density and RGB color information, which allow shape delineation and estimation of tree health status. This comprehensive analysis demonstrates that this procedure provides a practical way to inventory tree stands and individuals in urban areas.

Stochastic noise modelling of kinematic orbit positions in the Celestial Mechanics Approach

Abstract. Gravity field models may be derived from kinematic orbit positions of Low Earth Orbiting satellites equipped with onboard GPS (Global Positioning System) receivers. An accurate description of the stochastic behaviour of the kinematic positions plays a key role to calculate high quality gravity field solutions. In the Celestial Mechanics Approach (CMA) kinematic positions are used as pseudo-observations to estimate orbit parameters and gravity field coefficients simultaneously. So far, a simplified stochastic model based on epoch-wise covariance information, which may be efficiently derived in the kinematic point positioning process, has been applied. We extend this model by using the fully populated covariance matrix, covering correlations over 50 min. As white noise is generally assumed for the original GPS carrier phase observations, this purely formal variance propagation cannot describe the full noise characteristics introduced by the original observations. Therefore, we sophisticate our model by deriving empirical covariances from the residuals of an orbit fit of the kinematic positions. We process GRACE (Gravity Recovery And Climate Experiment) GPS data of April 2007 to derive gravity field solutions up to degree and order 70. Two different orbit parametrisations, a purely dynamic orbit and a reduced-dynamic orbit with constrained piecewise constant accelerations, are adopted. The resulting gravity fields are solved on a monthly basis using daily orbital arcs. Extending the stochastic model from utilising epoch-wise covariance information to an empirical model, leads to a – expressed in terms of formal errors – more realistic gravity field solution.

Topside Ionosphere of NeQuick2 and IRI‐2016 Validated by Using Onboard GPS Observations From Multiple LEO Satellites

AbstractAn accurate description of the topside ionosphere is of great importance for space weather applications. Currently, there are two commonly used empirical ionosphere models, that is, NeQuick2 and IRI‐2016, that can be used to describe the topside ionosphere. In this study, topside total electron content (TEC) values derived from low Earth orbit (LEO) satellites are selected to validate the topside ionosphere predictions of the NeQuick2 and IRI‐2016 models from 2008 to 2018. The results show that these two models both underestimate the topside ionosphere. Compared with the global topside ionosphere model estimated by LEO‐based global positioning system (GPS) observations with orbital altitudes of 500 and 800 km, NeQuick2 and IRI‐2016 have similar performances over different latitudes and longitudes. The daily mean root mean square (RMS) values are approximately 4.10 and 2.10 TEC units (TECU) for the region above 500 and 800 km, respectively. The results assessed by the GPS‐TEC technique indicate that the performance of the NeQuick2 model is slightly better than that of the IRI‐2016 model. The daily mean bias mainly ranges from −1.5 to 1.5 and −3.0 to −1.0 TECU for the regions above 500 and 800 km, respectively. The RMS values for the corresponding parts are 3.50 and 2.74 TECU. Finally, the validation results of the dSTEC method show that the mean bias is −0.43 and −0.20 TECU above 500 and 800 km, respectively. The mean RMS values are 2.04 and 0.72 TECU for the corresponding regions.

High-precision orbit determination for a LEO nanosatellite using BDS-3

The Tianping-1B is a 20-kg low earth orbit nanosatellite with a commercial multi-GNSS receiver based on a microelectromechanical system. This receiver collects concurrent code and phase dual-frequency measurements from the global positioning system (GPS) and the second and third generations of the BeiDou Global Navigation Satellite System (i.e., BDS-2 and BDS-3). However, BDS-3 signals with pseudorandom noise code numbers greater than 32 cannot be received. In this study, onboard GPS and BDS measurements from Tianping-1B are collected for days 133–147 of 2019. The performance of the onboard BDS-3 measurements is analyzed, and the potential of the BDS-3-based precise orbit determination (POD) for Tianping-1B is assessed. The carrier-to-noise-density ratio of the BDS-3 is higher than that of the BDS-2 and approaches that of the GPS. The BDS-3 has a smaller code multipath error than the BDS-2 and the GPS and therefore a higher quality of code measurements. The results of the overlap comparison show a GPS-based orbit consistency below 3.5 cm in three dimensions (3D) and below 1.2 cm in the radial direction. The mean of satellite laser ranging validation residual RMS is 1.7 cm. The orbit obtained using onboard BDS measurements is assessed using the GPS-based orbit as a reference: The mean 3D root mean square (RMS) difference between the BDS-3-only-based POD and the reference orbit is 4.57 cm. Thus, a sub-dm-level orbit can be achieved using only onboard BDS-3 measurements. A POD with slightly higher precision than the BDS-3-only-based POD is obtained using both BDS-3 and BDS-2 measurements, using higher weights for the BDS-3 data than the BDS-2 data. Then, the combined BDS-3/GPS POD is performed: The RMS difference between this joint orbit and the GPS-based orbit is 1 cm in 3D. This study can be used as a reference for the development of BDS-3 based POD, which will approve in accuracy and precision upon completion of BDS-3.

In-Orbit Results and Attitude Analysis of the SNUGLITE Cube-Satellite

SNUGLITE (Seoul National University Global navigation satellite system Laboratory satellITE) is a two-unit cube satellite (CubeSat) with dimensions 10 × 10 × 23 cm that requires an attitude system for missions and ground station telecommunication. A linear-quadratic-Gaussian-based optimal attitude system for the CubeSat platform has been developed using low-cost sensors, with the in-orbit verification of the attitude system being is one of main study objectives. Since launch, the SNUGLITE CubeSat has continuously broadcast in-orbit status information. In this study, a methodology for the analysis of in-orbit attitude estimation results using received data is presented, and this was achieved by comparing two sun-pointing vectors, i.e., the sun-pointing vector calculated using estimated attitude with the positions of the sun and the satellite and the reference vector generated by the power levels of the solar panels. Because the satellite position was required for the attitude analysis, the verification of the performance of the own-developed on-board Global Positioning System (GPS) receiver is also briefly described. Analyses indicate that the attitude estimation of the SNUGLITE CubeSat has achieved an in-orbit real-time pointing accuracy with a root mean square of 6.1°.

Performance Comparison of KOMPSAT-5 Precision Orbit Determination with GRACE

The Korean Multipurpose Satellite-5 (KOMPSAT-5) launched on 22 August 2013 was equipped with a global positioning system (GPS) receiver for precision orbit determination (POD). Even though the GPS receiver of KOMPSAT-5 shares the same heritage as the BlackJack receiver onboard in Gravity Recovery and Climate Experiment (GRACE) satellites, KOMPSAT-5 has a lower orbital position accuracy (~10 cm) compared with GRACE (~2 cm). The reduced dynamic and kinematic methods are applied for POD of KOMPSAT-5 and GRACE to investigate the GPS observation quality due to the satellite operation concept and hardware design. The results are analyzed in terms of the number of observations and their spatial distribution, GPS signal quality, and orbital position accuracies. The results show that the frequent attitude maneuvers of KOMPSAT-5 affect the quality of the GPS signals and solutions obtained from the kinematic method compared with that determined from the reduced-dynamic method. The onboard patch GPS antenna installed in KOMPSAT-5 and its geometrical position resulted in more erratic measurement residuals by 140% compared with the choke ring antenna of GRACE. The POD accuracy is dependent on the hardware design and regular attitude tilting for the synthetic aperture radar (SAR) imaging even though the same GPS receiver performances.

Personal Exposure Estimates via Portable and Wireless Sensing and Reporting of Particulate Pollution.

Low-cost, portable particle sensors (n = 3) were designed, constructed, and used to monitor human exposure to particle pollution at various locations and times in Lubbock, TX. The air sensors consisted of a Sharp GP2Y1010AU0F dust sensor interfaced to an Arduino Uno R3, and a FONA808 3G communications module. The Arduino Uno was used to receive the signal from calibrated dust sensors to provide a concentration (µg/m3) of suspended particulate matter and coordinate wireless transmission of data via the 3G cellular network. Prior to use for monitoring, dust sensors were calibrated against a reference aerosol monitor (RAM-1) operating independently. Sodium chloride particles were generated inside of a 3.6 m3 mixing chamber while the RAM-1 and each dust sensor recorded signals and calibration was achieved for each dust sensor independently of others by direct comparison with the RAM-1 reading. In an effort to improve the quality of the data stream, the effect of averaging replicate individual pulses of the Sharp sensor when analyzing zero air has been studied. Averaging data points exponentially reduces standard deviation for all sensors with n < 2000 averages but averaging produced diminishing returns after approx. 2000 averages. The sensors exhibited standard deviations for replicate measurements of 3–6 µg/m3 and corresponding 3σ detection limits of 9–18 µg/m3 when 2000 pulses of the dust sensor LED were averaged over an approx. 2 min data collection/transmission cycle. To demonstrate portable monitoring, concentration values from the dust sensors were sent wirelessly in real time to a ThingSpeak channel, while tracking the sensor’s latitude and longitude using an on-board Global Positioning System (GPS) sensor. Outdoor and indoor air quality measurements were made at different places and times while human volunteers carried sensors. The measurements indicated walking by restaurants and cooking at home increased the exposure to particulate matter. The construction of the dust sensors and data collected from this research enhance the current research by describing an open-source concept and providing initial measurements. In principle, sensors can be massively multiplexed and used to generate real-time maps of particulate matter around a given location.

Improving the GRACE Kinematic Precise Orbit Determination Through Modified Clock Estimating.

Utilizing global positioning system (GPS) to determine the precise kinematic orbits for the twin satellites of the Gravity Recovery and Climate Experiment (GRACE) plays a very important role in the earth’s gravitational and other scientific fields. However, the orbit quality is highly depended on the geometry of observed GPS satellites. In this study, we propose a kinematic orbit determination method for improving the GRACE orbit quality especially when the geometry of observed GPS satellites is weak, where an appropriate random walk clock constraint between adjacent epochs is recommended according to the stability of on-board GPS receiver clocks. GRACE data over one month were adopted in the experimental validation. Results show that the proposed method could improve the root mean square (RMS) by 20–40% in radial component and 5–20% in along and cross components. For those epochs with position dilution of precision (PDOP) larger than 4, the orbits were improved by 50–70% in radial component and 17–50% in along and cross components. Meanwhile, the Allan deviation of clock estimates in the proposed method was much closer to the reported Allan deviation of GRACE on-board oscillator. All the results confirmed the improvement of the proposed method.

Precise Orbit Determination for the FY-3C Satellite Using Onboard BDS and GPS Observations from 2013, 2015, and 2017

Using the FengYun-3C (FY-3C) onboard BeiDou Navigation Satellite System (BDS) and Global Positioning System (GPS) data from 2013 to 2017, this study investigates the performance and contribution of BDS to precise orbit determination (POD) for a low-Earth orbit (LEO). The overlap comparison result indicates that code bias correction of BDS can improve the POD accuracy by 12.4%. The multi-year averaged one-dimensional (1D) root mean square (RMS) of the overlapping orbit differences (OODs) for the GPS-only solution is 2.0, 1.7, and 1.5 cm, respectively, during the 2013, 2015, and 2017 periods. The 1D RMS for the BDS-only solution is 150.9, 115.0, and 47.4 cm, respectively, during the 2013, 2015, and 2017 periods, which is much worse than the GPS-only solution due to the regional system of BDS and the few BDS channels of the FY-3C receiver. For the BDS and GPS combined solution (also known as the GC combined solution), the averaged 1D RMS is 2.5, 2.3, and 1.6 cm, respectively, in 2013, 2015, and 2017, while the GC combined POD presents a significant accuracy improvement after the exclusion of geostationary Earth orbit (GEO) satellites. The main reason for the improvement seen after this exclusion is the unfavorable satellite tracking geometry and poor orbit accuracy of GEO satellites. The accuracy of BDS-only and GC combined solutions have gradually improved from 2013 to 2017, thanks to improvements in the accuracy of International GNSS Service (IGS) orbit and clock products in recent years, especially the availability of a high-frequency satellite clock product (30 s sampling interval) since 2015. Moreover, the GC POD (without GEO) was able to achieve slightly better accuracy than the GPS-only POD in 2017, indicating that the fusion of BDS and GPS observations can improve the accuracy of LEO POD. GC combined POD can significantly improve the reliability of LEO POD, simply due to system redundancy. An increased contribution of BDS to LEO POD can be expected with the launch of more BDS satellites and with further improvements in the accuracy of BDS satellite products in the near future.

The Influence of Geomagnetic Storm of 7‐8 September 2017 on the Swarm Precise Orbit Determination

AbstractWe analyze a recent geomagnetic storm event on 7‐8 September 2017 to investigate the impact of geomagnetic storm on the precise orbit determination (POD) of Swarm constellation. The storm time performance of POD is analyzed. The quality of Swarm orbits are severely degraded during the storm main phase on 8 September and the maximum precision degradation reached over 10 cm. The enhanced thermospheric mass density at Swarm altitude during the storm enlarges the atmosphere drag for low Earth orbit satellites, which makes main contributions to the storm time degradation of Swarm orbit. This negative effect of enhanced atmosphere drag on the orbit estimation is mostly suppressed by estimating a more frequent atmosphere drag parameter. The higher‐order ionospheric effects on the POD of Swarm are also analyzed. The vertical total electron content derived from the Swarm onboard Global Positioning System receiver presents a larger enhancement on the dayside at low latitude and midlatitude during the storm main phase. This leads to an increase in the high‐order ionospheric effects, especially in the second‐order terms. No evident precision improvement is observed after correcting the high‐order ionospheric effects. The results demonstrate that during this geomagnetic storm, the enhanced thermospheric neutral density serves as a stronger error source than the enhanced ionospheric plasma density for the low Earth orbit satellite orbit determination processing.

Geomagnetically Conjugate Observations of Equatorial Plasma Irregularities From Swarm Constellation and Ground‐Based GPS Stations

AbstractThe near‐polar orbit satellites of Swarm mission provide a good opportunity to investigate the conjugacy of equatorial plasma irregularities (EPIs) since their trajectories at low latitudes are basically aligned with fixed geographical longitude. However, the Swarm in situ electron density occasionally shows EPIs at only one hemisphere at this longitude. In this study, we provide detailed analysis of such EPI events from the in situ electron densities and onboard global positioning system (GPS) measurements of Swarm low pair satellites, and simultaneous GPS data from two geomagnetically conjugate ground stations at the Africa longitudes. The result indicates that when Swam in situ electron density sometime shows EPIs at only one hemisphere, the GPS scintillations are still observed from the Swarm onboard receiver and by the two conjugate ground stations. It implies that the EPIs should generally elongate along the geomagnetic flux tube. More than two‐year statistic results show that the onset time of scintillation in the northern station is on average 16 and 18 min earlier than that in the southern station for September equinox and December solstice in 2015, while for March equinox in 2016 the onset time of scintillation of northern station is about 11 min later than that of southern station, which indicates the asymmetry features of EPIs along the flux tube. Further analysis of nearly three‐year GPS data from two conjugate stations at the Asia longitudes, we find that during solar maximum years the local sunset time plays an important role for causing the difference of onset time of scintillation between two conjugate stations.

Locating Multiple GPS Jammers Using Networked UAVs

Recent technologies, such as, Internet of Things and cloud services, increases the usage of small and low-cost networked unmanned aerial vehicles (UAVs), which needs to be robust against malicious global positioning system (GPS) attacks. Due to the availability of low-cost GPS jammers in the commercial market, there has been a rising risk of multiple jammers and not just one. However, it is challenging to locate multiple jammers because the traditional jammer localization via multilateration is applicable for only a single jammer case. Also, during a jamming attack, the positioning capability of an on-board GPS receiver is compromised given its inability to track GPS signals. We propose a simultaneous localization of multiple jammers and receivers (SLMR) algorithm by analyzing the variation in the front-end signal power recorded by the GPS receivers on-board a network of UAVs. Our algorithm not only locates multiple jammers but also utilizes these malicious sources as additional navigation signals for positioning the UAVs. We design a Gaussian mixture probability hypothesis density filter over a graph framework, which is optimized using a Levenberg–Marquardt minimizer. Using a simulated experimental setup, we validate the convergence and localization accuracy of our SLMR algorithm for various cases, including attacks with a single jammer, multiple jammers, and a varying number of jammers. We also demonstrate that our SLMR algorithm is able to simultaneously locate multiple jammers and UAVs, even for a larger transmitted power of the jammers.