Pair Of Satellites Research Articles

Comparison of Autonomous Orbit Determination for Satellite Pairs in Lunar Halo and Distant Retrograde Orbits

<h3>Summary</h3> A comprehensive study on the autonomous orbit determination (AOD) performance of satellite pairs in halo orbits and distant retrograde orbits (DROs) is carried out. A factor called <i>dynamic and geometric dilution of precision</i> (DAGDOP) is proposed to simultaneously incorporate influences from the dynamics and geometry of satellite pairs. Based on the DAGDOP, the effect of different observation arcs on the AOD accuracy is investigated. Next, the AOD accuracy of three different types of satellite pairs—halo+halo, DRO+DRO, and halo+DRO—is systematically analyzed. The hybrid halo+DRO type shows the best overall accuracy. Finally, the AOD performance of the hybrid type is verified in a realistic model. Our studies find that the average AOD accuracy of the halo orbit is about 170 meters, and that of the DRO is about 190 meters. The relative time synchronization error of two satellites is less than 30 nanoseconds.

Spatiotemporal Characterization of Geophysical Signal Detection Capabilities of GRACE‐FO

AbstractThe intersatellite range measurements between the Gravity Recovery and Climate Experiment Follow‐On (GRACE‐FO) satellite pair enable mapping time variations in Earth's gravity field. The Laser Ranging Interferometer (LRI) and Microwave Instrument (MWI) provide independent range measurements. We compare gravity fields derived from the two instruments. The LRI captures time‐variable gravity field signals up to 21 mHz (165 km half‐wavelength) globally, while MWI reaches frequencies up to 15 mHz (230 km). Time‐variable mass variations free from annual cycles and trends are sensed up to 15 mHz (LRI) and 11 mHz (320 km) (MWI). Both instruments are shown to be excellent tools for detection and monitoring of sub‐monthly geophysical signals. Along‐track global maps of sub‐monthly signals are expected to shed light on the high‐frequency behavior of a large number of geophysical processes.

Development and Simulation of Motion Control System for Small Satellites Formation

In the paper, the problem of forming and maintaining the small satellites formation in the near-earth projected circular orbits is considered. The satellite formation reconfiguration and formation-keeping control laws are proposed by employing the passivity-based output feedback control. For the complete nonlinear and time-dependent dynamics of the relative motion of a pair of satellites in elliptical orbits, new combined control algorithms, including a consensus protocol, are proposed and analyzed. A comparison of the control modes using the passivity-based output feedback control and the proportional-differential controller with and without the consensus algorithm is given. On the basis of the passification method, the algorithm is obtained ensuring the stable motion of the slave satellite relative to the orbit of the master satellite. To improve the accuracy of the satellites’ positioning, a consensus protocol based on measurements of the relative positions of the satellites is proposed and studied. Computer simulations of the proposed algorithms for options to construct formations are provided for two projected circular orbits of 8 satellites, demonstrating the efficiency of the proposed control schemes. It is shown that the resulting passivity-based output feedback control provides better accuracy than the PD controller. It is also shown that the use of the consensus protocol further increases the positioning accuracy of the satellite constellation.

Geodetic observations for constraining mantle processes in Antarctica

Abstract Geodynamic processes in Antarctica such as glacial isostatic adjustment (GIA) and post-seismic deformation are measured by geodetic observations such as global navigation satellite systems (GNSS) and satellite gravimetry. GNSS measurements have comprised both continuous measurements and episodic measurements since the mid-1990s. The estimated velocities typically reach an accuracy of 1 mm a −1 for horizontal velocities and 2 mm a −1 for vertical velocities. However, the elastic deformation due to present-day ice-load change needs to be considered accordingly. Space gravimetry derives mass changes from small variations in the inter-satellite distance of a pair of satellites, starting with the GRACE (Gravity Recovery and Climate Experiment) satellite mission in 2002 and continuing with the GRACE-FO (GRACE Follow-On) mission launched in 2018. The spatial resolution of the measurements is low (about 300 km) but the measurement error is homogeneous across Antarctica. The estimated trends contain signals from ice-mass change, and local and global GIA signals. To combine the strengths of the individual datasets, statistical combinations of GNSS, GRACE and satellite altimetry data have been developed. These combinations rely on realistic error estimates and assumptions of snow density. Nevertheless, they capture signals that are missing from geodynamic forward models such as the large uplift in the Amundsen Sea sector caused by a low-viscous response to century-scale ice-mass changes.

Ground electron calibration of Charged Particle Detectors onboard GECAM satellite

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a pair of small satellites designed to monitor the Gamma-Ray Bursts (GRBs), some of them are possibly associated with Gravitational Wave Events (GWE). Each satellite includes twenty-five Gamma-Ray Detectors (GRDs) and eight Charged Particle Detectors (CPDs). GRDs are used to detect X-rays and gamma-rays to realize the observation and location of significant events such as GRBs or other X-ray Bursts (XRBs). Meanwhile, CPDs are used to help GRDs distinguish between GRBs and charged particle events (the majority are electrons) in space. In this paper, we present the electron calibration results of the CPDs, including the energy linearity, energy resolution, angular response, detection efficiency and response in high count rates. Moreover, we use the calibration spectra to verify and correct the mass model of CPD in Geant4 simulation. After the correction, the simulated spectra are in good agreement with the measurements.

Investigation of temporal baseline effect on DEMs derived from COSMO Sky-Med data

Digital elevation models (DEM) are indispensable elements of sensitive earth science studies. It is important the production and usage of DEMs. The science of remote sensing offers scientists an important source of data on this subject. Radar data, which is an active remote sensing system, has an important capacity in this regard. DEM production using InSAR data has been widely used in the literature in the last decade. The temporal baseline parameter, which is an important factor in data generation from InSAR pairs, also affects the final products. In this study, it is aimed to examine the usability of these data by producing short (4days), medium (84 days) and long (440 days) baseline DEMs using InSAR pairs of COSMO Sky-Med satellite. At the same time, photogrammetric DEMs were produced with unmanned aerial vehicles (UAV) in selected pilot areas. The DEMs produced were evaluated in 4 land surface types, namely plain-bare, agricultural, urban and rugged area. In addition, by performing statistical analyzes such as RMSE, MAE, the accuracy of the produced DEMs compared to the DEMs produced with UAV was examined. The results showed that short and medium baseline data give more accurate results than long baseline InSAR pairs. Increasing the temporal baseline, increases the amount of error in the DEMs produced. Also, the effect of land surface types on the produced DEMs was revealed in the results of the study.

Next Generation Gravity Mission Elements of the Mass Change and Geoscience International Constellation: From Orbit Selection to Instrument and Mission Design

ESA’s Next Generation Gravity Mission (NGGM) is a candidate Mission of Opportunity for ESA–NASA cooperation in the frame of the Mass Change and Geosciences International Constellation (MAGIC). The mission aims at enabling long-term monitoring of the temporal variations of Earth’s gravity field at relatively high temporal (down to 3 days) and increased spatial resolutions (up to 100 km) at longer time intervals. This implies also that time series of GRACE and GRACE-FO can be extended towards a climate series. Such variations carry information about mass change induced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere, land and solid Earth and will complete our picture of global and climate change. The main observable is the variation of the distance between two satellites measured by a ranging instrument. This is complemented by accelerometers that measure the nongravitational accelerations, which need to be reduced from ranging measurements to obtain the gravity signal. The preferred satellite constellation comprises one satellite pair in a near-polar and another in an inclined circular orbit. The paper focuses on the orbit selection methods for optimizing the spatial sampling for multiple temporal resolutions and then on the methodology for deriving the engineering requirements for the space segment, together with a discussion on the main mission parameters.

Investigation of Doppler Collision Effects in Kinematic Conditions and its Mitigation for NavIC System

NavIC (Navigation with Indian Constellation) constellation consists of four geosynchronous and three geostationary satellites and it is developed by ISRO, India. It provides position, velocity and timing services. Doppler Collision (DC) is a phenomenon where tracking errors are introduced in the measurements due to cross-correlation between the satellites. If relative doppler between satellites is less than the code loop bandwidth, then DC occurs. In this paper, to analyze DC impact on NavIC, the Doppler shift of each GEO satellite is calculated, the most effected GEO satellite pair is identified. In high dynamic applications like missile launches the effect of DC is very significant. In order to investigate DC in high dynamic conditions a simulation of trajectory path of the receiver is considered. For the precise position estimation, the contribution of DC error will be high, so it needs to be minimized. An efficient algorithm is developed to mitigate the DC using narrow correlator design of the receiver. Using the proposed algorithm, the DC duration for 1C-1G satellite pair has been reduced from 50min 21sec to 2sec in static conditions. Whereas in dynamic conditions from 4h20min 28sec to 5sec with DLL bandwidth of 4Hz and 0.1 chip spacing of receiver design.

Analyses of Laser Propagation Noises for TianQin Gravitational Wave Observatory Based on the Global Magnetosphere MHD Simulations

Abstract TianQin is a proposed space-borne gravitational-wave (GW) observatory composed of three identical satellites orbiting around the geocenter with a radius of 105 km. It aims at detecting GWs in the frequency range of 0.1 mHz–1 Hz. The detection of GW relies on the high-precision measurement of optical path length at the 10−12 m level. The dispersion of space plasma can lead to the optical path difference (OPD, Δl) along the propagation of laser beams between any pair of satellites. Here, we study the OPD noises for TianQin. The Space Weather Modeling Framework is used to simulate the interaction between the Earth magnetosphere and solar wind. From the simulations, we extract the magnetic field and plasma parameters on the orbits of TianQin at four relative positions of the satellite constellation in the Earth magnetosphere. We calculate the OPD noise for single link, Michelson combination, and Time-Delay Interferometry (TDI) combinations (α and X). For single link and Michelson interferometer, the maxima of ∣Δl∣ are on the order of 1 pm. For the TDI combinations, these can be suppressed to about 0.004 and 0.008 pm for α and X. The OPD noise of the Michelson combination is colored in the concerned frequency range; while the ones for the TDI combinations are approximately white. Furthermore, we calculate the ratio of the equivalent strain of the OPD noise to that of TQ, and find that the OPD noises for the TDI combinations can be neglected in the most sensitive frequency range of TQ.

The Onset of Chaos in Permanently Deformed Binaries from Spin–Orbit and Spin–Spin Coupling

Abstract Permanently deformed objects in binary systems can experience complex rotation evolution, arising from the extensively studied effect of spin–orbit coupling as well as more nuanced dynamics arising from spin–spin interactions. The ability of an object to sustain an aspheroidal shape largely determines whether or not it will exhibit nontrivial rotational behavior. In this work, we adopt a simplified model of a gravitationally interacting primary and satellite pair, where each body’s quadrupole moment is approximated by two diametrically opposed point masses. After calculating the net gravitational torque on the satellite from the primary, as well as the associated equations of motion, we employ a Hamiltonian formalism that allows for a perturbative treatment of the spin–orbit and retrograde and prograde spin–spin coupling states. By analyzing the resonances individually and collectively, we determine the criteria for resonance overlap and the onset of chaos, as a function of orbital and geometric properties of the binary. We extend the 2D planar geometry to calculate the obliquity evolution. This calculation indicates that satellites in spin–spin resonances undergo precession when inclined out of the plane, but they do not tumble. We apply our resonance overlap criteria to the contact binary system (216) Kleopatra, and find that its satellites, Cleoselene and Alexhelios, may plausibly be exhibiting chaotic rotational dynamics from the overlap of the spin–orbit and retrograde spin–spin resonances. While this model is, by construction, generalizable to any binary system, it will be particularly useful for studies of small bodies in the Solar System, whose irregular shapes make them ideal candidates for exotic rotational states.

Early detection of bark beetle infestation in Norway spruce forests of Central Europe using Sentinel-2

In the past decade, massive outbreaks of bark beetles (Ips spp.) have caused large-scale decline of coniferous-dominated, prevailingly managed forests of Central Europe. Timely detection of newly infested trees is important for minimizing economic losses and effectively planning forest management activities to stop or at least slow outbreaks. With the advancement of Copernicus services, a pair of Sentinel-2 satellites provides a unique remote sensing data source of multi-temporal observations in high spatial resolution on the scale of individual forest stands (although not allowing for individual tree detection). This study investigates the potential for using seasonal trajectories of Sentinel-2 bands and selected vegetation indices in early detection of bark beetle infestation (so–called green-attack stage detection) in Norway spruce monoculture forests in the Czech Republic. Spectral trajectories of nine bands and six vegetation indices were constructed for the 2018 vegetation season using 14 satellite observations from April to November to distinguish four infestation classes. We used a random forest algorithm to classify healthy (i.e., stands not infested) and infested trees with various trajectories of decay. The seasonal trajectories of vegetation indices separated the infestation classes better than did the individual bands. Among the most promising vegetation indices we identified the tasselled cap wetness (TCW) component and normalized difference index constructed from near and shortwave infrared bands. Analysing the inter-annual change of the indices was more promising for early detection than is single-date classification. It achieved 96% classification accuracy on day of year 291 for the tested data set.The algorithm for early detection of tree infestation based on the assessment of seasonal changes in TCW was applied on the time series of Sentinel-2 observations from 2019 and its outputs were verified using field observations of forest conditions conducted on 80 spruce forest plots (located in spruce monoculture stands). The overall accuracy of 78% was achieved for the separation of healthy and green-attack classes. Our study highlights the great potential of multi-temporal remote sensing and the use of shortwave infrared wavelengths for early detection of spruce forest decline caused by bark beetle infestation.

Building the Galilean moons system via pebble accretion and migration: a primordial resonant chain

ABSTRACT The origins of the Galilean satellites – namely Io, Europa, Ganymede, and Callisto – is not fully understood yet. Here we use N-body numerical simulations to study the formation of Galilean satellites in a gaseous circumplanetary disc around Jupiter. Our model includes the effects of pebble accretion, gas-driven migration, and gas tidal damping and drag. Satellitesimals in our simulations first grow via pebble accretion and start to migrate inwards. When they reach the trap at the disc inner edge, scattering events and collisions take place promoting additional growth. Growing satellites eventually reach a multiresonant configuration anchored at the disc inner edge. Our results show that an integrated pebble flux of ≥2 × 10−3 MJ results in the formation of satellites with masses typically larger than those of the Galilean satellites. Our best match to the masses of the Galilean satellites is produced in simulations where the integrated pebble flux is ∼10−3 MJ. These simulations typically produce between three and five satellites. In our best analogues, adjacent satellite pairs are all locked in 2:1 mean motion resonances. However, they have also moderately eccentric orbits (∼0.1), unlike the current real satellites. We propose that the Galilean satellites system is a primordial resonant chain, similar to exoplanet systems as TRAPPIST-1, Kepler-223, and TOI-178. Callisto was probably in resonance with Ganymede in the past but left this configuration – without breaking the Laplacian resonance – via divergent migration due to tidal planet–satellite interactions. These same effects further damped the orbital eccentricities of these satellites down to their current values (∼0.001). Our results support the hypothesis that Io and Europa were born with water-ice rich compositions and lost all/most of their water afterwards. Firmer constraints on the primordial compositions of the Galilean satellites are crucial to distinguish formation models.

What Can We Expect from the Inclined Satellite Formation for Temporal Gravity Field Determination?

The assumption of implementing a Bender-type mission with two pairs of inline satellites by two institutions, one in the inclined satellite formation (ISF) and the other in the polar satellite formation (PSF), has been proven as an optimal selection for the next-generation gravimetric mission (NGGM). The prerequisite of this assumption is to ensure the combination as well as the independency of the missions in the PSF scenario as well as in the ISF scenario. To promote NGGM, we implement a close-loop simulation study to evaluate the performance of the stand-alone ISF mission. In the spectral domain, the ISF estimations show extremely poor quality for zonal and near-zonal coefficients due to the absence of observations over the polar regions, while the sectorial and near-sectorial coefficient estimations show approximately 71 ~ 77% noise reductions when compared with the PSF estimations. In the spectral domain, the ISF mission presents its superior capability in detecting the Earth’s mass variations within its observational areas than the PSF mission. The improvements of ISF are also obtained over the transition zones (50°N ~ 70°N and 50°S ~ 70°S) with 41% noise reductions. In addition, the general better performance of gravity estimations via ISF may promote the potential geoscience applications over the small river basins, the coastal ice sheet and the earthquake areas with better accuracy and finer resolution. The simulation results demonstrate the feasibility of implementing a stand-alone mission in the inclined orbits, and the further possibility of promoting a Bender-type mission via a profound cooperation by two institutions.

General relativistic effects acting on the orbits of Galileo satellites

The first pair of satellites belonging to the European Global Navigation Satellite System (GNSS)—Galileo—has been accidentally launched into highly eccentric, instead of circular, orbits. The final height of these two satellites varies between 17,180 and 26,020 km, making these satellites very suitable for the verification of the effects emerging from general relativity. We employ the post-Newtonian parameterization (PPN) for describing the perturbations acting on Keplerian orbit parameters of artificial Earth satellites caused by the Schwarzschild, Lense–Thirring, and de Sitter general relativity effects. The values emerging from PPN numerical simulations are compared with the approximations based on the Gaussian perturbations for the temporal variations of the Keplerian elements of Galileo satellites in nominal, near-circular orbits, as well as in the highly elliptical orbits. We discuss what kinds of perturbations are detectable using the current accuracy of precise orbit determination of artificial Earth satellites, including the expected secular and periodic variations, as well as the constant offsets of Keplerian parameters. We found that not only secular but also periodic variations of orbit parameters caused by general relativity effects exceed the value of 1 cm within 24 h; thus, they should be fully detectable using the current GNSS precise orbit determination methods. Many of the 1-PPN effects are detectable using the Galileo satellite system, but the Lense–Thirring effect is not.

Living with Neighbors. III. The Origin of the Spin–Orbit Alignment of Galaxy Pairs: A Neighbor versus the Large-scale Structure

Abstract Recent observations revealed a coherence between the spin vector of a galaxy and the orbital motion of its neighbors. We refer to the phenomenon as “the spin–orbit alignment (SOA)” and explore its physical origin via the IllustrisTNG simulation. This is the first study to utilize a cosmological hydrodynamic simulation to investigate the SOA of galaxy pairs. In particular, we identify paired galaxies at z = 0 having the nearest neighbor with mass ratios from 1/10 to 10 and calculate the spin–orbit angle for each pair. Our results are as follows. (a) There exists a clear preference for prograde orientations (i.e., SOA) for galaxy pairs, qualitatively consistent with observations. (b) The SOA is significant for both baryonic and dark matter spins, being the strongest for gas and the weakest for dark matter. (c) The SOA is stronger for less massive targets and for targets having closer neighbors. (d) The SOA strengthens for galaxies in low-density regions, and the signal is dominated by central–satellite pairs in low-mass halos. (e) There is an explicit dependence of the SOA on the duration of interaction with its current neighbor. Taken together, we propose that the SOA witnessed at z = 0 has been developed mainly by interactions with a neighbor for an extended period of time, rather than tidal torque from the ambient large-scale structure.

Initial Assessment of Galileo Triple-Frequency Ambiguity Resolution between Reference Stations in the Hong Kong Area

The European Global Navigation Satellite System Galileo is gradually deploying its constellation. In order to provide reliable navigation and position services, the effectiveness and reliability of ambiguity resolution between reference stations is necessary in network real-time kinematic (NRTK). The multifrequency signal of Galileo could much enhance the ambiguity resolution (AR) reliability and robustness. In this study, to exploit full advantage of this, the geometry-free (GF) TCAR and ionospheric-free (IF) triple-carrier ambiguity resolution (TCAR) methods were utilized in solving the ambiguity in the Hong Kong area, which is an ionosphere disturbance active area, and compared with each other. The IF TCAR method was then used to combine multi-systems to improve Galileo E1 AR performance, which is named as the combined IF (CIF) TCAR method. Three experiments were carried out in the Hong Kong area and the results showed that the Galileo-only system could fix ambiguities on all satellite pairs correctly and reliably by the IF TCAR method, while the GF TCAR method showed a weaker performance. The wide-lane (WL) convergence time of the IF TCAR method is improved by about 37.6%. The IF TCAR method with respect to the GF TCAR method could improve the WL accuracy by 21.6% and the E1 accuracy by 72.7%, respectively. Compared with GPS-only TCAR or Galileo-only TCAR, the ambiguity accuracy and the convergence time of the CIF TCAR method, which combines GPS and Galileo, could be improved by about 25.7% and 47.1%, respectively.

Solar activity dependence of medium-scale traveling ionospheric disturbances using GPS receivers in Japan

In order to reveal solar activity dependence of the medium-scale traveling ionospheric disturbances (MSTIDs) at midlatitudes, total electron content (TEC) data obtained from a Global Positioning System (GPS) receiver network in Japan during 22 years from 1998 to 2019 were analyzed. We have calculated the detrended TEC by subtracting the 1-h running average from the original TEC data for each satellite and receiver pair, and made two-dimensional TEC maps of the detrended TEC with a spatial resolution of 0.15° × 0.15° in longitude and latitude. We have investigated MSTID activity, defined as delta I/overline{I}, where delta I and overline{I} are standard deviation of the detrended TEC and the average vertical TEC within the area of 133.0°–137.0° E and 33.0°–37.0° N for 1 h, respectively. From each 2-h time series of the detrended TEC data within the same area as the MSTID activity, auto-correlation functions (ACFs) of the detrended TEC were calculated to estimate the horizontal propagation velocity and direction of the MSTIDs. Statistical results of the MSTID activity and propagation direction of MSTIDs were consistent with previous studies and support the idea that daytime MSTIDs could be caused by atmospheric gravity waves, and that nighttime MSTIDs were caused by electro-dynamical forces, such as the Perkins instability. From the current long-term observations, we have found that the nighttime MSTID activity and occurrence rate increased with decreasing solar activity. For the daytime MSTID, the occurrence rate increased with decreasing solar activity, whereas the MSTID activity did not show distinct solar activity dependence. These results suggest that the secondary gravity waves generated by dissipation of the primary gravity waves propagating from below increase under low solar activity conditions. The mean horizontal phase velocity of the MSTIDs during nighttime did not show a distinct solar activity dependence, whereas that during daytime showed an anticorrelation with solar activity. The horizontal phase velocity of the daytime MSTIDs was widely distributed from 40 to 180 m/s under high solar activity conditions, whereas it ranged between 80 and 200 m/s, with a maximum occurrence at 130 m/s under low solar activity conditions, suggesting that gravity waves with low phase velocity could be dissipated by high viscosity in the thermosphere under low solar activity conditions.

Co-estimating geomagnetic field and calibration parameters: modeling Earth\u2019s magnetic field with platform magnetometer data

Models of the geomagnetic field rely on magnetic data of high spatial and temporal resolution to give an accurate picture of the Earth’s internal magnetic field and its time-dependence. The magnetic data from low-Earth orbit satellites of dedicated magnetic survey missions such as CHAMP and Swarm play a key role in the construction of such models. Unfortunately, there are no magnetic data available from such satellites after the end of the CHAMP mission in 2010 and before the launch of the Swarm mission in late 2013. This limits our ability to recover signals on timescales of 3 years and less during this gap period. The magnetic data from platform magnetometers carried by satellites for navigational purposes may help address this data gap provided that they are carefully calibrated. Earlier studies have demonstrated that platform magnetometer data can be calibrated using a fixed geomagnetic field model as reference. However, this approach can lead to biased calibration parameters. An alternative approach has been developed in the form of a co-estimation scheme which consists of simultaneously estimating both the calibration parameters and a model of the internal part of the geomagnetic field. Here, we go further and develop a scheme, based on the CHAOS field modeling framework, that involves co-estimation of both internal and external geomagnetic field models along with calibration parameters of platform magnetometer data. Using our implementation, we are able to derive a geomagnetic field model spanning 2008 to 2018 with satellite magnetic data from CHAMP, Swarm, secular variation data from ground observatories, and platform magnetometer data from CryoSat-2 and the GRACE satellite pair. Through a number of experiments, we explore correlations between the estimates of the geomagnetic field and the calibration parameters, and suggest how these may be avoided. We find evidence that platform magnetometer data provide additional information on the secular acceleration, especially in the Pacific during the gap between CHAMP and Swarm. This study adds to the evidence that it is beneficial to use platform magnetometer data in geomagnetic field modeling.

Impact of the third frequency GNSS pseudorange and carrier phase observations on rapid PPP convergences

New GNSS signals have significantly augmented positioning service and promoted algorithmic innovations such as rapid PPP convergence. With the emerging of multifrequency signals, it becomes essential to thoroughly explore the contribution of third frequency pseudorange and carrier phase toward PPP. In this study, we research the role of the third frequency observations on accelerating PPP convergence, commencing from both stochastic and functional models. We first constructed the stochastic model depending on the observation noise and then introduced two uncombined functional models with respect to different inter-frequency bias (IFB) estimation strategies. The double-differenced residuals based on a zero baseline were used to evaluate the signal noises, which were 0.09, 0.07, 0.11, 0.01 and 0.09 m for Galileo E1/E5a/E5b/E5/E6 pseudorange and 0.24, 0.31 and 0.05 m for BeiDou B1/B2/B3. Besides, carrier phase observations E5a/E5/E6/B1I/B3I shared a comparable signal noise of 0.002 m, while the signal noises of E1/E5b/B2I were 0.003 m. Both BeiDou-2/Galileo and Galileo-only float PPP were implemented based on the dataset collected from 25 stations, spanning 30 days. Triple-frequency Galileo PPP achieved convergence successfully in 19.9 min if observations were weighted according to observation precision, showing a comparable performance of dual-frequency PPP. Meanwhile, the convergence time of triple-frequency float PPP was further shortened to 19.2 min when satellite pair IFBs were eliminated by estimating a second satellite clock. While the improvement of triple-frequency float PPP was marginal, triple-frequency PPP-AR using signals E1/E5a/E6 shortened the initialization time of the dual-frequency counterpart by 38%. Moreover, the performance of triple-frequency PPP-AR kept almost unchanged after we excluded the third frequency pseudorange observations. We thus suggest that the contribution of the third frequency to PPP mainly rests on ambiguity resolution, favored by the additional carrier phase observations.

Joint PAZ &amp; TanDEM-X Mission Interferometric Experiments: Interoperability and Products

This article analyzes the potential synergies between the two German satellites of the TerraSAR-X/TanDEM-X (TSX/TDX) mission and the Spanish PAZ satellite. Since both platforms are almost identical and they were launched in the same orbital plane, comparable images of the same area under the same geometry may be regularly acquired by both systems. This allows the simultaneous exploitation of both missions for building larger time series and improving the revisit time of 11 days for each mission up to 4 or 7 days while combining both. The geometric accuracy and the interferometric performance when combining both sensors are investigated, considering different acquisition parameters and land cover types. For this article several images are acquired over German and Spanish calibration test sites. The results obtained when combining acquisitions from satellites of the TSX/TDX and PAZ missions show no loss of performance with respect to image pairs of the same satellite, indicating that both missions might be exploited synergistically in order to improve current monitoring applications or maybe to allow newer application with tighter requirements in terms of revisit time or temporal coverage. In order to explore all these possibilities, a call for scientific proposals will be jointly opened by the coordinators of the scientific activities of both missions in order to explore the simultaneous exploitation of TSX/TDX and PAZ data.