The accurate localization of gamma-ray bursts (GRBs) remains a crucial task. Historically, improv... more The accurate localization of gamma-ray bursts (GRBs) remains a crucial task. Historically, improved localizations have led to the discovery of afterglow emission and the realization of their cosmological distribution via redshift measurements; however, a more recent requirement comes with the potential of studying the kilonovae of neutron star mergers. Gravitational wave detectors are expected to provide locations to not better than 10 square degrees over the next decade. With their increasing horizon for merger detections the intensity of the gamma-ray and kilonova emission also drops, making their identification in large error boxes a challenge. Thus, a localization via the gamma-ray emission seems to be the best chance to mitigate this problem. Here we propose to equip some of the second-generation Galileo satellites with dedicated GRB detectors. This saves costs for launches and satellites for a dedicated GRB network, the large orbital radius is beneficial for triangulation, and...
In the framework of the ESA GSTP study “Accurate Orbit Determination of Space Debris with Laser T... more In the framework of the ESA GSTP study “Accurate Orbit Determination of Space Debris with Laser Tracking / Tasking” both two-color and multi-static laser tracking data was acquired. We validate these observations and assess orbits computed on their basis. In this context, we deliver insight into the technology’s accuracy and precision by comparing observed versus computed measurements from reference orbits and by assessing their high-frequency components with regard to object geometries. Moreover, based on post-fit and cross-validation residuals of our orbit products we validate the overall processing procedure which comprises data filtering, estimating force model coefficients from Two-LineElements (TLEs), as well as the actual orbit determination process. Eventually, further comparisons with external orbit data support our statements about the validity and quality of our data and orbit products, respectively.
Recent progress in the domain of time and frequency (T/F) standards requires important improvemen... more Recent progress in the domain of time and frequency (T/F) standards requires important improvements of existing time distribution links. Among these, the accuracy of time transfer is actually an important part of the concerns in order to establish and maintain time & space references from ground and/or space facilities. Several time transfers by laser link projects have been carried out over the past 10 years with numerous scientific and metrological objectives. Satellite Laser ranging (SLR) has proven to be a fundamental tool, offering a straightforward, conceptually simple, highly accurate and unambiguous observable. Depending on the mission, LR is used to transmit time over two-way or one-way distances from 500 to several millions of km. The following missions and their objectives employed this technique: European Laser Timing (ELT) at 450 km, Time Transfer by Laser Link (T2L2) at 1,336 km, Laser Time Transfer (LTT) at 36,000 km, Lunar Reconnaissance Orbiter (LRO) at 350,000 km, and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) at tens of million km. This article describes the synergy between SLR and T/F technologies developed on the ground and in space and as well as the state of the art of their exploitation. The performance and sources of limitation of such space missions are analyzed. It shows that current and future challenges lie in the improvement of the time accuracy and stability of the time for ground geodetic observatories. The role of the next generation of SLR systems is emphasized both in space and at ground level, from the point of view of GGOS and valuable exploitation of the synergy between time synchronization, ranging and data transfer.
Precise point positioning with integer ambiguity resolution requires precise knowledge of satelli... more Precise point positioning with integer ambiguity resolution requires precise knowledge of satellite position, clock and phase bias corrections. In this paper, a method for the estimation of these parameters with a global network of reference stations is presented. The method processes uncombined and undifferenced measurements of an arbitrary number of frequencies such that the obtained satellite position, clock and bias corrections can be used for any type of differenced and/or combined measurements. We perform a clustering of reference stations. The clustering enables a common satellite visibility within each cluster and an efficient fixing of the double difference ambiguities within each cluster. Additionally, the double difference ambiguities between the reference stations of different clusters are fixed. We use an integer decorrelation for ambiguity fixing in dense global networks. The performance of the proposed method is analysed with both simulated Galileo measurements on E1 and E5a and real GPS measurements of the IGS network. We defined 16 clusters and obtained satellite position, clock and phase bias corrections with a precision of better than 2 cm. Keywords Network solution • Satellite phase biases • Satellite position and clock corrections • Ambiguity fixing Kalman filter Kalman filter Ambiguity fixing Ambiguity fixing Kalman filter Ambiguity fixing Least−squares Estimation Ambiguity fixing Cluster 2 Cluster 1 reference cluster Combination of individual clusters Kalman filter Cluster C Ambiguity fixing Cluster optimization Coordinates of reference stations position, clock and phase bias corrections of satellites combined position, clock and phase bias corrections of all satellites
2006 IEEE International Frequency Control Symposium and Exposition, 2006
Carrier Phase GPS observations between a geodetic receiver at the Physikalisch-Technische Bundesa... more Carrier Phase GPS observations between a geodetic receiver at the Physikalisch-Technische Bundesanstalt (PTB) and two geodetic receivers at the USNO are processed using applications and extensions of the GIPSY and Bernese GPS Software packages. Their results are compared with Two Way Satellite Time and Frequency Transfer (TWSTFT) data. It is found that algorithms that eliminate day-boundary effects require careful handling in the presence of receiver instrumental delays. Depending upon the approach chosen, time differences of several ns and frequency differences of up to 100 ps/day can develop between solution types.
GPS satellite orbits available from the International GNSS Service (IGS) show a consistent radial... more GPS satellite orbits available from the International GNSS Service (IGS) show a consistent radial bias of up to several cm and a particular pattern in the Satellite Laser Ranging (SLR) residuals, which are suggested to be related to radiation pressure mismodeling. In addition, orbit-related frequencies were identified in geodetic time series such as apparent geocenter motion and station displacements derived from GPS tracking data. A potential solution to these discrepancies is the inclusion of Earth radiation pressure (visible and infrared) modeling in the orbit determination process. This is currently not yet considered by all analysis centers contributing to the IGS final orbits. The acceleration, accounting for Earth radiation and satellite models, is introduced in this paper in the computation of a global GPS network (around 200 IGS sites) adopting the analysis strategies from the Center for Orbit Determination in Europe (CODE). Two solutions covering 9 years (2000-2008) with and without Earth radiation pressure were computed and form the basis for this study. In previous studies, it has been shown that Earth radiation pressure has a non-negligible effect on the GPS orbits, mainly in the radial component. In this paper, the effect on the along-track and cross-track components is studied in more detail. Also in this paper, it is shown that Earth radiation pressure leads to a change in the estimates of GPS ground station positions, which is systematic over large regions of the Earth. This observed "deformation" of the Earth is towards North-South and with large scale patterns that repeat six times per GPS draconitic year (350 days),
The Center for Orbit Determination in Europe (CODE) has been involved in the processing of combin... more The Center for Orbit Determination in Europe (CODE) has been involved in the processing of combined GPS/GLONASS data during the International GLONASS Experiment (IGEX). The resulting precise orbits were analyzed using the program SORBDT. Introducing one satellite's positions as pseudo-observations, the program is capable of fitting orbital arcs through these positions using an orbit improvement procedure based on the numerical integration of the satellite's orbit and its partial derivative with respect to the orbit parameters. For this study, the program was enhanced to estimate selected parameters of the Earth's gravity field. The orbital periods of the GPS satellites are-in contrast to those of the GLON-ASS satellites-2 : 1 commensurable (P Sid : P GPS) with the rotation period of the Earth. Therefore, resonance effects of the satellite motion with terms of the geopotential occur and they influence the estimation of these parameters. A sensitivity study of the GPS and GLON-ASS orbits with respect to the geopotential coefficients reveals that the correlations between different geopotential coefficients and the correlations of geopotential coefficients with other orbit parameters, in particular with solar radiation pressure parameters, are the crucial issues in this context. The estimation of the resonant geopotential terms is, in the case of GPS, hindered by correlations with the simultaneously estimated radiation pressure parameters. In the GLONASS case, arc lengths of several days allow the decorrelation of the two parameter types. The formal errors of the estimates based on the GLONASS orbits are a factor of 5 to 10 smaller for all resonant terms.
ABSTRACT At the altitude of the CPS satellites the most important non-gravitational perturbation ... more ABSTRACT At the altitude of the CPS satellites the most important non-gravitational perturbation is caused by the solar radiation pressure acting on the satellite body and its solar panels. The development of high-fidelity radiation pressure models may be motivated by the following observation: The GPS satellites are orbiting in a 2:1-commensurability with the Earth's rotation which causes resonance. The expected sensitivity to specific coefficients of the geopotential is, however, significantly reduced by strong correlations of these parameters with radiation pressure parameters. Sophisticated radiation pressure models rely on a precise knowledge of the satellite's attitude which does not only affects the location of the antenna phase center or the phase windup of the signal carrier but, through radiation pressure, also the orbital dynamics. PRN 23, whose attitudinal behaviour was modified early in 2002 is an interesting case. Due to this change an impressive improvement in the orbit quality could be achieved.
The time transfer techniques used to generate TAI are currently the TWSTFT (TW), GPS C/A, and GPS... more The time transfer techniques used to generate TAI are currently the TWSTFT (TW), GPS C/A, and GPS P3. About 19% of all TAI laboratories possess P3, 12% possess TW backed up with P3, i.e., in total only 31% of the TAI laboratories transfer 100% of the primary frequency standards (PFS) including all the Cs fountains and 80% of the total weighted clocks for TAI. GPS carrier phase (CP) data are co-products of P3 receivers and they are never used in TAI. An effective strategy to improve TAI is to strengthen these TW+P3 links by using the already available CP information. We first review the classical TAI time transfer techniques and solutions of different analysis strategies of the CP data, namely the IGS [1], the AIUB [2], and the NRCan PPP [3]. To evaluate all the different time transfer techniques, we carried out an extensive comparison between the above techniques, over different periods from hours to 4 months and over different distances on inner- resp. inter-continental baselines b...
The altimetry satellite JASON-1 carries several independent instruments for precise orbit determi... more The altimetry satellite JASON-1 carries several independent instruments for precise orbit determination such as a DORIS receiver, a GPS receiver, and an SLR reflector. DORIS and GPS are both based on microwave phase measurements and not fundamentally different. A new analysis technique for processing DORIS observations has been developed which follows a GPS-like approach. This new method is used to
ABSTRACT QZS-1, the first satellite of the Japanese Quasi Zenith Satellite System (QZSS) was laun... more ABSTRACT QZS-1, the first satellite of the Japanese Quasi Zenith Satellite System (QZSS) was launched in September 2010. Transmission of the standard codes started in December 2010 and the satellite was declared healthy in June 2011. Five stations of the COoperative Network for GIOVE Observation (CONGO) were upgraded to provide QZSS tracking capability. These five stations provide the basis for the precise orbit determination (POD) of the QZS-1 spacecraft. The stability and consistency of different orbital arc lengths is analyzed based on orbit fit residuals, day boundary discontinuities, and Satellite Laser Ranging residuals. As QZS-1 simultaneously transmits navigation signals on three frequencies in the L1, L2, and L5 band, different ionosphere-free linear combinations can be formed. The differences of the orbits computed from these different observables (ionosphere-free linear combination of L1/L2 and L1/L5) as well as the stability of the differential code biases estimated within the POD are studied. Finally, results of the attitude determination based on the navigation signal transmission from two different antennas onboard QZS-1 are presented.
We demonstrate the stabilization of an all-in-fiber polarization maintaining semiconductor satura... more We demonstrate the stabilization of an all-in-fiber polarization maintaining semiconductor saturable absorber mirror (SESAM) mode locked frequency comb oscillator with an intra-cavity waveguide electro-optic phase modulator (EOM) to a narrow linewidth HeNe laser over 46 hours. The high feedback bandwidth of the EOM allows a coherent optical lock with an in-loop integrated phase noise of 1.12 rad (integrated from 10 Hz to 3 MHz) from the carrier signal. No piezo fiber stretcher was required to guarantee long-term stabilization, preventing mechanical degradation of the optical fibers and enabling a long lifetime of the oscillator. As an application a hybrid stabilization scheme is presented, where a comb tooth is phase locked to a longitudinal mode of the large ring laser "G" located at the Geodatic Observatory Wettzell. The hybrid stabilization scheme describes the optical lock of the frequency comb to the G laser and the simultaneous compensation of the ring laser frequency drift by comparing the comb repetition rate against an active H-maser reference. In this context the ring laser reached a fractional Allan deviation of 5 • 10 −16 at an integration time of 16384 s.
Sentinel-1A is the first satellite of the European Copernicus programme. Equipped with a Syntheti... more Sentinel-1A is the first satellite of the European Copernicus programme. Equipped with a Synthetic Aperture Radar (SAR) instrument the satellite was launched on April 3, 2014. Operational since October 2014 the satellite delivers valuable data for more than two years. The orbit accuracy requirements are given as 5 cm in 3D. In order to fulfill this stringent requirement the precise orbit determination (POD) is based on the dual-frequency GPS observations delivered by an eight-channel GPS receiver. The Copernicus POD (CPOD) Service is in charge of providing the orbital and auxiliary products required by the PDGS (Payload Data Ground Segment). External orbit validation is regularly performed by comparing the CPOD Service orbits to orbit solutions provided by POD expert members of the Copernicus POD Quality Working Group (QWG). The orbit comparisons revealed systematic orbit offsets mainly in radial direction (approx. 3 cm). Although no independent observation technique (e.g. DORIS, SLR) is available to validate the GPS-derived orbit solutions, comparisons between the different antenna phase center variations and different reduceddynamic orbit determination approaches used in the various software packages helped to detect the cause of the systematic offset. An error in the given geometry information about the satellite has been found. After correction of the geometry the orbit validation shows a significant reduction of the radial offset to below 5 mm. The 5 cm orbit accuracy requirement in 3D is fulfilled according to the results of the orbit comparisons between the different orbit solutions from the QWG.
Troposphere zenith delays estimated from observations of the Global Positioning System (GPS) prov... more Troposphere zenith delays estimated from observations of the Global Positioning System (GPS) provide valuable information about the neutral atmosphere. On the other hand, troposphere long time series of operational GPS analyses are degraded by model changes, in particular as regards the tropospheric mapping function, hydrostatic a priori delays, the elevation cut-off angle and the reference frame used for datum definition.
The accurate localization of gamma-ray bursts (GRBs) remains a crucial task. Historically, improv... more The accurate localization of gamma-ray bursts (GRBs) remains a crucial task. Historically, improved localizations have led to the discovery of afterglow emission and the realization of their cosmological distribution via redshift measurements; however, a more recent requirement comes with the potential of studying the kilonovae of neutron star mergers. Gravitational wave detectors are expected to provide locations to not better than 10 square degrees over the next decade. With their increasing horizon for merger detections the intensity of the gamma-ray and kilonova emission also drops, making their identification in large error boxes a challenge. Thus, a localization via the gamma-ray emission seems to be the best chance to mitigate this problem. Here we propose to equip some of the second-generation Galileo satellites with dedicated GRB detectors. This saves costs for launches and satellites for a dedicated GRB network, the large orbital radius is beneficial for triangulation, and...
In the framework of the ESA GSTP study “Accurate Orbit Determination of Space Debris with Laser T... more In the framework of the ESA GSTP study “Accurate Orbit Determination of Space Debris with Laser Tracking / Tasking” both two-color and multi-static laser tracking data was acquired. We validate these observations and assess orbits computed on their basis. In this context, we deliver insight into the technology’s accuracy and precision by comparing observed versus computed measurements from reference orbits and by assessing their high-frequency components with regard to object geometries. Moreover, based on post-fit and cross-validation residuals of our orbit products we validate the overall processing procedure which comprises data filtering, estimating force model coefficients from Two-LineElements (TLEs), as well as the actual orbit determination process. Eventually, further comparisons with external orbit data support our statements about the validity and quality of our data and orbit products, respectively.
Recent progress in the domain of time and frequency (T/F) standards requires important improvemen... more Recent progress in the domain of time and frequency (T/F) standards requires important improvements of existing time distribution links. Among these, the accuracy of time transfer is actually an important part of the concerns in order to establish and maintain time & space references from ground and/or space facilities. Several time transfers by laser link projects have been carried out over the past 10 years with numerous scientific and metrological objectives. Satellite Laser ranging (SLR) has proven to be a fundamental tool, offering a straightforward, conceptually simple, highly accurate and unambiguous observable. Depending on the mission, LR is used to transmit time over two-way or one-way distances from 500 to several millions of km. The following missions and their objectives employed this technique: European Laser Timing (ELT) at 450 km, Time Transfer by Laser Link (T2L2) at 1,336 km, Laser Time Transfer (LTT) at 36,000 km, Lunar Reconnaissance Orbiter (LRO) at 350,000 km, and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) at tens of million km. This article describes the synergy between SLR and T/F technologies developed on the ground and in space and as well as the state of the art of their exploitation. The performance and sources of limitation of such space missions are analyzed. It shows that current and future challenges lie in the improvement of the time accuracy and stability of the time for ground geodetic observatories. The role of the next generation of SLR systems is emphasized both in space and at ground level, from the point of view of GGOS and valuable exploitation of the synergy between time synchronization, ranging and data transfer.
Precise point positioning with integer ambiguity resolution requires precise knowledge of satelli... more Precise point positioning with integer ambiguity resolution requires precise knowledge of satellite position, clock and phase bias corrections. In this paper, a method for the estimation of these parameters with a global network of reference stations is presented. The method processes uncombined and undifferenced measurements of an arbitrary number of frequencies such that the obtained satellite position, clock and bias corrections can be used for any type of differenced and/or combined measurements. We perform a clustering of reference stations. The clustering enables a common satellite visibility within each cluster and an efficient fixing of the double difference ambiguities within each cluster. Additionally, the double difference ambiguities between the reference stations of different clusters are fixed. We use an integer decorrelation for ambiguity fixing in dense global networks. The performance of the proposed method is analysed with both simulated Galileo measurements on E1 and E5a and real GPS measurements of the IGS network. We defined 16 clusters and obtained satellite position, clock and phase bias corrections with a precision of better than 2 cm. Keywords Network solution • Satellite phase biases • Satellite position and clock corrections • Ambiguity fixing Kalman filter Kalman filter Ambiguity fixing Ambiguity fixing Kalman filter Ambiguity fixing Least−squares Estimation Ambiguity fixing Cluster 2 Cluster 1 reference cluster Combination of individual clusters Kalman filter Cluster C Ambiguity fixing Cluster optimization Coordinates of reference stations position, clock and phase bias corrections of satellites combined position, clock and phase bias corrections of all satellites
2006 IEEE International Frequency Control Symposium and Exposition, 2006
Carrier Phase GPS observations between a geodetic receiver at the Physikalisch-Technische Bundesa... more Carrier Phase GPS observations between a geodetic receiver at the Physikalisch-Technische Bundesanstalt (PTB) and two geodetic receivers at the USNO are processed using applications and extensions of the GIPSY and Bernese GPS Software packages. Their results are compared with Two Way Satellite Time and Frequency Transfer (TWSTFT) data. It is found that algorithms that eliminate day-boundary effects require careful handling in the presence of receiver instrumental delays. Depending upon the approach chosen, time differences of several ns and frequency differences of up to 100 ps/day can develop between solution types.
GPS satellite orbits available from the International GNSS Service (IGS) show a consistent radial... more GPS satellite orbits available from the International GNSS Service (IGS) show a consistent radial bias of up to several cm and a particular pattern in the Satellite Laser Ranging (SLR) residuals, which are suggested to be related to radiation pressure mismodeling. In addition, orbit-related frequencies were identified in geodetic time series such as apparent geocenter motion and station displacements derived from GPS tracking data. A potential solution to these discrepancies is the inclusion of Earth radiation pressure (visible and infrared) modeling in the orbit determination process. This is currently not yet considered by all analysis centers contributing to the IGS final orbits. The acceleration, accounting for Earth radiation and satellite models, is introduced in this paper in the computation of a global GPS network (around 200 IGS sites) adopting the analysis strategies from the Center for Orbit Determination in Europe (CODE). Two solutions covering 9 years (2000-2008) with and without Earth radiation pressure were computed and form the basis for this study. In previous studies, it has been shown that Earth radiation pressure has a non-negligible effect on the GPS orbits, mainly in the radial component. In this paper, the effect on the along-track and cross-track components is studied in more detail. Also in this paper, it is shown that Earth radiation pressure leads to a change in the estimates of GPS ground station positions, which is systematic over large regions of the Earth. This observed "deformation" of the Earth is towards North-South and with large scale patterns that repeat six times per GPS draconitic year (350 days),
The Center for Orbit Determination in Europe (CODE) has been involved in the processing of combin... more The Center for Orbit Determination in Europe (CODE) has been involved in the processing of combined GPS/GLONASS data during the International GLONASS Experiment (IGEX). The resulting precise orbits were analyzed using the program SORBDT. Introducing one satellite's positions as pseudo-observations, the program is capable of fitting orbital arcs through these positions using an orbit improvement procedure based on the numerical integration of the satellite's orbit and its partial derivative with respect to the orbit parameters. For this study, the program was enhanced to estimate selected parameters of the Earth's gravity field. The orbital periods of the GPS satellites are-in contrast to those of the GLON-ASS satellites-2 : 1 commensurable (P Sid : P GPS) with the rotation period of the Earth. Therefore, resonance effects of the satellite motion with terms of the geopotential occur and they influence the estimation of these parameters. A sensitivity study of the GPS and GLON-ASS orbits with respect to the geopotential coefficients reveals that the correlations between different geopotential coefficients and the correlations of geopotential coefficients with other orbit parameters, in particular with solar radiation pressure parameters, are the crucial issues in this context. The estimation of the resonant geopotential terms is, in the case of GPS, hindered by correlations with the simultaneously estimated radiation pressure parameters. In the GLONASS case, arc lengths of several days allow the decorrelation of the two parameter types. The formal errors of the estimates based on the GLONASS orbits are a factor of 5 to 10 smaller for all resonant terms.
ABSTRACT At the altitude of the CPS satellites the most important non-gravitational perturbation ... more ABSTRACT At the altitude of the CPS satellites the most important non-gravitational perturbation is caused by the solar radiation pressure acting on the satellite body and its solar panels. The development of high-fidelity radiation pressure models may be motivated by the following observation: The GPS satellites are orbiting in a 2:1-commensurability with the Earth's rotation which causes resonance. The expected sensitivity to specific coefficients of the geopotential is, however, significantly reduced by strong correlations of these parameters with radiation pressure parameters. Sophisticated radiation pressure models rely on a precise knowledge of the satellite's attitude which does not only affects the location of the antenna phase center or the phase windup of the signal carrier but, through radiation pressure, also the orbital dynamics. PRN 23, whose attitudinal behaviour was modified early in 2002 is an interesting case. Due to this change an impressive improvement in the orbit quality could be achieved.
The time transfer techniques used to generate TAI are currently the TWSTFT (TW), GPS C/A, and GPS... more The time transfer techniques used to generate TAI are currently the TWSTFT (TW), GPS C/A, and GPS P3. About 19% of all TAI laboratories possess P3, 12% possess TW backed up with P3, i.e., in total only 31% of the TAI laboratories transfer 100% of the primary frequency standards (PFS) including all the Cs fountains and 80% of the total weighted clocks for TAI. GPS carrier phase (CP) data are co-products of P3 receivers and they are never used in TAI. An effective strategy to improve TAI is to strengthen these TW+P3 links by using the already available CP information. We first review the classical TAI time transfer techniques and solutions of different analysis strategies of the CP data, namely the IGS [1], the AIUB [2], and the NRCan PPP [3]. To evaluate all the different time transfer techniques, we carried out an extensive comparison between the above techniques, over different periods from hours to 4 months and over different distances on inner- resp. inter-continental baselines b...
The altimetry satellite JASON-1 carries several independent instruments for precise orbit determi... more The altimetry satellite JASON-1 carries several independent instruments for precise orbit determination such as a DORIS receiver, a GPS receiver, and an SLR reflector. DORIS and GPS are both based on microwave phase measurements and not fundamentally different. A new analysis technique for processing DORIS observations has been developed which follows a GPS-like approach. This new method is used to
ABSTRACT QZS-1, the first satellite of the Japanese Quasi Zenith Satellite System (QZSS) was laun... more ABSTRACT QZS-1, the first satellite of the Japanese Quasi Zenith Satellite System (QZSS) was launched in September 2010. Transmission of the standard codes started in December 2010 and the satellite was declared healthy in June 2011. Five stations of the COoperative Network for GIOVE Observation (CONGO) were upgraded to provide QZSS tracking capability. These five stations provide the basis for the precise orbit determination (POD) of the QZS-1 spacecraft. The stability and consistency of different orbital arc lengths is analyzed based on orbit fit residuals, day boundary discontinuities, and Satellite Laser Ranging residuals. As QZS-1 simultaneously transmits navigation signals on three frequencies in the L1, L2, and L5 band, different ionosphere-free linear combinations can be formed. The differences of the orbits computed from these different observables (ionosphere-free linear combination of L1/L2 and L1/L5) as well as the stability of the differential code biases estimated within the POD are studied. Finally, results of the attitude determination based on the navigation signal transmission from two different antennas onboard QZS-1 are presented.
We demonstrate the stabilization of an all-in-fiber polarization maintaining semiconductor satura... more We demonstrate the stabilization of an all-in-fiber polarization maintaining semiconductor saturable absorber mirror (SESAM) mode locked frequency comb oscillator with an intra-cavity waveguide electro-optic phase modulator (EOM) to a narrow linewidth HeNe laser over 46 hours. The high feedback bandwidth of the EOM allows a coherent optical lock with an in-loop integrated phase noise of 1.12 rad (integrated from 10 Hz to 3 MHz) from the carrier signal. No piezo fiber stretcher was required to guarantee long-term stabilization, preventing mechanical degradation of the optical fibers and enabling a long lifetime of the oscillator. As an application a hybrid stabilization scheme is presented, where a comb tooth is phase locked to a longitudinal mode of the large ring laser "G" located at the Geodatic Observatory Wettzell. The hybrid stabilization scheme describes the optical lock of the frequency comb to the G laser and the simultaneous compensation of the ring laser frequency drift by comparing the comb repetition rate against an active H-maser reference. In this context the ring laser reached a fractional Allan deviation of 5 • 10 −16 at an integration time of 16384 s.
Sentinel-1A is the first satellite of the European Copernicus programme. Equipped with a Syntheti... more Sentinel-1A is the first satellite of the European Copernicus programme. Equipped with a Synthetic Aperture Radar (SAR) instrument the satellite was launched on April 3, 2014. Operational since October 2014 the satellite delivers valuable data for more than two years. The orbit accuracy requirements are given as 5 cm in 3D. In order to fulfill this stringent requirement the precise orbit determination (POD) is based on the dual-frequency GPS observations delivered by an eight-channel GPS receiver. The Copernicus POD (CPOD) Service is in charge of providing the orbital and auxiliary products required by the PDGS (Payload Data Ground Segment). External orbit validation is regularly performed by comparing the CPOD Service orbits to orbit solutions provided by POD expert members of the Copernicus POD Quality Working Group (QWG). The orbit comparisons revealed systematic orbit offsets mainly in radial direction (approx. 3 cm). Although no independent observation technique (e.g. DORIS, SLR) is available to validate the GPS-derived orbit solutions, comparisons between the different antenna phase center variations and different reduceddynamic orbit determination approaches used in the various software packages helped to detect the cause of the systematic offset. An error in the given geometry information about the satellite has been found. After correction of the geometry the orbit validation shows a significant reduction of the radial offset to below 5 mm. The 5 cm orbit accuracy requirement in 3D is fulfilled according to the results of the orbit comparisons between the different orbit solutions from the QWG.
Troposphere zenith delays estimated from observations of the Global Positioning System (GPS) prov... more Troposphere zenith delays estimated from observations of the Global Positioning System (GPS) provide valuable information about the neutral atmosphere. On the other hand, troposphere long time series of operational GPS analyses are degraded by model changes, in particular as regards the tropospheric mapping function, hydrostatic a priori delays, the elevation cut-off angle and the reference frame used for datum definition.
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Papers by U. Hugentobler