The Extended Tracking Network and Indications of Baseline Precision and Accuracy in the North Andes

The CASA UNO Global Positioning System (GPS) experiment (January-February 1988) included an extended tracking network which covered three continents in addition to the network of scientific interest in Central and South America. The repeatability of long baselines (400-1000 km) in South America is improved by up to a factor of two in the horizontal vector baseline components by using tracking stations in the Pacific and Europe to supplement stations in North America. In every case but one, the differences between the mean solutions obtained using different tracking networks was equal to or smaller than day-to-day rms repeatabilities for the same baselines. The mean solutions obtained by using tracking stations in North America and the Pacific agreed at the 2-3 millimeter level with those using tracking stations in North America and Europe. The agreement of the extended tracking network solutions suggests that a broad distribution of tracking stations provides better geometric constraints on the satellite orbits and that solutions are not sensitive to changes in tracking network configuration when an extended network is used. A comparison of the results from the North Andes and a baseline in North America suggests that the use of a geometrically strong extended tracking network is most importanwt hen the network of interest is far from North America

Plate Motions in the North Andean Region

Repeated geodetic measurements with the Global Positioning System (GPS) provide direct measurements of displacements due to plate motions and active crustal deformation in Central America and northern South America, an area of complex interaction of the Nazca, Cocos, Caribbean and South American plates. The displacement rates for the period 1988-1991, obtained from the results of the first three Central And South America (CASA) GPS campaigns, are in general agreement with the predictions of the NUVEL-1 plate motion model, but there are differences in detail between the observations and the model. The Nazca-North Andes convergence rate vector measured by GPS is different from the NUVEL-1 vector at 95% confidence. The difference implies that the North Andes are moving northward relative to South America. The measured convergence between the Caribbeanp late and the North Andes suggests that the southern margin of the Caribbean plate is located in the South Caribbean deformed belt. The April 1991 Costa Rica earthquake and the Cocos-Caribbean convergence rate determined by GPS suggest the possibility of significant ongoing deformation between Central America and the stable interior of the Caribbean plate. Our GPS results are consistent with deformation of the overriding plates at the convergent margins of Central and South America and confirm that active convergence is occurring around much of the southern margin of the Caribbean plate, from Colombia west to Costa Rica. Costa Rica and Panama are not part of the stable Caribbean plate. Instead, the South Caribbean deformed belt and the North Panama fold belt probably represent the southern margin of the Caribbean plate

Bounding the Residual Tropospheric Error by Interval Analysis

GNSS integrity monitoring requires proper bounding to characterize all ranging error sources. Unlike classical approaches based on probabilistic assumptions, our alternative integrity approach depends on deterministic interval bounds as inputs. The intrinsically linear uncertainty propagation with intervals is adequate to describe remaining systematic uncertainty, the so-called imprecision. In this contribution, we make a proposal on how to derive the required intervals in order to quantify and bound the residual error for empirical troposphere models, based on the refined sensitivity analysis via interval arithmetic. We evaluated experimentally the Saastamoinen model with (i) a priori ISO standard atmosphere, and (ii) on-site meteorological measurements from IGS and Deutscher Wetterdienst (DWD) stations as inputs. We obtain consistent and complete enclosure of residual ZPD errors w.r.t IGS ZPD products. Thanks to the DWD dense network, interval maps for meteorological parameters and residual ZPD errors are generated for Germany as by-products. These experimental results and products are finally validated, taking advantage of the high-quality tropospheric delays estimated by the Vienna Ray Tracer. Overall, the results indicate that our strategy based on interval analysis successfully bounds tropospheric model uncertainty. This will contribute to a realistic uncertainty assessment of GNSS-based single point positioning

Towards an International Height Reference Frame Using Clock Networks

Establishing an International Height Reference Frame (IHRF) has been a major goal of the International Association of Geodesy (IAG) for a long time. One challenge is to obtain the vertical coordinates, i.e., geopotential numbers, of the reference stations with high precision and global consistency. A promising approach is using clock networks, which are powerful in precisely obtaining geopotential or height differences between distant sites through measuring the gravitational redshift effect by comparing clocks’ frequencies. We propose a hybrid clock network following a specific hierarchy. It includes stationary clocks as the backbone of the frame and transportable clocks for regional densifications. The vertical coordinates of the clock stations can be straightforwardly referenced to the unique benchmark by various long-distance frequency transfer techniques, like using optical fibers or free-space microwave and laser links via relay satellites. Another practical way towards an IHRF is to unify all local height systems around the world. Clock networks are considered as an alternative to classical geodetic methods. The idea was verified through closed-loop simulations. We found that the measurements acquired by a few 10−18 clocks, three or four in triangular or quadrangular distributions for each local system, are sufficient to adjust the discrepancies between local datums and the systematic slopes within local height networks

On the Limits of State-of-the-Art GNSS Receivers in Frequency Transfer

GNSS frequency transfer (FT) based on precise point positioning delivers instability values down to sub-10−16 between two modern receivers. In the present study we investigate the technical limits such receivers impose on FT by means of a dedicated experiment at Germany’s national metrology institute (PTB). For this purpose, four geodetic receivers, two of the same type each, were all connected to one single antenna and fed by the highly stable UTC (PTB) frequency signal. Since all error sources affecting the satellite signals are the same for all receivers, they cancel out when forming receiver-to-receiver single differences (SDs). Due to the fact that the remaining SD carrier phase ambiguities can be easily fixed to integer values, only the relative receiver clock error remains in the SDs. We assess the instability of three different receiver combinations, two with the same receiver type (intra-receiver) and one with different types (inter-receiver). The intra-receiver pairs reach lower instability values faster than the inter-receiver combination, which is in part caused by the different signal tracking modes of the receivers. To be specific, the 10−18 instability range was only reached by the intra-receiver pairs, whereas the inter-receiver combination already hits its noise floor at about 1.5 ⋅ 10−17. In addition, our analysis of using different observation type combinations only shows small differences regarding the link instability

Isostatic Compensation and Conduit Structures of Western Pacific Seamounts: Results of Three-Dimensional Gravity Modeling

Detailed three-dimensional polygonal prism models of two large western Pacific seamounts show that the 135 mgal difference in the observed sea surface gravity over the two can be best explained by similar mean densities (2.6 gjcm3) and crustal thickening under one seamount (Airy isostatic compensation). Observed calculated residuals are further reduced by including dense (2.9 gjcm3) vertical feeder pipes or volcanic conduits in the models. Dense conduits or fracture zones 5 to 17 km in diameter are located under many, if not all, craters on volcanic islands and seamounts. Results from the detailed seamount studies can be generalized using exact expressions for the on-axis vertical component of gravity for cones or frustrums of cones. Seamount isostatic compensation levels can then be rapidly estimated by iteratively inverting the on-axis gravity. The estimation algorithm is independent of mechanical assumptions regarding oceanic lithosphere and is particularly useful for the rapid evaluation of large data sets. The results and associated uncertainties are comparable to those of the detailed three-dimensional models and frequency domain studies. As predicted by cooling plate models, the estimated Airy (local) compensation levels p. for seamounts are inversely proportional to the root ofthe seafloor age at the time of loading t: p.(%) = 68 -5.6t1/ 2• A map of depth-corrected on-axis gravity values for western Pacific seamounts indicates that seamounts with similar p. \u27fBlues tend to form clusters

Multipath Characterization Using Ray-Tracing in Urban Trenches

Multipath in urban environments still represents a great challenge for Global Navigation Satellite System (GNSS) positioning as it is a degrading factor which limits the attainable accuracy, precision and integrity. In an urban trench, the dense building structures in the vicinity of the antenna cause reflections of the satellite signals resulting in multipath errors. Various work has been presented for simulating reflections for stations under laboratory conditions, yet the simulative analysis of multipath propagation in urban environments is currently developing. In this contribution, we enhanced an existing Ray-Tracing algorithm which identifies potentially multipath affected satellite signals. So far, it calculates reflection points on a plane ground and estimates the resulting multipath error. We extended it for the urban area case by introducing a 3D city building model with possible reflections on all surfaces of the buildings. Based on the geometry between the antenna position, satellite position and the reflection surface, the extra path delays, the characteristics of the propagation channel and the signal amplitudes are calculated. The resulting multipath errors are then estimated from the discriminator function using state of the art correlator parameters and antenna models. For a validation, the simulation results are compared with code-minus-carrier combination from a real GNSS experiment in a dense urban area in Hannover. We find that the simulated multipath errors fit the observations in terms of the amplitude, but with uncertainties in the building model, the multipath wave length is too large. The distance to the reflection surface is a key factor which influences the multipath wavelength

Estimation of Earth Rotation Parameter UT1 from Lunar Laser Ranging Observations

Since 1969 Lunar Laser Ranging (LLR) data have been collected by different observatories and analysed by various analysis groups. LLR is providing the longest time series of any space geodetic technique for studying the Earth-Moon dynamics. In recent years, observations have been carried out with larger telescopes and at infra-red (IR) wavelength, resulting in a better distribution of precise LLR data over the lunar orbit and the observed retro-reflectors on the Moon. The increased number of high-accuracy observations allows for more accurate determination of Earth Orientation Parameters (EOPs) from LLR data compared to previous years. In this study we focus on ΔUT1 results from different constellations and compare our LLR solution to the IERS EOP C04 series

LUH-GRACE2018

In this contribution, we present the LUH-GRACE2018 time series of monthly gravity field solutions covering the period January 2003–March 2016. The solutions are obtained from GRACE K-Band Range Rate (KBRR) measurements as main observations. The monthly solutions are computed using the in-house developed GRACE-SIGMA software. The processing is based on dynamic orbit and gravity field determination using variational equations and consists of two main steps. In the first step, 3-hourly orbital arcs of the two satellites and the state transition and sensitivity matrices are dynamically integrated using a modified Gauss-Jackson integrator. In this step, initial state vectors and 3D accelerometer bias parameters are adjusted using GRACE Level-1B reduced-dynamic positions as observations. In the second step, normal equations are accumulated and the normalized spherical harmonic coefficients up to degree and order 80 are estimated along with arc-wise initial states, accelerometer biases and empirical KBRR parameters. Here KBRR measurements are used as main observations and reduced-dynamic positions are introduced to solve for the low frequency coefficients. In terms of error degree standard deviations as well as Equivalent Water Heights (EWH), our gravity field solutions agree well with RL05 solutions of CSR, GFZ and JPL