G41B-0622
The foliage effect on the height time series from permanent GPS stations
The effect of deciduous trees growing above antenna height was investigated for permanent Global Positioning System (GPS) stations. Signal blockage due to foliage and branches was found to have the same effect as an increased elevation cutoff angle, thus causing a change in the computed position. Height estimates are mostly affected, and they decrease due to tree growth. Empirical Orthogonal Function (EOF) analysis was performed on the height time series from five test sites, and two stations surrounded by trees showed a similar EOF mode of signal. We computed the signal availability as ratios of the complete to possible observations, and found that the ratio decreases as trees grow and has seasonal variation. The observation ratios were higher during the winter months when the leaves had fallen. Similar seasonal variation was observed in multipath error and signal attenuation due to foliage. The multipath error index MP2 was computed with a TEQC program, and MP2 values were increased at a significant rate at sites with growing trees. Signal attenuation was analyzed using 1-ċ uncertainties from the estimation process of daily GPS data processing. While 1-ċ uncertainties did not show any seasonal variations at sites without trees, they were highly dependent on the seasonal change of foliage condition when deciduous trees are near the antenna.
G41B-0623 INVITED
Site-Dependent Electromagnetic Effects in High-Accuracy Applications of GNSS
Global Navigation Satellite Systems, GNSS are used in many applications demanding millimetre-level accuracy in positioning. Such applications include monitoring of crustal movements. The GNSS technique also facilitates estimates of the tropospheric water vapour content, an important parameter in numerical weather predictions and climate research. The accuracy of estimated parameters is however degraded by many error sources. Some of those are related to the satellite system and the ground-based receivers. During 2006 the International GNSS Service, IGS implemented absolute antenna calibration for both satellite antennas and the different antenna types used at the ground-based stations. The use of such calibration values improves the accuracy as antenna type related errors are mitigated. However, unmodeled error sources still remain since a antenna couples electromagnetically with its surrounding environment. The absolute antenna calibrations, to be implemented by the IGS, will however not compensate for site- dependent effects. To further improve the accuracy, the site-dependent effects and their dependency on the direction of the observation need to be identified and removed in the GNSS data analysis. In this presentation the site-dependent error sources have been studied for the stations in the Swedish permanent GNSS network, SWEPOS, as well as some stations in the global IGS network. Strong similarities in terms of site-dependent effects were found. Differences in the site characteristics, caused by multipath and different antenna surroundings imply that an individual calibration of each station may be required needed. We have developed and evaluated two different methods for calibration of site dependent effects.
G41B-0624
GPS Multipath in Urban Environments
Multipath, where a GNSS signal arrives by more than one path, is considered one of the last unmodeled errors remaining in GNSS. Multipath is of great concern because the additional path length traveled by the incoming signal biases the satellite-receiver range and therefore determination of position. Siting a GNSS station in an urban area, in the immediate vicinity of large reflecting objects such as rooftops, buildings, asphalt and concrete parking lots, grassy fields, and chainlink fences, is both a multipath nightmare and a necessary evil. We note that continuously-operating GNSS stations are becoming increasingly common in urban areas, which makes sense as these stations are often installed in support of civil infrastructure (e.g. departments of transportation, strong motion monitoring of buildings in earthquake-prone areas, surveying networks). Urban stations are well represented in geodetic networks such as the CORS (United States) and GeoNet (Japan) networks, with more stations likely to be installed in the coming years. What sources and types of urban multipath are the most detrimental to geodetic GPS positioning? Which reflecting objects are assumed to be a major source of multipath error, but the GPS data show otherwise? Are certain reflecting environments worse for specific applications, i.e. kinematic vs. static positioning? If forced to install a GNSS station in a highly reflective environment, is it possible to rank objects for their multipath severity? To answer these questions, we provide multipath examples taken from continuously- operating GNSS stations sited in urban environments. We concentrate on some of the most common obstacles and reflecting objects for urban sites – rooftops, parking lots, and fences. We analyze the multipath signature of these objects as manifested in the GPS phase, pseudorange, and signal-to-noise ratio (SNR) observables, and also examine multipath impacts on the precision and accuracy of GPS-derived positions.
G41B-0625
GNSS Absolute Antenna Calibration in the Field at the National Geodetic Survey
Geodetic GNSS applications now routinely demand millimeter precision and extremely high levels of accuracy. To achieve these accuracies, measurement and instrument biases at the centimeter to millimeter level must be understood. One of these biases is the antenna phase center, the point of signal reception for a GNSS antenna. It has been well established that phase center patterns differ between antenna models and manufacturers, and can be affected by the presence of a radome or other installation-specific factors. As GNSS geodesy increasingly moves toward real-time applications and high-rate or kinematic positioning, it is all the more important to have the most complete picture of antenna phase center variations possible, as a function of both elevation and azimuth. To meet the needs of the geodetic GNSS community, the National Geodetic Survey (NGS) has constructed an absolute antenna calibration facility. Located in Corbin, Virgina, adjacent to the NGS relative antenna calibration site, this facility uses field measurements and actual GNSS satellite signals to determine antenna phase center patterns. A pan/tilt motor changes the orientation of the antenna under test, so that signals are received and their phase center subsequently measured for a wide range of angles. The NGS phase center models are generated for all possible azimuths and over 90 to -30 degrees elevation angles; negative elevation angles are included to support studies where valid signals are received below the antenna's horizon. Ultimately, this facility will be used to measure antenna phase center variations of commonly-used geodetic GNSS antennas, as well as antennas submitted by users. The phase center patterns will be publicly available and disseminated in the ANTEX format. We present information on the current status of and future plans for the NGS antenna calibration facility. We outline the observation models and software used to generate absolute calibrations, and provide examples of antenna calibrations from the NGS facility.
G41B-0626 INVITED
Noise characteristics of short drilled and deep drilled braced monuments in the PBO continuous GPS network
The EarthScope Plate Boundary Observatory (PBO) GPS network consists of 1,100 continuously operating stations, 880 of which were built between 2003 and 2008 to a standard set of specifications. With a few exceptions, all built stations incorporate a Trimble NetRS receiver (firmware v.1.1) and a Trimble TRM29659.00 choke ring antenna, so differences in station performance due to GPS equipment should be negligible. Station monuments are split almost exclusively between slightly modified versions of the Wyatt- Agnew deep drilled braced and the SCIGN short drilled braced types, both of which are considered to be the state-of-the-art in stable GPS monumentation. Deep drilled braced monuments are designed to anchor the GPS antenna to a depth of more than 3 m in order to isolate the antenna from expansion/contraction of the near subsurface from temperature or moisture variability, but installation costs are substantially higher than for shallow monuments. Determining the gain in stability due to deeper anchoring would benefit design decisions for future networks. We test the assumption that deep drilled braced monuments offer superior stability over short drilled braced by examining the noise characteristics of a large subset of the PBO GPS network. We apply analytical techniques widely used to study GPS noise, examining the amplitudes of seasonal and sub-seasonal cycles and various stochastic noise processes in detrended vertical and horizontal time series. Seasonal forcing of the near subsurface is correlated with seasonal variations in site-specific effects such as multipath and regional effects such as groundwater recharge, complicating the analysis. Restricting the analysis to these two monument types allows us to isolate shallow subsurface effects to the greatest extent possible, not only to assess the relative performance of deep versus shallow monumentation, but also to weigh the absolute magnitude of shallow effects against that of other noise sources.
G41B-0627
GPS antenna monuments and mounts supported by UNAVCO: Options and Effectiveness
Many different monumentation types and antenna mounts have been used in UNAVCO-supported projects for campaign, semi-permanent and long-term continuous GPS sites. We describe nine monuments and mounts currently in popular use in UNAVCO-supported projects as options to the greater scientific community, and compare results from these monuments and mounts for data quality. The designs range in height from 0 to 3 meters; substrates into which they are installed include soil, bedrock, and concrete; and costs range from approximately $30 to $15000. In many places outside the US, logistical, economical, and material restraints make installation of deep- and shallow-drilled braced monuments at best difficult and at worst impossible. Is a $1000 deep-drilled braced monument significantly better than a $150 stainless steel mast set into bedrock? We make a first order comparison of commonly used monument and mount types using existing site data from the science community. Preliminary results suggest that the effect of monument type on multipath is outweighed by the effect of the site environment.
G41B-0628
PBO Nucleus Project Summary: The Successful Integration of 209 Existing GPS Sions in the Plate Boundary Observatory
Tectonic and earthquake research in the US has
experienced a quiet revolution over the last decade
precipitated by the recognition that slow-motion
faulting events can both trigger and be triggered by
regular earthquakes. Transient motion has now been
found in essentially all tectonic environments, and the
detection and analysis of such events is the
first-order science target of the EarthScope Project.
Because of this and a host of other fundamental
tectonics questions that can be answered only with
long-duration geodetic time series, the 1100-station EarthScope Plate Boundary
Observatory (PBO) network was designed to
leverage 445 existing continuous GPS stations
whose measurements extend back over a decade.
The irreplaceable recording history of these stations
will accelerate EarthScope scientific return by
providing the highest possible resolution. This
resolution will be used to detect and understand
transients, to determine the three-dimensional
velocity field (particularly vertical motion), and to
improve measurement precision by understanding
the complex noise sources inherent in GPS.
The PBO Nucleus project supports the operation,
maintenance and hardware upgrades of a subset of
the six western U.S. geodetic networks until they are
subsumed by PBO. Uninterrupted data flow from
these stations will effectively double the time-series
length of PBO over the expected life of EarthScope,
and has created, for the first time, a single
GPS-based geodetic network in the US. The other
existing sites remain in operation under support from
non-NSF sources (e.g. the USGS), and EarthScope
continues to benefit from their continued operation
On the grounds of relevance to EarthScope
science goals, geographic distribution and data
quality, 209 of the 432 existing stations were selected
as the nucleus upon which to build PBO. Conversion
of these stations to a PBO-compatible mode of
operation was begun under previous funding, and as
a result data now flow directly to PBO archives and
processing centers while maintenance, operations,
and meta-data requirements are continue to be
upgraded to PBO standards. At the end of this
project all 209 stations have been fully incorporated into
PBO, meeting all standards for new PBO construction
including data communications and land use permits.
Funds for operation of these stations have been
included in approved budgets for PBO's Operations and Maintenance phase.
The data from these stations serve a much larger
audience than just the few people who work to keep
them operating. This project is now collecting the
data that will be used by the next generation of
solid-earth researchers for at least two decades.
Educational modules are being developed by a team
of researchers, educators, and curriculum
development professionals, and are being
disseminated through regional and national
workshops. An interactive website provides the
newest developments in tectonics research to K-16
classrooms.
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G41B-0629
PBO Facility Construction: GPS Network Completed
The Plate Boundary Observatory (PBO), part of the larger NSF-funded EarthScope project, will study the three-dimensional strain field resulting from active plate boundary deformation across the Western United States. The PBO construction phase is now completed, which involved the reconnaissance, permitting, installation, documentation, and maintenance of 891 permanent GPS stations and the upgrade of 209 existing stations in five years. Some of the GPS construction highlights from the project will be presented. These highlights include the San Simeon earthquake response, Mount Saint Helens volcano emergency response, the Magnitude 6.0 Parkfield earthquake emergency response, and GPS and tiltmeter installations on Augustine, Akutan, and Unimak Island in Alaska.