G41C-0630 INVITED
National Coordination Office for Space-Based PNT
In December 2004, President Bush issued the US Policy on space-based positioning, navigation, and timing
(PNT), providing guidance on the management of the Global Positioning System (GPS) and other space-
based PNT systems. The policy established the National Executive Committee (EXCOM) to advise and
coordinate federal agencies on matters related to space-based PNT. Chaired jointly by the deputy
secretaries of defense and transportation, the EXCOM includes equivalent level officials from the
Departments of State, the Interior, Agriculture, Commerce, and Homeland Security, the Joint Chiefs of Staff,
and the National Aeronautics and Space Administration (NASA). A National Coordination Office (NCO)
supports the EXCOM through an interagency staff. Since establishing the EXCOM and NCO in 2005, the
organizations have quickly grown in influence and effectiveness, leading or managing many interagency
initiatives including the development of a Five-Year National Space-Based PNT Plan, the Space-Based PNT
Interference Detection and Mitigation (IDM) Plan, and other strategic documents. The NCO has also
facilitated interagency coordination on numerous policy issues and on external communications intended to
spread a consistent, positive US message about space-based PNT. Role of the NCO - The purpose of the
EXCOM is to provide top-level guidance to US agencies regarding space-based PNT infrastructure. The
president established it at the deputy secretary level to ensure its strategic recommendations effect real
change in agency budgets. Recognizing such high-level officials could only meet every few months, the
president directed the EXCOM to establish an NCO to carry out its day-to-day business, including overseeing
the implementation of EXCOM action items across the member agencies. These range from the resolution of
funding issues to the assessment of strategic policy options. They also include the completion of specific
tasks and documents requested by the EXCOM co-chairs. The NCO has established several processes for
achieving interagency consensus on policy issues, including specialized staff-level working groups and an
assistant secretary–level Executive Steering Group. The NCO also established a process for interagency
coordination of US government communications related to space-based PNT, including speeches,
presentations, and other externally released documents. The goal is to ensure government-wide consistency
and accuracy in public and international statements made about space-based PNT. This is particularly
important in addressing false or misleading information about GPS in the public arena. Conclusion - In less
than three years, the NCO evolved from an idea into a highly active organization with substantial influence
within the space-based PNT community. NCO efforts have helped build the EXCOM into an effective
mechanism for raising issues to the attention of senior leadership and ensuring their guidance gets
implemented. While it is still a work in progress, the NCO has many significant accomplishments. The
organization will continue to mature as ongoing US government space-based PNT activities continue to bear
fruit.
http://www.pnt.gov
G41C-0631 INVITED
A Recommendation on SLR Ranging to Future Global Navigation Satellite Systems
The multi-agency US Geodetic Requirements Working Group has recommended that Satellite Laser Retro- reflectors be installed on GPS III satellites as a principal component of the Positioning, Navigation, and Timing mandate of the Global Positioning System. The Working Group, which includes NASA, NGA, NOAA, NRL, USGS, and the USNO, echoes the Global Geodetic Observing System recommendation that SLR retro- reflectors be installed on all GNSS satellites. It is further recommended that the retro-reflectors conform to and hopefully exceed the minimum standard of the International Laser Ranging Service for retro-reflector cross sections of 100 million square meters for the HEO GNSS satellites to insure sufficiently accurate ranging by the global network of satellite laser ranging systems. The objective of this recommendation is to contribute to the improvement in the International Terrestrial Reference Frame, and its derivative the WGS84 reference frame, through continuing improvements in the characterization of the GPS orbits and clocks. Another objective is to provide an independent means of assessing the interoperability and accuracy of the GNSS systems and regional augmentation systems. The ranging to GNSS-mounted retro-reflectors will constitute a significant new means of space-based collocation to constrain the tie between the GPS and SLR networks that constitute over 50% of the data from which the ITRF is derived. The recommendation for the installation of SLR retro-reflectors aboard future GPS satellites is one of a number of efforts aimed at improving the accuracy and stability of ITRF. These steps are being coordinated with and supportive of the efforts of the GGOS and its services such at the VLBI2010 initiative, developing a next generation geodetic network, near real-time GPS positioning and EOP determination, and numerous efforts in the improvement of geodetic algorithms for GPS, SLR, VLBI, DORIS, and the determination of the ITRF. If past is prologue, the requirements of accuracy placed upon GNSS systems will continue to evolve at a factor of ten per decade for the lifetime of the GPS III, extending to 2025 and beyond. Global societal priorities such as sea level change measurement already require a factor of ten or more improvement in the accuracy and stability of the ITRF. Increasing accuracy requirements by civilian users for precision positioning and time keeping will certainly continue to grow at an exponential rate. The PNT accuracy of our GNSS systems will keep pace with these societal needs only if we equip the GNSS systems with the capability to identify and further reduce systematic errors.
G41C-0632
CNES-CLS IGS Analysis Center Activities
GRGS and CLS teams process regularly GPS data from a worldwide network of IGS permanent stations. We
compute precise GPS orbits together with Earth rotation parameters and stations coordinates at the highest
level of precision using the CNES-GRGS GINS software. Our solutions are being submitted to the
International Earth Rotation Service since January 2004 and to the IGS since July 2007.
This poster gives a summary of our processing strategy as well as the status of the quality of our products. In
addition, the source of a systematic scale factor between our orbit solution and the IGS ones is discussed.
Special interest is paid on the radiation pressure modelization which includes in our case the Earth albedo
contribution. The processing of SLR tracking data from GPS satellite SVN35 and 36 is included in this study.
http://igsac-cnes.cls.fr
G41C-0633
Status of IGS Ultra-Rapid Products for Real-Time Applications
Since November 2000 the International GNSS Service (IGS) has produced Ultra-rapid (IGU) products for near real-time and real-time applications. They include GPS orbits, satellite clocks, and Earth rotation parameters for a sliding 48-hr period. The first day of each update is based on the most recent GPS observational data from the IGS hourly tracking network. At the time of release, these observed products have an initial latency of 3 hr. The second day of each update consists of predictions. So the predictions between about 3 and 9 hr into the second half are relevant for true real-time uses. Originally updated twice daily, the IGU products since April 2004 have been issued four times per day, at 3, 9, 15, and 21 UTC. Up to seven Analysis Centers (ACs) contribute to the IGU combinations: Astronomical Institute of the University of Berne (AIUB), European Space Operations Center (ESOC), Geodetic Observatory Pecny (GOP), GeoForschungsZentrum (GFZ) Potsdam, Natural Resources Canada (NRC), Scripps Insitution of Oceanography (SIO), U.S. Naval Observatory (USNO). This redundancy affords a high measure of reliability and enhanced orbit accuracy. IGU orbit precision has improved markedly since late 2007. This is due to a combination of factors: decommissioning of the old, poorly behaved PRN29 in October 2007; upgraded procedures implemented by GOP around the same time, by SIO in spring 2008, and by USNO in June 2008; better handling of maneuvered satellites at the combination level starting June 2008; and stricter AC rejection criteria since July 2008. As a consequence, the weighted 1D RMS residual of the IGU orbit predictions over their first 6 hr is currently about 20 to 30 mm (after a Helmert transformation) compared to the IGS Rapid orbits, averaged over the constellation. The median residual is about 15 to 20 mm. When extended to the full 24 hr prediction period, the IGU orbit errors approximately double. Systematic rotational offsets are probably more important than random errors due to limitations in EOP predictions, especially UT1, reaching up to about 35 mm RMS (equatorial at GPS altitude) about the Z axis. The observed orbits in the first half of each IGU update have WRMS residuals of about 10 to 12 mm. Note that while the precision of the Rapid orbits is around 7 to 9 mm (compared to the IGS Finals) discontinuities between successive daily orbits imply an inaccuracy in the IGS orbits of at least 21 mm WRMS. So it is likely that the observed IGU orbits are nearly comparable in accuracy to the Rapids and that the current IGU orbit predictions are not worse by more than a factor of two or so. Only four ACs (ESOC, GFZ, NRC, USNO) contribute estimates of the satellite clocks, which limits the robustness and quality of the IGU clock products. Because the stochastic component of clock variations is not predictable, errors for the second-half IGU clock predictions grow quickly to the same level as the broadcast navigation values. But the IGU observed clocks have typical errors just about double that of the Rapids. The scatters of precise point positions using the IGU observed products are only slightly greater than for the Rapids.
G41C-0634
The Realisation and Validation of the Galileo Terrestrial Reference Frame (GTRF)
After a time of uncertainty the European Global Navigation Satellite System (GNSS) Galileo is going to be
realized in the next years. Two test satellites – GIOVE A and B – have been placed in the orbit to validate
various signals and component.
One basic element of the overall Galileo system is the so-called Galileo Terrestrial Reference Frame (GTRF)
as the basis for all Galileo products and services. The realization and maintenance of such a TRF has been
given to an external consortium, named the Galileo Geodetic Service Provider (GGSP), which consists of
seven institutions under the lead of GeoForschungsZentrum Potsdam. The project is funded within the sixth
framework programme (FP6) of the European Union and managed by the European GNSS Supervisory
Authority (GSA). It will last until May 2009.
The GTRF will be a realisation of the International Terrestrial Reference Frame (ITRF) on a position
precision level of 3 cm (2 sigma). Since the GTRF will already be required by the time when the first Galileo
signals are going to be emitted during the In-Orbit-Validation (IOV) phase, an initial realisation of the GTRF
has to be based on other positioning data, notably GPS.
In addition to the GTRF, the GGSP will generate additional products and information, such as Earth Rotation
Parameters, satellites orbits, clocks for satellites and stations.
The presentation describes the strategy for the GTRF realisation following the "state of the art" TRF
implementation. Since the Galileo tracking stations, named Galileo Sensor Stations (GSS), will form a sparse
global network, it is necessary to densify the network with additional stations to get the highest possible
precision and stability for the GTRF. The connection to the ITRF is realized and validated by IGS stations,
which are part of the ITRF, and especially by local ties to other geodetic techniques like satellite laser
ranging and VLBI. Results from the first analysis campaigns will be shown with special concern to the so-
called Galileo Experimental Sensor Stations (GESS) which form the ground network for GIOVE satellites.
G41C-0635
Results of Recent Operational Validation of More Frequent Earth Orientation Parameter Updates to GPS
For years, the need for more frequent Earth Orientation Parameter Prediction (EOPP) updates to GPS satellites has been discussed and the recent small polar motion loops have only reinforced that need. Recently, the U.S. Air Force's 2nd and 19th Space Operations Squadrons agreed to an operational validation of more frequent satellite updates. This agreement resulted in a four week operational validation of this process. For the first two weeks EOPP updates were done on a semi-weekly basis and for the last two weeks the updates were done on a daily basis. This presentation will present the results of this successful validation and the resulting operational changes to the frequency of EOPP updates.
G41C-0636 INVITED
Considerations for Future IGS Receivers
Future International GNSS Service (IGS) receivers are considered against the backdrop of GNSS signal modernization and the IGS's goal of further improving the accuracy of its products. The purpose of this paper is to provide the IGS ---and any other group that uses geodetic-quality GNSS receivers---with a guide to making decisions about GNSS receivers. Modernized GNSS signals are analyzed with a view toward IGS applications. A schedule for minimum IGS receiver requirements is proposed. Features of idealized conceptual receivers are discussed. The prospects for standard commercial receivers and for software-defined GNSS receivers are examined. Recommendations are given for how the IGS should proceed in order to maximally benefit from the transformation in GNSS that will occur over the next decade. There are two reasons why it makes sense for the IGS to study GNSS receivers that will be integrated into its network in the coming years. First, the new GNSS signals that will come on line over the next decade will render current IGS receivers obsolete, so it is prudent to examine receiver options going forward. Second, the push to improve the accuracy of IGS products beyond current limits demands greater accuracy in the models used to describe receiver measurements. As a result, the IGS must demand from vendors more transparency into receiver firmware or adoption of user-specified algorithms. This paper considers future IGS receivers from four different points of view. Section 2 looks at modernized GNSS signals and their benefits for the IGS. Section 3 surveys the range of expected receiver capability. Section 4 considers current and future commercial geodetic-quality receivers. Section 5 considers software GNSS receivers as an alternative to less reconfigurable traditional receivers. Section 6 lays out the authors' recommendations to the IGS.
G41C-0637
Near Real-Time Processing and Archiving of GPS Surveys for Crustal Motion Monitoring
We present an inverse instantaneous RTK method for rapidly processing and archiving GPS data for crustal
motion surveys that gives positional accuracy similar to traditional post-processing methods. We first stream
1 Hz data from GPS receivers over Bluetooth to Verizon XV6700 smartphones equipped with Geodetics, Inc.
RTD Rover software. The smartphone transmits raw receiver data to a real-time server at the Scripps Orbit
and Permanent Array Center (SOPAC) running RTD Pro. At the server, instantaneous positions are
computed every second relative to the three closest base stations in the California Real Time Network
(CRTN), using ultra-rapid orbits produced by SOPAC, the NOAATrop real-time tropospheric delay model, and
ITRF2005 coordinates computed by SOPAC for the CRTN stations. The raw data are converted on-the-fly to
RINEX format at the server. Data in both formats are stored on the server along with a file of instantaneous
positions, computed independently at each observation epoch. The single-epoch instantaneous positions are
continuously transmitted back to the field surveyor's smartphone, where RTD Rover computes a median
position and interquartile range for each new epoch of observation. The best-fit solution is the last median
position and is available as soon as the survey is completed. We describe how we used this method to
process 1 Hz data from the February, 2008 Imperial Valley GPS survey of 38 geodetic monuments
established by Imperial College, London in the 1970's, and previously measured by SOPAC using rapid-static
GPS methods in 1993, 1999 and 2000, as well as 14 National Geodetic Survey (NGS) monuments. For
redundancy, each monument was surveyed for about 15 minutes at least twice and at staggered intervals
using two survey teams operating autonomously.
Archiving of data and the overall project at SOPAC is performed using the PGM software, developed by the
California Spatial Reference Center (CSRC) for the National Geodetic Survey (NGS). The importation of raw
receiver data, site metadata and antenna height information is performed using PGM client software running
on the same PDA running RTD Rover or laptop, and uploaded to the PGM server where the raw data are
converted to RINEX format. The campaign information is then published online, where all of the campaign
information can be accessed such as start and stop times, equipment information, RINEX and solution SINEX
files, observer information and baseline information for network adjustments.
http://sopac.ucsd.edu/projects/impvall2008.html
G41C-0638
High rate GPS positioning , JASON altimetry and marine gravimetry : monitoring the Antarctic Circumpolar Current (ACC) through the DRAKE campaigns.
The Drake campaign which took place from Jan 14, 2006 – 08 Feb, 2006 has been a very successful mission in collecting a wide range of GPS and marine gravity data all along JASON altimetry ground track n° 104. The same campaign will be repeated in 2009 along 028 and 104 JASON-2 ground track. The Drake Passage (DP) chokepoint is not only well suited geographically, as the Antarctic Circumpolar Current (ACC) is constricted to its narrowest extent of 700 km, but observations and models suggest that dynamical balances are particular effective in this area. Furthermore the space geodesy observations and their products provided from several altimetry missions (currently operating ENVISAT, JASON 1 and 2, GFO, ERS and other plannified for the future such as Altika, SWOT) require the cross comparison with independent geodetic techniques at the DP. The current experiment comprises a kinematic GPS and marine gravimetry Cal/Val geodetic approach and it aims to : validate with respect to altimetry data and surface models such a kinematic high frequency GPS technique for measuring sea state and sea surface height (SSH), compare the GPS SSH profiles with altimetry mean dynamic topography (MDT) and mean sea surface (MSS) models, give recommendations for future "offshore" Cal/Val activities on the ground tracks of altimeter satellites such as JASON-2, GFO, Altika using the GNSS technology etc. The GPS observations are collected from GPS antennas installed on a wave-rider buoy , aboard the R/V "Polarstern" and from continuous geodetic reference stations in the proximity. We also analyse problems related to the ship's attitude variations in roll, pitch and yaw and a way to correct them. We also give emphasis on the impact of the ship's acceleration profiles on the so called "squat effect" and ways to deal with it. The project will in particular benefit the GOCE mission by proposing to integrate GOCE in the ocean circulation study and validate GOCE products with our independent geodetic data set. The high rate GPS SSH solutions are derived using two different GPS kinematic software, GINS (CNES) and TRACK (MIT).
G41C-0639
GNSS Data Modelling to Determine the Tropospheric Wet-Delay From Ground Based Observations
Microwave signals of the GNSS satellites (GPS, GLONASS and in future GALILEO) are time delayed when passing the atmosphere. Based on this signal delay, e.g. the humidity distribution within the troposphere can be determined. We investigate how the atmospheric precipitable water content (derived from GNSS data) can be used within an operational Nowcasting system (INCA). It has been proved that e.g. passing weather fronts can be analysed much better by introduced GNSS derived tropospheric wet delays because this data is influenced by changes in humidity in the free atmosphere, whereas the data at the meteorological ground stations reacts to these changes only with a time delay. This allows to forecast heavy rainfall causing potentially local floodings more reliable and to narrow down the affected region. In this presentation we focus on several modelling aspects (as listed below) related to the derivation of precipitable water from ground based GNSS observations: (1) Derive GNSS wet delays with a mixed network/PPP strategy (2) Investigate the value of processing hybrid GPS+GLONASS data (3) Investigate the influence of different mapping functions (4) Demonstration of the enhancement of the prognosis in the numerical forecast model (5) Delivery of the estimates in close-to real time
G41C-0640
GNSS Precise Point Positioning Approach Based on Dual Code Linear Combination
The first Block IIR-M satellite has launched in 2005 which denoted that the conventional GPS stepped into the modernized GPS phase. The new C/A code of GPS IIR-M satellites which have modulated in L2 frequency band named as L2C offers the general users an opportunity to raise the positioning techniques, such as by adding L2C with C/A and P-code to form the ionosphere-free (IF) linear combination model for precise point positioning (PPP) approach that is proposed in this study. The experimental GPS data is coming from four stations of international L2C network, such as PGC5, SJRP, NYAC, and HRAC, which observations including L2C are in RINEX O-file format to test the proposed model of precise point positioning. There are two tests: the first one is the distinctive analysis between single point positioning (SPP) and precise point positioning and the second is the analysis of positioning accuracy with single/multi-epoch PPP by adding L2C observations. The single- and multi-epoch PPP test respectively calculates the positions of experimental network in 30-, 300-, 600-, and 900-second interval. The outcomes show that positioning accuracy of PPP can promote 50% contrast to SPP. Another finds that by adding the more L2C observations, the higher positioning accuracy. It can reach 25.14% in horizontal direction and 24.50% in vertical direction.
G41C-0641
Ambiguity Resolution in Precise Point Positioning for Sub-cm Precision With Hourly Data
Precise Point Positioning (PPP) has become a recognized and powerful tool for scientific analysis of Global Positioning System (GPS) measurements. Until recently, integer ambiguity resolution for PPP at a single station has been considered difficult, due to the non-integer uncalibrated hardware delays (UHD) originating in receivers and satellites. Fortunately, recent studies on this have shown that if these UHD can be determined precisely from a GPS network in advance, then ambiguity resolution at a single station is possible. In this study, we adopt the innovative method proposed by Ge et al. (2007), but with a refinement in which the fractional parts of single-difference narrow-lane UHD between satellites are determined within each of their passes over a regional network. We show that the total hourly position accuracy can be improved after ambiguity resolution by ~80%, ~30%, and ~40% to 0.5 cm, 0.5 cm, and 1.4 cm for the East, North and Up components, respectively, and that the accuracy of hourly zenith troposphere delays can also be improved by ~20%. Furthermore, PPP ambiguity resolution can be carried out for sites several thousand kilometers from the sites used to compute the UHD. We argue that the success of PPP ambiguity resolution demonstrates that many geodetic, geophysical and environmental monitoring applications requiring sub-daily or hourly positioning can benefit from this approach. Finally, following on from our results we propose a concept for a future online processing service based on PPP with integer ambiguity fixing.