G11A-01
National Positioning, Navigation, and Timing Architecture Study
The purpose of the National Positioning, Navigation and Timing (PNT) Architecture effort is to help guide future PNT system-of-systems investment and implementation decisions. The Assistant Secretary of Defense for Networks and Information Integration and the Under Secretary of Transportation for Policy sponsored a National PNT Architecture study to provide more effective and efficient PNT capabilities focused on the 2025 timeframe and an evolutionary path for government provided systems and services. U.S. Space-Based PNT Policy states that the U.S. must continue to improve and maintain GPS, augmentations to GPS, and back-up capabilities to meet growing national, homeland, and economic security needs. PNT touches almost every aspect of people´s lives today. PNT is essential for Defense and Civilian applications ranging from the Department of Defense´s Joint network centric and precision operations to the transportation and telecommunications sectors, improving efficiency, increasing safety, and being more productive. Absence of an approved PNT architecture results in uncoordinated research efforts, lack of clear developmental paths, potentially wasteful procurements and inefficient deployment of PNT resources. The national PNT architecture effort evaluated alternative future mixes of global (space and non space-based) and regional PNT solutions, PNT augmentations, and autonomous PNT capabilities to address priorities identified in the DoD PNT Joint Capabilities Document (JCD) and civil equivalents. The path to achieving the Should-Be architecture is described by the National PNT Architecture's Guiding Principles, representing an overarching Vision of the US' role in PNT, an architectural Strategy to fulfill that Vision, and four Vectors which support the Strategy. The National PNT Architecture effort has developed nineteen recommendations. Five foundational recommendations are tied directly to the Strategy while the remaining fourteen individually support one of the Vectors, as will be described in this presentation. The results of this effort will support future decisions of bodies such as the DoD PNT and Civil Pos/Nav Executive Committees, as well as the National Space-Based PNT Executive Committee (EXCOM). http://www.acq.osd.mil/nsso/pnt/pnt.htm
G11A-02
Maintenance of the Geodetic Reference Frame in the Global Positioning System
In the Global Positioning System (GPS) measurements of the satellite coordinates and the underlying World Geodetic System 1984 (WGS 84) reference frame are derived from observables such as pseudorandom noise (PRN) signals, and carrier phase, which are referenced to on-board atomic clocks. Systematic errors exist in both the estimated satellite coordinates and the reference frame. The reference frame utilizes external inputs in the form of International Terrestrial Reference Frame (ITRF) coordinates and constrains the results to be compatible with the ITRF coordinates for a set of global reference stations. The ITRF is, in turn, obtained from the combined analysis of GPS, Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) data. The current realization of the reference frame could be described as circular in that an independent method of external verification is currently not available. To ensure the continued successful operation of the GPS it is necessary to have the capability of analyzing systematic errors by an independent means from current radiometric observables and data from foreign sources. In practice, accuracy of the standards used for measurement should be better than the expected, required operational measurement accuracy by a factor of ten to ensure that the desired requirement is met. Currently, the accuracy of both the ITRF and the WGS 84 is estimated to be on the order of 1 to 2 parts per billion, leading to expected drifts of 0.6 to 1.2 cm per year. The experience of the last three decades has indicated an approximate improvement by a factor of ten per decade. Therefore, while current accuracy of the ITRF and WGS 84 reference frames marginally meets civilian and military requirements, it is very likely that, within the lifetime of GPS III, the accuracy of the reference frames will be unable to meet the anticipated requirements. This report examines to address the emerging capability gap. These include: (1) Increased Frequency of Satellite Clock Updates; (2) Radiometric Tracking at Higher Frequency; (3) Laser Tracking; (4) Astronomical Geolocation; and (5) GPS Intersatellite Links.
G11A-03
On the Accuracy of IGS Orbits
In order to explore the reliability of IGS internal orbit accuracy estimates, we have compared the geocentric satellite positions at the midnight epoch between consecutive days for the period since November 5, 2006, when the IGS changed its method of antenna calibration. For each pair of orbits, day "A" has been fitted to the extended CODE orbit model (three position and three velocity parameters plus nine nuisance solar radiation parameters), using the IGS05 Final orbits as psuedo-observations, and extrapolated to epoch 24:00 to compare with the 00:00 epoch from the IGS05 Final orbits of day "B". This yields a time series of orbit repeatability measures, analogous to the classical geodetic test for position determinations. To assess the error introduced by the fitting and extrapolation process, the same procedure has been applied to several days dropping the 23:45 epoch, fitting up to 23:30, extrapolating to 23:45, and comparing with reported positions for 23:45. The test differences range between 0 and 10 mm (mean = 3 mm) per geocentric component with 3D differences of 3 to 10 mm (mean = 6 mm). So, the effect of the orbit fitting-extrapolation process nearly always adds insignificant noise to the day- boundary orbit comparisons. If we compare our average 1D position differences to the official IGS accuracy codes (derived from the internal agreement among combined orbit solutions), root-sum-squared for each pair of days, the actual discontinuities are not well correlated with the expected performance values. If instead the IGS RMS values from the Final combination long-arc analyses (which also use the extended CODE model) are taken as the measure of IGS accuracy, the actual orbit discontinuties are much better represented. This is despite the fact that our day- boundary offsets apply to a single epoch each day and the long-arc analyses consider variations over a day (compared to the satellite dynamics determined over the full week). Our method is not well suited to probe the extent to which systematic effects dominate over random orbit errors, as indicated by satellite laser ranging residuals, but eclipsing satellites often display the most problematic behavior. Users must always be skeptical of GPS-based formal error estimates for all quantities, not just orbits, and should be aware that non- Gaussian errors are common. Nevertheless, the IGS SP3 accuracy codes appear to misrepresent the actual orbit errors in many cases. A better metric would probably be the IGS long-arc RMS values rather than the RMS agreement among solution submissions.
G11A-04 INVITED
IGS Clock Products for Accurate Geodetic and Timing Applications
The performance of any GNSS is intimately related to the characteristics of the satellite clocks, so an understanding of the clock behavior is vital. The accurate products of the IGS enable daily point positions to the sub-cm level and continuous global clock comparisons to the sub-ns level. Time transfers are less accurate than associated positioning because of: 1) difficult-to-measure hardware delays; 2) the limiting pseudorange measurement errors. Both factors arise from characteristics of the pseudorange signals, which are easily degraded by multipath and other effects. The behavior of the satellite clocks are also be important. Over sub-daily intervals, IGS products show that approximate power-law stochastic processes govern all GPS clocks. The Block IIA Rb and Cs clocks obey random walk noise, with the Rb clocks up to nearly an order of magnitude more stable. Due to the high-frequency noise of the onboard Time Keeping system in the newer Block IIR and IIR-M satellites, their Rb clocks are dominantly white noise up to a few 1000 s with standard deviations of 90 to 180 ps. Superposed on this random background, periodic signals are present at four harmonic frequencies, n × (2.0029 ± 0.0005) cycles per day for n = 1, 2, 3, and 4. The equivalent fundamental period is 11.9826 hours, which surprisingly differs from the reported mean GPS orbital period of 11.9659 hours by 60 ± 11 s. We cannot account for this apparent discrepancy but note that a clear relationship between the periodic signals and the orbital dynamics is evidenced for some satellites by modulations of the spectral amplitudes with eclipse season. The Cs clocks are more strongly affected than the Rb clocks. All four harmonics are much smaller for the IIR/IIR-M satellites than for the older blocks. The strong 12- and 6-hour periodics in most GPS clocks dictate that these variations should be modeled in all high-accuracy applications, such as for timescale formation, interpolation of IGS clock products for higher-rate GPS positioning, and predictions for real-time uses. Modeling will also overcome the effect at 6 hours due to the conventional neglect of the Earth's oblateness in time transformations and is more effective than modifying the relativistic modeling since other causes of 6-hour periodics are usually greater. After accounting for the 12- and 6-hour harmonics, and the usual quadratic variations, it is possible to interpolate or extrapolate the residual stochastic clock time series with minimal degradation for the IIA Rb satellites up to about 30 s or up to 5 minutes within the accuracy as the IGS products. The white noise of the IIR/IIR-M Rb clocks limits their interpolation and prediction to about 135 ps over intervals shorter than 1000 s. The high level of random walk variance for the IIA Cs clocks immediately overwhelms the IGS measurement noise for any interval. In any future improved GNSS clock systems, it would be beneficial to reduce the white noise floor of the Time Keeping System by at least a factor of five. For short-term performance comparable to the level expected for the Galileo H-masers, further reduction of the GPS clock instabilities would be necessary. If methods can be found to attenuate the GPS harmonics at the satellites, that would be useful, otherwise modeling of these effects is probably effective and does not require any understanding of the driving mechanisms.
G11A-05
Recent Developments in the Processing of Global GPS Data at the Jet Propulsion Laboratory
The Jet Propulsion Laboratory (JPL) has a long history of processing global GPS data for various geodetic and orbit determination applications, and serves as an Analysis Center for the International GNSS Service. In this paper, we describe recent developments in the GPS data processing software system at JPL, GIPSY-OASIS, and the derived GPS data products. We detail the resulting improvements in accuracy of the primary products, namely the estimates of the precise GPS orbits and clocks, Earth orientation parameters, and zenith troposphere at the GPS sites, as well as the impact on various geodetic and orbit determination applications. We provide details of the changes to the models and capabilities of the GIPSY-OASIS software system, and describe modifications to the JPL products that are provided to the international community.
G11A-06 INVITED
Opportunities in Atmosphere Monitoring Offered by Modernized/New Global Navigation Satellite Systems
L-band RF signals experience propagation delays dependent on pressure, temperature and humidity in the neutral atmosphere. This effect can be measured using GNSS receivers, and information extracted about atmospheric properties, particularly water vapour. Over the past decade, meteorologists have exploited GNSS as an atmospheric remote sensing tool, with applications in weather forecasting and climate change. The availability of Galileo signals, when combined with those from the modernized GPS (and/or GLONASS), will enable more accurate estimates of water vapour using ground-based receivers, with higher temporal and spatial resolution. By deploying modernized/new GNSS receivers onboard low-Earth orbiters, vertical profiles of atmospheric temperature and humidity may be derived with improved accuracy over current GPS-based methods. A GPS/Galileo approach would effectively double the number of observations available at a given epoch, allowing reliable precipitable water vapour estimation (PWV) over a shorter batch processing interval (e.g. ten minutes or less) than currently used for ground-based GPS networks. Increased redundancy will allow improved detection of outliers, particularly orbit errors, which are currently a limiting factor in near real-time processing. This resolution would allow new opportunities for detecting and monitoring severe weather such as hail storms, thunderstorms and tornados. Additional observations in a GPS/Galileo approach could be used to resolve azimuthal asymmetries and higher- order spatial variations. The availability of triple-frequency observations for Galileo and modernized GPS can be used to eliminate residual higher order ionospheric effects and provide more accurate input observables for PWV estimation. Overall, the availability of modernized/new GNSS signals for atmosphere monitoring will improve accuracies of ground-based moisture estimates by as much as 50 percent – through improved observation accuracy, better geometry, and higher temporal and spatial resolution. For radio occultation applications using low-Earth orbiters, vertical profiles may be obtained at higher altitudes through elimination of higher order ionosphere residual errors using triple-frequency observations. The availability of multiple-frequency signals from new/modernized GNSS will also improve observation quality and humidity profiling at lower altitudes (atmospheric boundary layer) where current methods based on GPS are limited by atmospheric attenuation of the L2 signal.
G11A-07
Combined processing of observations from different Global Navigation Satellite Systems
The upcoming the Galileo GNSS and the modernisation of the GPS and Glonass systems offers many exciting opportunities and challenges in the field of geosciences in the next decade. However, in order to obtain any positive effects on our geodetic and geophysical estimates the different GNSS systems will have to be observed by multi system receivers that track all systems on all available frequencies. Furthermore, these receivers should not introduce any biases between the tracked GNSS observations. In addition to this we need analysis software that can efficiently handle these multi-system and multi-frequency observations in one single estimation process. Over the last two years ESOC has put a significant effort into its Napeos processing software. This software is now capable of combined processing of SLR, DORIS, GPS, GLONASS, and GIOVE-A data. It is routinely used for a large number of tasks within ESOC, e.g., Envisat POD, GIOVE-A orbit predictions for SLR, and for the ESOC contributions to the Galileo Geodetic Service Provider. Furthermore, it will soon officially be used for generating all the ESOC products for the International GNSS Service (IGS). In our presentation we will show results from our combined GNSS analysis, both the combination of GPS and GLONASS as well as the combination of GPS and GIOVE-A. We will focus on the challenges and we were, and in part still are, faced with when combining the data of different GNSS. We will demonstrate that at present both GLONASS and GIOVE-A do not offer any benefits for our estimates. We will conclude our contribution with a discussion on the requirements which need to be fulfilled to be able to really benefit from a combined processing of multi Global Navigation Satellite Systems.
G11A-08
The GPS Analysis and Positioning Software (GAPS): A Present and Future Tool for Processing GNSS Data
Precise Point Positioning (PPP) is one of the existing techniques to determine point coordinates using a GNSS receiver. In this technique, observations carried out by a single receiver are used in order to determine the three coordinate components, as well as other parameters, such as the receiver clock error and total neutral atmosphere delay. The technique is said to be "precise" because precise information, such as satellite orbits and clock errors, is used in the data processing (because of this, one should have in mind that the usage of precise orbits, clocks and other precise products is an implicit procedure whenever the term "PPP" is used). More than that, PPP is "precise" also because the resulting parameters are precise (and accurate). The fact that the observation model used for accurate error modelling has to take into consideration the several effects present in GPS signals, and that observations are undifferenced (there are no differences between receivers nor between satellite measurements), makes PPP a powerful data analysis tool which is sensible to a variety of parameters. The PPP application developed at UNB (University of New Brunswick), which is called GAPS (GPS Analysis and Positioning Software), has been designed and built in order to take advantage of precise products, resulting in a data analysis tool for determining parameters other than position, receiver clock error and neutral atmosphere delay. These other estimated parameters include ionospheric delays, code biases, satellite clock errors, and code multipath among others. In all cases, the procedures were developed in order to be suitable for real-time as well as post-processing applications. The ionospheric delay estimation uses a spherical ionospheric shell model, in which the vertical delays are described by means of a zenith delay at the station position and two horizontal gradients. This estimation makes use of carrier-phase measurements only. The use of precise orbits and clocks is a key element for the quality control of the data which goes into the ionospheric estimation filter. The code multipath estimation is based on the assumption that the several effects present in code measurements are dealt with within PPP, but strongly based on carrier-phase measurements. Based on this, these effects can be removed from pseudorange measurements, and the leftover effect is essentially the code multipath plus receiver noise. Another effect which afflicts pseudorange measurements is the code bias. The code biases are important because satellite clock data products are computed using a certain arbitrary convention of observation type, such as P1 code measurements rather than C1 code. One of the analysis tools of GAPS produces values of the satellite code biases, based on a positioning observation model, as opposed to being based on a satellite clock estimation observation model as is usually the case when bias values are provided to users. Regarding satellite clock error estimates, GAPS was enhanced in order to provide estimates of satellite clock offsets. This tool was created aiming at a suitable approach for real-time carrier-phase-based satellite clock estimation. GAPS is available online via a web interface, through the University of New Brunswick website. In addition to signal analysis outputs, GAPS provides state-of-art PPP results, including position, receiver clock errors, and neutral atmosphere delays, in static or kinematic mode. Future plans for enhancing GAPS to process data from modernized GPS and other GNSS constellations will also be discussed as will the possibilities of using GAPS as a real-time tool.