Since 1994, the International GNSS Service (IGS) has provided precise GPS orbit products to the scientific community with increased precision and timeliness. Many national geodetic agencies and GPS users interested in geodetic positioning have adopted the IGS precise orbits to achieve centimeter level accuracy and ensure long-term reference frame stability. Relative positioning approaches that require the combination of observations from a minimum of two GPS receivers, with at least one occupying a station with known coordinates are commonly used. The user position can then be estimated relative to one or multiple reference stations, using differenced carrier phase observations and a baseline or network estimation approach. Differencing observations is a popular way to eliminate common GPS satellite and receiver clock errors. Baseline or network processing is effective in connecting the user position to the coordinates of the reference stations while the precise orbit virtually eliminates the errors introduced by the GPS space segment. One drawback is the practical constraint imposed by the requirement that simultaneous observations be made at reference stations. An alternative post-processing approach uses un-differenced dual-frequency pseudorange and carrier phase observations along with IGS precise orbit products, for stand-alone precise geodetic point positioning (static or kinematic) with centimeter precision. This is possible if one takes advantage of the satellite clock estimates available with the satellite coordinates in the IGS precise orbit/clock products and models systematic effects that cause centimeter variations in the satellite to user range. Furthermore, station tropospheric zenith path delays with mm precision and GPS receiver clock estimates precise to 0.03 nanosecond are also obtained. To achieve the highest accuracy and consistency, users must also implement the GNSS-specific conventions and models adopted by the IGS. This paper describes both post-processing approaches, summarizes the adjustment procedure and specifies the Earth and space based models and conventions that must be implemented to achieve mm-cm level positioning, tropospheric zenith path delay and clock solutions.
The International GNSS Service (IGS), formerly the International GPS Service, is a voluntary collaboration of more than 200 contributing organizations in more than 80 countries. The IGS global tracking network of more than 300 permanent, continuously-operating GPS stations provides a rich data set to the IGS Analysis Centers, which formulate precise products such as satellite ephemerides and clock solutions. IGS Data Centers freely provide all IGS data and products for the benefit of any investigator. This paper focuses on the advantages and usage of the IGS precise orbits and clocks.
Currently, up to eight IGS Analysis Centers (AC) contribute daily Ultra-rapid, Rapid and Final GPS orbit and clock solutions to the IGS combinations. The daily computation of global precise GPS orbits and clocks by IGS, with centimeter precision, facilitates a direct link within a globally integrated, reference frame which is consistent with the current International Terrestrial Reference Frame (ITRF). Since 2000 the ultra-rapid product originally designed to serve meteorological applications and support Low Earth Orbiter (LEO) missions, has been made available. The ultra-rapid product has since become useful to many other real-time and near real-time users, as well. For more information on the IGS combined solution products and their availability see the IGS Central Bureau Products Page.
For GPS users interested in meter level positioning and navigation, a simple point positioning interface combining pseudorange data with IGS precise orbits and clocks (given at 15 min intervals) can be used (e.g. Héroux et al., 1993; Héroux and Kouba, 1995). Since May 2, 2000 when Selective Availability (SA) was switched off these products also satisfy GPS users observing at high data rates in either static or kinematic modes for applications requiring meter precision. This is so, because the interpolation of the 15-min SA-free satellite clocks given in IGS sp3 files is possible at the precision level a few dm. Furthermore, since December 26, 1999, separate, yet consistent, clock files, containing separate combinations of satellite/station clocks at 5-min sampling intervals have been available and on November 5, 2000, the clock combinations became the official IGS clock products (Kouba and Springer, 2000). The 5-min clock sampling allows an interpolation of SA-free satellite clocks well below the dm level (Zumberge and Gendt, 2000). In order to keep clock interpolation errors at or below the cm-level, starting with GPS Week 1410 (January 14, 2007), the IGS Final clock combinations also include additional clock files with 30-sec sampling. For GPS users seeking to achieve geodetic precision, sophisticated processing software packages such as GIPSY (Lichten et al., 1995), BERNESE (Dach et al., 2007) and GAMIT (King and Bock, 1999) are required. However, by using the IGS precise orbit products and combining the GPS carrier phase data with nearby IGS station observations, geodetic users can achieve precise relative positioning consistent with the current global ITRF, with great ease and efficiency and with relatively simple software. For example, differential software packages provided by receiver manufacturers may also be used, as long as they allow for the input of the station data and orbit products in standard (IGS) formats and conform to the international (IGS and IERS) conventions and standards (see Section 5.3).
The precise point positioning (PPP) algorithms based on un-differenced carrier phase observations have been added to software suites using un-differenced observations such as GIPSY (Zumberge et al., 1997) and more recently even the traditional double-differencing software package such as the BERNESE has been enhanced also to allow precise point positioning. Users now have the option of processing data from a single station to obtain positions with centimeter precision within the reference frame provided by the IGS orbit products. PPP eliminates the need to acquire simultaneous tracking data from a reference (base) station or a network of stations. It has given rise to centralized geodetic positioning services that require from the user a simple submission of a request and a valid GPS observation file (see e.g., Ghoddousi-Fard and Dare, 2005). An alternative approach is an implementation of simple PPP software that effectively distributes processing by providing portable software that can be used on a personal computer. This software then takes full advantage of consistent conventional modeling and the highly accurate global reference frame, which is made available through the IGS orbit/clock combined products.
For both relative and PPP methods that utilize IGS orbit/clock products, there is no need for large and sophisticated global analyses with complex and sophisticated software. This is so because the IGS orbit/clock products retain all the necessary information of the global analyses that have already been done by the IGS ACs, using the state of art knowledge and software tools. Thus, the users of the IGS products in fact take full advantage of the IGS AC global analyses, properly combined and quality checked, all in accordance with the current international conventions and standards.
Even though, strictly speaking, it is illegitimate to combine solutions that are based on the same observation data set, the combinations of Earth Rotation Parameters (ERP) and station coordinate solution submissions have been successfully used by the International Earth Rotation and Reference Systems Service (IERS) for many years. Such combinations typically result in more robust and precise solutions, since space technique solutions are quite complex, involving different approaches and modeling that typically generate a random-like noise which is then averaged out within the combination process. This approach is also valid for the combination of IGS orbit solutions as clearly demonstrated by Beutler et al., (1996) who have also shown that, under certain conditions, such orbit combinations represent physically meaningful orbits as they still satisfy the equations of motions. Furthermore, when the AC weights resulting from orbit combinations are used in the corresponding ERP combinations (as done by IGS before February 27, 2000), the crucial consistency between the separately combined orbits and ERP solutions is maintained.
The IGS combined orbit/clock products come in various flavors, from the Final, Rapid to the Ultra-Rapid, which became officially available on November 5, 2000 (GPS Week 1087, MJD 5183). The IGS Ultra-Rapid (IGU) products replaced the former IGS predicted (IGP) orbit products (IGS Mail #3229). The IGS combined orbit/clock products differ mainly by their varying latency and the extent of the tracking network used for their computations. The IGS Final orbits (clocks) are currently combined from up to eight (seven) contributing IGS ACs, using six, largely independent, software packages (i.e. BERNESE, GAMIT, GIPSY, NAPEOS (Dow et al., 1999), EPOS (Gendt et al., 1999) and PAGES (Schenewerk et al., 1999). The IGS Final orbit/clocks are usually available before the thirteenth day after the last observation. The Rapid orbit/clock product is combined 17 hours after the end of the day of interest. The latency is mainly due to the varying availability of tracking data from stations of the IGS global tracking network, which use a variety of data acquisition and communication schemes, as well as different levels of quality control. In the past, the IGS products have been based only on a daily model that required submissions of files containing tracking data for 24-hour periods. In 2000, Data Centers have been asked to forward hourly tracking data to accelerate product delivery. This new submission scheme was required for the creation of an Ultra-Rapid product, with a latency of only a few hours, that should satisfy the more demanding needs of most real-time users, including the meteorological community and LEO (Low Earth Orbiter) missions. It is expected that IGS products will continue to be delivered with increased timeliness in the future (Weber et al., 2002). Development of true real-time products, mostly satellite clock corrections, is underway within the IGS Real-Time Pilot Project. For more information on the IGS products and their possible applications see e.g. Neilan et al., (1997); Kouba et al., (1998) and Dow et al., (2005).
Figure 1: Weighted orbit RMS of the IGS Rapid (IGR) products and AC Final orbit solutions during 1994-2009 with respect to the IGS Final orbit products. (COD - Center for Orbit Determination in Europe, Switzerland; EMR - Natural Resources Canada; ESA - European Space Agency; GFZ - GeoForschungsZentrum Potsdam, Germany; JPL - Jet Propulsion Laboratory, U.S.A.; MIT- Massachusetts Institute of Technology, U.S.A.; NGS - National Geodetic Survey, NOAA, U.S.A.; SIO - Scripps Institute of Oceanography, U.S.A.). The newest AC - GRG (CNES, CLS and GRG of France) is not shown here, since currently (May 2009), it is not yet included in the IGS combinations. (Courtesy of the IGS ACC)
From Figure 1, one can see that over the past 15 years the precision of the AC Final orbits has improved from about 30 cm to about 1 - 2 cm, with a concomitant improvement in the IGS Final combined orbit. It is also interesting to note that the IGS Rapid orbit combined product (IGR), with less tracking stations and faster delivery times, is now more precise than the best AC Final solutions. The precision of the corresponding AC/IGS ERP solutions has shown similar improvements since 1994. One element that has received less attention is the quality of the GPS satellite clock estimates included in the IGS orbit products since 1995. Examining the summary plots for IGS Final clock combinations at the IGS AC Coordinator (ACC) web site (http://acc.igs.org), one can notice that after small biases are removed, the satellite clock estimates produced by different ACs now agree with standard deviations of 0.02 - 0.06 nanosecond (ns) or 1 - 2 cm. This is also consistent with the orbit precision. Any biases in the individual IGS satellite clocks will be absorbed into the phase ambiguity parameters that users must adjust. The precise GPS orbits and clocks, weighted according to their corresponding precision (sigmas), are the key prerequisites for PPP, given that the proper measurements are made at the user site and the observation models are implemented correctly.
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