IGS Products and Applications Jan Kouba Geodetic Survey

IGS Products and Applications Jan Kouba Geodetic Survey Div. NRCan Canada

Content • IGS Combined Products • Use of IGS Products • • IGS Products Limitations and Solutions Future Developments – – – – Objectives Description Precision/availability Consistency Relative Positioning Precise Point Positioning/Navigation Compatibility Scientific Applications

IGS PRODUCTS • Support practical positioning and scientific applications of GPS, such as – surveying, navigation and national geodetic control networks – monitoring of Earth rotation, deformation and atmosphere (trop. & ion. ) – easy access and contributions to ITRF, time transfers, etc. • Provides convenient and precise “IGS reference system” through – the core IGS combined products: orbits, Earth rotation parameters (ERP), station positions/velocities and satellite/station clocks – additional combined products, consistent with the core products: tropospheric zenith path delays (ZPD), ionospheric TEC (total electron count) maps and associated rec. /satellite diff. calibration biases (DCB) – IGS combined orbits (since 1994) ; clocks and ERP (since 1995); station positions (since 1996) and tropospheric ZPD (since 1997) • IGS Products are more precise and reliable than the individual AC solutions (up to 8 Analysis Center solutions combined)

IGS Product Description • Core Combined Products: orbits/clocks/ ERP/station positions – IGS Final orbit, clock and ERP combined solutions (12 day delay) – IGS Rapid orbit, clock and ERP combined solutions (17 hour delay) – IGS Predicted orbit and clock combined solutions (1 hour before the observation day) – Nine Associate ACs (AACs) produce regional station position solutions in SINEX format – Up to eight ACs, AACs contribute global orbit, ERP, global station position (SINEX) solutions and up to five ACs clock solutions • Combined Zenith Path Delay (ZPD) – seven ACs, produced by GFZ, since Jan. 1997 – Weekly files by site, 2 hour resolution (four weeks delays) • Combined Global Ionosphere Maps of Total Electron Content (TEC) – Five ACs, AACs, Combined TEC and DCBs (Differential Code Biases) in IONEX format, produced by ESA since June 1998 – Daily files, 2 hour resolution (four weeks delay)

Analysis Center Functions • IGS Analysis Centers (AC): are the basis of all IGS products, improved IGS products are largely due to AC improvements! – Typically performs continuously (daily): three types of global solutions (Predicted, Rapid and Final) utilizing complex and sophisticated software, often developed in house over many years – Conforming to IGS/IERS standards, but innovations are encouraged. – Documentation on an each AC(‘ccc’) approach is at the IGS CB Archives: http: //igscb. jpl. nasa. gov/igscb/center/analysis/ccc. acn – Continuously involved in research & developments: striving for the best solutions and sharing scientific results amongst AC colleagues • Helpful and healthy competitions amongst all ACs • Associate Analysis Centers (AACs): – typically do not perform all global solutions, e. g. the Rapid only, or only certain specific solution products, such as: – RNAAC (Regional Network AAC), GNAAC (Global Network AAC), IAAC (Ionospheric AAC) etc.

IGS PRODUCT Table IGS Combined Product Precision Product Available Interval Predicted Rapid Final Real-time 17 hours 12 days 15 min 50 cm 10 cm 5 cm Positions Velocities Weekly solutions 2 -4 weeks 7 days 3 -5 mm 1 -3 mm/y Rapid PM Final PM Rapid UT /LOD Final UT /LOD Tropospheric ZPD Ionosph. grid TEC 17 hours 12 days 17 hours 10 days <4 weeks 1 day 2 hours . 2 mas. . 1 mas. 10 ms. 05 ms 4 mm 1 TEC unit . 4 mas/d. 2 mas/d. 06 ms/d. 03 ms/d Satellite Orbits&clocks IGS Combined (Prelim. ) Station Earth Rotation Parameters orbits Precision parameters clocks 30 ns. 5 ns. 3 ns rates/LOD (~10 cm)

IGS Orbit Improvement 1994 to Mid 1999

IGS PRODUCTS (AC performance)

IGS Orbit Comparison

Earth Rotation Parameters

Consistency of IGS Products • Consistent with the current international conventions – International Earth Rotation Service (IERS) Conventions 1996 – International Terrestrial Frame (ITRF)(currently ITRF 97) • IGS CORE products (orbits/clock/ERP/stations) are mutually consistent – Since December 1998, combination of minimally constrained AC solutions – Nearly exact ITRF transformations (e. g. for the ITRF 97 -ITRF 96 and the future changes) – ITRF realizations used for IGS Products (for more details see the presentation on: IGS ITRF Realization and Transformations) : • • • ITRF 92 ITRF 93 ITRF 94 ITRF 96 ITRF 97 - Jan 2, 1994(Wk 0730) to Dec 31, 1994 (Wk 0781) Jan 1, 1995(Wk 0782) to Jun 29, 1996 (Wk 0859) Jun 30, 1996(Wk 0860) to Feb 28, 1998 (Wk 0946) Mar 1, 1998(Wk 0947) to July 31, 1999 (Wk 1020) Aug 1, 1999(Wk 1021) to present (since Feb 27, 2000 IGS ITRF 97 (SINEX)) – IGS satellite clock solutions are aligned to the GPS time via broadcast clock corrections:

USE of IGS PRODUCTS • IGS Products greatly enhance and simplify (no need for complex global GPS solutions) all GPS analyses and applications such as: • Positioning/navigation – Global, Regional and Local • Relative positioning with respect to the IGS station Polyhedron of about 200 stations at the mm/ppb level (IGS orbits/ERP and station positions can be fixed even for baselines 2000 km and longer). No need for precise satellite clocks (nearly eliminated)! • Absolute, precise point positioning (ppp) worldwide, with respect to the “IGS Reference Frame” (IGS orbit/clocks/ERP held fixed) at mm precision level • Absolute, precise point navigation worldwide (ground, air, LEO satellites - no base stations needed!), at a cm precision level • Global Change Monitoring: solid Earth, atmosphere and oceans – ERP monitoring – plate motion and crustal deformation monitoring, volcanoes, etc. – monitoring of atmosphere (troposphere and ionosphere) • Precise time and freq. transfers (future contributions to UTC)

Relative Positioning • Relative position precision at ppb/mm – IGS Orbits, ERP, base station position solutions can be held fixed at the IGS solutions for all, even extreme, baseline lengths at ppb/mm precision level (in ITRF insensitive to ERP) – When using double differencing, potentially more robust (than precise point positioning), the IGS products ease data cleaning, ambiguity fixing – Complete covariance/correlation matrix available (e. g. for in SINEX format) – Suitable for continental, regional, national ITRF densification (e. g. EUREF normal equation matrix stacking of RNAAC SINEX solutions) • Warning: fixing IGS orbits/ERP implies very precise reference orientation and scale even for (SINEX) unconstrained solutions! (I. e. no stations fixed) • For long baselines (~ 1000 km) ITRF orientation/scale changes need to be taken into account prior normal matrix stacking – Insensitive to incompatibilities and clock errors • Insensitive to clock errors and all small (cm) ITRF changes for baselines <1000 km

Relative Positioning

Precise Point Positioning • • • IGS combined Orbits, clocks held fixed in single point solutions – Equivalent to back substitution into complex, global (IGS) solutions An efficient, flexible and convenient interface/access to the “IGS reference frame” – mm precision level for static point positioning worldwide, no base (IGS Polyhedron) station positions/data required – cm precision level for (epoch by epoch, ground, air, LEO sarellite) navigation, no base (DGPS) station positions/data required. – Efficient and precise solutions for station clocks and tropospheric ZPDs Disadvantages (wrt to double differencing/relative positioning) – More difficult to clean/edit data and/or to fix integer phase ambiguities – Sensitive to clock errors and to incompatibilities (see below) and even small reference (ITRF) changes; orbits/clocks/ERP must be consistent! – Correlation/variance matrix (amongst stations) lost and not available, thus not well suited for national/continental ITRF (SINEX) densification

Precise Point Positioning Note: in near future, the Ultra Rapid IGS orbits (Ultra R ) <12 h prediction) is to replace the IGS Predicted (IGP) Orbits (<48 h prediction)

Precise Point Positioning (PPP)

Precise Navigation

Comparison of Point Positioning Results at Three Stations

Compatibility IGS CORE products (orbits/clock/ERP/stations) – Mutually compatible and consistent since Dec 1998 – Consistency of orbits and clocks is of utmost importance for precise point positioning – IGS station (SINEX) solutions are compatible/consistent with IGS antenna offsets (station log files) and the antenna calibration (phase center) tables (IGS_01. pcv) at IGSCB Archives – IGS satellite clock solutions are based on/compatible with the following assumptions/conventions: • Periodical special relativistic correction • Cross correlation type of pseudoranges : - 2. X. V/c 2 applied : C/A; P 2’= C/A + (P 2 -P 1) : P 1, P 2 (After April 2, 2000, Wk 1056) • Adopted satellite antenna offsets (X, Y, Z m) – Block II/IIA : 0. 279 0. 000 1. 023 (in the satellite fixed frame) – Block IIR : 0. 000 • no L 1 -L 2 satellite/receiver calibration delayed applied – Ionospheric TEC grid maps, satellite/receiver calibration biases must be of the same solutions and • corresponding mapping function and DCB’s must be used for all ionospheric applications (to derive slant ionospheric delays)

Compatibility Solid Earth tides have once and twice daily periods; can reach up to 300 mm in height and up to 50 mm in latitude and longitude.

Compatibility Ocean Loading Effects have once and twice daily periods; can reach up to 50 mm in height and up to 10 mm in latitude and longitude.

Scientific Applications • Earth Orientation Parameter (EOP) monitoring – IGS Polar Motion rates of unprecendeted precision (0. 1 mas/day) and resolution (1 day, or even 2 hours for CODE AC solutions!) – Continuously available with delays of less than 24 h (IGS Rapid) – Independent short period (<30 days) nutation determination (by CODE AC) • Global Change Studies – Correlation studies of ERP rates Atmospheric (AAM) and Oceanic Angular Momentum (OAM) – Water vapor studies (TZPD, MET LEO satellite missions) – Gravity LEO satellite missions (CHAMP, GRACE, etc. ) – Ionospheric studies and correlation with EOP, atmosphere, etc. • Precise time/frequency transfers – Current limitation - temperature calibration/control – Potentially at a few tens pico seconds or below the 10 -15/day level

Scientific Applications Correlation between Polar Motion Rates (Xrt, Yrt (not shown)) and OAM+AAM is nearly always better than 0. 8 (except for the low signal period at or below 0. 25 mas/d)

Water Vapor Estimates Comparison at Station Potsdam Partial Water Vapor (PWV) from GPS at IGS station POTS (Germany) agrees with Water Vapor Radiometer (WVR), well within one mm (Courtesy of GFZ).

Limitations and Solutions • IGS Combined Orbits : • IGS Combined ERP: • IGS Combined Clocks (see also compatibility): – In 24 h files with 15 min sampling (0 -23: 45 GPS time); can be easily concatenated to span daily boundaries; easily be fitted with low order polynomials for continuous representation – Small daily discontinuities (at the dm level) – Possible orbit origin offsets (biases at the cm level) – Improvements at the AC level, I. e. smaller daily AC orbit discontinuities (<dm) and smaller orbit origin offsets (< cm) required – May be biased at the. 1 mas level due to AC ERP biases – New IGS SINEX ERP combination should help (started Feb 20, 2000) – Daily clock discontinuities (< 1 ns) – New- improved station clock combinations: improved editing, orbit/clock consistency, smaller daily AC clock solution discontinuities

Recent Changes and Improvements • IGS Preliminary SINEX Combined Products became official – From Feb 20/00 (GPS Week 1050), aligned to ITRF 97 ( using min. constraints) – See IGS Mail #2740 for more details • New IGS ITRF 97 realization (through the IGS SINEX combined solution • • IGS 00 P 04. snx product) – From Feb 27/00 (GPS Week 1051); delay increased from 10 to 12 days – Improved ITRF 97 realization for the IGS Final orbits/ERP/clocks – No noticeable changes to the IGS users (no transformation, no steps!) – See IGS Mail #2750 and #2751 for more details New Pseudorange bias convention starts on Apr 02/00(wk 1056) – P 1, P 2 replace the current convention: C/A, P 2’= C/A + (P 2 -P 1) – Important for IGS clock users (precise point positioning, time transfers) – See IGS Mail #2744 for more details and the adopted C/A-P 1 biases IGS Ultra-Rapid Comb. Orbits/clocks testing started Mar 2/00!

Future Developments • Improved IGS ITRF realization (see the next presentation) – Weekly, Preliminary SINEX combinations, official since Feb 20, 2000 – Up to 250 IGS Polyhedron stations IGS Combined SINEX solutions – Continuous plate motion monitoring/quality check (under way) • Ultra Rapid products Pilot Project (<50 cm, testingsince March 2, 2000) – 3 hour delivery delays, may replace both IGR & IGP in near future – 48 h orbits, twice daily, increased reliability for prediction portions (<12 h) • Improved IGS satellite/station clock combinations (April 2000 ) – 5 min. Station/sat. clock combinations, improved editing, orbit/clock consistency – Clock alignment to UTC (IGS/BIPM Time transfer Pilot Project) – To support LEO POD and precise, cm navigation at any interval • Ionospheric (global) TEC products to become official ?

QUESTIONS? Jan Kouba Geodetic Survey Division Natural Resources Canada (NRCan) 615 Booth Street Ottawa, Canada K 1 A OE 9 kouba@geod. emr. ca Tim Springer (The IGS AC Coordinator) Astronomical Institute, Univ. Bern Springer@aiub. unibe. ch For details on IGS Products, see the IGS Annual Reports http: //igscb. jpl. nasa. gov/overview/pub. html