The International GNSS Service: In the Service of Geoscience and the Geospatial Industry

Transcription

1 International Global Navigation Satellite Systems Society IGNSS Symposium 2007 The University of New South Wales, Sydney, Australia 4 6 December, 2007 The International GNSS Service: In the Service of Geoscience and the Geospatial Industry Chris Rizos School of Surveying and Spatial Information Systems University of New South Wales, Kinsington, Sydney, NSW 2052, Australia Tel:(02) c.rizos@unsw.edu.au ABSTRACT The IGS is an international activity with more than 200 participating organisations in over 80 countries celebrating over 13 years of successful service ( The IGS primarily develops the scientific research and analysis of long-term, highly precise and accurate Earth observations using the technologies of the Global Navigation Satellite Systems (GNSS), primarily the U.S. Global Positioning System (GPS). The mission of the IGS recently revised at the IGS Strategic Planning Meeting held in December 2006 is: to provide the highest-quality GNSS data and products in support of the terrestrial reference frame, Earth rotation, Earth observation and research, positioning, navigation and timing and other applications that benefit society The IGS will continue to support the International Association of Geodesy's (IAG initiative to develop an approach to coordinate cross-technique global geodesy for the next decade - the Global Geodetic Observing System (GGOS which focuses on the needs of global geodesy at the mm-level. The IGS activities are fundamental to scientific disciplines related to climate, weather, sea-level change, and space weather. However, the IGS also supports many other apllications, including precise navigation, machine automation, surveying and mapping. This presentation will discuss the IGS strategic plan and future directions, and describe the many working groups and pilot projects as the world anticipates a truly multi-system GNSS. How the Real-Time IGS Pilot Project may integrate with Australia s AuScope Project, and in turn with numerous state government and private CORS stations, will be a particular focus. Key words: IGS; IAG; GGOS; AuScope; CORS networks; geosciences

2 1. INTRODUCTION The International GNSS Service (IGS) was established in January 1994 as a service of the International Association of Geodesy (IAG) (IAG, 2007). Since June 1992, the IGS originally known as the International GPS Service for Geodynamics, from 1999 simply the International GPS Service, and finally since March 2005 simply as the International GNSS Service has been making freely available to interested users precision GPS satellite orbit and other products (IGS, 2007; Beutler et al, 1999; Dow et al, 2004, 2005; Slater et al, 2004). The IGS operates as a voluntary, non-commercial, confederation of about 200 institutions world-wide (see Figure 1), self-governed by its members, managed on a day-to-day basis by the Central Bureau under the policy guidance of the Governing Board. Each participating organisation contributes its own resources: there is no central source of funding. Since the IGS Governing Board adopted in December 2001 its current Strategic Plan, covering the years , a number of developments taking place inside and outside the IGS has made it necessary to revise the plan. A reflection process was initiated at the 2004 Workshop in Bern, Switzerland, continued through a dedicated session at the 2006 Darmstadt Workshop, Germany, culminating in a one and a half day meeting of a specially appointed Strategic Planning Committee in Pasadena, California, in September 2006, and a two day Strategic Planning Retreat of the Governing Board in San Francisco, Calfornia, in December A new IGS Strategic Plan for the years has resulted from this process. The IGS collects, archives, and distributes GPS and GLONASS observation data sets of sufficient accuracy to meet the objectives of a wide range of scientific and engineering users. These data sets are analysed and combined to form the IGS products shown in Table 1 (IGS, 2007). IGS products support scientific activities such as improving and extending the International Terrestrial Reference Frame (ITRF) maintained by the International Earth Rotation and Reference Systems Service (IERS); monitoring deformations of the solid Earth and variations in the liquid Earth (sea level, ice sheets, etc.) and in Earth rotation; determining orbits of scientific satellites; and monitoring the troposphere and ionosphere. Typical accuracies and latencies of the various products are indicated in Table 1. The primary IGS product is the GPS satellites IGS Final Orbit, now at the cm accuracy level (see Figure 2 for the evolution of orbit accuracy from the early days of the IGS to the present). 2. THE CHANGING GNSS LANDSCAPE 2.1 The GLONASS service While the IGS product range has been mainly concerned with GPS, since 1998 GLONASS products were developed, initially in connection with the International GLONASS Experiment (IGEX) of 1999 (Slater et al, 2004). This continued seamlessly, from 2001, through the International GLONASS Service (IGLOS), a pilot project of the IGS, which reached a successful conclusion in December and thus could be dissolved - when the GLONASS products (raw data and derived products) were integrated into the mainstream IGS data flow (see Table 1). The participation during the past two years of the Russian Institute IAC in the IGS GLONASS orbit combination has considerably improved the quality of these products. Improvements in the GLONASS geodetic reference frame PZ 90 are in part due to assimilation of IGS stations well-determined in the ITRF.

3 Figure 1: Global distribution ofstations of the IGS network Figure 2: Improvement of IGS Final Orbits with time (since GPS week 700)

4 2.2 The IGS and Galileo More recently the IGS has been actively following the development of the European Galileo system, in three general areas (Dow et al, 2007). Firstly, the IGS GNSS Working Group and its individual members are involved in bringing to the attention of the new GNSS system developers the experience gained over the past decade and a half in the IGS concerning orbit models, antenna phase calibrations, standardisation of data formats and other matters. This applies to future generation GPS as well as to Galileo, and possibly other systems such as China s Beidou/Compass, India s Regional Navigation Satellite System (IRNSS), and Japan s Quasi-Zenith Satellite System (QZSS). Secondly, European IGS participants contributing to the global IGS ground station network (ESOC and GeoForschungsZentrum Potsdam - GFZ) have been working with the European Satellite Navigation Industries (ESNIS, formerly Galileo Industries) and ESA to set up and operate the network of sensor stations (GIOVE Experimental Sensor Stations - GESS) to track the experimental GIOVE satellites. This network, totalling 13 stations, including 11 on well tested IGS sites, has been fully operational since early The third area in which IGS has been able to contribute its expertise is in the Galileo Geodetic Service Provider (GGSP) Prototype. This is a project funded by the European GNSS Authority (formerly the Galileo Supervisory Authority - GSA), with technical management support from ESA, with the objective of designing and developing a system capable of providing and maintaining over the projected 20 year lifetime of the Galileo system a geodetic reference frame, to be known as the Galileo Terrestrial Reference Frame (GTRF). This frame shall be within 3cm (2-sigma) of the standard International Terrestrial Reference Frame (ITRF) (ITRF, 2007), to which the IGS contributes on a routine basis the consolidated GPS input. Three European IGS Analysis Centres (University of Bern, GFZ, ESOC), as well as key European institutes involved in the IERS (BKG, IGN), supported by Canadian and Chinese organisations (NRCan, University of Wuhan), are working in a consortium led by the GFZ. A first realisation of the GTRF, based on GPS data from selected IGS sites, is currently being validated. Updates will be based on inclusion (in addition) of GPS data from the GIOVE network and then GIOVE data itself. As the Galileo Sensor Stations (GSS) planned for the Galileo In-orbit Validation Phase become available, data from those sites will also be processed and included. Further details of the GGSP can be found in (Gendt et al, 2007). 2.3 Towards real time products The IGS has been developing the capability for real time data streaming from the ground station network for some years. Currently up to 60 stations are providing data, with a latency of the order of a few seconds (RTIGS, 2007). A recent development was the initiation of a Real Time Pilot Project (RT-PP), which has the following objectives: Manage and maintain a global IGS real time GNSS tracking network. Enhance and improve selected IGS products. Generate new real time products. Investigate standards and formats for real-time data collection, data dissemination and delivery of derived products. Both the RTIGS and the NTRIP protocols will be assessed as to their suitability. Monitor the integrity of IGS predicted orbits and GNSS status. Distribute observations and derived products to real time users. Support Network DGPS/RTK operations.

5 Encourage cooperation among real time activities, particularly in IGS densification areas. The Call for Participation in the RT-PP (see IGS, 2007) requests proposals for Real time Tracking Stations Real time Data Centres Real time Analysis Centres Real time Associate Analysis Centres Real time Analysis Centre Coordinator Real time Network Management and Monitoring Real time Users for Assessment, Evaluation and Feedback The RT-PP will gather and distribute real time data and products associated with GNSS satellite constellations. The primary products envisioned are multi-frequency observation data and precise satellite clocks and orbits made available in real time. These products will be freely available to participants, and external users, for any purpose, in accordance with the IGS open data policy. An important objective of the RT-PP will be to support and promote the development of real time applications. The RT-PP will operate for a period of up to 3 years. Annual reviews will be conducted by the IGS Governing Board to assess the project's progress towards achieving its goals and objectives. Australia s response to the RT-PP has been coordinated by Geoscience Australia (GA), and will include the contributions of several university departments, private industry and state government agencies, in addition to GA. 2.4 IGS and the Global Geodetic Observing System (GGOS) In parallel with these IGS internal developments, the IGS has been working with the IAG on the design of the Global Geodetic Observing System (GGOS) (GGOS, 2007), which would federate the activities and products of the IAG services and commissions (IAG, 2007) in order to provide the contribution of geodesy to the Global Earth Observing System of Systems (GEOSS) now being established by the inter-governmental Group on Earth Observations (GEO). The International Terrestrial Reference Frame (ITRF, 2007), its future development and its correct and consistent use is a central issue of the GGOS initiative. The IGS, with its prime concern for high accuracy and high reliability processing of the signals of the GNSS constellations and as provider of the consolidated inputs of the GNSS contribution to the ITRF, will necessarily play a key role in GGOS. The work of the IGS and its constituent elements has become even more relevant, as global societal issues such as climate change, global mass transport, sea level rise, measuring surface geodynamics at a range of spatial scales, geohazard prediction and monitoring, and natural disaster mitigation (earthquakes, volcanoes, tsunamis, etc.) gain more prominence in continuing efforts to better understand the Earth System in which we live. 2.5 The IGS Strategic Plan Much of the IGS Strategic Plan (see IGS, 2007) remains valid for the coming years. Nevertheless a new Plan was developed, under the guidance of a professional facilitator who had been involved in the first Strategic Planning Retreat in A mission statement and six long-term goals were formulated (see text box).

6 MISSION The International GNSS Service provides the highest-quality GNSS data and products in support of the terrestrial reference frame, Earth rotation, Earth observation(s) and research, positioning, navigation and timing and other applications that benefit society. LONG-TERM GOALS 1. Serve as the premier source of the highest-quality GNSS related standards (conventions), data and products, openly available to all user communities. 2. Attract leading-edge expertise to pursue challenging, innovative projects in a collegial collaborative and creative culture. 3. Incorporate and integrate new systems, technologies, applications and changing user needs into IGS products and services. 4. Facilitate the integration of IGS into GGOS and other more broadly based Earth observing and global navigation systems and services. 5. Maintain an international federation with committed contributions from its members, and with effective leadership, management and governance. 6. Promote the value and benefits of IGS to society, the broader scientific community, and in particular to policy makers and funding entities. Based on these, the new Plan identifies three key strategies: 1. Deliver world-standard quality GNSS data and products to all users globally with leading-edge expertise and resources. 2. Develop, integrate, and participate with new and changing GNSS systems and user needs to continuously improve IGS services and to provide value to a broad range of users. 3. Continuously improve the effectiveness of IGS management and governance to support future growth. The broad objectives remain unchanged, however a significant number of the derived actions are new. 3. GNSS GEODETIC INFRASTRUCTURE At all spatial scales, from global to the local, continuously operating GNSS reference stations are being established to address a range of applications. The IGS network is the übernetwork of reference stations to which many national and local networks are tied. 3.1 National geodetic GPS networks GPS geodesy was the first civilian application of the U.S. Department of Defense s Global Positioning System. It was also the first example of a civilian innovation the use of integrated carrier phase measurements for the determination of position parameters to a relative accuracy of about 1 part per million. The inter-receiver distances were at first several tens of kilometres apart (being the average distance between first order geodetic control groundmarks). However, GPS was also proving itself to be an effective space geodesy technique for

7 measuring crustal motion due to tectonic and other geophysical phenomena, and progressively the distances between GPS receivers increased to hundreds and then thousands of kilometres, while simultaneously the relative accuracies went up hence ensuring cm-level relative accuracy within GPS receiver networks, even as inter-receiver distances grew significantly. GPS is now the premier tool for modern geodesy, and relative accuracies at the parts per billion (ppb) level are routinely achieved (IGS, 2007; GGOS, 2007). These GPS geodetic stations inevitably became permanent reference stations for: (a) the monitoring of the station motion itself (due to geodetic effects effectively ushering in the era of 4-dimensional geodesy), (b) realising modernised geocentric geodetic datums at the national level, and (c) the densification of geodetic control networks using GPS techniques. However, as GPS became an indispensible geodetic technology, government agencies looked for ways to replace traditional geodetic networks initially with groundmarks surveyed using GPS technology, and then increasingly with networks of permanent GPS reference stations. This trend from groundmarks surveyed using carrier phase-based GPS techniques commencing in the 1980s to today s networks of GPS receivers supporting high accuracy positioning, anytime and increasingly in real time, has been generally justified on the basis of improved efficiency. For example, one of the reasons cited by government agencies for replacing passive networks of groundmarks with active networks of permanent GPS receivers is the lowered maintenance of the network (there are typically far less GPS stations than groundmarks and even if they need to be re-established using the permanent GPS receiver network such a maintenance task is very cost efficient). Another is that the national geodetic datum can be propagated to all other GPS surveys using reference network data. Almost without exception the first permanent GPS receiver networks were established by geodetic/surveying agencies. 3.2 Hierarchy of permanent GPS networks Within the IGS hundreds of globally distributed GPS receivers have been operating on a continuous basis for over ten years. The data they have collected have been used in progressive realisations of the geocentric International Terrestrial Reference Frame (ITRF) the latest of which is ITRF2005 (ITRF, 2007). Many countries have over the last decade or so redefined their national datums to be compatible with an ITRF reference frame, by typically linking primary stations and/or groundmarks to the ITRF via the nearest IGS reference stations. These national datums are geocentric, and as far as most users are concerned they are equivalent to the GPS datum WGS84. Many countries have also established active primary/geodetic networks of GPS reference stations to monitor the stability and integrity of their national datums. This is particularly the case for countries located on or near tectonic plate boundaries, that cause their datum (or to be more correct, the realisation of their datum in the form of 3D coordinates of groundmarks and reference stations) to undergo deformation over time, as in Japan, USA, New Zealand, Indonesia, etc. Even countries that do not directly experience the crustal deformation arising from tectonic plate motion/collision that challenges a national datum s internal integrity, consider permanent GPS receiver networks (e.g. established by the national geodetic agency) as infrastructure to support national and international geodetic studies. However, all such infrastructure until relatively recently did not have a real-time data transfer or processing capability. In the 1990s, when the establishment of such permanent receiver networks was justified on geodetic grounds, national networks were similar to IGS stations. That is, although operated on a 24/7 basis, the data was only periodically downloaded from each

8 receiver, and were transmitted daily to an archive or data centre. From there the data was available to users for post-processing. Archived RINEX files from both IGS stations and national GPS reference networks were (and still are) accessed by any user via the Internet. All IGS data has been, and continues to be, available at no cost. Although some GPS receiver network operators charged fees for their RINEX files, the trend is to increasingly make such data available for free. If users were: (a) satisfied with post-processed coordinate results (i.e. they did not want real time coordinates), and (b) were fortunate to be carrying out a GPS survey or positioning task close to a reference station, then users could benefit from such data in two ways: Download data from the nearest GPS reference station(s), for the time period of their own survey, and then process this data together with their own receiver-collected data, using their own software. Rather than the user managing all the data files and doing their own data processing, there are several free web engines that can accept data upload by a user, combine it with nearby IGS station data, and carry out the processing for them (examples, AUSPOS, 2007; OPUS, 2007; SCOUT, 2007). Note that no distinction is made between data sourced from an IGS station, or from any other GPS receiver network. The data is provided in receiver-independent format (RINEX). Both modes provide the user with significant savings, as they can obtain high relative accuracy coordinate results without the need to operate their own reference station(s). Nevertheless it is sometimes useful to consider the hierarchy of permanent GPS reference stations: (1) Tier 1 being the IGS stations, (2) Tier 2 the primary national geodetic network (e.g. the Australian Regional GPS Network - ARGN), and (3) Tier 3 the state and private GPS networks. For some applications the source of the GPS data is irrelevant. However, other applications seeking the highest accuracy and/or integrity may only use data from Tier 1 and perhaps Tier 2 stations/networks. That national GPS receiver networks can satisfy GPS surveying applications came to be viewed as an important justification for the provision of geodetic infrastructure in its own right. Note, this can be considered an extra benefit of a Tier 2 permanent network operated by a national geodetic agency (the primary justification always being that the network allows the national geodetic framework to be monitored, as in the case of Geoscience Australia, National Resources Canada, and the National Geodetic Survey in the U.S.). For other states or agencies in Australia or North America, however, state-established Tier 3 networks are rarely justifiable on geodetic grounds, and hence supporting professional (surveyors, engineers, etc.) users so that they can carry out high accuracy GPS surveys with greater efficiency may be the sole justification. 3.3 The era of real time GPS networks With the advent of GPS-RTK techniques in the early 1990s, carrier phase-based GPS technology finally could be seriously considered a surveying tool. Productivity increased to such a degree that private survey companies could invest in the receiver equipment (Rizos, 2002). At first surveyors operated their own reference stations, and the radio links used to transmit reference receiver data to the user or rover unit. In this way full control was exercised over the positioning system, and the rover unit provided an immediate coordinate for timecritical GPS applications such as engineering construction, detail surveys, precision agriculture, etc. However, to ensure high productivity GPS-RTK (i.e. rapid on-the-fly

9 ambiguity resolution OTF-AR) there were many constraints, not the least of which were: (a) that all GPS receivers (reference and rover) must have dual-frequency tracking capability, and (b) the inter-receiver distance should be less than ten or so kilometres. These are significant constraints and the impact was: A GPS-RTK system was the most expensive of all GPS technologies. Reference receivers were set up on an ad hoc basis, only for the duration of the survey. Proprietary formats and protocols proliferated. Communication links were point-to-point, not broadcast (in contrast to DGPS services). No sharing of reference receiver data was possible. The most serious implication was that it was difficult for any agency or private organisation to justify the establishment of a network of GPS reference stations with inter-receiver spacing of the order of 20km covering an entire region, state or even nation (so that reference-rover distances could be kept to under ten kms in order to ensure rapid OTF-AR). This mode of single-base RTK was soon joined, from the late-1990s, by the so-called network-rtk approach, where the spatially correlated atmospheric and satellite errors (orbit and clock) could be better mitigated using several GPS reference stations surrounding the rover receiver. Rizos et al (2000) identified some advantages of network-rtk over single-base RTK: Rapid static and kinematic GPS techniques can be used over baselines many tens of kilometres in length. Instantaneous (i.e. single-epoch) OTF-AR algorithms can be used for GPS positioning, at the same time ensuring high accuracy, availability and reliability for critical applications. Rapid static positioning is possible using low-cost, single-frequency GPS receivers, even over tens of kilometres. The greatest impact on GPS receiver infrastructure was that network-based techniques enabled cm-accuracy positioning with reference receiver spacing of between kms, even in real time. Such less-dense reference station spacing could now be considered feasible as surveying infrastructure, and by the late 1990s and early 2000s many government and private network operators became interested in the economics of network-rtk. In this new era of real-time data streaming, the number of reference stations contributing GPS data over the Internet grows daily. The IGS and some national geodetic GPS reference stations provide their data streams for free (e.g. Geoscience Australia, National Resources Canada). Other government and private GPS networks may charge fees. The marketplace for GPS data is therefore increasingly confusing. 3.4 The Australian AuScope GNSS network The National Collaborative Research Infrastructure Strategy (NCRIS) has funded the development of AuScope at a cost of approximately $65 million over the next 3-5 years, to establish the geoscientific tools for monitoring the Australian continent. Within AuScope there is a budget ($17.8 million) allocated to improve the National Geospatial Reference Framework (NGRF), by investing in geodetic infrastructure such as VLBI and SLR stations, absolute graviemtry, and over 100 GNSS stations. Although pproximately one third of the NGRF budget is allocated to GNSS, state government agencies will contribute an equal amount of funding. Figure 3 shows indicative GNSS sites of the NGRF. The NGRF GNSS sites will therefore be an equal number of Tier 2 stations (it is unlikely that any of these new stations will belong to the IGS network), and Tier 3 stations. All stations

10 will be equipped with GNSS capabale receivers (GPS and Glonass tracking, with upgrade possibility to also track the Galileo satellites), will have real time data streaming capability, and high quality monumentation. While the Tier 2 stations will provide all the real time data free to all users (in line with the Commonwealth government s open data policy), it is unlikely that Tier 3 station data will be as freely available. However, all these GNSS stations, when combined with data from the ARGN stations and the state-level GNSS networks will be a substantially improve the efficiency and reliability of high accuracy positioning in Australia. Figure 3: Planned distribution of new AuScope GNSS stations REFERENCES AUSPOS, Australian online GPS processing service, see accessed 21 September Beutler, G., Rothacher, M., Schaer, S., Springer, T.A., Kouba, J., Neilan, R.E., The International GPS Service (IGS): An interdisciplinary service in support of Earth sciences. Adv. Space Res. 23(1999), Dow, J.M., Gendt, G., Moore, A., Neilan, R.E., Weber, R., The International GPS Service What s next? 10 th anniversary assembly charts future directions. Proc. of ION GNSS 2004, Long Beach, CA, USA, September, Dow, J.M., Neilan, R.E., Gendt, G., The International GPS Service: Celebrating the 10 th anniversary and looking to the next decade. Adv. Space Res. 36(2005) Dow, J.M., Neilan, R.E., Weber, R., Gendt, G., Galileo and the IGS: Taking advantage of multiple GNSS constellations. Adv. Space Res. 39(2007),

11 Gendt, G., Soehne, Rothacher, M., and GGSP Prototype Team, Realisation and maintenance of the Galileo Terrestrial Reference Frame (GTRF). Proc. of 1st Colloquium on Scientific & Fundamental Aspects of the Galileo Programme, Toulouse, France, 1-4 October 2007, in press. Global Geodetic Observing System (GGOS) website: accessed 21 September International Association of Geodesy (IAG) website: accessed 21 September International GNSS Service (IGS) websites: or igscb.jpl.nasa.gov, accessed 21 September International Terrestrial Reference Frame (ITRF), see IERS website: accessed 21 September OPUS, U.S. National Geodetic Survey s online processing user service, accessed 21 September Real Time IGS (RTIGS) website: accessed 21 September Rizos, C., Making sense of the GPS techniques. Chapter 11 in Manual of Geospatial Science and Technology, J. Bossler, J. Jenson, R. McMaster & C. Rizos (eds.), Taylor & Francis Inc., ISBN , Rizos, C., Han, S., Ge, L., Chen, H.Y., Hatanaka, Y., & Abe, K., Low-cost densification of permanent GPS networks for natural hazard mitigation: first tests on GSI's Geonet network. Earth, Planets & Space, 52(10), SCOUT, Scripp s coordinate update tool, csrc.ucsd.edu/cgi-bin/scout.cgi, accessed 21 September Slater, J.A., Weber, R., Fragner, D., The IGS GLONASS Pilot Project Transitioning an experiment into an operational GNSS service. Proc. of ION GNSS 2004, Long Beach, CA, USA, September Table 1: IGS Product Summary Accuracy Latency Updates Sample Interval GPS Satellite Ephemerides/ Satellite & Station Clocks Broadcast orbits ~160cm Sat. clks ~7ns real time -- daily Ultra- orbits ~10cm Rapid four x real time (predicted Sat. clks ~5ns daily half) 15 min Ultra- orbits <5cm Rapid 3 hours four x (observed Sat. clks ~0.2ns daily half) 15 min orbits <5cm 15 min Rapid Sat. & 17 hours daily 0.1ns Stn. clks 5 min Final orbits <5cm 15 min Sat. & ~13 days weekly <0.1ns 5 min Stn. clks GLONASS Satellite

12 Ephemerides Final 15cm 2 weeks weekly 15 min Geocentric Coordinates of IGS Tracking Stations (>130 sites) Final horizontal 3mm positions vertical 6mm 12 days weekly weekly Final horizontal 2mm/yr velocities vertical 3mm/yr 12 days weekly weekly Ultra- Rapid (predicted half) Ultra- Rapid (observed half) Earth Rotation Parameters PM 0.3mas PM rate 0.5mas/day LOD PM PM rate LOD 0.06ms 0.1mas 0.3mas/day 0.03ms PM <0.1mas Rapid PM rate <0.2mas/day LOD 0.03ms PM 0.05mas Final PM rate <0.2mas/day LOD 0.02ms Atmospheric Parameters Final tropospheric zenith path delay 4mm Ultra-Rapid tropospheric zenith path delay 6mm Final Ionospheric TEC grid Rapid Ionospheric TEC grid real time 3 hours 17 hours ~13 days < 4 weeks 2-3 hours four x daily four x daily daily weekly weekly every 3 hours 2-8TECU ~11 days weekly 2-9TECU <24 hours daily four x daily (00,06,12,18 UTC) four x daily (00,06,12,18 UTC) daily (12 UTC) daily (12 UTC) 2 hours 1 hour 2 hours; 5 deg (lon) x 2.5 deg (lat) 2 hours; 5 deg (lon) x 2.5 deg (lat)