Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.gao.spb.ru/english/personal/malkin/publ/zm07j.pdf Äàòà èçìåíåíèÿ: Thu Mar 3 10:06:03 2011 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 03:15:36 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: propulsion

GGOS Working Group on Ground Networks and Communications
M. Pearlman Harvard-Smithsonian Cen ter for Astrophysics (CfA) , Cambridge, MA 02138, USA Z. A ltamimi Institut GÈographique National, 77455 Marne- la-V allee, Fran ce N. Beck Geodetic Survey D ivision ­ Natural Resources Canada, Ottawa, ON K1A O E9, Canada R. Forsberg Danish National Sp ace Cen ter , DK-2100 Copenhagen, Den mark W. Gur tner Astronomical Institute Un iversity of Bern, Bern, CH-3012, Sw itzer land S. K enyon National Geosp atial-Intellig ence Agen cy, Arnold, MO 63010-6238, USA D. Behrend, F.G. Lemoine, C. Ma, C.E. Noll, E.C. Pavlis NASA Goddard Sp ace Fligh t Cen ter, Greenbelt MD 20771-0001, USA Z. Malk in Institute of Applied Astronomy, St. Petersburg, 191187, Russia A.W. Moore, F.H . W ebb, R.E. Neilan Jet Propulsion Laboratory, California Institute of Technolog y, Pasad ena CA 91109, USA J.C. Ries Cen ter for Space Research, Th e Un iversity of Tex as, Austin TX 78712, USA M. Rothacher GeoForschnungsZen trum Po tsdam, Potsdam, D-14473, G ermany P. W illis Institut GÈographique National, 94160 Saint- Mande, Fran ce Jet Propulsion Laboratory, California Institute of Technolog y, Pasaden a CA 91109, U SA

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Abstract. Properly designed and structur ed groundbased geodetic networks materialize the ref erence systems to support sub-mm global ch ange measurements over space, time and evolving technologies. Ov er th is past y ear, the Ground Networks and Commun ications Working Group (GN&C WG) has been organized under the G lobal Geodetic Observ ing System (GGOS) to work with the IAG measuremen t services ( the IGS, ILRS, IVS, IDS and IGFS) to develop a strategy for building, integr ating, and maintain ing the fundamental n etwork of instruments and supporting infrastru cture in a sustainab le way to satisfy the long-term (10-20 year) requirements id entified by the GG OS Science Council. Activities of th is Working Group include the investigation of the status quo and the developmen t of a plan for full network integration to support improvements in terrestrial referen ce frame establishment and maintenance, Earth or ientation and gravity field monitoring, precision orbit deter min ation, and other geodetic and gravimetric applications r equired for the long-ter m observation of global change. Th is integration process includ es the dev elopment of a n etwork of fundamen tal stations with as many co- located techniqu es as possible, with precisely deter mined in tersystem vectors. This network would explo it the strengths of each technique and min imize the w eaknesses wher e possible. This paper discusses the organ ization of the working group, the work done to date, and future tasks. Keywords. Glob al G eodetic Observing System, GGOS, GEOSS, IAG, GPS, SLR, VLBI, DORIS, Gravity , Tides, Geoid

1.1 Significance of the Terrestrial Ref erence Frame Space geodesy provides pr ecise position, velo city and gravity on Earth, w ith reso lution from local to global scales. The terrestr ial referen ce system defines th e terr estr ial refer ence frame (TRF) in which positions, velocities, and gravity ar e reported. The r eferen ce surface for height reckoning, the geo id, is defin ed through the adopted gravity model, which is ref eren ced to the TRF. Th e TRF is therefor e a sp ace geod esy product th at links every observab le quantity , product and geophysical parameter on Earth. I ts position, or ientation and evolution in space and time ar e the basis through which w e connect and co mpare such measuremen ts over space, time, and evolv ing technologies. It is the means by which w e ver ify th at observed temporal chang es are geophysical signals rather than artifacts of the measurement system. It provides the foundation for much of the spacebased and ground-based observations in Earth science and global chang e, in clud ing remote monitoring of sea level, sea surface and ice surface topography, crustal deformation, temporal gravity variations, atmospher ic circu lation, and direct measurement of so lid Ear th dynamics. A precise TRF is also essential for in terp lanetary navig ation, astronomy and astrodynamics. The realization of the TRF for its most demanding app lications requ ires a mix of technologies, str ategies and models. D ifferen t observational methods have differ ent sensitiv ities, strengths and sources of error. The task is complicated by the dynamic character of Ear th's surface, which deforms on time scales of seconds to millennia and on spatial scales from lo cal to glob al. 1.2 The Role of GGOS In early 2004 under its new organization , th e International Association of G eodesy (IAG) established the GGOS project to coordin ate geodetic resear ch in support of scientific applications and d isciplines ( Rummel, 2002). GGOS is intend ed to in tegrate d ifferen t geodetic techniques, models and approaches to provide better consistency , long-term r eliab ility , and understanding of geodetic, geodynamic, and global change processes. Through the IAG's measuremen t services (IGS1, ILRS2, IVS3, IDS4, and IGFS5),
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1 Intr oduction
The Ground N etworks and Communications Working Group (GN&C WG) of the Global Geodetic Observing System (GGOS) is ch arged with developing a str ategy to design, in tegr ate, and main tain the fundamen tal space g eodetic network. In this report, we review the signif icance of geodetic networks and the GGOS project, and summar ize th e presen t state of as w ell as fu ture improvements to and requ irements on space geodetic networks, serv ices, and products. The approach of the WG and preliminary conclusions follow.

International GNSS Serv ice, International G PS Service 2

former ly

the


GGOS will ensure the robustness of the three aspects of geodesy : geometry and kin ematics, Earth orientation, and static and time-v arying gravity field. It will iden tify geodetic products and establish requirements on accuracy, time reso lution, and consisten cy. The project will work to coordin ate an integr ated global geod etic n etwork and implemen t compatib le standards, models, and p arameters. A fundamental asp ect of GGOS is the establishment of a glob al n etwork of stations with co-located techniques, to provide the strongest reference frames. GGOS will provide the scientific and infrastructural b asis for all global ch ange research and provide an interface for geodesy to the scientific community and to society in gener al. GGOS will striv e to ensure th e stab ility and r eady access to the g eometric and gravimetric ref erence frames by establish ing uninterrupted time series of state-of- the-ar t g lobal observations. As shown in Figure 1, G GOS is organized in to working groups headed by a Project Bo ard and guided by a Science Council th at helps define th e scientific requir ements to which GGOS w ill respond.

Fig. 1. GGOS Organization

1.3 Role of the Ground N etworks and Communicat ions Working Group The ground network of GGOS is fundamental since all GGOS data and products emanate from this infrastru cture.

The Char ter of th e Ground Networks and Commun ications Working Group (GN&C) within GGOS is to develop a strategy to design, integrate and maintain th e fundamen tal geodetic network of instruments and supporting infrastructure in a sustainab le w ay to satisfy the long-ter m (10-20 years) requiremen ts iden tif ied by the GGOS Science Council. A t th e base of GGOS ar e th e sensors and observatories situated around the world providing the timely, precise and fundamen tal data essen tial for creating the GGOS products. Primary emphasis must be on sustain ing the infrastructure needed to maintain evolving g lobal ref erence frames wh ile at the same time ensuring support to the scientific applications' requir ements. Opportunities to better integr ate or co- locate with the infrastructure and commun ications networks of the many other Earth Observation disciplines now organizing under th e G lobal Earth Observation System of Systems (GEOSS) should be taken into accoun t (Group on Earth Observations, 2005). Recognizing th at the infrastructure and operations co llectively contr ibuting to the Serv ices of the IAG are possible solely due to the volun tary contributions of th e globally distributed collaborating agen cies and their interest in maximized system performance and sustainable long term eff icien t operations, the Working Group is made up of representatives of the measuremen t services plus o ther en tities th at are cr itical to guiding th e activ ities of the Working Group: · IGS: Angelyn Moore, Norman Beck · ILRS: Mike Pearlman, W erner Gurtn er · IVS: Chopo Ma, Zinovy Malkin · IDS: Pascal W illis · IGFS: Rene Forsb erg, Stev e K enyon · ITRF and Local Survey: Zuheir Altamimi, Jinling Li · IERS Technique Combination Research Cen ters: Marcus Ro thacher · IAS (future International A ltimetry Service) : Wolfgang Bosch · Data Cen ters: Carey No ll · Data Analysis: Erricos Pav lis, Frank Lemoine, Frank Webb , John Ries, D irk Behrend

2 Gl obal Geod etic Networ k Infrastr uctur e
All infrastructur e, and products of GGOS and i made possib le through contributions of national 3 resu lting analysis and ts constituent services ar e the goodwill voluntary ag encies and institutions

2 3

International Laser Ranging Service International VLBI Service for Geod esy and Astrometry 4 International DO RIS Service 5 International Grav ity Field Service


and are coordin ated by the IAG governance mech anisms. The ground network of GGOS includ es all the sites th at have instruments of the IAG measuremen t services either perman ently in place or regularly occupied by portable in struments. Some sites hav e more than one space geodesy technique co- located, and knowledge of the pr ecise v ectors between such co-located instrumen ts (known as "local ties") is essen tial to full and accurate use of these colocations. Analysis centers use the ground networks' data for various purposes including positioning, Earth orientation parameters ( EOP), the TRF, and th e gravity f ield. The ground stations of the satellite techniques provide data for precise orbit deter min ation (POD). The indiv idual sites' reference poin ts of the contr ibuting space g eodesy networks are the fiducial po ints of th e TRF. 2.1 IA G Measurement Services Each field we meas service coordinates its own network, including stations and supporting infrastructure. Her e will rev iew the curren t status of each urement service.

for geodynamics, remo te sensing (altimeter, SAR, etc.), grav ity f ield determination, g eneral relativity, verification of GN SS orbits, and eng ineer ing tests (Pearlman et al., 2002). Satellite altitudes r ange from a few hundreds of kilo meters to GPS altitude (20K kilometers) and the Moon. The n etwork includ es forty laser ranging stations, two of which routinely range to four targets on the Moon. Satellites are added and deleted from th e ILRS track ing roster as new programs are initiated and old programs ar e co mpleted. Th e co llected d ata ar e archiv ed and disseminated via two cen ters, and several an alysis centers voluntarily and routinely deliv er products for TRF, EOP, POD, and gravity modeling and d evelopmen t. 2.1.3 IVS The Intern ational V LBI Service for Geodesy and Astrometry (IVS) was estab lished in 1999 and currently consists of 74 per manen t co mponents: coordinating center , operation centers, n etwork stations, correlators, analysis cen ters, and technology development cen ters. Th e IVS observing network in cludes about 30 regularlyobserving IVS stations and 20-30 co llaborating stations p articipating in selected IVS programs on an irregular basis ( Behrend and Baver , 2005). 24hour sessions twice per w eek as well as o ther less frequent sessions ar e used to determine th e complete set of EOP (polar motion, celestial pole coordinates, UT1-U TC) , station coordinates and velocities, and th e positions of the radio sources. Daily 1-hour single baseline sessions are used to monitor Universal Time (UT1) with low latency (Schlueter et al., 2002). 2.1.4 ID S The Intern ational DORIS Service (IDS) was created in 2003 (Tavernier et al., 2005). Th e current ground track ing network is composed of 55 stations allowing an almost continuous tracking of the current f ive satellites (SPO T-2, -3 and -4 used for remote sensing applications, Jason-1 and Envisat used for satellite altimetry). Th e main applications of the DORIS system are pr ecise orbit deter min ation, geodesy and geophysics (Willis et al., 2005). Using improved grav ity Ear th models derived from the GRA CE mission (Tapley et al., 2004), DORIS week ly station positions can now be regular ly obtain ed at th e 10 mm lev el (W illis et al., 2004). DORIS data are available at the two IDS Data Center sin ce 1990 (SPOT-2). In 1999 a 4

2.1.1 IGS The foundation of th e International GNSS Service (IGS, formerly th e International GPS Service) is a global network of more than 350 permanent, continuously operating, geodetic-quality GPS and GPS/GLO NASS sites. The station data ar e archiv ed at three g lobal d ata cen ters and six regional d ata centers. Ten analysis centers regular ly process the data and con tribute products to the analysis center coordin ator, who produces th e official IGS comb ined orbit and clock products. Timescale, ionospher ic, tropospheric, and ref erence frame products ar e analogously formed by specialized coordin ators for each. Mor e th an 200 institu tes and organ izations in mor e than 80 countries contribu te voluntarily to the IGS, a service begun in 1990. The IGS intends to integrate future GNSS signals (su ch as G alileo) into its activ ities, as demonstr ated by the successfu l integr ation of GLONASS. (Kouba et al., 1998; Beu tler et al., 1999; Dow , 2003). 2.1.2 ILR S The International Laser Ranging Service (ILRS) currently tr acks 28 r etroref lector- equipped satellites


DORIS Pilot Exp (Tavernier et al., IDS. Th e Fr ench leading role in the 2.1.5 IGFS

er iment was created by the IAG 2002) lead ing gradually to th e sp ace agency ( CN ES) has th e IDS .

implementation of modern methods and additional sharing of communications facilities and infrastru cture.

3 Syner gy of the Obser ving Techni ques
The International Gravity Field Service (IGFS) w as created in 2003 to provide coordination and standard ization for grav ity f ield modeling. I t supports the IAG scientific and outreach goals and therefore GGOS, through activ ities such as collecting data for fundamental gravity field observation networks (e.g., a global abso lute reference n etwork, co-lo cated w ith satellite stations and other geodetic observation techniques), d ata collection and release of mar ine, surface and airborne gravity data for improved global model development (e.g ., EG M96 (Lemo ine et al., 1998)), and advocating consisten t standards for grav ity f ield models across th e IAG services. Establishing n ew methodology and science applications, par ticularly in th e integr ation and v alidation of d ata from a variety of sources, is anoth er focus of the service. The IGFS is composed of a var iety of primary service en tities: Bureau GravimÈtrique International (BGI), International Geoid Service (IGeS) , International Cen ter for Earth Tides (ICET) , and International Cen ter for Global Ear th Models (ICGEM), w ith the National G eospatial-Intelligence Agency (NGA) p articipating as an IGFS Technical Cen ter. 2.2 C ommunications Transmission of data from the network instrumen ts to data centers and processing or analysis centers is a function cr itical to all the techniques. For th e satellite services, data transmission is normally via primar ily the Internet through terrestr ial or satellite communications networks. Du e to the volume of data (terabytes p er station per 24 hrs) , VLBI d ata are currently shipped on recorded media, but transmission of data via high speed fiber is a future goal. Control and coordination information is also routinely and primar ily sen t via Internet. Sites ar e often situ ated wh ere suitab le access to communications networks, and ideally Internet, exists. In some cases, however , connectiv ity must be installed at ex isting sites. Co mmunications costs are borne by the op erating ag encies, which in remote areas is often at consid erable expense. The GN&C WG will investigate the possib ility of improving efficiency through coordinated 5 At the d awn of sp ace ag e about half a century ago, the individual nation al classical systems that wer e then dominating geodesy started slow ly to b e replaced by in itially crude global equiv alents ( e.g., the SAO Standard Earth models), and later on, when the first satellite nav igation constellations lik e TRAN SIT became av ailab le, by more sophisticated "World G eodetic Systems" (e.g., the US DoDdeveloped WGS60, 66, 72, and WGS84). As space techniques prolifer ated throughout th e world, it soon became apparen t that the optimal approach would be to make use of all av ailab le systems, and to shar e th e burden of th e development through intern ational coordination and cooperation . Th is section rev iews the synerg istic contr ibutions of space geodetic techniques to var ious products. 3.1 The Terrest rial R ef erence Frame The dramatic improvement of space g eodesy techniques in th e eighties, th anks to NA SA's Crustal Dynamics Project and Europe's WEGEN ER Project, has drastically in creased th e accur acy of TRF determination. Howev er, none of the space geod esy techniques alone is able to provide all th e necessary parameters for the TRF datum def inition (origin , scale, and orientation). While satellite techniques are sensitive to Ear th's center of mass, VLBI is not. The scale is dependen t on the modeling of some physical par ameters, and the abso lute TRF orientation (unobservable by any technique) is arbitr ary or conventionally def ined through specif ic constraints. Th e utility of mu ltitechnique combin ations is ther efore recognized for the TRF implementation, and in par ticular for accur ate datum defin ition. Since the creation of the International Ear th Rotation and Ref erence Systems Serv ice (IERS) , the curren t imp lemen tation of the International Terrestrial Referen ce Fr ame (ITRF) has been based on suitab ly weighted multi- technique combin ation, incorporating indiv idual TRF solu tions der ived from space geodesy techniques as w ell as lo cal ties of co-location sites. Th e IERS h as recen tly initiated a new effort to improve the quality of ties at existing co-location sites, cru cial for ITRF development.


The par ticular str engths of each observing method can compensate for weaknesses in others. SLR d efin es th e ITRF2000 geocentric orig in, which is stab le to a f ew mm/d ecade, and SLR and VLBI define the absolu te scale to around 0.5 ppb/decad e (equivalent to a sh ift of approximately 3 mm in station heights) (A ltamimi et al., 2002). Measuremen t of geocenter motion is under refinement by the an alysis cen ters of all satellite techniques. The d ensity of the IGS n etwork provides easy and rigorous TRF access wor ld-wide, using precise IGS products and facilitates th e implementation of the rotational time evolution of the TRF in order to satisfy the No-Net- Rotation condition over tecton ic motions of Earth's crust. DORIS contr ibutes a geographically welldistr ibuted network, th e long-term permanency of its stations, and its early decision to co-lo cate with other track ing systems. The TRF is heavily dependen t on the quality of each n etwork and suffers w ith any network degradation over time. The current distribu tion and quantity of co-location sites as depicted on Figure 2 (in particular sites w ith three and four techniques) is sub-optimal.

celestial pole position and UT1, and VLBI also defines th e ICRF (Internation al Celestial Ref erence Frame) ( Ma et al., 1998), whose f iducial objects (mostly quasars) have no detectab le physical motion across th e sky becau se of their great distance. Th e two-decade V LBI data set contr ibutes a long time ser ies of po lar motion, U T1 and celestial pole position. Satellite techniques (GPS, SLR and DORIS) measure polar motion and length of day relative to th e orbital planes of the satellites track ed. In practice, recen t polar motion time series are d erived from GPS with a h igh degree of automation, and predictions of U T1 r ely on G PS length of day and atmospher ic ex citation functions. 3.3 Gravity, Geoid, and Vert ical Datum Gravity is importan t to many scien tif ic and engineering disciplines, as w ell as to so ciety in general. I t describes how th e "v ertical" direction changes from one location to ano ther, and similar ly, it def ines at each point the equ ipoten tial surface; therefore, it describes how "water flows". G lobal scale models of terr estr ial grav ity and geoid (Lemo ine et al., 1998) are now routinely deliv ered on a monthly basis by missions lik e GRA CE, with a resolution of 200 km or so, and high accuracy (Tapley et al., 2004). The addition of surface gravity observations can extend the reso lution of these mod els down to tens of kilometers in areas of dense networks. Worldw ide datab ases of absolute and relative gravity, airborne and mar ine grav ity ar e collected and maintained by IGFS. Astronomicallydriven temporal v ariations of gravity ( Earth , ocean and atmo spheric tides) ar e also a product of th is and other IAG services. The co mbination of all th is information is cru cial in precisely d etermining instantaneous position on Earth or in orbit, th e direction of the v ertical and the height of any poin t on or around Earth, and the computation of precise orbits for near-Ear th as well as interplanetary spacecraf t. Similar ly, the v ertical d atu m is th e common refer ence for science, engin eer ing, mapping and navigation problems. A chieving a globally consistent ver tical datum of v ery high accur acy has b een a pr ime g eodetic problem for decad es, and only recen tly ( thanks to missions lik e CHA MP and GRA CE) is a successful result in reach . Strength ening and main taining a close link between the "g eometric" and "gravimetr ic" reference frames is of par amount impor tance to th e goals of GGOS.

Fig. 2. Distribution of space geodesy co-location sites since 1999.

3.2 Earth Orient ation Paramet ers Earth orien tation parameters measure th e orientation of Earth w ith r espect to iner tial sp ace (which is requir ed for satellite orbit d etermination and spacecraf t navig ation) and to the TRF, which is a precondition for long-term monitoring. Polar motion and UT1 track changes in angular momen tum in the flu id and solid componen ts of the Earth sy stem driv en by phenomena like w eather patterns, ocean tid es and circulation, post-glacial rebound and great ear thquakes. Th e celestial pole position, on the oth er hand, is d ependent on th e deep structure of Earth. On ly VLBI measur es 6


3.4 Precise Orbit D et ermination Precise orbit determination is one of the principal applications of the satellite techn iques (GPS, SLR, DORIS), and has direct application to many different scien tif ic d isciplines such as ocean topography mapping, measurement of sea level change, deter min ation of ice sheet height change, precise geo-ref erencing of imag ing and remote sensing data, and measur emen t of site deformation using synthetic aper ture r adar (SAR) or GPS. Th e techniques hav e evolv ed from meter- lev el orbit deter min ation of satellites such as LA GEOS in th e early 1980's to cm- lev el today . The computation of precise orbits allow s these satellite tr acking data to be used for grav ity f ield deter min ation (both static and time-var iable) and the estimation of other geophysical par ameters such as post glacial rebound, ocean tidal parameter s, precise coordinates of tracking sites, or the measurement of geocen ter motion. Precise orb it d etermination, wh ich r equires precise UT1 and gravity models, underpins th e analysis that in parallel has resulted in improved station coordinate estimation, and th ereby improved realizations of the TRF (e.g., ITRF2000); There is close synergy b etw een POD and TRF realization. The density of d ata av ailab le from GPS ( and in th e future from oth er GNSS in cluding G alileo) allows the estimation of reduced-dynamic or kinematic orbits w ith radial accur acy of a few cm even on low-altitude satellites such as CHA MP and GRA CE. Only a few satellites carry mu ltiple track ing systems, but space-based co- location is invalu able. The d etailed in ter compar ison of orbits computed independen tly from SLR, DORIS, and GPS data confirms that Jason-1 orbits have a onecm rad ial accur acy (Lu thcke et al., 2003). These techniques are comp lementary; the pr ecise but inter mittent SLR tracking of altimeter satellites, such as Envisat or TOPEX /Poseidon, is complemented by the dense tr ack ing available from the DORIS network. SLR tracking of the GPS, GLONASS or future Galileo satellites is and w ill be vital to calibrating GNSS satellite b iases and assuring the realization of a high quality TRF.

geodetic measur emen t accuracy to be roughly a factor of 5 to 15 below today's levels. G iven that the TRF and global geod esy ar e now accurate to th e order of 1 cm (or 5-15 mm for diff eren t quantities) and 2 mm/yr, w e foresee near-term utility in global measurements w ith absolu te accur acies at or below 1 mm and 0.2 mm/yr. Corresponding levels of improvement ar e required for Earth orien tation and gravity.

5 Evol uti on of the Techni ques
Each of th e GGOS Services techniques envisions technological and operational advances that w ill enhance measuremen t capability. So me advances are curren tly b eing implemen ted wh ile other s are in the process of design or developmen t. In add ition, each techn ique-related service is seek ing to improve not only data quality and precision , but also reliability of data and product deliv ery, performance, continuity , station stability, and d ata latency (which in the case of GNSS includ es realtime). While making these improvements, contributors seek oper ational efficiencies in order to minimize costs. 5.1 GN SS Geodetic GNSS has already evolv ed from GPSonly operations to inclusion of GLONASS, and upgrades to nex t-generation receivers will allow full benefit from modernized GPS sign al structures, Galileo signals, and G LONASS signals. Studies leading to improved handling of calibration issu es such as local signal effects ( e.g., multip ath) and antenn a phase patterns ar e underw ay, as ar e initiatives to fill r emain ing network gaps, particularly in the south ern hemisphere. Elsewhere, station density is less problematic and the focus has sh ifted to consolidation of supplementary instrumentation such as strain meters and meteorological sensors. 5.2 Laser Ranging Newly designed and imp lemen ted laser ranging systems op erate semi- autonomously and autonomously at kiloher tz frequen cies, providing faster satellites acquisition, improved data yield, and extend ed range capability, at substan tially reduced cost. Improved control systems per mit much more eff icien t pass interleav ing and new higher reso lution event- timer s deliver p icosecond timing. Th e higher reso lution will mak e two7

4 Futur e Requir ements
The measur ement requ irements for GGO S will b e set by th e GGOS Project Board with guidance from the Science Council ( Rummel, 2002). Until these requirements are formally specif ied , we judge th e practical useful target for the TRF and sp ace


wavelength op eration for atmosph eric refraction delay recov ery more pr actical and applicable for model v alidation. The current laser rang ing network suffers from weak geographic distr ibution, particularly in Africa and th e southern h emisphere. The co mprehensive fundamental network should includ e additional co-located sites to f ill in this gap. Improved satellite r etroref lector array designs will reduce uncer tainties in cen ter-of-mass corrections, and optical transponders currently under development offer opportunities for extraterr estr ial measurements. 5.3 VLBI The VLBI component of the future fundamen tal network will b e th e next-g eneration system now undergoing conceptual developmen t. Critical elemen ts include fast slewing; high efficiency 10-12 m diameter antennas; ultra w ide bandwidth front ends with continuous RF coverag e; d igitized back ends with selectable frequen cy segments cover ing a substan tial portion of the RF bandwid th; data r ate improvements by a factor of 2­16; a mixture of disk-based recording and high speed network data transfer , near r eal time correlation among networks of processors, and r apid automated gen eration of products. Better geographic distr ibution, especially in th e sou thern hemisphere, is requ ired. 5.4 DORIS The DO RIS tr ack ing network is being modernized using third-gener ation antennae and improvemen ts to beacon monumentation (Tav ernier et al., 2003; Fagard, in preparation). Effor ts are underw ay to expand th e network to fill in g aps in ex isting coverage. DO RIS beacons ar e also being dep loyed to support altimeter calibration, co- location with other geodetic techniqu es, or specif ic shor t-term experiments. A sp ecific IDS working group is selecting sites and occupations for such campaigns, using additional DO RIS beacons provided by CNES to th e IDS. 5.5 Gravity Gravity observations are mo st sensitive to height changes; th ey ther efore provide an obvious w ay to define and con trol th e vertical datum. A uniformlydistr ibuted n etwork of r egular ly cross-calibrated absolute gravimeters supported by a well-designed relative measuremen t network that will be repeated ly observed at regular intervals, and a sub8

network of continuously operating superconducting tidal grav imeters ar e expected in a fundamen tal network of co-located techniques. These per manen t networks should be augmen ted with targeted airborne and ship campaigns to collect data over large areas that are devo id of gravimetric observations. A well-d istr ibuted global data set of surface d ata is n ecessary to calibrate and valid ate products of the recen t (CHA MP and GRACE) and upcoming (GO CE) high-accur acy and -r esolu tion missions. Even tually , grav imetry w ill n eed to devise a method analogous to InSAR, to continuously "map" changes in the f ield with resolution many orders of magn itude h igher than currently achievable from any geopotential mapping mission.

6 Appr oaches to Network Desi gn
The fin al d esign of the GGO S network must tak e into consider ation all of th e app lications including the geometr ic and gravimetr ic r eferen ce frames, EOP, POD, geophysics, oceanography, etc. W e w ill first consider th e TRF, since its accuracy influ ences all oth er GGOS products. Early steps in the process are: 1. Define the cr itical contr ibutions that each technique provides to the TRF, PO D, EOP, etc. 2. Characterize the improvements that could be anticipated over the n ext ten y ears w ith each technique. 3. Examine th e effect in the TRF and Earth orientation resu lting from the lo ss of a significan t par t of the curren t network or observation program. 4. Using simulation techniques, quantify th e improvement in the TRF, Earth or ientation and other key products as stations are added and station capab ility ( co-location, data quan tity and quality) is improved. W e w ill also exp lore the b enefit of adding n ew SLR targets. 6.1 Impact of N etwork D egradat ion on the TRF Preliminary results (Govind, 2005) indicate th e origin drift caused by removal of one station, Yarragadee (Austr alia), from SLR an alysis. Th e drift is about 0 .6, 1 and 1mm/yr over the or igin components around the three axes X, Y Z, respectively . This drift is at least three times larger than requiremen ts for high -precision Ear th science


applications such as sea level chang e and o ther geophysical processes. 6.2 Eff ect of Syst em and N et work Degradation on Other GGOS Products The TRF is a primary space g eodesy product, but it is also the basis on which every other product is referenced. As such, degrad ation in its def inition and maintenance inf luences th e quality of these other products and services, such as EOP, geo center motion, temporal g lobal grav ity var iations, and PO D . The degradation can originate in two ways: geometric ch anges ( as those shown by the example of sec. 6.1) and changes in the type, amount and spatiotemporal distr ibution of the observations. In practice what happens is a combin ation of both. To quantify the resultant errors is not an easy task because ther e are inf inite possib le var iations in th e network of TRF stations, supporting techniques, and selection of data. Ex amination of particular station deletions th at either happened in practice or had been proposed indicates (Pav lis and KuzmiczCieslak, 2005) that ev en moder ate degrad ations impact r esults significan tly mor e than their quoted accur acies. This confir ms the present ILRS n etwork is not robust to any contr action ; the smallest perturbation of the system yields large uncon trolled changes in the products. The closing of the Arequ ipa and Haleakala SLR sites for example, degrad ed origin, or ientation and scale of th e by 3-4 times the standard dev iation of the relev ant p arameters. I mpact on geo center motion was almost two times worse. Temporal gravity v ariations ar e less sensitive du e to their nature as proxies of global scale changes, but wer e still degr aded by sever al standard d eviations. On th e positive side, for a modest improvement from an old TRF (ca. 1995) to the current one (ITRF2000), POD-based products (su ch as altimeter der ived Mean Sea Level) improved by 30 p ercen t. Much mor e work is required to assess th e effects of such ch anges in the tracking networks of all space geodesy techn iques, and their comb ined effect on the final products. Th e sizes of these separate networks and th e inf inite possible variations in their design, ov erlap and oper ation, and the quality of their data and the targets used for collecting their observ ations co mplicate th is task, but a few well- thought-through scenar ios w ill b e tested with futur e simu lations.

6.3 Improvements in t he TRF and Other K ey Products Expected advances in instrumentation, as descr ibed in section 5, will cause improvements in the TRF and the var ious products, bu t the accuracy needed for future science applications will requir e optimization of the ground network. Simulation capab ilities will b e developed th at w ill allow for evalu ation and optimization of the locations of potential sites. In addition, th e benefit of introducing a few new SLR targ ets n eeds to be evalu ated. Target inter action w ith th e curren t larg e LAG EOS satellites is one of the principal limitations in mmlevel SLR, and smaller targets would support the necessary accuracy . New low er-altitude targets would allow mor e observation opportunities per day, incr eased probability of track ing from lowerpower systems (particularly during daylight) and a more accur ate deter min ation of th e Earth's mass center, cr itical for both controlling th e drift in th e origin of the TRF as well as observing the seasonal geocen ter motions associated with large-scale mass transport within the Earth system. Simu ltaneously , enh anced p erformance of each of the indiv idual techniques should result as each technique's data and analysis outputs ar e further combined and compar ed and even tually integrated.

7 Sustaini ng the Ground Network Over the Long Term
The measurement techniques services h ave each main tained th eir own networks and supporting infrastru cture, routin ely producing data, but suffer from severe budget constr ain ts of the voluntarily contributing ag encies that prev ent appropriate main ten ance and development of physical and computational assets. This d egradation of th e observing network capability coin cid es w ith high value science investigations and missions, such as sea level studies from ocean and ice-sheet altimetry missions, eroding their scientific r eturn and limiting their ab ility to meet th e mission goals. Many of the elements of the current networks are funded from year to y ear and dep end upon specific activ ities. Stations are of ten f inanced for cap ital and main ten ance and op erations costs through research budgets, which may not constitu te a long-term commitmen t. Sudden changes in funding as priorities and organizations change h ave resu lted in devastating impacts on station and network performance. On the other h and, missions and long 9


term projects hav e assumed that the n etworks w ill be in place at no cost to th em, fully functioning when their requir ements need fulfillment. GGOS will be proactive in helping to persuade funding sources th at th e networks ar e interdependen t infrastru cture th at n eeds long term, stab le support. The GGOS community must secure long-term commitmen ts from sponsoring and contributing agencies for its evo lution and operations in order to support its user s with high-quality products. In view of the difficulties in securing long-lasting and stable financial support by th e in ter ested par ties, n ew financial models for th e networks must b e developed . This Working Group w ill work w ith th e Strategy and Funding Working Group to develop an approach. Since th e present n etworks must support current as well as future requir ements, the GGOS n etwork must evolv e withou t interruption of data and data products. In p articular, the TRF r elies on a long continuous history of data for its stability and robustness. N ew and upgraded systems, changes in stations lo cations, and chang es in the way products are formed must be planned and phased so th at th e impacts ar e w ell do cumen ted and w ell understood. The an alysis and simu lation procedures being undertaken by the Working Group w ill identify network voids and shortcomings. Th e Ground Networks and Co mmunications Working Group, in concer t with the other GGOS entities, will work with agencies and in ternational organ izations toward f illing in these g aps.

science and associated eng ineer ing and societal concerns.

Acknowl edgments
The au thors would like to acknowledge the support of IAG services (IGS, I LRS, IVS, IDS, IGFS, and IERS) and th eir par ticipating organizations. Par t of this work w as carried out at th e Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronau tics and Space Administration.

Refer ences
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8 Summary
A perman ent geod etic n etwork of comp lemen tary yet in terd ependent space geodetic techn iques is critical for geod etic and geophysical applications and underpins the G lobal Earth Observation System of Systems. Thanks to the g enerous and volun tary contributions of many national agen cies and institu tions around the world, th e IAG has been able to coordinate g lobal collaborations for geodetic technique based serv ices from which all benef it. There is a strong need for coordination of th e planning, funding and oper ation of fu ture integrated geodetic networks to maximize performance in meeting evolving requiremen ts while tak ing into accoun t the n eed for sustain able infrastructure and efficien t operations. Th e GGOS Ground Networks & Communications Working Group has initiated studies, wh ich w ill guide the services in infrastru cture p lanning for optimal ben efit to Earth 10


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