GPS Networks: The Practical Side

Eos, Vol. 72, No. 6, February 5, 1991 E o s ,

T R A N S A C T I O N S ,

A M E R I C A N

G E O P H Y S I C A L

U N I O N

VOLUME 72, NUMBER 6 FEBRUARY 5, 1991 PAGES 49-64

Ground rupture of Iran's catastrophic June 20, 1990 earthquake (Ms = 7.7, Mb=6.4, Io=X), looking east. The Rudbar-Tarom earthquake in northwest Iran, along with its aftershocks, claimed more than 40,000 Hues (according to official reports), left more than 500,000 homeless, and de­ stroyed three cities (Rudbar, Manjil, and Lowshan). It also completely de­ stroyed 700 villages, slightly damaged 300 more, and caused $7 million worth of damage in Gilan and Zanjan provinces, southwest of the Caspian Sea. The strongest earthquake to hit a densely populated urban area of Iran this century, the shock disrupted virtually every aspect of urban life. Concern­ ing the quake itself, the meizoseismal area showed evidence of both earth­ quake faulting and folding. The earthquake was associated with at least three main discontinuous en-echelon complex systems of fault segments ex­ tending more than 80 km, indicating that the main shock was a complex multiple-event with at least three main subevents. The mechanism at the surface was left-lateral oblique high-angle reverse faulting.—Contributed by Manuel Berberian, San Diego, Calif. PAGE 4 9

GPS Networks: The Practical Side PAGES 4 9 , 5 5 - 5 6

Michael Bevis Setting up a Global Positioning System (GPS) network presents a number of practi­ cal problems in terms o f techniques, equip­ ment, fieldcraft, logistics, organization, poliMichael Bevis, North Carolina State University, Box 8208, Raleigh, NC 27695-8208

tics, and funding. T h e s e issues are just as important to address as the more purely sci­ entific aspects of GPS research. The National S c i e n c e Foundation (NSF) sponsored a 1-day workshop in San Fran­ c i s c o on D e c e m b e r 3, 1989, the first ever to focus on the practical side of organizing and This page may be freely copied.

managing GPS campaigns and networks. More than 75 geophysicists, geodesists, and geologists from the U.S.A., Canada, England, France, Greece, J a p a n , Australia, and Costa Rica attended. Most represented universities that participate in the University Navstar Con­ sortium (UNAVCO), or organizations such as the National Aeronautics and S p a c e Adminis­ tration (NASA), the U.S. Geological Survey (USGS), the National Geodetic Survey (NGS), and their foreign counterparts that engage in collaborative research with UNAVCO scien­ tists. T h e workshop served as the first forum to share insights and experiences from past campaigns, to a s s e s s present strengths and w e a k n e s s e s , and to address key technical and organizational issues in GPS/crustal mo-

Eos, Vol. 72, No. 6, February 5, 1991 tion research in the 1990s—all of which are reported in this article.

GPS Observations and Monuments Selecting sites and placing monuments for a GPS network require great care, and the novice GPS planner should seek advice on the relevant issues and techniques. When­ ever possible monuments should b e set in solid stable rock, and they should also be visible (placed in areas with clear skies above 15° elevation, with no rnultipath tar­ gets nearby), a c c e s s i b l e , apt to remain intact for a long time, and placed where land own­ ership would help insure the monument's survival. Many if not most existing geodetic monuments, especially those outside the U.S., are unsuitable as GPS marks b e c a u s e they do not satisfy all of these criteria. Setting benchmarks at a site with a suit­ able area of exposed bedrock is relatively straightforward. Several GPS groups have adopted as a marker a stainless steel pin that is friction-fitted and/or cemented into a hole drilled in solid rock using a portable drill. Low-profile and visually unimpressive marks of this kind tend to e s c a p e vandals' attention, while e m b o s s e d brass disks and monuments that project above the ground are more likely to be vandalized or acciden­ tally damaged. Placing monuments at sites without bedrock is far more difficult. T h e s e monuments "float in dirt" and are often, if not always, unstable. While the new NGS 3-D GPS Benchmark s e e m s to be a near-opti­ mum approach to building marks in soil and unconsolidated sediment, many researchers, particularly those working in environments with severe logistical problems, simply do not have the resources to construct such elaborate monuments. More research is needed to determine if there are low-cost, easily deployed alternatives in which stability is not greatly compromised. The conserva­ tive position is that if any kind of mark is set in sediment, its stability is questionable.

print array with an aperture of 1 km or more. And if environmental and logistical circum­ stances are right, kinematic GPS surveys c a n be very efficient as well as sufficiently a c c u ­ rate. The S p a c e Geodetic Measurement Sites (SGMS) S u b c o m m i s s i o n recently started a newsletter focusing on placing monuments, site stability, site surveying, and cataloging. To contribute to this publication, or to join the mailing list, write SGMS Subcommission, Code 6 0 1 , NASA GSFC, Greenbelt, MD 2 0 7 7 1 . Several aspects of survey implementation require improvement. S o m e processors of GPS campaign data said at the workshop that global fiducial tracking networks must b e c o m e more reliable; too many surveys have b e e n spoiled by erratic fiducial track­ ing. Field workers emphasized the need for better in-field communication systems, espe­ cially in remote and logistically difficult envi­ ronments. Perhaps the next generation of satellite-linked transceivers ("telephones") will be small and inexpensive enough to be incorporated into GPS field units. More em­ phasis should also be placed on nearly real­ time data quality c h e c k s , to discover prob­ lems before leaving the survey area. Several workshop participants recommended that more thought be given to simplifying and reducing the cost of shipping GPS gear. S o m e researchers warned about mixing different kinds of GPS receivers in a field campaign; if absolutely necessary, the survey should b e designed s o that o n e or more baselines are measured repeatedly with dif­ ferent combinations o f receivers, so the ef­ fect of non-uniform equipment can be as­ sessed empirically. Careful attention should be focused on the way different c l a s s e s of receivers sample and time-tag the GPS sig­ nal. In response to the question of how many days a given station should b e observed in a GPS campaign, several groups working in domestic networks in the U.S. and Canada found 2-day occupations reasonable, whereas most groups involved in interna­ tional projects (and particularly those work­ ing in sparse networks and with poor interstation c o m m u n i c a t i o n s ) have adopted minimum occupation-times of 3^1 days.

Participants at the San Francisco work­ shop widely agreed that more attention and resources must be applied to site surveys. In all but very dense networks, it is essential to tie e a c h monument in three dimensions to a set of nearby recovery marks. This survey might employ purely conventional tech­ All the workshop participants accepted niques, or utilize GPS (in the static, semithe need to develop and follow standard static, or kinematic m o d e s ) . Several groups field procedures, including the meticulous felt that monuments set in sediment should completion of carefully designed field logs. be tied into a multiscale "footprint" array so Unlike most scientific data, survey data ac­ that stability of the network, and h e n c e the quired for crustal motion studies b e c o m e main mark, can be a s s e s s e d by repeated more valuable as they age. Without standard procedures and careful documentation, we footprint surveys. A conventional surveying would s o o n forget precisely how a given apparatus such as a total station may be data set related to a physical system of mon­ more accurate or cost-effective than GPS for uments. Researchers must reconcile this surveying a system of recovery marks whose need for standardization, which implies inaperture (or extent) is less than 100 m. And conventional techniques are required for sur­ varying procedures, with their wish to take advantage of rapid developments in receiver veys in which s o m e recovery marks are not hardware, processing software, and satellite "GPS-visible," or for surveys designed to es­ tablish the offset vector from an old GPS sta­ constellation and signal structure. Clearly, standardized procedures require periodic tion marker that was abandoned after envi­ upgrading. In this s c h e m e most field workers ronmental c h a n g e rendered the marker GPSinvisible. On the other hand in many settings would follow procedures developed and documented by groups such as NGS or UN­ GPS is the better tool for surveying a foot­ This page may be freely copied.

AVCO. Meanwhile, technology development groups within these organizations would be experimenting with new procedures that would take advantage of evolving hardware and other advances. O n c e a year or so, a group like UNAVCO would publish a new set of standard procedures that researchers would simultaneously adopt. This would re­ quire meticulous documentation of each quantum leap in designated standard proce­ dure, and those who operate GPS receivers should attend training camps fairly fre­ quently to ensure that they are operating ac­ cording to the current standard.

International Campaigns American research groups working in other countries should involve local Earth scientists and surveyors in all aspects of planning, site selection, placing monuments, and observation. Local surveyors are prac­ ticed in the art of finding existing monu­ ments; they c a n also employ legal mecha­ nisms that promote the preservation of monuments, and are generally better than geophysicists at making site maps. Geolo­ gists are usually better than surveyors at as­ sessing site stability, however, while few re­ searchers in any field are familiar with the concept of constructing monuments for long term stability at the 1-mm level. Although local surveyors and scientists c a n be very helpful in identifying potential GPS sites, it is always good for an individual with prior experience in GPS/crustal motion research to participate in final site selection. Local col­ laborators are also very valuable for logisti­ cal support, c u s t o m s clearance, and other such considerations. Since any collaboration should provide benefits for both sides, Amer­ ican investigators must make an effort to help transfer GPS technology to local geo­ detic surveyors, to include local scientists in publication efforts, and to publish when pos­ sible in local journals. Several large interna­ tional GPS projects funded by NSF have de­ veloped a policy of training local scientists and surveyors in the use of GPS equipment, with the intention of gradually transferring the task of performing the observations from American scientists to in-country collabora­ tors. This c a n greatly reduce the cost of a scientific project, and at the same time assist the transfer o f a valuable new technology to the host country.

GPS Data Processing and Analysis So far, processing GPS data has taken considerably longer than acquiring it, as evi­ denced by the often lengthy delay between a field campaign and submission of a manu­ script describing the results. Data processing has not yet b e c o m e routine; often, 10% of the data requires 9 0 % of the processing time, which must frequently include recover­ ing from errors in the survey or fiducial tracking data. A major problem in the past, the removal o f cycle-slips, has eased re­ cently b e c a u s e reliable automatic cycle-slip removal software is now available, and be­ c a u s e the latest GPS receivers produce far fewer cycle slips to begin with. Another ma-

Eos, Vol. 72, No. 6, February 5, 1991 jor problem in the past was the somewhat erratic availability of global fiducial tracking data, which partly reflected the laissez-faire, fund-it-as-you-can effort behind global track­ ing networks such as the Cooperative Inter­ national GPS Network (CIGNET). Several groups c o m m e n t e d that fiducial tracking net­ works are improving with time, both b e c a u s e GPS receivers are b e c o m i n g more reliable, and b e c a u s e the growth of these networks is providing much-needed redundancy. A third time-consuming aspect of data processing is the reformatting or resampling of data. This problem has b e e n worsening as the diversity of receivers in use increases. B e c a u s e s o m e types of GPS receivers may use or process the GPS signal differently— using a different time-tag than others, for example—it may be necessary to revise es­ tablished data processing software to make it compatible with s o m e new piece of hard­ ware. As the nominal accuracy associated with GPS processing continues to improve, ongo­ ing comparisons should be made with Very Long Baseline Interferometry (VLBI) and Sat­ ellite Laser Ranging (SLR) to validate GPS solutions. More control experiments are also needed to test the use of different GPS re­ ceivers, including using mixed receivers. Researchers also need a routinely main­ tained and readily available global-referencesystem for use with GPS observations and solutions. The International Earth Rotation Service (IERS) is already charged with this task, but several workshop participants ar­ gued that IERS does not yet provide the in­ formation GPS users require. IERS represen­ tatives at the workshop acknowledged that they must develop products more useful and better suited to the needs of GPS research­ ers. One contention sparked controversy— that processsing of GPS data is 5 years be­ hind that of VLBI b e c a u s e too many uncoor­ dinated groups are developing processing algorithms and software. It was suggested that two or three specialized groups do all GPS data processing for geodesy/geodynamics researchers, as well as maintain and im­ prove their processing software. In favor of this, it was argued that such an infrastruc­ ture would e n a b l e geophysicists to c o n c e n ­ trate on interpreting results rather than proc­ essing data. In opposition, it was said that such concentrated effort would not provide adequate validation, particularly when there is already a processing bottleneck and data are being acquired at a phenomenally in­ creasing rate. One large GPS campaign c a n produce a data set c o m p a r a b l e in size to the entire VLBI database. Several participants also expressed an aversion to a "big sci­ e n c e " approach to GPS. Presently, GPS re­ ceiver-technology is very decentralized and widely dispersed, like personal computer technology, but the notion of two or three national GPS processing centers invokes the highly centralized but moribund situation of the mainframe computer. The c o n f e r e n c e participants widely agreed that processing is a major bottleneck

between acquiring field data and publishing survey results. This problem is worsening as the number of receivers grows very rapidly, while the number of processors does not. Several researchers argued that the pressure to submit proposals with modest budgets makes it very difficult to obtain financial sup­ port for graduate students who would spe­ cialize in GPS data processing. On the other hand s o m e university and NSF personnel questioned the desirability of a geophysics graduate student working nearly full-time on GPS data-processing. It was countered that many a low-temperature-physics student spends a great amount of time designing, building, modifying, maintaining, and utiliz­ ing cryostats, yet this is not perceived as in­ appropriate or unnecessary.

Data Archiving and Availability Since many GPS data sets acquired to study crustal motion will gain value with age, it is important to carefully preserve these data and all ancillary information, such as the operator field logs and site re­ ports. Participants agreed that much greater resources must be applied to routinely ar­ chiving GPS survey data. At present large government a g e n c i e s such as NASA and NGS are constructing archives for their own GPS data, while most universities rely on the UN­ AVCO archiving facility, run by Judah Levine, to preserve their data. Whenever possible data archives should be run by organizations that might b e expected to exist at least as long as the data retains its value, or by orga­ nizations that will pass on their data prop­ erly to a longer-lived archiving organization when this b e c o m e s - n e c e s s a r y . Most aca­ d e m i c researchers felt that duplicating ar­ chiving facilities inside the United States was not inappropriate, b e c a u s e s o m e intellectual competition among nascent archiving groups is helpful, given the conceptual and techni­ cal complexity of the task. Smaller organiza­ tions such as UNAVCO tend to quickly de­ velop low cost solutions to the problems of data archiving, while large and established organizations such as NGS or NASA are more likely to provide very long-term ar­ chiving. An organization like NGS, however, faces the problem o f how to pay for ar­ chiving data for researchers outside the sur­ vey, s i n c e user fees simply discourage po­ tential customers. Several university researchers raised the problem of how to fund the archiving of NSF-funded project data. One option is for e a c h researcher to put a line item in his/her NSF budget, but most participants s e e m e d to prefer that NSF fund the UNAVCO archive directly. Field logs are a very important part of any campaign data set. Several experienced field workers pointed out that logs should b e designed to b e filled out as easily as possi­ ble, and should ask only for necessary infor­ mation. Nearly all participants agreed, for example, that there is no point for an opera­ tor to stay awake all night to record wet and dry temperatures several times, b e c a u s e no o n e had ever used such information. Also, a

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log format should not be redundant, b e c a u s e the typical operator will ignore all but the first o c c u r e n c e of a query—which is a dan­ gerous habit to encourage. The necessity to tediously photocopy field logs is another source of irritation, yet a single campaign might generate 2 0 0 logs of three-four pages each. A project m e m b e r may make three copies of a complete set of logs, only to receive an additional request the following week. Paper logs also tend to fall apart and get lost if copied excessively, and extremely bored operators of photocopy m a c h i n e s c a n lose pages or mix originals and c o p i e s . One solution is to optically scan the original logs, and to use very-high-den­ sity storage media such as optical disks, with raw data, the scanned site descriptions, and field logs on o n e or a few optical disks. Copying the entire data set would then b e relatively painless and error-free. Participants agreed that a project should archive, at a minimum, the raw GPS data, the field logs, and the site descriptions. But many felt that the project should also archive global tracking data acquired during the campaign. Others stated that it would b e useful to archive preprocessed data (after cycle slips had been removed) and even processing products such as the precise ephemeris associated with a given set of baseline solutions. One of the workshop's most contentious issues was how long a research group should b e allowed to keep GPS data to itself. NASA representatives said their policy is that data should enter a public domain archive as s o o n as possible, reflecting the philoso­ phy that data acquired with government funding best serves s c i e n c e and society this way. Many individual researchers or groups, however, argue that they deserve the first shot at analyzing and publishing data they acquired, or the systems of a c a d e m i c re­ wards and funding will strongly discourage them from acquisition. NSF allows propri­ etary data periods of 1 year, or even longer, in such areas as marine geology and geo­ physics. But the interdisciplinary winds in NSF are blowing in favor of the open data policy already espoused by NASA. Data avail­ ability is further complicated by the fact that American participants in an international project may not b e able, or willing, to im­ pose restrictions on foreign c o l l e a g u e s . A strict open-data policy, however, may dis­ qualify s o m e scientists from participating in interesting and worthwhile international projects. Even after vigorous and broad dis­ cussion, very little c o n s e n s u s emerged at the workshop on this topic. Several avenues for c o m p r o m i s e surfaced upon o c c a s i o n . It is true that no agency would want research groups to dump data into a public domain archive without investigating data quality or the adequacy of associated documentation. Availability in s o m e c a s e s may b e a "pseu­ do-issue," however, for those who acquired the data are much better-prepared and more strongly motivated to analyze it first. On the other hand, o n e research group d e s c r i b e d . how it received three separate requests for its data, within weeks after a large interna-

Eos, Vol. 72, No. 6, February 5, 1991 tional campaign ended. The topic obviously requires wider discussion.

Quality Control As the number of receivers available rap­ idly increases, more groups are mounting survey campaigns independent of experi­ e n c e d organizations such as UNAVCO, and NSF is c o n c e r n e d that survey quality may slide. Professional surveying organizations such as NGS are used to following docu­ mented standard procedures, but many sci­ entists are not. T h e NSF may soon require that research groups proposing to perform crustal motion measurements adopt standard procedures developed at UNAVCO, or to make a very strong c a s e for self-imposed quality control. Several participants recom­ mended that scientists should seek advice from UNAVCO while developing GPS propos­ als. It was suggested that a c a d e m i c research groups mounting a GPS campaign should start with a training c a m p for all operators. During field campaigns great effort must be made to fill out log sheets meticulously. It was widely acknowledged that site descrip­ tions are not given enough attention. Also, it is essential that ancillary instruments like tribracs—not only GPS receivers—should be calibrated before and recalibrated after e a c h campaign. Any university group operating independently of UNAVCO should include an individual who c a n perform and document instrument calibration and maintenance. It was suggested that field logs should contain a s p a c e for instrument calibration dates.

Present and Future Roles of UNAVCO The UNAVCO Facility has served univer­ sity researchers in two complementary ways: by providing GPS equipment, training, ad­ vice, operational support, data analysis and archiving services; and by developing and testing high-accuracy GPS equipment and procedures. With three TI-4100 receivers obtained in 1986, the UNAVCO Facility has supported campaigns in the Caribbean, New England, California, Yellowstone, Montana, Colorado, Iceland, Greenland, Hawaii, South America, the South Pacific, Mexico, Turkey, Greece, Indonesia, and Ascension Island. UNAVCO has also collaborated with other a g e n c i e s that own GPS equipment, most of the time to mutual advantage. In 1 9 8 7 - 1 9 8 8 , the lack of receivers was the major limit on GPS sur­ veys; by the end of 1989, however, these re­ ceivers had decreased fivefold in price and were widely available. At the time of the workshop UNAVCO was in the p r o c e s s of acquiring about 3 0 additional receivers, many of which will re­ main 6 months a year in an instrument pool maintained by the UNAVCO Facility. Not only will receivers b e more widely available than in the past, but they will also be smaller, lighter and easier to use—precipitating major

shop attendees did not always.agree on the directions of change, it is fairly clear that UNAVCO will b e asked to take on more roles than in the past. Since UNAVCO engineers will spend rela­ tively less effort running campaigns and more on training others to do so, the consor­ tium will have to place more emphasis on documenting standard operating procedures it develops. The number and variety of in­ struments that the facility must maintain and calibrate will require a major increase in re­ sources, and UNAVCO's role in archiving must also b e greatly expanded. The technol­ ogy-development function at the facility is likely to expand as the mixing of various receivers and antennae produces problems, especially given "selective availability" (the intentional degradation of the GPS signal by the Department of Defense). Finally, NSF has already indicated that UNAVCO will play a greater role in survey quality control. As many receivers b e c o m e available, GPS/crustal motion researchers will likely find their work is limited more by funding problems than by a paucity of receivers. It is increasingly necessary for the UNAVCO com­ munity, as distinct from its facility, to de­ velop a stronger identity, and to engage in university and funding politics as an authori­ tative body representing many universities. This community must act coherently in order to win more resources for itself, and s o to realize the enormous scientific potential of GPS geodesy.

Emergency Response Should there b e preplanned measure­ ment s c e n a r i o s that can b e activated auto­ matically during a large earthquake? Discus­ sion on this topic followed a USGS workshop focusing on emergency response to the Loma Prieta earthquake. Key ques­ tions included: Should o n e respond at all? What questions can b e answered by a quick response? If it is desirable to respond only to large earthquakes in a given setting, how should o n e define the critical magnitude? Who should coordinate the response? If a coordinated response is undertaken, what about individual investigators who want to "do their own thing?" If an area of significant seismic activity is covered by o n e or more GPS networks, the project teams should develop contingency plans for a variety of major earthquake sce­ narios, guided by the s c i e n c e plans driving GPS projects. Although s o m e scientific and many logistical problems will arise after a large earthquake that c a n b e solved only at the time, s o m e m e a s u r e s can b e taken in advance—such as circulating a set of bench­ mark descriptions for a given network or area to all likely partners in an emergency response—that will help a rapid response o n c e it has been deemed necessary. Whether or not UNAVCO will respond to a major earthquake will b e determined by the consortium's Steering Committee after con­ sultations.

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Interagency Cooperation The first few years of GPS/crustal motion research have s e e n extraordinary formal and informal cooperation between individuals and institutions, necessitated in part by the shortage of GPS receivers, but also inspired by a c o m m o n "pioneer spirit." Some partici­ pants felt that the widespread acquisition of GPS receivers would tend to make research groups more self-contained and less prone to cooperate, while others felt that the pres­ sure exerted on the funding agencies by pro­ posals from a rapidly expanding GPS c o m ­ munity will in fact require more coordination to avoid a wasteful duplication of effort and resources. In the recent past university re­ searchers frequently have been supported by more than o n e agency (such as NSF, NASA, USGS, and NGS). In most c a s e s this kind of cooperation has b e e n promoted "from be­ low" by a project's principal investigators, often over the course of more than o n e GPS campaign. One question raised was whether funding a g e n c i e s should set up more formal mecha­ nisms for joint review and funding of GPS research. NSF and NASA-HQ representatives indicated that c l o s e r coordination between these a g e n c i e s is likely. S o m e scientists ex­ pressed c o n c e r n that too much emphasis on interagency coordination may threaten the "small s c i e n c e " character of GPS research. A s p o k e s m a n of the Geological and Geodetic Surveys of Canada—organizations that have cooperated a great deal with the U.S.—indi­ cated that informal rather than formal c o o p ­ eration has often b e e n more successful. On the other hand several attendees felt that interagency cooperation can foster concerted efforts to solve outstanding problems such as the funding shortage for GPS processingspecialists and their students, or, at the in­ ternational level, for expanded and more reliable global tracking networks. At the c l o s e of the workshop, participants generally concurred that occasional work­ shops focusing on practical aspects of GPS research would b e useful, and that NSF, UN­ AVCO, or s o m e other body should consider convening another meeting in a couple of years. It will b e interesting to see how our c o n c e r n s , experiences and priorities evolve during the coming d e c a d e .

Acknowledgments The author thanks his fellow members of the workshop steering committee—R. Ware, M. Mayhew, R. Reilinger, B . Schutz, and B . Minster—and a l s o B . Creamer, J . Freymuller, D. Agnew, J . Bosworth, K. Kastens, and D. J a c k s o n for providing their notes on the pro­ ceedings.

2nd SEDI Symposium Held PAGES 4 9 , 5 0 Geomagnetism and paleomagnetism were the emphasis of the second SEDI (Study of the Earth's Deep Interior) symposium, "Re­ versals, Secular Variation and Dynamo Theo-