10 1521-9615/02/$17.00 © 2002 IEEE COMPUTING IN SCIENCE & ENGINEERING

TECHNOLOGY NEWST E C H N O L O G Y N E W S A N D R E V I E W S

satellites was orbiting Earth, providing military personnel withprecise positioning data for such tasks as troop movements, ar-tillery targeting, and missile guidance. Many of the most im-pressive military videos seen over the past 15 years have partiallyresulted from the Navstar satellite constellation’s success.

Fortunately for us civilians, the government has also allowednonmilitary users access to their multibillion-dollar invest-ment. Thus, the technology originally designed to enable amissile to find a particular building hundreds of miles from itslaunch site can now help you find your way to an unknowndestination hundreds of miles from your home.

Perhaps even more important for our community is thewide variety of scientific and engineering applications towhich we can apply this technology. Any scientist or engineerwhose job description includes fieldwork should consider aGPS unit as invaluable as a calculator or a laptop computer.Whether you are responsible for maintaining an array ofpower distribution equipment, collecting geological samples,or taking meteorological readings, the GPS system can be apowerful tool in your professional life.

The basics of GPS operationGPS technology determines a user’s position by calculating

the distance from several satellites whose locations are pre-cisely known. If you know, for example, that you are 10,500miles from a particular satellite, then you can state with cer-tainty that your location is somewhere on a sphere whose cen-ter is the satellite and whose radius is 10,500 miles. If youknow that your location is also 11,000 miles from a secondsatellite, you can further restrict your location to a secondsphere. The intersection between these two spheres is a cir-cle. Knowing your distance from a third satellite would fur-ther limit your possible location to the surface of a third

sphere; the intersection of three spheres is, at most, two points. Typically, one of these two points will be on or near the

Earth’s surface, so if you know your approximate altitude,you can identify your location with just three satellites.However, GPS is designed to help control airborne systemsas well as ground-based ones, so a fourth satellite can dis-tinguish between these two remaining points, providing al-titude in addition to latitude and longitude.

Thus, determining a user’s location on or near the Earth’ssurface can be reduced to determining the distance to each ofthree or four satellites whose positions are precisely known.Each satellite used for GPS contains an atomic clock that keepsvirtually perfect time. The satellites use these clocks to sendpseudorandom sequences of numbers over their radio anten-nas. The satellite antennas operate at a frequency of 1575.42MHz and at a power of approximately 50 watts. Each of the 21active GPS satellites has its own well-known algorithm for cal-culating a particular pseudorandom number sequence, so atany particular time, we can determine the number that each ac-tive satellite is transmitting. The remaining three satellites arekept in reserve in case one of the primary satellites fails.

A typical handheld GPS unit also contains a precise clock,although most do not contain an atomic clock. The unit cal-culates the same pseudorandom sequence of numbers thatthe satellites calculate and transmit. By measuring the delaybetween when it calculates that a satellite transmits a par-ticular number and when the GPS unit receives that num-ber, and then dividing by the speed of light, the unit can cal-culate its distance from the satellite.

Two potential problems arise in this calculation. First, asatellite’s orbit is not perfect. It occasionally needs to fire asmall thruster to correct for small orbital imperfections. If atypical non-GPS satellite’s location is imperfect by, say, 100feet, this doesn’t greatly affect its correct operation. However,if a GPS satellite’s position is off by 100 feet, it will substan-tially degrade the receiver’s precision. We can overcome thisproblem using almanac and ephemeris data. Each satellite con-stantly broadcasts almanac data, giving its ideal location.Ephemeris data corrects this ideal almanac data, accountingfor small fluctuations in the satellite’s path. The base stationcontrolling the satellite determines ephemeris data and broad-

FINDING YOUR WAY WITH THE GARMINGPS V By Douglas Tougaw

GLOBAL POSITIONING SYSTEM (GPS)

TECHNOLOGY BEGAN WITH THE

LAUNCH OF THE FIRST NAVSTAR SATELLITE IN

1978. BY 1994, A FULL CONSTELLATION OF 24

Editors: Donald L. Shirer, donshirer@earthlink.net

Douglas Tougaw, doug.tougaw@valpo.edu

MAY/JUNE 2002 11

casts it up to the satellite, which transmitsit along with its almanac data andpseudorandom sequence.

The second problem with the basic al-gorithm for determining the distance toa satellite is the imprecision of the localclock onboard the GPS unit. A quartzclock might have more than sufficient precision for most ap-plications, but light travels approximately one foot per nanosec-ond, so the requirements for precisely measuring time in thisapplication are incredibly strict. We can overcome this by us-ing an additional satellite. Four satellites are sufficient for de-termining 3D location given a perfect local clock, so a fifthsatellite can correct small errors to the local clock.

Imagine, for example, that you had precisely determinedyour location in space using four satellites, but that the lo-cation you determined was not on the sphere a fifth satellitedefined. This error can only be due to fluctuations in thespeed of light (more on this later) or the local clock’s impre-cision. An iterative adjustment of the local clock phase canminimize the error seen by all five satellites. Even more pre-cise corrections can be made when we use more than fivesatellites. Currently, the best commercially available GPSunits can monitor up to 12 satellites at once.

It is important to note that the government has not com-pletely released GPS technology into the public domain. Ofcourse, they still control all the satellites, so they can adjustthe signals being transmitted to suit their needs. In fact, thecivilian and military GPS signals are transmitted at differentfrequencies, so one could be disabled without affecting theother. Until May 2000, a strategy known as selective availabil-ity introduced random timing errors into the civilian GPS sig-nals to prevent them from being precise enough for enemytroops to use against the United States. Since the governmentdisabled SA, precision of civilian GPS has improved tremen-dously. The introduction of the Wide-Area AugmentationSystem has further improved GPS precision.

Wide-Area Augmentation SystemWAAS is a GPS enhancement that accounts for local vari-

ations in the composition of the Earth’s ionosphere and tro-posphere. Because light travels at different speeds throughmaterials of differing composition, there will always be someerror in the distance measured to a particular satellite. Usingan array of 25 ground reference stations that are at preciselysurveyed locations throughout the US, we can determine theeffect of these local atmospheric variations. We can then cal-culate and broadcast (via satellite) the corrections to all

WAAS-enabled GPS units in the country. For example, suppose I calculate my location in the tradi-

tional way using five or more satellites, but I am very near aWAAS correction station that has identified a known 10-footerror in the location it is calculating from the same satellites.I can subtract that 10-foot error from my calculated positionto make my measurement more precise, canceling the effectof atmospheric variations.

With SA disabled, WAAS-enabled GPS units can typicallymeasure position with an accuracy of approximately 10 to16 feet. This is the best precision currently available usingcommercially available handheld GPS units.

The Garmin GPS V unitThe GPS V unit is Garmin’s (www.garmin.com) most re-

cent model. It is 5.0 × 2.3 × 1.6 inches, weighs just nineounces, and has a built-in high-resolution LCD screen withbacklighting (see Figure 1). It contains 19 Mbytes of non-volatile user memory in addition to an unspecified (but sub-stantial) amount of system memory that comes prepro-grammed with the locations of every major US road, highway,interstate, town, and city.

We can power it for up to 25 hours on four AA batteries,although using backlighting substantially decreases this time.It also comes with a serial cable for connection to a laptop ordesktop PC and a 12-volt power cable for powering by an au-tomobile accessory outlet.

The GPS V takes approximately five minutes to acquire asatellite lock in a “cold start” (one in which it has been turnedoff for more than a few days). If it has only been turned offfor a short time period, it still has information about thesatellite almanac and can quickly determine the satellites itexpects to be currently visible. In such a “warm start,” theGPS V can reacquire its satellites in a matter of seconds.

The GPS V has 12 parallel channels designed to indepen-dently track up to 12 satellites at once. One of the user inter-face’s main pages shows which satellites should be visible andwhich ones are actually being used to calculate the unit’s posi-tion. In my experience, you can usually expect the device tolock on to approximately eight satellites. Fortunately, becausethese transmissions are satellite-based, there are few “dead

Figure 1. The Garmin GPS V unit.

Garmin International1200 East 151st StreetOlathe, Kansas 66062+ 1 800 800 1020www.garmin.com

zones” such as those encountered with cellular phone net-works. However, these are line-of-sight signals, meaning thatGPS cannot typically be used indoors, underground, or un-derwater. I have had some luck with indoor reception usingthis unit, but it is definitely less reliable.

Used correctly, precision is typically excellent, and the GPSV helpfully calculates an approximate error range on the posi-tion it delivers. This error range varies as the atmospheric con-ditions change but is usually in the range of 15 to 20 feet. An-other nice GPS V feature is that it can collect many samplesfrom the same location and integrate them together to providehigher precision on that one point’s location. When my GPScollected and integrated data at a particular point for 10 min-utes, it collected 600 samples. The predicted error of each in-dividual measurement was 19 feet, but the predicted error ofthe integrated data was just 7.6 feet. For most nonsurveying ap-plications, this precision is more than sufficient.

I found the screen very easy to read, although those whohave trouble reading fine print might want to consider a unitwith a larger screen. The resolution was excellent (256 × 160pixels), and variable backlighting and contrast provide a

The University of South Florida

Computational Scientist/Computing Consultant

The University of South Florida is developing a Research Oriented Computing Center to provide researchers access to High Performance Computing. As part of this initiative we are seeking a computational scientist/computing consultant.

The scientist/ consultant will be responsible for helping the faculty of the University make use of the HPC facilities. Other duties will include assisting the development of the Computing Center. The scientist/ consultant will be required to assess the computing needs of the faculty and make implementation recommendations. The scientist/consultant will work with the support staff to improve computing resources. Additionally the scientist/ consultant will be expected to continue their own research and stay current in both their research area and HPC. This is a full time position with full funding for three years. At that time funding drops to half time level with the remainder to come from the candidate’s own research.

The candidate should have a Ph.D. in a applied subject ,such as; physics, chemistry, biology, geology, finance, mathematics, or other computational science field. The applicant should have a strong publication record in this area. The candidate must have demonstrable experience in the design, implementation and testing of scientific and parallel programming, including familiarity with MPI libraries. The applicant should have experience or in the use of high performance cluster computers. Strong analytic and interpersonal communication skills are essential.

To apply please send resume/curriculum vita with a cover letter to Dan Majchrzak, via email dan@usf.edu, or mail to:Dan Majchrzak, University of South Florida, 4202 E. Fowler Ave LIB618,Tampa,FL 33620. (USF IS AN AFFIRMATIVE ACTION/EQUAL ACCESS/EQUAL OPPORTUNITY INSTITUTION)

Abstract Deadlines — In fairness to all potential authors, late abstracts will not be accepted.June 5, 2002: for abstracts sent via fax or mail � June 19, 2002: for abstracts sent via the MRS Web site

2002 MRS FALL MEETING

Exhibit and

Polymers and Biomaterials

A: Defect-Mediated Phenomena in Ordered PolymersB: Polymer/Metal Interfaces—Fundamentals, Properties,

and ApplicationsC: Bio-Inspired Nanoscale Hybrid SystemsD: Electronics on Unconventional Substrates—

Electrotextiles and Giant-Area Flexible Circuits

Nanomaterials and Technology

E: Physics and Technology of Semiconductor Quantum DotsF: Nanocrystalline Semiconductor Materials and DevicesG: Spatially Resolved Characterization of Local Phenomena

in Materials and NanostructuresH: Three-Dimensional Nanoengineered AssembliesI: Nanomaterials for Structural ApplicationsJ: Nano- and Microelectromechanical Systems

(NEMS and MEMS) and Molecular Machines

Electronic and Photonic Materials

K: Silicon Carbide—Materials, Processing, and DevicesL: GaN and Related AlloysM: Progress in Semiconductor Materials II—

Electronic and Optoelectronic ApplicationsN: Novel Materials and Processes for Advanced CMOSO: Microphotonics III—Materials and Applications

Spin, Superconductivity, and Ferroelectricity

P: Novel Aspects of Spintronic Materials and DevicesQ: Magnetoelectronics—Novel Magnetic Phenomena

in NanostructuresR: Advanced Characterization of Artificially Structured

Magnetic MaterialsS: Advances in Superconductivity—Electronics and

Electric Power Applications from Atomically Engineered Microstructures

T: Crystalline Oxides on SemiconductorsU: Ferroelectric Thin Films XI

Surfaces, Interfaces, and Membranes

V: Interfacial Issues for Oxide-Based ElectronicsW: Morphological and Compositional Evolution of Thin FilmsY: Surface Engineering 2002—Synthesis, Characterization,

and ApplicationsZ: Structure-Property Relationships of Oxide Surfaces

and InterfacesAA: Membranes—Preparation, Properties, and Applications

Metals, Alloys, and Inorganics

BB: Defect Properties and Related Phenomena inIntermetallic Alloys

CC: Supercooled Liquids, Glass Transition, and BulkMetallic Glasses

DD: Solid-State Chemistry of Inorganic Materials IVEE: Solid-State IonicsFF: Materials for Fuel Cells and Fuel ProcessorsGG: Fiber-Reinforced Cementitious CompositesHH: High-Temperature Thermal Spray Coatings—

Thermal Barrier CoatingsII: Scientific Basis for Nuclear Waste Management XXVI

Materials Science and Society

JJ: The Undergraduate Curriculum in MaterialsScience Education (MSE)

KK: Femto and Attosecond Phenomena in MaterialsLL: Rapid Prototyping Technologies IIIMM: Granular Material-Based TechnologiesNN: Virtual Symposium—Molecular ElectronicsX: Frontiers of Materials Research

SYMPOSIA MEETING ACTIVITIES

Symposium Tutorial Program

Available only to meeting registrants, the symposiumtutorials will concentrate on new, rapidly breakingareas of research.

Exhibit and Research Tools Seminars

A major exhibit encompassing the full spectrumof equipment, instrumentation, products, software,publications, and services is scheduled forDecember 3–5 in the Hynes Convention Center,convenient to the technical session rooms.Research Tools Seminars, an educational seminarseries that focuses on the scientific basis andpractical application of commercially available,state-of-the-art tools, will be held again this fall.

Publications Desk

A full display of over 730 books, plus videotapesand electronic databases, will be available at theMRS Publications Desk.

Symposium Assistant Opportunities

Graduate students planning to attend the 2002MRS Fall Meeting are encouraged to apply for aSymposium Assistant (audio-visual assistant)position.

Employment Center

An Employment Center for MRS meeting attendeeswill be open Tuesday through Thursday.

www.mrs.org/meetings/fall2002/

The 2002 MRS Fall Meeting will serve as a key forum for discussion of interdisciplinary leading-edge materials research from around the world.Various meeting formats—oral, poster, round-table, forum and workshop sessions—are offered to maximize participation.

Member ServicesMaterials Research Society

506 Keystone DriveWarrendale, PA 15086-7573

Tel 724-779-3003Fax 724-779-8313

E-mail: info@mrs.org

For additional meeting information,visit the MRS Web site at

www.mrs.org/meetings/or contact:

DECEMBER 2–6BOSTON, MA

02-0019

NewNew� Materials Development� Characterization Methods� Process Technology

great deal of control, letting the useradjust to many different environments.A color screen would, of course, bewelcome, but it would come at a highercost in terms of both power consump-tion and list price. For the purposes ofthis device, a four-level grayscalescreen is sufficient.

The interface to the PC is convenientand quite intuitive. The unit’s purchaseprice includes the opportunity to “un-lock” detailed map information for oneof seven regions of North America. Ad-ditional regions can be unlocked at anadditional expense. This “feature” frus-trated me, because I live quite near theboundary between two regions. I’m surethat for most users this will not be a greatproblem, but it would be more conve-nient if two regions could be unlockedwith the purchase of the GPS unit.

The MapSource software lets the userupload information from the GPS unitto the PC, including the location ofmarked points and precise tracks that theuser followed. This is probably one ofthe most useful features for scientificusers, because it can precisely mark thelocation of a particular specimen, mea-surement, or job site.

In addition, the MapSource softwarecan download detailed maps (withinyour chosen region), selected routes,and waypoints directly to the GPS unit.Just as with devices such as Palm Pilots,I found it much more efficient to domost of my work on the PC and down-load the results to the GPS unit.

The user interface consists of theLCD screen, eight buttons, and a singleeight-direction rockerpad. Unlike manyengineers, I tend to read a new piece ofequipment’s instruction manual cover-to-cover before I turn it on, but I didn’tfind that helpful here. The manual ap-pears to be well written, but I learnedmore about the correct operation of thisdevice in a 10-minute test than I did inan hour of reading about it. The layout

of the buttons is not intuitive but is eas-ily learned. At least two of the buttonsseem redundant and could be removedin future models.

The most frustrating thing I encoun-tered in approximately 15 hours of test-ing is the wrist strap connection’s poordesign. This might seem trivial, but thewrist strap connects directly to the batterycover. If you lift the metal tab on thiscover and rotate it 90 degrees (which canhappen quite easily if you carry the deviceby the wrist strap), the batteries automat-ically eject themselves quite forcefully.This happened twice during my testing,and one of these times I was unable to lo-cate all the batteries after they fell out. Ofcourse, removing power from the systemrequires a reacquisition of the satellitesand a confirmation of the new location.This is more of a nuisance than anythingelse, but I would recommend throwingaway the wrist strap and carrying the unitin your pocket when it is not in use.

T he Garmin GPS provides excel-lent precision, a flexible user in-

terface, and detailed built-in mappingcapabilities for a reasonable price. (Thelist price is US$499, and several autho-rized resellers offer discounts over theInternet.) It is unlikely that GPS tech-nology will improve substantially onthe currently available precision with-out the use of much more advanced(and expensive) add-on units, so if yourwork could benefit from precise knowl-edge of your current location, I highlyrecommend the Garmin GPS V.

Douglas Tougaw is an assistant professor and

the department chair at the Department of

Electrical and Computer Engineering at Val-

paraiso University, Indiana. Contact him at the

Dept. of Electrical and Computer Engineering,

Valparaiso Univ., Valparaiso, IN 46383; doug.

tougaw@valpo.edu; http://diamond.gem.valpo.

edu/~dtougaw.

MAY/JUNE 2002

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