CS 128/ES Lecture 10b1 GPS

CS 128/ES Lecture 10b2 A guide to GPS theory …

CS 128/ES Lecture 10b3 … and practice. Recreational GIS inputs Surveying Transportation and of course, the military

CS 128/ES Lecture 10b4 A brief history of GPS… Forerunners - LORAN-C: marine navigation. Used radio beacons along shoreline. Localized coverage and low accuracy (CEP ~ 200 m) - TRANSIT: developed by U. S. Navy. Used 6 satellites, low orbits. Global coverage & high accuracy (sub-meter) but “fix” took hours to days Department of Defense: 1970s study showed ~120 navigation systems in use. Proposed a single system, called NAVSTAR

CS 128/ES Lecture 10b5 The NAVSTAR system GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996  Military wanted portable, fast, passive positioning system  Navstar I launched Feb ’78. Now 24 satellites + “spares”  Global, 24/7 coverage by at least 4 satellites  Satellites carry atomic clocks

CS 128/ES Lecture 10b6 GPS basics Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

CS 128/ES Lecture 10b7 1, 2 … Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

CS 128/ES Lecture 10b8 3! Differential GPS Explained. J. Hurn. Trimble Navigation, 1993  Technically 4 satellites are necessary, but normally only 1 of the 2 points is on the geoid  But, as we’ll see later, a 4 th satellite is still useful

CS 128/ES Lecture 10b9 How to time the signal?  Standard EDMs bounce a signal off a reflector and measure time of return  But, GPS requires a low-power transmission and passive receivers  Imagine 2 people with synchronized stop watches, standing 1 mile apart. A gun fires near 1, and each records the time when they hear the shot. What do the 2 times tell you?

CS 128/ES Lecture 10b10 Pseudocode GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993  A string of pseudo-random bits  Predetermined sequence – can be generated by the satellite and the receivers

CS 128/ES Lecture 10b11 Code correlation GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996

CS 128/ES Lecture 10b12 But wait: for $19.95 you get all these extra codes …  C/A (coarse acquisition) code: - short (1 ms) & slow (1.023 Mbps) - meant to enable receivers to get a crude “fix” quickly  P (precision) code: - long (267 d) & fast (10.23 Mbps) - permits sub-meter accuracy

CS 128/ES Lecture 10b13 Why use the P code? Differential GPS Explained. J. Hurn. Trimble Navigation, 1993  C/A code bits are ~ 1 µs wide  Even a good receiver can be out of phase with the code by 1-5%  1% phase error ~ 3 m position error

CS 128/ES Lecture 10b14 Still not sure? There’s more …  Each code is broadcast on 2 frequencies, the L1 and L2 bands  “Dual frequency” receivers utilize the frequency difference between L1 & L2 to compensate for atmospheric distortions – more on that later  Mucho expensive

CS 128/ES Lecture 10b15 Sources of error 1.[Selective availability] 2.Clock errors 3.Ephemeris errors 4.Atmospheric delays 5.Multipath effects 6.Receiver errors Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

CS 128/ES Lecture 10b16 A small clock error matters! Microwaves travel at approximately the speed of light: 300,000 km per second  A clock error of only 1 µs could produce a positional error of ~ 300 m!  How can a $100 GPS receiver have a clock that stays accurate to the µs??

CS 128/ES Lecture 10b17 Voila – the 4 th satellite! Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 A 2-D example: Clocks synchronized Clocks not synchronized

CS 128/ES Lecture 10b18 If the 4 ranges don’t overlap: Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993  Receiver adjusts its clocks until they do  [Actually: done by algebra, not trial & error]  The time on a $100 GPS unit is really accurate!

CS 128/ES Lecture 10b19 Ephemeris errors GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 Trimble Navigation Ephemeris: mathematical description of an orbit

CS 128/ES Lecture 10b20 Atmospheric delays Differential GPS Explained. J. Hurn. Trimble Navigation, 1993  Signal slowed by: - charged particles in the ionosphere - water vapor in the troposphere  Dual-frequency receivers can correct for ionosphere delays

CS 128/ES Lecture 10b21 Multi-path errors Top: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 Bottom:  Worse: - near buildings, other obstructions - satellites near horizon: use “elevation mask”  Better: - more sophisticated antenna - ground plane to block low-angle reflections

CS 128/ES Lecture 10b22 Typical error “budget” (m)* Source: Standard GPS Differential GPS Satellite clocks1.5~ 0 Ephemeris errors2.5~ 0 Ionosphere delays Troposphere delays Multi-path (reflections)0.6 Receiver errors0.3 Typical totals10 – Differential GPS Explained. J. Hurn. Trimble Navigation * Horizontal position; vertical errors typically 2x or greater

CS 128/ES Lecture 10b23 Improving GPS accuracy DOP and mission planning Differential GPS Surveying-grade GPS

CS 128/ES Lecture 10b24 DOP & “mission” planning Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993  Dilution of precision: a small number of satellites or poor positions degrades accuracy  Advance software lets you plan when to use GPS for maximum accuracy

CS 128/ES Lecture 10b25 “mission” planning software Pathfinder Office. Trimble Navigation

CS 128/ES Lecture 10b26 Differential GPS Differential GPS Explained. J. Hurn. Trimble Navigation, 1993  Fixed base station: - knows its location - records any shifts in its readings in correction file  Differential corrections: - real time - post-processing

CS 128/ES Lecture 10b27 Real-time DGPS GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1989  Radio link with base station  U. S. Coast Guard beacons  WAAS

CS 128/ES Lecture 10b28 Coast Guard beacons Trimble’s Beacon on a Belt

CS 128/ES Lecture 10b29 WAAS  Wide Area Augmentation System  Designed by FAA & DOT for general aviation  25 ground base stations collect DGPS data & uplink to 2 geostationary satellites, 1 over east coast, the other over the west coast

CS 128/ES Lecture 10b30 Effectiveness of WAAS  Corrects for satellite orbit & clock errors, plus ionosphere & troposphere distortions  Capable of improving accuracy to < 3 m for WAAS enabled receivers  Vertical accuracy is not yet sufficient for landing airplanes at uninstrumented airports, the original program objective  The eastern satellite (#35) is low on the SE horizon and signal can be hard to receive

CS 128/ES Lecture 10b31 Post-processing  GPS receiver logs all signals received during data collection phase  Data log compared to similar record from a base station receiver  U. S. NGS operates a series of public base stations and an internet file processing system (OPUS) for free postprocessing

CS 128/ES Lecture 10b32 Survey-grade GPS  Uses high-quality, dual-frequency receivers and DGPS  Carrier phase processing - utilizes the L1 or L2 carrier signal, 1.2 / 1.5 GHz  Can achieve accuracies of < 1cm  Expensive, complex, and requires long periods of data collection at each station Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

CS 128/ES Lecture 10b33 The Future of GPS  Everyday utilities (cell phones, car navigation systems)  Integrated GIS/GPS units  GPS stalking??  ???