GPS CS 128ES 228 Lecture 10 b 1

GPS CS 128/ES 228 - Lecture 10 b 1

A guide to GPS theory … www. usace. army. mil CS 128/ES 228 - Lecture 10 b 2

… and practice. n Recreational n GIS inputs n Surveying n Transportation n and of course, the military CS 128/ES 228 - Lecture 10 b 3

A brief history of GPS… n 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 n Department of Defense: 1970 s study showed ~120 navigation systems in use. Proposed a single system, called NAVSTAR CS 128/ES 228 - Lecture 10 b 4

The NAVSTAR system § 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 GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 § Satellites carry atomic clocks CS 128/ES 228 - Lecture 10 b 5

GPS basics Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 6

1, 2 … Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 7

3! § 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 Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 8

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 228 - Lecture 10 b 9

Pseudocode § A string of pseudo-random bits § Predetermined sequence – can be generated by the satellite and the receivers GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 10

Code correlation GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 CS 128/ES 228 - Lecture 10 b 11

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 228 - Lecture 10 b 12

Why use the P code? § 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 Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 13

Still not sure? There’s more … § Each code is broadcast on 2 frequencies, the L 1 and L 2 bands § “Dual frequency” receivers utilize the frequency difference between L 1 & L 2 to compensate for atmospheric distortions – more on that later § Mucho expensive CS 128/ES 228 - Lecture 10 b 14

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 228 - Lecture 10 b 15

A small clock error matters! n 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 228 - Lecture 10 b 16

Voila – the 4 th satellite! A 2 -D example: Clocks synchronized Clocks not synchronized Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 17

If the 4 ranges don’t overlap: § Receiver adjusts its clocks until they do § [Actually: done by algebra, not trial & error] § The time on a $100 GPS unit is really accurate! Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 18

Ephemeris errors Ephemeris: mathematical description of an orbit Trimble Navigation GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 CS 128/ES 228 - Lecture 10 b 19

Atmospheric delays § Signal slowed by: - charged particles in the ionosphere - water vapor in the troposphere § Dual-frequency receivers can correct for ionosphere delays Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 20

Multi-path errors § 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 228 - Lecture 10 b Top: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 Bottom: www. gpsw. co. uk 21

Typical error “budget” (m)* Source: Standard GPS Differential GPS Satellite clocks 1. 5 ~0 Ephemeris errors 2. 5 ~0 Ionosphere delays 5. 0 0. 4 Troposphere delays 0. 5 0. 2 Multi-path (reflections) 0. 6 Receiver errors 0. 3 Typical totals 10 – 15 1 -2 Differential GPS Explained. J. Hurn. Trimble Navigation * Horizontal position; vertical errors typically 2 x or greater CS 128/ES 228 - Lecture 10 b 22

Improving GPS accuracy n DOP and mission planning n Differential GPS n Surveying-grade GPS CS 128/ES 228 - Lecture 10 b 23

DOP & “mission” planning § Dilution of precision: a small number of satellites or positions degrades accuracy § Advance software lets you plan when to use GPS for maximum accuracy Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 24

“mission” planning software Pathfinder Office. Trimble Navigation CS 128/ES 228 - Lecture 10 b 25

Differential GPS § Fixed base station: - knows its location - records any shifts in its readings in correction file § Differential corrections: - real time - post-processing Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10 b 26

Real-time DGPS § Radio link with base station § U. S. Coast Guard beacons § WAAS GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1989 CS 128/ES 228 - Lecture 10 b 27

Coast Guard beacons Trimble’s Beacon on a Belt www. navcen. uscg. gov/dgps/coverage/NYork. htm www. trimble. com CS 128/ES 228 - Lecture 10 b 28

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 228 - Lecture 10 b www. garmin. com/about. GPS/waas. html 29

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 228 - Lecture 10 b 30

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 www. ngs. noaa. gov/OPUS/What_is_OPUS. html CS 128/ES 228 - Lecture 10 b 31

Survey-grade GPS § Uses high-quality, dual-frequency receivers and DGPS § Carrier phase processing - utilizes the L 1 or L 2 carrier signal, 1. 2 / 1. 5 GHz § Can achieve accuracies of < 1 cm Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 § Expensive, complex, and requires long periods of data collection at each station CS 128/ES 228 - Lecture 10 b 32

www. garmin. com/about. GPS/waas. html The Future of GPS § Everyday utilities (cell phones, car navigation systems) § Integrated GIS/GPS units § GPS stalking? ? § ? ? ? CS 128/ES 228 - Lecture 10 b 33