how gps works

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Slide 1:

Introduction to NAVSTAR GPS Charlie Leonard, 1999 (revised 2001, 2002)

The History of GPS:

Feasibility studies begun in 1960’s. Pentagon appropriates funding in 1973. First satellite launched in 1978. System declared fully operational in April, 1995. The History of GPS

How GPS Works:

How GPS Works

Three Segments of the GPS:

Control Segment Space Segment User Segment Three Segments of the GPS Monitor Stations Ground Antennas Master Station

Control Segment:

Kwajalein Atoll US Space Command Control Segment Hawaii Ascension Is. Diego Garcia Cape Canaveral Ground Antenna Master Control Station Monitor Station

Space Segment:

Space Segment

User Segment:

Military. Search and rescue. Disaster relief. Surveying. Marine, aeronautical and terrestrial navigation. Remote controlled vehicle and robot guidance. Satellite positioning and tracking. Shipping. Geographic Information Systems (GIS). Recreation. User Segment

Four Basic Functions of GPS:

Position and coordinates. The distance and direction between any two waypoints, or a position and a waypoint. Travel progress reports. Accurate time measurement. Four Basic Functions of GPS

Position is Based on Time:

Position is Based on Time T + 3 Distance between satellite and receiver = “3 times the speed of light” T Signal leaves satellite at time “T” Signal is picked up by the receiver at time “T + 3”

Pseudo Random Noise Code:

Pseudo Random Noise Code Receiver PRN Satellite PRN Time Difference

What Time is It?:

What Time is It? Zulu Time Military Time (local time on a 24 hour clock) Universal Coordinated Time Greenwich Mean Time Local Time: AM and PM (adjusted for local time zone) GPS Time + 13* * GPS Time is ahead of UTC by approximately 13 seconds

Signal From One Satellite:

Signal From One Satellite The receiver is somewhere on this sphere.

Signals From Two Satellites:

Signals From Two Satellites

Three Satellites (2D Positioning):

Three Satellites (2D Positioning)

Triangulating Correct Position :

Triangulating Correct Position

Three Dimensional (3D) Positioning:

Three Dimensional (3D) Positioning

Selective Availability (S/A):

Selective Availability (S/A) The Defense Department dithered the satellite time message, reducing position accuracy to some GPS users. S/A was designed to prevent America’s enemies from using GPS against us and our allies. In May 2000 the Pentagon reduced S/A to zero meters error. S/A could be reactivated at any time by the Pentagon.

Sources of GPS Error:

Sources of GPS Error Standard Positioning Service (SPS ): Civilian Users Source Amount of Error Satellite clocks: 1.5 to 3.6 meters Orbital errors: < 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters Selective Availability (see notes) User error: Up to a kilometer or more Errors are cumulative and increased by PDOP.

Receiver Errors are Cumulative!:

Receiver Errors are Cumulative! User error = +- 1 km System and other flaws = < 9 meters

Sources of Signal Interference:

Sources of Signal Interference Earth’s Atmosphere Solid Structures Metal Electro-magnetic Fields

Using GPS Receivers for Positioning and Navigation:

Using GPS Receivers for Positioning and Navigation

GPS Navigation Terminology:

XTE (CDI) N (000 0 ) (0 0 ) N Desired Track (DTK) (x º ) Active Leg Distance to Waypoint Bearing (X 0 ) Present Location Speed Over Ground (SOG) Tracking (TRK) (x º ) Active GOTO Waypoint GPS Navigation Terminology Course Made Good (CMG) (CMG) (x º ) Active From Waypoint

GPS Navigation: On the Ground:

Active GOTO Waypoint Bearing = Course Over Ground (COG) = Cross Track Error (XTE) = Location Where GOTO Was Executed Bearing = 65 0 COG = 5 0 XTE = 1/2 mi. Bearing = 78 0 COG = 350 0 XTE = 1/3 mi. Bearing = 40 0 COG = 104 0 XTE = 1/4 mi. Active Leg N GPS Navigation: On the Ground

Position Fix:

Position Fix A position is based on real-time satellite tracking. It’s defined by a set of coordinates. It has no name. A position represents only an approximation of the receiver’s true location. A position is not static. It changes constantly as the GPS receiver moves (or wanders due to random errors). A receiver must be in 2D or 3D mode (at least 3 or 4 satellites acquired) in order to provide a position fix. 3D mode dramatically improves position accuracy.

Waypoint:

Waypoint A waypoint is based on coordinates entered into a GPS receiver’s memory. It can be either a saved position fix, or user entered coordinates. It can be created for any remote point on earth. It must have a receiver designated code or number, or a user supplied name. Once entered and saved, a waypoint remains unchanged in the receiver’s memory until edited or deleted.

Planning a Navigation Route:

Planning a Navigation Route Start = Waypoint

How A Receiver Sees Your Route:

How A Receiver Sees Your Route

GPS Waypoint Circle of Error:

GPS Waypoint Circle of Error X

GPS Dilution of Precision and Its Affects On GPS Accuracy:

GPS Dilution of Precision and Its Affects On GPS Accuracy

GPS Satellite Geometry:

GPS Satellite Geometry Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration. Dilution of Precision (DOP) reflects each satellite’s position relative to the other satellites being accessed by a receiver. There are five distinct kinds of DOP. Position Dilution of Precision (PDOP) is the DOP value used most commonly in GPS to determine the quality of a receiver’s position. It’s usually up to the GPS receiver to pick satellites which provide the best position triangulation. Some GPS receivers allow DOP to be manipulated by the user.

Ideal Satellite Geometry:

Ideal Satellite Geometry N S W E

Good Satellite Geometry:

Good Satellite Geometry

Good Satellite Geometry:

Good Satellite Geometry

Poor Satellite Geometry:

Poor Satellite Geometry N S W E

Poor Satellite Geometry:

Poor Satellite Geometry

Poor Satellite Geometry:

Poor Satellite Geometry

Differential GPS:

Differential GPS

Real Time Differential GPS:

DGPS Site x +30, y +60 x +5, y -3 True coordinates = x +0, y +0 Correction = x -5, y +3 DGPS correction = x +(30-5) and y +(60+3) True coordinates = x +25, y +63 x -5, y +3 Real Time Differential GPS DGPS Receiver Receiver

Slide 39:

NDGPS Ground Stations National Differential Global Positioning System Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.

Slide 40:

NDGPS Ground Stations National Differential Global Positioning System Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.

Slide 41:

Wide Area Augmentation System Geostationary WAAS satellites GPS Constellation WAAS Control Station (West Coast) Local Area System (LAAS) WAAS Control Station (East Coast)

Slide 42:

How good is WAAS? + - 3 meters +-15 meters With Selective Availability set to zero, and under ideal conditions, a GPS receiver without WAAS can achieve fifteen meter accuracy most of the time.* Under ideal conditions a WAAS equipped GPS receiver can achieve three meter accuracy 95% of the time.* * Precision depends on good satellite geometry, open sky view, and no user induced errors.

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