GLOBAL POSITIONING SYSTEMS

Knowing where and when

OverviewContinuous position fixes

Worldwide coverage

Latitude/Longitude/Height

Centimeter accuracy

Accurate time

The History of GPS (or, Global Navigation Satellite Systems—GNSS)

Feasibility studies begun in 1960’s.

Pentagon appropriates funding in 1973.

First satellite launched in 1978.

System declared fully operational in April, 1995.

Typical GPS Applications

Location - determining a basic position

Navigation - getting from one location to another

Tracking - monitoring the movement of people and things

Mapping – creating and updating maps/GIS

Timing - bringing precise timing to anywhere in the world

Global Positioning SystemsThe ‘default’ Global Positioning System is the US NavStarsystem, developed by the US Department of Defence. The first satellites were sent into orbit in 1978, although GPS as we currently recognize it wasn’t available until after 1989.

The Russian Federal Space Agency developed their own system, GLONASS, around the same time as the US system. (here)

China has its own system—BeiDou—in 2000, and currently is establishing a NavStar-like system (by 2020). (here)

Europe is also developing its own system—Galileo—started in 2011, with plans to complete the system by 2020. (here)

India (IRNSS) and Japan (QZSS) also have regional GPS in place.

GPS Segments (or components)

Space Segment: satellite constellation(24+ active satellites in space).

Control Segment: ground stations located on earth (originally 5, now up to 16).

User Segment: GPS receiver units that receive satellite signals and determine receiver location from them.

Space segment

Control segment

User segment

Space Segment

Space segment is the satellite constellation.31 satellites with a minimum of 24 operating 98% of the time (22 additional satellites have ‘died’ over the years) (currently, 27 are active in the ‘constellation’ in order to provide more accurate fixings)6 orbital planes (inclination ~55°)Circular orbits~20,000 km above the Earth's surface 11 hours 58 minute orbital period Visible for approximately 5 hours above the horizon

Control Segment: Ground Monitoring Stations

Falcon AFBColorado Springs, CO

Master Control Monitor Station

HawaiiMonitor Station

Ascension IslandMonitor Station

Diego GarciaMonitor Station

KwajaleinMonitor Station

GPS Satellite Orbits

Orbits of GPS satellites need to be continually updated because they do not stay circular without adjustments.

Adjustments are required because:Other objects exert gravitational force on each satellite (e.g., sun, moon).The effect of gravity is non-uniform (geoide).Radiation pressure (due to solar radiation).Atmospheric drag.Other effects.

Third Component: User Segment

User segment: the receivers that have been designed to decode signals transmitted from satellites for purposes of determining position, velocity or time.

Receiver must perform the following tasks:

• select one or more satellites in view • recover navigational data• acquire GPS signals • measure and track signal

User Segment

•Military•Search and rescue•Disaster relief•Surveying, geophysics (continental drift)•Marine, aeronautical and terrestrial navigation•Remote controlled vehicle and robot guidance•Satellite positioning and tracking•Shipping•Geographic Information Systems (GIS)•Recreation (e.g., geocaching)

Important Concepts

Satellites transmit Ephemeris and Almanac data—obtained from the monitoring stations—to the GPS receivers.

Ephemeris data contains precise orbital information about the satellite, the current status of it (healthy or unhealthy), the current date and time. This part of the signal is essential for determining a position.

Almanac data tells GPS receiver where each GPS satellite should be at any time throughout day. Each satellite transmits almanac data showing orbital information for that satellite and for every other satellite in the system.

The fundamental principle behind the GPSystemis that time can be used to determine position.

Therefore, the most important element of the GPSystem is determining the Time Of Arrival (TOA) of the signal in the GPS unit.

Determining position

TOA Concept

GPS use the concept of time of arrival (TOA) of signals to determine user position.

This involves measuring the time it takes for a signal transmitted by an emitter (satellite) at a known location to reach a user receiver.

Time interval (signal propagation time) is multiplied by the speed of the signal (speed of light) to obtain satellite-to-receiver distance.

Pseudo Random Number Code [PRNC}

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”

Synchronizing Clocks

In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond.

Accurate time measurements are required. If it is off by a thousandth of a second, at the speed of light, that translates into almost 200 miles of error.

Synchronizing Clocks

To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in each receiver.

But atomic clocks cost somewhere between $50,000 and $100,000, which makes them too expensive for everyday consumer use.

The scientists behind the GPSystem developed a clever solution to this problem. Every satellite contains an expensive atomic clock, but each receiver uses an ordinary quartz clock, which is constantly reset.

Synchronizing Clocks

In a nutshell, the receiver looks at incoming signals from three or more satellites and gauges its own inaccuracy (which = determining its exact position).

The actual process of determining the position is called trilateration

B

D?C

E

Synchronizing Clocks

When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point.

Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly.

Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.

Trilateration

- One time measurement narrows down our position to the surface of a sphere

- Second time measurement narrows it down to intersection of two spheres

- Third time measurement narrows it to just two points

Trilateration

- Fourth time measurement will decide between the two points

Fourth measurement will only go through one of the two points

In certain situations, 3 measurements are enough (2 dimensional position: latitude and longitude; e.g., when on a boat). One point will be obviously incorrect: out in space or moving at high speed.

A fourth measurement will provide a 3 dimensional position: latitude, longitude and elevation

Determining the clock correction

Showing only 2 spheres for clarity,but the principle is the same for 4.

However, because the GPS unit’s clock is off, even with 4 satellites you will not initially identify a single point.

Clock error

Synchronizing Clocks

The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point.

Based on this, it resets its clock to be in sync with the satellite's atomic clock.

The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.

Knowing Satellite Locations

In order to properly synchronize clocks and figure out which PRNC signal it is listening to, the receiver has to know where the satellites actually are.

This isn't particularly difficult because the satellites travel in very high and predictable orbits.

Pseudo Random Number Code [PRNC}

Using Almanac Information

The GPS receiver stores an almanac that tells it where every satellite should be at any given time.

Things like the pull of the moon and the sun do change the satellites' orbits very slightly.

However, the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals (ephemeris).

Ephemeris

Two Types of Errors

Errors can be categorized as intentional and unintentional.

Intentional errors: government can and does degrade accuracy of GPS measurements. This is done to prevent hostile forces from using GPS to full accuracy.

Policy of inserting inaccuracies in GPS signals is called Selective Ability (SA). SA was single biggest source of inaccuracy in GPS. SA was deactivated in 2000 (but could be turned back on should the US military decide to do so, although the newer NavStar satellites have been designed without SA).

Sources of Unintentional Timing Errors

Atmospheric interactions

Ionosphere (a band of charged particles)

Troposphere (our weather)

Earth’s Atmosphere

Solid Structures

Metal

Electro-magnetic FieldsMultipath signals

Typical Errors

Source of Error Typical Error in Meters (per satellite)

Satellite Clocks 1.5Orbit Errors 2.5Ionosphere 5.0Troposphere 0.5Receiver Noise 0.3Multipath 0.6SA 30

Errors are cumulative and increased by PDOP.

User error = ± 1 km

System and other flaws = < 9 meters

Receiver errors are cumulative

Data quality parameters

Positional Dilution of Precision (PDOP)

Horizontal DOP

Vertical DOP

Relative positions of satellites can affect error and reduce accuracy standards

Y

N

PDOP

Good PDOP ≤ 4

Bad PDOP ≥ 8

Ideal Satellite Geometry

N

S

W E

Good Satellite Geometry

Good Satellite Geometry

DOP

Poor Satellite Geometry

N

S

W E

Poor Satellite Geometry

Poor Satellite Geometry

DOP

Differential GPS

A technique called differential correction can yield accuracies within 1-5 meters, or even better, with advanced equipment.

Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point.

Because physical location of base station is known, a correction factor can be computed by comparing known location with GPS location determined by using satellites.

Differential GPS

Differential GPS Improvements

Source Uncorrected With Differential

Ionosphere 0-30 meters Mostly Removed Troposphere 0-30 meters All Removed Signal Noise 0-10 meters All Removed Orbit Data 1-5 meters All Removed Clock Drift 0-1.5 meters All Removed Multipath 0-1 meters Not Removed Receiver Noise ~1 meter Not RemovedSA 0-70 meters All Removed

Wide Area Augmentation System

Geostationary WAAS satellites

GPS Constellation

WAAS Control Station (West Coast)

WAAS Control Station (East Coast)

Local Area Augmentation System (LAAS)

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.

Using GPS Data

With GPS lat/lon, and its built-in clock, the receiver can give you several pieces of valuable information:

o How far you've traveled (odometer) o How long you've been traveling o Your current speed (speedometer) o Your average speed o A "bread crumb" trail showing you exactly where you have

traveled on the map o The estimated time of arrival at your destination if you

maintain your current speed

SummaryGPS has fundamentally changed how location is determined

Continuous position fixes

Worldwide coverage

Latitude/Longitude/Height

Centimeter accuracy

Accurate time anywhere