ESSC 541-542 Lecture 4.14.05

1

ESSC 541-542 Lecture 4.14.05

2

What is GPS?The Global Positioning System (GPS) is a satellite based navigational aid. Essentially it is a radio positioning navigation and time transfer system. It provides accurate information on position, velocity and time of an object or a platform at any moment, anywhere in the globe.

A Constellation of Earth-Orbiting Satellites Maintained by the United States Government for the Purpose of Defining Geographic Positions On and Above the Surface of the Earth. It consists of Three Segments:

Control Segment

Space Segment

User Segment

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

•Control Segment

•Space Segment

•User Segment

Three Segments of the GPSThree Segments of the GPS

•Monitor Stations

•GroundAntennas

•Master Station

•Kwajalein Atoll

•US Space Command

Control SegmentControl Segment

•Hawaii

•Ascension Is. •Diego Garcia

•Cape Canaveral

•Ground Antenna•Master Control Station •Monitor Station

•Falcon AFB, Colorado springs

ESSC 541-542 Lecture 4.14.05

6

Control Segment: Maintaining the System

(5) Monitor Stations

• Correct Orbitand clockerrors• Create new navigation message

• Observeephemerisand clock

Falcon AFBUpload Station

7

Space Segment 24+ satellites

– 6 planes with 55° inclination

– Each plane has 4-5 satellites

– Broadcasting position and time info on 2 frequencies

– Constellation has spares Very high orbit

– 20,200 km– 1 revolution in approx. 12 hrs– Travel approx. 7,000 mph

Considerations– Accuracy – Survivability– Coverage

ESSC 541-542 Lecture 4.14.05

8

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

ESSC 541-542 Lecture 4.14.05

10

How the system works

Space Segment24+ Satellites

The Current Ephemeris is Transmitted to UsersMonitor

Stations• Diego Garcia• Ascension Island• Kwajalein • Hawaii• Colorado Springs GPS Control

Colorado Springs

End User

ESSC 541-542 Lecture 4.14.05

11

ESSC 541-542 Lecture 4.14.05

12

ESSC 541-542 Lecture 4.14.05

13

ESSC 541-542 Lecture 4.14.05

14

ESSC 541-542 Lecture 4.14.05

15

ESSC 541-542 Lecture 4.14.05

16

ESSC 541-542 Lecture 4.14.05

17

ESSC 541-542 Lecture 4.14.05

18

Triangulation

Satellite 1 Satellite 2

Satellite 3 Satellite 4

ESSC 541-542 Lecture 4.14.05

19

Position is Based on TimePosition 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”

ESSC 541-542 Lecture 4.14.05

21

Distance Measuring

Rate = 186,000 miles per second (Speed of Light)

Time = time it takes signal to travel from the SV to GPS receiver

Distance = Rate x Time

Each satellite carries around four atomic clocks

Uses the oscillation of cesium and rubidium atoms to measure time

Accuracy?

plus/minus a second over more than 30,000 years!!

The whole system revolves around

time!!!

ESSC 541-542 Lecture 4.14.05

22

SV and Receiver Clocks

• SV Clocks– 2 Cesium & 2 Rubidium in each SV– $100,000-$500,000 each

• Receiver Clocks– Clocks similar to quartz watch– Always an error between satellite

and receiver clocks ( t)

• 4 satellites required to solve for x, y, z, and t

ESSC 541-542 Lecture 4.14.05

23

• PROBLEM– Can’t use atomic

clocks in receiver

• SOLUTION– Receiver clocks

accurate over short periods of time

– Reset often– 4th SV used to

recalibrate receiver clock

Cesium Clock = $$$$$$$!!!

Size of PC

4

ESSC 541-542 Lecture 4.14.05

24

PRN code

25

•GPS Satellite Signals

•• The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers.•• Three binary codes shift the L1 and/or L2 carrier phase. The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz bandwidth.•The C/A code repeats every 1023 bits (one millisecond). There is a different C/A code PRN for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil SPS.•• The P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a very long (seven days) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys. The P (Y)-Code is the basis for the PPS. The Navigation Message also modulates the L1-C/A code signal. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters.

ESSC 541-542 Lecture 4.14.05

26

Breaking the Code

Transmission Time

Receiver

The Carrier Signal...

combined with…

The PRN code...

produces the

Modulated carrier signal which is transmitted... demodulated...

And detected by receiver, Locked-on, but With a time delay...

Time delay

Satellite

Pseudo Random Noise CodePseudo Random Noise Code

•Receiver PRN

•Satellite PRN

•Time Difference

ESSC 541-542 Lecture 4.14.05

28

ESSC 541-542 Lecture 4.14.05

29

Accuracy and Precision in GPS

• Accuracy– The nearness of a measurement to the

standard or true value

• Precision– The degree to which several

measurements provide answers very close to each other.

What affects accuracy and precision in GPS?

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 InterferenceSources of Signal Interference

•Earth’s Atmosphere

•Solid Structures

•Metal •Electro-magnetic Fields

33

Sources of Error• Clock Error

– Differences between satellite clock and receiver clock

• Ionosphere Delays– Delay of GPS signals as they pass through the layer of charged ions and free electrons known as the ionosphere.

• Multipath Error–Caused by local reflections of the

GPS signal that mix with the desired signal

Direct

Sig

nal

Ref

lect

ed S

igna

l

GPSAntenna

Reflected Signal

Hard Surface

Satellite

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 GeometryIdeal Satellite Geometry•N

•S

•W •E

Good Satellite GeometryGood Satellite Geometry

Good Satellite GeometryGood Satellite Geometry

Poor Satellite GeometryPoor Satellite Geometry•N

•S

•W •E

Poor Satellite GeometryPoor Satellite Geometry

Poor Satellite GeometryPoor Satellite Geometry

ESSC 541-542 Lecture 4.14.05

41

Sources of Error• Geometric Dilution of

Precision (GDOP) – Describes sensitivity of receiver to changes in the geometric

positioning of the SVs

• The higher the DOP value, the poorer the measurement

QUALITY DOP

Very Good 1-3Good 4-5Fair 6Suspect >6

•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 GPSReal Time Differential GPS

•DGPS Receiver•Receiver

ESSC 541-542 Lecture 4.14.05

43

Differential GPS • Method of removing errors that affect GPS

measurements

• A base station receiver is set up on a location where the coordinates are known

• Signal time at reference location is compared to time at remote location

• Time difference represents error in satellite’s signal

• Real-time corrections transmitted to remote receiver– Single frequency (1-5 m)– Dual frequency (sub-meter)

• Post-Processing DGPS involves correcting at a later timeReference location

Remote location

= Error

www.ngs.noaa.gov/OPUS

Online post-processing

•Wide Area Augmentation System•Wide Area Augmentation System•Geostationary WAAS satellites

•GPS Constellation

•WAAS Control Station (West Coast)

•Local Area System (LAAS)

•WAAS Control Station (East Coast)

ESSC 541-542 Lecture 4.14.05

45

Wide Area Augmentation System (WAAS)

• System of satellites and ground stations that provide GPS signal corrections

• 25 ground reference stations across US

• Master stations create GPS correction message

• Corrected differential message broadcast through geostationary satellites to receiver

• 5 Times the accuracy (3m) 95% of time

• Only requires WAAS enabled GPS

•How good is WAAS?•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.

ESSC 541-542 Lecture 4.14.05

47

ESSC 541-542 Lecture 4.14.05

48

ESSC 541-542 Lecture 4.14.05

49

ESSC 541-542 Lecture 4.14.05

50

ESSC 541-542 Lecture 4.14.05

51

ESSC 541-542 Lecture 4.14.05

52

ESSC 541-542 Lecture 4.14.05

53

Differential GPS

ESSC 541-542 Lecture 4.14.05

55

ESSC 541-542 Lecture 4.14.05

56