(C) Leila Z. Ribeiro,

(1)

ECE 739 – Fall 2003

Satellite Communications

Lecture 13

GPS & Future Trends

Dr. Leila Z. Ribeiro December 1, 2003

2

Agenda

• Global Positioning System (GPS)

– Introduction

– Triangulation

– Measuring distance from a satellite\ – Timing

– Satellite position in space

– Delay errors and additional error sources – Applications and Limitations

• Future Trends on Satellite Communications

Global Positioning System (GPS)

4

GPS Creation

• The U.S. Department of Defense (DoD)

decided that the military had to have a very

precise form of worldwide positioning

• And fortunately they had the kind of money

(2)

5

What is GPS?

• Global Positioning System

• Worldwide radio-navigation system consisting of a constellation of 24 satellites and multiple ground stations.

• Uses satellites as reference points to calculate positions accurate to a matter of meters (advanced forms of GPS can achieve centimeter accuracy!) • GPS receivers miniaturized and becoming very

economical and accessible to the end users • Uses include cars, boats, planes, construction

equipment, movie making gear, farm machinery

6 • 24 satellites/6 planes • Carrier frequencies L1=1575.42 MHz L2=1227.60 MHz

GPS System Overview

Copyrighted. Reproduce with permission only.

7

GPS Satellites

• Name: NAVSTAR (Rockwell International) • Altitude: 10,900 nautical miles (MEO) • Weight: 1900 lbs (in orbit)

• Size: 17 ft with solar panels extended • Orbital period: 12 hours

• Orbital plane: 55 degrees to equatorial plane • Planned Lifespan: 7.5 years

• Current constellation: 24 Block II production satellites

• Future satellites: 21 Block IIrs (Martin Marietta) 8

• Also known as the “Control Segment”

• Monitor the GPS satellites, checking both their operational health and their exact position in space • Master ground station transmits corrections for the

satellite's ephemeris constants and clock offsets • The satellites can then incorporate these updates in the

signals they send to GPS receivers

• Five Monitor Stations: Hawaii, Colorado Springs, Ascension Island, Diego Garcia, Kwajalein

(3)

9

1. The basis of GPS is "triangulation" from satellites (formally speaking, “trilateration”)

2. To "triangulate," a GPS receiver measures distanceusing the travel time of radio signals

3. To measure travel time, GPS needs very accurate timing 4. Along with distance, the receiver needs to know exactly where the satellites are in space; high orbits and careful monitoring contribute to this accuracy

5. Finally the receiver must correct for any delaysthe signal experiences as it travels through the atmosphere

We will look at each concept

How GPS Works…Basics

10

• Use satellites in space as reference points for

location on Earth

• How does knowledge of distance from three

(or more) satellites allow for position

determination? …

1. Triangulation from Satellites

11

1. Position is calculated from distance measurements (ranges) to satellites

2. Mathematically we need four satellite ranges to determine exact position

3. Three ranges would be enough if we reject ridiculous answers or use other auxiliary information

4. Another range is required for technical reasons to be discussed later

Triangulation Basics

12

• Suppose we measure our distance from a satellite and find it to be 11,000 miles (we have not said how to do this yet)

• This knowledge narrows down our position from the whole universe to a sphere shell of radius 11,000 miles from the satellite

Distance to One Satellite

11,000 miles

We are somewhere on this sphere shell

(4)

13

• Now we measure distance to a second satellite, and find it is 12,000 miles away

• This knowledge produces a second sphere centered at the second satellite

• So, we are on the “circle” where the two spheres intersect

11,000 mile

sphere 12,000 mile sphere

Add a Second Satellite

We are somewhere on this circle

14

• Suppose a third measurement yields 13,000 miles • Now our position is narrowed down to one of two points

- where the third sphere cuts through the circle that's the intersection of the first two spheres

11,000 mile sphere 12,000 mile sphere 13,000 mile sphere Three measurements put us at one of these two points

Now Add a Third Satellite

15

1. By ranging from three satellites we can narrow our position to just two points in space

2. To decide which one is our true location we could make a fourth measurement; but usually one of the two points is a ridiculous answer (for instance, too far from Earth or an impossible altitude) and can be rejected without a measurement

3. A fourth measurement will be useful for another reason

Summary - Triangulation

16

• Solution: Distance is measured by timing how

long it takes for a signal sent from the satellite

to arrive at the receiver

• Speed of light: c = 300,000 km/sec • Distance to satellite is d = c x Td

The problem is measuring the travel time

2. Measuring Distance From a Satellite

(5)

17

• A pseudo-random code (PRC) is transmitted from each satellite

• Physically it's a pseudo-random sequence of "on" and "off" pulses

• Receiver knows the time of transmission of the sequence

• By synchronizing the received sequence with a locally generated sequence, the receiver can identify the relative delay between the satellite and its location

Measuring Travel Time (1)

18

Measuring Travel Time (2)

Transmission from satellite

Reception at GPS receiver

Td= Time elapsed This time is known;

precisely

synchronized with universal time (UT)

Same code generated at receiver at same time

This time is computed by correlating the received code with the receiver generated code

19

How Does the Receiver Synchronize? (1)

• Receiver generates the same code as the satellite at

the same time

• PRCs have good auto-correlation properties • When a PRC is convolved with a time-shifted

version of itself, a peak occurs when the two signals are aligned, else only low energy noise occurs

• The time at which the peak occurs is a measure of the delay, T_d

20

How Does the Receiver Synchronize? (2)

Receiver generated code Reception at GPS receiver Td= Time elapsed … …

(6)

21

• The complex patterns assure that

synchronism with other interfering signal

will not occur

• Each satellite has its own unique PRC,

allowing unique satellite identification => all

satellites can use the same frequency (a form

of code division multiplexing)

Why Use a PRC Sequence (1)

22

• Pseudo-random sequences also make it more

difficult for a hostile force to jam the system

(in fact the PRC gives the DoD a way to

control access to the system)

• Most importantly, the PRC gives a

spread-spectrum effect, allowing the receiver to

“amplify” the signal at de-spreading (employ

spreading gain); this enables use of

economical GPS receivers (portable units with

low gain antennas)

Why Use a PRC Sequence (2)

23

• GPS satellites transmit signals on two carrier frequencies

– The L1 carrier is 1575.42 MHz and carries both the status message and a pseudo-random code for timing – The L2 carrier is 1227.60 MHz and is used for the more

precise military pseudo-random code

• Navigation message: low frequency signal added to the L1 codes that gives information about the satellite's orbits, their clock corrections and other system status

GPS Signals

24

• C/A (Coarse Acquisition) code – Modulates the L1 carrier at a 1 MHz rate – Repeats every 1023 chips, rate is 1.024 Mbps – Each satellite has a unique C/A code – Basis for civilian GPS use

• P (Precise) code

– Modulates both the L1 and L2 carriers at a 10 MHz rate – It repeats on a seven day cycle, rate is 10.24 Mbps – Intended for military users and can be encrypted – When it's encrypted it's called "Y" code

– More difficult for receivers to acquire than C/A code (C/A usually acquired first)

(7)

25

1. Distance to a satellite is determined by measuring how long a radio signal takes to reach the user from that satellite 2. To make the measurement we assume that both the satellite

and the user’s receiver are generating the same pseudo-random codes at exactly the same time (Universal Time) 3. By comparing how late the satellite's pseudo-random code

appears compared to the receiver's code, the receiver determines how long the signal took to reach it

4. Multiply that travel time by the speed of light and you've got distance

Summary - Measuring Distances

But to measure time delay perfect synchronism is required!

26

• Timing is critical: 1 ms error => 200 mile error! • Remember that both satellite and receiver need to

precisely synchronize their pseudo-random codes to make the system work

• Timing is almost perfect on the satellite because it has incredibly precise atomic clocks on board • But what about receivers

on the ground?

3. Timing

27

• If our receivers had atomic clocks (which cost upwards of $50K to $100K) GPS would be un-economical

• Solution to this problem is to make an extra satellite measurement

• Key element of GPS and also means every GPS receiver is essentially an atomic-accuracy clock • In other words: Three perfect measurements or four

imperfect measurements can locate a point in 3-D space

Timing at Receivers (1)

28

• Perfect clocks => three measurements would

give receiver’s position

• Imperfect clocks => a fourth (cross-check)

measurement will NOT intersect the first three

• Receiver can detect time offset from UT which

can be subtracted from all measurements, giving

a single point intersection

• Thus, the receiver has synced with UT =>

atomic accuracy

(8)

29

• Once receiver has the timing correction, it is applied to all measurements

• Result is very precise positioning

• One consequence of this principle is that any GPS receiver needs at least four channels so that it can make the four measurements

simultaneously

Timing at Receivers (3)

30

1. Accurate timing is the key to measuring distance to satellites

2. Satellite timing is accurate because it has atomic clocks on board

3. Receiver clocks don't have to be too accurate because an extra satellite range measurement can remove errors

But for the triangulation to work, besides distance we need the knowledge of satellite position

Summary - Timing

31

• All GPS receivers (on the ground) have

programmed “almanacs” to tell them where in

the sky each satellite is at any moment

4. Satellite Position in Space

32

• Orbits constantly monitored by the DoD

• They use precise radar to monitor each

satellite's exact altitude, position and speed

• Errors in position caused by gravitational

pulls from the moon and sun and by the

pressure of solar radiation on the satellites

• Errors are usually very small because of high

orbit (MEO), but for accuracy they must be

taken into account

(9)

33

• Satellite's exact positions are measured and

then relayed back to the satellite itself;

satellite always transmits correct position

information (and timing information)

• GPS signal also contains a navigation

message with ephemeris information as well

as timing information

Monitoring Satellite Position (2)

34

1. To use the satellites as references for range

measurements we need to know exactly where they are

2. GPS satellites are at high orbits (MEO) and are very predictable

3. Minor variations in their orbits are measured by the DoD

4. The error information is sent to the satellites, to be transmitted along with the timing signals

Summary - Satellite Position

35

• Recall: Distance to satellite is d = c x Td

• Equation is simplified: speed of light is only constant, and equal to 300,000 km/s in a vacuum

• Reality: GPS signal traverses charged particles of the ionosphere and water vapor in the troposphere and gets slowed down

• Result is errors similar to having a bad clock, a delay error

5. Correcting Additional Errors

36

• Method 1: Model atmospheric conditions to predict ‘typical’ delays

– Provides limited improvement because atmospheric conditions are rarely typical

• Method 2: Compare relative speeds of two different signals (a dual frequency measurement)

– As light passes through a given medium, low frequency signals get "refracted" or slowed more than high frequency signals

– Comparing the delays induced in the different carrier frequencies (L1 and L2), can correct for atmospheric delay – Requires very sophisticated receiver since only the military

has access to signals on L2

(10)

37

• Multipath error:The signal may bounce off various local obstructions before it gets to receiver.

• Atomic clock imperfections (small but not zero) • Position detection errors

• Geometric dilution of precision (GDOP)

• Intentional errors (removed in 2000) by the DoD – Called “Selective Availability (SA)”

– To ensure that no hostile force or terrorist groups could use GPS to make accurate weapons

Other Sources of Error

38

• A geometric error

• Receiver usually has more satellites than needed to find it’s position, so receiver chooses among them • It should not choose closely spaced satellites =>

intersecting circles will cross at shallow angles, increasing error margin

• It should choose widely separated satellites, => circles intersecting at near right angles, minimizing the error region

• Good receivers determine which satellites will give lowest GDOP

Geometric Dilution of Precision (GDOP)

39

1. Earth's ionosphere and atmosphere cause delays in the GPS signal that translate into position errors 2. Some errors can be factored out using mathematics

and modeling

3. The configuration of the satellites in the sky should be chosen to minimize errors

4. Differential GPS can eliminate almost all error

Summary – Correcting Errors

40

• Differential GPS

–A second receiver at a known location monitors variations in GPS signal and transmits to first receiver

–First receiver can correct its calculations for better accuracy

–High cost to implement second receiver –Essential in civil applications before SA

removed.

(11)

41

• Carrier-phase GPS

– Uses high frequency carrier to improve resolution of timing measurements

• Augmented GPS

– Use of a GEO satellite as a relay station to transmit differential corrections and GPS satellite status information

– The GEO satellite can provide corrections across an entire continent

GPS Flavors (2)

42

• C/A (civil): About 10 meters

• P (military): Can get down to centimeter with

use of differential GPS techniques

GPS Accuracy

43

• Civilian location: Determining a basic position • Tracking: Monitoring movement of people/objects • Timing: Providing atomic clock precision

• Military: Primary targeting and navigation system for U.S. armed forces

• Surveying: Mapping and locating land areas • Vehicular navigation: On board cars

• Ship navigation: Especially in coastal and inland waters

• Aircraft navigations/landing: With development of Augmented GPS by FAA

GPS Applications

44

• Receiver must have line of sight to four or more

satellites

• Cannot work indoors or if sky is blocked (by

buildings or other solid obstructions)

• Accuracy in vertical dimension is lower than in

horizontal (altitude)

• C/A code may be vulnerable to interference and

jamming

(12)

Future Trends in Satellite Communications

46

• Bigger/heavier GEO satellites with multiple roles • More direct broadcast TV and radio satellites • Point-to-multipoint broadcasting applications • Expansion into (higher) Ka, V, Q bands

• Massive growth in data services fueled by internet demand (overtaking voice in 2000+)

• Directional, self-steering phased array antennas at mobile

– Allows frequency reuse

– 6 dB gain => Quadruple capacity

Current Trends in Satellite

Communications

47

Influencing Factors

• Competition: Fiber optics, cellular and mobile communications systems

• GEO orbit congestion • Regional and global interests

• Commercial and political environments

48

Growth in Satellite-Based Internet

• Mobile applications: Broadband internet access over

mobile (3G)

– GSO or NGSO options

– Currently supported with limitations, e.g. low data rates • Fixed applications (Direct-to-home): Typically