Satellite System

Azizol Bin Abdullahazizol@fsktm.upm.edu.my (A2.04)

Rujukan: Text Book Chapter 11

The First Satellite The Soviet Sputnik

satellite was the first to orbit Earth, launched on October 4, 1957.

Sputnik was a 23-inch (58-cm), 184-pound (83-kg) metal ball.

After 92 days, gravity took over and Sputnik burned in Earth's atmosphere.

Satellite-Related Terms Earth Stations – antenna systems on or near earth Uplink – transmission from an earth station to a

satellite Downlink – transmission from a satellite to an

earth station Transponder – electronics in the satellite that

convert uplink signals to downlink signals

Ways to CategorizeCommunications Satellites Coverage area

Global, regional, national Service type

Fixed service satellite (FSS) Broadcast service satellite (BSS) Mobile service satellite (MSS)

General usage Commercial, military, amateur, experimental

Geometry Terms Elevation angle - the angle from the

horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite

Minimum elevation angle Coverage angle - the measure of the portion

of the earth's surface visible to the satellite

Minimum Elevation Angle Reasons affecting minimum elevation angle

of earth station’s antenna (>0o) Buildings, trees, and other terrestrial objects

block the line of sight Atmospheric attenuation is greater at low

elevation angles Electrical noise generated by the earth's heat

near its surface adversely affects reception

Classification of Satellite Orbits Circular or elliptical orbit

Circular with center at earth’s center Elliptical with one foci at earth’s center

Orbit around earth in different planes Equatorial orbit above earth’s equator Polar orbit passes over both poles Other orbits referred to as inclined orbits

Altitude of satellites Geostationary orbit (GEO) Medium earth orbit (MEO) Low earth orbit (LEO)

Frequency Bands Available for Satellite Communications

Satellite Link Performance Factors Distance between earth station antenna and

satellite antenna For downlink, terrestrial distance between earth

station antenna and “aim point” of satellite Displayed as a satellite footprint (Figure 9.6)

Atmospheric attenuation Affected by oxygen, water, angle of elevation, and

higher frequencies

Satellite Footprint

Satellite Network Configurations

Capacity Allocation Strategies Frequency division multiple access

(FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)

What is Global Positioning System(GPS) ?

3D Coordinates for Fixed and Moving Objects on Earth A Nanosecond accurate universal time. Information and data about the atmosphere around the

globe Time & Location data for military purposes Managed and controlled by the U.S. Defense Dept.

Global Positioning System (GPS)

Started development in 1973 First four satellites launched in 1978 Full Operational Capacity (FOC) reached on July 17, 1995 System cost of $12 billion GPS provides both civilian and military positioning globally GPS comprised of three “segments”

Space Segment (the satellites) Ground Segment (the ground control network) User Segment (GPS receiver and their users)

GPS Satellite NAVSTAR GPS

satellite

The GPS System ArchitectureOverview

The GPS System Architecture The Space Segment

The Space-Segment consists of the combination of satellites that orbit the globe. The space segment of the NAVSTAR constellation includes at least 24 orbiting satellites

Orbiting Earth every 12 hours

Located at 20.200km from

Earth’s Surface

Orbiting Earth every 12 hours

Located at 20.200km from

Earth’s Surface

60 degrees apart,

55 degrees to

the Equator

60 degrees apart,

55 degrees to

the Equator

24 or more GPS Satellites

Called a Constellation

(e.g. NAVSTAR)

24 or more GPS Satellites

Called a Constellation

(e.g. NAVSTAR)

Space SegmentSpace Segment

The GPS System Architecture The Space Segment

The GPS Constellation consists of 24 satellites distributed on 6 orbital planes positioned at 55 degree inclination. There are 4 satellites in each orbital plane.

The orbits are at distances exceeding 20,200 km above the surface of Earth.

The GPS System Architecture The Space Segment

The GPS System Architecture The Space Segment

The GPS System Architecture The User Segment

The User –Segment consists of all agencies, organizations, businesses, and individuals who use GPS.

GPS ReceiversCommunication devices

that receive and analyze

signals from the

GPS Constellation.

GPS ReceiversCommunication devices

that receive and analyze

signals from the

GPS Constellation.

End User Software

Applications (e.g. surveying)

End User Software

Applications (e.g. surveying)

PeopleIndividuals, researchers,

businessmen, …etc.

PeopleIndividuals, researchers,

businessmen, …etc.

User SegmentUser Segment

The GPS System Architecture The User Segment

The user -segment consists of all agencies, organizations, businesses, and individuals who use GPS.

The transportation industry makes the most use of GPS primarily for global navigation.

The GPS System Architecture The Control Segment

Control stations is where satellite atomic time, orbits, and other data are monitored and corrected if necessary.

The Master Control station for the NAVSTAR constellation of the 24 satellites is located in the Falcon AFB Colorado Springs - USA.

Updates, Upgrades,

CorrectionsAuthorization and

Access Control

Updates, Upgrades,

CorrectionsAuthorization and

Access Control

Different Ground Controls

Stations

Located in USA, Hawaii,

Diego Garcia, & other

places.

Different Ground Controls

Stations

Located in USA, Hawaii,

Diego Garcia, & other

places.

Master Control Station(Air Force Base)

Colorado, U. S. A.

Master Control Station(Air Force Base)

Colorado, U. S. A.

Control SegmentControl Segment

The GPS System Architecture The Control Segment

GPS Master Control and Monitor Station Network

Using the GPS Technology GPS Services (The U.S. Federal Radio Navigation

Plan)

Precise Positioning Service (PSS) Horizontal Accuracy (X, Y Coordinates): 22

meters. Vertical Accuracy (Z Coordinate): 27.7 meters Time Accuracy: 100 nanoseconds (10-7 seconds) Authorized Access Only (using signal encryption)

Military Applications and Authorized Government Agencies & Organizations

Using the GPS Technology GPS Services (The U.S. Federal Radio Navigation

Plan)

Standard Positioning Service (SPS) Horizontal Accuracy (X, Y Coordinates): 100

meters Vertical Accuracy (Z Coordinate): 156 meters Time Accuracy: 340 nanoseconds (3.4 x 10-7

seconds) Access to All Users (without restrictions or

charges)

Civil Applications (Individuals, Researchers, Businesses, …etc.)

Using the GPS TechnologyCalculating Positions: Sat-

Ranges Circles in plain colors represent GPS

satellite ranges with respect to a moving object on the ground, on the sea, or in the air. The position and tie of the GPS device (I.e. receiver) is where the satellite ranges intersect.

In reality, the physical satellite ranges are those shown in dotted lines. The difference is actually the clock bias to compensate for the delay in receiving the GPS signals.

These satellite ranges are used internally by the GPS device to calculate the position of the device at the time the GPS signals from the four satellites are received.

Note: A GPS device might be able to receive signals from many satellites but only 4 are needed to determine its position and time (i.e. Position Fix).

Using the GPS Technology How To Get A Position

Need signal from at least four satellites for 3D position

One SV provides a time reference

Distance to three remaining satellites is determined by observing the GPS signal travel time from SV to the receiver

With three known points, and distances to each, we can determine the GPS receiver’s position (trilateration)

Using the GPS TechnologyCalculating Positions -

ECEF(XYZ) The Earth Centered, Earth Fixed

(ECEF) coordinate system is a 3D frame of reference. In this frame, everything on the ground, under the ground, or in the air has a unique set of (X,Y,Z) coordinates.

A GPS satellite signal includes information about “orbital elements” of the satellite. This information is known as the "Ephemeris Data". The collection of these Ephemeris Data from each of the GPS Constellation satellites is known as the "Almenac".

A GPS receiver (i.e. device) receives signals from satellites and computes their position in the ECEF coordinate system using the “orbital elements” specified in the "Almenac". Once these positions are computed, the position of the GPS receiver itself (i.e. the intersection of the satellite ranges) can be computed.

Using the GPS TechnologySources of Position Errors

Atmosphere Ionosphere & Troposphere Selective Availability - Intentional

(Most Frequent) Noise Errors causing Data Errors Blunder Errors - Satellite Loss of

Orbit (Very Rare)

Using the GPS TechnologyCorrecting the Errors : Differential -

GPS The lack of precision in GPS

position caused primarily by errors can be minimized to a certain extent using a method known as "Differential GPS".

This method consists of using an additional GPS device at a fixed base station whose position is already known. The shift in position of a mobile GPS device (due to errors) can be corrected to some extent using the known position of the fixed GPS device.

Note: Position shift correction using Differential GPS is only partial. It does however in some situations improve precision quite considerably.

GPS Applications Industry & Business

Geography: Mapping & Surveying Agriculture Telecommunications Transportation & Vehicle Tracking

Aviation (Military & Civil), Land Transport, Marine Transport, Space

Travel,…etc.

Science & Research Archeology Atmospheric Science and the Environment Geodesy Geology & Geophysics

Leisure & Recreation Sea, Air, Land

GPS User Devices

GPS User Devices

GPS User Devices