Contains the transponder - a key component for satellite comms
Receives signals from transmitting Earth station
Filters and translates signals
Redirects translated signals to satellite’s transmitting antenna
Bus
Carries the payload and all its equipment into space
Physical platform holding the satellite’s parts together
Provides electrical power, navigation, control & propulsion to satellite
Contains equipment allowing satellite to communicate with Earth (remote control)
Orbits
Satellites orbit at different heights
Low Earth Orbit (LEO)
Height: 160 - 1,400 km
Coverage: Diameter of ~8,000 km
Examples: Satellite phone
Medium Earth Orbit (MEO)
Height: 10,000 - 15,000 km
Coverage: Diameter of 10,000 - 15,000 km
Examples: Weather, GPS satellites
Geostationary Earth Orbit (GEO)
Height ~36,000 km
Coverage: Diameter of ~10,000 km
Examples: Television
Propagation Delay
Amount of time taken for signal to travel from sender to receiver
Example
Assuming distance from ground level to satellite is 36,000 km. For direct transmission,
Propagation time
= 36,000,000 m / speed of light
= 36,000,000 m / 300,000,000 m/s
= 0.12 seconds (1 way)
Footprint
Ground area where the satellite’s transponders offer coverage
Footprint Maps
Shows the estimated minimal satellite dish diameter required or signal strength in each area
Measured in dBW
Footprint Coverage
Global: Covers largest possible portion of the Earth that can be viewed from the satellite
Regional: Covers certain areas (e.g. a continent)
Spot Beam: Covers a small area
Frequency Bands
Satellite communications mostly use C-, Ku- and Ka- bands (GHz)
Comparison between bands
Applications
Trunk Telephony
Used by telephone operators
Carry out long-distance telephone communications
Intercontinental
Complement / bypass submarine cables
Low density areas where satellite can connect to local telephone exchange
Mobile Satellite Telephony
To make telephone calls from wherever user is located
Lacks coverage over sparsely populated / uninhabited places
Types of Satellites used
LEO-based Globalstar/ Iridium
MEO-based Inmarsat
GEO-based Thuraya
Data, Broadband and Multimedia Services
Transfer of data over satellite networks
Data collection / broadcasting
Image & video transfer
Voice
2-way computer transactions & data inquiries
Integrates IP-based satellite communication systems seamlessly to Internet
End-user services for Small Office and Home (SOHO)
Satellite broadband connectivity provides internet access
Able to reach everywhere, even places with poorly developed infrastructure
Subscriber request routed through land-based line
Download data routed via satellite directly to Earth station
One-way Internet ServiceTwo-way Internet Service
Broadband Global Area Network (BGAN)
A portable satellite Earth station the size of a laptop
Placed at desired location to receive & transmit signals from & to the Inmarsat satellites orbiting Earth
Satellites relay data signal from BGAN to an Earth station with connectivity to Internet Service Provider
Users at location can access the Internet via devices wirelessly connected to the BGAN
Global Navigation Satellite System-based Electronic Road Pricing
a.k.a. GNSS-based ERP
Functions
Conduct & enforce congestion charging, including distance-based charging
Collect & disseminate traffic data
Provide other value-added services such as electronic road-side parking
Gantry-based ERP
Currently in-use
Annual operating costs risen by 80% over the last decade
Large part of system coming to end of life-cycle
Physical gantries take up land
Advantages of GNSS-based ERP
Distance-based pricing along congested roads
Motorists charged according to distance travelled
Allows every vehicle to become a sensor for LTA to develop a better, real-time picture of the traffic situation
Same data can be broadcasted to motorists to help them better plan their journeys and avoid congested roads
GPS
a.k.a. Global Positioning System
Details
A worldwide radio-navigational system formed from a constellation of 24 (this number is dynamic) or more satellites and their ground station
These satellites circulate in 6 orbital planes, with 4 satellites in each plane
These constellation of satellites provides the user (or GPS receiver) with 5 to 8 reachable satellites from any point on the Earth at any one time
GPS uses ‘man-made stars’ as reference points to calculate its position accurately (to the closest meters)
Why
Originated from US DOD as a mean to get the exact location of a missile’s launch (often from sea) and the exact enemy’s location
After the Cold War, GPS has been commercialised for use in navigational systems
How it works
The Basis of GPS is “triangulation” from satellites
To locate the position of the receiver, 2 methods can be used:
2D Triangulation
Exact Position of an object can be identified if its distances from 3 other positions are known
Steps:
Find the relative position of receiver with reference to Satellites 1, 2 and 3 using the triangulation method
To use the triangulation method, need to measure the distance from each satellite to the receiver, d1, d2, d3
To measure d1, d2, d3 the formula shown below is used:
The GPS receiver measures distance using the travel time of the radio waves
To get absolute position of the receiver, an “almanac” which tells the receiver the exact position of the satellites, is required
3D Triangulation
Satellites that are tracking a receiver are in orbits of the Earth
Instead of a series of circles (2D), a series of spheres is used, because the radii from the 3 satellites go off in all directions
Steps:
If the GPS receiver knows it’s 10km from satellite A in the sky, it could be anywhere on the surface of a huge imaginary sphere within a 10km radius
If it knows it is 15km from satellite B, it can overlap the first 10km sphere with another larger 15km sphere. The spheres interest to form a perfect circle
If it knows the distance of a third satellite, we get a third sphere which intersects the circle at two points
The Earth acts as the fourth sphere. As a result, only one of the two points will be on the surface of the planet, and we can eliminate the point in space. Hence only 3 satellites are sufficient for calculation
Distance from satellite to receiver can be measured using the formula:
To measure travel time, GPS needs to have a very accurate timing
Provisions have to be made for delays as signals travel through the atmosphere in all kinds of weather conditions
As GPS satellites circle our planet, they all ‘sing’ two endless ‘songs’ in unison
These ‘songs’ are loops of code and data called L1 and L2
Beam in all directions over the 1.57542 GHz frequency
Beam through a low power, 50 watt radio transmitter
Audiences are tens of millions of GPS receivers
Phones
Cars
Watches
Optional Information
Each radio transmission has 3 pieces of information:
Pseudorandom Number
A pseudorandom code identifies which satellites are transmitting
The satellites’ PRN from 1 through 32 is displayed on the receiver to indicate which satellites are detected
Ephemeris Data
Satellites’ ephemeris data keeps ground stations posted on:
health of the satellite
current time and date (maintained by onboard atomic clocks)
Almanac Data
Almanac data tells GPS receivers the locations of all the GPS satellites at any time throughout the day
Older GPS receivers have a single channel
Channel have to be divide its time rotating among satellites, picking up signals from each
New GPS receivers have a parallel, multichannel design
Between 5 - 12 receiver circuits
Each locked onto a particular satellite
All of them operates at the same time
When a receiver circuit detects a satellite’s broadcast, the GPS device uses the ephemeris and almanac data to set its own clock and save the data for use when called upon to calculate its own position
When the unit’s receiver locked onto at least 4 satellites, the receiver can begin navigating through the process of trilateration
To find a GPS satellite
We need to know where the GPS satellites so we can use them as reference points
The US airforce injects each GPS satellites into a precise orbit, according to the GPS master plan
On the ground, all GPS receivers will receive almanac data that that tell them where each satellite is, moment by moment
GPS satellites constantly monitored by Department of Defence (DoD). Precise radar is used to check each satellites’ exact altitude, position and speed
Key Features of GPS Applications
Location
System must be able to locate the source and target object accurately at any point under any kind of weather conditions
Navigation
This process guide the source object to reach the destination or target (E.g. a vehicle weather conditions)
Tracking
This process of monitoring the movement of the object as it moves from its origin to its target
Maps
To aid navigation, maps should be built and loaded into the GPS (e.g. vehicle navigation system)
Timing
The system should be able to determine how long it takes for object to reach its destination
GPS Application Examples
Geo-information for airplane passengers
How far the plane has travelled from the starting point (odometer)
How long the plane has been travelling
The current speed of the plane (speedometer)
Average speed of plane
A ‘bread-crumb’ trail of the map showing the passengers exactly where the plane has travelled
Estimate time of arrival at the destination if the plane maintains the current speed
Pop Quiz!
A GPS receiver receives signals from three satellites A, B and C. Location and horizontal distance of the satellites are:
Satellite TableMap
Find the position of the GPS receiver, marking it on the map and giving its coordinates below.
