1. Satellite Systems 1st session ends at p.30
2nd session ends around 15min earlier, then talk about exam.
Satellite Systems

2. Outline Introduction Types of Satellite
Characteristic of Satellite Systems
Satellite System Infrastructures
Call Setup
GPS
Inter-Satellite Routing

3. Introduction Satellites being above the earth can cover a larger area.
The information to be transmitted from a mobile user should be correctly received by a satellite and forwarded to one of the earth stations (ESs).
This puts the limitation that only Line of Sight (LOS) communication is possible.

4. Application Areas of Satellite System
Traditionally
Meteorological satellites
Radio and TV broadcast satellites
Military satellites
Satellites for navigation and localization (e.g GPS)
Telecommunications
Global telephone connections
Global mobile communication
Backbone for global networks
Connections for communication in remote areas

5. Types of Satellite Systems
Four different types of satellite orbits have been identified depending on the shape and diameter of each orbit:
GEO (Geostationary Earth Orbit) at 36,000 kms above earth’s surface.
LEO (Low Earth Orbit) at 500-1,500 kms above earth’s surface.
MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit) at 6,000-20,000 kms above earth’s surface.
HEO (Highly Elliptical Orbit).

6. Orbits of Different Satellites

7. Earth-satellite Parameters for a Stable Orbiting Path

8. Earth-Satellite Parameters
The orbits could be elliptical or circular.
Rotation time depends on the distance between the satellite and the earth.
For satellites following circular orbits, applying Newton’s gravitational law:
Fg (attractive force) = mg (R/r)2
Fc (centrifugal force) = mr2
 = 2f
Where, m= mass of the satellite
g= gravitational acceleration (9.81 m/s2)
R= radius of the earth (6,370 kms)
r= distance of the satellite to the center of earth
 = angular velocity of satellite
f = rotational frequency

9. Earth-Satellite Parameters
For the orbit of the satellite to be stable, we need to equate the two forces.
Thus,
gR2
r  3
2f  2

10. Inclination The plane of the satellite orbit with respect to earth
Satellite plane and earth plane
Inclination
The plane of the satellite orbit with respect to earth

11. Footprint and Elevation
Satellite can adjust it antenna to fix its footprint for a period of time.
Footprint and Elevation
The area inside the circle on earth is called the footprint of the beam.
There are two types of footprint: moving footprint and fixed footprint (fixed only for a period of time).
Elevation: Angle ε between center of satellite beam and surface of earth.
Minimal elevation: Elevation needed to at least communicate with the satellite.
Fixed footprint 
Footprint
Moving footprint

12. Distance from Satellite to MS
s is the distance
Distance from Satellite to MS
Satellite s h MS
Rsin  R
Rcos R

13. Satellite Communication
GPS is to calculate s given delay!
Satellite Communication
The figure shows the path s taken for communication from a MS to the satellite.
The time delay is a function of various parameters and is given by:
where, R= radius of the earth
h = orbital altitude
 = satellite elevation angle c= speed of light

14. Frequency Bands for Up/Down links
C: for 1st generation of satellites, has become over crowded
Ku and Ka are more popular now
LIS bands is now used by GEO/LEO satellites
Frequency Bands for Up/Down links
The satellites operate in different frequencies for up/down links
Band
Uplink (GHz)
Downlink (GHz) C Ku Ka
LIS
C band frequencies have been used in the first generation satellites and has become overcrowded because of terrestrial microwave networks using these frequencies.
Ku and Ka bands are becoming more popular, even though they suffer from higher attenuation due to rain

15. Transmission Power Characteristics
Satellites receive signals at very low power levels (less than 100 picowatts) which is one to two orders of magnitude lower than the terrestrial receivers (1 to 100 micro watts)
The received power is determined by four parameters:
Transmitting Power
Gain of transmitting antenna
Gain of receiving antenna
Distance between transmitter and receiver
The atmospheric conditions cause attenuation to the transmitted signal and the loss is given by:
L = (4rf/c)2
where r is the distance from transmitter to receiver, f is the carrier frequency

16. Atmospheric Attenuation
Attenuation is smaller as elevation angle is larger (the satellite is more at upright position above the receiver).
Atmospheric Attenuation
Atmospheric Attenuation as a function of the elevation angle

17. Basic Parameters of Satellite Systems
Satellites weigh around 2,500 kgs.
The GEO satellites are at an altitude of 35,768 kms which orbit in equatorial plane with 0 degree inclination.
They complete exactly one rotation per day.
The antennas are at fixed positions and use an uplink band of 1, ,660.5 MHz and downlink in the range of 1,530-1,559 MHz.
Ku band frequencies (11 GHz and 13 GHz) are employed for connection between the BS and the satellites.

18. Characteristics of Satellite Systems
A satellite typically has a large footprint – 34% of earth’s surface is covered.
When a satellite is close to north/south pole, it is undesirable to relay communication, as it is far away from the equator (latency is high and signal quality is poor).
LEO satellites are classified into little and big satellites:
Little LEO satellites are smaller in size, in the frequency range MHz (uplink) and MHz (downlink).They support only low bit rates (in the order of 1 kb/s) for two way messaging.
Big LEO satellites have adequate power and bandwidth to provide mobile services like data transmission, paging etc. It uses frequency range 1, ,626.5 MHz (uplink) and 2, ,500 MHz (downlink).
LEOs orbit around 500-1,500km above the earth’s surface and have latency around 5-10ms.
MEOs orbit at a height of about 5,000 – 12,000km, and have a latency around 70-80ms.

19. A Typical Satellite System
Satellite, base-station, mobile users
MUL: mobile user link
GWL: gateway link
ISL: inter-satellite link
Footprint and spotbeams…
A Typical Satellite System

20. Satellite System Infrastructure
When a MS establishes a connection with a satellite using a LOS beam, it connects the rest of the world by the underlying infrastructure and backbone network.
The satellites are controlled by the BS located at the surface of the earth which serves as a gateway.
Inter satellite links can be used to relay information from one satellite to another, but they are still controlled by the ground BS.
The footprint of a satellite beam is the area where a MS can communicate with the satellite.

21. Satellite System Infrastructure
-> diversity
Satellite System Infrastructure
There are losses in free space and also due to atmospheric absorption of the satellite beams.
Rain also causes attenuation to signal strength when GHz and GHz bands are used to avoid orbital congestion.
The satellite’s beam may be temporarily blocked due to flying objects or the terrain of the earth’s surface.
Therefore a concept known as “diversity” is used to transmit the same message through more than one satellite.

22. Satellite Path Diversity
Diversity of signal: enhance the quality of received signal (sophisticated encoding/decoding is required)
Satellite Path Diversity

23. Satellite System Architecture
MSC
/VLR
Earth Station (ES)
Gateway
EIR AUC HLR SUMR
PSTN
Public fixed and mobile networks
Mobile Link
Satellites MS

24. Satellite System Architecture: MSC handoff
/VLR
Earth Station (ES)
Gateway
EIR AUC HLR SUMR
PSTN
Public fixed and mobile networks
Satellite acts as a relay node between MSs and ES
ES performs functions similar to Base Station System (BSS)
ES keeps track of MSs located in the area by using HLR-VLR pair
When a MS moves to another ES’ area, the MSC performs handoff, requesting MS’ home MSC to update the VLR pointing to the new MSC
ES maintains SUMR (Satellite User Mapping Register) via communication with satellites.
SUMR records the satellites whose signals cover the ES
SUMR maps the satellite assigned to each MS in its area
These ESs are also connected to the PSTN backbone so that calls to/from regular household phones can also be established.

25. Note: the MS is far away from ES, so that it cannot connect to ES directly (the earth is curve, signal cannot propagate over 100 km over earth surface). That’s why MS has to use satellite as relay to reach ES
Setup of Calls to MS
MSC
/VLR
Earth Station (ES)
Gateway
EIR AUC HLR SUMR
PSTN
Public fixed and mobile networks
Mobile Link
Satellites MS
For an incoming call, the gateway reaches the closest ES.
The ES uses HLR/VLR (and SUMR) to identify satellite that serves the most recently known location of the MS.
The satellite uses a paging channel to inform the MS about the incoming call and the radio resources for the uplink/downlink connection.
The connection is established between the MS and the fixed phone

26. Setup of Calls Initiating from MS
MSC
/VLR
Earth Station (ES)
Gateway
EIR AUC HLR SUMR
PSTN
Public fixed and mobile networks
Mobile Link
Satellites MS
For a call originated from a MS, the MS accesses the shared control channel of an overhead satellite (it keeps track of MS via beacon signal)
The satellite informs the ES for authentication of the MS. The satellite knows which ES serves which active MSs under its footprint
The ES then allocates a traffic channel to the MS via the satellite and also informs the gateway about additional control information.

27. Characteristic of satellite: 1) high moving speed, making handoff frequent; 2) the predictable moving pattern of satellite makes the handoff easier; 3) satellite is only a relay node between MS and ES
Types of Handoff
Spotbeam handoff (IntraSatellite): Spotbeam/cell handoff. A satellite’s footprint is divided into spotbeams/cells. Handoff occurs when MS moves from one cell to another
Satellite handoff (InterSatellite): As MSs are mobile and most satellites are not geo-synchronous, handoff occurs from one satellite to another under control of the ES when a MS moves from one satellite’s footprint to another
Satellite-ES handoff: As satellite moves relative to the ground, Satellite-ES handoff occurs when a satellite’s control is handed over from one ES to another
InterSatellite Link (ISL) handoff: Due to the change of connectivity among satellites, the links connecting satellites are up and down. When a links is about to be down, ISL handoff occurs. Many satellite systems don’t have ISLs, thus no ISL handoff (we will not discuss it further)

28. Spotbeam/cell Handoff (IntraSatellite)
MS movements is negligible compared with satellite’s move
Spotbeam/cell Handoff (IntraSatellite)
The footprint of a satellite is divided into spotbeams (cells) for frequency reuse (the same set of frequencies can be reused in non-neighboring cells)
As satellite moves (Note: MS mobility is negligible compared with ground speed of satellite), a MS changes from one cell to another, handoff occurs (i.e., new frequency assignment)
As spotbeam is small, this handoff occurs very frequent (every 1-2 min). The handoff is within the same satellite

29. 1) Satellite keeps track of MS in its footprint via beaconing; 2) user 2 moves from satellite B to C
Satellite Handoff
When a MS (e.g., user2) changes the area from one satellite’s footprint to another due to satellite’s movement, the MS is handoff to another satellite
Satellites keep beaconing MSs inside its footprint, communicates with ES to update the SUMER (Satellite User Mapping Register)
Note: MS cannot reach ES directly. The connection between MS and ES is via the relay of the satellite.
All decision making is done by ES, leaving satellite light-weighted.

30. Satellite-ES handoff
When a satellite’s footprint moves from one ES to another, handoff occurs
It also needs to update the information of SUMR in both old and new ESs, so each ES keeps track of the MSs within its area and which satellite maps to which MS
It involves the change of IP address of MSs for IP-based communications, because ES ‘A’ and ES ‘B’ allocate different sets of IP addresses

31. Global Positioning System (GPS)
Used in applications such as military targeting, navigation, tracking down stolen vehicles, guiding civilians to the nearest hospital, exact location of the callers for E-911 emergency.
GPS system consists of a network of 24 orbiting satellites called “NAVSTAR” placed in 6 different orbital paths with 4 satellites in orbital plane.
The orbital period of these satellites is 12 hours.
The first GPS satellite was launched in Feb
Each satellite is expected to last approx. 7.5 years.

32. GPS Nominal Constellation of 24 Satellites in 6 Orbital Planes

33. Principle of GPS Localization
? How do I get accurate \delta?
Principle of GPS Localization
GPS is based on Triangulation Technique:
Satellites broadcast signal periodically, containing a timestamp t
A GPS receiver (MS) receives signal at t+δ. It can calculate its distance d to satellite by δ. It knows it is on the sphere centered from satellite ‘A” with radius of d
The MS is also a point on another imaginary sphere with a second satellite ‘B’ at its center. The MS is somewhere on the circle formed by the intersection of the 2 spheres.
With the measurement of distance from a third satellite ‘C’ the position of the receiver is narrowed down to just 2 points on the circle. One of these points is imaginary and is eliminated.
Therefore the distance measured from 3 satellites is sufficient to determine the position of the GPS receiver on earth.

34. Ephemeris: location information of the satellite
GPS (Cont’d)
The GPS signal (message) consists of a ‘Pseudo-Random Code’ (PRN), ephemeris and navigation data.
Satellites are referred to by their PRN ranging from The PRN code identifies which satellite is transmitting.
The ephemeris data corrects errors caused by gravitational pulls from the moon and sun on the satellites.
The navigation data is the information about the located position of the GPS receiver.

35. Limitations of GPS
Distance measurements may vary as the values of signal speed vary in atmosphere.
Propagation delay due to atmospheric condition affects accuracy.
Effects of Multi-path fading and shadowing are significant.
In GPS, multi-path fading occurs when the signal bounces off a building or terrain.
No GPS signals inside a building.

36. Some Applications of GPS
User Group
Application Area
U.S. Military
Maneuvering in extreme conditions and navigating planes, ships, etc.
Building the English channel
Checking positions along the way and making sure that they meet in the middle
General aviation and commercial aircrafts
Navigation
Recreational boaters and commercial fishermen
Surveyors
Reduces setup time at survey site and offers precise measurements

37. Inter-Satellite Routing: Space and Time Routing
Graph is connected over time by links
4 satellites in 4 orbits:
source A wants to send a message to B at time t = 0.
P1: A to D at t = 0, wait at D until t = 2, and D to C at t = 2 and C to B at t = 3.
P2: A waits until t = 2 and delivers directly to B at t = 2.

38. Difference of Inter-Satellite Routing and Traditional Routing
To di, next hop could go via H at t-2 to t, and go via L at t’

39. Network Model: time-varying links
G(t) = (V, E(t)), where V is the set of nodes and E(t) is a time-varying set of links between these nodes
Lij(t) represents whether a communication link is present between nodes vi and vj at time t
γ is transmission range of satellites, Pi(t) is the geographic position of node vi at time t

40. Network Model: message transmission
Message transmission s  v1  v2  v3  v4  d
Need link active long enough for a transmission (depending on message size)

41. Spatial-Temporal Links
Each layer of this graph has one copy of every node in the network and a column of vertices corresponds to a single node.
Temporal links: connect “time-copies” of the same node between consecutive intervals of time.
Traversing a temporal link denotes “carrying” a message by a node
Temporal links: nodes along the same column

42. Space-Time Graph: Link LAC of Different Sizes
Spatial links
A link is able to transmit message of different sizes. For example, the spatial links A1->C2, A2->C3 and A3->C4 are all assigned color number 1 as they represent a delay of one units. Similarly, A1->C3 and A2->C4 are colored number 2 as they correspond to message transmission delays of two units.

43. Routing using Space-Time Graph
Given a request (vi, vj, t, m) to route a message of time m (in the unit of transmission time) from vi to vj at time t
Find a path in the space-time graph from vertex vit to vjt’ of color c (size c), (c – 1)τ < m < cτ. Note: t’ is unknown
The shortest path is the path that produces the shortest delay |t’ – t|
The shortest path computation is based on dynamic programming
Since the transmission time required m, we can make the graph containing only edges with size c.
Then, we do shortest path routing on the graph. But, what is the destination node? The destination is unknown, bcs we don’t know which time-plane we end in the space-time graph!
A naïve solution is to compute shortest path to all nodes vjt’, where 0 < t’ < n, n is the total number of snapshots (planes), which is much larger than the number of satellites -> dynamic programming

44. Shortest Path Routing using Dynamic Programming
Dk(i,j,t,c) denotes the delay of shortest path from vi and vj at t computed in kth iteration
Initialization phase:
for each direct edge (vit, vj(t+c)), set D0(i,j,t,c) = c;
if there is no direct edge between vi and vj at t, but D0(i,j,t+1,c) has some finite value d, D0(i,j,t,c) = d+1*
Dynamic programming is used to iteratively compute the shortest path between vi and vj in the entire Space-time graph
Note*: it comes from the delay invariant:
D(i,j,(t-x),c) ≤ D(i,j,t,c) + x
Each iteration expands the path length
D0 is for path of length 1 hop.

45. Dynamic Programming Formulation for shortest path
A typo in formula D(i,k,t) -> D(i,k,t,c)
k is the length of path

46. The Shortest Path Routing Algorithm

47. Summary Satellite System Infrastructures
Call Setup and Handoff in Satellite Systems
GPS
Inter-Satellite Link Routing