Satellite Systems Communication: An Introduction

1. Modulo 16 Discovery, Identification, Localization (Global Positioning System and Galileo) Igor Bisio igor.bisio@unige.it Master Universitario di II Livello "Internet of Things and Big Data" A.A.2018-2019 Università degli Studi di Genova - www.master-iot.it

2. Introduction to Satellite Systems

3. Communication Satellite • A Communication Satellite can be looked upon as a large microwave repeater; • It contains several transponders which listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals.

4. Motivation to use Satellites

5. Satellite Missions

6. Satellite System Elements Space Communications Control Tracking, Telemetry and Control

7. Ground Segment • Collection of facilities, Users and Applications • Earth Station = Satellite Communication Station (Fixed or Mobile)

8. Satellite Signals • Used to transmit signals and data over long distances: • Weather forecasting; • Television broadcasting; • Internet communication; • Global Positioning Systems.

9. Types of Satellite Orbits • Based on the inclination, i, over the equatorial plane: • Equatorial Orbits above Earth’s equator (i=0°); • Polar Orbits pass over both poles (i=90°); • Other orbits called inclined orbits (0°<i<90°); • Based on Eccentricity: • Circular with centre at the earth’s centre; • Elliptical with one foci at earth’s centre;

10. Types of Satellite based Networks • Based on the Satellite Altitude: • GEO – Geostationary Orbits; • 36000 Km = 22300 Miles, equatorial, High latency; • MEO – Medium Earth Orbits; • High bandwidth, High power, High latency; • LEO – Low Earth Orbits; • Low power, Low latency, More Satellites, Small Footprint; • HEO – Highly Elliptical Orbits;

11. GEO - Geostationary Orbit • In the equatorial plane; • Orbital Period = 23 h 56 m 4.091 s; • Satellite appears to be stationary over any point on equator: • Earth Rotates at same speed as Satellite; • Radius of Orbit r = Orbital Height + Radius of Earth; • Avg. Radius of Earth = 6378.14 Km; • 3 Satellites can cover the earth (120° apart).

12. LEO - Low Earth Orbits • Circular or inclined orbit with < 1400 km altitude: • Satellite travels across sky from horizon to horizon in 5 - 15 minutes => needs handoff; • Earth stations must track satellite or have Omni directional antennas; • Large constellation of satellites is needed for continuous communication (66 satellitesneeded to cover earth); • Requires complex architecture; • Requires tracking at ground.

13. HEO - Highly Elliptical Orbits • HEOs (i = 63.4°) are suitable to provide coverage at high latitudes (including North Pole in the northern hemisphere); • Depending on selected orbit two or three satellites are enough for continuous time coverage of the service area.

14. Satellite Orbits

15. Earth’s Atmosphere

16. The Positioning Problem:Geometrical Fundamentals

17. 2D Positioning Problem: First Approach

18. 2D Positioning Problem: Second Approach

19. 3D Positioning Problem

20. Global Positioning Systems

21. GPS • What is it? • How does it work? • Errors and Accuracy • Ways to maximize accuracy • System components

22. GPS • Stands for Global Positioning System • GPS is used to get an exact location on or above the surface of the earth (1cm to 100m accuracy). • Developed by DoD and made available to public in 1983. • GPS is a very important data input source. • GPS is one of two (soon to be more) GNSS – Global Navigation Satellite System

23. GPS Uses • GPS uses can be divided into five categories: • Location – positioning things in space • Navigation – getting from point a to point b • Tracking –monitoring movements • Mapping – creating maps based on those positions • Timing – precision global timing

24. GPS Uses • Agriculture • Navigation (air, sea, land) • Engineering • Military operations • Unmanned vehicle guidance • Mapping

25. GPS • GPS uses satellites in space as reference points for locations here on earth

26. GPS 11 monitor stations help satellites determine their exact location in space. Five original stations: • Hawaii • Ascension Island • Diego Garcia • Kwajalein • Colorado Springs (control)

27. 12,500 km 11,200 km 11,500 km How does GPS work? • GPS receiver determines its position relative to satellite “reference points” • The GPS unit on the ground figures out its distance (range) to each of several satellites

28. How Does GPS Work? • Considering a planar geometry we need at least 3 satellites as reference points • Position is calculated using trilateration, similar to triangulation but with spheres. In this case at least four sphere are needed.

29. How Does GPS Work? • This method assumes we can find exact distance from our GPS receiver to a satellite. HOW??? • Simple answer: see how long it takes for a radio signal to get from the satellite to the receiver. • We know speed of light, but we also need to know: Distance = Velocity * Time • When the signal left the satellite • When the signal arrived at the receiver

30. How Does GPS Work? • The difficult part is measuring travel time (~.06 sec for an overhead satellite) • This gets complicated when you think about the need to perfectly synchronize satellite and receiver. (A tiny synch error can result in hundreds of meters of positional accuracy)

31. How Does GPS Work? • To do this requires comparing lag in pseudo-random code, one from satellite and one generated at the same time by the receiver. • This code has to be extremely complex (hence almost random), so that patterns are not linked up at the wrong place on the code. Sent by satellite at time t0 Received from satellite at time t1

32. Accuracy Depends On: • Time spent on measurements • Location • Design of receiver • Relative positions of satellites • Use of differential techniques

33. Sources of Error • Gravitational effects • Atmospheric effects • Obstruction • Multipath • Satellites Position (Geometry)

34. Errors and Accuracy • Gravitational pull of other celestial bodies on the satellite, affecting orbit • Atmospheric effects - signals travel at different speeds through ionosphere and troposphere. Both of these errors can be partly dealt with using predictive models of known atmospheric behavior and by using Differential GPS.

35. Errors and Accuracy • Obstruction - Signal blocked or strength reduced when passing through objects or water. • Weather • Metal • Trees • Buildings • Multipath – Bouncing of signals may confuse the receiver.

36. Errors and Accuracy • Satellite Positions: • Number of satellites available • Elevations or azimuths over time

37. Errors and Accuracy • PDOP – Positional Dilution of Precision • Optimal accuracy when PDOP is LOW; • Mainly due to poor satellite geometry. High PDOP Low PDOP

38. Differential GPS • Increase accuracy dramatically; • DGPS uses one stationary and one moving receiver to help overcome the various errors in the signal; • By using two receivers that are nearby each other, within a few dozen km, they are getting essentially the same errors.

39. How does DGPS work? • Can do this because precise location of stationary receiver is known, and hence, so is location of satellite • Once it knows error, it determines a correction factor and sends it to the other receiver.

40. How does DGPS work?

41. Assisted GPS (AGPS) • Equivalent to DGPS but it employs differential information over the Internet.

42. What is Galileo: (Mission High Level Definition) • Galileo is the European contribution to GNSS; • Galileo is a global infrastructure, comprising: • A constellation of MEO satellites; • Its associated Ground Segment; • Galileo is independent from, yet interoperable with other existing global radio-navigation systems, notably GPS; • Galileo is a civil system operated under public control; • The Galileo programme also includes the development of receivers, applications & services; • The Galileo programme is currently jointly managed & financed by the EC and ESA.

43. What is Galileo • Galileo open services should be competitive with comparable services of next generation GPS; • Galileo will offer new features to improve and guarantee services (integrity); • Galileo will serve a wide range of applications through the provision of 5 reference services.

44. Galileo Components & System overview

45. Galileo architecture • 3 main components: • The global component (constellation and ground control segment); • The regional component (integrity monitoring networks); • The local component (augmentations to improve availability, accuracy or integrity); • Other components: • User segment (terminals); • Service centres.

46. The Global component: Constellation • 26 MEO Satellites + 2 to bel lunched; • 3 Orbital planes; • 23616 km altitude.

47. The Global component: Ground segment • 1 Global network of 20 GALILEO Sensor Stations (GSS); • 2 GALILEO Control Centres (GCC) implemented on European ground: • Compute the satellites orbits; • Compute the integrity information; • Synchronise the time signal of all satellites and ground stations clocks; • Manage the alarms; • Interface with service centres; • Interface with regional integrity networks.