PART0 » History » Version 37

COLIN, Tony, 03/14/2016 10:05 AM

1 3 COLIN, Tony
h1. PART 1 : An introduction to Navigation.
2 3 COLIN, Tony
3 16 COLIN, Tony
{{toc}}
4 3 COLIN, Tony
5 15 COLIN, Tony
---
6 12 COLIN, Tony
7 15 COLIN, Tony
p((. Before anything else, it is necessary to tackle the origin of navigation and its applications, the creation of the current GNSS with a comparison of different systems and finally, describing the main principles of GPS.
8 3 COLIN, Tony
9 3 COLIN, Tony
---
10 3 COLIN, Tony
11 3 COLIN, Tony
h2. 1 - History of Navigation.
12 3 COLIN, Tony
13 37 COLIN, Tony
_*Navigation* is defined as a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another. The field of navigation includes four general categories: land navigation, marine navigation, aeronautic navigation, and space navigation._
14 17 COLIN, Tony
A brief evolution of Navigation through history is suggested in the following. 
15 4 COLIN, Tony
16 1 COLIN, Tony
p=. !33.jpg! !34.jpg! !35.jpg! !37.jpg! !36.jpg!
17 17 COLIN, Tony
*Figure 1* : Lighthouse __________________________ *Figure 2* : Magnetic compass ________________________ *Figure 3* : Sextant __________________ *Figure 4* : Chronometer _______________ *Figure 5* : TomTom GPS
18 3 COLIN, Tony
19 1 COLIN, Tony
h3. a- Visual navigation.
20 1 COLIN, Tony
21 17 COLIN, Tony
Seamark and landmark references *[Figure 1]* where among the first entities to help to find a direction. However, this system works with a limited range, a limited accuracy, a low availability of marks.
22 17 COLIN, Tony
Note that stars has been used for centuries, but are only available during night and clear sky.
23 1 COLIN, Tony
24 17 COLIN, Tony
h3. b- Navigation with classic instruments.
25 3 COLIN, Tony
26 17 COLIN, Tony
Then, come the Magnetic compass *[Figure 2]*, the Sextant *[Figure 3]* as navigation tools.
27 17 COLIN, Tony
The last one allows the measurement of the height of stars above the horizon providing longitude inaccurately i.e. it was the first use of ephemeris and time reference.
28 17 COLIN, Tony
Improvements has been done by combining it with Harrison's clock (Chronometer) *[Figure 4]* providing latitude and longitude with acceptable accuracy.
29 3 COLIN, Tony
30 3 COLIN, Tony
---
31 3 COLIN, Tony
32 3 COLIN, Tony
h2. 2 - Global Navigation Satellite System (GNSS).
33 3 COLIN, Tony
34 3 COLIN, Tony
h3. a- History.
35 3 COLIN, Tony
36 21 COLIN, Tony
- *Sputnik* in *1957* : first satellite in the world, orbit estimated using the received signal frequency via Doppler effect.
37 21 COLIN, Tony
- *TRANSIT* from *1958* operational in 1964 : first satellite navigation system for the US Navy.
38 21 COLIN, Tony
- *GPS* program started at *1973* providing position anywhere at anytime.
39 3 COLIN, Tony
40 3 COLIN, Tony
h3. b- Current definition.
41 3 COLIN, Tony
42 19 COLIN, Tony
_*Global Navigation Satellite System (GNSS)* is defined as a system of satellites that provide autonomous spatial positioning on Earth with global coverage.
43 3 COLIN, Tony
It allows small electronic receivers to determine their location (longitude/latitude/altitude) to high precision (within a few meters) using time signals transmitted along a line of sight by radio from satellites. 
44 3 COLIN, Tony
Global coverage for each system is generally achieved by a satellite constellation of 20–30 medium Earth orbit (MEO) satellites spread between several orbital planes. The actual systems vary, but use orbital inclinations of >50° and orbital periods of roughly 12 hours._
45 3 COLIN, Tony
46 3 COLIN, Tony
---
47 3 COLIN, Tony
48 3 COLIN, Tony
h2. 3 - Comparison of systems.
49 3 COLIN, Tony
50 22 COLIN, Tony
Various global satellite navigation systems has been developped in different countries and while some of them are currently used, others are in preparation. The following array sums up these systems and underlines their differences :
51 22 COLIN, Tony
52 20 COLIN, Tony
p=. !31-3.png!
53 23 COLIN, Tony
*Figure 6* : Comparison of GNSS systems.
54 22 COLIN, Tony
55 25 COLIN, Tony
An illustration of their orbits in terms of altitude and revolution period is proposed :
56 23 COLIN, Tony
57 1 COLIN, Tony
p=. !30.png!
58 23 COLIN, Tony
*Figure 7* : Orbits of GNSS systems.
59 3 COLIN, Tony
60 3 COLIN, Tony
---
61 3 COLIN, Tony
62 3 COLIN, Tony
h2. 4 - Our system : GPS details.
63 3 COLIN, Tony
64 1 COLIN, Tony
h3. a- Space Segment.
65 3 COLIN, Tony
66 27 COLIN, Tony
The official GPS website *[5]* describes the evolution of the space segment by improving the satellites' capabilities :
67 27 COLIN, Tony
68 3 COLIN, Tony
p=. !26.png!
69 26 COLIN, Tony
*Figure 8* : Evolution of the GPS satellites with their characteristics through time
70 3 COLIN, Tony
71 1 COLIN, Tony
Here is an animation depicting the GPS constellation with the number of available satellites through time :
72 24 COLIN, Tony
73 24 COLIN, Tony
p=. !28-2.gif!
74 26 COLIN, Tony
*Figure 9* : GPS constellation
75 3 COLIN, Tony
76 3 COLIN, Tony
h3. b- Ground Control Segment.
77 3 COLIN, Tony
78 29 COLIN, Tony
The Ground Control Segment handles different aspect of the constellation :
79 29 COLIN, Tony
- Continuous tasks : Tracks GPS satellites, estimates clock and orbit, keeps GPS time, uploads data that describes clock & orbit for each satellite.
80 29 COLIN, Tony
- Infrequent tasks: Commands small maneuvers to maintain orbit, small clock corrections, major relocations to compensate any satellite failure.
81 3 COLIN, Tony
82 3 COLIN, Tony
h3. c- Delivery of Navigation messages.
83 3 COLIN, Tony
84 32 COLIN, Tony
Considering the exchange of data : Clock & Ephemeris are sent by the Ground Segment to Space Segment, while Spread Spectrum ranging signals & navigation data sent by Space Segment to User & Ground Segment.
85 3 COLIN, Tony
86 1 COLIN, Tony
h3. d- Signals.
87 3 COLIN, Tony
88 31 COLIN, Tony
As a simple model, the GPS signal can be decompose in navigation data D(t) spread via code C(t) and modulated on a carrier frequency f(t) :
89 31 COLIN, Tony
90 5 COLIN, Tony
p=. !2.png!
91 26 COLIN, Tony
*Figure 10* : Signal decomposition
92 3 COLIN, Tony
93 3 COLIN, Tony
h3. e- Ranging.
94 3 COLIN, Tony
95 34 COLIN, Tony
The basic concept of localization relies on the measurement of 3 distances from 3 different entities (satellites orbiting the Earth). Knowing the positions of these entities, it is possible determine the desired location with a certain accuracy.
96 33 COLIN, Tony
Parameters have to be measured :
97 33 COLIN, Tony
- Propagation time : Time of transmission - Time of reception
98 33 COLIN, Tony
- Distance between satellite and receiver \approx Propagation time x Speed of light
99 3 COLIN, Tony
100 34 COLIN, Tony
The process used is so-called *trilateration*, which can be defined as _the way of determining absolute or relative locations of geometric points by measurement of distances, using the geometry of circles, spheres or triangles. :_
101 1 COLIN, Tony
1 measurement : 1 sphere of radius D1 centered in Sat1
102 1 COLIN, Tony
2 measurements : intersection of 2 spheres  (D1, Sat1) (D2, Sat2) = Circle 
103 3 COLIN, Tony
3 measurements : intersection of 3 spheres (D1, Sat1) (D2, Sat3) (D2, Sat3) = 2 points (1 often aberrant) 
104 3 COLIN, Tony
105 34 COLIN, Tony
These measurements are depicted on the figure :
106 1 COLIN, Tony
107 1 COLIN, Tony
p=. !20.png!
108 1 COLIN, Tony
*Figure 11* : Pseudo-Ranging method.
109 34 COLIN, Tony
110 34 COLIN, Tony
The official GPS website also provides an explicit scheme of how it works under : attachment:"29.pdf"
111 34 COLIN, Tony
112 34 COLIN, Tony
_Note that in reality, a 4th measurement is necessary in order to find the clock bias between the clock of the satellite and the clock of the receiver._
113 3 COLIN, Tony
114 1 COLIN, Tony
h3. f- GNSS Receivers.
115 3 COLIN, Tony
116 36 COLIN, Tony
Classical GNSS receivers are divided into 4 important steps : Acquisition, Tracking, Navigation data decoding and Localization computation as explained on *Figure 12 and 13* extracted from *[1]* :
117 35 COLIN, Tony
118 1 COLIN, Tony
p=. !21.png!
119 35 COLIN, Tony
*Figure 12* : Receiver diagram.
120 1 COLIN, Tony
121 3 COLIN, Tony
p=. !22.png!
122 35 COLIN, Tony
*Figure 13* :
123 3 COLIN, Tony
124 1 COLIN, Tony
h3. g- GNSS Measurements.
125 3 COLIN, Tony
126 3 COLIN, Tony
p=. !23.png!
127 26 COLIN, Tony
*Figure 14* : 
128 3 COLIN, Tony
129 3 COLIN, Tony
p=. !25-2.png!
130 26 COLIN, Tony
*Figure 15* : 
131 3 COLIN, Tony
132 3 COLIN, Tony
p=. !24.png!
133 26 COLIN, Tony
*Figure 16* : 
134 3 COLIN, Tony
135 3 COLIN, Tony
p=. !25.png!
136 26 COLIN, Tony
*Figure 17* : 
137 3 COLIN, Tony
138 3 COLIN, Tony
*References :* 
139 3 COLIN, Tony
*[1]* M. Sahmoudi, Introduction to satellite positioning & multi-sensor navigation, 2016
140 3 COLIN, Tony
*[2]* K. Borre, D. M. Akos, N. Bertelsen, P. Rinder, S. H. Jensen, A software-defined GPS and GALILEO receiver
141 3 COLIN, Tony
*[3]* https://en.wikipedia.org/wiki/Satellite_navigation
142 3 COLIN, Tony
*[4]* https://en.wikipedia.org/wiki/Global_Positioning_System
143 1 COLIN, Tony
*[5]* http://www.gps.gov/systems/gps/