PART3 » History » Version 73

JANVIER, Thibault, 03/23/2016 12:01 PM

1 46 JANVIER, Thibault
h1. PART 5 : Implementation and results.
2 2 COLIN, Tony
3 20 COLIN, Tony
{{toc}}
4 2 COLIN, Tony
5 47 JANVIER, Thibault
This part is dedicated to explain and illustrate the main steps that have been done in order to retrieve a navigation signal and to compute the position of the receiver from the pseudo-range measurements of 4 satellites. [[PART|PART 2]], [[PART2|PART 3]] and [[PART41|PART 4]] provided a theoretical background on GPS signals, the main blocks of a GPS receiver and methods to compute the position using the navigation data. Now, it is time to put this theoretical knowledge into practice.
6 47 JANVIER, Thibault
As lots of VIs have been created, a UML diagram has been drawn to illustrate the structure of the overall code. For each main step that will be described, a link to the UML diagram will be provided, the different VIs to be taken into consideration will be listed, a few key points regarding the implementation will be clarified and the results will be displayed.
7 2 COLIN, Tony
8 2 COLIN, Tony
---
9 2 COLIN, Tony
10 4 COLIN, Tony
h2. 1 - Starting point.
11 2 COLIN, Tony
12 49 JANVIER, Thibault
h3. a - Quid about LabVIEW.
13 1 COLIN, Tony
14 49 JANVIER, Thibault
Labview is a modular dataflow programming language that relies on block diagrams interconnected with each other. Each block diagram represents a node, a function that can be run as soon as its inputs are available. Then, it computes outputs used as inputs by other nodes. Any created program can be reused as a node in a higher-level program, hence the modularity of Labview. While running a program, each input can be controlled thanks to a user interface known as a “front panel”. Likewise, every output can be monitored and displayed using the front panel. As a matter of fact, the front panel is closely linked to the program called the “block diagram”. Indeed, every primary input of the block diagram are defined and controlled via the front panel.
15 1 COLIN, Tony
16 49 JANVIER, Thibault
h3. b - Receiver scheme and milestones.
17 34 COLIN, Tony
18 49 JANVIER, Thibault
The software-defined GPS receiver to be implemented should take as an input a sampled signal that has been received and down-converted by an RF receiver front-end. As our RF receiver front-end was missing an antenna, we used a recorded GPS raw signal from the output of an RF front end (from CD *[1]* under GNSS_signal_records/GPSdata-DiscreteComponents-fs38_192-if9_55.bin). In order to read properly the data that are contained in the recorded file, one needs to know the sampling frequency and the intermediate frequency of the raw signal.
19 49 JANVIER, Thibault
Sampling frequency = 38.192 MHz
20 49 JANVIER, Thibault
Intermediate frequency = 9.55 MHz
21 50 JANVIER, Thibault
The following diagram sums up the general structure of the GPS receiver to be implemented.
22 1 COLIN, Tony
23 50 JANVIER, Thibault
p=. !SDR_GPS_Receiver.png!
24 73 JANVIER, Thibault
*Figure 5.1*: General receiver scheme.
25 34 COLIN, Tony
26 34 COLIN, Tony
h3. c - Local C/A code generation.
27 34 COLIN, Tony
28 43 COLIN, Tony
The files involved are :
29 55 COLIN, Tony
- _CA_Code.vi_ : attachment:"SnapCACode.png"
30 55 COLIN, Tony
- _CA_generatorG1.vi_ : attachment:"SnapG1.PNG"
31 56 COLIN, Tony
- _CA_generatorG2.vi_ : attachment:"SnapG2.png"
32 4 COLIN, Tony
33 51 JANVIER, Thibault
These VIs allow generating a PRN sequence according to the satellite ID and the sampling frequency. Indeed, the newly generated PRN sequence has to be multiplied with the incoming signal. Knowing that the period of one PRN sequence is 1 ms, the length of the generated sequence has to be adjusted according to the sampling frequency. With a sampling frequency of 1.023 MHz, a sampled PRN sequence with 1023 samples is generated. With a sampling frequency of 38.192 MHz, a sampled PRN sequence with 38192 samples is generated.
34 51 JANVIER, Thibault
35 52 JANVIER, Thibault
p=. !PRN_sequence.png!
36 52 JANVIER, Thibault
*Figure 5.2*: Example of a locally generated PRN sequence.
37 52 JANVIER, Thibault
38 53 JANVIER, Thibault
In Figure 5.2, a PRN sequence corresponding to satellite 21 has been generated with a sampling frequency of 38.192 MHz. This PRN sequence lasts 1 ms (a zoom has been done on the figure in order to see the sequence of chips).
39 52 JANVIER, Thibault
40 4 COLIN, Tony
---
41 4 COLIN, Tony
42 4 COLIN, Tony
h2. 2 - Acquisition.
43 1 COLIN, Tony
44 43 COLIN, Tony
*See the UML Diagram of Section 2 under :* attachment:"Acquisition.png"
45 43 COLIN, Tony
46 43 COLIN, Tony
The files involved are :
47 44 COLIN, Tony
- _Main_Acquisition.vi_ : attachment:"SnapAcquisition.PNG"
48 44 COLIN, Tony
- _Acquisition_subVI.vi_ : attachment:"SnapAcquisitionSub.png"
49 55 COLIN, Tony
- _CA_Code.vi_ : attachment:"SnapCACode.png"
50 43 COLIN, Tony
51 58 JANVIER, Thibault
In [[PART2|PART 3]], three different methods for acquisition have been described. Each of these methods exhibits different performances regarding the execution time and the accuracy of the frequency and code phase estimations. It has been explained that the parallel code phase search acquisition is less time consuming than the others. Likewise, it is more accurate regarding the estimation of the code phase, which will facilitate the convergence of the DLL towards its settling state. Thereby, the parallel code phase search acquisition will be implemented.
52 59 JANVIER, Thibault
53 57 JANVIER, Thibault
The parallel code phase search acquisition requires to search among the frequencies around the intermediate frequency of the raw signal because of the Doppler that is induced by the motion of the satellite relatively to the receiver. This searching process is finite, meaning that the range of ± 10 kHz around the intermediate frequency has to be swept with a frequency step. Such a frequency step induces a frequency mismatch between the actual frequency of the raw signal and the local frequency of the receiver. This frequency mismatch translates into losses during the acquisition process. A common GPS receiver exhibits 1 dB of loss during the acquisition process. Such a loss is reached provided that:
54 57 JANVIER, Thibault
55 57 JANVIER, Thibault
p=. !Loss_acquisition.png!
56 57 JANVIER, Thibault
57 57 JANVIER, Thibault
Where ∆f is the maximum frequency mismatch and T_coh is coherent time of integration.
58 57 JANVIER, Thibault
Given that we integrate over 1 ms, which corresponds to the period of one PRN sequence, the maximum frequency mismatch should be less than 500 Hz. By using a frequency step of 500 Hz, the searching process has to go through 41 different frequencies to cover the range of ± 10 kHz around the intermediate frequency and the maximum frequency mismatch is 250 Hz. Thus, the acquisition process exhibits less than 1 dB of losses.
59 1 COLIN, Tony
60 59 JANVIER, Thibault
The acquisition process is carried out over 1 ms which corresponds to the period of one PRN sequence. Nonetheless, the code phase is not known during the acquisition process. As a result, the integration time is likely to overlap over two actual incoming PRN sequences as it is illustrated on the following figure.
61 59 JANVIER, Thibault
62 60 JANVIER, Thibault
p=. !Acquisition_Duration.png!
63 60 JANVIER, Thibault
*Figure 5.3*: Acquisition - Integration duration.
64 60 JANVIER, Thibault
65 61 JANVIER, Thibault
As mentioned in [[PART|PART 2]], one navigation bit contains 20 PRN sequences and last 20 ms. As one does not know whether the acquisition process is aligned with an incoming PRN sequence, the acquisition might be carried out while there is a navigation bit transition. In that case, the result from the acquisition is erroneous and the receiver might consider that a satellite is not visible whereas it actually is (worst case when the navigation bit transition occurs at the middle of the acquisition process). To overcome this issue, the acquisition process is run twice on two successive milliseconds. By doing so, at least one acquisition iteration does not include any navigation transition bit.
66 59 JANVIER, Thibault
67 61 JANVIER, Thibault
Once the navigation bit transition has been taken care of, the acquisition is successful when a satellite is visible and one is provided with a coarse estimation of the carrier frequency of the GPS raw signal as well as its code phase. Nevertheless, the frequency error can be up to 250 Hz. Such an error cannot be handled by the PLL once the acquisition outputs are given as inputs to the tracking block. Consequently, the frequency estimation has to be refined. As the code phase is known with a good accuracy thanks to the acquisition, it is possible to remove it from the raw signal. If it is removed over 1 ms of the raw signal, there is no bit transition and only the carrier of the raw signal is left (see Figure 3.6 of [[PART2|PART 3]]). Then it is possible to refine the estimation of the carrier by computing the Fourier transform of the carrier.
68 1 COLIN, Tony
69 64 JANVIER, Thibault
The link and the figure below illustrate the front-panel of the Main_Acquisition VI.
70 62 JANVIER, Thibault
71 63 JANVIER, Thibault
p=. attachment:"Main_Acquisition_1.png"
72 1 COLIN, Tony
73 64 JANVIER, Thibault
p=. !Main_Acquisition_2.png!
74 64 JANVIER, Thibault
*Figure 5.4*: Acquisition results.
75 4 COLIN, Tony
76 4 COLIN, Tony
---
77 4 COLIN, Tony
78 1 COLIN, Tony
h2. 3 - Tracking.
79 45 JANVIER, Thibault
80 43 COLIN, Tony
*See the UML Diagram of Section 2 under :* attachment:"Track.png"
81 43 COLIN, Tony
82 44 COLIN, Tony
The files involved are :
83 1 COLIN, Tony
- _Main_Carrier_Tracking.vi_ : attachment:"SnapTracking.png"
84 44 COLIN, Tony
- _CalcLoopCoeff.vi_ : attachment:"SnapCalcLoopCoeff.PNG"
85 1 COLIN, Tony
86 66 JANVIER, Thibault
The theory of the PLL and DLL has been detailed in [[PART2|PART 3]]. In order to assess the carrier phase error and the code phase error, several discriminators can be used for the PLL and the DLL. The discriminators used for the PLL and the DLL are given in the following table:
87 4 COLIN, Tony
88 67 JANVIER, Thibault
p=. !Table_PLL_DLL_discriminators.png!
89 67 JANVIER, Thibault
*Table 5.1*: Choice of PLL and DLL discriminators.
90 67 JANVIER, Thibault
91 68 JANVIER, Thibault
The DLL aims at updating the code phase of the locally generated PRN sequences for them to be perfectly aligned with the incoming PRN sequences. A PRN sequence is supposed to span over 1 ms. Therefore, the updating process of the DLL will be carried out over 1ms and the signal will be cut into slots of 1ms for the computation. Nonetheless, because of the Doppler shift that is induced by the motion of the satellite relatively to the receiver, the duration of an incoming PRN sequence might vary around 1 ms. Thus, a phase shift is introduced by that Doppler shift on the next millisecond to process. The DLL analyses this phase shift and adjusts the size of the window (=slot) for the next iteration. This way, the updating process vary around 1 ms and keep track of the code phase.
92 69 JANVIER, Thibault
The following links and figure illustrate the front-panel of the Main_Tracking VI (/!\ The Main_Acquisition VI has to be run first in order to have the right inputs for the Main_Tracking VI).
93 1 COLIN, Tony
94 73 JANVIER, Thibault
p=. attachment:"Main_Tracking_1.png"
95 73 JANVIER, Thibault
attachment:"Main_Tracking_2.png"
96 73 JANVIER, Thibault
attachment:"Main_Tracking_3.png"
97 73 JANVIER, Thibault
!Main_Tracking_4.png!
98 73 JANVIER, Thibault
*Figure 5.5*: Retrieved Signal at the output of the Tracking Block of the SDR GPS receiver.
99 4 COLIN, Tony
100 4 COLIN, Tony
---
101 4 COLIN, Tony
102 4 COLIN, Tony
h2. 4 - Navigation Data decoding.
103 13 COLIN, Tony
104 70 COLIN, Tony
* From this Section, the work that has been done is additional to the project.*
105 70 COLIN, Tony
106 10 COLIN, Tony
*See the UML Diagram of Section 4 under :* attachment:"NavigationData.PNG".
107 29 COLIN, Tony
108 1 COLIN, Tony
h3. a - Delimiting subframes.
109 29 COLIN, Tony
110 55 COLIN, Tony
The files involved are :
111 44 COLIN, Tony
- _FindPreamble.vi_ : attachment:"SnapFindPreamble.png"
112 55 COLIN, Tony
- _TestFindPreamble.vi_ : attachment:"SnapTestFindPreamble.PNG"
113 44 COLIN, Tony
- _GenerateFrame.vi_ : attachment:"SnapGenerateFrame.png"
114 1 COLIN, Tony
- _ParityCheck.vi_ : attachment:"SnapParityCheck.PNG"
115 1 COLIN, Tony
116 70 COLIN, Tony
findPreamble.vi finds the first preamble occurrence *[1 -1 -1 -1 1 -1 1 1]* in the bit stream of each channel. The preamble is verified by check of the spacing between  preambles (6sec) and parity checking of the first two words in a subframe. At the same time function returns list of channels, that are in tracking state and with valid preambles in the nav data stream.
117 70 COLIN, Tony
This program has been tested via TestFindPreamble.vi for an arbitrary frame and perfectly delimits the subframes as it is depicted :
118 70 COLIN, Tony
119 23 COLIN, Tony
p=. !Preamble1.PNG! !Preamble2.PNG!
120 26 COLIN, Tony
121 29 COLIN, Tony
p((((. *Figure 5. :* Cross-correlation between navigation frame and local preamble. *Figure 5. :* Subframes with index of delimitation.
122 23 COLIN, Tony
123 70 COLIN, Tony
Still for certain real signals, false alarm of preamble occurence is detected even with protection such as the parity check and the 6sec distance.
124 70 COLIN, Tony
125 71 COLIN, Tony
p=. !RxyRealFrame.PNG!
126 71 COLIN, Tony
127 70 COLIN, Tony
It prevents to find correctly the delimitation of the different subframes : refinement need to be implemented for such cases.
128 70 COLIN, Tony
129 29 COLIN, Tony
h3. b- Decoding ephemeris and information within the frame.
130 29 COLIN, Tony
131 55 COLIN, Tony
The files involved are :
132 44 COLIN, Tony
- _Ephemeris.vi_ : attachment:"SnapEphemeris.png"
133 44 COLIN, Tony
- _BinaryArrayToDecimal.vi_ : attachment:"SnapBinaryArrayToDecimal.PNG"
134 44 COLIN, Tony
- _twosComp2dec.vi_ : attachment:"SnapTwosComp2dec.PNG"
135 4 COLIN, Tony
- _ParityCheck.vi__ : attachment:"SnapParityCheck.PNG"
136 4 COLIN, Tony
- _TestEphemeris.vi_
137 1 COLIN, Tony
138 70 COLIN, Tony
Function Ephemeris.vi decodes ephemerides and TOW from the given bit stream. The stream (array) in the parameter _Navigation bits_ must contain 1500 bits. The first element in the array must be the first bit of a subframe. The subframe ID of the first subframe in the array is not important.
139 70 COLIN, Tony
Extracted ephemeris data are as mentionned in PART 4.
140 70 COLIN, Tony
141 4 COLIN, Tony
---
142 43 COLIN, Tony
143 10 COLIN, Tony
h2. 5 - Elementary blocks for localization.
144 14 COLIN, Tony
145 4 COLIN, Tony
*See the UML Diagram of Section 5 under :* attachment:"Localization.PNG"
146 31 COLIN, Tony
147 1 COLIN, Tony
h3. a - Satellite position.
148 31 COLIN, Tony
149 55 COLIN, Tony
The files involved are :
150 44 COLIN, Tony
- _SatellitePosition.vi_ : attachment:"SnapSatellitePosition.png"
151 44 COLIN, Tony
- _TestSatellitePosition.vi_ : attachment:"SnapTest_satellite_position.PNG"
152 23 COLIN, Tony
- _Check_time.vi_ : attachment:"SnapCheckTime.PNG"
153 31 COLIN, Tony
154 70 COLIN, Tony
Computation of satellite coordinates X,Y,Z at _TransmitTime_ for given ephemeris _Ephemeris data_. Coordinates are computed for one satellite by applying the algorithm of PART 4.
155 70 COLIN, Tony
_TestSatellitePosition.vi_ goal is to test _SatellitePosition.vi_ with ephemeris data collected from GPS signal by University of Colorado *[2]* . The figure illustrates the computed satellite orbit level with respect to Earth radius level : 
156 70 COLIN, Tony
157 8 COLIN, Tony
p=. !SatPos.PNG!
158 32 COLIN, Tony
*Figure 5. :* Interface with ephemeris as input and illustration of the satellite position.
159 1 COLIN, Tony
160 32 COLIN, Tony
h3. b - Pseudoranges.
161 32 COLIN, Tony
162 8 COLIN, Tony
The file involved is :
163 33 COLIN, Tony
_PseudorangesComputation.vi_
164 1 COLIN, Tony
165 70 COLIN, Tony
PseudorangesComputation.vi finds relative pseudoranges for one satellite at the specified millisecond of the processed signal which has been stored in the file _absoluteSample_sat(i).txt_ after the tracking function. The pseudoranges contain unknown receiver clock offset. It can be found by the least squares position search procedure in the following.
166 70 COLIN, Tony
167 33 COLIN, Tony
h3. c - Least Square solution for position determination.
168 33 COLIN, Tony
169 55 COLIN, Tony
The files involved are :
170 44 COLIN, Tony
- _LeastSquarePosition.vi_ : attachment:"SnapLeastSquare.png"
171 44 COLIN, Tony
- _SatelliteRotationECEF.vi_ : attachment:"SnapSatelliteRotationECEF.PNG"
172 44 COLIN, Tony
- _toTopocentric.vi_ : attachment:"SnaptoTopocentric.PNG"
173 55 COLIN, Tony
- _CartesianToGeodetic.vi_ : attachment:"SnapCartesianToGeodetic.PNG"
174 1 COLIN, Tony
- _TroposphericCorrection.vi_ : attachment:"SnapTropospheric.PNG"
175 43 COLIN, Tony
176 70 COLIN, Tony
Function LeastSquarePosition.vi calculates the Least Square Solution based on the demonstration of PART 2 - Section 2.
177 70 COLIN, Tony
 Moreover it is also necessary to apply different change of coordinates system such as _toTopocentric.vi_ , _CartesianToGeodetic.vi_ and rotation _SatelliteRotationECEF.vi_ 
178 70 COLIN, Tony
Finally, the tropospheric correction for pseudoranges accuracy is computed in this part.
179 70 COLIN, Tony
180 70 COLIN, Tony
Hint for coordinate systems :
181 71 COLIN, Tony
182 71 COLIN, Tony
p=. !ECEF.png!
183 70 COLIN, Tony
ECEF (Earth-Centered, Earth-Fixed)
184 71 COLIN, Tony
185 72 COLIN, Tony
p=. !GeodeticSphere.PNG!
186 70 COLIN, Tony
Flattened Sphere for reference ellipsoid coordinates (Geodetic)
187 70 COLIN, Tony
188 43 COLIN, Tony
h2. 6 - Receiver position computation.
189 43 COLIN, Tony
190 43 COLIN, Tony
*See the UML Diagram of Section 6 under :* attachment:"Receiver.PNG"
191 43 COLIN, Tony
192 55 COLIN, Tony
The files involved are :
193 55 COLIN, Tony
- _ComputeReceiverPosition.vi_ : attachment:"SnapReceiver.png"
194 44 COLIN, Tony
- _NavigationProcess.vi_ : attachment:"SnapNavigationProcess.PNG"
195 55 COLIN, Tony
- _CartesianToGeodeticForUTM.vi_ : attachment:"SnapCartesianToGeodeticForUTM.PNG"
196 43 COLIN, Tony
- _CartesianToUTM.vi_ : attachment:"SnapCartesianToUTM.png"
197 43 COLIN, Tony
198 70 COLIN, Tony
For the sake of structuring, a block _NavigationProcess.vi_ includes _FindPreamble.vi_ , _Ephemeris.vi_ , _SatellitePosition.vi_ in order to fine-tuned the process for one satellite from frame delimitation to satellite position determination.
199 70 COLIN, Tony
_ComputeReceiverPosition.vi_ is the *main VI* . It combines navigation process of different satellite, loops on pseudoranges measurement points and deduces receiver position via Least Square VI and the associated DOP (Dilution Of Precisions).
200 70 COLIN, Tony
Then, the receiver coordinates are converted into UTM coordinates with _CartesianToGeodeticForUTM.vi_ and _CartesianToUTM.vi_
201 70 COLIN, Tony
202 70 COLIN, Tony
Hint for UTM coordinate system :
203 70 COLIN, Tony
204 71 COLIN, Tony
p=. !UTMzones.jpg!
205 70 COLIN, Tony
206 2 COLIN, Tony
h2. 7 - Complete UML Diagram of the receiver.
207 41 COLIN, Tony
208 6 COLIN, Tony
*The UML diagram with real size is available under :* attachment:"UMLDiagram.png"
209 40 COLIN, Tony
210 40 COLIN, Tony
Here is a small overview of the structure :
211 40 COLIN, Tony
212 40 COLIN, Tony
p=. !UMLDoverview.PNG!
213 2 COLIN, Tony
214 3 COLIN, Tony
---
215 3 COLIN, Tony
216 3 COLIN, Tony
*References :* 
217 1 COLIN, Tony
*[1]* K. Borre, D. M. Akos, N. Bertelsen, P. Rinder, S. H. Jensen, A software-defined GPS and GALILEO receiver
218 70 COLIN, Tony
*[2]* http://www.colorado.edu/geography/gcraft/notes/gps/ephclock.html