GNSS Receivers, One Step Deeper

Transcription

1 DANISH GPS CENTER GNSS Receivers, One Step Deeper Kai Borre, Head of DGC Darius Plaušinaitis Danish GPS Center, Aalborg, Denmark

2 The Signal Reception Problem The GNSS signal can be received only when: The frequency of the local carrier replica matches the frequency of the carrier in the received signal The PRN replica code is well aligned in time to the PRN code in the received signal There are number of parameters, that influence how precisely these signals must match to obtain desired processing qualities Incoming signal Integrator () 2 Correlation result Carrier wave replica PRN code replica 2013 Danish GPS Center 2

3 How Carrier Correlation Works Correlation Danish GPS Center 3

4 How Code Correlation Works Incoming code Generated code Correlation Danish GPS Center 4

5 Receiver Channel States An example of basic receiver states and transitions between states Examples of additional states (or state flags): re-acquisition, PLL lock, bit synchronization, frame synchronization, ephemerides received, high dynamics, data wipe-off, 2013 Danish GPS Center 5

6 GNSS Signal Acquisition Purpose of acquisition Find satellites (signals) visible to the receiver Estimate coarse value for C/A code phase Estimate coarse value for carrier frequency Refine carrier search result if it is needed for the chosen tracking (receiver) design Acquisition in high sensitivity receivers might also find bit boundaries The search space can be reduced if the receiver has some a priori knowledge about visible GNSS signals Re-acquire signals if tracking was interrupted 2013 Danish GPS Center 6

7 GNSS Signal Acquisition Correct values of the code phase (signal alignment in the time domain) and the carrier frequency will yield a high correlation between the locally generated signal and the received GNSS signal 2013 Danish GPS Center 7

8 Weak Signal Acquisition The weak signal acquisition process is an extension of the basic acquisition: Coherent integration period is increased Non-coherent integration period is increased M () 2 M () 2 K Correlation result 2013 Danish GPS Center 8

9 Non-Coherent Acquisition Non-coherent acquisition snapshot was made by student group 1049 (2005) 2013 Danish GPS Center 9

10 Weak Signal Acquisition Aids Additional Information that reduces search space Precise GNSS time Approximate position Ephemerides (or at least almanac data) Present GNSS signal parameters (Doppler etc.) Hardware acceleration Multiple physical correlators limited application A classical GPS channel is using 2-3 complex correlators Other algorithms (parallel processing, FFT etc.) SirfStarIV correlators u-blox 6 over 2 million effective correlators 2013 Danish GPS Center 10

11 Carrier Tracking Loop I = 1 2 D( n)cos( φ ) D( n)cos( ω n) if cos( ω n + φ) if sin( ω n + φ) if φ = tan 1 I Q Q = 1 D( n)sin( φ ) Danish GPS Center 11

12 Code Tracking Idea Received code Early Prompt Late Locally generated copies of the code 1 Correlation Delay in chips, time 2013 Danish GPS Center 12

13 Noncoherent DLL Integrate & dump I E I Integrate & dump I P Integrate & dump I L Incoming signal Local oscillator E P L PRN code generator E P L Inputs for the discriminator 90 Integrate & dump Q E Q Integrate & dump Q P Integrate & dump Q L 2013 Danish GPS Center 13

14 Tracking Results Output from the 6 correlators, when the the tracking is locked Amplitude 3 x 107 Inphase Code Correlators I L 2 I P 2 I E Time (ms) Q prompt Discrete-Time Scater Plot Amplitude 3 x 107 Quadrature Code Correlators Q L 2 Q P 2 Q E I prompt Time (ms) 2013 Danish GPS Center 14

15 Tracking Errors Due To Multipath The multipath signal is a delayed and attenuated copy of the direct signal. There can be several (M) multipath signals. M x( t) = A ( t) D( t τ i ( t)) C( t τ i ( t))cos(2π ( f0 i= 1 + v ( t)) + ϕ ( t)) i i i + n( t) The figures show the constructive and destructive interference of just one multipath signal 1 1 Correlation 0.5 Correlation Delay in chips 0 Delay in chips Danish GPS Center 15

16 GNSS Signal Bandwidth and the Measurement Precision Relation Frequency Frequency 2013 Danish GPS Center 16

17 Receiver Tracking Channel I Integrate & dump Integrate & dump I E I P Output (nav. data bit stream) Integrate & dump I L E P L Incoming signal Q PRN code generator E P L Code loop filter Integrate & dump Integrate & dump Q E Q P Code loop discriminator Output (code phase and count of complete codes) 90 Integrate & dump Q L Local oscillator Carrier loop filter Carrier loop discriminator Output (carrier phase) 2013 Danish GPS Center 17

18 Examples Of Raw Nav. Data An example of a strong signal. The bit transitions are clearly visible. Q prompt Discrete-Time Scatter Plot I prompt Prompt I output (strong signal) Time (ms) An example of a weak signal. The bit transitions are not so clear. Q prompt Discrete-Time Scatter Plot I prompt Prompt I output (weak signal) Time (ms) 2013 Danish GPS Center 18

19 An Example Of a GPS Sub-frame 2013 Danish GPS Center 19

20 First Words Of a Subframe 2013 Danish GPS Center 20

21 GPS Navigation Data Contents 2013 Danish GPS Center 21

22 Error Detection And Correction Three types of techniques that deal with bit errors in transmitted/received signals: Error detection: CRC, parity check Error detection an correction: parity check, FEC Techniques to mitigate loss or corruption of a series of bits (burst errors): block interleaving 2013 Danish GPS Center 22

23 An Example Of Interleaved Data Corruption , , , , , , Deinterleaving, , , , , , , , , , 2013 Danish GPS Center 23

24 GNSS Software Defined Radios (SDR) And Other Alternatives 2013 Danish GPS Center 24

25 Basic Facts Radio communication today: multi-standard, multi-frequency communication in a single, low power, compact device E.g. today s mobile phone use Bluetooth, GSM (3 bands), GPRS, EDGE, 3G, 3.5G, 4G, WLAN, GPS, FM, DVB and more Devices continue to become smaller A need for fewer hardware components This means that the hardware in the device must be reused for several different purposes Today s devices have powerful DSP capabilities Intelligent radios need to handle all this 2013 Danish GPS Center 25

26 Basic Facts (GNSS Receivers) GNSS positioning is also becoming multi-standard and multi-frequency GPS II, GPS modernizations: M code and L2C, L5 signals, L1C(GPS III) Galileo GLONASS + modernized signals QZSS (Japan), IRNSS (India), and Beidou (China) SBAS systems: EGNOS, WAAS, MSAS, GAGAN Today GNSS receivers often are part of devices which have other radios too (hardware reuse) 2013 Danish GPS Center 26

27 GNSS SDR Partitioning Hardware Traditional Receiver Radio front-end (analogue) ADC Correlators (Channels) Channel loop closure, Positioning Position Software Defined Receiver Radio front-end (analogue) ADC Correlators (Channels) Channel loop closure, Positioning Position Ideal Software Receiver ADC Correlators (Channels) Channel loop closure, Positioning Position Software 2013 Danish GPS Center 27

28 Solutions To use general hardware per new signal or ASIC (Application Specific Integrated Circuit) To use reconfigurable hardware FPGA (Field Programmable Gate Array), etc. To use DSP (Digital Signal Processor) To use a general purpose processor (CPU) x86, ARM, etc Danish GPS Center 28

29 Comparison of Solutions Performance Unit Price ASIC FPGA DSP GPP ASIC FPGA GPP DSP Flexibility Power consumption The figures show only a general, rough picture. Other issues to consider in platform choice: development time, development cost, development tools, learning curve. Today DSPs are squished from both sides by GPPs and FPGAs 2013 Danish GPS Center 29

30 SDR Practical Conclusions Very, very flexible Matlab enables to write and test algorithms very quickly (real life example: 6-8 lines in Matlab vs lines in VHDL) Can be very slow (e.g. pure Matlab version) Matlab version of a GPS receiver is a few hundred times slower than real-time Matlab & C code is close to real time (4 channels on a fast PC) Real-time GNSS SDR implementations exist (written in C) for embedded and for PC applications. A very fast PC can process about 5 x 12 channels in real-time (2011) 2013 Danish GPS Center 30

31 SDR Advantages A very convenient educational tool Quick prototyping A demo acquisition for Galileo in less than an hour Students have converted the GPS SDR to EGNOS and Galileo SDRs in ~6 month SDR enables alternative positioning methods (e.g. non-real time) Easy exploration of particular signal cases (anomalies) or algorithms because the GNSS signal record can be replayed again and again 2013 Danish GPS Center 31

32 GNSS Snapshot Idea GNSS antenna Data Aiding DGPS SBAS Multi-frequency, multisystem GNSS receiver Compact, low power snapshot device (rover) RF front-end Amplifier Mixer Frequency synthesizer A/D Wireless or other type data delivery Software that does GNSS signal processing, derives measurements and does the actual position computation PVT solution Receiver clock Signal recording Additional tasks can be precision improvement or GNSS signal validation Memory 2013 Danish GPS Center 32

33 GNSS Snapshot Technique The rover devices can be a low power type devices (on the opposite the ordinary GPS is a very power consuming device) The rower device is relatively GNSS system independent, and GNSS modernization independent The server software implements nearly all GNSS system dependent signal processing parts one place to update system capabilities The server software can have more time, power and also other types of resources to do position estimation The server software can implement signal authentication, validation checks using all available resources Usually not for true real-time applications 2013 Danish GPS Center 33

34 SDR Demo 2013 Danish GPS Center 34

35 Current Receiver Development Future Research and development High sensitivity Multi-system & multifrequency receiver Multipath mitigation Further SDR development GNSS integrity Integration of other kinds of positioning 2013 Danish GPS Center 35

36 The ML507 Setup GNSS front-end Battery adapter 2013 Danish GPS Center 36

37 Simulink Model The adaptor block inside calls nearly unmodified C code of the FPGA receiver 2013 Danish GPS Center 37

38 Matlab SDR Plots 15 Acquisition results Not acquired signals Acquired signals Acquisition Metric PRN number (no bar - SV is not in the acquisition list) Correlation Real correlation result from GNSS SDR Correlation Samples (time) Theoretical correlation Code Offset [chips] x Danish GPS Center 38

39 Matlab SDR postprocessing.m script probesignal.m tracking.m acquisition.m plottracking.m USB driver (C) Recorder application (C, C++) postnavigation.m findpreambles.m ephemeris.m satpos.m leastsquarepos.m Coordinate transformations Signal record file (1 byte per sample) plotnavigation.m 2013 Danish GPS Center 39

40 Commercial DGPS Performance 2013 Danish GPS Center 40

41 SDR Modification For Galileo One sub-frame 2013 Danish GPS Center 41

42 Thank You For Your Attention DANISH GPS CENTER Danish GPS Center 42

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