GNSS Receivers, Introduction

Transcription

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

2 Recap on GNSS GNSS satellites GPS Block 2R, Lockheed Martin Time tags Ephemerides Ionosphere data Almanac etc. L1, E1, L2, L5, E5, Uplink GNSS control station GNSS monitor stations GNSS: GPS Galileo GLONASS Compass 2013 Danish GPS Center 2

3 Global Navigation Satellite Systems GPS (II 1980) GLONASS (1993) COMPASS Galileo ~ s-90 s the first professional, GPS + GLONASS receivers 2011 launch year for the first consumer, mobile phone GPS + GLONASS receiver chips (from Qualcomm, Broadcom, ST- Ericsson, u-blox and others) Danish GPS Center 3

4 GNSS Receiver Structure Visualization Mapping Other applications User GNSS application Coord. transformation (optional) NMEA 0183 or binary interface GNSS receiver Coordinates transformation Position computation Tracking Acquisition 2013 Danish GPS Center 4

5 TTFF Stand Alone Case Receiver activation Code phases of received signals are known: relative measurements are available TOW is known: pseudoranges can be computed (receiver clock error is unknown) Satellite positions can be computed: receiver position can be computed From seconds to minutes 0 6s From 18 to 30 seconds time Signal acquisition Tracking lock Bit sync. Frame sync.* Collection of ephemeris data* * May require multiple reception of the same data when BER is high (for weak signals) 2013 Danish GPS Center 5

6 Receiver Functional Blocks GNSS antenna(s) GNSS Receiver Other sensors (IMU, baro etc.) Display, keyboard Amplifier RF front-end(s) Mixer Frequency synthesizer Receiver clock A/D Correlators (channels) Carrier generators Code generators Power supply Receiver, navigation processor Battery powered clock and memory 2013 Danish GPS Center 6

7 GNSS Antenna Types Consumer Professional Professional for very high precission applications Antenna arrays are used for special applications (military, airports and other) 2013 Danish GPS Center 7

8 The RF Front-end GNSS antenna(s) Amplifier RF front-end(s) Mixer Frequency synthesizer Receiver clock GNSS Receiver A/D Converts RF GNSS signals to a lower frequency Display, keyboard digital signals Other sensors (IMU, Baro etc.) Correlators (channels) Generates clock signals that are Receiver, used in navigation all aspects processor of a GNSS receiver Carrier generators Code generators Varying Battery possibilities powered for configuration clock and of memory the signal reception chain Power supply 2013 Danish GPS Center 8

9 GNSS Receiver Clock Receiver clock is used for local signal creation and for time keeping (receiver time) Crystal oscillators have good short term stability, low phase noise There is some sensitivity to temperature changes Various types of crystal oscillators exist, but the most popular for GNSS applications are TCXO temperature-compensated crystal oscillator (0.5-3ppm typically are used for GPS) OCXO oven-controlled crystal oscillator (~0.001ppm) Chip scale atomic clock is a new product in the market with the best precision in this size class (< ppm) 2013 Danish GPS Center 9

10 Consumer Receivers Primary concerns Low cost Low power Small size That it works everywhere Easy integration Precision typically <3-10m 2013 Danish GPS Center 10

11 Receivers For Surveying Typically these are dual frequency devices with dedicated antennas Always used in DGPS mode Today typically a network of receivers is employed for DGPS Precision is about 1cm, but can be also <1mm Price is about 10000$ 2013 Danish GPS Center 11

12 An Example From AAU Station 2013 Danish GPS Center 12

13 Aviation The main information of interest is horizontal position GNSS vertical position is not so good and an airplane has more precise altitude instruments The main difference from an ordinary GNSS receiver a certified aviation grade GNSS receiver must reliably indicate when it is not reliable Airports may have their own monitoring receivers 2013 Danish GPS Center 13

14 Aalborg EGNOS Station 2013 Danish GPS Center 14

15 Receivers for Space Applications Receiver architecture: Flexible configuration of up to 24 tracking channels L1 and L2 frequencies, C/A code, P(Y) code Interfaces: UART (RS-422) TC/TM interface MIL-STD-1553B TC/TM interface extension possible 1 PPS output (RS-422) Secondary power interface 2 antenna inputs Navigation Solution Accuracy: Position (1σ, 3d): <5 m Velocity (1σ): <1 cm/s time offset 1PPS (1σ)?: < 50 ns Time to first fix: Hot start < 5 s Warm Start < 90 s Cold start < 20 min. Physical / Environment: Size: 300x240x50 mm Weight: < 1,3kg Operating temperature: -25º C to +60º C Radiation: Cumulative dose >20 krad (Si) Power Consumption: < 8W (TBC) 2013 Danish GPS Center 15

16 Receivers for Space Applications Low cost 12 Channel L1 C/A code space GPS receiver for small satellites for $17,900 Mass: 20g (40g with screens) Design lifetime (LEO): 7 Years Radiation Tolerance: >10kRad (Si) Total Dose Interfaces : Serial data interface; Pulse-per-second Position to 10m (95%) Velocity to 15cm/s (95%) Typical TTFF (warm) 50s Typical TTFF (cold) 550s 5V Supply, 0.8W 70 x 45 x 10mm 2013 Danish GPS Center 16

17 Marine Applications Used for position, heading, rate of turn, speed and other measurements in the sea and in the ports Usually part of a total navigation system of the ship (INS, compass, autopilot, AIS etc.) DGPS, and Kalman filters are often used 2013 Danish GPS Center 17

18 Timing Receivers For science For network synchronization Internet, mobile and other Can synchronize Time Frequency Can be combined with additional clocks in the same package 2013 Danish GPS Center 18

19 Alternative Systems Research and development continues how to adapt or modify existing systems to provide positioning services Legacy ground based systems (no perspectives) WiFi (very limited precision capabilities) Mobile Networks (does not meet today s GNSS precision level; new protocol versions are under development) TV (DVB) signals based Proprietary, local (for example LOCATA) New methods based on GPS+LEOS (for example Boeing Timing & Location) 2013 Danish GPS Center 19

20 GNSS Measurements 2013 Danish GPS Center 20

21 Tasks To Perform For Position Computations In a GNSS Receiver Acquire GNSS signals Track all acquired satellite signals Decode the navigation messages from all satellites Do measurements of transmission time (in other words take snapshots of all signal, time tracking counters) Correct transmission time Compute satellite position at transmission time Compute pseudoranges to all satellites Compute the receiver position based on pseudoranges and satellite positions (include compensations for various signal delays) 2013 Danish GPS Center 21

22 Range And Pseudorange The true (geometrical) range between satellite k and receiver i is denoted as ρ i k The range can be expressed through satellite signal travel time (in GPS time) k i c( t t The receiver can measure only the sum of the true range and the signal delay i k ) P k i Geometrical range k k i c( dti dt ) Clock errors Troposphere delay T k i I k i e Ionosphere delay k i Other errors (including multipath) 2013 Danish GPS Center 22

23 GNSS Signal The GNSS signal is also a ruler For GPS case: 1 sub-frame = 300 bits = 6 sec 1 bit = 20 spreading codes 1 spr. Code = 1023 chips = 1 ms 1 spr. Code 300 km 1 chip 1μs (at Mc/s) 300 m 1 carrier wave at 1.5 GHz = m Data frames and sub-frames Contained data: Satellite status (health etc.) Satellite clock corrections Satellite coordinates (ephemerides) Ionosphere correction GNSS to UTC time conversion Almanac The Speed of Light m/s Subframe start mark (preamble) and a time tag 2013 Danish GPS Center 23

24 Data For Pseudorange Measurements The pseudorange measurement procedure must know when a GNSS signal was transmitted Therefore navigation data processing must provide information at which code start a subframe was detected and what is TOW of that subframe Number of complete bits Number of complete codes Number of chips TLM HOW Navigation data of the sub-frame t k = s GPS time 2013 Danish GPS Center 24

25 Time of Transmission All satellites transmit signals at the same time Due to different distances between receiver and satellites the GPS (GNSS) signals will arrive at receiver at different time instances CH: 1 CH: 2 CH: 3 CH: 4 t 1 = s t 4 = s GPS time t 2 = s t 3 = s 2013 Danish GPS Center 25

26 Position Basic Computation Ephemerides P k i k k i c( dti dt ) T k i I k i e k i Satellite clock correction dt k and position computation dt k Satellite coordinates k i c( t t i k ) Measurements t k at t i = t GPS + dt i Pseudorange construction P k i = (t i - (t k + dt k )) c (may also include DGPS and other corrections) LS or Kalman filter Solution X, Y, Z dt i Velocity Ext. measurements 2013 Danish GPS Center 26

27 Receiver Measurements A GNSS receiver does a number of measurements Signal time of transmission (code) measurements Carrier phase based measurements Doppler based measurements Signal to noise ratio measurements RF interference measurements Some receivers can estimate multipath Professional receivers do measurements on two or more carrier frequencies Measurements on all channels are done at the same time instance (epoch) 2013 Danish GPS Center 27

28 Ambiguity Resolution Computational Models (1) A one-way code observation on frequency L1 between receiver i and satellite k is characterized by: P k 1,i = ρ k i + c dt i dt k + T k i + I k i + noise. A single difference of code observations on L1 between two receivers i and j: P k 1,i P k 1,j = + T k k i + I i ρ k i + c dt i dt k ρ k j c dt j dt k T k k j I j + noise = ρ k i ρ k j + c dt i dt j + T k i T k j + I k i I k j + noise. Note that the satellite clock offset term dt k cancels! 2013 Danish GPS Center 28

29 Ambiguity Resolution Computational Models (2) Next we calculate the double difference between two receivers i and j and two satellites i and l: P k k l l 1,i P 1,j P 1,i P 1,j = ρ k i ρ k j + c dt i dt j + T k i T k j + I k k i I j ρ i l ρ j l + c dt i dt j + T i l T j l + I i l I j l ρ i k ρ j k ρ i l + ρ j l +T i k T j k T i l + T j l + I i k I j k I i l + I j l + noise Or with obvious notation kl P 1,ij kl = ρ 1,ij kl + T 1,ij kl + I 1,ij + noise. + noise = We observe that all clock offsets cancel in a double difference! That was exactly the purpose of making this linear combination of the four observed one-ways Danish GPS Center 29

30 Ambiguity Resolution Computational Models (3) The standard deviation of a C/A-code observation is σ C/A code = 3 m and that one of a P-code observation σ P code = 0.3 m. Geodetic GPS receivers additionally observe the phase of the carrier wave. A phase observation Φ i k t is the difference in phase from a signal generated at the same frequency as pseudorange and multiplied by the wave length λ. The basic equation is Φ i k t = ρ i k I i k + T i k + c dt i t dt k t τ i k + λ φ i t 0 φ k t 0 + λn i k + noise Danish GPS Center 30

31 Ambiguity Resolution Computational Models (4) The new terms are the ambiguitiesn i k between satellite k and receiver i, and the nonzero initial phases φ k t 0 and φ i t 0. Finally the double difference for the phase is kl Φ 1,ij kl = ρ 1,ij I kl ij + λ 1 N k 1,i l N 1,i N k 1,j l N 1,j The standard deviation of a phase observation is σ phase = 3 mm. + noise. The important observation to make is that in double differences N kl ij is an integer. Knowing the correct integer and with phase σ phase = 3 mm it becomes possible to estimate the baseline between i and j at centimeter level Danish GPS Center 31

32 GNSS Distance Measurement Errors 2013 Danish GPS Center 32

33 GNSS Signal Propagation Signal generation GNSS Satellite Amplifier Antenna Signal changes during propagation: Attenuations Frequency and phase offsets Signal delays Reflections Free space Atmosphere Ionosphere Troposphere Antenna GNSS Receiver Amplifier Danish GPS Center 33

34 Measurement Corrections Models are used in standalone case Differential GPS is using measurements from a second GPS receiver (called the base station) to correct the distance measurements. Professional user can use a network of GPS base stations (monitoring stations) WAAS provides Ionosphere correction data and also DGPS type corrections WAAS monitoring stations use also other types of atmosphere measurements A-GPS can provide DGPS corrections 2013 Danish GPS Center 34

35 The Menu of Future GNSS Signals Originally GPS and GLONASS had one signal on one carrier for civil applications Future GNSS offer system diversity and frequency diversity System GLONASS Signal L1 Carrier frequency [MHz] COMPASS B Galileo E GLONASS L1 OC/SC GPS L Galileo E COMPASS B GLONASS L2 GPS L Component Type Data rate [sps/bps] Modulation Chipping rate [Mcps] Code length [chips] OF standard Data -/ BPSK SF high accur. Military 5.11 B1-CD 100/50 Open B1-CP -/- B1 Authorized 100/50 -/- MBOC(6,1,1/11) BOC(14,2) A PRS cosboc(15,2.5) Full length [ms] B Data, SOL 250/ CBOC(6,1,1/11) C Pilot, SOL -/ * C/A Data -/50 BPSK P(Y) BPSK days 7 days Military M BOC(10,5) A PRS cosboc(10,5) B Data 1000/500 BPSK(5) C Pilot -/ * B3 B3-AD Authorized 100/50 B3-AP -/- -/500 QPSK(10) BOC(15,2.5) L2 CM Data 50/25 or -/50 TM and BPSK L2 CL Pilot -/ P(Y) BPSK days 7 days Military M BOC(10,5) GNSS L1 (carrier) spectrum Figure source GPS World GLONASS L3 OC QBSK(10) GLONASS L3 OF/SF OF standard Data -/ BPSK SF high accur. Military COMPASS B Galileo E5 ( ) E5a E5b GPS L B2aD 50/25 B2aP -/- B2bD Open 100/50 B2bP -/- a-i Data 50/25 AltBOC(15,10) * a-q Pilot -/ * AltBOC(15,10) b-i Data, SOL 250/ * 4 4 b-q Pilot, SOL -/ * I Data 100/50 1 QPSK Q Pilot -/ Danish GPS Center 35

36 Space Based Augmentation Systems Wide Area Augmentation System (WAAS), USA European Geostationary Navigation Overlay Service (EGNOS) System for Differential Correction and Monitoring (SDCM), Russia GPS And Geo-Augmented Navigation (GAGAN) system, India Quasi-Zenith Satellite System (QZSS), Japan Multi-functional Satellite Augmentation System (MSAS), Japan 2013 Danish GPS Center 36

37 Receiver Changes I More frequency bands (radio front-ends), but not all may be needed for all applications Extra channels required More complex channels than for GPS Galileo memory codes need extra resources Legacy GLONASS is using FDMA and a different approach to satellite position data GPS and Galileo have compatible satellite ephemerides representation and computation More complex navigation data processing (FEC, interleaving etc.) 2013 Danish GPS Center 37

38 Receiver Changes II More navigation information will be transmitted More detailed inter-signal delay information Open signals authentication for Galileo was under consideration Differential corrections More powerful CPU is needed due to extra channels and more complex PVT computation A likely consequence of increased complexity the receiver will require more power This is somehow alleviated by other receiver technology improvements 2013 Danish GPS Center 38

39 Low Cost GNSS Receivers Device cost, size, power consumption and integration price reduction are the primary drivers Part of the receiver market are solutions for car navigation some receivers come with INS integration features INS integration is likely to spread also in other markets 2013 Danish GPS Center 39

40 Professional Receivers More added features, communication possibilities, service integration Receivers exploit the system and frequency diversity already today Continuation (relatively slow) of size and cost reduction Tendency to contain a full set of channels per GNSS system for maximum performance 2013 Danish GPS Center 40

41 GNSS Development Schedule GPS II Test & deploym. of L2C, staged roll-out of CNAV Galileo launch Sys. testbed v1/v2 IOV Deployment GLONASS-M (launched until 2012) Test & deployment of L5 Test & deployment GPS III GLONASS Full Operational Capability (FOC) GLONASS-K1 New: L3OC (CDMA), SAR SDCM design/tests Galileo operational 18 SV OC L2C Full Operational Capability (FOC) Test & deployment of L1C L5 FOC GLONASS-K2 (KM after 2015) New: L1OC, L3OC, L1SC, L2SC (CDMA), SAR SDCM fully deployed L1C FOC GPS III FOC New signals FOC In the first decades GPS and GLONASS evolved slowly In the last decade the development of space and ground control segments is intense COMPASS COMPASS 1 (end date unknown) COMPASS 2 test & depl. COMPASS 2/3, regional service; global service depl. COMPASS 3 FOC Danish GPS Center 41

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

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