GPS and other GNSS signals GPS signals and receiver technology MM10 Darius Plausinaitis

GPS Signals MM10-MM15  MM10 | GPS and other GNSS signals  MM11 | GPS signals - Code Generation and Carrier Generation  MM12 | GPS signals - Acquisition of the GPS Signal  MM13 | GPS signals - Code Tracking and Carrier Tracking  MM14 | GPS signals - Navigation Data Decoding  MM15 | GPS signals - Calculation of Pseudoranges and Positions

Today's Subjects  GPS Signal – Codes, carriers, navigation data – Signal Bandwidth  Overview of today's and future GNSS signals  Spread Spectrum Technique – PRN Codes – Correlation and other signal properties  GPS Signal Generation

GPS Signals  Transmission frequencies: – L1 = MHz = 154 x MHz – L2 = MHz = 120 x MHz – (Upgrade) L5 = MHz = 115 x MHz (for civil use) – (Upgrade) New military signal (M-code) and a new civil signal (L2CS)

GPS Signal Spectrum

GPS signal  C/A codes – Chipping rate of Mcps – Length of 1023 chips – Chip duration ~ 1µs ~ wave length 300 m – Repeats every millisecond – 32 different sequences assigned to GPS satellites  P(Y) codes – Chipping rate of Mcps – Length ˜10 14 chips – Chip duration ~ 0.1µs ~ wave length 30 m – Repeats every week – Anti-spoofing

GPS Navigation Data  Bit-rate of 50bps  Ephemerides  Satellite clock information  Satellite health and accuracy  Almanac  Repeated every 12.5 minutes  More details in MM14

Other GNSS signals

WAAS and EGNOS  Provide facilities to obtain better position accuracy by: – Correction of ephemeredes errors – Providing more accurate Ionospheric model  GPS C/A type signals (same modulation, frequency and spreading codes)  Much higher data rate (500sps bps)  Forward Error Correction  Much lower Doppler (<210Hz instead of 5kHz)  EGNOS is designed as a support system for GALILEO

Galileo  More signals transmitted on each frequency (comparing to today’s GPS)  Longer spreading codes  Data less signals  BOC modulation  Forward Error Correction  Block Interleaving (bit scattering) - to make the long data losses manageable.  Uplink emergency signal

GLONASS  Two frequencies  More accurate comparing to GPS in stand alone applications.  Separate carrier frequency per satellite.  Mcps civil signal and 5.11 Mcps military spreading codes  12 satellites operating

GNSS signals – today and future  Relative locations of GNSS signals GPSGALILEOGLONASS E5a E5b L5 E5 L2 E6L1E2E1 L MHz L MHz L2 EGNOS and WAAS are using GPS L1 GNSS systems:

Direct Sequence Spread Spectrum (DSSS) technique

DSSS Technique  Used for Code Division Multiple Access (CDMA) systems: – All users transmit on the same frequency – The frequency spectrum of the signal is spread with a noise like code – Spreading codes have very low cross-correlation and are unique for every user – Transmission bandwidth is much higher than information bandwidth (but several users can share the same band) – Resists jamming – Very low interference with other signals because of large bandwidth and low power

Pseudo Random Noise (PRN)  Noise-like properties  Very low cross-correlation with other signals  PRN sequences (codes) are almost orthogonal  High auto correlation only at 0 lag and very low cross correlation  PRN codes are created by shift registers of length n  Length of PRN sequence is calculated as: N DS = 2 n -1

Spreading operation  Data signal is multiplied by a PRN code (XOR operation for binary signals)  The result signal has PRN like properties  An example of a spreading operation and the BPSK modulation: 1 bit period1 chip period Data bits DSSS code chips Data * DSSS code Carrier Carrier after BPSK

Frequency spectrum plot  Wide band signal is less affected by narrow band interferences  A high power narrow band interferences are spread by the de-spread operation (at the receiver) to low power high bandwidth interference.  Hard to detect DSSS type signal without correct codes  Hard to jam

Encoding / Decoding Multiplication- Spreading Multiplication- De-spreading or Correlation PRN code generator Narrow band signal Wideband signal

Block diagram of signal generator

Literature       Read Interface Control Documents for detailed description of the GNSS signals

Questions and Exercises