Transcript GPS and other GNSS signals

GPS and other GNSS signals
GPS signals and receiver technology
MM10
Darius Plausinaitis
[email protected]
GPS Signals MM10-MM15
 MM10 | GPS
 MM11 | GPS
Generation
 MM12 | GPS
 MM13 | GPS
 MM14 | GPS
 MM15 | GPS
Positions
and other GNSS signals
signals - Code Generation and Carrier
signals
signals
signals
signals
- Acquisition of the GPS Signal
- Code Tracking and Carrier Tracking
- Navigation Data Decoding
- Calculation of Pseudoranges and
http://gps.aau.dk/educate/receiverTechnologyPart3.htm
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 = 1575.42 MHz = 154 x 10.23 MHz
– L2 = 1227.6 MHz = 120 x 10.23 MHz
– (Upgrade) L5 = 1176.45 MHz = 115 x 10.23 MHz (for civil use)
– (Upgrade) New military signal (M-code) and a new civil signal (L2CS)
GPS Signal Spectrum
GPS signal
 C/A codes
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Chipping rate of 1.023 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
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Chipping rate of 10.23 Mcps
Length ˜1014 chips
Chip duration ~ 0.1µs ~ wave length 30 m
Repeats every week
Anti-spoofing
GPS Navigation Data
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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 - 250 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.
 0.511 Mcps civil signal and 5.11 Mcps military spreading
codes
 12 satellites operating
GNSS signals – today and future
 Relative locations of GNSS signals
EGNOS and WAAS
are using GPS L1
E5
E5a
L5
E6
E2
L1
E1
E5b
L2
L2
L1
L1
1610 MHz
1194 MHz
GNSS systems:
GALILEO
GLONASS
GPS
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: NDS= 2n -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 period
Data bits
DSSS code chips
Data * DSSS code
Carrier
Carrier after BPSK
1 chip period
Frequency spectrum plot
Power
Narrowband waveform
Noise floor
Spread waveform
Frequency
 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
Narrow
band
signal
MultiplicationSpreading
PRN code
generator
Wideband
signal
MultiplicationDe-spreading or
Correlation
PRN code
generator
Narrow
band
signal
Block diagram of signal generator
Literature
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http://www.navcen.uscg.gov/gps/modernization/
http://gps.faa.gov/Programs/WAAS/waas.htm
http://www.esa.int/esaNA/galileo.html
http://www.esa.int/esaNA/egnos.html
http://www.glonass-center.ru/
Read Interface Control Documents for detailed description
of the GNSS signals
Questions and Exercises