GPS and other GNSS signals

GPS and other GNSS signals

GPS signals and receiver technologyMM10

Darius Plausinaitisdpl@gps.aau.dk

GPS Signals MM10-MM15GPS 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

http://gps.aau.dk/educate/receiverTechnologyPart3.htm

Today's SubjectsToday'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 SignalsGPS 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 SpectrumGPS Signal Spectrum

GPS signalGPS signal C/A codes

– 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– Chipping rate of 10.23 Mcps– Length ˜1014 chips– Chip duration ~ 0.1µs ~ wave length 30 m– Repeats every week– Anti-spoofing

GPS Navigation DataGPS 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 signalsOther GNSS signals

WAAS and EGNOSWAAS 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

GalileoGalileo 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

GLONASSGLONASS 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

GNSS signals – today and future

Relative locations of GNSS signals

GPSGPSGALILEOGALILEO GLONASSGLONASS

E5aE5a E5bE5b

L5L5

E5E5

L2L2

E6E6 L1L1E2E2 E1E1

L1L1

1194 MHz1194 MHz

L1L1

1610 MHz1610 MHz

L2L2

EGNOS EGNOS andand WAAS are using WAAS are using GPS L1GPS L1

GNSS GNSS systems:systems:

Direct Sequence Spread Spectrum (DSSS)

technique

Direct Sequence Spread Spectrum (DSSS)

technique

DSSS TechniqueDSSS 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)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 operationSpreading 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 1 chip period

Data bits

DSSS code chips

Data * DSSS code

Carrier

Carrier after BPSK

Frequency spectrum plotFrequency spectrum plot

Narrowband waveform

Spread waveform

Noise floor

Frequency

Po

we

r

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 / DecodingEncoding / Decoding

Multiplication- Spreading

Multiplication- De-spreading or Correlation

PRN code generato

r

PRN code generato

r

Narrowband signal

Narrowband signal

Wideband signal

Block diagram of signal generator

Block diagram of signal generator

LiteratureLiterature 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 ExercisesQuestions and Exercises