African School on Space Science Consumer GNSS Receiver Design & comparison with ionospheric scintillation studies Reference: Chapters 2,3 of: A-GPS; Assisted GPS, GNSS & SBAS, van Diggelen. Chapters 11,12 of: Global Positioning System, Misra & Enge
GPS (Civilian) Signal at the Satellite PRN Code Data includes: Almanac, Ephemeris, HOW 1575.42 MHz ~ C/A Code 1 Mbps Data 50 bps BPSK signal
Standard GPS receiver architecture + noise locally generated copies of PRN code RF Front End local oscillator ~ IF NCO received PRN code + noise Correlate integrate Σ correlation peak - f e
Standard GPS receiver architecture BASEBAND BLOCK REPEATED ONCE PER CHANNEL locally generated copy of PRN code code delay + noise RF Front End IF received PRN code + noise NCO + noise NCO - f e received PRN locally code generated copy of PRN code + noise received PRN locally code generated copy of PRN code - f e Correlate code delay Correlate integrate Σ code delay Correlate integrate Σ integrate Σ correlation peak correlation peak correlation peak local oscillator ~ NCO - f e
Tri-band front end 1561.098 1575.42 1602 Band Separation Filters A/D A/D A/D Gain Control and Filters Gain Control and Filters Gain Control and Filters To Baseband Processing
Search space 2014, Frank van Diggelen
Acquisition space review: Background Real-time animation of standard GPS search of freq/code space. Click picture to play
Search Engine Evolution (1) Broadcom Samples Correlator 1 Samples Correlators 1-4 Correlator 2 Results Correlators 5-8 Results Correlator 3 Correlators 9-12 Correlator n Correlators n-m Gen 1 Gen 2 Processing ca. 1993 Samples Correlators 1-2046 Samples Sample Storage FFT Multiply IFFT Correlators 2047-4092 Results Gen 4 Results Correlators 4092-6138 Correlators n-m Gen 3 2014, Frank van Diggelen Matched Filter Processing FFT Processing
Search Engine Evolution (2) Broadcom Coarse-Time Acquisition Sensitivity (@ fixed TTFA of 10s) vs. number of code-epoch bins -130 dbm Magellan 1992 these are actual receivers built over the last 20 years -140 dbm -150 dbm 1999 SiRF Global Locate 2002 Broadcom One search bin -160 dbm 2012 10-3 10-2 10-1 1 10 100 Number of full code-epoch bins that can be searched in parallel 2014, Frank van Diggelen * With A-GPS assistance data: ± 100 ppb frequency, ± 2 s time, ± 3km position, ephemeris
Broadcom GPS Broadcom One bin 2) Search all over 100 bins in parallel 1) Search all code delays simultaneously
Contributors to frequency offset 8.4 khz (satellite motion) 0.15 khz / 100 km/h (receiver speed) 1.5 khz/ppm (oscillator) 0.1 khz / 100km (init. position) 0.0008 khz / s (init. Time) 2014, Frank van Diggelen Dependence on oscillator stability Does this mean the consumer market will lead to better oscillators?
Reminder of receiver design BASEBAND BLOCK REPEATED ONCE PER CHANNEL locally generated copy of PRN code code delay + noise RF Front End IF received PRN code + noise NCO + noise NCO - f e received PRN locally code generated copy of PRN code + noise received PRN locally code generated copy of PRN code - f e Correlate code delay Correlate integrate Σ code delay Correlate integrate Σ integrate Σ correlation peak correlation peak correlation peak local oscillator ~ NCO - f e
Assisted GNSS (1) Satellite nav data from the internet
Assisted GNSS (2) Reduced search space quicker acquisition higher sensitivity Location Server Assistance: Acquisition assist Almanac Ephemeris Frequency Time Position frequency (khz) reduce search space: frequency code delay code-delay (chips)
Long Term Orbits (LTO) (aka Extended Ephemeris) Location Server Assistance: Acquisition assist Almanac Ephemeris Frequency Time Position frequency (khz) reduce search space: frequency code delay code-delay (chips) Ephemeris is calculated for many days into the future
Broadcom, LTO Server, Unique Android Visitors, in 24 hours 2014, Frank van Diggelen Greenland: 388 Russia: 1.1M UK: 27M USA: 37M Spain: 0.9M Vatican City: 12 China: 80M North Korea: 6 South Korea: 1.6M Tuvalu: 1 Brazil: 36M South Africa: 0.5M
Morocco: 136,000 Mauritania: 1,000 Algeria: 40,000 Tunisia: 38,000 Libya: 21,000 Egypt: 454,000 Broadcom, LTO Server, Unique Android Visitors, 24 hours Cape Verde: 450 Senegal: 14,000 Gambia: 650 Mali: 2,000 Niger: 750 Burkina: 2,000 Guinea Bissau: 100 Guinea : 350 Sierra Leone: 300 Liberia: 200 Benin: 500 Cote Divoire: 9,000 Ghana: 6,000 Togo: 150 Nigeria: 18,000 Cameroon: 2,000 St Tome & Principe: 80 Equatorial Guinea: Gabon: 1,000 Congo: 300 DRC: 1,000 Angola: 17,000 Namibia: 17,000 Chad: 20 CAR: 80 Zambia: 1,000 Zimbabwe: 3,000 Uganda: 1,000 Rwanda: 300 Burundi: 100 Botswana: 2,000 Malawi: 1,000 Tanzania: 4,000 Mozambique: 5,000 Sudan: 57,000 Eritrea: 10 Djibouti: 150 Kenya: 62,000 Ethiopia: 1,000 Somalia: 200 Comoros: 90 Mayotte: 400 Madagascar: 2,000 Seychelles: 700 Mauritius: 14,000 Reunion: 14,000 2014, Frank van Diggelen Swaziland: 300 Lesotho: 150 South Africa: 484,000 Above 1,000 rounded to nearest 1,000 Below 1,000 rounded to 50 Below 100 rounded to 10
Back to search space with A-GNSS 8.4 khz (satellite motion) 0.15 khz / 100 km/h (receiver speed) 1.5 khz/ppm (oscillator) 0.1 khz / 100km (init. position) 0.0008 khz / s (init. Time)
Frequency assistance Cell towers have oscillators that are known to ±100ppb A cell-phone communicating with a tower can calibrate its internal oscillator to ±100ppb
Back to search space with A-GNSS 8.4 khz (satellite motion) ± 0.15 khz/100ppb 0.15 khz / 100 km/h (receiver speed) 1.5 khz/ppm (oscillator) 0.1 khz / 100km (init. position) 0.0008 khz / s (init. Time) Result: remaining search space is a fraction of a khz, easily within the capabilities of modern receivers. And so the trend is towards worse (= cheaper) oscillators in consumer products L. 2014, Frank van Diggelen
OSCILLATORS & IONO... 2014, Frank van Diggelen
Studying ionospheric scintillation Measuring phase scintillation: must remove effects of receiver oscillator Frequency jumps are not tolerable: 20 MHz OCXO (Bad) 10 MHz OCXO (Good) Sigma Phi - Radians 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 ph 1 ph3 ph10 ph30 ph60 Phase Density @ 1 Hz Offset Thermal Noise Contribution Frequency Jumps Feb 10 1998-70 -75-80 -85-90 -95-100 -105-110 Spectral Power @ 20.473 MHz - db Sigma Phi - Radians 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 ph 1 ph3 ph10 ph30 ph60 Spectral Density @ 1 Hz Thermal Noise Contribution Strong Multipath SV 1 - Feb 97 Loss of Lock -70-75 -80-85 -90-95 -100-105 -110 Spectral Power @ 10 MHz - db 0.1-115 0.1-115 0-120 175000 180000 185000 190000 195000 200000 205000 GPS Time-Of-Week - Seconds 0 373200 376800 380400 384000 387600 391200 394800 GPS Time Of Week - Seconds -120 Conclusion: higher frequency OCXO showed jumps of the order of 1 rad/s in measured phase 0.1 ppb Crystal Oscillator Noise Effects on the Measurement of Ionospheric Phase Scintillation Using GPS, A.J. Van Dierendonck & Quyen Hua IEEE Frequency Control Symposium. May 1998
Oscillator summary Typical frequency jumps in different types of oscillators OCXO ~$100 TCXO ~$0.50 TSX ~$0.25 cost (USD) 0.1 1 10 ppb Summary: for consumer products to measure iono scintillation effect on phase you would need (at least) to change the crystal oscillator.
Measuring scintillation using observed C/No 2014, Frank van Diggelen
MEASURING TEC... 2014, Frank van Diggelen
State of the art, and trends Current consumer GNSS is multi-frequency, but across different systems (therefore different satellite clocks) However, the trend is towards L1 and L5 In the next decade you may see consumer products measuring multi GNSS systems on dual frequencies (L1, L5)
Summary You ve seen consumer GNSS designs and trends half for your general knowledge half relevant to your work Consumer products have some (small) overlap with GNSS for space science today And may be quite useful in years to come