GNSS - Fast Acquisition with Low Signal Level

[GPS Receiver Start-Up and Operational Modes]

• Receiver operational modes: cold start, warm start, hot start, reacquisition, assisted GPS, self-assisted GPS, DGPS

[Factory Start]

• Receiver never used since fabrication

• Receiver not powered for more than a year.

• No information at all on GPS satellite. The GPS receiver chip has never been fully operated after its semiconductor fabrication.

• Almanac download alone takes 12.5 minutes.

• TTFF = 13.5 min.

[Cold Start]

• Receiver not powered for 8-12 hours.

• Almanac in non-volatile memory is valid.

• Invalid ephemeris (older than 4 hours): ephemeris data is broadcast every 30 seconds.

• Receiver position not valid: away from previous fix by more than 100km

• Receiver time: unknown

• Receiver works through an internal list of all satellites acquiring each SV in view in turn. Acquisition time is the longest among all start-up modes.

• TTFF = 30-40s @ 2011

• Forced cold start: wrong information (very old almanac) on SV will make receiver spend more time than in the total cold start mode. In this case, a forced cold start is necessary.

[Warm Start]

• Receiver not powered for 30 minutes.

• Valid almanac

• Invalid ephemeris (older than 4 hours)

• Receiver position: approximately known (within 100km of the last fix)

• Receiver time: approximately known (GPS has been active in the last three days or RTC has been on by backup power).

• Receiver immediately detects overhead SVs but needs to download current ephemeris data.

• TTFF = 30-40s @ 2011 (not much different from cold-stat time)

[Hot Start]

• Receiver not powered for 15 minutes.

• Valid almanac

• Valid ephemeris data for at least 5 SVs (less than 4 hours old)

• Receiver position: not changed (exactly known)

• Receiver time: exactly known

• Receiver rapidly tracks overhead SVs and needs to download a minimum of data.

• TTFF = 1s @ 2011

[Reacquisition]

• Receiver always being powered. After blockage for up to 10s while tracking SV signals.

• TTFF < 1s @ 2011

[Assisted GPS(AGPS)]

• Receiver always being powered and assistance data always received.

• Relay information (= assistance data) to GPS receiver to help shorten acquisition time.

- SV ephemeris (orbit parameters): mandatory

- SV clock corrections: mandatory

- Other error corrections: ionospheric, tropospheric

- Accurate local time: produces 3dB sensitivity improvement

- GPS receiver (UE=user equipment) location

- Visible satellites

- Relative code delay offsets

- Doppler frequencies

- DGPS corrections (optional)

- Navigation message bits: produces 3dB SNR improvement

• GPS receiver maximum distance from server station: 150km

• Benefits:

- Search space is drastically reduced. 100-1000 times faster in all SNR conditions

- SNR (code phase tracking) improvement: 25dB, operation in low signal and heavy multipath environments (foliage, indoors, urban canyons).

- TTFF almost independent of SNR.

- Indoor operation of GPS is possible.

- Accuracy: 4m open sky, 20-50m(indoors, urban canyons)

[Self-Assisted GPS]

• Receiver did not work for last 4 hours -> Warm stat when re-powered.

• Extend this 4-hr limit by precisely predict (based on planetary physics) the SV trajectory from the last ephemeris data up to 3 days (72 hours).

• TTFF = 3s @ 2011