RTC Time Fraction Calibration for Fast GNSS TTFF
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Solution Overview
Problem
Existing GPS receivers face challenges in achieving fast time to first fix (TTFF) due to the lack of accurate local time at power-on, which is necessary for signal acquisition and ephemeris calculations, especially at low signal levels, as conventional real-time clocks lack the required time resolution and have significant time drift, making them unreliable for assisting signal acquisition without decoding the Z-count.
Innovation Solution
A GPS receiver with an always-on real-time clock (RTC) and an RTC calibrator that uses the difference between GPS time and RTC time to determine an RTC time fraction and drift, calibrating the RTC time for accurate local time estimation, thereby assisting in the first fix without relying on the Z-count.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional real-time clocks are used to provide local time at power-on, then power consumption is reduced, but time resolution and accuracy deteriorate significantly
Solution Approach 1:
The system performs preliminary calibration of the RTC against GPS time during periods when GPS signals are available. The calibration data (time fraction and drift rate) is stored in memory, so that when power is turned on, the pre-calibrated RTC can provide accurate time assistance without consuming continuous power for high-precision timekeeping.
Solution Approach 2:
The patent introduces calibration parameters (time fraction and drift rate) as intermediaries that bridge the gap between the low-precision RTC and the high-precision GPS time. These parameters allow the RTC to be adjusted and expanded to provide accurate time assistance for signal acquisition and ephemeris calculations.
2Device complexity
If conventional real-time clocks are used to provide local time at power-on, then device complexity is reduced, but time drift increases making the clock unreliable for signal acquisition
Solution Approach 1:
The RTC performs self-calibration by comparing its time against GPS time when signals are available. The system automatically calculates time fraction and drift rate, stores these calibration parameters, and uses them to correct future RTC time readings. This self-service approach maintains reliability without requiring external intervention or complex additional hardware.
Solution Approach 2:
The patent replaces the need for continuous mechanical/time-based precision keeping with a computational correction approach. Instead of relying on the RTC hardware to inherently provide accurate time, the system uses software-based calibration and time expansion algorithms to achieve the required precision.
3Measurement precision
If Z-count decoding is used to obtain accurate GPS time, then time accuracy is improved, but time to first fix increases
Solution Approach 1:
The system performs preliminary calibration of the RTC against GPS time during periods when GPS signals are available. The calibration data (time fraction and drift rate) is stored in memory, so that when power is turned on, the pre-calibrated RTC can provide accurate time assistance without consuming continuous power for high-precision timekeeping.
Solution Approach 2:
The patent implements a dynamic time expansion process that takes the coarse RTC time and expands it to the fine GPS time scale using the calibrated drift rate and time fraction. This dynamic correction allows the system to achieve Z-count level accuracy without actually decoding the Z-count, thereby reducing time to first fix.
Data Source
AI summary
A generic navigation satellite system (GNSS) signal receiver having a fast time to first fix by calibrating a low power always-on real time clock (RTC). The receiver includes an RTC calibrator having a fraction calculator. The RTC calibrator may also include a time expander. Before the receiver is powered off, the fraction calculator uses the fine resolution of GNSS time for determining a time fraction for RTC time. When the receiver is powered back on, the time expander uses an estimate of RTC time drift during the time that GNSS receiver had power off and the time fraction for calibrating and increasing the resolution of the RTC time for an RTC time tick. A signal navigation processor uses the calibrated RTC time for assisting a first fix with code phase search, integration time periods, resolution of epoch integer and/or location-in-space of GPS satellites.