Hardware curve flattening of crystal oscillator temperature drift followed by software compensation for residual offset
View Patent ↗A method for correcting frequency errors in the local reference frequency crystal oscillator of a GPS receiver begins with a step in which the ordinary temperature response of a crystal is characterized. A hardware circuit is incorporated with that crystal to mirror that same characteristic. A temperature sensor is used to control a compensation applied. The combination outputs a fairly flat temperature response, e.g., near zero delta-frequency change over the operating temperature range. But this flat response has an unknown offset from the true frequency. A next step finds a precision timing reference, such as when the receiver locks onto and tracks GPS satellites. The receiver then knows time precisely and a navigation software can compute the offset accurately. A value is stored for that offset in memory. Such value is thereafter recalled at each cold start and is used by software to remove the expected frequency offset.
1. A global positioning system (GPS) receiver method for improving time-to-first-fix performance, comprising:
before being installed for use in a navigation radio receiver as a reference local oscillator, characterizing the ordinary frequency versus temperature response of a crystal over its operating temperature range, wherein any deviations from a nominal frequency, but not a single average offset of said nominal frequency from a true frequency are modeled by a characteristic curve;
during operation, using an electronic temperature-correction circuit with an opposite characteristic curve to said characteristic curve modeled for said crystal to in a hardware-only combination with said crystal oscillator to continually produce a substantially flat temperature response of near zero delta-frequency change over said operating temperature range, but such flat response has an unknown single offset from a true frequency;
if a calculation of crystal offset frequency error is available, storing in memory a value representing such said single offset from a true frequency;
using a reference frequency with said near zero delta-frequency change over said operating temperature range produced by said combination of said local reference oscillator and said electronic temperature-correction circuit in a GPS digital signal processor to acquire and track GPS satellite transmissions in spite of there being an unknown single offset from a true frequency in said reference local oscillator;
during operation, in navigation software position fix calculations to initially acquire and track GPS satellite transmissions, removing in software said single offset from a true frequency by fetching and using said value stored in said memory, wherein initial carrier frequency uncertainty is reduced;
after acquiring and tracking GPS satellite transmissions, deriving a precision timing reference that is used to calculate said unknown single offset from a true frequency in said reference local oscillator, and storing a value in said memory to represent said calculation of crystal offset frequency error; and
thereafter recalling at each cold start said stored value and using it in software to remove an expected frequency offset.
2. A method of crystal oscillator temperature-frequency error correction, comprising:
flattening a characteristic S-curve of delta-frequency error of a crystal oscillator modeled over an operating temperature range solely with an electronic hardware circuit guided by a collocated temperature sensor; and
using a GPS navigation receiver for removing a single flat-over-temperature delta-frequency error for an entire operating temperature range from an output of the previous step with a computer software that processes digitized samples of downconverted GPS radio transmissions.
3. An improved GPS receiver, having
a radio frequency stage for down-converting transmissions to digitized samples which are then demodulated by a GPS navigation software;
a crystal local oscillator that has a previously modeled characteristic variation at its output frequency that depends on its operating temperature;
a temperature sensor to measure said operating temperature;
the improvement comprising:
an electronic temperature curve flattening circuit connected to the crystal local oscillator that actually eliminates said characteristic variation according to a measurement it obtains from said temperature sensor, and that leaves an actual single offset from a true frequency that spans the entire operating temperature; and
a GPS navigation software program connected to process said digitized samples and to virtually remove said offset from a true frequency according to a value calculated and stored in a previous acquisition and tracking that provided a precision timing reference.
4. The GPS receiver of claim 3 , wherein said value is consulted by the GPS navigation software program at receiver initialization to begin a carrier frequency search.