Detecting lack of movement to aid GNSS receivers
Embodiments of the invention provide a method of detecting movement to aid GNSS receivers. By detecting when the user is stationary, the Doppler frequency estimation can be corrected or the SNR can be boosted more both of which lead to improved performance. The embodiments allow a GNSS receiver to process signals in when the signal level would otherwise be too low—for example indoors. The embodiments can improve performance when one or more satellites are temporarily blocked but one or more satellites are still being tracked.
1. A method for estimating the clock drift, said method comprising:
receiving a signal from a satellite;
using only a portion of said signal corresponding to when said user is stationary;
forcing inputs to a correlator engine to zero;
estimating a Doppler frequency of said satellite;
receiving an assistance Doppler frequency from an aiding source that is given under the assumption that said user is stationary;
comparing said estimated Doppler frequency to said assistance Doppler frequency when said user is stationary; and
using the difference in the two Doppler frequencies to estimate clock drift.
2. The method of claim 1 , further comprising correcting a receiver Doppler estimate using said estimated clock drift.
3. The method of claim 1 , wherein negligible motion is present and said method further comprising using a plurality of low-cost sensors to identify stationary from negligible motion.
4. The method of claim 1 , wherein the output from said plurality of low-cost sensors are calibrated or un-calibrated.
5. The method of claim 4 , further comprising using the sensor outputs to increase an integration time.
6. The method of claim 4 , further comprising using the sensor outputs to increase an integration time even if said user motion is not negligible by only using said signal when said user is stationary.
7. A method for estimating the clock drift, said method comprising:
handshaking between a receiver and a mobile platform to allow said receiver to estimate a clock quality;
using said estimate of clock quality to increase an integration time;
receiving a signal from a satellite;
estimating a Doppler frequency of said satellite;
receiving an assistance Doppler frequency from an aiding source that is given under the assumption that negligible motion is present;
using a plurality of low-cost sensors to identify stationary or negligible motion, wherein the output from said plurality of low-cost sensors are calibrated or uncalibrated;
comparing said estimated Doppler frequency to said assistance Doppler frequency when negligible motion is present; and
using the difference in the two Doppler frequencies to estimate clock drift.
8. An apparatus comprising:
a processor for processing a signal;
a motion detector coupled to said processor for providing measurements about a user's movement, wherein said motion detector comprises at least one low-cost sensor;
a clock drift estimator coupled to said processor, for computing an estimate of a clock drift:
an aiding source coupled to processor for providing Doppler information; and
an alternative position estimator coupled to said processor, said clock drift estimator and said aiding source for providing a rough estimate of a user's current position.
9. The apparatus of claim 8 further comprising an adder for combining outputs from said clock drift estimator and aiding source to provide a Doppler frequency to said processor.
10. The apparatus of claim 8 , wherein said alternative position estimator uses cellular signals from a plurality of base stations to get an approximation of position.
11. The apparatus of claim 8 , wherein said alternative position estimator uses a proximity to another device which is communicating said device position or whose position is otherwise known.
12. The apparatus of claim 8 , wherein said alternative position estimator using an assistance network could communicate an approximate position to the GNSS receiver.
13. The apparatus of claim 8 , wherein said motion detector determines when said user is stationary or has negligible motions so that only a portion of said signal is used for processing.
14. A method of reducing a search space for a Doppler frequency of a satellite signal, said method comprising:
a sensor module sensing a plurality of user dynamics using a plurality of low-cost sensors,
searching over a plurality of time delays and Doppler frequency hypotheses around an initial estimate of each; and
limit searching to a small number of Doppler frequency hypotheses around a given Doppler frequency.
15. The method of claim 14 , wherein said initial estimate is from an assistance source.