Satellite positioning system receivers with microelectromechanical systems oscillators

• US 10,107,919 B1
• Filed: 03/01/2016
• Issued: 10/23/2018
• Est. Priority Date: 03/01/2016
• Status: Active Grant

• Current Assignee: L3harris Interstate Electronics Corporation (L3Harris Technologies, Inc.)
• Original Assignee: Interstate Electronics Corp. (L3Harris Technologies, Inc.)
• Inventors: Chapman, David Duane, Alexander, Steven B.
• Primary Examiner(s): Liu, Harry K
• Application Number:
  US15/057,858
• Time in Patent Office: 966 Days
• Field of Search: 34235777
• CPC Class Codes: G01S 19/35 Constructional details or hardware or software details of the signal processing chain
• CPC Class Codes: G01S 19/37 Hardware or software details of the signal processing chain

Abstract

Apparatus and methods permit the use of a microelectromechanical systems (MEMS) oscillator in a satellite positioning system receiver, such as a Global Positioning System (GPS) receiver. Techniques to ameliorate jitter or phase noise disadvantages associated with MEMS oscillators are disclosed. For example, a receiver can use one or more of the following techniques: (a) use another source of information to retrieve ephemeris information, (2) perform advanced tight coupling, and/or (3) use a phase-locked loop to clean up the jitter or phase noise of the MEMS oscillator. With respect to advanced tight coupling, an advanced tight coupling processor can include nonlinear discriminators which transform I and Q data into linear residual measurements corrupted by unbiased, additive, and white noise. It also includes an amplitude estimator configured to operate in rapidly changing, high power noise; a measurement noise variance estimator; and a linear residual smoothing filter for input to the navigation filter.

Claims

1. An apparatus comprising a global positioning satellite (GPS) receiver, the apparatus comprising:

a microelectromechanical systems (MEMS) oscillator configured to generate a first clock signal as an output;

a frequency synthesizer configured to receive the first clock signal from the MEMS oscillator as an input and configured to generate a local oscillator signal as an output;

a downconverter configured to downconvert one or more received radio frequency (RF) signals using the local oscillator signal to generate an intermediate frequency (IF) signal;

an analog-to-digital converter configured to receive the IF signal and generate a digital IF signal;

a plurality of channel signal processors configured to despread GPS signals from the digital IF signal and generate pseudo-range estimates to space vehicles, wherein a channel signal processor of the plurality of channel signal processors comprises a carrier numerically controlled oscillator (NCO) and a code NCO, wherein the carrier NCO and the clock NCO are configured to receive an NCO clock signal based on the first clock signal from the MEMS oscillator as an input; and

a navigation processor configured to extract at least position information based at least on the plurality of pseudo-range estimates.

2. The apparatus of claim 1, wherein the first clock signal from the MEMS oscillator is provided as an input to the carrier NCO and the code NCO such that the first clock signal is the NCO clock signal.

3. The apparatus of claim 2, wherein the apparatus does not include a crystal oscillator.

4. The apparatus of claim 1, wherein the GPS receiver is integrated with a mobile phone, and wherein the plurality of channel signal processors and the navigation processor are configured to receive ephemeris information originating from a cellular network.

5. The apparatus of claim 1, wherein the GPS receiver is integrated with a mobile phone, and wherein the plurality of channel signal processors and the navigation processor are configured to receive ephemeris information originating from a wide area network.

6. The apparatus of claim 1, wherein the plurality of channel signal processors and the navigation processor are configured to retrieve ephemeris information from a data store, wherein the data store is populated with ephemeris information originating from a source external to the apparatus.

7. The apparatus of claim 1, further comprising:

a phase-locked loop configured to receive the first clock signal from the MEMS oscillator as an input, and to generate the NCO clock signal as an output, wherein the NCO clock signal has less phase noise relative to the first clock signal.

8. The apparatus of claim 7, wherein the second oscillator comprises a crystal oscillator.

9. The apparatus of claim 1, wherein one or more channel signal processors of the plurality of channel signal processors further comprises an advanced tight coupling (ATC) tracking processor configured to calculate and provide at least range, range rate, and range acceleration residuals to a navigation Kalman filter.

10. The apparatus of claim 1, wherein one or more channel signal processors of the plurality of channel signal processors further comprises an extractor configured to extract vehicle range and carrier phase residual errors, the extractor comprising a discriminator configured to compute at least two nonlinear transformations to generate linear measurements of pseudorange and carrier phase errors.

11. The apparatus of claim 10, wherein the extractor is configured to extract vehicle range and carrier phase residual errors in the presence of interference signals, wherein the extractor comprises a linear estimator.

12. The apparatus of claim 10, wherein the extractor further comprises a tracking processor for each satellite which accepts early, late, and on-time I and Q data from the GPS signal tracker and outputs vehicle to satellite range and/or range rate residual measurements to a navigation Kalman filter, wherein the tracking processor comprises:

at least one amplitude estimator to accept I and Q data and output estimates of the I and Q amplitudes for L1 and L2 signals;

at least one discriminator to transform I and Q data into measurements of satellite range and carrier phase error residuals; and

at least one phase error estimator to process discriminator measurements for input to the navigation Kalman filter.

13. The apparatus of claim 12, wherein the tracking processor is configured to filter the Is and Qs for a period of time corresponding to an amount of time that a navigation data bit is expected to be constant.

14. The apparatus of claim 12, wherein the filtered Is and Qs signals are provided to one or more nonlinear transformations configured according to their linearity and additive noise properties, comprising:

a range residual transformation, made up of the early and late Is and Qs to produce a measurement that is a linear function of range residual error;

a range rate residual transformation, made up of the prompt Is and Qs, to produce a measurement that is a linear function of range rate residual error with the square of the correlation function remaining as a multiplier; and

an estimator of the I and Q amplitude and noise variance to define the transformed measurements and their noise statistics, wherein the noise on each discriminator is substantially additive, substantially zero mean, and substantially white with a variance given by a deterministic equation dominated by the variance of the I and Q noise.

15. The apparatus of claim 12, wherein the at least one phase error estimator comprises a three-state phase error estimator configured to process the range and range rate transformed measurements are processed in order to obtain residual error estimates at approximately one second intervals for the navigation Kalman filter.

16. The apparatus of claim 15, wherein the three-state phase error estimator comprises a Kalman filter comprising three phase error states (L1 phase, phase rate, and phase acceleration), which are propagated assuming that the phase acceleration is constant;

a linear dynamics equation with a constant transition matrix;

four measurements for each satellite;

range and range rate for L1 and L2 signals with measurement noise that is substantially unbiased;

a measurement noise matrix dominated by the I and Q noise variance in low signal to noise environments; and

a measurement noise filter input variance increased to account for non-gaussian measurement noise.

17. The apparatus of claim 16, wherein the three-state phase error estimator is configured to handle large magnitude noise-induced swings in the measurements.

18. The apparatus of claim 10, wherein the extractor further comprises an amplitude estimator configured to produce a nonrandom multiplier for the range and range rate residual error measurements during periods of high signal interference, which helps the phase error estimator perform properly in a high noise environment as follows:

when the interference noise is low, the I and Q amplitude measurement is projected back through a system gain to estimate the constant satellite signal amplitude and

when the noise increases, the satellite signal amplitude estimate is projected forward through the system gain to estimate the I and Q amplitude.

19. The apparatus of claim 10, wherein the extractor further comprises an I and Q Noise Variance Estimator further comprising a running average filter configured to compute the I and Q noise variance using a separate outbound correlator channel for each satellite that is sufficiently away from the code that it contains substantially random noise.

20. The apparatus of claim 10, wherein the extractor further comprises an I and Q Noise Variance Estimator further comprising a running average filter configured to compute a sample variance of noise in one or more outbound satellite correlator channels.

21. The apparatus of claim 10, wherein the extractor further comprises an amplitude estimator configured to use an approximately-constant satellite signal amplitude at the antenna as the state to be estimated.

22. The apparatus of claim 1, wherein one or more channel signal processors of the plurality of channel signal processors is further configured to adapt to changing levels of interference without knowledge of a GPS navigation data bit.

23. The apparatus of claim 1, further comprising:

a strapdown inertial measurement unit (IMU);

a navigation Kalman filter configured to process vehicle range and carrier phase residual signals to generate vehicle position, velocity, attitude, and time estimates along with IMU error estimates;

a strapdown navigation processor configured to integrate the IMU measurements between the navigation Kalman filter processing intervals, wherein the strapdown navigation processor is configured to receive navigation state error estimates from the navigation Kalman filter, and wherein the strapdown navigation processor is configured to receive navigation data from the IMU; and

a satellite navigation message decoder configured to provide satellite position, velocity, and acceleration (PVA) data to a processor-implemented satellite line of sight projection estimator and to the navigation Kalman filter;

wherein the processor-implemented satellite line of sight projection estimator to project navigation data onto the satellite line of sight, generate range and range rate measurements, and provide these measurements to one or more Numerically Controlled Oscillators configured to provide code and carrier signals for GPS signal phase de-rotation and correlation.

24. The apparatus of claim 23, wherein one or more satellite signals and IMU measurements are combined to estimate the vehicle trajectory and associated system error terms using a maximum likelihood estimate of one or more GPS signal parameters.

25. The apparatus of claim 1, wherein the plurality of channel signal processors are further configured to maintain code lock to the GPS signals, but are not configured to track carrier phases of the GPS signals or demodulate navigation messages.