Simulating Phase-Coherent GNSS Signals
A method and apparatus for simulating radio-frequency Global Navigation Satellite System (GNSS) signals that are carrier-phase and code-phase aligned with ambient GNSS signals at a user-specified location in the vicinity of the simulator. Such phase alignment allows the synthesized signals to be made to appear substantially the same as the authentic signals to a target receiver, allowing the target receiver to transition seamlessly between authentic and simulated signals. The method is embodied in a device, a phase-coherent GNSS signal simulator, which can be implemented on a digital signal processor for embedded applications.
1 . A GNSS phase-coherent signal simulator comprising:
a GNSS signal receiver operable to receive one or more ambient radio-frequency GNSS signals;
a GNSS signal simulator operable to generate one or more simulated radio-frequency GNSS signals;
a timing oscillator coupled to both the GNSS signal receiver and the GNSS signal simulator; and
a control module operable to align both a carrier phase and a code phase of the one or more simulated GNSS signals respectively with a carrier phase and a code phase of the one or more ambient radio-frequency GNSS signals at a predetermined three-dimensional position offset and a predetermined velocity offset relative to a reference location on the GNSS phase-coherent signal simulator.
2 . The GNSS phase-coherent signal simulator of claim 1 , wherein at least one of the position offset and the velocity offset is substantially zero.
3 . The GNSS phase-coherent signal simulator of claim 1 , wherein the position offset and velocity offset are individually selectively variable.
4 . The GNSS phase-coherent signal simulator of claim 1 , wherein the position offset is less than about 10 km.
5 . The GNSS phase-coherent signal simulator of claim 1 , wherein the signal simulator is configured to compensate for at least one of a signal processing latency and an analog RF bias.
6 . The GNSS phase-coherent signal simulator of claim 1 , wherein the signal simulator is operable to align the simulated GNSS signal with the ambient GNSS signal within about one tenth of a carrier wavelength.
7 . The GNSS phase-coherent signal simulator of claim 1 , wherein the reference location comprises an input antenna location.
8 . The GNSS phase-coherent signal simulator of claim 1 , wherein position and velocity offset are selected to align the code and carrier phases along a trajectory.
9 . A method of combining a plurality of GNSS signals, the method comprising:
receiving, at a GNSS signal receiver, one or more ambient radio-frequency GNSS signals;
generating, with a GNSS signal simulator, one or more simulated radio-frequency GNSS signals;
receiving, at both the GNSS signal receiver and the GNSS signal simulator, a common timing reference; and
aligning, via a control module, both a carrier phase and a code phase of the one or more simulated radio-frequency GNSS signals respectively with a carrier phase and a code phase of the one or more ambient radio-frequency GNSS signals at a predetermined three-dimensional position offset and a predetermined velocity offset relative to a reference location on the GNSS phase-coherent signal simulator.
10 . The method of claim 9 , wherein the aligning includes compensating for analog RF biases.
11 . The method of claim 9 , further comprising calibrating the GNSS signal simulator to account for signal processing latency.
12 . The method of claim 9 , wherein the velocity offset is selected for carrier phase and code phase alignment at a stationary point.
13 . The method of claim 9 , wherein the velocity offset is selected for continuous carrier phase and code phase alignment along a trajectory of a moving point.
14 . The method of claim 9 , wherein the simulated GNSS signal is aligned with the ambient GNSS signal within one tenth of a carrier wavelength.
15 . The method of claim 9 , further comprising estimating a Doppler frequency offset.
16 . The method of claim 15 , wherein the Doppler frequency offset is set to within about 1 Hz.
17 . The method of claim 9 , wherein the respective carrier and code phases are each aligned at a centimeter-level.
18 . A method of phase aligning GNSS signals via a GNSS signal simulator, the method comprising:
receiving one or more ambient radio-frequency GNSS signals;
generating one or more simulated radio-frequency GNSS signals; and
aligning both a carrier phase and a code phase of the one or more simulated radio-frequency GNSS signals respectively with a carrier phase and a code phase of the one or more ambient radio-frequency GNSS signals at a predetermined three-dimensional position offset and a predetermined velocity offset relative to a reference location on the GNSS signal simulator.
19 . The method of claim 18 , further comprising providing a timing reference to a GNSS receiver and the GNSS simulator via an oscillator to facilitate phase alignment.