A satellite navigation system allows a user to determine his/her geographical position in three dimensions (longitude, latitude and altitude), his/her speed and the time. Satellite navigation systems are known under the names of GPS, GALILEO or GLONASS.
Such a satellite navigation system comprises a constellation of satellites in orbit. Each satellite emits a satellite signal consisting of a carrier with a determined frequency, modulated by a navigation message and an identification code called pseudo-random code. The navigation message notably contains data on the ephemeris of the satellite. The pseudo-random code is a pseudo-random cyclic binary signal specific to the satellite. Each satellite has its own pseudo-random code and its own carrier frequency. The pseudo-random code spreads out the satellite signal over a wide frequency band and embeds the satellite signal in the background noise. This limits the interferences between the satellite signals and the sensitivity to external perturbations.
In order to detect a satellite signal, the receiver generates a local code by means of a code generator driven by a digitally controlled oscillator, the local code reproducing the pseudo-random code of the satellite signal, and correlates the received signal with the local code.
In an initial acquisition phase, in which the receiver tries to detect the satellite signal, it is necessary to synchronize the locally generated local code with the received pseudo-random code of the satellite. To do this, the receiver comprises a code loop (or DLL, “Delay Lock Loop”) for subordinating the code oscillator. The code loop comprises a suitable code discriminator for receiving the result of the correlation, in order to determine a code error depending on the result of the correlation and for sending a corresponding code correction signal to the code oscillator.
The satellite navigation receiver receives the satellite signal in a direct line-of-sight and optionally, depending on the environment, in an indirect line-of-sight, for example after reflection on the ground, the sea or on buildings. These multiple paths perturb the detection of the satellite signal and are sources of error in calculating the positioning of the receiver.
To overcome this drawback, in the acquisition phase it is known to correlate the satellite signal with a combination of punctual, advanced and/or delayed local codes, so as to obtain a correlation function with which the pseudo-random code received in direct line-of-sight may be better discriminated.
FR 2 739 695 discloses a Double Delta correlator which will be described in more detail hereafter. The theoretical correlation function of the Double Delta correlator has a narrow capture area surrounded by two so-called false lock-onareas, in which the correlation coefficient is zero.
The Double Delta correlator is efficient for an initial code error located in the capture area. Nevertheless, if the initial code error is located in a false lock-on area, the code loop operates in an open loop, which leads to a stationary measurement error.
Further, in a practical embodiment, the band pass of the receiver is not infinite so that the actual correlation function has secondary zeros in the false lock-on areas, over which the code loop may be locked on.
In order to overcome this drawback, FR 2 974 914 proposes a receiver comprising a Double Delta correlator associated with a false lock-on detector.
Nevertheless, this requires adaptation of the receiver for implementing the false lock-on detector, which is not necessarily possible upon updating a receiver.