A passive keyless entry system including an in-vehicle apparatus installable on a vehicle and a portable apparatus, the in-vehicle apparatus including: a plurality of antennas (111, 112, 113) for generating a plurality of respective magnetic fields (H1, H2, H3), an in-vehicle receiver for communicating with a portable transmitter of the portable apparatus, an in-vehicle control unit for controlling the plurality of antennas (111, 112, 113) and for generating an authorization signal based on information received from the portable apparatus, the portable apparatus including: a receiving antenna having three mutually orthogonal coils (141x, 141y and 141z), a portable transmitter for communicating with the in-vehicle receiver, a portable control unit for controlling the receiving antenna and the portable transmitter, the portable apparatus being configured for measuring the absolute values only of the 3-D components of at least two of the magnetic fields (H1, H2, H3) to define at least two respective projection vectors (m1, m2, m3) of the magnetic fields (H1, H2, H3) into the positive octant of an R 3 space, the in-vehicle control unit or the portable control unit being configured for computing at least one computed angle (α m, α m13, α m23) between the two projection vectors (m1, m2, m3) of the magnetic fields (H1, H2, H3), the in-vehicle control unit or the portable control unit being configured for comparing the at least one computed angle (α m, α m13, α m23) with an angular threshold value (α t), the in-vehicle control unit being configured for preventing the authorization signal when all the computed angles (α m, α m13, α m23) are lower than the angular threshold value (α t). a plurality of antennas (111, 112, 113) for generating a plurality of respective magnetic fields (H1, H2, H3), an in-vehicle receiver for communicating with a portable transmitter of the portable apparatus, an in-vehicle control unit for controlling the plurality of antennas (111, 112, 113) and for generating an authorization signal based on information received from the portable apparatus, a receiving antenna having three mutually orthogonal coils (141x, 141y and 141z), a portable transmitter for communicating with the in-vehicle receiver, a portable control unit for controlling the receiving antenna and the portable transmitter, the portable apparatus being configured for measuring the absolute values only of the 3-D components of at least two of the magnetic fields (H1, H2, H3) to define at least two respective projection vectors (m1, m2, m3) of the magnetic fields (H1, H2, H3) into the positive octant of an R 3 space, the in-vehicle control unit or the portable control unit being configured for computing at least one computed angle (α m, α m13, α m23) between the two projection vectors (m1, m2, m3) of the magnetic fields (H1, H2, H3), the in-vehicle control unit or the portable control unit being configured for comparing the at least one computed angle (α m, α m13, α m23) with an angular threshold value (α t), the in-vehicle control unit being configured for preventing the authorization signal when all the computed angles (α m, α m13, α m23) are lower than the angular threshold value (α t).