A method for determining a change of a physical parameter (Δ ε, ΔT) with the correct sign, the method comprising: - coupling a first pulse sequence into an optical fiber comprising Rayleigh scatterers in at least in one portion, the first pulse sequence including a plurality (m+1) of at least substantially coherent optical pulses (Π p) of identical pulse duration (τ d) in the optical fiber, wherein an optical frequency (ν p = ν 0 + Δν p) of the first pulse sequence changes strictly monotonically; - determining a first power spectrum (I(t ref, t z, Δν p)), comprising measuring a respective power (I(t ref, t z)), of a signal component backscattered by the Rayleigh scatterers for each of the optical pulses (Π p) of the first pulse sequence; - coupling a second pulse sequence into the optical fiber, the second pulse sequence corresponding at least substantially to the first pulse sequence, a permutation of the first pulse sequence, a part of the first pulse sequence or a permutation of a part of the first pulse sequence, and/or an optical frequency (ν p) in the second pulse sequence changing strictly monotonically; - determining a second power spectrum (I(t s, t z, Δν p)), comprising measuring a respective power (I(t s, t z)) of a signal component backscattered by the Rayleigh scatterers for each of the optical pulses (Π p) of the second pulse sequence; and - determining, with the correct sign, a change of a physical parameter (Δ ε, ΔT), of the optical fiber, comprising determining an offset (Δv m) between the second power spectrum and the first power spectrum, wherein coupling of the respective pulse sequence comprises monotonically decreasing or increasing a feed current (i) and/or a temperature of a continuous-wave laser coupled to the optical fiber. - coupling a first pulse sequence into an optical fiber comprising Rayleigh scatterers in at least in one portion, the first pulse sequence including a plurality (m+1) of at least substantially coherent optical pulses (Π p) of identical pulse duration (τ d) in the optical fiber, wherein an optical frequency (ν p = ν 0 + Δν p) of the first pulse sequence changes strictly monotonically; - determining a first power spectrum (I(t ref, t z, Δν p)), comprising measuring a respective power (I(t ref, t z)), of a signal component backscattered by the Rayleigh scatterers for each of the optical pulses (Π p) of the first pulse sequence; - coupling a second pulse sequence into the optical fiber, the second pulse sequence corresponding at least substantially to the first pulse sequence, a permutation of the first pulse sequence, a part of the first pulse sequence or a permutation of a part of the first pulse sequence, and/or an optical frequency (ν p) in the second pulse sequence changing strictly monotonically; - determining a second power spectrum (I(t s, t z, Δν p)), comprising measuring a respective power (I(t s, t z)) of a signal component backscattered by the Rayleigh scatterers for each of the optical pulses (Π p) of the second pulse sequence; and - determining, with the correct sign, a change of a physical parameter (Δ ε, ΔT), of the optical fiber, comprising determining an offset (Δv m) between the second power spectrum and the first power spectrum,