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, <EOS>