Method for determining at least one transmission phase difference of a radar that comprises at least two juxtaposed transmission channels (10 1, 102, 10 3) and at least one reception channel, said radar being arranged so that each transmission channel produces a transmission radiation (R 1) modulated in accordance with a modulation that is allocated to said transmission channel, and which is orthogonal to the modulation that is allocated to each other one of the transmission channels, and said radar being arranged so that, when the reception channel detects a return signal, said return signal is filtered in accordance with each modulation in order to isolate a component of the return signal that has said modulation, the method being characterised in that it comprises the following steps: /1/ activating the transmission channels (10 1, 10 2, 10 3) to produce the modulated transmission radiations (R 1), and activating the reception channel to detect the return signal, and then filtering said return signal in accordance with each modulation; /2/ separately for each modulation: determining a value of a phase difference between transmission and reception, referred to as transmission-reception phase difference (ΔΦ 01, ΔΦ 02, ΔΦ 03) for said modulation, which exists between the transmission radiation (R 1) produced and modulated in accordance with said modulation and the component of the return signal that was isolated in accordance with the same modulation, said transmission-reception phase difference being determined for signal passage points constant within the radar for each transmission channel (10 1, 10 2, 10 3) and for all the components of the return signal; then /3/ determining a value of a phase difference that exists between the transmission radiations as produced by two of the transmission channels (10 1, 10 2, 10 3) of the radar, referred to as transmission phase difference (Φ 02, Φ 03) for said two transmission channels, from a difference between the values of the transmission-reception phase differences (ΔΦ 01, ΔΦ 02, ΔΦ 03) that were determined in step /2/ separately for each of the modulations of said two transmission channels. /1/ activating the transmission channels (10 1, 10 2, 10 3) to produce the modulated transmission radiations (R 1), and activating the reception channel to detect the return signal, and then filtering said return signal in accordance with each modulation; /2/ separately for each modulation: determining a value of a phase difference between transmission and reception, referred to as transmission-reception phase difference (ΔΦ 01, ΔΦ 02, ΔΦ 03) for said modulation, which exists between the transmission radiation (R 1) produced and modulated in accordance with said modulation and the component of the return signal that was isolated in accordance with the same modulation, said transmission-reception phase difference being determined for signal passage points constant within the radar for each transmission channel (10 1, 10 2, 10 3) and for all the components of the return signal; then /3/ determining a value of a phase difference that exists between the transmission radiations as produced by two of the transmission channels (10 1, 10 2, 10 3) of the radar, referred to as transmission phase difference (Φ 02, Φ 03) for said two transmission channels, from a difference between the values of the transmission-reception phase differences (ΔΦ 01, ΔΦ 02, ΔΦ 03) that were determined in step /2/ separately for each of the modulations of said two transmission channels.