The Doris instrument is part of the satellite payload. It comprises a MVR receiver that measures radial velocity (see Doppler effect), an omnidirectional antenna and an ultrastable oscillator. This last instrument is a kind of ultrastable clock enables to acquire extremely precise radial velocity (of the order of 0.3 mm/s) and consistent with a high-precision orbitography. Every 10 seconds, it measures the Doppler shift in the frequency of radio signals transmitted by beacons at 401.25 MHz and 2 036.25 MHz. Measuring the 401.25 MHz signal shift is vital for reducing ionospheric propagation errors. Radiofrequency signals are disturbed as they go through the ionized upper layers of the atmosphere. Higher frequencies are less sensitive to ionospheric disturbances, which is why the main measuring frequency was set at 2036.25 MHz and a second frequency at 401.25 MHz for error corrections.