Received: 09 Mar 2021 – Accepted for review: 10 Mar 2021 – Discussion started: 11 Mar 2021
Abstract. The acquisition of atmospheric wind profiles on a global scale was realized by the launch of the Aeolus satellite, carrying the unique Atmospheric LAser Doppler INstrument (ALADIN), the first Doppler wind lidar in space. One major component of ALADIN is its high-power, ultraviolet (UV) laser transmitter which is based on an injection-seeded, frequency-tripled Nd:YAG laser and fulfills a set of demanding requirements in terms of pulse energy, pulse length, repetition rate as well as spatial and spectral beam properties. In particular, the frequency stability of the laser emission is an essential parameter which determines the performance of the lidar instrument, as the Doppler frequency shifts to be detected are on the order of 108 smaller than the frequency of the emitted UV light. This article reports the assessment of the ALADIN laser frequency stability and its influence on the quality of the Aeolus wind data. Excellent frequency stability with pulse-to-pulse variations of about 10 MHz (root mean square) is evident for over more than two years of operations in space despite the permanent occurrence of short periods with significantly enhanced frequency noise (> 30 MHz). The latter were found to coincide with specific rotation speeds of the satellite's reaction wheels, suggesting that the root cause are micro-vibrations that deteriorate the laser stability on time scales of a few tens of seconds. Analysis of the Aeolus wind error with respect to ECMWF model winds shows that the temporally degraded frequency stability of the ALADIN laser transmitter has only minor influence on the wind data quality on a global scale, which is primarily due to the small percentage of wind measurements for which the frequency fluctuations are considerably enhanced. Hence, although the Mie wind bias is increased by 0.3 m·s−1 at times when the frequency stability is worse than 20 MHz, the small contribution of 4 % from all wind results renders this effect insignificant (< 0.1 m·s−1) when all winds are considered. The impact on the Rayleigh wind bias is negligible even at high frequency noise. Similar results are demonstrated for the apparent speed of the ground returns that are measured with the Mie and Rayleigh channel of the ALADIN receiver. Here, the application of a frequency stability threshold that sorts out wind observations with variations larger than 20 MHz changes the accuracy of the Mie and Rayleigh ground velocities by less than 0.15 m·s−1.
The long term laser frequency stability is first reported for spaceborne high energy solid-state laser. The difference performance between under-ground and in- orbit is implemented. The enhanced frequency noise due to the satellite’s reaction wheels is discovered. Two year's global frequency stability of laser is present. It is very significant for future frequency-stability spaceborne laser development. Aeolus wind error in both Mie and Rayleigh Channel due to the enhanced frequency noise is analyzed. The wind error can be accepted for the ECMWF mode.
It is better that the mechanics of the frequency noise enhancement in the master oscillator due to micro-vibration is given.
The work assesses the frequency stability of the laser transmitters on-board Aeolus and discusses its influence on the quality of the global wind data. Excellent frequency stability of the space lasers is evident, although enhanced frequency noise occurs at certain locations along the orbit due to micro-vibrations that are introduced by the satellite’s reaction wheels. The study elaborates on this finding and investigates the extent to which the enhanced frequency noise increases the wind error.
The work assesses the frequency stability of the laser transmitters on-board Aeolus and...