ALADIN laser frequency stability and its impact on the Aeolus wind error

Abstract

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, and 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 2 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 timescales of a few tens of seconds. Analysis of the Aeolus wind error with respect to European Centre for Medium-Range Weather Forecasts (ECMWF) model winds shows that the temporally degraded frequency stability of the ALADIN laser transmitter has only a 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 Mie 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 filters out wind observations with variations larger than 20 or 10 MHz improves the accuracy of the Mie and Rayleigh ground velocities by only 0.05 and 0.10 m s−1, respectively, however at the expense of useful ground data.

Validation of Aeolus wind profiles using ground-based lidar and radiosonde observations at La Réunion Island and the Observatoire de Haute Provence

“…This separation between orbital phases is done because of the aforementioned distance to collocation disparities. Nonetheless, several long-term cal/val activities showed significant phase-dependent differences in the determined biases of Aeolus wind measurements (Wu et al, 2021, Iwai et al, 2021Lux et al, 2021;Baars et al, 2020;Rennie and Isaksen, 2020;Geiß et al, 2019; Krisch and the Aeolus DISC, 2020), further assessing the need for an independent study on both cases. The correlation plot of the Aeolus wind is shown in Fig.…”

“…To do so, we showed how the distance to collocation had small effects on the bias in Sect.4.2 and how the time difference had a significant impact on the standard deviation in Sect.4.3. Some error sources were pointed out by previous research, e.g., hot pixels and dark current anomalies (Weiler et al, 2021a), Rayleigh wind errors introduced by angular variations (Lux et al, 2022, Lux et al, 2018, Lux et al, 2020a, vibrations introduced by the satellite platform, which affects the Q-switched master oscillator cavity length (Lux et al, 2020b), photon shot noise (Liu et al, 2006), micro-vibrations due to critical rotation speeds of the satellite's reaction wheels (Lux et al, 2021), mechanical disturbances generated by reaction wheels of the class of those embarked on Aeolus (Le., 2017), linear drift in the illumination of the Rayleigh/Mie spectrometers, and the telescope M1 mirror temperature variations Weiler et al, 2021b). In section 4.1, we observe a steep increase in the random error above 25 km, which can be further observed in fig.…”

Spectral performance analysis of the Aeolus Fabry–Pérot and Fizeau interferometers during the first years of operation

“…In this paper, the attention is directed to the receiver side of the system. A more detailed description of ALADIN is given in ESA (2008) and Reitebuch et al (2018); the laser transmitters as well as their frequency stability in space are discussed by Lux et al (2020aLux et al ( , 2021.…”

Retrieval improvements for the ALADIN Airborne Demonstrator in support of the Aeolus wind product validation

“…The two campaigns not only considerably extended the available dataset to validate Aeolus wind observations in different geographical regions and under different atmospheric conditions in terms of cloud types and dynamics, but also served to assess the accuracy and precision of the Aeolus wind product in different phases of the mission. In particular, the switch-over from the first flight model laser (FM-A) to the redundant laser FM-B just after AVATARE in June 2019 marked a caesura, as the FM-B delivered much higher emit energy and hence ensured higher signal-to-noise ratio of the backscatter return compared to the end of the FM-A period (Lux et al 2020b;Lux et al, 2021). Moreover, the satellite instrument was found to be more susceptible to changes in the thermal environment than expected before launch showing orbital and seasonal variations in the beam alignment, and hence in the wind bias (Martin et al, 2021;Chen et al, 2021).…”

“…Thanks to these modifications, UV frequency stability of 1.8 MHz and timing stability below 100 ns were simultaneously achieved, thus forming the basis for highly accurate wind measurements. For comparison, the frequency stability of the ALADIN laser is on the order of 8 to 10 MHz (Lux et al, 2021). However, it was found that other error sources limited the accuracy and precision of the A2D wind results.…”