The sounds of a helicopter on Mars

The sounds of the Ingenuity Helicopter flying in the Martian atmosphere are among the most notable recordings of the microphone on the SuperCam instrument on the Mars 2020 Perseverance Rover. Distinct acoustic signatures of the helicopter were recorded on the 4th, 5th, 6th,and 8th flights: prior to this, simultaneous microphone and helicopter operations had not been verified in the testbed, and generally since these early flights the helicopter has been too far away for its emissions to be detectable given CO2absorption in the Mars atmosphere.The detected signatures are around 84 ​Hz and (occasionally) at 168 ​Hz, at the blade crossing frequency and its first harmonic. Several higher harmonics were prominent in hover tests in short-range recordings in a test chamber on Earth; these are attenuated by CO2absorption at the 50m-plus ranges on Mars. Doppler shift of the 84 ​Hz signal can be measured and is consistent with the trajectory measured with Ingenuity's navigation camera and inertial navigation unit, and documented by Perseverance's cameras.A striking feature of the sound recordings is an unanticipated deep modulation of the signals with nulls spaced by around 15–20s, superposed on the simple and expected decline in amplitude with distance. We have evaluated and rejected models of multipath sound interference as requiring implausibly strong near-surface temperature gradients. We find instead that the modulation appears to be the signature of a slight asynchrony between the rotation rates of the two coaxial rotors, such that the blade-crossing azimuth rotates slowly during flight, resulting in a ‘lighthouse’ sweeping of the radiated sound pattern. Analysis of blade orientations seen in the shadow of the helicopter observed in down-looking navigation images supports this model.

“机智号”直升机在火星大气中飞行的声音是“火星2020毅力号”探测器上超级相机仪器麦克风最显著的录音之一。在第4、5、6和8次飞行中记录到了直升机独特的声学特征：在此之前，在测试台上尚未验证麦克风和直升机同时运行的情况，而且一般来说，自这些早期飞行以来，由于火星大气中的二氧化碳吸收，直升机飞得太远，其发出的声音无法被探测到。检测到的特征频率约为84赫兹，（偶尔）为168赫兹，分别是桨叶交叉频率及其一次谐波。在地球上的一个测试室进行的短距离悬停测试中，几个高次谐波很明显；在火星上50多米的距离处，这些谐波因二氧化碳吸收而减弱。可以测量84赫兹信号的多普勒频移，它与“机智号”导航相机和惯性导航单元测量的轨迹一致，并由“毅力号”的相机记录下来。录音的一个显著特征是信号出现了意想不到的深度调制，零点间隔约为15 - 20秒，叠加在振幅随距离简单且预期的下降之上。我们已经评估并否定了多径声音干扰模型，因为它需要不合理的强近地表温度梯度。我们发现，这种调制似乎是两个同轴旋翼旋转速率之间轻微不同步的特征，使得桨叶交叉方位角在飞行过程中缓慢旋转，从而导致辐射声模式像“灯塔”一样扫动。对俯视导航图像中观察到的直升机阴影中桨叶方向的分析支持这一模型。