A strong candidate for use in future missions to map time variations in the Earth's gravity field is laser heterodyne measurements between separate spacecraft. At the shortest wavelengths that can be measured in space, the main accuracy limitation for variations in the potential with latitude is expected to be the frequency stability of the laser. Thus the development of simple and reliable space-qualified lasers with high frequency stability appears to be an important goal for the near future. In the last few years, quite high stability has been achieved by locking the second harmonic of a Nd:YAG laser to a resonant absorption line of iodine molecules in an absorption cell. Such a laser system can be made quite robust, and temperature related frequency shifts can be controlled at a low value. Recent results from laboratory systems are described. The Allan standard deviation for the beat between two such lasers was 2 x 10(-14) at 10 s, and reached 7 x 10(-15) at 600 s. [References: 12]
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机译：在未来的任务中用于绘制地球重力场中的时间变化图的一个强有力的候选人是在单独的航天器之间进行激光外差测量。在空间中可以测量的最短波长下，预期纬度随电势变化的主要精度限制是激光器的频率稳定性。因此，开发具有高频稳定性的简单可靠的符合太空要求的激光器似乎是近期的重要目标。在最近几年中，通过将Nd：YAG激光器的二次谐波锁定到吸收池中碘分子的共振吸收线上，已经获得了很高的稳定性。可以使这种激光系统非常坚固，并且可以将与温度有关的频移控制在较低的值。描述了实验室系统的最新结果。两个这样的激光器之间的节拍的艾伦标准偏差在10 s为2 x 10（-14），在600 s达到7 x 10（-15）。 [参考：12]
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