The racetrack beam routing system for GRACE Follow-On
The simplest way to measure the inter-satellite distances in GRACE Follow-On would be a measurement directly along the line-of-sight between the centres of gravity of the two spacecraft. In GRACE Follow-On, however, this space is already occupied by the microwave emitter and receiver link. The laser ranging instrument, then, must fit around the existing microwave ranging instrument. Importantly, the laser beam must also be routed around the microwave instrument, and it is this requirement which leads to the use of the innovative ‘racetrack’ beam routing system. At the heart of the racetrack beam routing system is the so-called triple mirror assembly, or TMA, which is based on a corner cube retroreflector.
A corner cube has the property of reflecting light back into the same direction it came from, regardless of which way the cube itself is pointing. This is illustrated in Figure 1. The incoming beam, shown in red, undergoes three reflections and comes out in the direction indicated by the yellow beam. The incoming and the outgoing beam are exactly parallel if the corner cube is manufactured correctly. The distance travelled by the light is also independent of the cube orientation.
The principle of a corner cube is also used in retro-reflectors commonly found on bikes, cars, clothes, road markings and even on the moon or on artificial satellites. In these applications the retro-reflectors help to improve the amount of light reflected back to the observer, so the object can be seen more easily when a light is shined at it. They typically consist of many small corner-cubes, which allow to cover a larger area and thereby to reflect more light back to the observer.
For the application on board the GRACE Follow-On spacecraft it is also important to place the corner of the cube in the centre of gravity of the spacecraft, since this is the point whose motion we would like to measure. This, of course, is not actually possible, as the centre of gravity is already occupied by another (very important) spacecraft instrument: the accelerometer. However, we can take advantage of the fact that only a small portion of the whole cube is actually used for the reflection and leave out the rest of the material. This achieves two goals simultaneously: it allows significant weight savings, since we can just omit most of the cube, and it allows to place the virtual corner of the cube exactly at the spacecraft centre of gravity. The resulting configuration is shown in Figure 2. It is called a triple-mirror assembly, or TMA for short. It is made from a carbon fibre tube, with high quality mirrors attached to the two ends at the appropriate angles to form the necessary portion of a corner cube retroreflector.
Here at ANU Centre for Gravitational Physics we will first test the stability of the TMA, to verify that it satisfies the performance requirements for the mission. We will also perform environmental testing to determine if the TMA can survive the heavy vibrations of a rocket launch and still fulfil the demanding requirements on the relative parallelism of the two beams after the launch.
The length stability of the TMA is critical because the GRACE mission measures the distance between two satellites to calculate the gravity field of the earth. If the length of the tube changes, than this will lead to a shift of the intersection point of the three mirror surfaces and thereby to an apparent change of the distance of the two satellites. The distance measurements are very accurate, and so very demanding requirements are put on the length stability of the TMA: the length can not change by more than one thousandth of the diameter of a human hair during one minute without spoiling the measurement.
This is to be accomplished using a set up very similar to the actual length measurement on board the two satellites when they are in orbit around earth. This setup employs two TMAs, each of which represents one of the two spacecraft. This setup is shown in Figure 3.
Light from a laser represented as red lines in the Figure 3 is sent through the left TMA and interferes with the light from a second laser, represented in orange on the input beam splitter of the second TMA. This allows to measure length changes of the left hand TMA. At the same time, part of the light from the laser represented in orange travels through the second TMA and interferes with light from the other laser in the upper middle part of the figure. Combining these two very sensitive interferometric measurements allows one to measure the length fluctuations of both TMAs and thus to assess their suitability for the application in GRACE follow-on.