## 2 Mission Requirements and Experimental Platform  
When spacecrafts are launched into orbit, the relative attitudes between these spacecrafts are not adjusted, meaning the pointing uncertainty of the laser beams in generally of about 10 mrad magnitude [37]. As a result, interferometry cannot be performed. In order to implement scientific interferometry processes, three stable laser links should be established first. After the establishment of a laser link between two spacecrafts, their precise positions will be known. Then, the establishment of the other two laser links will be simple.  
The establishment of inter-satellite laser links and the interferometry of inter-satellite ranging are two key processes in inter-satellite laser interference. The specifications for these two key technologies in the Taiji program are a pointing jitter of 10 \(\mathrm{nrad}/\surd\mathrm{Hz}\) and ranging noise of 8 \(\mathrm{pm}/\surd\mathrm{Hz}\), both at a distance of 3 \(\times\) 10\({}^{6}\) km. In the Micius mission for quantum communication [38], as an intensity-sensitive system, the object of detection is the laser intensity, on which the communication information is modulated. The requirement for the pointing jitter for the satellite-to-ground laser link is better than 1 \(\mathrm{\SIUnitSymbolMicro rad}\), with an orbital altitude of 500 Km. After the establishment of the laser link, communication information is obtained by demodulating the laser intensity. This is state of the art until now; however, the Taiji program is a phase-sensitive system. A laser beam is emitted from the remote spacecraft. After the establishment of the inter-satellite laser link, a beat signal is produced on the receiving spacecraft, showing the interference of the received laser beam and a local laser beam. By measuring the phase of the beat signal, the gravitational waves can be retrieved; thus, the nrad-level for laser link establishment and the pm-level for laser interferometry ranging should be considered at the same time. For more difficult challenges, the ultimate goals are achieved in two steps. In the current step, a pointing jitter of 100 \(\mathrm{nrad}/\surd\mathrm{Hz}\) and ranging noise of 100 \(\mathrm{pm}/\surd\mathrm{Hz}\) are listed as the key specifications.  
For inter-satellite laser interference in the Taiji mission, the technologies used in the three triangle arms are basically the same. Additionally, after the establishment of a laser link for one arm, the other two are easy. As such, in order to verify the technologies and mitigate the risks of the Taiji program, a semi-physical simulation experimental platform was designed for two spacecrafts. Testing of the above-mentioned specifications is performed at the current step. Most of the key inter-satellite laser interference technologies for the triple set can also be verified. As depicted in Figure 3, two spacecraft simulation platforms with the ability for micro-vibration and high-resolution attitude adjustments are used to bear the laser interference payloads. Each spacecraft simulation platform is made up of a precision active vibration isolation platform and a piezo-driven hexapod. With the latter placed on the former, the two abilities mentioned above can be realized. The dual-spacecraft simulation platforms are placed between two freely suspended test mass simulators. The semi-physical simulation test for inter-satellite laser interference is carried out in a dual vacuum tank system. The dual vacuum tank system, which is composed of two vacuum tanks with a heat sink and a vacuum pipeline, is used to simulate the cosmic vacuum and cold-black environment where the two spacecrafts are located. With the QPDs (quadrant photodiodes) located on the laser interference payloads, both the pointing jitter of 100 \(\mathrm{nrad}/\surd\mathrm{Hz}\) and ranging noise of 100 \(\mathrm{pm}/\surd\mathrm{Hz}\) can be tested.