ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) is a mission concept with three spacecraft -one near L1/L2 point, one with an inner solar orbit and one with an outer solar orbit, ranging coherently with one another using lasers to test relativistic gravity, to measure the solar system and to detect gravitational waves. ASTROD I with one spacecraft ranging optically with ground stations is the first step toward the ASTROD mission. In this paper, we present the ASTROD I payload and accelerometer requirements, discuss the gravitational-wave sensitivities for ASTROD and ASTROD I, and compare them with LISA and radio-wave Doppler-tracking of spacecraft.

Advances in laser physics and its applications have triggered the proposition and development of Laser Astrodynamics. In carrying out research projects on Laser Space Programs, it is necessary to process the laser signal sent back from remote spacecraft. After traveling an extremely long distance, the power of this signal is greatly reduced. Weak-light phase-locking is the key technique used for signal amplification in these space projects. After the returning laser beam is collected by telescope, it is used to phase-lock a local laser oscillator. The local laser then carries the phase information of the remote spacecraft laser. we used diode-pumped non-planar ring cavity Nd:YAG lasers to serve as the remote weak-light laser and the local strong-light laser. We then built an optical phase-locked loop to phase-lock them. The weak-light laser signal was simulated using ND (neutral density)-filters to decrease the light intensity. In the phase detection, we used balanced detection to eliminate laser intensity noise and improve the S/N ratio. Combining this with an appropriate loop filter, we were able to control the laser frequency and improve the phase-locking ability. We phase-locked a 2 nW weak-light beam and a 2 mW strong-light beam with a 57 mrads(rms)phase error. The locking duration was very long. Locking of a 200 pW and a 2007thinsp;μW light beam, with phase error of 200 mrad (rms) and duration of over 2 hours was achieved. The phase error for locking a 200 μW to a 20pW light beam was 160 mrad (rms). The locking duration was also longer than 2 hours. the last locking performed was carried out with a 2 pW and a 200 μW light beam. The phase error and the locking duration were 290 mrad(rms) and 1.5 min respectively.