  ASTROD is a relativity mission concept encompassing multi-purposes. One of
its main purposes is to detect gravitational waves sensitive to low-frequency
band similar to LISA, but shifted to lower frequencies. In this aspect, ASTROD
would complement LISA in probing the Universe and study strong-field black hole
physics. Since ASTROD will be after LISA, in the Cosmic Vision time-frame
2015-2025, a ten-fold improvement over LISA accelerometer noise goal would be
possible, allowing us to test relativistic gravity to 1 ppb and improve the
gravitational-wave sensitivity. In this paper, we address to this possible
improvement, especially in the frequency range below 0.1 mHz. We look into
possible thermal noise improvement, magnetic noise improvement, spurious
discharging noise improvement and local gravitational noise improvement. We
discuss various possibilities of lower-frequency gravitational-wave responses
and their significance to potential astrophysical sources.
