One of the key components for improving positioning accuracy of the global navigation satellite system (GNSS) is the stability of the on-board microwave reference. Therefore, we have proposed a spaceborne photonic microwave generator which is composed of a frequency-stabilized laser and an optical frequency comb whose frequency stability is much higher than that of currently-used rubidium atomic clocks. We use an iodine-stabilized laser as the optical frequency reference whose frequency is down-converted to the microwave region by using an optical frequency comb without degradation [1]. A nonlinear amplifying loop mirror (NALM)-type figure-8 mode-locked fiber laser has been selected as the oscillator of the optical frequency comb. The cavity of our figure-8 mode-locked fiber laser consists of all polarization-maintaining (PM) fibers, which shows less sensitivity against external disturbances such as mechanical vibrations and temperature fluctuations, and is expected to be robust and stable in long-term operation. However, the phase noise of conventional figure-8 mode-locked fiber lasers has been worse than that of the other-type fiber mode-locked laser (NALM figure-9 or nonlinear polarization rotation) [2]. Moreover, figure-8 type mode-locked fiber laser requires the initial pulses for starting mode-locking operation [3]. Our laser has a combination of high- and low-concentration Erbium-doped fibers as the gain media, and with the help of a non-reciprocal phase shifter, the self-start of the mode-locking operation is successfully realized in which a series of pump power control sequence generates stable mode-locking (Fig. 1). The net cavity dispersion of our mode-locked laser is designed to be -0.01 $\text{ps}^{2}$ to achieve stretched- pulse mode-locking. The self-start success rate of our mode-locked fiber laser reaches more than 99.94 % in more than 1000 trials where our self-start sequence is automatically repeated by the computer-controlled system. Furthermore, the disturbance test (maximum acceleration of 10 $\mathrm{m}/\mathrm{s}^{2}$ ) and the thermal vacuum test ( $\mathrm{T}=-20\text{to} 65^{\circ}\mathrm{C}$ at the pressure of 10 ⁻⁵ Pa) have been applied to our mode-locked fiber laser which simulates the space environment. In both environment tests, the mode-locking operation is stably maintained, and more than 90% of the self-start success rate has been achieved. The $f_{\text{ceo}}$ beat note is detected by using $f-2f$ interferometer. The measured linewidth of $f_{\text{ceo}}$ beat note is less than 100 kHz (Fig. 2), which is comparable with our figure-9 mode-locked fiber laser and is low enough for our stable microwave generation system. The detail of the phase noise investigation and the results of radiation test will be also presented.