  We present the frequency stability performances of a vapor cell Rb clock
based on the pulsed optically pumping (POP) technique. The clock has been
developed in the frame of a collaboration between INRIM and Leonardo SpA,
aiming to realize a space-qualified POP frequency standard. The results here
reported were obtained with an engineered physics package, specifically
designed for space applications, joint to laboratory-grade optics and
electronics. The measured frequency stability expressed in terms of Allan
deviation is $1.2 \times 10^{-13}$ at 1 s and achieves the value of $6 \times
10^{-16}$ for integration times of 40000 s (drift removed). This is, to our
knowledge, a record result for a vapor-cell frequency standard. In the paper,
we show that in order to get this result, a careful stabilization of microwave
and laser pulses is required.
