Why were chip-scale atomic clocks (CSAC) made?

The development of CSAC enabled ultra-miniaturized and ultra-low power atomic clocks for high-security Ultra High Frequency (UHF) communication and jam-resistant GPS receivers

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.