  We describe the development of a simple atomic magnetometer using $^{87}$Rb
vapor suitable for Earth magnetic field monitoring. The magnetometer is based
on time-domain determination of the transient precession frequency of the
atomic alignment around the measured field. A sensitivity of 1.5 nT/$\sqrt{Hz}$
is demonstrated on the measurement of the Earth magnetic field in the
laboratory. We discuss the different parameters determining the magnetometer
precision and accuracy and predict a sensitivity of 30 pT/$\sqrt{Hz}$
