I got myself a 2005 edition of the Garmin eTrex GPS receiver a week ago (available at Sim Lim Square and Sim Lim Tower for less than S$200). This is like a high frequency radio receiver (tuned to a frequency of 1575.72 MHz) detecting signals sent by 29 satellites hovering overhead at an altitude of 20,200 km. If you are able to get the signal from four satellites, the...