The Global Positioning System (GPS)#1 built by the US miltitary is the de facto satellite-based navigational system used by many civilians today. Many other systems exist for navigational assistance including those that are land-based or augmented with land-based systems. This site shall focus mainly on the US-made GPS system#2. We are not aware of any publicly-accessible land-based augmentation system to the GPS in Singapore and nearby regional countries. As such, we shall not discuss much about such systems - a notable one of which is the US' WAAS.
How the GPS came into civilian use
In September 1983, a Korean Air Lines passenger jet flying from New York to Seoul strayed into Soviet airspace. This was a time, when the U.S.S.R. was still in a cold war with the other world superpower, the USA.
Soviet fighter jet planes scrambled into the skies to confront the passenjer jet, which probably appeared to be a foreign spy plane to the Soviet force. Moments later, Korean Air Lines KAL 007 was shot down over the Sea of Japan killing all 269 passengers and crew.
Due to this incident, then-President of the United States, Ronald Reagan, announced that the Global Positioning System (GPS) being built by the military at the time, would be available for civilian use once it was completed. He believed the tragedy would have been avoided had the navigational error made by the crew of KAL 007 been corrected with a global positioning system available to them.
In January 1994, the GPS system reached full operational status when the complete constellation of the required 24 satellites was put into orbit around the Earth.
On 2 May 2000, the full GPS signal was made open to the public. This allowed civilian GPS receivers to decode the same quality signals as the military and achieve a higher positional precision capability than with the degraded signals before this.
#1 More accurately, the system is known as the NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System).
#2 "GPS system" seems like a tautology ("system" is repeated). However, the acronym 'GPS' is already in such common use that it has become a word itself. As such, we shall use both "GPS" and "GPS system" interchangeably to refer to the system (which comprises satellites, protocols, receiver devices etc.).