October 2000, Issue 123

Navigating With GPS

GPS FUNDAMENTALS

GPS became available in 1978 with the successful launch of NAVSTAR 1. NAVSTAR 1 was the first of four NAVSTAR satellites launched that year, creating an operational satellite navigation system for the military. Then in 1982, Russia launched a system called GLONASS.

GPS satellites are incredible instruments. Each satellite contains four atomic clocks that operate on a level of one second of error in three million years. This degree of precision time keeping is required so each satellite can operate autonomously yet remain synchronized. GPS satellites transmit ranging codes based on a signal’s time of arrival, not position and motion.

These satellites, which are at known locations at all times, transmit on two L-band carrier signals. The satellite’s receiver marks the difference between the time the signal was sent and received, and multiplies the difference by the signal speed (close to the speed of light). Using ranging code from four satellites, a GPS receiver can calculate its own position in three-dimensional space, including the receiver’s velocity.

The NAVSTAR system breaks down navigation into two domains, Standard Positioning Service (SPS) and Precise Positioning Service (PPS). PPS accuracy is published at 21-m horizontally and 29-m vertically. The early NAVSTAR SPS was so accurate that it was considered a threat, so the gap between SPS and PPS was intentionally widened. The accuracy level of the SPS was decreased to 100 m in the horizontal plane and 160 m in the vertical plane. The decrease, called selective availability (SA), introduced error into the satellite orbital data and time transmissions.

SA made life more difficult for commercial GPS-based navigation systems. One hundred meters (roughly 300¢) of accuracy isn’t bad, but if you’re trying to develop a precise hand-held or automotive navigation system, more accuracy is needed. To the delight of the navigation community, the U.S. government turned off SA on May 1, 2000. Instead of 100 m, accuracy now is within 10 to 30 m in the horizontal plane and slightly more in the vertical plane.

Now, the floodgate is open for new and highly accurate GPS applications based on latitude, longitude, and time. GPS receivers turn up in everything from wristwatches to locomotives.

Latitude and longitude are fundamentals of navigation. Sometimes it’s difficult to remember which is which. I use the mnemonic "it’s a long way from the North Pole to the South Pole." Longitude lines run from the North Pole to the South Pole and are measured in half circles from the Royal Greenwich Observatory in Greenwich, UK. Longitude lines run from 0° to 180° east and 0° to 180° west (see Figure 1).

Figure 1—Longitude lines run east and west from pole to pole. Latitude lines run north and south, parallel to the equator.

Latitude lines run in parallel from the equator to the North and South Poles. Latitude lines run from 0° at the equator to 90° at the North and South Poles. As the lines of latitude get closer to the poles, they become smaller. This presents a problem when trying to use a two-dimensional distance formula, as I’ll explain later.