erver) An IBM mainframe feature that lets TN3270 clients access LPD/LPR printers via the SNA/VTAM network. See TN3270
ystem) A satellite-based radio navigation system run by the U.S. Department of Defense, officially known as NAVSTAR GPS (see also GLONASS
). Originally created for the military, today, GPS receivers are built into all smartphones and millions of cars and trucks. See in-dash navigation
, Android Auto
and portable GPS
GPS vs. the "Nav"
Technically, the GPS is the satellite system, whereas a "navigation" system or "nav" is the GPS receiver that picks up the signals. In practice, most people refer to their navs as their GPS.
Three or Four Signals Are Required
At least four satellites are on the horizon at all times, which is sufficient to compute the current latitude, longitude and elevation anywhere on earth to within 15 to 70 feet (see latitude
). If only three signals are present due to interference, elevation cannot be derived. If fewer than three, the location cannot be computed unless the nav is built into the vehicle and interfaces with the speedometer (see in-dash navigation
The Satellite System
In six different orbits approximately 12,500 miles above the earth, 24 medium-earth orbit (MEO) satellites circle the earth every 12 hours. They constantly transmit their current time based on atomic clocks and current location on two frequencies in the L-band (L1: 1575.2 MHz; L2: 1227.6 MHz). Most navs pick up L1, while more advanced receivers pick up both for greater accuracy.
The nav calculates the distance to the satellites by comparing the times the transmitted signals were sent with the times received. By knowing the precise locations of the satellites at any given moment, the nav computes the current coordinates using trilateration, a method similar to how ship captains navigated for centuries (see triangulation
First Launched in 1978
In this decades-old system, satellites are periodically replaced. In addition, worldwide government and commercial monitoring networks use earth stations to improve accuracy; for example, enabling farm equipment to plant ultra-precise rows of crops (see GPS augmentation system
). See social navigation
, vehicle tracking
, reality view
, GPS augmentation system
, mobile positioning
An Early Car Nav
In 1996, Sony's NVX-F160 was one of the first vehicle navigation systems, and it was able to find the nearest restaurant and hotel. (Image courtesy of Sony Corporation.)
GPS in the Woods
Navigation App in a Laptop
Portable navigation works everywhere. The flat object pointing up is the antenna. See portable GPS
The Delorme Street Atlas offers extensive details including satellites in range (#15 just lost signal), elevation (Elv), azimuth (Az) and signal-to-noise ratio (dB). NET means: N=navigation satellite, E=ephemeris data available and T=being tracked (yellow object on dashboard is the GPS antenna).
It Doesn't Get Better Than This
Tesla's navigation system is unlike any other. The 17" touchscreen displays more map area than any other nav screen.