James J. Spilker

A new positioning system using television synchronization signals

The technique discussed herein may be used to position a range of wireless devices that require location information when in inclement urban conditions, including PDAs, laptops, cellular phones, asset-tracking devices and radios for emergency response personnel. We make use of synchronization signals that are part of the standard for Television set forth by the Advanced Television Systems Committee. Consequently, the technique described herein requires no changes to the television broadcast stations. The signal can accommodate robust indoor positioning where the Global Positioning System (GPS) fails, since the television synchronization signals typically have a power advantage over GPS of more than 40 dB. In addition, the effects of multipath are substantially mitigated since the signals have a bandwidth of roughly 6 MHz, and substantially superior geometry for triangulating lateral position to that which GPS can typically provide in inclement environments. A wide range of VHF and UHF frequencies have been allocated to television stations; consequently, there is redundancy built into the system to protect against deep fades on particular channels. In addition, unlike GPS, the synch signals are not affected by transmitter Doppler, ionospheric propagation delays, or data that is modulated onto the signals. In overview, the technology exploits the considerable Digital TV infrastructure to achieve more reliable, accurate and rapid positioning than can be achieved with existing technologies.

Global Positioning System (GPS)

The Global Positioning System, commonly referred to as GPS, is a worldwide, satellite-based positioning and timing system that allows suitably equipped radio receivers to locate themselves in four dimensions, latitude, longitude, altitude, and time, anywhere there is a reasonably clear view of the sky. The system is also known as NAVSTAR, a convenient nickname that is not an acronym. The GPS system was developed, deployed, and is currently operated by the U.S. Air Force. GPS enables precision weapon delivery for all branches of the U.S. Department of Defense, as well as allied nations. Additionally, GPS supports civilian positioning and was always intended to support civil operations. The complete satellite constellation and ground support equipment that make up GPS was declared “operational” in December 1994, although civil use of the developmental signals started in the early 1980s. The fundamental operation is as follows: the 24 GPS satellites are uploaded from the ground with their current and predicted positions (called ephemeris or orbital parameters). Small corrections of their space-borne atomic clocks are also uploaded. This information is broadcast to the user as a data modulation on an L-band signal (1575 MHz for most civilian users) that doubles as a precise, one-way ranging signal. Ranging is achieved by synchronizing the start time of a pseudorandom sequence of bits transmitted from the GPS satellites at an accuracy of about one nanosecond (10−9 s). Three very important results are achieved by this implementation. First, this makes GPS ranging a one-way signal that allows an infinite number of users to receive the signal and compute their position without saturating the GPS system. Additionally, this makes the GPS receiver passive, so that it does not radiate radio-frequency (RF) energy. Last, by receiving four or more satellite signals, users can synchronize their local clocks to GPS time, obviating the need for a very high quality and very expensive atomic clock in the receivers. The design objectives of the GPS system were to provide a continuously available, worldwide, all-weather, three-dimensional precision navigation system for both military and civilian users on land, at sea, or in the air (or even in space). Selected application of the GPS system are discussed. Many topics presented are on the cating edge of research. Keywords: Global Positioning Systems (GPS); satellites; history; TRANSIT; timation; 621B; predecessors; operation; multiateration; signal structure; space segment; ground control; navigational data; user; ranging errors; error analysis; differential GPS; national differential GPS; user; ranging errors; error analysis; differential GPS; national differential GPS; Wide Area Augmentation System; Local Area Augmentation System; survey; crustal motion; aviation; vehicle tracking; precision munitions; space applications; Galileo