1. Field of the Invention
The present invention relates to wireless communications systems, and particularly to wireless communications systems using aircraft to determine aircraft position and for monitoring aircraft position.
2. Description of the Related Art
Locating mobile platforms is vital for many applications and consequently attracts much attention. Radio positioning or use of radio waves to locate mobile platforms includes both non-cooperative techniques (e.g., radar) and cooperative techniques wherein mobile platforms receive only, transmit only, or both receive and transmit (e.g., GPS, Teletrac, or EPLRS, respectively).
All of these techniques rely on radio wave propagation time between transmitter and receiver. Most systems based on these techniques employ reference sites with fixed, known geolocations as a basis for locating mobile platforms although some systems use mobile reference platforms with locations separately determined, e.g., state of the art literature describes a means for determining locations for satellite reference platforms used in a positioning system such as GPS.
In using GPS, each user (mobile platform) makes simultaneous or near-simultaneous time-of-arrival measurements on signals arriving from at least four different GPS satellites. These measurements resolve unknown user platform parameters (px, py, pz and t) because satellite ephemeris are approximately known and GPS satellites are synchronized (i.e., their relative clock offsets are known). However, specialized GPS receivers are necessary to perform GPS-related geodetic determinations.
In using Teletrac, each of four or more reference sites makes simultaneous time-of-arrival measurements on signals arriving from the mobile platform. Together, these measurements resolve unknown user platform parameters (px, py, pz and t) because reference site geodetic positions are known, and these sites are synchronized (i.e., their relative clock offsets are known). Some systems of this type use measurements, not necessarily time-of-arrival measurements, made at different times (e.g., Doppler frequency measurements on different orbital passes for SARSAT) to determine locations.
Position Location Reporting System (PLRS) relies on two-way time-of-arrival measurements to develop range measurements subsequently converted to position fixes based on three participating platforms with known locations (references) as well as barometric altitude for each mobile platforms. Mobile platforms with position fixes can serve as references for determining positions for other mobile platforms as they enter the system. All computation of position occurs at master stations, and mobile platforms know their positions only if master stations provide this information to these platforms.
Four reference platforms are sufficient to provide absolute position for asynchronous participants in position location systems (e.g., GPS): three to resolve position and one for participant timing (clock) offset. Reference platforms may be mobile (like GPS satellites) provided their positions (and offsets) are calculable by user platforms. Among position location systems using reference sites with known geolocations as a basis for locating mobile platforms, position determination for cellular phones has enjoyed great popularity recently due largely to government-mandated E911 requirements.
As already described, other systems use two-way ranging, between mobile platforms and reference platforms only, to locate mobile platforms. These systems reduce the minimum number of reference sites from four to three for three-dimensional position location or from three to two for two-dimensional position location, because paired two-way measurements (platform j to platform k and vice versa) eliminate timing offsets. Use of two-way ranging (or round trip delay measurements) has ancillary benefits such as eliminating the need for highly accurate clocks for mobile platforms.
Alternatively, use of additional measurements such as angle-of-arrival measurements can also reduce the number of fixed sites required to as few as two for a three-dimensional position location. Especially where ground-based reference sites support position location over vast areas, reducing the number of sites required is important in controlling infrastructure cost.
Available systems cannot reduce the number of reference sites below two regardless of the technique(s) employed without significant sacrifices in performance (whether position location quality or time to first fix), depending upon the approach taken and mobile platform parameters.
Further, presently available systems all rely on outside systems, such as GPS, to perform position determination. However, GPS systems and signals may not always be available due to weather, satellite degradation or failure, or other factors.
It can be seen, then, that there is a need in the art for an independent system for determination of geodetic position without the use of, and independent of, GPS or other external signals. It can also be seen that there is a need in the art for making position information for all mobile platforms available externally without recourse to an external communications system as required when using GPS for developing mobile platform locations.