1. Field of the Invention
Embodiments of the present invention generally relate to satellite position location systems and, more particularly, to a method and apparatus for managing time in a Global-Navigation-Satellite System.
2. Description of the Related Art
Global-Navigation-Satellite-System (GNSS) receivers, such as Global Positioning System (GPS) receivers, use measurements from several satellites to compute position. GNSS receivers normally determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide the distance from the receiver to each of the satellites that are in view of the receiver.
For example, each GPS signal available for commercial use utilizes a direct sequence spreading signal defined by a unique pseudo-random noise (PN) code (referred to as the coarse acquisition (C/A) code) having a 1.023 MHz spread rate. Each PN code bi-phase modulates a 1575.42 MHz carrier signal (referred to as the L1 carrier) and uniquely identifies a particular satellite. The PN code sequence length is 1023 chips, corresponding to a one millisecond time period. One cycle of 1023 chips is called a PN frame or epoch.
GPS receivers determine the time delays between transmission and reception of the signals by comparing time shifts between the received PN code signal sequence and internally generated PN signal sequences. These measured time delays are referred to as “sub-millisecond pseudoranges”, since they are known modulo the 1 millisecond PN frame boundaries. By resolving the integer number of milliseconds associated with each delay to each satellite, then one has true, unambiguous, pseudoranges. A set of four pseudoranges together with a knowledge of absolute times of transmission of the GPS signals and satellite positions in relation to these absolute times is sufficient to solve for the position of the GPS receiver. The absolute times of transmission (or reception) are needed in order to determine the positions of the GPS satellites at the times of transmission and hence to compute the position of the GPS receiver.
Accordingly, each of the GPS satellites broadcasts information regarding the satellite orbit and clock data known as the satellite navigation message. The satellite navigation message is a 50 bit-per-second (bps) data stream that is modulo-2 added to the PN code with bit boundaries aligned with the beginning of a PN frame. There are exactly 20 PN frames per data bit period (20 milliseconds). The satellite navigation message includes ephemeris data, which identifies the satellites and their orbits, as well as absolute time information (also referred to herein as “GPS time”, “satellite time”, or “time-of-day”) associated with the satellite signals. The absolute time information is in the form of a second of the week signal, referred to as time-of-week (TOW). This absolute time signal allows the receiver to unambiguously determine a time tag for when each received signal was transmitted by each satellite.
In some GPS applications, the signal strengths of the satellite signals are so low that either the received signals cannot be processed, or the time required to process the signals is excessive. As such, to improve the signal processing, a GPS receiver may receive assistance data from a network to assist in satellite signal acquisition and/or processing. For example, the GPS receiver may be integrated within a cellular telephone and may receive the assistance data from a server using a wireless communication network. This technique of providing assistance data to a remote mobile receiver has become known as “Assisted-GPS” or A-GPS.
In some A-GPS systems, the wireless communication network that provides the assistance data is not synchronized to GPS time. Such non-synchronized networks include time division multiple access (TDMA) networks, such as GSM networks, universal mobile telecommunications system (UMTS) networks, North American TDMA networks (e.g., IS-136), and personal digital cellular (PDC) networks.
In these types of networks, absolute time information is presently obtained at the base stations of such wireless networks using co-located location measurement units (LMUs). Each of these conventional LMUs includes a GPS receiver that is used to receive and decode the TOW information from the satellites in view of the base stations that are near the LMU. The conventional LMU then computes an offset between GPS time and the local time at such base stations. The offset is then supplied to the base stations, which in turn, use the offset to correct their local time. One disadvantage associated with conventional LMUs is that the wireless communication network typically includes many thousands of base stations, thus requiring many conventional LMUs. Providing a large number of conventional LMUs is significantly expensive, and is thus undesirable.
Therefore, there exists a need in the art for a method and apparatus that manages time within an assisted GNSS without employing conventional LMUs.