An important application of the global positioning system enables users to determine the remote location of assets, that incorporate a transceiver, through an appropriate software application. For example, a surface transport tractor-trailer may automatically report its position to a proprietary dispatch system, determining position via the GPS constellation. The GPS constellation is a group of at least 24 GPS satellites, currently 28 GPS satellites, that orbit the earth and provide location information to GPS systems. Another application provides location determination capabilities for cellular phones for the United States Federal Communications Commission (FCC) wireless Enhanced 911 (E911) program. In order to report its position, a remote unit must “know” where it is. In order to do this, the remote unit acquires its position through interaction with a minimum of four GPS Satellites.
The NAVSTAR (Navigation Satellite Timing and Ranging) Global Positioning System is a space-based radionavigational system that provides a dual-use global positioning and navigation service to military and civilian users. NAVSTAR is managed by the Interagency GPS Executive Board (IGEB), and is co-chaired by the United States Department of Defense and the United States Department of Transportation. Information is based on a nominal 24-satellite constellation at an altitude of 20,184 m, with satellites distributed equally in six orbital planes separated by 60°. The array of satellites is known as the GPS constellation. Signal services provided are L1-C/A at 1575.42 MHz and L2-C/A at 1227.6 MHz. Additionally, a new L2 Civil Signal (L2CS) at 1227.6 MHz will become operational by 2008 and a Safety-of-Life signal L5 at 1176.45 MHz is intended to be operational by 2013. The civilian GPS standard positioning service (GPS-SPS) is designed to provide global coverage with between five and eight visible satellites from any location. Global availability averages better than 99.94%.
The NAVSTAR System uses two techniques to improve GPS receiver performance; Code Division Multiple Access (CDMA) as a means to allow different satellites to transmit on the same frequency with limited interference, and direct sequence-spread spectrum (DSSS) as a means to increase resistance to interference and recover damaged ranging data. The GPS broadcast has three components: Carrier Wave, Ranging Codes and Navigation Message. The NAVSTAR System operates at a system dock frequency of 10.23 MHz, which is a sub-multiple of the L1 carrier frequency (1575.42 MHz=154 *10.23 MHz). The GPS L1 carrier broadcast message is the modulo-2 spread of the 50 bps NAV bit-train and a Pseudorandom Noise (PRN) Code. PRN-Codes have the characteristics of random noise, but are a sequence defined by a 1023-chip maximal sequence Bi-Phase Shift Key (BPSK) modulation (i.e. alternating 1s and 0s). PRN-Code sequences are generated with two 10-bit Linear Feedback Shift Registers (LFSRs); the output is combined by an exclusive-OR (XOR) addition; the signal advances with each new value created during the dock cycle.
Thirty-six unique PRN-sequences (also known as Gold-Codes) may be generated in this manner, ensuring that no two PRN-sequences will match. PRN-Code sequences depend on the G2-register “tap” combinations (or seed values) used to initialize the operation, and the G1-register polynomial that defines the LFSR. Code-correlating receivers extract the Navigation Message from the Carrier by generating PRN reference sequences to identify SVs by PRN-Code matches. When the patterns are synchronized the receiver mathematically extracts the embedded Navigation Message by modulo-2 recovery from the carrier link frequency. The C/A-Code provides an unambiguous reference for a receiver to determine carrier signal travel-time (by clock offset); as well as, pseudorange based on the C/A-Code “chip-period.” Mobile receivers use satellite ephemeris (Keplerian parameters) broadcast in the Link Carrier Frequency as their reference for determining satellite position, when used in conjunction with pseudorange, enables PVT Solution. NAVSTAR data broadcasts contain satellite ephemeris parameters based on the U.S. military World Geodetic System (WGS-84 G1150). Reference frame receiver calculations are based on Earth-Centered Earth-Fixed (ECEF) (X,Y,Z,t) Coordinates. A GPS Solution is transformed automatically in a single-step to the more intuitive, and more commonly used, geodetic-coordinate system of Latitude, Longitude and Altitude.
For geo-location positioning, a GPS Receiver must find and acquire signals transmitted from a minimum number of GPS Satellites, typically four, unless augmented to eliminate clock bias. Each satellite space vehicle has its own Pseudo Random Number (PRN) Code to uniquely identify it. Each satellite transmits satellite ephemeris, i.e. Keplerian parameters, and timing chip sequence enabling remote units to derive satellite pseudorange and ultimately position-velocity-time (PVT) solution. Consequently, remote units may autonomously determine their latitude, longitude and altitude, reporting the results to a user through some form of software application programming interface.
Generally, a remote unit determines the general health and relative position of the GPS Satellites through the GPS navigation messages. The GPS navigation message is a continuous 50-bits/second data stream modulated via a spread spectrum sequence onto the carrier signal of each satellite. The navigation message is a telemetry message transmitted in frames. A GPS frame is 1500 bits long, and takes 30 seconds to be transmitted. Every satellite starts transmission of a frame precisely on the minute and half minute according to its own clock. Each frame consists of five subframes. Subframe 1 includes dock correction parameters and perimeters used for correction of atmospheric delays. Subframes 2 and 3 contain high accuracy ephemeris and dock offset data. A handover data word, or HOW, is also included. Subframe 4 is reserved for special messages which may be included in the data, and subframe 5 contains Almanac data. Almanac data includes information relating to dock corrections, ephemerides (the plural of ephemeris) and atmospheric delays for the normal compliment of twenty-four satellites. This data allows the remote unit to select four satellites that will be required for calculating a navigation solution. Subframes 4 and 5 are “subcommutated.” The data to be transmitted in each of subframes 1, 2 and 3 data comprises a number of bits that do not exceed the number of bits in the subframe. Therefore, subframe 1, 2 and 3 data can each be transmitted within one frame. However, a frame has sufficient length to transmit about 4% of subframe data or subframe 5 data. Consequently, 25 consecutive frames of subframe 4 and 5 data must be collected before the receiver has all of the unique data content being transmitted by a satellite.
Typically, uploads are provided to a GPS satellite once every 24 hours. A Master Control Station (MCS) sends the satellite all the data that the satellite will transmit during the next 24 hours and may also include data for a time period going farther out. An upload contains roughly 16 subframe 1, 2, and 3 data sets. Each subframe 1, 2 and 3 data set is transmitted for up to two hours. The MCS is operated by the United States Air Force 50th Space Wing's 2nd Space Operations Squadron at Schriever AFB, Colorado.
In order to acquire the satellite position, a remote receiver must receive the ephemeris and Almanac data. Based on the amount of data and the 50-bits/second data rate, a nominal transfer time is 90 seconds for ephemeris data and 12½ minutes for an Almanac. A receiver must be powered during the time it is receiving the GPS data. The current generation GPS tracking systems on trucks have a hard-wired vehicle power-source with battery back up; in general, supplying power to this type of system is not an issue. Cellular phones are periodically recharged by a user. Therefore, GPS functionality is easily included in a cellular phone that will be frequently recharged. Again, supplying the GPS system is not an issue.
However, it may be desired to place a GPS system in an application in which the system is not going to be powered by a battery that is substantially continuously recharged or in which the system will not be attended by a user for recharging. Power requirements take on a new significance. Batteries must be provided whose capacity, and hence size and expense, must be increased commensurate with the desired length of operation of the system between maintenance intervals. Expense and reliability issues are multiplied when a number of assets are temporarily stored at one location. Where assets need to be tracked separately by a GPS device associated with each item.
The prior art has traditionally required several minutes for a GPS device to orient itself after a “cold start.” The requirements for extended operation of the device to formulate its position after a “cold start,” continuous tracking or position updating greatly increase the amount of power required. Prior art attempts try to achieve power savings have included semiconductor sub-miniaturization and selecting the slowest possible embedded processors. This approach is inherently flawed and will not enable wireless untethered GPS appliances due to slow performance, requiring long times-to-first-fix TTFF. Such a device must, therefore, track continuously with all internal clocks operating; to power-down the internal clocks and power-up requires an additional waiting period.
Devices including GPS technology have been utilized for remotely reporting location information by users or Software Application Programming Interface. They may also report other information. These devices do not address the power requirement issues relating to the operational requirements of GPS or the transmitter power budget for transmitting location information. The above-discussed transportation location systems have not traditionally included security features to prevent improper modification of location information sent by a GPS station to a base station.
Also, above-discussed transportation location systems have not traditionally included security features to prevent improper modification of location information sent by a GPS appliance to an end-user or operational control center. The prior art has required that that the ephemeris data transmitted by the GPS Constellation be provided to calculate a PVT Solution.