Patent Application
20060238419
This invention relates generally to satellite positioning systems (SPS), and more particularly, to a method and apparatus for aiding positioning of an SPS receiver.
For a fast location fix, a GPS receiver (also interchangeably referred to as an SPS receiver) relies heavily on the aiding provided by GPS satellite frequency, precise time used by the GPS satellites, approximate position of the GPS receiver, and ephemeris (a table giving the coordinates of a celestial body at a number of specific times during a given period).
The more accurate these parameters are the better the GPS receiver will perform. Typical uncertainty for GPS frequency aiding is +/−0.5 ppm, for precise time +/−100 us, and for approximate position +/−30 Km from a reference point such as a transmission tower. There is very little area for improvement with the frequency and time parameters, but the approximation error on the position of the GPS receiver is quite large and can be improved.
Embodiments in accordance with the invention provide a method and apparatus for aiding positioning of an SPS receiver.
In a first embodiment of the present invention, a satellite positioning system (SPS) receiver operates according to a method having the steps of (a) measuring a distance between the SPS receiver and a transmission source according to a radio frequency (RF) signal transmitted by the transmission source, (b) calculating an approximate location on Earth from the distance and a location of the transmission source, and (c) determining a location fix of the SPS receiver on Earth using the approximate location.
In a second embodiment of the present invention, a satellite positioning system (SPS) receiver has a computer-readable storage medium. The storage medium has computer instructions for measuring a distance between the SPS receiver and a transmission source according to a radio frequency (RF) signal transmitted by the transmission source, calculating an approximate location on Earth from the distance and a location of the transmission source, and determining a location fix of the SPS receiver on Earth using the approximate location.
In a third embodiment of the present invention, a selective call radio (SCR) has a radio transceiver for exchanging messages with a communication system, an SPS receiver for locating the SCR on Earth, a memory, and a processor for controlling operations of the memory, the radio transceiver and the SPS receiver. The processor is programmed to measure from the radio transceiver a distance between the SCR and a transmission source of the communication system according to a radio frequency (RF) signal transmitted by the transmission source, calculate an approximate location on Earth from the distance and a location of the transmission source, and cause the SPS receiver to determine a location fix of the SCR on Earth using the approximate location.
While the specification concludes with claims defining the features of embodiments of the invention that are regarded as novel, it is believed that the embodiments of the invention will be better understood from a consideration of the following description in conjunction with the figures, in which like reference numerals are carried forward.
The SCR 100 operates according to method 200 depicted in
The signal strength can be determined from an RSSI (Received Signal Strength Indication) reading provided by conventional means used in the radio transceiver 102. The transmission power and the location of the transmission source 301 can be transmitted to the SCR 100 in the RF signal (or from prior signals), or alternatively, can be pre-stored in the memory 108 of the SCR 100 as predetermined information.
It will be appreciated that other loss thresholds can be selected, e.g., −90 dB. However, as the loss threshold is lowered the uncertainty of determining a distance between the SCR 100 and the transmission source 301 increases. Additionally, rather than having a first and a second distance, more distance estimates can be determined. For example, a line of sight path loss equation can be used as follows:
PL=−(32.44+20*LOG(D)/3.25/1000)+20*LOG(f)),
where PL (path loss) is determined from the RSSI reading of step 202, f is the known carrier frequency of the RF signal, and D is the distance to the transmission source 301. Using this equation it can assumed that the best signal the SCR 100 receives would be a line of sight signal. Although this equation can provide more distance estimates than the two-distance approach mentioned above, the knee-curvature of path loss shown
In step 212, the distance 302 measured in step 201 can be used to calculate an approximate location of the SCR 100 relative to the known position of the transmission source 301. The approximate location of the SCR 100 in turn can be used in step 214 by the SPS receiver 104 as a reference position to accelerate the determination of a location fix of the SCR 100.
The approximate location 302 of the SCR 100 also provides other improvements in determining a location fix in step 214. For example, in
To review the EPE on a location fix by location fix basis, all EPE's for the first 4 SV fixes are averaged from the 2nd fix up to the 10th fix for every session. This data is compared against approximate position uncertainty. The results are shown in
In a supplemental embodiment, method 200 can be improved with steps 216-222. In step 216, the location fix of step 214 can be stored in the memory 108 as a prior location fix. In step 218, a duration is measured between a start time of determining a new location fix and the prior location fix. From this duration a travel distance of the SCR 100 can be determined. The travel distance can be computed according to a velocity of the SCR 100 and the duration. In a first embodiment, the travel velocity can be estimated by the SCR 100 by conventional means such as by gross changes in position of the SCR 100 tracked by conventional triangulation means. Alternatively, a maximum velocity (e.g., 120 mph) can be established to address a worst-case scenario. The travel distance can then be calculated from an estimated distance traveled plus the EPE at the estimated distance (see
For example, a fixed velocity of 120 mph or 2 miles/min can be assumed as the travel velocity of the SCR 100. Knowing that 30 Km is approximately 19 miles, it would take 9 minutes to reach a 30 Km boundary. Assume also a duration of 60 seconds is measured in step 218. Since 120 mph is approximately 54 m/sec, the SCR 100 would have traveled approximately 3.2 Km. Adding a typical EPE to this change in distance, it is evident that the result would most always be less than 30 Km. Hence, in step 222 the prior position fix would be used as an aid to the SPS receiver 104 to determine the new location fix. On the other hand, had the travel distance exceeded the uncertainty threshold (of 30 Km in this instance), then steps 201-214 can be repeated to aid the SPS receiver 104. The foregoing method can be used as a supplemental embodiment to improve the time to locate the SCR 100. Additionally, in portable applications, this method can be used to extend the duration between fixes to improve the battery life of the SCR 100 by shutting power by way of the power supply 112 to portions of the SCR 100 between location fixes.
It should be evident to the reader by now that the present invention can be realized in hardware, software, or a combination thereof. Additionally, the present invention can be embedded in a computer program executed by the processor 114 of the SCR 100, which comprises all the features enabling the implementation of the methods described herein, and which enables said SCR 100 to carry out these methods. A computer program in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form. Additionally, a computer program can be implemented in hardware as a state machine without conventional machine code as is typically used by CISC (Complex Instruction Set Computers) and RISC (Reduced Instruction Set Computers) processors.
It should also be evident that the present invention may be used for many applications. Thus, although the description is made for particular arrangements and methods, the intent and concept of the invention is suitable and applicable to other arrangements and applications not described herein. For example, the above descriptions refer to an SCR 100 operating according to the embodiments of method 200. Alternatively, method 200 can be applied to an SPS receiver 104 alone (i.e., operating an independent device). It would be clear therefore to those skilled in the art that modifications to the disclosed embodiments described herein can be effected without departing from the spirit and scope of the invention.
Accordingly, the described embodiments ought to be construed to be merely illustrative of some of the more prominent features and applications of the invention. It should also be understood that the claims are intended to cover the structures described herein as performing the recited function and not only structural equivalents. Therefore, equivalent structures that read on the description are to be construed to be inclusive of the scope of the invention as defined in the following claims. Thus, reference should be made to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.
