Method and apparatus for using satellite status information in satellite positioning systems

ES 2 298 259 T3

DESCRIPTION

Procedure and apparatus for using satellite status information in satellite positioning systems.

Field of the invention

The present invention relates generally to the field of satellite positioning systems (SPS), such as global positioning system (GPS) receivers, and more particularly to SPS signal processing.

Background of the invention

Global Positioning System (GPS) receivers typically determine their position by calculating arrival times of signals transmitted simultaneously from a multiplicity of GPS (or NAVSTAR) satellites. These satellites transmit, as part of their message, both satellite positioning data and clock timing data, called "ephemeris" data. The process of searching for and acquiring GPS signals, reading the ephemeris data for a multiplicity of satellites, and calculating the location of the receiver from this data takes time, often requiring several minutes. In many cases, this long processing time is unacceptable and also severely limits battery life in miniaturized portable applications.

GPS reception systems have two main functions. The first is the calculation of the pseudo-ranges of the various GPS satellites, and the second is the calculation of the position of the receiver using these pseudo-ranges and satellite timing and ephemeris data. Pseudo-ranges are simply the arrival times of satellite signals measured by a local clock. This definition of pseudorange is also sometimes referred to as a code phase. The ephemeris and satellite timing data is extracted from the GPS signal once it is acquired and tracked. As discussed above, collecting this information typically takes a relatively long time (18 seconds to several minutes) and must be done with a good received signal level in order to achieve low error rates.

Most GPS receivers use correlation procedures to calculate pseudo ranges. These correlation procedures are carried out in real time, often with hardware correlators. GPS signals contain repetitive, high-rate signals called pseudo-random sequences (PN). The codes available for civil applications are called Approach / Acquisition (C / A) codes, and have a bit-phase inversion transmission rate, or “chipping” rate, of 1.023 MHz and a repetition period. of 1023 code elements for a code period of 1 millisecond. The code sequences belong to a family known as Gold codes, and each GPS satellite emits a signal with a unique Gold code.

For a signal received from a given GPS satellite, after a baseband downconversion process, a correlation receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates, or filters low pass, the product in order to get an indication of the presence of the signal. This process is called a "correlation" operation. By sequentially adjusting the relative timing of this stored replica with respect to the received signal, and observing the correlation output, the receiver can determine the time delay between the received signal and a local clock. The initial determination of the presence of such an output is called "acquisition." Once the acquisition occurs, the process enters the “tracking” phase in which the local reference timing is adjusted small amounts in order to maintain a high correlation output. The correlation output during the tracking phase can be viewed as the GPS signal with the pseudo-random code removed, or, in common terminology, "de-spread." This signal is narrow band, with a bandwidth consistent with a 50 bits per second bivalent phase shift modulated (BPSK) data signal that is superimposed on the GPS waveform.

This correlation acquisition process is time consuming, especially if the received signals are weak. To improve acquisition time, most GPS receivers use a multiplicity of correlators that allow a parallel search for correlation peaks.

GPS receiving equipment is typically designed to receive GPS signals in open spaces since satellite signals are lines of sight and therefore can be blocked by metal and other materials. Enhanced GPS receivers provide signal sensitivity that enables tracking of GPS satellite signals in confined spaces, or in the presence of weak multipath signals or signals that are pure reflections. However, the ability to acquire such weak GPS signals usually causes other problems. For example, simultaneous tracking of strong and weak signals can cause the receiver to lock onto a cross-correlated signal, which is not a real signal. Instead of finding a weak true peak, a stronger cross-correlated peak can be acquired. Keeping track of a weak satellite signal does not guarantee that it is a direct signal. This weak signal can be a reflected signal or a combination of direct and indirect signals. The combined signals are called multipath signals. The path of the reflected signal is normally longer than the path of the direct signal. This difference in path length causes the reflected signal time of arrival measurement to be normally delayed or the corresponding code phase measurement to contain a positive offset. In general, the magnitude of the deviation is proportional to the relative delay between the paths

ES 2 298 259 T3 reflected and direct. The possible absence of a direct signal component makes existing multipath mitigation techniques (such as a narrow correlator or a strobe correlator) obsolete.

The GPS navigation message is the information transmitted to a GPS receiver from a GPS satellite. It is in the form of the 50 bits per second data stream that is modulated on the GPS signals.

The data message is contained in a data frame that is 1500 bits long. It has five subframes each of which contains GPS system time. Each subframe consists of 10 words of 30 bits each. Subframes 1 through 3 repeat every 30 seconds. There are twenty-five pages of data that appear in sequence in the fourth and fifth subframes; one every 30 seconds. Therefore, each of these twenty-five pages repeats every 750 seconds.

Subframes 4 and 5 contain two types of status or operational data for GPS satellites: (a) each of the 32 pages containing the clock / ephemeris related almanac data provides a satellite operational status word eight bits relative to the satellite from which they carry their almanac data, and (b) the twenty-fifth page of subframes 4 and 5 together contain six-bit operational status data for up to 32 satellites. Additional satellite operation data is provided in subframe 1. Subframe 1 is part of the ephemeris data set.

Typically, a GPS receiver will receive information regarding the status (eg "operation") of a satellite and then process the GPS signals by not acquiring and not tracking malfunctioning satellites while acquiring and tracking GPS signals from satellites working properly. Alternatively, standalone GPS receivers can be designed to acquire and track well-functioning and malfunctioning satellites but avoid using malfunctioning signals in the location calculation after the operational status data has been read from the ephemeris message. from a signal from a malfunctioning satellite. No attempt has been made in the prior art to use the information relating to the operation of a satellite to try to avoid using the cross-correlated results or to detect such errors. Also, no attempt has been made to ensure that satellite operating status is available for GPS receivers that do not have direct access to satellite operating status. (Direct access is from SPS satellites or from data previously downloaded from SPS satellites).

Summary of the invention

According to the present invention there is provided a method for processing the satellite positioning systems according to claim 1.

"In sight" as used herein is broadly defined to indicate SPS satellites in sight of: an SPS reference receiver; a network of SPS reference receivers; a mobile SPS receiver; or a cell or group of cells of a cellular radio system. However, these are provided merely as examples. Therefore, it should be noted that an SPS satellite can be considered "in sight" if the signals can be received by almost any SPS receiver.

According to the present invention there is also provided a computer readable medium containing executable computer program instructions. The computing medium causes the data processing system to perform a procedure for processing satellite positioning system (SPS) signals when program instructions are executed by a data processing system. The procedure includes attempting to acquire SPS signals from all SPS satellites in view regardless of whether the SPS satellites in view are malfunctioning, and receiving performance data that specifies which of the SPS satellites in view are malfunctioning. The procedure includes testing measurements obtained from the acquisition of SPS signals, for all SPS satellites in sight, for a cross-correlation between two different SPS satellites, where the test is carried out regardless of whether a satellite in sight SPS has malfunction.

According to the present invention there is also provided an apparatus according to claim 10. According to the present invention there is provided an apparatus according to claim 11.

The disclosed apparatus includes a mobile SPS receiver coupled to a second receiver. The mobile SPS receiver receives the SPS signals from the transmitter in the second receiver and analyzes the SPS satellites in view for a cross-correlation condition where a properly functioning SPS satellite signal is influenced by a malfunctioning SPS satellite signal , the malfunctioning SPS satellite signal and the cross-correlated SPS satellite signal being removed or corrected before a location is calculated from the SPS satellites in view by the mobile SPS receiver. In one embodiment, the transmitter and the second receiver are compatible with a cellular radio system.

Brief description of the drawings

The present invention is illustrated by way of example and is not limited in the figures of the accompanying drawings, in which the same reference numerals indicate similar elements.

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Figure 1 illustrates a procedure for using satellite positioning system (SPS) status data to improve a position calculation.

Figure 2 illustrates a variety of satellite positioning system (SPS) satellites "in sight".

Figure 3 illustrates a cellular communication system having a plurality of cells each serving a cell site, and each of which is coupled to a cellular switching center.

Figure 4 shows a representation of a cell-based information source that provides an association between Doppler information sets at specified times with respect to cell service areas and / or cell sites.

Figure 5 illustrates an implementation of a base station system according to an embodiment of the present invention.

Figure 6 illustrates an example of a combined SPS receiver and communication system according to an embodiment of the present invention.

Detailed description

In the following detailed description of embodiments of the invention, reference is made to the accompanying drawings in which the same references indicate similar elements, and in which specific embodiments in which the invention may be practiced are shown by way of illustration . These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of the invention is defined only by the appended claims.

A method and apparatus for using satellite status information in SPS systems are disclosed. In an exemplary embodiment, a mobile SPS receiver receives SPS signals from SPS satellites in sight and determines (or attempts to determine) pseudo-ranges for all (in most cases) satellites in sight. Information regarding the status of the satellites (eg satellite operation) is also collected and this information is used when processing the SPS signals received by the mobile SPS receiver. Since, in most cases, the mobile SPS receiver determines code phases / pseudo-ranges for all satellites (even for malfunctioning satellites) it is possible to determine a cross-correlation between a weak malfunctioning satellite GPS signal and a strong malfunctioning satellite GPS signal. Once such a cross-correlated result is identified, it may be possible to eliminate a cross-correlated measurement or attempt to correct it. Thus, according to one embodiment, a mobile SPS determines code phase / pseudorange measurements for satellites known to malfunction.

Satellite operating information can be monitored by GPS reference receiver (s) that transmit (s) this information to GPS server (s) (e.g., See the location servers described in US Patent No. 6,208,290 and these servers can either have this information for all satellites in view or (a) be used locally to process pseudorange measurements received from a mobile GPS receiver (in the case in which the mobile GPS receiver does not calculate the position of the mobile but the location server, or another system in a network, calculate position) or (b) transmitted to the mobile GPS receiver (eg, via a cellular phone that is paired to and hosted with the mobile GPS receiver) for use by the mobile GPS receiver. Typically, according to this embodiment, the GPS reference receivers acquire and track all GPS satellites in view whether they are working properly or malfunctioning; this will allow in one embodiment the detection (by means of a location server or by means of a mobile GPS receiver itself) of cross correlations between signals from properly functioning and incorrectly functioning satellites.

"On sight", as used in this detailed description, is intended to be flexible. Accordingly, "sight satellites" or "sight SPS satellites" can be used to denote sight of the entire wide area reference network (WARN) or any subset of WARN. For example, a non-exclusive list would include the view of a single SPS reference receiver, the view of a plurality of SPS reference receivers, the view of a mobile SPS receiver, the view of a cell or any group of cells of a cellular (wireless) radio network. Figure 2 shows various "SPS satellites in view". Referring to Figure 2, a wide area reference network viewing zone 200 is shown where satellites (SV) 202, SV 204, SV 206, SV 208, SV210, SV 212, SV 214, SV 216, SV 218, SV 242, SV 244, SV 246, SV 248, SV 252, SV 254, and SV 256 are all within sight of the WARN. A subset of these satellites are within view of the reference receiver viewing area 240, eg, SV 242, SV 244, SV 246, and SV 248. Another subset of these satellites is in view of a second reference receiver viewing area 260, for example, SV 210, SV 212, SV 214, SV 216, and SV 218. Another subset of these satellites is in view of a third reference receiver viewing area 270, eg, SV 208, SV 210, SV 252, SV 254, and SV 256. Another subset of satellites is in view of a mobile SPS receiver viewing area 250, for For example, SV 252, SV 254, and SV 256. There is no implicit limitation by the selection of the satellites included in the viewing area 240, 250, 260, or the viewing area 270 or the size of the viewing areas. The viewing areas were chosen for illustration purposes only.

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In one embodiment, the operating state of "all" SPS satellites in view of the WARN viewing area 200 is acquired. Operational information from a subset of the satellites could be transmitted to a mobile SPS receiver based on the satellites "in sight" of the mobile SPS receiver location. "All" SPS satellites used in this document are dependent on the particular acquisition implementation. For example, considering an elevation mask, or a signal-to-noise ratio mask for the interface mask, etc. Therefore, "everything" is built flexibly. The GpS reference receiver (s) may collect satellite status information that is more current than the corresponding information in the satellite ephemeris (or almanac) information, and this updated status information may be transmitted to the servers tracking devices and / or mobile GPS receivers for use in accordance with the various embodiments of the invention. In one embodiment, the operational status information can be received directly from the satellite. In another embodiment, the location server (s) may obtain updated status information based on the information and / or measurements received from the GPS reference receiver (s). The updated status information may be a function of the quality of status (QoS) associated with the location request. For a given quality level, the location server may determine that a satellite that is marked as functioning well in the ephemeris and / or almanac data may not provide the desired level of precision. In such a case, the location server may update its operational status information and send it to the remote SPS receiver (s) and optionally store it for further processing. An example of this processing may be the use of this updated operating state to determine whether the satellite should be used in the position calculation, which is performed at the location server. Alternatively, the determination and updating of the operational status information can be performed by the mobile SPS receiver. An example of a network of GPS reference receivers that can be used to provide this updated status information is described in copending US patent application in conjunction with present serial number 09 / 067,407, filed April 28, 1998, now US Patent No. 6,215,441, which is hereby incorporated herein by reference.

Figures 1,5,6 and 7a of US Patent 6,208,290 have been included herein for clarity and correspond to Figures 3,4, 5 and 6 respectively. Referring to the figures herein, Figure 3 illustrates various cells within a cellular service area in which the mobile SPS receiver can receive helper data from a communication network. In one embodiment, the communication network includes a location server, which may be represented by 26 in Figure 3. Figure 4 may also contain satellite operational status information (not shown) transmitted to the mobile SPS receiver via the network of communications. For example, an SPS receiver in one viewing area may receive operational status information from one set of satellites and an SPS receiver in another viewing area may receive operational status information from another set of satellites in the communication system. A base station illustrated in Figure 5 can associate with the reference receiver viewing areas shown in Figure 2 and support the communication system by including cells within the cellular service area.

In one embodiment, FIG. 6 illustrates a cellular telephone 375, which is coupled to a mobile SPS receiver 376 with a communication system transceiver 378 to receive help information including the operational status of communication system satellites. In one embodiment, the location server determines a cell site or group of cell sites that is in communication with the mobile SPS receiver 376, based on the cell site or group of cell sites. The location server can determine a group of satellites in view and then operational information for all such satellites in view can be transmitted to the cellular telephone 375 and received by the communication transceiver 378. The mobile SPS receiver 376 can calculate its location based on the received help data or the location server can determine the location of the mobile SPS receiver.

The operating data given in subframes 1,4 and 5 of some elites may differ from those shown in subframes 4 and / or 5 of another group of satellites since the latter may be updated at a different time.

Only the operation summary in subframe 5 is updated at the time of loading the almanac. During prolonged operations the operation summary may become out of date due to the passage of time since the last charge.

This situation is further aggravated when the GPS receiver is indoors or in other signal restrictive environments. As much as 17 dB of additional signal sensitivity may be required to be able to receive satellite operational status bits directly from the satellites. The GPS receiver can measure the pseudo-range to the satellites based on the signal that is 17 dB weaker than the signal containing the satellite operating status bits (50 bits per second). Receiving satellite operational status bits may also require continuous satellite signal tracking from 18 seconds to several minutes, which is generally not possible in signal restrictive environments. Several critical performance aspects related to GPS radiolocation can be affected by outdated or unavailable satellite operating information. One aspect is related to the use of signal (s) from malfunctioning satellites in determining the location. It is commonly known that the use of malfunctioning satellite signal (s) can result in significant position errors.

If the operation summary becomes outdated and does not accurately reflect the state of the GPS constellation, the time to first fix (TTFF) for a GPS receiver may be delayed. This is due to the time spent on

ES 2 298 259 T3 possible acquisition of malfunctioning satellites that were marked as "correct operation" by the stored operation summary. Attempts to acquire well-functioning satellites that are marked as malfunctioning may also fail. As a result, lengthy operations without correct and timely satellite status information updates can jeopardize the missions of those users who require an effective TTFF. A fast TTFF is critical for emergency services and also affects battery standby and talk times. In a wireless assisted GPS mode, a wireless network can provide correct and timely satellite operating information for all mobile GPS receivers communicating with such a network.

Another reason for satellite operation aid is protection against false satellite measurements. In heavily hampered signal environments, it is quite common for GPS satellite signals to be received with a very high dynamic range. Receiving GPS signals with signal strengths that differ by more than about 17 dB can cause a GPS receiver to acquire a cross-correlated signal rather than a relatively weaker actual signal. A procedure that can be used to detect and possibly correct or eliminate a cross-correlation measurement is described in co-pending U.S. Patent Application Serial No. 09 / 241,334, filed February 1, 1999, which is incorporated by herein by reference. However, for a GPS receiver to detect the presence of cross-correlated signals, all signals from both well-functioning and malfunctioning satellites would have to be acquired. A problem would arise if a strong "malfunction" satellite signal were cross-correlated with a weak "malfunction" satellite signal. Regardless of the presence of a "malfunction" signal, a GPS receiver may not be able to detect a cross-correlation condition. Another problem can arise if only a subset of all visible satellites are acquired regardless of operational status. If any of the acquired satellites in this subset show weak signal behavior, then this may also be the case that the cross-correlation condition may not be detected.

In one embodiment of this invention, the GPS reference receiver (s) that provide reference data for location server (s) (also referred to as a position determination entity (PDE) in CDMA cellular phone systems and central Mobile Location Service (SMLC) in GSM cellular phone systems) acquires and tracks all satellites in sight: working properly and malfunctioning. Additionally, GPS technologies (for example, a GPS receiver) integrated with or connected to wireless devices (for example, a cell phone or a two-way paging device) also acquire and track all satellites in sight: operating correct and malfunctioning. In a Wireless Assisted GPS (WAG) mode (for example, see the examples described in co-pending U.S. Patent Application Serial No. 08 / 842,559, filed April 15, 1997), the server (s) Location (s) can provide "working" status information to mobiles communicating with a wireless network served by the location server (s). This operational status information may accompany any other help information provided by the location server (s). In general, the help information enables rapid acquisition of GPS signals in highly restrictive signal environments. In order to achieve such performance improvements, the help information may specify the satellites to be searched, the estimated time of arrival of these signals, and the expected frequency (Doppler) of the signals. This help information can be provided to improve a three-dimensional search for a satellite signal. Other types of assistance such as ephemeris and / or almanac, approximate location or time information may be provided to assist with location calculation and / or rapid acquisition of satellite signals. When satellite signals are acquired, pseudo-ranges, Dopplers, and other satellite signal measurements are analyzed for cross-correlation conditions. In order to perform this analysis, help should be provided and measurements should be made for all satellites in view: good and bad. In this embodiment, the satellite performance information is used to aid in the detection of a cross-correlation condition and the cross-correlated and / or "badly running" satellites are then analyzed to determine if they should be included in the calculation process. location or correct. In an alternative embodiment, where the help information is provided only for well-functioning satellites (the operation of the satellite is implied by the inclusion of the particular satellite in the satellite list) and the current satellite operation status information and valid is not available for the mobile GPS receiver, the mobile may try to acquire only properly functioning satellites (for example, the satellites included in the list). In this case, a possible presence of a potentially "strong" malfunction satellite potentially cross-correlated with relatively weaker malfunction satellites may not be known to the mobile and therefore may not be proven. The information related to the current satellite constellation (including the list of visible satellites and correct operating status) may not be known as it cannot be guaranteed that the GPS receiver will have the necessary information, that it is current and available (for example, stored in his memory). The use of undetected cross-correlated signals can lead to large position errors which therefore affect the quality of the location service.

The following example provides an illustration of the present invention. Data was collected in an urban canyon with a resulting horizontal position error of 61.4 meters. The SV PRN # 10 satellite (line 2 of Table 1) was removed from the solution as it was identified as cross-correlated with the SV PRN # 17 satellite (line 3 of Table 1). If the SV PRN # 10 satellite were used in the solution, the error in its pseudo-range measurement of -37,903 meters would have caused a horizontal error greater than 50 kilometers.

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This example also illustrates the need to provide help information for all satellites in view. If the SV PRN # 17 satellite had been declared malfunctioning and the wireless network did not provide support for all four satellites, the mobile GPS receiver would not have been able to detect the cross-correlation and the SV PRN # 10 satellite would have been used by inputting from that Thus the horizontal error greater than 50 kilometers, described above, in the position estimation of the mobile GPS receiver, an unacceptable result.

TABLE 1

Cross correlation between satellites SV PRN17 and SV PRN10

<td>SV PRN</td><td>Azimuth (degrees)</td><td>Elevation (degrees)</td><td>Output SNR (dB)</td><td>Error (meters)</td><td>Used in the solution</td>

<td> 6</td><td> 128</td><td> 63</td><td> 49</td><td> -1,1</td><td>Yes</td>

<td> 10</td><td> 44</td><td> 8</td><td> 14</td><td> - 37903</td><td>Do not</td>

<td> 17</td><td> 330</td><td> 65</td><td> 52</td><td> -0,6</td><td>Yes</td>

<td> 22</td><td> 295</td><td> 28</td><td> 19</td><td> 92,2</td><td>Yes</td>

Alternatively, the operating information may be received (or may have been received) directly from the satellites and this operating information may be used in the same way described herein as the operating information that is received from a transmitter at a location in cell.

Operational information can be transmitted from a cell-site by broadcasting this information to all satellites in view of a cell-phone base station ("cell-site"). Alternatively, it can be provided to a cell phone on request (on demand). The operating information can be transmitted from the cell phone base station to the cell phone which then provides the operating information to a GPS receiver that is coupled to the cell phone. In case the information is transmitted on demand, A location server can determine the appropriate information (e.g., up-to-date operation) based on a cell site that is in cellular radio / wireless communication with the phone and the cell site information can be used to determine an approximate location to be used to determine satellites in view of that location and is then caused to transmit (in one case) updated operational information to those satellites to the cell phone which in turn provides the information to the mobile GPS receiver for use in processing SPS signals in the GPS receiver. SPS signal processing may include cell phone position determination. Otherwise, the location server may retain the updated operating information or the information used to determine the updated operating information and use it to process the pseudo-ranges (for example, correlation measurements that specify code phases) received from the GPS receiver. mobile in order to determine the position of the mobile GPS receiver. In both cases, pseudoranges, estimated Doppler, and other measurements are determined even for known malfunctioning GPS satellites so that cross-correlations can be detected as described herein. For example, a GPS receiver may receive updated operating information from a cell site but still acquire GPS signals from a GPS satellite that was indicated to have malfunction in the transmitted updated operating information. In the example, if the receiver can also calculate the location information, it can use the updated operating information to determine which satellites can be used in the solution. Co-pending U.S. application in conjunction with present Serial Number 08 / 842,559, filed April 15, 1997, describes a procedure for identifying a cell site that is in wireless communication with a cellular phone and which then determines assistive data from the cell phone. satellite for satellites in view based on an approximate location obtained from the identification of this cell site. This method can be used with the present invention if the satellite assist data in this case is either from satellite operation (for example, based on the satellite almanac) or from updated satellite operation (for example, more current than existing satellite almanac message information regarding satellite operation).

Figure 1 shows an example according to an embodiment of the present invention. In this example, the location server provides helper data to the mobile GPS receiver and then receives measurements back from the mobile GPS receiver, and these measurements are then used to resolve the position of the mobile GPS receiver. The procedure shown in Figure 1 assumes a certain sequence in which the location server provides helper data via a cell site to the mobile device, and then the mobile device attempts to acquire all satellites in view, including all satellites in view. satellites in sight of correct operation and malfunction, and then tests for cross-correlations on all correlation measurements that result from acquiring the satellite signals. It will be appreciated that a different sequence of operations may also be employed in accordance with the present invention. For example, the GPS receiver may attempt to acquire all satellites in view before receiving the help data. Also, the cross-correlation test can be performed on the localization server after

ES 2 298 259 T3 for the GPS receiver to determine pseudo-range data and transmit this data and other data necessary for the cross-correlation test back to the location server which can then perform the cross-correlation tests. In another alternative embodiment, the mobile device can perform its own position calculations and thus operations 6, 7 and 8 of Figure 1 are performed by a processing unit in the combined GPS / mobile phone receiver device. In this case, step 5 in which data is transmitted to the location server is not necessary. In yet another alternative embodiment, the location server may broadcast the appropriate operation assist data rather than providing it on request to a mobile device. In this case, the broadcast can be done from the selected cell site (s) and no location server may be required except the processor (s) located at the cell site (s). cell-site (s) determining the necessary operational aid data based on the satellites in view at the cell-site. This approach may also be applicable to the on-demand delivery described herein.

Although satellite operation information can be obtained from the reception, demodulation and decoding of the operation data in the SPS signals from the SPS satellites, and used in accordance with the various embodiments of the invention, It is also possible to use GPS receivers and processing systems that monitor (for example receive and measure characteristics of SPS signals) SPS signals and determine from measured characteristics of SPS signals that an SPS satellite is malfunctioning (even if the data demodulated and decoded operating conditions in the SPS signals of this satellite or other satellites indicate that this satellite has correct operation) or vice versa.

In this discussion, embodiments of the present invention have been described with reference to application in the US global positioning system (GPS) system, which is an example of an SPS system. However, it should be apparent that these procedures are equally applicable to other satellite positioning systems, such as the Russian Glonass system. Thus, the term "GPS" used herein includes such alternative satellite positioning systems, including the Russian Glonass system. Also, the term "GPS signals" includes signals from alternative satellite positioning systems.

Furthermore, although embodiments of the present invention have been described with reference to GPS satellites, it will be appreciated that the teachings are equally applicable to positioning systems using pseudo-satellites or a combination of satellites and pseudo-satellites. Pseudo-satellites are ground-based transmitters that emit a PN code (similar to a GPS signal) modulated on an L-band (or other frequency) carrier signal, generally synchronized with GPS time. Each transmitter can be assigned a unique PN code to allow identification by a rover. Pseudo-satellites are useful in situations where GPS signals from an orbiting satellite might not be available, such as mine tunnels, buildings, urban canyons, or other closed areas. The term "satellite", as used herein, is intended to include pseudo-satellites or pseudo-satellite equivalents, and the term GPS signals, as used herein, is intended to include GPS-like signals from pseudo-satellites. or equivalents of pseudo-satellites.

It will be appreciated that the procedures described in conjunction with the figures may be carried out in machine executable instructions, eg software. The instructions can be used to cause a general-purpose or special-purpose processor that is programmed with the instructions to perform the operations described. Alternatively, the operations could be performed by specific hardware components that contain hard-wired logic to perform the operations, or by any combination of programmed computer components and custom hardware components. The procedures may be provided as a computer program product that may include a computer-readable medium that has instructions stored therein that can be used to program a computer (or other electronic devices) to perform the procedures. For the purposes of this specification, the term "machine-readable medium" should be taken to include any medium that can store or encode a sequence of instructions for execution by the machine and that causes the machine to perform any one of the methodologies. of the present invention. Accordingly, the term "machine-readable medium" should be taken to include, but is not limited to, solid state memories, optical and magnetic discs, and carrier wave signals. Furthermore, it is common in the art to speak of software, in one way or another (for example, program, procedure, process, application, module, logic ...), as performing an action or causing a result. Such expressions are simply a shortened way of saying that running the software by a computer causes the computer's processor to perform an action or produce a result.

Therefore, a novel method and apparatus for using satellite status data is described. Although the invention is described herein with reference to specific preferred embodiments, many modifications thereof will readily occur to those skilled in the art. Accordingly, all of the aforementioned variations and modifications are included within the proposed scope of the invention as defined by the following claims.