Method of improving a vehicle emergency call network

A method of improving a vehicle emergency call network used during an enhanced 911 call, which is especially useful in environments having degraded GPS signals. The present method utilizes an independent position enhancement algorithm, such as a dead reckoning algorithm, and converts the output of that algorithm into a compatible format compatible with a position determining entity (PDE). Some independent position enhancement algorithms return output in the form of position information (typically, two- or three-dimensional coordinates), however, most PDEs only accept information in the form of pseudo-ranges. Thus, the present method provides a technique in which the advantages of an independent position enhancement algorithm are enjoyed, yet a compatible output is sent to the PDE.

TECHNICAL FIELD

The present invention generally relates to vehicle GPS devices, and more particularly, to vehicle GPS devices that are used in conjunction with vehicle emergency call networks to provide vehicle position information during an enhanced 911 or E911 call.

BACKGROUND OF THE INVENTION

In recent years, vehicle GPS devices have grown in popularity and are now widely available in a variety of forms, including different types of In-Vehicle Navigation Systems (IVNSs). These systems are primarily based on a Global Positioning System (GPS) which was founded by the U.S. Department of Defense and consists of a constellation of twenty-four satellites working in conjunction with five base stations. The satellites orbit the Earth and transmit precise timing data to GPS receivers located on Earth. If strong signals from three or more satellites are received, then a latitude and longitude (two-dimensional) position can be determined; if strong signals from four or more satellites are received, then a latitude, longitude and altitude (three-dimensional) position can be calculated.

In addition to providing an occupant with navigation-related information, the IVNS can also transmit important vehicle position information during an emergency call. Telecommunication companies have successfully implemented enhanced 911 (E911) services throughout the country, giving the public fast and easy access to a Public Safety Answering Point (PSAP) which is in turn connected to various local emergency responders. E911 systems automatically send certain information such as the caller's location to the PSAP so that it can dispatch emergency services to the caller's location without requiring the possibly panicked caller to convey their location. Traditional land-line telephone systems utilize the telecommunication company's records to lookup an address based upon the caller's phone number. However, this technique does not provide meaningful location information for E911 calls originating from a wireless telecommunication system, such as an IVNS or a mobile phone.

Therefore, in order to enable E911 systems to obtain quick and accurate information during an emergency wireless call, the federal government enacted wireless E911 rules. One of the purposes of the new E911 rules is to enable the PSAP to obtain as much helpful information as possible during an emergency call originating from a wireless INVS; particularly, information relating to the current position of the vehicle.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method for improving a vehicle emergency call network. The method comprises the steps: (a) utilizing an independent position enhancement algorithm to determine enhanced position information, (b) converting the enhanced position information into enhanced pseudo-range information, and (c) sending the enhanced pseudo-range information to a position determining entity (PDE).

In accordance with another aspect of the invention, there is also provided a method for improving a vehicle emergency call network. This method includes the steps of: (a) utilizing a GPS receiver unit to receive GPS information from at least one satellite, (b) utilizing the GPS information and an independent position enhancement algorithm to determine enhanced position information that is generally in the form of coordinates, (c) converting the enhanced position information into enhanced pseudo-range information that has a set of errors purposely inserted therein, and (d) sending the enhanced pseudo-range information to a position determining entity (PDE) in an electronic message that conforms with an IS-801 format.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIG. 1, there is shown an example of a known vehicle emergency call network10that receives and processes vehicle position information during an enhanced-911 or E911 emergency call. The vehicle position information enables network10to locate a distressed vehicle12and to provide emergency responders with the vehicle's location so they can be quickly dispatched to the scene. According to this particular embodiment, vehicle emergency call network10generally includes a vehicle navigation device or GPS receiver unit14, a vehicle communications device16, a wireless communications network18, a Position Determining Entity (PDE)20, a Public Safety Answering Point (PSAP)22, and one or more emergency responders24-28. It should be recognized that emergency call networks, such as that shown here, are generally known in the art, thus the following paragraphs simply provide a brief overview of the structure and operation of one exemplary system.

GPS receiver unit14acquires GPS information from one or more satellites36and can be one of numerous types of devices known in the art, including devices used in telematics-based systems or autonomous systems. A telematics-based vehicle navigation system generally communicates with a system back-end such as a remote call center in order to perform certain navigation related services, while an autonomous vehicle navigation system generally utilizes road data and other information stored locally on a CD or DVD in order to perform similar services. Vehicle communications device16is preferably equipped with a wireless modem for data communication via wireless communications network18, which can be a cellular network. PDE20is a hardware component that utilizes the vehicle position information provided by vehicle communications device16to locate the position of GPS receiver unit14and to relay that position to PSAP22. Currently, most PDEs only accept information in a pseudo-range format and are thus unable to process other types of information formats. In this particular embodiment, PDE20is connected to PSAP22via a wired connection, however, the two components could be connected by a wireless connection as well. PSAP22is a government controlled call center responsible for answering emergency calls and relaying the request for help to an appropriate emergency responder, such as a local police department24, fire department26or ambulance service28.

When an emergency E911 call is initiated, GPS receiver unit14gathers GPS information from several satellites36so that the current location of the vehicle can be determined. Each of the satellites36transmits a ‘navigation message’ that includes a Coarse Acquisition (C/A) code for modulating the carrier signal by spreading it over a 1 MHz bandwidth. The C/A code is a repeating Pseudo Random Noise (PRN) code that uniquely identifies the particular satellite transmitting the message. A complete navigation message includes twenty-five data frames (1500 bits/data frame), each data frame includes five sub-frames (300 bits/sub-frame), and each sub-frame is populated with a different piece of information. From this GPS information, GPS receiver unit14is able to generate pseudo-range information for each satellite transmission that it receives. Depending on how the particular emergency call network is setup, the GPS receiver unit can either make error corrections to the pseudo-range information before sending it to the PDE, or it can send uncorrected pseudo-range information knowing that the PDE will subsequently take those errors into account. These corrections typically pertain to errors such as clock synchronization errors, atmospheric transmission delay errors, and Earth rotational errors, to name but a few. The term ‘pseudo-range information’ is broadly defined as information generally relating to the calculated distance between a particular satellite and the GPS receiver unit, and can either be corrected or uncorrected for errors such as these. Once pseudo-range information is calculated, it is sent to PDE20via vehicle communications device16and wireless communications network18.

The PDE then uses the pseudo-range information to determine position information in the form of coordinates (usually two- or three-dimensional coordinates, but not necessarily); the position information corresponds to the place where the pseudo-range information from a number of different satellites intersects. Knowing the position information for GPS receiver unit14enables PDE20to identify the closest or otherwise most appropriate PSAP22for responding to the emergency call. For instance, if GPS receiver unit14was located within the territory of a certain PSAP X, instead of alerting all PSAPs (there are over 6,500 in the United States) of the emergency call, only PSAP X would be alerted. PSAP X would then compare the position information to electronically stored road data so that an actual street address, where available, could be generated and provided to the appropriate emergency responders24-28. With an accurate street address in hand, the emergency responders can quickly and efficiently locate and assist the occupants of vehicle12.

As previously mentioned, PDE20only accepts information from vehicle communications device16in a pseudo-range format which, of course, limits the type of information that can be provided to the PDE. If the vehicle communications device were to send the PDE information in a format other than pseudo-ranges, then upgraded hardware and/or software would be needed in order to process this information. System upgrades such as these can be costly and time consuming. Thus, the following paragraphs describe a method that improves vehicle emergency call network10by utilizing an independent position enhancement algorithm to provide enhanced output in a non pseudo-range format, yet does so without requiring an upgrade to the PDE.

Method of Improving a Vehicle Emergency Call Network

With reference toFIG. 2, there is shown an embodiment of a method50for improving a vehicle emergency call network, such as network10just described. Generally, method50improves the accuracy and reliability of vehicle position information by employing an independent position enhancement algorithm in conjunction with the GPS information provided by satellites36. The independent position enhancement algorithm relies upon information generated by a non-GPS device, such as an on-board gyroscope or wheel speed sensor, and is therefore effective even in degraded GPS signal environments. For example, tall buildings (urban canyons), tunnels and other barriers can obscure line-of-sight communication with satellites36and thereby inhibit GPS receiver unit14from receiving sufficient and accurate GPS information. Although independent position enhancement algorithms can be quite useful in environments such as these, some of these algorithms only provide output in the form of position information (typically, two- or three-dimensional coordinates) while most PDEs can only receive pseudo-range information. Method50addresses this issue by providing a technique in which the advantages of the independent position enhancement algorithm are enjoyed, yet a compatible output is sent to PDE20.

Beginning with step52, GPS receiver unit14preferably receives GPS information from one or more available satellites36. It should be noted, the term ‘GPS information’ not only includes information provided by the Global Positioning System which is maintained and operated by the U.S. Department of Defense, but it also includes information provided by other global positioning networks, such as Galileo. In step54, GPS receiver unit14utilizes the GPS information to determine pseudo-range information for each of the transmitting satellites; the pseudo-range information may or may not be corrected for certain errors depending on the particular setup of the emergency call network, as previously explained. The precise manner in which error corrections are made is known in the art and is expressed in the Interface Control Document, which is hereby incorporated in its entirety be reference. ARINC Research Corporation,Interface Control Document—NAVSTAR GPS Space Segment/Navigation User Interfaces(Oct. 10, 1993); <http://www.navcen.uscg.gov/pubs/gps/icd200/icd200cw1234.pdf>.

Once pseudo-range information has been generated, step56applies an independent position enhancement algorithm to this information in order to generate enhanced position information. Although a number of different independent position enhancement algorithms may be used, one example of a suitable algorithm is the dead reckoning algorithm described in the article Turn, Turn, Turn—Wheel-Speed Dead Reckoning for Vehicle Navigation, which is hereby incorporated by reference. Curtis Hay,Turn, Turn, Turn—Wheel-Speed Dead Reckoning for Vehicle Navigation, Curtis Hay, GPS World, pgs. 37-42 (October 2005); http://www.gpsworld.com/gpsworld/data/articlestandard/gpsworld/402005/183484/article.pdf>. The dead reckoning algorithm generally uses a known starting point and independent wheel-speed sensors to measure both the distance traveled by the vehicle and the vehicle's heading. Thus, the position of the vehicle calculated by the dead reckoning algorithm can be used in conjunction with the GPS information so that a more accurate overall location can be calculated. Again, this two-fold approach of using both GPS information and independently derived information from a non-GPS device is particularly useful in environments, such as those mentioned above, where there is insufficient GPS information. As previously mentioned, PDE20is only able to receive and process pseudo-range information, thus the enhanced position information outputted by step56must be converted.

Step58converts the enhanced position information (typically, two- or three-dimensional coordinates) generated by the independent position enhancement algorithm into enhanced pseudo-range information so that it is compatible with PDE20. Turning now toFIG. 3, a more detailed flowchart of conversion step58is shown. Following execution of the independent position enhancement algorithm, step80is provided with the following pieces of information: 1) enhanced position information describing the calculated position of the GPS receiver unit, 2) the GPS receiver unit's two-dimensional velocity (speed and heading), 3) a reference time at which the enhanced position information was determined, and 4) ephemeris data for the various satellites. Preferably, information pieces 1)-3) each come from the independent position enhancement algorithm, while 4) is provided in the GPS information originally sent by satellites36.

Step82determines whether or not the ephemeris data is valid. The ephemeris data describes the particular orbits of each of the satellites36, and is constantly being updated by the GPS system. Typically, ephemeris data is updated every two to six hours so that slight variations in the orbital behavior of satellites36can be taken into account. If the ephemeris data is invalid, then method50cannot proceed because there is not enough data available on satellites36at the reference time mentioned above; namely, data pertaining to satellite position and clock correction parameters. An example of when the ephemeris data would likely be invalid is when steps52-56are executed, but then the vehicle is then turned off for more than, for example, eight hours. When the operator restarts the vehicle and execution of method50continues, the ephemeris data would likely be considered outdated and thus invalid. In the event that the ephemeris data is invalid, method50would be exited and the process would need to be reinitiated; if the ephemeris data is valid, then the method proceeds to step84.

Step84determines which of the satellites36(there are a total of twenty-four in the GPS constellation) would have been visible to GPS receiver unit14at the reference time, assuming that nothing was obstructing their transmission (hereafter, referred to as the ‘visible satellites’). As previously mentioned, the reference time provided in step80is the point in time at which the enhanced position information was determined by the independent position enhancement algorithm. By knowing the ephemeris data for each of the satellites36, step84is generally able to determine where each of those satellites were at the reference time, and more importantly which of those satellites would have been visible to GPS receiver unit14. Generally, the GPS receiver unit can see between eight to twelve satellites at any one time.

Step86checks the health status of each of the visible satellites identified in the previous step. Occasionally, a satellite suffers a malfunction with a thruster or experiences some other technical difficulty which causes the ephemeris data to be inaccurate. In order to alert users of this inaccuracy, the GPS information includes a health status bit that indicates the status of each of the satellites36. Accordingly, step86makes sure that each of the visible satellites has a healthy status.

Step88calculates enhanced pseudo-range information for each of the visible satellites based upon the enhanced position information previously determined. Put differently, the independent position enhancement algorithm has already made its best determination as to the location of the vehicle, however, the output is in the form of enhanced position information and PDE20only accepts information in pseudo-range format. Thus, step88works backwards and calculates an enhanced pseudo-range for each of the visible, healthy satellites; some of which may not have actually transmitted GPS information to GPS receiver unit14because of an obstruction in the transmission path. These enhanced pseudo-ranges are calculated so that when PDE20processes them, it will arrive at the same enhanced position information previously determined.

In order to calculate an enhanced pseudo-range for a particular satellite, the following formula is utilized:
PRi=c(TOTi−TOR)+εi(Eqn. 1.0)

where,PRiis the enhanced pseudo-range between the ithsatellite and GPS receiver unit14; c is the speed of light in free space;TOTiis the time of transmission for the ithsatellite;TORis the time of reception and is set to the reference time provided in step80; and εiis the error due to the Earth's rotation during the time-period betweenTOTiandTOR. The speed of light c and the time of receptionTORare both known, and the variablesTOTiand εican be determined from the following equations:

where,Ri(TOR) is the range between GPS receiver unit14and the ithsatellite at the time of reception; and δiis the ithsatellite clock offset from GPS system time. It will be appreciated by those skilled in the art, that the variableRi(TOR) utilizes the enhanced position information previously calculated and that the variableTOTiis corrected for clock synchronization and atmospheric transmission delay errors.

The error value εi, which is also referred to as the Sagnac effect, is due to the rotation of the Earth during the time of signal transmission. During the time of that signal transmission, a clock in the vehicle navigation device14would experience a finite rotation with respect to the resting reference frame at the Earth's geocenter. The Sagnac effect, εi, is determined by rotating the vehicle navigation device14based on the signal transit time and the Earth's rotational velocity. More thorough information regarding the above-described calculations can be found inThe Interface Control Document, which is already incorporated by reference.

According to this particular embodiment, step88purposely inserts a set of errors (clock synchronization errors, atmospheric transmission delay errors and Earth rotation errors) into the enhanced pseudo-range information. This purposeful error insertion is done in order to offset anticipated error corrections subsequently made by the PDE. Stated differently, method50expects PDE20to make corrections to the enhanced pseudo-range information for one or more errors, however, since the enhanced pseudo-range information was derived from enhanced position information, these error corrections are unnecessary. Thus, if no error insertions were made, the PDE would attempt to correct non-existent errors and thus inadvertently introduce errors into the otherwise accurate enhanced pseudo-range information. It should be recognized, the errors listed above are simply examples of correctable errors, and method50could be designed to correct for a different set of errors other than those described herein. Furthermore, the emergency call network could be provided so that the PDE does not correct for any errors, as mentioned above. In which case, step88would not purposely insert errors into the enhanced pseudo-range information because there would be no need to offset subsequent actions taken by the PDE. Once step88has determined enhanced pseudo-range information for each of the healthy, visible satellites, the method proceeds to step90.

In step90, a Doppler frequency f is determined for each healthy, visible satellite and is based on the relative velocity between the GPS receiver unit14(previously provided in step80) and the transmitting satellite36. The Doppler frequency f appears positive or negative depending on whether the satellite is moving towards the GPS receiver unit or away from it, and a frequency f of zero appears when the satellite transitions from moving towards unit14to moving away from it, or vice-versa. Again, the equations for determining the Doppler frequency f are described inThe Interface Control Documentand are known in the art.

Step92determines a number of different operating parameters, some of which are discretionary. For instance, it is oftentimes required to provide a satellite carrier-to-noise ratio which is indicative of the strength of the signal being transmitted by the satellite36. Typically, this parameter is useful if PDE20is utilizing a weighted least squares approach to improve position accuracy, however, it is not pertinent to method50because enhanced pseudo-ranges are being produced from a known position and the method is not actually tracking a satellite. Thus, a healthy carrier-to-noise ratio, such as 40 dB-Hz, will be assigned for each of the healthy, visible satellites. Another parameter that may need to be determined is the satellite multi-path indicator, which is usually determined by GPS receiver unit14. Since method50does not rely on measured ranges, this parameter is not applicable and will therefore be assigned a value indicating that it is not available. Lastly, a pseudo-range RMS error is the root mean square error of the pseudo-range measurement for each of the healthy, visible satellites. The range of the RMS error is typically between 0.5 m to 112 m, however, a value of 0.5 m will be assigned for all healthy, visible satellites because no actual measurement is being made. It should be appreciated that other parameters and/or calculations could be made in order to complete step92, and that one or more of the above-mentioned parameters could be omitted. Once step92is completed, the conversion step60is finished and control passes to step62.

Referring back toFIG. 2, step62gathers all of the information provided in the previous steps and packages it into a format suitable for sending it to PDE20. According to a preferred embodiment, the information is packaged into an electronic message that conforms with the IS-801 format, as is appreciated by those skilled in the art, and is sent to PDE20. Of the various pieces of information populating an IS-801 message, are the enhanced pseudo-ranges previously described. The PDE20will then process the IS-801 message and calculate the position of GPS receiver unit14; a position that corresponds to the enhanced position information previously calculated.

It is to be understood that the foregoing description is not a description of the invention itself, but of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. For example, the method of improving a vehicle emergency call network described above could be used with one of a number of other networks and is not specifically limited to the emergency vehicle call network10that is shown inFIG. 1. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims.