Abstract:
A system for interconnecting multiple GPS receivers via a single line to simultaneously communicate both serial data and precise timing pulses to numerous GPS receivers using a time multiplexed data format.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to global positioning systems (GPS), and more particularly relates to communication to multiple GPS receivers. 
     BACKGROUND OF THE INVENTION 
     In the past, designers of GPS receiver systems have frequently used one GPS receiver to provide precise time information to another GPS receiver. Typically, the time source GPS is coupled to the time receiving GPS receiver by a serial data interface and a timing interface line which provides a timing pulse (or pulse train). When two or more time receiving GPS receivers are used, then separate data and time interface lines would be run to each time receiving GPS receiver. In other cases, the time receiving GPS receivers might be ganged together by connecting separate data and time sync interface lines from the output of a broadcasting GPS receiver to the input of multiple GPS receivers. 
     In some applications, it may be desirable to include many GPS receivers in a single deployable unit. For example, numerous surveillance units may be dropped in an area to be monitored, via an airdrop. In such cases, to conserve battery life, the GPS receivers may be turned off until just before deployment. In such cases, it may be desirable to quickly initialize the many GPS receivers by a single time standard (such as a reference GPS receiver used by the airplane navigation system) prior to the airdrop. 
     While these prior art interfaces have been used extensively in the past, they have several drawbacks. First of all, the direct wiring approach becomes increasingly cumbersome as the number of time receiving GPS receivers grows. Additionally, the ganged approach is undesirable because as the number of time receiving GPS receivers grows, the number of connections grows. With each additional connection, there is an increased potential for a failed connection, which would affect all “downstream” GPS receivers. Furthermore, separate interfaces for serial data and time synchronization pulses doubles the total number of connections required. This becomes more burdensome when a larger number of receivers must be initialized. 
     Consequently, there exists a need for improvement in systems and methods for communicating with multiple GPS receivers. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to facilitate ease of communication with multiple GPS receivers. 
     It is another object of the present invention to provide an enhanced method of communicating to a network of GPS receivers. 
     It is a feature of the present invention to utilize a single serial line to interconnect numerous GPS receivers. 
     It is another feature of the present invention to include a single 2-to-1 multiplexor coupled to a source of a precise time pulse and serial data. 
     It is an advantage of the present invention to better enable sharing of information with a network of GPS receivers. 
     It is another advantage of the present invention to reduce weight in a multiple GPS system. 
     It is yet another advantage of the present invention to reduce the potential for single connection faults that could break communication paths to multiple GPS receivers. 
     The present invention is an apparatus and method for communicating information with GPS receivers, which is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features, and achieve the already articulated advantages. The present invention is carried out in a “direct wire” in a sense that the number of direct independent wires has been greatly reduced. 
     Accordingly, the present invention is a system and method for communicating information with multiple GPS receivers, which uses a single serial data/timing line interconnecting multiple GPS receivers. This line alternates between data transmission and time synchronization pulses functions in a time multiplexed fashion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the preferred embodiments of the invention, in conjunction with the appended drawings wherein: 
     FIG. 1 is a simplified diagram of a multiple GPS receiver system of the prior art. 
     FIG. 2 is a simplified diagram of a multiple GPS receiver interconnection system of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawings wherein like numerals refer to like matter throughout, there is shown a multiple GPS receiver interconnection system of the prior art, generally designated  100 , having a source of precise time and serial data, labeled a GPS master  102 . GPS master  102  includes a serial data port  104  for transmitting data, such as current location for use in initialization, and a timing information port  106  to provide precise timing information. Also shown are time and data receiving GPS receivers  110 ,  120  and  130 , each of which is coupled to GPS master  102  by independent direct connections. 
     Throughout this document, GPS is used as an example of a broader set of position determination schemes which could include, but are not limited to: Glonass, Loran, or any other system which requires precise and serial data to be transferred from one location to another. 
     Now referring to FIG. 2, there is shown a multiple GPS receiver interconnection system of the present invention, generally designated  200 , including a source of precise time pulse and serial data  202 . Source of precise time pulse and serial data  202  selects, in a time multiplexed manner, between either the periodic pulse shown in periodic timing pulse pattern  205  on time pulse line  204  or the serial data  207  on serial data line  206 , to transmit on the single combined data and timing communication line  212  with optional line driver  211 . Line driver  211  may be preferred, but not necessarily required. In many cases, the source of precise time pulse and serial data  202  will be another GPS receiver. The time and data receiving GPS receivers  220 ,  230 ,  240 , etc., as well as the source of precise time pulse and serial data  202 , default to using the single combined data and timing communication line  212  for serial data transfer. When the source of precise time pulse and serial data  202  needs to send time information to the time and data receiving GPS receivers  220 ,  230 ,  240 , etc., then the source of precise time pulse and serial data  202  computes the time of the first pulse which will be transmitted, and sends a message to the data receiving GPS receivers  220 ,  230 ,  240 , etc., to inform them of the time and to instruct them to switch modes, so as to accept the periodic timing pulse pattern  205 . Source of precise time pulse and serial data  202  then disconnects the single combined data and timing communication line  212  from the asynchronous serial data  207  on asynchronous serial data line  206  via the select line  208  and then connects single combined data and timing communication line  212  to the periodic timing pulse pattern  205  on time pulse line  204 . This switching between time pulse line  204  and asynchronous serial data line  206  is accomplished by 2-to-1 multiplexor  210 . The preferred embodiment of the 2-to-1 multiplexor  210  is a device which duplicates one of two inputs on its output pin. Which input is duplicated depends on logic state of the channel section input. Another embodiment of the 2-to-1 multiplexor  210  includes two switches. One switch would tie the serial data line  206  to single combined data and timing communication line  212 , and the other switch would tie the time pulse line  204  to the single combined data and timing communication line  212 . Only one switch would be on at any moment in time. Another embodiment of the 2-to-1 multiplexor  210  is two logic outputs which are tied together where at least one of the two outputs is in high-impedance state at any moment in time. The output of the 2-to-1 time multiplexor  210  may be buffered by an optional line driver  211  and line receiver  224 ,  234 ,  244 , etc. 
     It can be assumed that periodic timing pulse pattern  205  is a pulse that is output at once per second, or any other desirable rate, such as when the universal coordinated time (UTC) second rollover occurs, a convenient event in a GPS receiver. When the time transfer is complete, the source of precise time pulse and serial data  202  disconnects the periodic timing pulse pattern  205  on time pulse line  204  and waits a predetermined idle time to signal completion before commencing transfers of asynchronous serial data  207  to the time and data receiving GPS receivers  220 ,  230 ,  240 , etc. 
     The data on single combined data and timing communication line  212  is graphically represented by time multiplexed combined time pulse and asynchronous data signal  213 . 
     From the viewpoint of first time and data receiving GPS receiver  220 , second time and data receiving GPS receiver  230 , and third time and data receiving GPS receiver  240 , etc., which may be viewed as identical receivers operating in parallel, the invention functions as follows: 
     For example, first time and data receiving GPS receiver  220  receives the incoming information on single combined data and timing communication line  212 , possibly through the optional line driver  211  and optional line receiver  224 . The incoming information on single combined data and timing communication line  212  is connected to both the first time tagged interrupt  221  via the first internal time tagged interrupt line  226  and to first asynchronous serial port  227  via the first asynchronous serial port connection line  228 . The first time and data receiving GPS receiver  220  defaults to assuming that incoming data is being used for serial data communication and disables any interrupts from occurring on the first time tagged interrupt  221 . Serial communications occur as normal and until first time and data receiving GPS receiver  220  receives a time transfer message from the source of precise time pulse and serial data  202 . When first time and data receiving GPS receiver  220  receives a time transfer message from the source of precise time pulse and serial data  202 , it takes the time of a pulse from the serial data message. Then the first time and data receiving GPS receiver  220  disables serial input to first asynchronous serial port  227  and enables first time tagged interrupt  225 . First time and data receiving GPS receiver  220  initializes its internal clocks with the time transmitted in the data message  207  and uses the incoming data time pulse  205  to synchronize that time with the first receiver  220 &#39;s own internal time. While receiving the timing pulse  205 , the first time and data receiving GPS receiver  220  may also perform precise frequency calibration of its internal time standard. The source of precise time pulse and serial data  202  outputs a pulse at a known repetition rate or at a rate specified in the serial data message  207 . The first time and data receiving GPS receiver  220  measures the elapsed time between pulses with its time tagged interrupt  221  and compares this to the known elapsed time as specified (or agreed upon) repetition rate of time pulse  205 . The difference between the actual elapsed and the elapsed as measured by the first time data receiving GPS receiver  220  can be used to compute frequency correction to the first GPS receiver  220 &#39;s time standard. After the first time and data receiving GPS receiver  220  has received no pulses for a predetermined idle time, it switches back to serial data mode and continues serial communication as normal. 
     Also shown in FIG. 2 are second time and data receiving GPS receiver  230  and third time and data receiving GPS receiver  240 , which include second GPS receiver connection line  232  and third GPS receiver connection line  242  respectively, and second optional line receiver  234  and third optional line receiver  244  respectively, which are respectively coupled to second time tagged interrupt  235  and third time tagged interrupt  245  via second bridged internal time tagged interrupt line  236  and third bridged internal time tagged interrupt line  246 . Second time and data receiving GPS receiver  230  and third time and data receiving GPS receiver  240  also include third serial port  247  and third asynchronous serial port connection line  248 , as well as second serial port  237  and second serial port connection line  238 . It is presently preferred to use an asynchronous data stream; however, the present invention is intended to cover both synchronous and asynchronous data streams. 
     Other variations of the present invention are also contemplated. For example, it is possible to have a separate dedicated line between the source of precise time pulse and serial data  202  and each of the GPS receivers  220 ,  230  and  240 , so that the precise time and serial data are not broadcast on a single line but transmitted individually to each receiver one at a time or all together in parallel. Another possible variation is that each data link uses two lines for bi-directional serial data flow between the receiver and source of precise time pulse and serial data  202 . The innovation of time multiplexing the precise time pulse and serial data on the same outgoing data line still applies. Another variation is the number of GPS receivers which receive time pulse and serial data  220 ,  230 , and  240  could vary from one to one thousand. 
     It is thought that the method and apparatus of the present invention will be understood from the foregoing description and that it will be apparent that various changes may be made in the form, construct steps and arrangement of the parts and steps thereof, without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred exemplary embodiment thereof.