Abstract:
A method of communicating to a RF tag having a low power mode and a scan mode with a radar and an interrogator. The method comprises the steps of alternating the RF tag between the low power mode and the scan mode and then transmitting a wake-up call to the RF tag with the radar. Next, the wake-up call is received from the radar by the RF tag when the RF tag is in the scan mode. Once the wake-up call has been received, the radar will transmit a downlink message to the RF tag. Upon receipt of the downlink message, the RF tag will send an uplinked message to the radar. After the uplink message has been sent to the radar, the RF tag will return to the low power mode. By alternating the RF tag between the low power mode and the scan mode, the power consumption of the RF tag is greatly reduced thereby increasing the battery life thereof.

Description:
STATEMENT OF GOVERNMENT RIGHTS 
     This invention was made with Government support under contract F30602-98-0257 awarded by the United States Air Force. The Government has certain rights in this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to radio frequency communications and more particularly to a radio frequency communication protocol between an interrogator and a tag. 
     Radio frequency (RF) tags are used for tracking of ground-based inventory by an overhead vehicle such as an aircraft or satellite. Upon interrogation by the radar of the aircraft or satellite, the radar responsive tag will transmit. a unique identifier. The unique identifier provides information about the tag. As such, RF tags are used to control inventory of assets on land and sea. 
     Typically, the RF tags are interrogated at unscheduled times. Accordingly, the RF tag must be turned on continuously to listen for the interrogator&#39;s signal. However, the tag&#39;s power consumption limits the tag&#39;s battery life to only a few hours when continuously listening for the interrogator. This deficiency is amplified by the fact that interrogation times are irregular and scheduling is impractical and/or impossible. The prior art RF tags typically remain in a state of readiness for interrogation thereby leading to a substantial amount of power consumption. 
     Prior art RF tags that allow unscheduled communications with an interrogator have limited signal processing and can only operate at short ranges. However, the limited signal processing ability of these prior art RF tags limits the tags to the processing of very strong signals. Weak and noisy signals are not useable with the prior art RF tag. This results in a limited operating range from as little as 1 meter to at most 100 meters for the prior art RF tag. 
     The present invention addresses the above-mentioned deficiencies in the prior art RF tags by providing a RF tag communications protocol between an interrogator and the RF tag. The communications protocol of the present invention provides a method whereby power consumption within the RF tag is adjusted thereby providing a power savings within the RF tag and extending the battery lifetime thereof. Additionally, the RF communications protocol of the present invention provides a method whereby the RF tag will respond only to an interrogator that is within a prescribed range. The RF communications protocol of the present invention provides a method of communication between an RF tag and an interrogator that is power efficient. 
     BRIEF SUMMARY OF THE INVENTION 
     A method of communicating to a RF tag having a low power mode and a scan mode with a radar and an interrogator. The method comprises the steps of alternating the RF tag between the low power mode and the scan mode and then transmitting a wake-up call to the RF tag with the radar. Next, the wake-up call is received from the radar by the RF tag when the RF tag is in the scan mode. Once the wake-up call has been received, the radar will transmit a downlink message to the RF tag. Upon receipt of the downlink message, the RF tag will send an uplink message to the radar. After the uplink message has been sent to the radar, the RF tag will return to the low power mode. By alternating the RF tag between the low power mode and the scan mode, the power consumption of the RF tag is greatly reduced thereby increasing the battery life thereof. 
     Typically, the RF tag is alternated between the low powered mode for 977 milliseconds and the scan mode for 23 milliseconds. The uplink message will not be sent to the radar until a prescribed period of time has passed. Typically, the prescribed period of time is specified in the downlink message received by the RF tag. Similarly, the RF tag will be returned to the low power mode after a prescribed period of time in order to conserve power. Typically, the prescribed period of time is about four hours. 
     In the preferred embodiment, a wake-up call is modulated with a linear frequency modulation. The radar will transmit the wake-up call in a left beam, a center beam, and a right beam. The pulse width of the left beam is 20 μs, the pulse width of the center beam is 30 μs, and the pulse width of the right beam is 40 μs. In the preferred embodiment, the downlink message and the uplink message are transmitted when the radar transmits a wake-up call in the center beam. In this respect, the RF tag will receive and transmit information to the radar when the beam from the radar is aligned with the RF tag. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These as well as other features of the present invention will become more apparent upon reference to the drawings wherein: 
     FIG. 1 illustrates a radar responsive tag system utilizing a tag communications protocol of the present invention; 
     FIG. 2 illustrates an alternative configuration of the radar responsive tag system shown in FIG. 1; 
     FIG. 3 is a system time line for the RF tag and radar shown in FIGS. 1 and 2; 
     FIG. 4 is a time line for the scanning of the RF tag of the present invention; 
     FIG. 5 is a RF tag state diagram for the present invention; 
     FIG. 6 is a diagram depicting the signal for the radar used with the present invention; 
     FIG. 7 is a diagram depicting a wake-up call signal for the present invention; and 
     FIG. 8 is a graph depicting the battery life of the RF tag for the communications protocol of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the present invention only and not for purposes of limiting the same, FIG. 1 illustrates a radar responsive tag system  10  utilizing a tag communications protocol of the present invention. The radar responsive tag system  10  comprises a radar  12  disposed on an aircraft or satellite. The radar  12  is in electrical communication with an interrogator  14  configured as computer software for the radar  12  of the aircraft or satellite. The radar  12  of the aircraft or satellite communicates via radio frequencies to a tag  16  that is disposed on the object to be identified. In this respect, the tag  16  can receive and transmit radio frequency (RF) signals to the radar  12  of the aircraft or satellite. The radar  12  can interrogate the tag  16  such that the tag  16  will transmit a unique identifier to the radar  12  that identifies the object. In the preferred embodiment, the radar responsive tag system  10  utilizing the communication protocol of the present invention can communicate with a medium range satellite that has a slant range in excess of one-hundred miles. Accordingly, the radar  12  can track ground based inventory by interrogating the tag  16  disposed on ground or sea assets. 
     Referring to FIG. 2, an alternative configuration of the radar responsive tag system  10  is shown. In the alternative configuration, the interrogator  14  is not an integral part of the radar  12  of the aircraft or satellite. In this respect, the interrogator  14  communicates with the radar  12  using a computer server  18  and a ground station  20 . The ground station  20  transmits the signals from the interrogator  14  via a data communication channel to the radar  12  of the satellite or aircraft. The interrogator  14  is connected to the ground station  20  via the computer server  18  and a telecommunication line  22 . Therefore, in the alternative configuration of the radar responsive tag system  10 , changes to the interrogator  14  (i.e., software) may be easily implemented because the interrogator  14  is located on the ground. 
     The fundamentals of the tag communications protocol for the radar responsive tag system  10  are shown in FIG.  3 . The tag  16  begins in a sniff mode  24  consisting of a low power sleep stage  26  and a scan stage  28 , as seen in FIG.  4 . In the sleep stage  26 , the tag  16  is in a power saving mode. Typically, the sleep stage  26  lasts for about 977 ms and the tag  16  consumes about 10 micro amps of current at 3 volts. The scan stage  28 , on the other hand, typically lasts about 23 ms such that the tag  16  consumes about 1 Watt of power. The sleep stage  26  and the scan stage  28  alternate during the sniff mode  24  until a valid wake-up call  30  is received during the scan stage  28  of the sniff mode  24 , as will be explained below. Accordingly, if the tag  16  does not receive a wake-up call  30  during the scan stage  28 , the tag  16  will return to the sleep stage  26 . 
     Referring to FIG. 5, once the tag  16  receives a valid wake-up call  30 , the tag  16  goes into a downlink mode and waits for a downlink message  32 . Typically, the radar  12  is programmable such that it will perform a wake-up call  30  and then send a downlink message  32  a prescribed number of times (e.g., three times). After the radar  12  has completed sending the wake-up call  30  and downlink message  32 , the radar  12  will wait a programmable amount of time (e.g., typically four hours) before transmitting a new wake-up call  30 . 
     Once the tag  16  receives a valid downlink message  32 , the tag  16  will wait a specified amount of time before transmitting an uplink message  34  to the radar  12 . This delay allows the tag  16  to skip over redundant wake-up calls  30  and multiple downlink messages  32  that are repeated in the message group. The time interval between receiving a downlink message  32  and transmission of an uplink message  34  may be specified in the downlink message  32  received by the tag  16 . 
     Once the tag  16  finishes sending an uplink message  34 , the tag  16  sets a programmable lock out timer that enables the tag  16  to ignore any new wake-up calls  30  and downlink messages  32 . The lock out timer is nominally set to four hours and prevents the batteries in the tag  16  from depleting as a result of multiple interrogations or false signals. Accordingly, this feature restricts the tag  16  from sending any new uplink messages  34  before a prescribed minimum time period has passed (i.e., about four hours). 
     At any point the tag  16  determines that the signal from the radar  12  is invalid, the tag  16  will enter the sleep stage  26 . This improves the power savings in the tag  16  when the signals from the radar  12  may be false or erroneous. The period of the sleep stage  26  may be programmable such that the tag  16  will be in the power savings mode for a prescribed period of time. 
     Referring to FIG. 5, the tag  16  transitions between multiple states. Accordingly, the tag  16  transitions between each major state as shown in FIG.  5  and described by the following table: 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Definitions of Tag State Transitions. 
               
             
          
           
               
                  Transition No. 
               
               
                   
               
               
                       1 
                 Sleep timer expires, begin scan for 
               
               
                   
                 interrogator. 
               
               
                 2 
                 Interrogator not found, return to 
               
               
                   
                 sleep. 
               
               
                 3 
                 Detects interrogator, look for 
               
               
                   
                 downlink. 
               
               
                 4 
                 Downlink fails, return to sleep. 
               
               
                 5 
                 Successful downlink; prepare for 
               
               
                   
                 uplink. 
               
               
                 6 
                 Uplink complete, return to sleep. 
               
               
                   
               
             
          
         
       
     
     In order to ensure that the tag  16  is receiving a signal from the main beam of the radar  12  and not a leakage signal from the side lobes of the radar  12 , the communications protocol of the present invention provides a method of informing the tag  16  whether or not it is receiving the main beam of the radar  12 . Referring to FIG. 6, the radar  12  will steer its antenna (i.e., transmission source) during the wake-up call  30  to a left position  36 , a center position  38 , and a right position  40 . Each of the positions  36 ,  38 ,  40  overlap the beam from the radar  12  at the −3 db power point. As will be explained below, the radar  12  expects to communicate with the tag  16  while the radar antenna and hence the beam from the radar  12  is in the center position  38 . Accordingly, the radar  12  will only transmit a downlink message  32  and the tag  16  will only transmit an uplink message  34  while the beam is in the center position  38 . 
     The wake-up call  30  from the radar  12  will be of a pulse width (PW) and linear frequency modulated (LFM) according to the following table: 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Typical Radar Modulation in a Wake-Up Call. 
               
             
          
           
               
                   
                           PW, 
                         LFM, 
               
               
                   
                 TIME 
                 FREQUENCY DEVIATION 
               
               
                   
                   
               
             
          
           
               
                 “L” 
                 20 μs 
                    100 MHz 
               
               
                 “C” 
                 30 μs 
                 100 MHz 
               
               
                 “R” 
                 40 μs 
                 100 MHz 
               
               
                   
               
             
          
         
       
     
     In this respect, the tag  16  will contain a suitable receiver to receive and measure the PW and LFM of the wake-up call  30 . 
     In the communications protocol of the present invention, the pulse width and linear frequency modulation of the wake-up call  30  is deviated in order to inform the tag  16  of the position  36 ,  38 , or  40  of the beam from the radar  12 . Accordingly, when the beam from the radar  12  is in the left position  36 , the wake-up call  30  will have a pulse width of approximately 20 μs, while a wake-up call  30  from a beam of the radar  12  in the center position  38  will have a pulse width of approximately 30 μs and a wake-up call  30  from a beam of the radar  12  in the right position  40  will have a pulse width of approximately 40 μs. The linear frequency modulation (LFM) of the wake-up call  30  will be approximately 100 MHz for the left position  36 , the center position  38 , and the right position  40 . The duration of the wake-up call  30  depends on the number of radar pulses and the pulse repetition frequency (PRF) thereof. Typically, the wake-up call  30  will contain 300 pulses with a PRF of 300 Hz thereby resulting in a wake-up call  30  that is approximately one second long. In the communications protocol of the present invention, the tag  16  will measure the signal strength, pulse width, and LFM deviation of the wake-up call  30  during the scan stage  28  of the sniff mode  24 . During the time that the tag  16  receives the wake-up call  30 , the tag  16  will receive approximately seven pulses from the radar  12 . This will result in slightly more than two sub-sequences of the wake-up call  30  shown in FIG.  7 . 
     The left position  36 , center position  38 , and right position  40  of the beam from the radar  12  generate a characteristic signal. Accordingly, the following table shows the signal strength that is received at the tag  16  for different distances between the tag  16  and the radar  12 . 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Signal Strength at the Tag. 
               
             
          
           
               
                 Relative 
                 Tag in 
                 Tag in 
                 Tag in 
               
               
                 distance 
                 left 
                 center 
                 right 
               
               
                 from tag 
                 beam, L 
                 beam, C 
                 beam, R 
               
             
          
           
               
                 to radar 
                   L 
                 C 
                 R 
                 L 
                 C 
                 R 
                 L 
                 C 
                 R 
               
               
                   
               
               
                 Far 
                 x 
                 — 
                 — 
                 — 
                 x 
                  — 
                 — 
                 — 
                 x 
               
               
                 Medium 
                 X 
                 x 
                 — 
                 x 
                 X 
                 x 
                 — 
                 x 
                 X 
               
               
                 Close 
                 X 
                 X 
                 x 
                 X 
                 X 
                 X 
                 x 
                 X 
                 X 
               
               
                   
               
               
                 X = Strong Signal  
               
               
                 x = Weak Signal  
               
               
                 —= No Signal  
               
             
          
         
       
     
     The tag  16  measures the relative strength of the signal from the radar  12  to determine the relative distance from the tag  16  to the radar  12 . Accordingly, the tag  16  can determine if the signal from the radar  12  has enough strength to be a valid signal. 
     Once the tag  16 , during the scan stage  28 , identifies a pulse sequence of sufficient strength according to Table 3, the tag  16  will next identify the corresponding PW and LFM according to Table 2. By determining the PW and LFM for the wake-up call  30 , the tag  16  determines whether the beam from the radar  12  is in the left position  36 , center position  38 , or right position  40 . Therefore, if the tag  16  determines that the wake-up call  30  has sufficient strength according to Table 3 and is from the center position  38  according to Table 2, then the tag  16  has received a valid wake-up call  30 . 
     After determining that the wake-up call  30  is valid, the tag  16  will start a programmable downlink wait timer that is set to the same duration as the wake-up call  30  (i.e., typically one second). If the programmable downlink wait timer of the tag  16  expires before a downlink message  32  is received thereby, then the tag  16  will resume the sniff mode  24 , as previously described. Otherwise, the tag  16  will read the downlink message  32  and transmit an uplink message  34 , as seen in transmission state number  5  of FIG.  5 . After transmitting the uplink message  34 , the tag will return to the low power sniff mode  24  as previously described. 
     As previously mentioned, the tag communications protocol of the present invention provides a power savings for the tag  16 . Typically, the tag  16  operates using standard 3 volt lithium batteries. When the tag  16  is in the sleep stage  26  it typically draws 10 micro amperes of current at three volts. During the scan stage  28 , the tag  16  nominally dissipates approximately one watt of power for a scan duration of 23 milliseconds occurring every second. The duration of the downlink message  32  is typically 1.1 seconds and occurs every four hours such that the power dissipated during the downlink message  32  is nominally two watts. Furthermore, the duration of the uplink message  34  is nominally ten seconds and occurs every four hours such that five watts is dissipated. 
     Therefore, energy utilization for the tag  16  is power x time. The average energy (E T ) used by the tag  16  is: 
     
       
           E   T   =E   SLEEP   +E   SCAN   +E   DOWNLINK   +E   UPLINK   (1) 
       
     
     The energy for each of the states (E SLEEP , E SCAN , E DOWNLINK , and E UPLINK ) is typically stated in units of Watt-hours and is shown below in Equations 2 to 6. The usage factor in Equations 2 to 6 typically has a range of between about 0 to 1 and indicates how much time the tag  16  spends in that corresponding mode. Typically, the usage factor is weighted for a four hour interval (i.e., the time between wake-up calls  30 ). The time duration of the uplink and downlink are nominally 1 second and 2 seconds, respectively. 
     
       
           E   SLEEP =3V×10 μA×977/1000 usage factor×1 hr=29.3 microWatt-hr  (2) 
       
     
     
       
           E   SCAN =1 Watt×23/1000 usage factor×1 hr=23 milliWatt-hr  (3) 
       
     
     
       
           E   UPLINK =5 Watts×(0.25×1/3600 usage factor)×1 hr=347 microWatt-hr  (4) 
       
     
     
       
           E   DOWNLINK =2 Watts×(0.25×2/3600 usage factor)×1 hr=278 microWatt-hr  (5) 
       
     
     
       
           E   T =sum of above=23.7 milliWatt-hr  (6) 
       
     
     The battery life (L) for the tag  16  using the communications protocol of the present invention, is given by the following equation: 
     
       
           L=E   AVAIL-BATT   /E   T =3 batteries×(3 V×1 Amp-hr)/ E   T =380 hours, or 15.8 days  (7) 
       
     
     Accordingly, using three batteries for the tag  16 , the battery life will be approximately 380 hours or 15.8 days. Referring to FIG. 8, it is concluded that a typical one second wake-up call  30  will correspond to a battery life of 16 days, as previously mentioned. Therefore the main factor in the life of the battery is the periodicity of the scan stage  28 . The communications protocol of the present invention provides longer battery life in the tag  16  because the tag  16  spends a majority of time in the low power sleep stage  26 . As seen by Equations 2 to 7, the main factor in the power consumption of the tag  16  is the frequency of the scan stage  28 , which as previously mentioned, is set by the sleep timer. Therefore, by changing the duration of future wake-up calls  30  and by sending a new value to the sleep timer via the downlink message  32 , the power savings of the tag  16  can be adjusted accordingly. Therefore, in the process of modifying the duration of the wake-up call  30 , the interrogator  14  of the radar  12  can change the power consumption in the tag  16 . The change in the power consumption of the tag  16  will follow the curve shown in FIG.  8 . 
     Additional modifications and improvements of the present invention may also be apparent to those of ordinary skill in the art. Thus, the particular combination of parts described and illustrated herein is intended to represent only a certain embodiment of the present invention, and is not intended to serve as limitations of alternative devices within the spirit and scope of the invention.