Abstract:
A wireless instrumentation system enables a plurality of low power wireless transceivers to transmit measurement data from a plurality of remote station sensors to a central data station accurately and reliably. The system employs a relay based communications scheme where remote stations that cannot communicate directly with the central station due to interference, poor signal strength, etc., are instructed to communicate with other of the remote stations that act as relays to the central station. A unique power management scheme is also employed to minimize power usage at each remote station and thereby maximize battery life. Each of the remote stations preferably employs a modular design to facilitate easy reconfiguration of the stations as required.

Description:
ORIGIN OF THE INVENTION 
   The invention described herein was made in the performance of work under a NASA contract and by an employee of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. §202, the contractor elected not to retain title. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a wireless instrumentation system that is particularly suited for acquiring measurement data from a number of remote station sensors and transmitting the data to a central station. In the preferred embodiment, the system is designed to operate with low power transceivers and other elements and includes an automatic data relaying scheme for accommodating communications with out-of-range transceivers. In addition, a power management scheme is employed for minimizing power consumption by the remote stations. 
   2. Description of the Background Art 
   A number of measurement devices and systems are currently in use around the launch pad and within the Vehicle Assembly Building (VAB) at the Kennedy Space Center (KSC) to monitor various operational parameters of the Space Shuttle and related systems. Most of these systems use cabling to support power and data transmission. In such systems, interaction at the sensor level is not a desirable option because of extensive cabling requirements. Centralized data acquisition systems have therefore been employed, which, unless provided with redundancy, have often been the source of single point failures. 
   These issues have led to the need for data acquisition system that can operate in close proximity to the Orbiter, External Tank and Solid Rocket Boosters, for example, without interfering with existing electronic systems. To eliminate cabling issues, the system would have to be a wireless radio frequency (RF) system that could be interfaced with various measurement devices and would include a plurality of remote sensor stations in communication with a central station. However, limitations on both power and physical size of the remote stations would be necessary to minimize radio interference with other electronic systems and space requirements. As a result, the remote stations would have to be implemented with low power transceivers that are powered by small exhaustible power sources, e.g. batteries. Very low RF power systems can be operated in close proximity to other systems virtually interference-free. However, low RF power inherently limits the operating range of the transceivers, thus making it difficult or impossible to insure reliable communications between each remote station and the central station. Furthermore, small batteries inherently imply short life which means that a power management scheme would be necessary to maximize energy utilization and battery life of each remote station. 
   SUMMARY OF THE INVENTION 
   To address the forgoing needs, the present invention provides a wireless instrumentation system that enables a plurality of low power wireless transceivers to transmit measurement data from a plurality of remote station sensors to a central data collection station accurately and reliably. The system is designed specifically to insure that sensor data can be received from the low power transceivers, even when the transceivers are affected by poor RF signal conditions or no RF signal due to physical placement outside the normal range of the central data collection station. In addition, a unique power management scheme is employed to minimize power usage at each remote station and thereby maximize battery life. 
   More particularly, the system comprises a plurality of stations, each of which includes a transceiver and a computer-based controller. At least one of the stations is selected to be a central station, while the remaining stations are each designated as a remote station. The station controllers employ a relay-based communications scheme that facilitates communicating with remote stations that are transmitting/receiving either weak or no RF signals to/from the central station. In the scheme, these weak stations are identified and automatically become satellite stations of another remote station that is capable of better communications. In an effort to increase the range of the system communications even further, additional levels are created that permit stations to relay data from one location to another through a number of the remote stations. This permits stations that are too far from the central station location to communicate indirectly with the central station. 
   In the communications scheme, the central station normally communicates with all of the remote stations using a conventional handshaking routine in which the central station sends a command to each remote station transceiver and then waits for a reply from each transceiver confirming that the command has been received. Mainly because of their low RF power properties, distant stations or those located in signal-poor areas where the communication drop rate is too high, may not be able to communicate consistently, if at all, with the central station. To detect such communication failures, the central station and the remote stations constantly monitor their communication health using a health check routine. The central station controller labels any of the remote stations that cannot be reached as “lost stations.” Thus, in the event that the central station fails to receive an acknowledgment reply from anyone of the remote station transceivers, the central station controller identifies that remote station as a lost station and begins a routine that will identify which of the other remote stations has the ability to communicate with the lost station. That remote station is then designated as a relay station to guarantee the communication between the central station and the lost station. The routine can continue on to more levels, effectively creating a chain of stations which relay data from one to another, to permit long-distance communications using relatively low power. 
   To enable continuous high-speed transmissions and to overcome frequency jamming, each transceiver is preferably capable of sending and receiving data on a plurality of transmit and receive frequencies. This permits multiple communication paths between the central station and the remote stations, and between the remote stations and any lost stations for simultaneous operations without interference. Under normal operations, the central station queries each remote station in the system using a “primary transmit” frequency, while each remote station replies in sequence on a “primary receive” frequency. All remote stations that communicate directly with the central station are designated primary stations. If all stations reply as expected, the central station will eventually cycle through each remote station and restart the process. 
   In the preferred embodiment, should a primary remote station be identified as a lost station, two things will occur. First, the lost station&#39;s controller, in response to not having received a polling command from the central station for a predetermined period of time, will change the transceiver&#39;s transmit and receive frequencies to an alternate frequency pair. Second, the central station controller will then begin the routine to find a primary station that has the ability to communicate with the identified lost station. On the next successive poll, the central station transmits data to all primary stations to identify the station that is lost and commands the first primary station in the poll sequence to retransmit a query to the lost station. That primary station controller then switches its transceiver to the alternate frequency pair, transmits a poll, and waits for a response. If the lost station responds, the primary station then stores the data and its controller switches back to the primary frequency pair until the central station polls it again. At this point, the primary station will send both its own data and that of the lost station to the central station using the primary frequency pair. With this response, the central station controller notes the successful communications loop and designates that primary station as the relay station for all future communications with that particular lost station. The central station controller continues to establish a direct communication with the lost station using the primary frequency or any of the alternate frequency pairs even when it has already identified a relay station. This process occurs automatically and autonomously in the system. 
   To extend the distance capability, it can be possible to place a tertiary station at a distance far enough away from the central and primary stations that it would have to rely on a secondary station as its only source of communications with the system. Such system layouts may be necessary, by design, rather than incidental due to malfunction. The operation sequences mimic that of a normal lost station routine but differ in that the level reaches one step further and utilizes other alternate frequency pairs for transmitting and receiving. Additional alternate frequency pairs could be employed to extend the chain of relay stations even further, though in actual practice, a chain consisting of the central station, a lost remote station and two relay stations between these should be sufficient. 
   The wireless instrumentation system also preferably employs an effective power management algorithm to achieve maximum data transfer with minimum power consumption. As noted before, the central station acts as a master station that repeatedly sends the commands to the remote stations until a response is received or timeout occurs. To save power, the remote stations remain in a low power consumption mode much of the time, but are powered up periodically to check whether a central station request for action is received. If so, the power management algorithm will maintain the power to the various circuit elements in the remote station until a reply has been transmitted back to the central station. Once this occurs, the algorithm powers the remote station back down until the arrival of the next time interval for checking for incoming commands from the central station. The power on/off duty cycle implemented by the algorithm can be readily adjusted either by the remote station controller itself or by the central station controller to allow for faster or slower communications, depending on the criticality of the particular parameter being monitored. 
   Preferably, the wireless system also employs a modular architecture in which a wireless core module and a power module are common for every remote station. An analog signal conditioning board is provided that is unique to each sensor. The modular design allows for only the analog board to be replaced if a special measurement must be accommodated, thus simplifying the installation for new measurements by requiring minimal hardware changes. Since the system does not require permanent power and communication cables, it becomes easily portable and reconfigurable. The modular architecture also allows for the inclusion of unique functions in the system. Examples of these functions are additional local data storage, local complex data processing, etc. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a wireless instrumentation system that is constructed in accordance with a preferred embodiment of the present invention; 
       FIG. 2  is a block diagram of a central communications station for the wireless system of  FIG. 1 ; 
       FIG. 3  is a block diagram of a remote communications station for the wireless system of  FIG. 1 ; 
       FIG. 4  is a block diagram illustrating the steps implemented for establishing communications between the central station and a lost remote station via relaying through a primary remote station; 
       FIG. 5  is a block diagram illustrating the steps implemented for establishing communications between the central station and a tertiary remote station via relaying through a primary remote station and a secondary remote station; 
       FIG. 6  is a time graph showing a power duty cycle implemented by a power management algorithm in each remote station; and 
       FIG. 7  is a time graph showing the power scheme applied to the remote station in response to commands received from the central station. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIGS. 1 and 2 , a wireless instrumentation system  10  is illustrated that can be configured to operate in accordance with a preferred embodiment of the present invention. The system  10  is in the form of a network and consists of a central station (CS)  12  and one or more remote stations (RS&#39;s)  14 . It should be noted that the system  10  represents a simple example of the type of communication system with which the concepts of the present invention can be employed, but other more complicated systems that employ more than one central station, for example, could be used as well. The CS  12  is the main controller of the system  10  in that the CS  12  manages all communications with the RS&#39;s  14 . To provide this functionality, the CS  12  employs a computer based controller  16  and one or more radio frequency (RF) transceivers  18 . The controller  16  can be implemented with any suitable processor, single board computer, personal computer, etc. and contains the software required for graphical user interface (GUI), data storage, data analysis, command and control and additional software required to monitor the RS&#39;s  14 . The CS  12  initiates and maintains the polling sequence of the RS&#39;s  14  and displays the data retrieved via RF from the RS&#39;s  14  on a display  19 . The RF transceiver  18  can be selected to fit a particular application or to comply with local regulations. For example, among the options for the RF communications are ultrahigh frequency (UHF) spread spectrum, Bluetooth, and IEEE 802.1 1. 
   The system  10  has the ability to perform like a web, providing the capability to share information among the RS&#39;s  14 . Communication between the CS  12  and the RS&#39;s  14 , as well as between the RS&#39;s  14  themselves is via a plurality of wireless links  20  as illustrated in  FIG. 1 . The system  10  can be centralized in which all communications go through the CS  12 , or it can be decentralized as shown in  FIG. 1  where each of the RS&#39;s  14  can communicate directly with each other. 
   Different approaches were investigated for the wireless communication links  20 . The type of wireless link to select depends on the particular application and on constraints placed by the environment where the system is installed. For instance, the wireless links  20  will vary according to the KSC environments they need to work on (specifically related to the Space Shuttle program, Space Station program, Future Vehicles program). Electromagnetic Compatibility (EMC) emissions are also part of the constraints. 
   The CS  12  and each of the RS&#39;s  14  monitor the condition of the communication links  20  that facilitate communication between the CS  12  and the RS&#39;s  14 . An RS  14  will be deemed a “lost station” if communication failures are detected. The communication failures can be temporary or permanent and could occur as a result of interference by other RF-emitting equipment, physical obstructions or faulty electronics. As will be discussed in greater detail in conjunction with  FIGS. 4 and 5 , the CS  12  and the RS  14  seek other communication paths to reestablish a good communication channel. If communication problems still persist, a faulty RS can be deactivated from the polling sequence. 
   The details of one of the RS&#39;s  14  are illustrated in  FIG. 3 . Each RS  14  is a modular device consisting of a number of modules- a core module  22 , a smart module  24  and a custom module  26 . In addition, a signal conditioning board  28  is employed that is specific to the characteristics of one or more sensors  30  to which the signal conditioning board  28  is connected. The main purposes of the RS  14  are to acquire data from the sensor (or sensors)  30 , to command an actuator, to excite a sensor, or to acquire an entire process and transmit the data to the CS  12 . The modular design of the RS  14  simplifies the installation for new measurements by requiring minimal hardware changes. 
   The core module  22  is the heart of the RS  14  and includes a microcontroller  32 , an RF transceiver module  34 , a memory  36  and one or more DC-DC power regulators  38 . The microcontroller  32  controls communications with the CS  12  and the other RS&#39;s  14 . Among other things, the microcontroller  32  performs RF health checks with the CS  12 . In addition, the microcontroller  32  implements a power management and scheduling scheme in accordance with an algorithm to be discussed in detail in conjunction with  FIGS. 6 and 7 . In this manner, the power that is supplied to the various modules or components of the RS  14  from the regulators  38  is controlled to maximize efficient power usage from an exhaustible power supply  40 , such as a battery, that forms part of the custom module  26 . The RS  14  can be installed in existing systems utilizing available externally supplied power or self-contained battery power. A communication and power bus  42  connects the components of the core module  22  to the other modules  24  and  26  and to the signal conditioning board  28 . 
   Preferably, the smart module  24  includes a digital signal processor (DSP)  50  and a memory  52  interfaced thereto for storing data received from the sensor  30  and generated by the DSP  50 . However, it should be understood that any other form of processor can be used in place of the DSP  50 . The DSP  50  together with the embedded software forms the “smarts” or intelligence of the RS  14  and is programmed as desired to process and analyze the data received from the sensor  30 . Without the DSP  50 , the RS&#39;s  14  can have some intelligence built into the software to perform real-time data analysis. However, the DSP  50  can perform more complicated functions. Analysis can include simple averaging of data, maximum and minimum detection, decimation, statistics, spectral analysis, digital filtering, trending, etc. 
   The custom module  26 , as the name implies, includes application a number of specific components  60  that are custom tailored for the particular RS. These application specific components can include, for example, an actuator, communication agent and a memory bank. In addition, these can include the battery banks  40  that power the RS  14  and will differ from RS to RS, depending upon the power requirements of the particular sensor and functions employed by the RS  14 . 
   To interface the analog voltage or current signal generated by the sensor  30  to the digital circuitry of the core and smart modules  22  and  24 , the signal conditioning board  28  includes a number of conditioning components  70 , such as amplifiers, filters, etc. These modify the analog output signal of the sensor  30  as necessary so that the signal can then be digitized by an A/D converter  72  and then sent through the communication bus  42  to the other components of the RS  14 . The signal conditioning board  28  can also provide sensor excitation if required. 
   The manner in which the CS  12  communicates with and receives data from each of the RS&#39;s  14  will now be described. Each RS  14 , as well as the CS  12 , has a unique identifier, such as an address, that is used by the CS  12  and each of the RS&#39;s  14  for facilitating communications between the two. The digitized sensor output that is generated by the A/D converter  72  is included by the RS  14  in a message structure for RF transmission. Each RS  14  can also be instructed by the CS  12  to serve as a “relay station” for other RS&#39;s that are placed in a “lost station” condition. A relay station acts as a repeater, thus assisting in the relaying of the information sent to and from the remote “lost station.” A chain of the relay stations can thus be created to relay information from sensors placed far apart from the CS  12 . Each RS  14  may also serve as a node master station (NMS) that is responsible for monitoring several of the other RS&#39;s  14  and reporting its findings to the CS  12 . 
   The present invention employs an algorithm that facilitates communications between two stations directly, as well as through indirect routes using stations as transponders or relays. In addition, the invention is designed to satisfy the requirement that communications remain fast with maximum reliability while a specified amount of data is being sent. In the preferred embodiment, each RS  14  is processor-based and is capable of sending and receiving data on primary and several alternate frequency pairs to enable continuous high-speed transmissions. This permits multiple communication paths for simultaneous operations without interference. 
   Also in the preferred embodiment, the CS  12  keeps track of all communication transactions. Under normal operations, the CS  12  queries each RS  14  in the system  10  using a first radio frequency. Each RS  14  replies in sequence on a second frequency until all of the RS&#39;s  14  are logged. All RS&#39;s  14  that communicate directly with the CS  12  are each designated as a primary remote station. If all stations reply as expected, the CS  12  will eventually cycle through each RS  14  and restart the process. 
   Mainly because of their low RF power properties, distant stations or those located in signal-poor areas where the communications drop rate is too high may become identified as “lost stations” if the signal between them and the CS  12  becomes lost. In the preferred embodiment, should a primary one of the RS&#39;s  14  fail to communicate with the CS  12  on two consecutive polls, the RS  14  will be flagged as a lost remote by the CS  12 . At the same time, the microcontroller  32  in the lost one of the RS&#39;s  14  will detect that a communication failure has occurred through expiration of a lost station timer that keeps track of how much time has passed since a polling command has been received from the CS  12 . If the timer expires, the microcontroller  32  switches transmit and receive radio frequencies for that RS  14  to a third frequency for receiving and a fourth frequency for transmitting a reply for reasons to be discussed next. 
   Having identified which of the RS&#39;s  14  is lost, the CS  12  will then begin an operation to find a primary one of the RS&#39;s  14  that has the ability to communicate with the identified lost station. The step-by-step process carried out is illustrated in  FIG. 4  and TABLE 1. It begins by incrementing the lost station count from 0 to 1. On the next successive poll, the CS  12  transmits data to all primary RS&#39;s to identify the station that is lost and commands the first primary RS  14 -P in the poll sequence to retransmit a query to the lost station (LS)  14 -L, in effect becoming a relay station to the CS  12  for the LS  14 -L. That primary RS  14 -P then transmits a poll signal on an alternate frequency pair and waits for a response. If the LS  14 -L communicates on that alternate frequency pair, the primary RS  14 -P then stores the data and switches to the primary frequency pair until the CS  12  polls it again. At this point, the primary RS  14 -P reports on the primary frequency pair with its own data, as well as that of the LS  14 -L. With this response, the CS  12  notes the successful communications loop and designates that primary RS  14 -P as the relay station for all future communications with that particular lost station LS  14 -L until the CS  12  successfully establishes a direct communication path with the LS  14 -L. The polling routine then continues normally for all regularly reporting RS&#39;s except these two RS&#39;s until a refresh poll is initiated by the CS  12 . 
   As an extension to this scenario, if the LS  14 -L does not reply to the designated primary RS  14 -P, then the CS  12  will duly note this and go to the next primary RS in the poll sequence, requesting it to perform the same operations until one of the primary stations eventually acknowledges communications with the LS  14 -L. If none of the primary RS&#39;s achieve contact, then the LS  14 -L will be flagged as defective and dropped from the poll events until either an automated or manual reset is initiated. When activated, this reset will restart the communications routine and the CS  12  attempts to reestablish direct communications with each RS  14  in the system as a primary RS. 
   
     
       
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Step 
               Data Transmitted or Received 
             
             
                 
                 
             
           
           
             
                 
               1) CS Transmits: 
               Primary Remote Address 
             
             
                 
                 
               “1” for Lost Remotes Count 
             
             
                 
                 
               Primary Remote Commands 
             
             
                 
                 
               Its Own Address for Reply 
             
             
                 
                 
               Secondary Remote Address 
             
             
                 
                 
               Secondary Remote Commands 
             
             
                 
               2) PR Receives: 
               Its Own Address 
             
             
                 
                 
               “1” for Lost Remote Count 
             
             
                 
                 
               Its Own Commands 
             
             
                 
                 
               Central Station Address for Reply 
             
             
                 
                 
               Lost Remote Address 
             
             
                 
                 
               Lost Remote Commands 
             
             
                 
               3) PR Transmits: 
               Lost Remote Address 
             
             
                 
                 
               “0” Lost Remote Count 
             
             
                 
                 
               Lost Remote Commands 
             
             
                 
                 
               It&#39;s Own Address for Reply 
             
             
                 
               4) LR Receives: 
               Its Own Address 
             
             
                 
                 
               “0” Lost Remote Count 
             
             
                 
                 
               Its Own Commands 
             
             
                 
                 
               Primary Remote Address for Reply 
             
             
                 
               5) LR Transmits: 
               Primary Remote Address 
             
             
                 
                 
               “0” for Lost Remote Count 
             
             
                 
                 
               Its Own Address for ID 
             
             
                 
                 
               Its Own Data 
             
             
                 
               6) PR Receives: 
               Its Own Remote Address 
             
             
                 
                 
               “0” for Lost Remote Count 
             
             
                 
                 
               Lost Remote Address 
             
             
                 
                 
               Lost Remote Data 
             
             
                 
               7) PR Transmits: 
               Central Station Address 
             
             
                 
                 
               “1” Lost Remote Count 
             
             
                 
                 
               Its Own Address for ID 
             
             
                 
                 
               Its Own Data 
             
             
                 
                 
               Last Remote Address 
             
             
                 
                 
               Lost Remote Data 
             
             
                 
               8) CS Receives: 
               Its Own Address 
             
             
                 
                 
               “1” Lost Remote Count 
             
             
                 
                 
               Primary Remote Address 
             
             
                 
                 
               Primary Remote Data 
             
             
                 
                 
               Lost Remote Address 
             
             
                 
                 
               Lost Remote Data 
             
             
                 
                 
               Flags this Primary Remote to be Repeater for 
             
             
                 
                 
               this Lost Remote on Future Polls 
             
             
                 
                 
             
           
        
       
     
   
   To extend the distance capability, it can be possible to place a tertiary station (TS) at a distance far enough away from the CS  12  and the primary RS&#39;s  14  that it would have to rely on a secondary station as its only source of communications with the system. Such system layouts may be necessary, by design, rather than incidental due to malfunction. The operation sequences mimic that of a normal lost station routine but differ in that the level reaches one step further and utilizes another alternate frequency pair to transmit and receive. This procedure, which involves relaying information from the CS  12  through a primary RS  14 -P and a secondary RS  14 -S to a tertiary RS  14 -T and then back, is illustrated in  FIG. 5  and TABLE 2. The CS  12  begins such an operation by informing the primary RS  14 -P that there are 2 lost stations. The primary RS  14 -P tells the secondary RS  14 -S that there is 1 lost station, and the secondary RS  14 -S then acts like a primary RS by sending a zero-lost-station count and command string to the tertiary RS  14 -T. 
   
     
       
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Step 
               Data Transmitted or Received 
             
             
                 
             
           
           
             
               1) CS Transmits: 
               Primary Remote Address 
             
             
                 
               “2” for Lost Remotes Count 
             
             
                 
               Primary Remote Commands 
             
             
                 
               Central Station Address for Reply 
             
             
                 
               Secondary Remote Address 
             
             
                 
               Secondary Remote Commands 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Commands 
             
             
               2) PR Receives: 
               Its Address 
             
             
                 
               “2” for Lost Remote Count 
             
             
                 
               Its Own Commands 
             
             
                 
               Central Remote Address for Reply 
             
             
                 
               Secondary Remote Address 
             
             
                 
               Secondary Remote Commands 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Commands 
             
             
               3) PR Transmits: 
               Secondary Remote Address 
             
             
                 
               “1” for Lost Remote Count 
             
             
                 
               Secondary Remote Commands 
             
             
                 
               Its Own Address for Reply 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Commands 
             
             
               4) SR Receives: 
               Its Own Address 
             
             
                 
               “1” for Lost Remote Count 
             
             
                 
               Its Own Commands 
             
             
                 
               Primary Remote Address for Reply 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Address 
             
             
               5) SR Transmits: 
               Address for Tertiary Remote 
             
             
                 
               “0” for Lost Remote Count 
             
             
                 
               Commands for Tertiary Remote 
             
             
                 
               Its Own Address for Reply 
             
             
               6) TR Receives: 
               Its Own Address 
             
             
                 
               “0” For Lost Remote Count 
             
             
                 
               Its Own Commands 
             
             
                 
               Secondary Remote Address for Reply 
             
             
               7) TR Transmits: 
               Secondary Remote Address 
             
             
                 
               “0” For Lost Remote Count 
             
             
                 
               Its Own Address for ID 
             
             
                 
               Its Own Data 
             
             
               8) SR Receives: 
               Its Own Address 
             
             
                 
               “0” for Lost Remote Count 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Data 
             
             
               9) SR Transmits: 
               Primary Remote Address 
             
             
                 
               “1” Lost Remote Count 
             
             
                 
               Its Own Address for ID 
             
             
                 
               Its Own Data 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Data 
             
             
               10) PR Receives: 
               Its Own Address 
             
             
                 
               “1” for Lost Remote Count 
             
             
                 
               Secondary Remote Address 
             
             
                 
               Secondary Remote Data 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Data 
             
             
               11) PR Transmits: 
               Central Station Address 
             
             
                 
               “2” for Lost Remote Count 
             
             
                 
               Its Own Address for ID 
             
             
                 
               Its Own Data 
             
             
                 
               Secondary Remote Address 
             
             
                 
               Secondary Remote Data 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Data 
             
             
               12) CS Receives: 
               Its Own Address 
             
             
                 
               “2” for Lost Remote Count 
             
             
                 
               Primary Remote Address 
             
             
                 
               Primary Remote Data 
             
             
                 
               Secondary Remote Address 
             
             
                 
               Secondary Remote Data 
             
             
                 
               Tertiary Remote Address 
             
             
                 
               Tertiary Remote Data 
             
             
                 
               Flags this Primary Remote to be Repeater for 
             
             
                 
               these Secondary and Tertiary Remotes on Future Polls 
             
             
                 
             
           
        
       
     
   
   Once the determination is made that a particular primary RS can communicate with a secondary RS, the CS  12  assigns that primary RS as a repeater station for the specific secondary RS until a refresh poll is sent. Polling for a secondary RS starts after two missed poll attempts to that station as a primary RS (first skipping attempts through primary RS&#39;s that have already been assigned as repeaters). 
   If more than two tertiary RS&#39;s exist, the CS  12  will stack the addresses of the stations and rotate the order of the stations to be contacted to ensure each possible sequence is attempted. For example, assume CS  12  has two tertiary RS&#39;s that are not communicating. The CS  12  will direct the secondary RS to communicate with the first tertiary station first. If there is no response, then the CS  12  will direct the secondary RS to try to communicate with the second tertiary station. Ultimately, the desire is to minimize the RS&#39;s use as repeaters as much as possible. 
   It should be noted that the system  10  can also be configured such that both the CS  12  and each of the RS&#39;s  14  can communicate with one another using any of a plurality of transmit/receive frequency pairs to avoid problems with frequency jamming, etc. For example, if a primary one of the RS&#39;s  14  attempts but fails to communicate as a relay station with a lost station, the relay station can then switch to another pair of alternate transmit/receive frequencies and repeat this process for all frequency pairs until even the original pair of frequencies may once again be tried. Thus, if the lost station fails to receive communications on a first alternate frequency pair within a given period of time, it can sequence through each additional frequency pair until either a frequency pair is found on which communications are received or the lost station terminates further communication attempts. 
   As mentioned previously, the power management algorithm in the microcontroller  32  in the RS  14  is responsible to power up and down the other boards in the RS  14  and any other circuitry associated with it. An adjustable duty cycle in the power management algorithm provides the on and off periods as illustrated in  FIG. 6  such that the on periods adjust in length depending upon whether the RS  14  is receiving, transmitting or processing information, while the off periods remain relatively constant. As a result, the total cycle T 2  shown in  FIG. 6  is longer than the cycle T 1 . The duty cycle information is stored in the microcontroller memory and can be adjusted based on the required sampling or control rate. The CS  12  can also adjust the duty cycle information of the RS&#39;s  14  to allow for faster or slower communications (higher and lower power consumption, respectively). 
   During operation of the system  10 , when the CS  12  requires information, a command is sent to the RS&#39;s  14  continuously as illustrated in  FIG. 7 , until a response is received from the RS  14  or the command times out. The RS  14  will not receive the command while it is in low power mode, but will receive the command as soon as it powers up during the power on period of the cycle. Upon reception of the command, the power management algorithm instructs the microcontroller  32  to maintain power to the RS  14 &#39;s components while it executes the received command. Once the command is executed and the response is sent back to the CS  12 , the power management algorithm causes the microcontroller  32  to power down the RS  14  until the next on cycle. 
   It should be noted that other auxiliary boards in the RS  14  could be powered up and down depending on the type of request sent by the CS  12 . Also, the duty cycle can be adjusted to achieve fast response times and reliable communication. Further, the power management parameters can be changed in real time to decrease response time and maximize communication bandwidth whenever needed. 
   Although the invention has been disclosed in terms of a preferred embodiment and variations thereon, it will be understood that numerous additional variations and modifications could be made thereto without departing from the scope of the invention as set forth in the following claims.