MXPA97009560A - Two-way communications system that provides a data communication service inalambri - Google Patents

Abstract

The present invention relates to a two-way wireless data communication system, comprising: an output message subsystem, an input message subsystem consisting of at least one radio frequency base station for receiving input messages; a frequency analysis processor (FAP) associated with each of the at least one radio frequency base station, each of the FAPs continually track, in their respective locations, the power levels received through a frequency group in a radio frequency input band, to determine an observed level of frequency availability for the frequency group of each base station, a propagation analysis processor (PAP) that determines a propagation probability for the frequency group; minus a remote field unit having an output message receiver and a tunable input message transmitter, and a remote control unit ol central, where the central control unit uses the output message subsystem to send an output message to the remote field unit, and uses the input message subsystem as an input link to receive an input message from the field unit, and where the output message includes data fields indicating an input time and an input carrier frequency in which the field unit must send each input message, and where the central control unit receives reports of the observed levels of frequency availability from the FAPs, and the central control unit also receives reports of the probabilities of the frequencies propagating from the PAP, and where the central control unit selects the input carrier frequency depending both on the levels of frequency availability according to, are reported by the FAPs, as the probabilities of the frequency that propagates as reported by the PAP, of m all the probability that the input message will be received by at least one of the base stations without interference from another communication system is maximized

Description

"TWO-WAY COMMUNICATIONS SYSTEM THAT PROVIDES A WIRELESS DATA COMMUNICATION SERVTHTO" This invention relates generally to radio communication systems, and in particular, to a low-cost wide-area data communication system, which provides reliable long-distance communication using a high-base station radio network. frequency (HF) that determine in advance which frequencies can be expected to be propagating and free. BACKGROUND OF THE INVENTION There is a vital and continuing need for wireless communication networks of various types. Certain wireless systems are focused on the need for reliable two-way data communications. Such networks do not need to support particularly high data exchange rates, but should provide communication over as wide a geographical area as possible, such as the continental United States or Europe. Unfortunately, many existing systems and still safe, costing many millions of dollars, have failures of one kind or another. Consider, for example, wireless wide area data networks that support communication between a mobile field unit and a base station. These networks either use terrestrial base stations or installed on satellite. Terrestrial systems can also be classified as one-way or two-way. One-way terrestrial systems, such as national-scale message networks such as Sky Tel, do not provide the ability for a remote user to send data. Although certain types of message networks do support two-way data transfer, they provide only limited geographic coverage. Additionally, such networks typically exhibit poor penetration of building structures due to the high carrier frequencies in which they operate. Other existing and proposed two-way terrestrial systems include cellular networks, mobile data networks such as RAM, ARDIS, emerging PCB networks EMBARC, and many others. While the data rates of these systems are typically quite high, each system requires users to be within a short distance, usually 20 miles or less, of the system's base station infrastructure. This infrastructure is extremely expensive, requiring hundreds of millions of dollars to build a network on a national scale. It can sometimes be cost effective to build such infrastructure in areas of high population density, and indeed, approximately 90% of the population of the United States can be supported by such systems, however, this land infrastructure only covers approximately 15-20 % of the country, geographically. It is simply not economical for providers of such services to install the required infrastructure in remote areas of low population density. Several satellite networks, both existing and proposed, have been designed to address the problem of poor geographic coverage. These satellite-based systems typically require a tremendous investment in infrastructure. The infrastructure is located in orbit where it can not be installed, maintained or replaced without large space launch vehicle costs. Additionally, mobile subscribers or field devices required to communicate with such satellite systems are relatively expensive. In addition, field devices need to be within the line of sight of the satellite, since they typically have high-range, visible electromagnetic receiving devices, such as dishes or long antennas. Such systems are therefore impractical for certain applications.

Consider the problem set faced by the manager of a fleet of rental cars. The assets for which the manager is responsible are highly mobile; in truth, they can be located virtually anywhere in the continental United States. The assets are easily stolen and thus expensive to insure. Assets can also become unproductive when a rental customer fails to return the vehicle to its proper location. Rental cars can "get lost" when there is poor communication between rental points and valuable time of the rental asset is then wasted. Another important point for fleet managers is the safety of their customers. Car rental drivers, and in fact, all drivers, could benefit from a system from which it would be possible to request emergency assistance at any time, from any location, without leaving the vehicle. Similar problems exist in other industries. For example, there is increasing pressure in the rail industry to move toward scheduled service, to facilitate "just in time" delivery in an effort to better compete with the trucking industry. To achieve this goal, the manager of a railroad system should ideally be able to quickly determine the location of each and every rail car on a regular basis, regardless of the location of the rail car. Then the optimal route and delivery time could then be accurately predicted. In both such fleet management applications, the fleet manager would very much like to be able to consult a remote device to determine its location, but at minimal cost. Existing systems do not fill this need. For example, the current mobile cellular telephone service carries with it a relatively high charge for connection time, travel charges, and monthly service fees, and fleet managers do not consider such systems as cost-effective. Other industries, such as the trucking and shipping industries could also benefit from cheap capacity and exactly find the location of shipping containers no matter where they are located. Any shipping container can contain thousands or potentially millions of dollars of valuable goods, and clearly, those responsible for the welfare of goods in transit would like to know where they are at all times.

Similar demands are made on remote meters, detector readings, facility monitoring, security, buoy monitoring, and other applications. One way to provide low cost communications service over long distances is to use shortwave radio links operating in the high frequency radio (HF) radio that is in the approximate range of 3 to 30 Mega Hertz (MHz). Radios that operate in this band have been in use for many years, and the required transmitter-receiver equipment is cheap to maintain and operate. The signals transmitted in HF frequencies can be carried for hundreds and even thousands of miles. However, there are certain well-known difficulties that make HF radio transmission unreliable. A first problem is rooted in the fact that HF provides long distance communication, over the horizon, bouncing the signal out of the Earth's ionosphere. Due to multiple atmospheric conditions, a set of phenomena that changes depending on location, time of day, time of year, and sunspot activity levels, different portions of the 3-30 MHz spectrum may propagate or may not propagate in different directions at any given time of the day. Thus, to provide reliable communication, the radio transmitter must make some accommodation for the fact that a selected carrier frequency may or may not be propagating between itself and the receiver. Second, of those frequencies that are propagating, the transmitter and receiver must also know which frequencies are free, that is, which frequencies are not in use by other equipment operating in the same band. This problem is not as easy to solve as it may seem. Although certain frequencies in the HF spectrum are dedicated in advance to certain known users, many other frequencies in the HF band remain available for on-demand use. Thus it can not be predicted with certainty when these frequencies will be or will not be occupied at any instant in time.

Traditionally, HF communication systems have relied on trial and error to find a frequency that is both propagating and clean. These systems thus only provide minimum reliability in terms of establishing a link from transmitter to receiver exactly when that link is desired. More advanced systems improve reliability using "sounding" techniques in conjunction with automatic link establishment algorithms (ALE). In these systems, the base station transmits on multiple frequencies, and the remote receivers listen to multiple frequencies on them. When the remote listens to the base station, it knows that the frequency that is heard was propagating. The remote then transmits on that frequency as soon as the base completes its transmission, before the frequency can be occupied by another user. Unfortunately, even ALE-like systems have several drawbacks. First, they are spectrally inefficient, since base stations must transmit on several frequencies. Second, remote units are more expensive than what would otherwise be required, because they need to contain agile frequency HF receivers as well as an HF transmitter. The capacity of the system, in terms of how many remote units can be supported, is limited due to the need to transmit on multiple frequencies at the same time. Finally, because a single HF base station coordinates the use of outbound links, the geographic coverage of such a system is limited to that which can be provided by a single base station and the reliability is minimized if that base station is not a region that is conducive to propagation.

DESCRIPTION OF THE INVENTION It is an object of this invention to provide a long-distance data communication system in which relatively small amounts of data can be withdrawn from highly mobile or very remote sources only on an infrequent basis, but in time close to real, at minimum cost. Another object is to provide highly reliable communication while minimizing interference with other communication systems that may be operating in the same band. The system should provide two-way communication, that is, it should be possible to communicate from a base station to a remote field unit, as well as from the field unit to the base station. Still another object of the invention is to provide omnipresent coverage over a wide geographic area, such as the continental United States, although requiring a minimum of capital expenditures for new infrastructure. Additionally, such a system should make use of simple and inexpensive field devices costing much less than, for example, a comparable satellite data terminal.

Field units should not require a direct line of sight with a base station unit for communication to be successful and reliable.

Field units should also be able to operate with battery power, eliminating external power supplies, as typically required for satellite based systems. Finally, the use of the system should cost the client much less than existing paging, cellular and satellite systems. Briefly, the invention is a communication system that provides omnipresent wireless data communication services, such as through the continental United States, using a network of only a few widely distributed radio base station locations. The radio base stations receive data from remote mobile field units using well-proven, long-range radio technology such as the one operating on short-wave carrier frequencies, including, for example, the high frequency radio (HF) band. A core unit of the network or mission operations center (MOC) controls the sites of base radio stations and field units from a central location. The MOC receives information from a Propagation Analysis Processor (PAP) that maintains a database of probabilities that a signal at a given frequency will propagate between each of a number of radio base station sites and each of many remote locations possible. The MOC also receives information from a Frequency Analysis Processor (FAP) associated with each radio base station indicating the HF frequencies that appear to be clean and thus available for pulse transmission. When a client uses a call station to request access to a remote field unit, the MOC first determines an available HF frequency and time gap for a particular field unit to transmit. This determination is made both from the propagation probability data reported by the PAP as well as from the free frequency data reported by the FAP. The MOC then issues an exit message to the field unit, requesting the field unit to report back any information it may have. The exit request message may be transmitted to the remote field unit using any inexpensive wireless infrastructure, such as the existing one-page page network infrastructure. The output message can also be communicated by other types of subsystems, such as cellular, satellite, or other means of radio transmission.

When the field unit receives the output message, it collects data to formulate an internal response message, such as reading data from its associated geographic location receiver, or reading other data that is available to it. The field unit then sends its response as an internal message back to the radio base stations at the indicated HF carrier frequency and time, in the form of a short duration pulse message. In a preferred embodiment, the internal message may be encoded for broadcast with a scheme such as spread spectrum modulation, to minimize the likelihood of interfering with other communications at nearby frequencies.

The system of the present invention thus consists of several different subsystems, including the call stations, the mission operation center (MOC) including the Propagation Analysis Processor (PAP), the output signaling network, the remote units of field, and the internal radio base station network including the Frequency Analysis Processors (FAP's). The call stations provide an interface for the system's customers. They include a platform such as a personal computer and modem, to accept a request from a client to communicate with a particular field unit by reporting the request to the MOC, receiving the report from the field unit from the MOC and then displaying the report. return to the client. The call stations are connected to the MOC through any convenient system, such as by modem connected to the public connection telephone network (PSTN). The MOC, which is also a computer, performs a number of tasks. Accepts requests for communications with the field units from the calling stations and sends the response of the field units back to the calling stations. The MOC also provides a central point of selection for the frequencies to be used by the internal message link. This is done by periodically communicating with the PAP, to increase a table of available frequencies with propagation probabilities for each radio base station from each of many possible remote locations.

The MOC also receives reports of internal link radio frequencies available from the FAPs and maintains a database of such frequencies and time gaps for which they are available. The MOC communicates with radio base stations via modems using appropriate low-cost land-based connections such as PSTN, leased or private telephone circuits, Very Small Aperture (VSAT) wireless terminal networks, or other cost-effective connections . In operation, upon receipt of a client request from a calling station, the MOC selects a frequency from the PAP database that has a maximum propagation probability to all radio base stations. The MOC then determines if that frequency was also reported as being a clean frequency by the FAPs. In other words, the clean frequency having the best probability of aggregate propagation for all stations is selected. The MOC then selects a time available from its database, and formulates an exit request message with the frequency and time selected as data arguments. The MOC then quickly distributes the exit request message to the remote field units in as short a time as possible, since the data is highly perishable. That is, the selected free channel can quickly be occupied, within a few seconds of its first identification. The output message is then sent to the output signaling link, with a request that the output message be sent to the field unit. This request to the output signaling link is typically sent via any convenient ground-based means, such as the PSTN, VSAT, or other type of data communication network. In a first mode, the outgoing message may be sent to the field unit by a target signaling link such as a paging center that is capable of alerting a particular remote unit. In a second mode, a list of frequency and time gap pairs can be transmitted to any number of remote units in the system. In such mode, any remote unit wishing to complete a call listens to the transmission of the outgoing message and then randomly selects, from among the various alternatives, a frequency in which to attempt to complete the internal call. In any mode, the MOC then alerts one or more of the associated base stations to wait for a response from the indicated field unit, at the specified frequency and time. Upon receipt of an internal message from one or more of the base stations, the MOC then sends the information in the message back to the calling station.

The MOC reports successful transmission to PAP. In the event that the internal message was not received in the expected time and frequency, the failure to communicate is reported to the PAP. A different frequency and time are then selected by the MOC, and another attempt is made to communicate with the field unit. Radio base stations perform several functions. First, to assist in determining the MOC of which frequencies are not occupied, each radio base station includes a Frequency Analysis Processor (FAP) that periodically verifies each internal link channel to determine if the channel is in use. This can be done, for example, by using an available radio receiver and continuously sweeping the HF band, by measuring a received power level in each channel, such as in each 3 KHz bandwidth. An estimate of the received power level can also be made by sampling subbands in each channel and integrating the power level of the signal detected in the subbands over time. In any case, the FAP identifies clean channels available. This can be done by comparing the power level in each channel with a threshold level of background noise and in other ways. Regardless of the technique used to identify clean channels, the FAP then periodically outputs this list of clean channels to the MOC so that the MOC can maintain its own frequency selection table. Additionally, the FAP can remove from the list of clean frequencies any frequency known to be prededicated for specific uses by regulatory agencies such as the FCC, which should be avoided. For example, certain well-known sound stations in the HF band, such as transmitting stations such as V, are removed from the list. In order to receive internal messages from the field units, the radio base stations also include a battery of tunable HF receivers and modems. When accepting an order from the MOC to wait for an internal message from a particular field unit, at a particular frequency and time, each base station then designates an HF receiver and modem from the battery, waits for the internal message to be received and then formulates a report back to the MOC.

For example, if an internal message is successfully received from the field unit, the data from the internal message is reported back to the MOC as the response message. If, however, no internal message is received at the indicated time and frequency, a failure of the internal link is reported back to the MOC. The Propagation Analysis Processor (PAP) is typically located at a central site, such as the same MOC site. The PAP estimates the probability by which an input signal will be correctly received at each radio base station, preferably using both ionospheric prediction analytical models and using real-time inputs that correspond to the observed performance of the system. The model for location of each radio base station is a database, or table, of time of day versus frequency with an expected spread being determined by a signal transmitted from a number of remote locations through the expected service area to each of the radio base stations. The propagation model can be created initially using a known ionospheric model program such as the IONCAP program developed by Link Corporation of Binghampton, New York. This model program, when given a remote location and a base station location, a predicted solar activity estimate, time of day, and antenna conformation, can mathematically predict which frequencies will propagate, that is, the model provides a probability of actually receiving a signal from the remote location at the location of the base station. As real data is received concerning the successes or failures of specific field units, at particular locations of the radio base station, the propagation model is then updated. The. Updates can be made, for example, using a weighted average of the old propagation data and the new propagation data observed. Periodically, the propagation model can be readjusted by running the ionospheric model calculations, such as on a daily basis. The propagation model can also be updated using data from known transmitters at known frequencies, such as the WWV transmitter in Ft, Collins, Colorado, as well as using polling receivers in each base station along with known sound transmitters or known field units .

Each of the field units receives an output message signal containing data representing an identification tag specific to that field unit, and a frequency and a time at which the field unit must originate the internal message. Other data may also be included in the internal message, as dictated by the particular application for the system. Upon receiving such an internal signal, the field units gather data to be reported back to the MOC, such as latitude and longitude from a geo-location system, or input data from other detectors or equipment connected to the field unit . The field unit then generates the internal message from these inputs and transmits the internal message at the frequency and time specified through the HF link to the base stations. A communication system according to the invention provides several advantages. The system is highly reliable. Determine in advance, scrutinizing each of the base stations in the internal message network in the clean transmission frequency having a high probability of propagating to each of the base stations. The system allows relatively small amounts of data to be withdrawn from highly mobile or very remote sources in real time, at minimum cost. By not setting frequency locations early, for the system, the system can adapt dynamically as changes in ionospheric conditions and system usage demands put into effect which frequencies are available and most likely will result in a successful transmission. Omnipresent coverage over a wide geographic area such as the continental United States is possible, such as when paging systems are used for outgoing messages.

The use of non-HF networks such as paging networks for the outgoing network not only eliminates the need for a receiver HF agile and complex frequency in the field unit site, but also dramatically increases the number of field units that can be supported. The system requires a minimum of capital investment for new infrastructure when such existing systems and networks are used. The system is spectrally efficient, since it does not require high-power sound transmitters. It is also as non-intrusive as possible, since it only uses frequencies that appear not to be in use in other systems at any given time. Additionally, by sending only short messages, in the order of a few seconds or something, it is ensured that even if a frequency that is in use by another system is inadvertently selected, such interference is minimized. Field units can operate with battery power as well as conventional paging receivers, since the transmission unit is activated only intermittently, and even then, only by an internal message pulse of short duration. BRIEF DESCRIPTION OF THE DRAWINGS The above and further advantages of the invention can be better understood by referring to the following description in conjunction with the accompanying drawings in which: Figure 1 is a block diagram of a radio communication network of two. routes according to the invention; Figure 2A illustrates the format of a message sent by a system client from a call station to a mission operations control unit (MOC); Figure 2B illustrates the format of an output message sent from the MOC to a field unit using a paging network; Figure 2C illustrates the format of an internal response message sent from the field unit to a remote base station (RBS) at particular times and frequencies within a shortwave radio band, and as sent from the RBS to the MOC; Figure 2D illustrates the format of the internal message as sent from the MOC back to the calling station; Figure 3 is a block diagram of an MOC control unit showing several databases, or tables, maintained by the MOC; Figure 4 is a flow chart of the operations executed by the MOC upon receipt of a client message requesting data from a particular field unit, and the operations performed to generate the outbound paging message; Figure 5 is a block diagram of an RBS control unit showing several databases it maintains; Figure 6 is a flow chart of the operations performed by a Frequency Analysis Processor (FAP) portion of the RBS to periodically update a database of available frequencies; Figure 7 is a flow chart of the operations executed by the RBS to verify reception of the internal message; Figure 8 is a block diagram of a Propagation Analysis Processor (PAP) showing a database of propagation probabilities that it maintains; Figure 9 is a flow chart of the operations executed by the PAP to maintain the database of probabilities of propagation; and Figure 10 is a map of the continental United States showing a grid indicating possible latitudes and longitudes used in the model maintained by the propagation analysis processor of Figure 8. DETAILED DESCRIPTION OF A PREFERRED MODALITY Turning now to the drawings, the Figure 1 illustrates a block diagram of a two-way wireless communications system 10 according to the invention. The system 10 includes a number, s, of call stations 20a, 20b, ... 20s (collectively, call stations 20), a data communication mechanism 25, a mission operations control center 30 (MOC) , a number P, of geographically dispersed exit message subsystems 40a, 40b ... 40p covering a wide geographic area, such as the continental United States or Europe, multiple mobile or remote field units 50a, 50b, 50c, 50d, 50x, 50y, 50z, and an internal message subsystem that it can make use of a number, such as approximately four, of radio base stations (RBS) 60a, 60b, 60c and 60d, which are also geographically dispersed to provide omnipresent coverage. Also according to the invention, the system 10 makes use of a Propagation Analysis Processor (PAP) 70, which maintains an estimate of the probability of a successful transmission on each frequency for each base station from each of the many of the many possible remote locations, as well as a set of Frequency Analysis Processors (FAP's) 65, which continuously determine which radio frequencies are free at each base station 60. The MOC 30, making use of data maintained by the PAP 70 and the FAP's 65, determines, on a "per-call" basis, a frequency a be used by field units 50 when communicating with radio base stations 60. This process will be discussed in detail below, but it is helpful to first understand the various components of system 10 and how they interact.

More particularly now, a client of the system 10 initiates a request for communication with a particular field unit 50 using the call station 20a. The request is then sent to MOC 30 through the data communication network 25. The data communication network 25 may preferably be a public telephone connection network (PSTN), as shown. However, private networks, Very Small Aperture Terminal (VSAT) networks, and other types of communication networks can be used. The MOC 30, in turn, sends the request to one or more of the output message systems 40. The output message systems 40 provide radio links 45a, 45b, ... 45z which are used for communication from the system 10 to the remote field units 50. These radio links are collectively referred to as the output links 45. In a preferred embodiment, the output links 45 can be provided by multiple paging subsystems 40. However, other systems such as such as nationwide paging systems, satellite networks, private radio networks and the like can be used to provide the outbound links 45. Thus, although the outbound message system 40 may in some cases be referred to here as a paging system, It should be understood that other types of outbound radio links can also be used. Radio communication from the field units 50 back to the system 10, which are implemented using the RBS's network 60, are referred to as the links as the internal links 55a, 55b, .... 55z. The internal links 55 may use shortwave radio links, encoded in extended spectrum, operating in the high frequency radio (HF) band. It is the purpose of PAP 70, as well as that of FAPs 65a, 65b, 65c and 65d to assist MOC 30 in determining radio carrier frequencies, or channels, to be used to establish internal links 55. In particular, the MOC determines a frequency and time for the field unit 50 to use it, which is reported by the PAP 70 as having the highest probability of successful propagation between the last known location for the field unit 50 to each of the RBSs 60. Additionally , the MOC 30 ensures that the selected frequency was reported as being free by at least some predetermined number of the base stations 60. To coordinate use of the internal links 55, the output message in the 45a output links consists of data indicating the frequency thus determined by the MOC 30 and a time at which a remote particular field unit such as unit 50a can signal to the radio base stations 60 with their information. At the indicated time and frequency, a message is received from the field unit 50a by one or more of the base stations 60, and the message is then sent to the MOC 30. The MOC 30 in turn, then supplies the requested data to the client at the calling station 20a through the network 25. As a result, a wireless communication system 10 according to the invention allows reliable communication on a near real-time basis across a broad geographical area such as be conveniently covered by a network of only a few short wave radio base stations 60. Because of which system 10 determines in advance a frequency that is capable of propagation and is presently free, reliability even in noisy media such as HF is possible. The architecture of the system also eliminates the need for expensive infrastructure, expensive for the client, and omnipresent as it is now required by cellular systems and other terrestrial systems as well as satellite systems. For example, existing paging subsystems may be used to provide the outbound links 45, and the network 25 may be the public telephone connection network. Internal links 55 are provided by well proven HF radio technology. The system is thus much more maintainable and less expensive to support than competing cellular, multi-emission, or satellite systems. Additionally, the radio systems in the remote field units 50 operate only when a client initiates a request for data from a call station 20. Thus, not only interference with other systems is minimized, but also the field units 50 can be designed to operate with minimum support power, as available, from battery or solar power, which is ideal in remote locations for which access to external power is not readily available, reliable or safe. Because the radio base stations use HF shortwave signaling, a direct line of sight between the base stations 60 and the remote field units 50 is not required, and thus the system 10 will operate properly even in situations where line propagation Direct vision is not possible, such as in stacked containers or in densely populated urban areas. System 10 thus exhibits greater availability and applicability than competing satellite systems. Before proceeding to a discussion of the PAP 70 in particular, each of these components of the communication system 10 will now be discussed at a higher level of technical detail so that one skilled in the art can more easily understand how to build and operate the system. MESSAGE FORMATS (Figures 2A to 2D) The format of a message 200 sent from the call station 20 to the MOC 30 is shown in Figure 2A. At a minimum, the message 200 includes at least one field data 200-1 indicating an identification code (ID) for the field unit 50a from which the client is requesting data. Additionally, however, other data to be sent to the field unit 50a from the calling station 20a may be contained in one or more output data fields 200-2. Figure 2B shows the format 245 of an output message sent through the output links 45 to the field units 50. The message 245 consists of the ID of a coded field unit 245-1 and output data 245- 4 as originated by the calling station. Additionally, message 245 includes a frequency field 245-2 and a time field 245-3 indicating a transmission frequency and a time of day at which remote unit 50a should signal to RBS 60. Figure 2C shows the format of the internal message 255 returned by the field unit 50a through the internal links 55, including an ID of the field unit, field 255-1, as well as data field 255-2, containing data being returned from the unit 50th field Such internal data 255-2 can, for example, in the case of a mobile field unit 50, include information concerning the position of the field unit in the form of latitude and longitude. However, it should be understood that the field unit 50 may be stationary and / or that other internal data types 255-2 may be sent, depending on the client's application. Finally, Figure 2D illustrates the message format 270 sent by the MOC as a response to the call station 20a. Message 270 includes the ID of field unit 270-1 if necessary, as well as internal data 270-2 returned. It should be understood that the illustration of the message formats in Figures 2A to 2D is not restrictive, and that the various fields 200-1, 200-2, 245-1, 255-1, 270-1, 270-2 they can occur in any order in each respective message. Each message 200, 245, 255, and 270, may also typically have additional fields such as header fields, check of sums, itinerary or synchronization information and other fields as normally required in any message communication system. CALL STATIONS 20 As mentioned above, the call stations 20a, 20b, .... 20s provide an rface for the clients to ract with the system 10. A typical unit of the stations 20a is materialized as a personal computer (PC) 21 having a well-known normal communication apparatus, such as a computer modem 20-2 for exchanging messages with the MOC 30 through the PSTN 25. The MOC also has a bank of computer modems 31-1, 31-2 ... 31-m for communication with multiple calling stations 20. The message requesting communication with a particular field unit 50a thus typically travels from the calling station 20a to the MOC 30 via temporary connection via the PSTN 25. MISSION OPERATION (MOC) 30 The MOC 30 also includes a computer, referred to as the 32 controller of the mission operation center (MOC), and multiple modems 31-1, 31-2, ... 31-m, 33- 1, 33-2, ... 33-4. The MOC uses the modems 31 for communications through the network 25 with at least the call stations 20 and paging centers 40. The MOC may also preferably use other modems 33-1, 33-2, 33-3 and 33- 4 to communicate with the base radio stations (RBSs) 60. However, because the MOC needs to communicate frequently with the RBSs, and since there are only a handful such as 4RBSs, the MOC can also use modems 33 that are connected to dedicated telephone circuits, such as leased lines, connected packet networks, or other cost-effective services, high long-line data rate. As mentioned briefly above, upon receipt of the client request message 200 from one of the modems 31, the MOC controller 32, determines a free and propagating frequency using data from the PAP 70 and the FAPs 65, and then send an exit message 45 containing data indicating that frequency as well as time for the field unit to use it for its response. The MOC controller 32 then removes the internal data from the RBSs 60 and sends the internal data to the indicated call station 20a. Figure 3 shows a more detailed block diagram of the MOC controller 32 and the various databases 32-5, 32-6, 32-7 and 32-8 that it maintains to complete these tasks. The controller 32 of the MOC includes the usual components of a computer system such as a central processing unit (CPU) 32-1, memory 32-2, storage disk 32-3, interface 32-4 input / output (I / O). The modems 31,33 communicate with the MOC via the 32-4 I / O interface. Because the MOC controller 32 is primarily responsible for coordinating communication between a number of different devices. The architecture of the computer system is selected to be an appropriate system operated switch or multitasking. To determine the frequencies to be used by field units 50, the MOC maintains a first database referring to a frequency table 32-5. This table includes a number of entries, n. An exemplary entry in this table consists of a frequency, fa, and a set of four power amplitudes Aa, l, Aa, 2, Aa, 3, Aa, 4 associated with each of the four radio base stations 60a, 60b , 60c, and 60d. An entry in Table 32-5 is made for each of a set of frequencies in the HF spectrum. These frequencies are taken from the set of free frequencies reported to the MOC controller 32 by the FAPs 65 as being free. Since free and propagating frequencies are used for a single communication, the entries in Table 32-5 change dynamically. The precise manner in which each FAP 65 determines an available frequency is discussed in detail below. Suffice it to say here that a given FAP, such as FAP 65a associated with RBS 60a (Fig.l), periodically reports a list fl, f2, ... fn of available frequencies, or open channels, which the RBS 60a is currently viewing , and a level A of amplitude of noise associated with each such frequency. Similarly, the other RBSs, 60b, 60c and 60d also periodically report their respective lists of frequencies and amplitude levels. As described below, the MOC controller 32 also uses probability propagation factors from a model maintained by the PAP 70 which makes use of known algorithms of the ionospheric model in conjunction with current system experience data in the process of selecting frequencies. A sub-game of the data maintained by the PAP 70 can be stored in a second table 32.6 in the MOC 32-2 memory. Each entry in Table 32-6 consists of a radio base station (RBS) location, a range of propagating frequencies, for example, as specified by a usable lower frequency (LUF) and a usable maximum frequency (MUF). or the some other way, a remote location in latitude (LAT) and longitude (LONG), and a propagation factor, P. A third table 32-7 is preferably used to keep track of the last known location of each unit of field 50 displayed. Each entry in this table 32-7 consists of a field unit ID code, and position information as lately reported by the field unit such as a latitude (LAT) and longitude (LONG). MOC controller 32 maintains and updates this table 32-7 as field units 50 are trained or removed from service and as internal messages are returned by each field unit indicating its latitude and longitude. As mentioned above, in the preferred embodiment, the output links 45 are provided by several paging subsystems 40. A fourth table 32-8 is used thus for location data of paging subsystem. Each entry in this table contains a range of latitudes and lengths covered by the paging subsystem as well as an identification code for each paging subsystem 40 associated with system 10. This table 32-8 is updated each time that arrangements for use are made. of several paging systems are made by the system operator 10. Table 32-8 can also include details of how the MOC controller 32 can access each paging system., such as modem telephone numbers, protocol types, and the like. It should be understood that table 32-8 is not needed if a nationwide multi-issue paging network is used to implement the system 10; however, if the system 10 keeps track of the location of the field unit 50 and does use conventional paging systems 40, it can offer its service at a low cost. A final table 32-9 is a pending message table. The entries in this table include data concerning each message in transit to one of the 50 field units, such as the ID of a field unit, the assigned time, t, and frequency, f, in which a response is expected, and other data that may be necessary to avoid conflicting access assignments to available channels.

Figure 4 is a flow chart of the operations executed by the MOC controller 32. Upon receipt of a request from a client to communicate with a remote unit 50 in step 401, the MOC first proceeds to determine an HF frequency to be used for the internal link 55. In step 402, a last known latitude and longitude they are determined for the field unit 50a indicated by the request message from the calling station 20a. This latitude and longitude can be determined using the ID of field unit 200-1 that was part of the customer request message, and executing a table look in table 32-7 of unit location. Next, in step 403, a set of likely frequencies to propagate from the indicated latitude and longitude are determined. This is done by executing another table look in table 32-6 of the subgame of propagation, to determine a probability of propagation, p, each of the RBSs 60 from a latitude and longitude that is the closest to the expected latitude and longitude. of the 50th field unit of interest. The frequency with the highest expected overall probability of success is then selected, in step 404, by comparing a sum of probabilities for each RBS 60.

It should be understood that more sophisticated techniques can be used, such as by computing a weighted sum of probabilities, P. In particular, if the MOC controller 32 maintains table 32-7 of the last known locations of the field unit, it can thus determine which RBS is most likely to receive the message from field unit 50, assuming that the field unit has not moved too far from its last known location. The MOC controller 32 can thus weigh the probability associated with the most probable RBS more heavily than the probability associated with the other RBSs. Next, at step 405, the MOC controller 32 selects one of the HF link frequencies that have been reported as being free on some or all of the radio base stations by the FAPs 65. This determination is made by comparing information in the table frequency 32-5. Other techniques can be used to refine the frequency selection process. For example, the MOC controller 32 can permanently exclude from the frequency selection process portions designated as necessary from the radio spectrum HF known to contain known fixed or interfered transmitters.

If a free frequency can not be found, the control returns to step 404 to select the frequency having the next most likely probability to propagate.

In step 406, the selected frequency is then removed from table 32-5 of available frequency. The system 10 is so designed that the MOC controller 32 uses the selected frequency within several seconds, and then abandons it, to avoid interference with other users of the HF spectrum. Minimizing the time between observing a free frequency and then selecting it for transmission is also key to successful communication, and that is why the FAPs are requested to report back to the MOC, to enable continuous updates of the available frequency table 32-5 in real time. When designing the system 10, a computational model was made of the probability of occurrence of conflicting use in the European environment. The European environment is typically more demanding than the average environment in the United States. The table below shows the probability that a frequency will be used by another conflicting user after the system has identified it as clean and before the transmission occurs.

Time elapsed from Probability of interference frequency selection 3 seconds 0.01 10 seconds 0.03 30 seconds 0.10 1 minute 0.15 6 minutes 0.63 The table above can thus be used to determine how often is the frequency availability table 32-5 it should be updated, depending on a probability desired interference with the internal link.

In any case, in step 407, table 32-9 of pending message is consulted to determine a time free, t, for the frequency, f, selected.

Once the time is selected, a new one is made entry in the pending message table 32-9 for the message output stream 245, in step 408.

Next, in step 409, the RBS's 60 are alerted to wait for an internal message 255 in the frequency, f, and time, t, determined.

In step 410, the MOC sends the paging message of exit 245 to the appropriate paging center. In particular, knowing the last latitude and longitude for the unit searched field 50a from field location location table 32-6, the identity of the paging system closest to the last known location of field unit 50a is determined, looking for the entries in table 32-8 of location of paging system. The outbound paging message 245 containing the indicated frequency, f, and time, t, is then sent to the nearest supposed paging system, requesting that the remote unit 50a be "voiced". This request to the paging system 40a is then sent through the network 25 (Figure i). In step 411, the MOC controller 32 then waits for a response from the field unit 50a to be reported by the RBS's 60 shortly after the indicated time t. Of course, since the controller 32 is a managed switch or multi-tasking, the controller 32 can actually perform many other tasks, such as serving service requests from other customer call stations 20, while waiting for the response from the unit 50a. In the event that the output message 245 extracts an appropriate response, in step 412 the internal data from the unit 50a is then reported to the calling station 20a in the form of the response message 270.

The fact of a successful message is also reported back to PAP 70, in step 413, so that PAP 70 can update its propagation probability model. In step 414, the corresponding entry in the pending message table is also removed. In the event that the exit message 245 does not extract the expected response from the field unit 50a, the MOC controller 32 assumes that the attempt to communicate the message has failed. In step 416, controller 32 reports this propagation failure back to PAP 70, which in turn updates its model. The pending message table is then updated by removing the corresponding entry, in step 416, and the MOC controller 32 then returns to step 404, to try to send the output message one more time. OUTPUT RADIO LINKS 45 AND INTERNAL RADIO LINKS 55 Returning briefly to Figure 1, all outbound links 45 preferably use licensed FCC communication means, such as the existing paging network infrastructure 40. However, such outgoing links 45 can also be provided by public or private carriers such as frequency modulated sub-carrier (FM) paging network systems, high frequency radio (HF) networks, or other suitable types of links. output, depending on the nature of field units 50. For example, if field units 50 are expected to be located in stacked containers, output links 45 should not be implemented using a communication methodology that requires line of sight. However, if the units in question are, for example, deployed in a remote registration application, communication by line of sight may be adequate. The internal links 55 make use of a network of radio stations 60 of high frequency (HF) making use of a technology that operates with carrier frequencies in the radio spectrum from 3 to 30 MHz. As mentioned above, to establish reliable communication in the internal HF links from the field units 50 to the RBS network 60, the remote field units 50 are instructed as to which frequency to use in the HF band. Additionally, as soon as the frequency is used for a short message it is then abandoned by the field unit 50. PAGE NETWORK 40 An exemplary output message subsystem may be a paging system 40a which is a normal paging system which can accept a request for a paging over the network 25. As is known in the art, such paging systems 40a include a modem 41 for accepting paging requests, a paging control center 42 which is typically a computer of some type, and a number of paging system transmitters 43, 43-1, ... 43-n. Given a paging request that includes a paging and message field unit ID, the paging subsystem 40a formats and then sends the outgoing paging message 245 in the conventional manner. The paging system 40a need not be a two-way system nor does it require any acknowledgment of paging reception from field units 50. FIELD UNITS 50. Continuing to pay attention to Figure 1, an exemplary field unit includes a receiver of output message such as a paging receiver 51, an HF transmitter 52, a field unit controller 53, and data collection apparatus such as a geo-location receiver 54 or other detector. The paging receiver is conventional, the field unit controller 53 is also a conventional controller device, such as a microcomputer.

The receiver 54 of the geo-location system can be any of the known types, such as global positional system (GPS) or Loran receiver. Upon receipt of a paging output message 45, an exemplary field unit 50a transmits an internal message back to the RBS network 60, such as an internal message 255 containing its current position or other data. The internal message 255 is transmitted to the carrier frequency and time that were indicated by the output message 245. The transmission time gaps in the internal communication links 55 can be synchronized using normal time-based universal data, as may be available from a GPS 54 receiver or another normal radio transmitter. The internal links HF 55 thus exhibit characteristic non-interference behavior since the remote field 50 units already know, before transmitting, which frequencies are not in use at particular times. The frequency is then quickly unoccupied after a single use by the MOC controller 32 thus making it available for other uses, such as its regularly authorized use. Additionally, field units transmit on only one of the open frequencies for a short period of time, of several seconds at most. In a preferred embodiment the field units 50 use a low power extended spectrum HF waveform, having a duration of about one (1) to ten (10) seconds. The waveform can, for example, be a waveform direct deployment, phase 8 key offset (PSK) with a bandwidth of 3KHz. And a chip rate of around 2400. This provides a data rate of approximately 75 bits per second for internal messages 255. It should be understood that other types of signal and modulation coding can be used, however. Interference to voice users in the HF spectrum is thus minimal since the noise pulse from the system 10, even if there is some interference, is similar to a typical HF channel fading. Other users of HF spectrum data are also typically equipped to handle second-time channel fades and have typically implemented ARQ or coded leaf patterns to avoid fading difficulties. Thus, other data users in the HF band should not notice the existence of the system 10. BASE RADIO STATIONS (RBS's) 60 Figure 5 is a block diagram of a typical base radio station (RBS) 60-1, consisting of of an RBS controller 61, a modem 62, a battery of modems HF 63, receivers HF 64, and a processor of analysis of frequency (FAP) 65. The controller RBS 61 is a conventional computer similar to the controller 32 of the MOC. The RBS controller 61 uses the 62 modem to exchange messages with the MOC controller 32. The RBS controller 61 maintains a real-time database of available frequency channels such as the frequency table 66. Each entry in the table 66 includes a frequency HF, f, and an observed noise power amplitude measurement A. The frequency table 66 can be maintained by a frequency analysis processor (FAP) 65 which periodically determines the identity of free operating HF frequencies, on a regular basis. The FAP 65 can do this using a sweep receiver, a spectrum analyzer or it can walk one or more of the tunable HF receivers 64 through the HF frequency band under the control of a computer or microcomputer.

In most cases, the FAP has a good chance of finding a frequency that is not occupied by another user. Assuming a worst case of time of day, such as at sunrise, about 2 MHz of the HF radio spectrum propagates at any given location. Given a 30% assumption of channel occupancy, which is based on empirical observations, the system 10 will typically have at least 466 available channels of the required 3 KHz bandwidth. The deployment of the HF modems 63 and associated HF receivers 64 is handled by the RBS controller 61 to monitor reception of internal messages from the field units at the specified frequencies and times from the MOC controller 32. To help in this process, a table 67 for displaying HF receivers is maintained. Each entry in this table 67 contains an HF receiver ID, and associated HF modem ID, serving the channel, a busy field, B, indicating whether the receiver / modem HF pair is presently assigned. If the occupied field indicates active status, the input also contains a frequency, f, and time, t, in which a message is expected for the HF pair, receiver / modem, as well as the ID of the field unit that is expected send the message The HF receivers 64 are adapted to receive the signal generated by the field units 50, which may be spread spectrum or other coding as already described. Figure 6 is a flow chart of the many possible implementations of operations executed by a control processor in the FAP 65. From an inactive state, in step 601 the FAP determines the identity of a possible next free HF frequency. In step 602, the FAP measures the power level received at that frequency, and in step 603, if the power level is sufficiently lower than a threshold amount, the FAP updates its internal table 66. In step 604 , the process is repeated until all the channels are checked. Frequency review typically occurs in small increments, such as 60 Hz, which are much smaller than other bandwidths of HF signals. Finally, at step 605, when requested by the MOC controller 32, the FAP 65 sends the available frequency information to the MOC controller 32 through the PSTN 25. In particular, the FAP 65 will typically send fixed frequencies that were observed being free at at least 3 kilohertz (KHz) contiguous.

Figure 7 is a flow chart of the operations executed by the RBS controller 61 to receive an internal message 255. From an inactive state 700, the RBS controller moves to a step 701 upon receiving a command from the MOC controller 31 to expect to receive a message from a particular field unit at a particular frequency and time. In step 702, the frequency, time, and ID of the field unit are read from the message of the MOC. In step 703, a free HF pair receiver and modem are identified by examining the local display table 67. The corresponding entry is then marked as busy and updated with the frequency, time and field unit ID information. The RBS then waits, in step 704, until time t approaches. Shortly before the time t, that is, with sufficient lead time to the time t to ensure complete active state of the selected HF receiver, the HF receiver pair and HF modem are activated in step 705. In step 706, it is then determined if an internal message was received from the indicated field unit 50a at time t. If so, in step 707 the RBS sends a report message back to the MOC which includes the data from the remote field unit in the internal message 255, along the ground-based communication link between the RBS and the MOC, as the internal paging response message 255. If, however, it is not received at the indicated time and frequency, a link failure is reported back to the MOC in step 708. PROCESSOR OF PROPAGATION ANALYSIS 70 Figure 8 is a block diagram of the Propagation Analysis Processor (PAP) 70. The PAP 70 is another computer consisting of a central processing unit (CPU) 70-1, memory 70-2, storage disk 70-3, and interface 32-4 input / output (I / O). The PAP 70 and MOC 32 can communicate using any type of convenient interface, or they can still be implemented in the same processor. The PAP 70 maintains a set of propagation tables 72-1, 72-2, 72-3, and 72-4, with a propagation table associated with each radio base station 60. Each propagation table 72 contains data values estimating the probability for an internal signal to be correctly received by that base station from a number of possible latitudes and longitudes, at different times of the day and in each of a number of frequency bands.

An exemplary table 72-1 associated with RBS 60a consists of a first input 73 indicating the latitude and longitude of the RBS 60a. Next, rows 74a, 74b, ... 74z, are created for each of many possible fields and locations. In one embodiment of the invention covering the continental United States, there may be 900 such possible locations, corresponding to the locations in a 30 x 30 grid that covers the service area, as shown in Figure 10. The entries in Table 72-1 are preferably maintained using both an analytical ionospheric prediction model, as well as using inputs in real time corresponding to the observed performance of system 10. For example, table 72-1 can be created initially using known ionospheric model programs such as the IONCAP program developed by Link Corporation of Binghampton, New York. The IONCAP model program, when given an hour of the day, transmitter and receiver location, estimated solar activity expected, antenna type, and other data, can mathematically predict a probability that given frequency ranges will propagate. Such an analytical propagation model program thus provides a probability, P, of actually receiving a signal at a base station from a particular field location at a given time of the day. The information is typically reported for all possible frequencies in the selected band. Thus, there are typically several rows in the table for each location. Additionally, since the propagation factors P are time dependent, an entry is made in the table for each hour of the day. The propagation model 72-1 can be updated as data relative to the current successes or failures of specific attempts to communicate between field units and particular radio base station locations are received. Updates can be made, for example, by using a weighted average of the old propagation data values and new observed propagation data values. Reports of successes or failures are received from controller 32 of the MOC, which reports successful and unsuccessful transmissions to PAP 70 (steps 412 and 415 of Figure 4). The propagation model 72-1 can be readjusted by re-processing the ionic modeling calculations, such as on a daily basis. The propagation model 72-1 can also be updated with data concerning known sound transmitters, polling transmitter and receiver pairs with other data as reported by the MOC controller 32. Figure 9 is a flow chart of the operations performed by the PAP CPU 70-1. From an inactive state in step 900, PAP 70 executes step 901, where the initial propagation model 72-1 is constructed. This is done by repeatedly running the propagation prediction program for different times of day and grid locations until the tables are complete for each of the four radio base stations. Next, in step 902, data concerning known sound emitters such as radio stations and the like in the vicinity of base stations 60 can be removed from tables 72. In step 903, the PAP 70 then waits for reports of transmission activity from controller 32 of the MOC.

If the MOC controller 32 reports a successful transmission, a decision is made in step 904 to proceed to step 905, where the propagation table 72 for the given base station is updated. This can be done, for example, by reading the value from table 72 for the corresponding frequency and time, and increasing the probability value in an appropriate manner, such as a weighted average of the last value and an incremental value. If, however, a failed transmission is reported, step 906 is executed, where the appropriate entry in the propagation table is reduced in value, again, preferably, by some weighted average technique. In any case, the control then goes to step 907, where it is determined whether it is time to reconstruct the propagation model. If it is not, as will be the case usually, PAP 70 then returns to step 903 to wait for a report of another transmission. If, however, it is time to reconstruct the tables in model 72, to what can be done on a daily basis, the control returns to step 901.

Claims (20)

1. CLAIMS 1. A two-way wireless data communication system, CHARACTERIZED by understanding: an output message subsystem; an internal message subsystem consisting of at least one radio frequency base station for receiving internal messages; a frequency analysis processor (FAP) associated with each of the at least one radio frequency base station, each of the FAPs continuously sampling, at their respective locations, the power levels received through a set of frequencies in an internal radio frequency band, to determine an observed level of frequency availability for the frequency set of each station: a propagation analysis processor (PAP) that determines a propagation probability for the frequency set; at least one remote field unit having an output message receiver and a tunable internal message transmitter; and a central control unit, where the central control unit uses the output message subsystem to send an output message to the remote field unit, and uses the internal message subsystem as an internal link to receive an internal message from the field unit, and where the output message includes data fields indicating an internal time and an internal carrier frequency in which the field unit must send each internal message, and where the central control unit receives reports of the observed levels of frequency availability from the FAPs and the central control unit also receives reports of the probabilities of the frequencies propagating from the PAP, and where the central control unit selects the internal carrier frequency depending on both the levels of frequency availability as reported by the FAPs as well as the probabilities of the frequency propagating as reported by the PAP, so that the The probability that the internal message will be received by at least one of the base stations without interference from another communication system is maximized.
2. 2. A system as in claim 1, characterized in that the internal message subsystem uses the high frequency (HF) of the radio frequency band as the internal radio frequency band.
3. 3. A system as in claim 2, characterized in that the output message subsystem is a paging subsystem.
4. 4. A system as in claim 1, characterized in that the PAP additionally comprises: database means, to maintain a propagation probability table, the propagation table containing a series of entries for each possible frequency and time of day .
5. 5. A system as in claim 2, CHARACTERIZED by comprising multiple internal links implemented by multiple radio base station receivers, and where the control unit allocates the carrier frequency depending on the observed availability of that carrier frequency in each of the multiple radio base station receivers.
6. 6. A system as in claim 4, CHARACTERIZED by comprising multiple internal links implemented by multiple radio base station receivers, and wherein the PAP maintains a propagation table for each of the base station receivers.
7. 7. A system as in claim 4, CHARACTERIZED in which the propagation table is initially written with propagation probability values determined by an analytical propagation model.
8. 8. A system as in claim 4, CHARACTERIZED in that the PAP additionally updates the propagation table as successful and failed internal message transmissions are reported by the MOC.
9. 9. A method of operating a communications system to provide omnipresent wireless data communication services between a mission operations center (MOC) and a plurality of remote field units, using a network of radio base station sites (RBS) ) widely distributed, the CHARACTERIZED method for understanding the steps of: A. At a customer call station, initiate a request to the MOC to communicate with a particular remote field unit, such as a request that the field unit determine and report your location; B. On the MOC site; i. Receive reports from a propagation analysis processor (PAP), the PAP reports indicating a probability that a list of radio frequencies will propagate from locations of field units to RBS sites; ii. Receive reports from frequency analysis processors (FAPs) located on the RBS sites, the FAP reports indicating a list of radio frequencies that are presently available for the unit to send an internal message back to one or more of the RBS sites; iii. Determine a frequency for the response of the field unit based on both the data reported by the PAP as well as the data reported by the FAP; iv. Determine a time available for the response of the field unit; v. Format the time and frequency for the response of the field unit in an outbound paging message; saw. Send the paging message to at least one of the paging systems. C. In at least one of the paging system sites, communicating the paging message to the field units through the paging system; D. In the selected field unit; i. Receive the paging message; ii. Formulate a response to the paging message by reading locally available data to the field unit; iii. Formulate a response as an internal message back to radio base stations; iv. Encode the response in the form of a short-duration impulse message, to minimize the likelihood of interfering with existing radio stations or other communications at frequencies near the selected frequency; v. At the indicated carrier frequency and time, transmit the internal message to the air; E. On the RBS site; i. Receive the internal message from the field unit from the air; and ii. Send the internal message back to the MOC; F. On the MOC site, send the internal message back to the client call station; and G. At the calling station, receive the internal message.
10. 10. A method according to claim 9, wherein the call stations are connected to the MOC through the land-based public telephone network (PSTN).
11. 11. A method as in claim 9, characterized in that the MOC is connected to paging systems through the land-based public telephone network (PSTN).
12. 12. A method as in claim 9, CHARACTERIZED in the MOC communicates with the RBS's through a terrestrial base connection such as the public telephone network.
13. 13. A method as in claim 9, characterized in that the MOC communicates with the RBS's through a terrestrial base connection such as leased private telephone circuits.
14. 14. A method as in claim 9, CHARACTERIZED by further comprising the step of: in the MOC, alerting one or more of the associated RBS's to wait for a response from the indicated field unit, at the specified frequency and time.
15. 15. A method as in claim 14, CHARACTERIZED by further comprising the steps of. H. In the RBS, if such an internal message is not received as alerted by the MOC, to report a failed internal message to the PAP; and I. In the PAP, update a table of expected propagation probabilities based on the failed internal message report.
16. 16. A method as in claim 14, CHARACTERIZED in that it further comprises the steps of: H. In the RBS, if an internal message is received as alerted by the MOC, report a successful internal message to the PAP; and I. In the PAP, update a table of expected probabilities of propagation based on the successful internal message report.
17. 17. A method as in claim 9, characterized in that each FAP executes additionally the step of periodically measuring a received power level in each possible internal frequency channel.
18. 18. A method as in claim 9, CHARACTERIZED by further comprising the step of, in the field unit, collecting data to be reported including a latitude and longitude from a geographic location system.
19. 19. A method as in claim 9, characterized in that the short duration internal message is encoded using extended spectrum modulation.
20. 20. A method as in claim 9, characterized in that the MOC communicates with the RBSs through a Very Small Aperture Terminal (VSAT) network.