Voice call group function for a satellite based air traffic control system

Voice call group functionality is provided in a satellite based air traffic control system to allow air traffic controllers and pilots of one or more aircraft to establish and maintain voice communication over a group call. A pilot of an aircraft may maintain voice communication with the same air traffic controller the entire duration of the flight over the entire globe. Voice communication between the pilot of an aircraft may be handed off from one air traffic controller to another by switching voice call groups. The voice call group functionality allows an air traffic controller to communicate simultaneously with pilots of different aircraft, and also allows pilots of different aircraft to communicate with each other.

BACKGROUND OF THE INVENTION
 The present invention relates generally to the field of aircraft
 communications, and more particularly to a system and method for providing
 voice call group functionality in a satellite based air traffic control
 system.
 Air traffic control (ATC) systems control the airspace and airchannels
 between airports. Present day ATC systems utilize an air traffic
 communication infrastructure that has been in place since the 1950s. In
 the United States, this infrastructure includes more than 400 airport
 towers, 185 terminal radar approach control sites (Tracons), and 20
 regional ATC centers. Voice communication between air traffic controllers
 and each aircraft remains, for the most part, entirely ground based.
 Airport towers communicate flight plans and instructions for take-off and
 landing to the aircraft while it is on the ground. Tracons monitor the
 aircraft and give flight instructions during take-off, approach and
 landing. Tracons typically monitor the aircraft up to 40 miles out of an
 airport in lower altitudes. Regional ATC centers take over control of the
 aircraft in high altitudes. Each regional ATC center maintains control
 over aircraft flying within its region, which may cover areas between 20
 and 200 miles wide. As an aircraft enters or leaves the region of control
 of a regional ATC center, the regional ATC center communicates with either
 the Tracon site or the regional ATC center having control over the
 airspace from which the aircraft is entering or to which the aircraft is
 leaving to coordinate a hand-off of control of the aircraft. Tracon sites
 monitor aircraft within its airspace using airport surveillance radar
 (ASR), which typically has a range of approximately 55 nautical miles.
 Regional ATC centers monitor aircraft within their airspace using air
 route surveillance radar, which typically has a range of approximately 200
 nautical miles.
 Voice communication between air traffic controllers and aircraft is
 important for exchanging information such as route changes, weather and
 safety alerts, landing instructions, and information relating to crew or
 equipment emergency situations. Voice communication between a local
 airport control tower, a tracon site, or a regional ATC center and en
 route aircraft is provided in present day ATC systems using AM radio
 signaling. Typically, however, voice communication between a local airport
 control tower, a tracon site, or a regional ATC center ends when an
 aircraft leaves the airspace controlled by a respective ground control
 site. Thus, there is no seamless voice communication between a particular
 ground site and an airborne aircraft. Voice communication handoffs must be
 coordinated by the ground control site from whose airspace the aircraft is
 leaving and the ground control site into whose airspace the aircraft is
 entering. Handoffs are coordinated via ground line communications links
 such as ground telecommunications or microwave links.
 Aircraft communication becomes even more complex when an aircraft crosses
 international borders. Each country or group of countries typically has
 its own ATC system and navigational infrastructure. This increases the
 complexity and therefore the reliability of ground-to-aircraft voice
 communication handoffs between different ATC systems. In addition, each
 ATC system may provide coverage of the entire country or group of
 countries, but more typically covers only a large part of it. Thus, some
 areas of some countries, and some areas between countries such as the
 airspace over oceans and the polar regions are uncovered. In uncovered
 areas, ground-to-aircraft communication may be slow and suffer more
 heavily from atmospheric interference, or may simply be unavailable.
 Present day ATC communications systems have many disadvantages. First,
 present day ATC communications systems cannot provide seamless voice
 communication between a particular ATC tower, tracon or ATC center and an
 aircraft during the entire duration of its flight. Second, voice
 communication is not globally available. If an aircraft flies over an
 uncovered territory, all voice communication may be lost. Another
 disadvantage of current ATC communications systems is that radio
 communication signals over existing links are susceptible to atmospheric
 interference. Finally, because ATC systems are fragmented worldwide, a
 given aircraft does not maintain voice communication with any single ATC
 system during the entire duration of its flight.
 Accordingly, a need exists for a method for maintaining voice communication
 between an aircraft and a ground air traffic control center for the entire
 duration of the aircraft's flight, if necessary, and thus independently of
 the location of the air traffic control center and the position of the
 aircraft. A need also exists for allowing a single air traffic controller
 or a pilot of an aircraft to simultaneously communicate with a group of
 pilots and or air traffic controllers.

DETAILED DESCRIPTION
 The system and method of the present invention provides a reliable
 instantaneous voice call group function for a satellite based air traffic
 control system. The present invention provides a solution to the problems
 of current day ATC ground-to-aircraft communications systems at
 significant cost reduction by utilizing a low-orbit global-coverage
 satellite communications network to reduce infrastructure investment and
 complexity. The creation of a satellite based ATC communications system
 that provides voice call group functionality allows air traffic
 controllers to maintain voice communication with the pilot of an aircraft
 throughout the entire duration of its flight. The invention further allows
 pilots of other aircraft and ATC centers of different regions to listen in
 on and or join in the conversation. The present invention also provides a
 method for authenticating and identifying the voice generator to protect
 against unauthorized phantom controllers. Additionally, the use of a
 global-coverage satellite communications network in the present invention
 eliminates difficulties involved in communication hand-offs between
 regional ATC systems. Moreover, the satellite based ATC voice call group
 function of present invention allows voice data to be transmitted and
 received virtually error-free without susceptibility to atmospheric
 interference.
 FIG. 1 is a diagram of an air traffic control (ATC) communications network,
 shown generally at 1. In accordance with the present invention, the ATC
 communication network 1 includes at least a satellite processing node 2,
 one or more avionics processing nodes 4, and a ground processing node 6.
 Satellite processing node 2 may comprise a plurality of sub-nodes, as for
 example a satellite communication network comprising a plurality of
 satellites in communication via inter-satellite links such as the
 IRIDIUM.TM. satellite communications network. Ground processing node 2 may
 also comprise a plurality of sub-nodes, as for example a local area
 network comprising a plurality of individual air traffic controller
 stations in communication with each other and each sharing a common
 satellite ground link 12 for communication with satellite processing node
 2.
 Voice signals may be passed between satellite processing node 2 and any one
 or more of a group of M avionics processing nodes 4 over satellite
 communication links 14. Similarly, voice signals may be passed between
 satellite processing node 2 and ground processing node 6 over a satellite
 ground link 12. Voice signals may be sent between ground processing node 6
 and one or more of the M avionics processing nodes 4 through satellite
 processing node 2. Thus voice signals sent from ground processing node 6
 may be transmitted over satellite ground link 12 to satellite processing
 node 2, and then over one of the satellite communication links 14 to one
 or more of the avionics processing nodes 4. Likewise, voice signals sent
 from an avionics processing node 4 may be transmitted over a satellite
 communication link 14 to satellite processing node 2, and then over
 satellite ground link 12 to ground processing node 6. Voice signals may
 also be transmitted between different ones of the avionics processing
 nodes 4 by routing the voice signals through satellite processing node 2.
 For example, voice signals sent from the first avionics processing node 4
 to the Mth avionics processing node 4 are transmitted over a first
 satellite communication link 14 to satellite processing node 2, and then
 over the same or a different satellite communication link 14 to the Mth
 avionics processing node 4.
 Satellite ground link 12 comprises at least one group communication
 channel, and preferably multiple group communication channels. In the
 embodiment shown, multiple group communication channels are provided by
 using any one of a number of multiple access protocols which include, but
 are not limited to, time division multiple access (TDMA), code division
 multiple access (CDMA), random division multiple access (RDMA), or a
 combination of such schemes. TDMA is a well known data transmission scheme
 in which a communication channel is divided into a plurality of time slots
 and in this case being the preferred embodiment of this system. In this
 preferred embodiment, each time slot may be assigned to a different voice
 call group. Each satellite communication link 14 also comprises at least
 one group communication channel, and preferably multiple group
 communication channels using the TDMA scheme as described above.
 The frequency and time slot of a given voice call group may very likely be
 different over channels in different links. For example, a voice call
 group may be assigned to one time slot in one group communication channel
 over satellite ground link 12 and to a completely different time slot and
 different group communication channel over a satellite communication link
 14. Accordingly, the satellite processing node 2 includes a communication
 controller 5 which operates to route voice signals between a group
 communication channel in satellite ground link 12 assigned to a particular
 voice group call to the corresponding group communication channel in a
 satellite communication link 14 assigned to the same voice group call.
 The ATC communication network 1 also includes a call group controller 9
 which manages the assignments of available group communications channels
 in satellite ground link 12 and satellite communication links 14 to each
 voice call group. Each voice call group requires a group communications
 channel in satellite ground link 12 and a group communications channel in
 one or more satellite communication links 14. Call group controller 9
 keeps track of available group channels in the satellite ground link 12
 and each of the satellite communication links 14 which may be assigned to
 new voice call groups. Call group controller 9 also monitors membership in
 each established voice call group, and may also be configured to grant or
 deny membership in various voice call groups. In the embodiment shown,
 call group controller 9 is implemented at ground processing node 6.
 However, call group controller 9 may alternatively be implemented at
 satellite processing node 2. Moreover, the functionality of the call group
 controller 9 may be fragmented and partially implemented at both the
 satellite processing node 2 and ground processing node 6.
 FIG. 2 is a block diagram of an air traffic control (ATC) system 100 in
 which the voice call group function of the present invention may be
 implemented. In the embodiment shown, ground processing node 6 comprises
 an air traffic control (ATC) center 60. ATC center 60 comprises a call
 group controller 90 and a plurality of air traffic controller (ATC)
 stations, each shown identically at 8. Call group controller 90 and ATC
 stations 8 are configured to share a satellite ground link 12 over which
 voice signals are sent and received. Accordingly, ATC center 60 in the
 embodiment shown also includes a satellite gateway 70. Satellite gateway
 70 includes a communications controller 72 for managing channel
 synchronization and access, and a transceiver 78 for transmitting and
 receiving signals over satellite ground link 12. Each ATC station 8 is
 configured to send and receive voice signals to and from satellite gateway
 70 over a local communication link 10. Call group controller 90 is
 configured to send and receive control and data messages to and from each
 of the ATC stations 8 and the satellite gateway 70 over local
 communication link 10. The control and data messages generated by and
 received by call group controller 90 may include, by way of example,
 channel access requests/grants, group communication channel availability,
 and authentication/identifier information. Call group controller 90 may be
 implemented using any conventional processor executing a call group
 control program in accordance with the desired functionality of such a
 program. Local communication link 10 may be implemented using any
 conventional optical, wired or wireless communication system. Each ATC
 station 8 is also preferably assigned to a particular voice call group,
 and is thus typically connected to its assigned group communication
 channel.
 In ATC system 100, satellite processing node 2 is preferably implemented
 with a satellite communications network 24 comprising a plurality of
 satellites, each identically shown at 20, in communication via
 inter-satellite links 22. Preferably, each satellite 20 includes a
 communications controller 50 which operates to route voice and data
 signals between corresponding group communication channels of different
 communication links 12, 14, 22 that are assigned to the same voice group
 call.
 As also shown in FIG. 2, ATC system 100 includes a plurality of avionics
 processing nodes 4. Each avionics processing node 4 in the embodiment
 shown comprises an avionics satellite communications unit 30 aboard an
 aircraft 40.
 FIG. 3 is a block diagram of a preferred embodiment avionics satellite
 communication unit 30. As shown in FIG. 3, the avionics satellite
 communication unit 30 preferably includes a satellite communications
 transceiver 32, a burst processor 33, a vocoder 35, a speaker 36, a
 microphone 37, an avionics unit controller 31, and a display unit 38 and
 input device 39. Satellite communications transceiver 32, which may be
 implemented with any transceiver having the capability to send and receive
 signals to and from a satellite 20 (e.g., a Motorola LST5 manufactured by
 Motorola, Inc.), receives an incoming signal from a satellite 20.
 Satellite communications transceiver 32 demodulates the actual incoming
 signal from the carrier signal. Burst processor 33 synchronizes the actual
 incoming signal into frames to extract digital packets. The avionics unit
 controller 31 removes the header information, determines what type of
 message is contained in the digital packet, and extracts the message. If
 the message is a voice message, avionics unit controller 31 sends the
 voice message to vocoder 35 where it is converted into an analog voice
 signal and output by speaker 36. If the message is a data message,
 avionics unit controller 31 processes the data message, and outputs any
 updates to display unit 38 when appropriate.
 FIG. 4 is a diagram illustrating a time slice 120 of a preferred TDMA
 embodiment of a single channel of any or all of links 10, 12, and 14 of
 FIG. 2. As shown, a single channel may be divided into N time slots, shown
 at 121-124 to support N group communication channels for N independent
 voice call groups. The number of slots N is typically determined by the
 bandwidth of the channel 120 and the minimum bandwidth required to
 transmit a single digital packet containing enough information to support
 voice communication across the channel.
 FIG. 5 illustrates a preferred embodiment of a digital packet used to carry
 a digitized voice or digital data message. As shown in FIG. 5, the digital
 packet, shown at 110, includes a header field 111, a channel management
 field 112, and a message body 113. The header field 111 contains
 information including routing information such as a source node
 identifier, a destination node identifier, a message type, and other
 pertinent information. The channel management field includes link control
 information such as the voice call group identifier used by the call group
 controller 90 for translating channels between different communication
 links. Message body 113 contains either voice or data. Header field 111
 preferably contains a message type indicator. Thus, as shown in FIG. 3,
 when a digitized packet is extracted from an incoming signal by burst
 processor 33, avionics unit controller 31 determines the message type from
 the header field 111. If the message type is a voice message, the message
 body 113 is sent to vocoder 35 for output by speaker 36. If the message
 type is a data message (e.g., control information such as the granting of
 a channel access request by the call group controller 90 of the
 communications channel network, or display information), the message body
 113 is processed by the avionics controller 31 and/or sent to an
 appropriate entity for processing.
 Once an aircraft 40 is assigned to a voice call group on a communications
 channel, the pilot of the aircraft 40 preferably remains mainly in a
 receive or "listening" mode. Occasionally, the pilot must join the
 conversation, or enter a "talk" mode. In this case, the pilot speaks into
 microphone 37 which converts the speech into an analog voice signal. The
 analog voice signal is sent through vocoder 35 where it is converted to a
 digitized voice message. The avionics unit controller 31 packetizes the
 digitized voice message and adds header information to create a digital
 packet. The digital packet is sent to burst processor 33, where it is
 queued and any necessary link control information is added to it. Burst
 processor 33 waits for its assigned user timeslot, and then sends the
 digitized packet to the satellite communications transceiver 32 for
 transmission over its assigned group communications channel.
 In one embodiment of the invention, each voice call group includes two
 channel timeslots in its assigned group communication channel over each
 link. In this embodiment, one timeslot is used as a dedicated controller
 uplink to allow the controller to speak at any time and always be heard by
 the other members of the same voice call group. The other timeslot is a
 conference bridge over which any and all voice call group members may both
 speak and listen.
 To avoid voice "collisions" when more than one pilot attempts to talk over
 a group communication channel simultaneously, a "talk" protocol is
 implemented in the voice call group function of the invention. This "talk"
 protocol is preferably implemented using a "push-to-talk" function that
 emulates the present day communications protocol that is implemented in
 line-of-site AM radio communication systems. Under this protocol, a pilot
 member of a voice call group acquires permission to talk over the group
 communications channel assigned to the voice call group by pressing a
 "push to talk" button, shown in FIG. 3 at 46.
 FIG. 6 is a flow chart illustrating a preferred embodiment implementation
 of a "push to talk" function. The method, shown generally at 200, of the
 "push to talk" function includes a first step 202 of determining whether
 the pilot wants to talk. As described previously, a pilot preferably
 remains in a listening mode unless and until the pilot has relevant and
 important information to convey via voice. Accordingly, while the pilot is
 in listening mode, the method remains in step 202. When it becomes
 necessary for the pilot to talk, the determination of whether an emergency
 exists is made in a step 204. If no emergency exists, the pilot determines
 whether anyone is currently talking over the group communication channel
 in a step 206. This is typically determined by listening to the channel to
 determine if anyone is currently speaking. If the channel is currently
 being accessed by another user, the pilot waits in a step 208 until nobody
 is talking over the channel. Once it has been determined that no one is
 talking over the group communication channel assigned to the voice call
 group, the pilot generates a channel access request in step 210,
 preferably by pressing a "push to talk" button 46. When the channel access
 request 210 is generated, the avionics satellite communication unit 30 in
 the pilot's aircraft 40 waits to see whether the request was granted in
 step 212. If the request was not granted, the pilot then waits in a step
 208, and then repeats steps 206 through 212. Once the request is granted,
 the pilot may then talk over the group communication channel in a step 214
 by speaking into a microphone, as shown at 37 in FIG. 3.
 Steps 202 through 214 may be followed in most circumstances. However,
 occasionally, an emergency condition exists and a pilot will need talk
 over the voice group channel immediately. In this case, it will be
 determined in step 204 that an emergency does exist. The pilot then
 generates an emergency access request in a step 220, which is sent to the
 call group controller 90 (see FIG. 3) controlling the group communications
 channel assigned to the voice call group. The call group controller 90
 preferably detects the high priority of the emergency access request and
 grants the emergency access request as long as no other message has higher
 priority. Preferably, the call group controller 90 signals to each member
 of the voice group call that an emergency exists and to clear the channel.
 The determination of whether the emergency access request is granted is
 made in step 222. If the emergency access request has not been granted,
 the pilot must wait in a step 224 and then repeat steps 220 through 224.
 Once the emergency access request has been granted, the pilot may then
 talk over the channel in step 214. As mentioned previously, the
 communications protocol employed in the preferred embodiment is preferably
 designed to emulate the current "push to talk" protocol used in present
 day line of sight AM radio frequency systems. However, it will be clear to
 those skilled in the art that the "push to talk" function may be
 implemented using a variety of other methods, implementations, and
 protocols. Accordingly, any method, implementation, or set of
 communication protocols which achieves the desired "push to talk"
 functionality is incorporated herein.
 Referring once again to FIG. 2, call group controller 90 maintains a voice
 call group register for recording each group communication channel,
 assigned voice call group, current voice call group member IDs,
 corresponding call start time of each voice call group member, and other
 such relevant information from which channel availability and usage may be
 derived. Call group controller 90 causes information contained in or
 derived from the voice call group register to be broadcast over the
 satellite communications links 14 and satellite ground link 12. Avionic
 satellite communication units 30 of each aircraft 40 and ATC stations 8
 receive this information. In a first embodiment, this information may be
 broadcast continuously over a designated channel in each link 12, 14.
 Alternatively, the information may be sent as a burst packet interleaved
 with voice message packets over each group communication channel. In this
 case, the burst packets must be interleaved at a low enough rate so as not
 to degrade the quality of the reconstructed voice signal as heard by the
 pilots and controllers. Preferably, the information contained in the burst
 packets or over a designated channel are transmitted in digital packet
 format similar to that used for transmitting digitized voice messages. In
 the avionics satellite communication device 30 of FIG. 3, these packets
 are received by the satellite communication transceiver 32, extracted by
 burst processor 33, and routed to display unit 38. Display unit 38
 preferably comprises a display processor 48 which processes the
 information from the call group controller 90 and suitably displays it on
 a display device 49. ATC stations 8 at the ATC center 60 similarly
 receive, process and display the information from the call group
 controller 90.
 FIGS. 7 and 8 are examples of one embodiment of the displays illustrating
 the type of information that may be collected and transmitted by call
 group controller 90 and formatted for display for air crew members and ATC
 staff. FIG. 7 is an example display which may appear before an air crew
 member is assigned to a voice call group. In this case, the air crew
 member may be presented with a menu of voice call groups, including the
 current availability for each voice call group, the ID and/or location of
 each voice call group's assigned controller, and the IDs of current users
 assigned to each voice call group. The screen shown in FIG. 7 allows an
 air crew member to select an available voice call group by entering the
 channel number corresponding to the requested voice call group. The call
 group controller 90 receives the request and either grants or denies the
 request. Reasons for denying the voice call group access request may
 include, for example, an improper authentication of the requesting air
 crew member or aircraft.
 FIG. 8 is an example display screen which may be displayed after an
 aircraft has been assigned to a voice call group. As shown in FIG. 8, this
 display screen displays voice call group specific information such as the
 member ID, registered member information such as the pilot's name,
 airline, flight number, any relevant navigation information, and who is
 currently speaking on the channel. In FIG. 8 the current speaker is
 indicated with an asterisk before the user ID.
 As previously described, a particular voice call group that requires
 communication over more than one link 12, 14 may be assigned to different
 channel frequencies and/or time slots in each independent link 12, 14.
 Accordingly, satellite processing node 2 is preferably provided with
 communications controller 50 which manages the routing of signals from one
 link to another. A similarly functioning communications controller may be
 implemented at any communications network node through which signals are
 routed from one communication link to another. FIG. 9 illustrates a block
 diagram of satellite communications controller 50 for a member satellite
 20 of a satellite communications network 24 implemented at satellite
 processing node 2. As shown in FIG. 9, satellite communications controller
 50 preferably includes a set of inter-satellite crosslink transceivers 52,
 a set of satellite ground link transceivers 54 and a set of satellite
 communication link transceivers 56. Each of these transceivers are
 controlled by a controller 58. A memory 53 is provided for storing voice
 call group channel and slot assignments and inter-link mappings.
 Controller 58 uses the assignments and mappings stored in memory 53 to
 route signals received over a given channel and time slot of one link to
 its corresponding voice call group assigned channel and time slot of
 another link. Communication controller 50 also preferably includes timer
 55 for use in synchronizing time slot frames when transmitting and
 receiving signals over transceivers 52, 54, 56. Data transmission
 protocols and synchronization schemes are known in the art and any
 suitable implementation is hereby incorporated herein.
 In present day air traffic control systems, regional air traffic
 controllers are stationed at fixed locations relative to the ground and
 spaced out across different geographical regions. Regional air traffic
 control centers thus typically provide control over and manage the
 airspace over a specific geographical region. FIG. 10 is a block diagram
 of an ATC system 300 illustrating two different regions of airspace,
 AIRSE A and AIRSE B, each under the control of a different ATC
 center, shown respectively at 306 and 308. Each ATC center 306 and 308
 include respective call group controllers 316 and 318 for managing voice
 call groups. An aircraft 340 provided with an avionics satellite
 communications unit 330 is preferably a member of a voice call group under
 the control of a call group controller 316 at ATC center A while it is
 within the regional airspace of AIRSE A. With the present invention it
 is possible for a given air traffic controller to maintain voice
 communication with a given aircraft for the entire duration of the
 aircraft's flight. This is especially true when the satellite processing
 node 2 of FIGS. 1 and 2, and 302 of FIG. 10, is implemented using a global
 coverage satellite communication network such as the IRIDIUM.TM. satellite
 communications system developed by Motorola, Inc. in Chandler, Ariz.
 However, in the interest of efficiency, costs and airspace sovereignty,
 this feature may not always be desirable. Thus, when aircraft 340
 approaches airspace boundary 305, it may be desirable to perform a handoff
 of communications with aircraft 340 from a controller at ATC center 306 to
 a controller at ATC center 308. FIG. 11 is a flow chart of a method for
 performing a communication handoff. The method, shown at 400, includes a
 first step 402 where a controller A at ATC center 306 in FIG. 10 sends a
 handoff request to a controller B at ATC center 308. Controller B either
 accepts or rejects the request in step 404. If the handoff request was
 rejected, then no communication handoff takes place from controller A to
 controller B. If controller B accepts the handoff request, controller A or
 controller B, depending on the implementation, sends a transfer handoff
 request to the pilot of aircraft 340 in a step 406. In the preferred
 embodiment, the pilot has the option of accepting or rejecting the
 transfer. The pilot accepts or rejects the transfer handoff request in
 step 408. If the handoff transfer request is rejected, in the preferred
 embodiment, no handoff transfer will take place. If the handoff transfer
 request was accepted, controller A transfers all necessary control
 sequence information of aircraft 340 to controller B in a step 410. In a
 step 412 call group controller 318 adds aircraft 340 to a new voice call
 group under its control. Call group controller 316 also removes aircraft
 340 from its original voice call group managed by call group controller
 316. In a step 416, the aircraft avionics satellite communications unit
 tunes the avionic satellite communication unit 330 of aircraft 340 to the
 group communication channel assigned to its new voice call group managed
 by call group controller 318.
 In the method 400 of FIG. 11, each of controller A, controller B and the
 pilot have the option to accept or reject hand off requests. In certain
 circumstances, however, it may be desirable not to give either the pilot
 or controller B the option of rejecting the request. Accordingly, the
 communication handoff method may be implemented differently to accommodate
 the requirements of the ATC system's handoff protocol.
 FIG. 12 is a flow chart illustrating a method, shown at 500, of a
 communication handoff initiated by either the pilot or the avionics
 satellite communication unit 330 of aircraft 340 of FIG. 10. A typical
 scenario in which a communication handoff may be initiated by a pilot is
 when the aircraft is approaching a geographical border, such as airspace
 boundary 305 shown in FIG. 10. According to method 500, a handoff is
 initiated by generating a handoff request in a step 502. The handoff
 request is sent to the requested controller (controller B) in a step 504.
 In a step 506 controller B either grants or denies the handoff request. If
 the handoff request is not granted, no handoff occurs and aircraft 340
 remains in its original voice call group. If the handoff request is
 granted, the avionic satellite communication unit 330 instructs, in a step
 508, original controller A to transfer all control sequence information of
 the aircraft 340 to controller B. In a step 510, call group controller 318
 adds aircraft 340 to a new voice call group. In a step 512, call group
 controller 316 removes aircraft 340 from its original voice call group. In
 step 514, avionic satellite communication unit 330 of aircraft 340 tunes
 to the new voice call group.
 A given call group controller may be implemented to collect billing
 information. FIG. 13 is a flow chart of a method, shown at 600, for
 determining time based billing costs for utilizing the services of an air
 traffic controller station. As shown in FIG. 13, the call group controller
 records the avionics satellite communications unit ID and the call start
 time at the time that the aircraft is added to a voice call group in step
 602. In a step 604, the call group controller records the call end time
 when it removes an avionics satellite communication unit from the voice
 call group. In a step 606, the call group controller sends the avionic
 satellite communication ID and call start and end times to a billing
 processor 92 (see FIG. 2). The billing processor 92 may determine the
 duration of the call from the start and end times and associate a charge
 with the corresponding avionic satellite communication ID of the aircraft.
 Other billing algorithms may also be used to determine user fees, such as
 position of aircraft/length of time in a given airspace, number of data
 packets transferred, or length of time the push to talk function is
 utilized.
 Although the invention has been described in terms of the illustrative
 embodiments, it will be appreciated by those skilled in the art that
 various changes and modifications may be made to the illustrative
 embodiments without departing from the spirit or scope of the invention.
 It is intended that the scope of the invention not be limited in any way
 to the illustrative embodiment shown and described but that the invention
 be limited only by the claims appended hereto.