TELECOMMAND SYSTEM

1
TELECOMMANDSYSTEM
2
TELECOMMANDING ARCHITECTURE
3
TELECOMMAND DATA STRUCTURES
4
CHANNEL SERVICE

• PHYSICAL LAYER

5
SENDING END SERVICE SPECIFICATION

• Inputs from the layer above
• Buffer of bits corresponding to a CLTU and
  control information.

• Outputs to the layer above
• Status of the physical telecommand channel.
• Outputs to the receiving end of the layer
• Modulated radio frequency waveforms.

• Internal functions
• Establishes the physical radio frequency path to
  the spacecraft.
• Radiates a buffer of data bits serially according
  to the PLOP requested by the layer above.

6
RECEIVING END SERVICE SPECIFICATION

• Inputs from the sending end of the layer
• Modulated radio frequency waveforms which have
  been radiated by a transmitting station.

• Outputs to the layer above
• Synchronized detected dirty symbol stream and
  status of the RF lock.
• Modulated carrier/subcarrier presence, used by
  layer above to select between Inactive and Search
  states.

• Internal functions
• Receives and detects the modulated
  carrier/subcarrier.
• Performs demodulation and symbol synchronization.
• Determines the state (Active/Inactive) of the
  physical TC channel.
• Performs symbol detection.
• Informs the layer above of status of the Physical
  layer.

7
STANDARD DATA STRUCTURES

• Acquisition Sequence
• The preferred minimum length is 16 bytes.
• It shall be alternating 1 and 0, starting
  with either a 1 or a 0.

• CLTU
• Each codeblock within the CLTU provides at least
  2 data transitions.
• If it required more frequent transitions, the
  CLTU must have been randomised as described in
  the Coding Layer.

• Idle Sequence
• It provides for maintenance of symbol
  synchronization in the absence of CLTUs.
• It is a sequence of alternating 1 and 0.
• The length is an unconstrained number of bits.

8
STANDARD PROCEDURES

• Carrier Modulation Modes (CMM)
• CMM-1 unmodulated carrier only.
• CMM-2 carrier modulated with Acquisition
  Sequence.
• CMM-3 carrier modulated with TC data.
• CMM-4 carrier modulated with Idle Sequence.

• Physical Layer Operations Procedures (PLOPs) ?
  PLOP-1
• It is a procedure for individually radiating
  CLTUs.
• Decoder is always forced into the Inactive state
  by deactivating the physical TC channel after the
  end of transmission of each CLTU.

• Physical Layer Operations Procedures (PLOPs) ?
  PLOP-2
• Decoder is in the Search state after each CLTU
  (channel not deactivated).
• Decoder is forced into the Inactive state only at
  the end of transmission of a series of CLTUs.
• It is recommended insert a minimum Idle Sequence
  of one byte between each CLTU.

9
CHANNEL SERVICE

• CODING LAYER

10
SENDING END SERVICE SPECIFICATION

• Inputs from the layer above
• Input Data to be included in a single CLTU and
  control instructions.
• Inputs from the layer below
• Status of the physical TC channel.

• Outputs to the layer above
• Status of the physical TC channel.
• Outputs to the layer below
• CLTUs and control instructions.

• Internal functions
• Conditions Input TC Data by randomising it if
  used by a mission.
• Adds fill as necessary to complete the last
  codeblock of the CLTU.
• Encodes the Input TC Data (TC Frames) into TC
  Codeblocks.
• Forms the TC Codeblocks into a CLTU by adding the
  Start and Tail Sequence.

11
RECEIVING END SERVICE SPECIFICATION

• Inputs from the layer below
• Synchronized detected dirty symbol stream,
  control information and status.

• Outputs to the layer above
• Clean decoded and derandomised (if used) TC
  data from each codeblock which have passed the
  decoder quality check.
• Decode Status (indicating start, continuity, end
  of data), control information.

• Internal functions
• Detects the CLTU Start Sequence subsequent
  codeblock are automatically synchronized by being
  contiguous.
• Makes an estimate to determine if an error has
  probably occurred.
• Optionally makes an estimate of the correct value
  and continues decoding.
• If a detected or uncorrectable error is
  encountered, leaves the Decode state and enters
  the Search state signals the stop of valid TC
  data.
• If modulation is lost, leaves the Decode state
  and enters the Inactive state.
• The valid TC data is derandomised (if used)
  before passing to the layer above.

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TC CODEBLOCK FORMAT

• (1) Information (k bits)
• Contains k information bits (k32,40,48, or 56
  bits) it shall be fixed.
• Information bits may be randomised.
• The preferred overall length n of the TC
  Codeblock is 64 bits (k56bits).
• (2.1) Parity Check Bits (7 bits)
• The encoding procedure for generating these
  parity bits is described later.
• (2.2) Appended Filler Bit (1 bit)
• Provide an overall Codeblock length which is an
  integer number of byte.
• This Filler Bit shall always be a zero.

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CLTU FORMAT

• (1) Start Sequence (16 bits)
• Pattern with low autocorrelation sidelobes
  1110101110010000
• (2) Encoded TC Data
• Consists of a set of TC Codeblocks.
• (3) Tail Sequence
• It shall have the same length as the TC
  Codeblocks that are being used.
• It is constructed specifically to be a
  noncorrectable sequence.
• It shall consist of leading octets having the
  pattern 11000101
• The last octet completes the tail sequence field,
  and always has the pattern 01111001

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RANDOMISATION PROCEDURE

• Used if a sufficient bit transition density is
  not ensured for the channel.
• Random sequence shall be generated using the
  following polynomial
• h(x)x8x6x4x3x2x1 (it repeats after 255
  bits)

• The BTG (Bit Transition Generator) is pre-set to
  the all-ones state
• It remains in this state until Start Sequence has
  been detected.
• It is reset in this state following a failure of
  the decoder.
• Derandomisation is applied to the successfully
  decoded TC data.
• Adders are Modulo-2 (EX-OR).

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ENCODING/DECODING PROCEDURE (1)

• The code used is a (63,56) modified BCH
• The generator polynomial is g(x)x7x6x21
• The same encoding also serves for shortened cases
  (k32,40,48) by forcing to zero the other (56-k)
  bits (virtual fill not outputted, nor
  transmitted).

• If the Input Data do not fit exactly within an
  integral number of Codeblocks, last octets of the
  information field may contain Fill bits (a
  sequence of alternating ones and zeros
  starting with a zero).
• Codeblocks may be decoded using the following
  mode
• Error-detecting mode (TED mode) one, two or
  three bits in error will be detected.
• Error-correcting mode (SEC mode) one bit in
  error will be corrected and two bits in error
  will be detected.

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ENCODING/DECODING PROCEDURE (2)

• The shift registers are initialized to zero.
• The switches are
• In position (1), while the k TC data bits are
  being transmitted.
• In position (2), for the seven parity bits.
• In position (3), for the appended fill bit.

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DATA ROUTING SERVICE

• TRANSFER LAYER

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SENDING END SERVICE SPECIFICATION (1)

• Inputs from the layer above
• TC Segments and control information.
• Inputs from the receiving end of the layer
• Information about the status of receipt of TC
  Frames using CLCW (Command Link Control Word).
• Inputs from the layer below
• Status of the physical layer.

• Outputs to the layer above
• Status of the data routing process and
  availability of VCs.
• Outputs to the receiving end of the layer
• Control Command TC Frames, which instruct the
  receiving end.
• Outputs to the layer below
• Buffer of TC data bits and control instructions.

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SENDING END SERVICE SPECIFICATION (2)

• Internal functions
• Encapsulates TC Segments into TC Frames.
• Translates control instructions received from the
  layer above into the appropriate set of
  operational procedures.
• Creates Control Command TC Frames to control the
  FARM (Frame Acceptance and Reporting Mechanism).
• Supervises the transfer of TC Frames by executing
  a FOP (Frame Operation Procedure).
• Retransmits TC Frames as required to rectify
  channel-induced errors.

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RECEIVING END SERVICE SPECIFICATION (1)

• Inputs from the layer above
• Information defining the ability of the layer
  above to accept more data.
• Inputs from the sending end of the layer
• Control Command TC Frames.
• Inputs from the layer below
• Clean octets of decoded TC data (only correct
  data).
• Indication of the start of the first valid octet
  of TC data (Data Start signal).
• Indication of the last valid octet of TC data
  (Data Stop signal).
• Control information describing the status of
  physical channel.

• Outputs to the layer above
• TC Segments which have been extracted from TC
  Frames.
• Outputs to the sending end of the layer
• CLCWs used by the FOP to control the transmission
  or retransmission of TC Frames.

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RECEIVING END SERVICE SPECIFICATION (2)

• Internal functions
• Responds to Control Command TC Frames received
  from the sending end.
• Performs the Frame Validation Check Procedure
  for all TC Frames and the Frame Acceptance and
  Reporting Mechanism for Type-A Frames.
• Creates reports (CLCWs) to the sending end
  describing the status of TC Frame acceptance.
• Processes TC Frame which have been retransmitted
  as required to rectify channel-induced errors.
• Extracts TC Segments and passes them to the layer
  above.

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COMMAND OPERATION PROCEDURE (COP)

• A COP consists of a pair of synchronised
  procedures FOP and FARM.
• COP-1 is a closed-loop TC protocol that uses
  go-back-n techniques.
• COP-1 provides two services
• Sequence-Controlled Service used for Type AD
  and BC Frames.
• Expedited Service used only for Type BD Frames.

• 1 ? WTX ? PWRX and WTX lt 256
• 2 ? WRX ? 254 always even integer
• PWRX NWRX WRX/2
• V(R) FARM counter

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TC FRAME FORMAT (1)

• (1.1) Version Number (2 bits)
• At present, only Version-1 is defined the bits
  are set to value 00.
• (1.2) Bypass Flag (1 bit)
• 0 Type-A TC Frame (normal frame acceptance
  checks of the FARM).
• 1 Type-B TC Frame (normal frame acceptance
  checks are bypassed).
• (1.3) Control Command Flag (1 bit)
• 0Data Frame (mode D) 1Control Command
  Frame (mode C).
• (1.4) Reserved Spares (2 bits)
• At present shall be set to value 00.
• (1.5) Spacecraft Identifier (10 bits)
• (1.6) Virtual Channel Identifier (6 bits)
• Provides 64 VC each VC has its own CLCW
  reporting.

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TC FRAME FORMAT (2)

• (1.7) Frame Length (10 bits)
• Contains the length C, in byte, between the
  first bit of the Frame Header and the last bit of
  the Error Control Field, expressed as C
  (nbyte - 1)

• (1.8) Frame Sequence Number N(S) (8 bits)
• It is an up-counting modulo-256 binary number
  used to check the sequentially of Type-A TC
  Frames in Type-B Frames it shall be all zeros.
• (2) Frame Data Field (variable but integer number
  of bytes)
• Length up to 1019 bytes (1017 bytes if Error
  Control Field is used).
• Contains Control Command (only UNLOCK and SET
  V(R)) or Data Unit.
• (3) Frame Error Control Field (optional 16 bits)
• Systematic binary (n,n-16) block code with
  polynomial g(x)x16x12x51
• Encoding the shift registers are initialized to
  1. (n-16) bits enter with A and B closed, and C
  open. Other 16clock with A0, B open, C closed.
• Decoding the shift registers are initialized to
  1. n bits enter (n-16) bits with B open and 16
  bits with B closed.

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ENCODER AND DECODER
Encoder
Decoder
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PERFORMANCE NOTES

• Binary Symmetric Channel (BSC) with AWGN is
  assumed.

• Frame rejection performance
• Required 10-3

• Undetected error performance
• Required 10-9

• ATED mode BSEC mode

• p 10-5 is assumed
• N 1 ? 40

• PFAPSA(1-PSA)PCA where
• PSA1-(1-p)16
• PCA1-(1-p)nN

• SEC Mode (BCH code only)
• PUE 10-10 ? 10-9

• PFBPSB(1-PSB)PCB where
• PSB1-(1-p)1616p(1-p)15
• PCB1-(1-p)nnp(1-p)n-1N

• TED Mode (BCH code only)
• PUE 10-16 ? 10-15

• p channel bit error rate
• n n of bits in Codeblock
• N n of Codeblocks into Frame

• SEC Mode (BCH CRC frame)
• PUE 10-21 ? 10-19

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CLCW FORMAT (1)

• (1.1) Control Word Type (1 bit)
• Always 0 for a CLCW.
• (1.2) CLCW Version Number (2 bits)
• At present, only Version-1 is defined the bits
  are set to value 00.
• (1.3) Status Field (3 bits)
• Free it may be used by Agencies for enhancements
  to TC operations.
• (1.4) COP In Effect (2 bits)
• At present, only COP-1 is defined the bits are
  set to value 01.
• (1.5) Virtual Channel Identifier (6 bits)
• Provides 64 VC each VC has its own CLCW
  reporting.
• (1.6) Reserved Spares (2 bits)
• At present shall be set to value 00.

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CLCW FORMAT (2)

• (2) Flags (5 bits)
• No RF Available 1 physical channel not
  available.
• No Bit Lock 1 no bit lock achieved.
• Lockout 1 FARM in lockout state (all Type-A
  Frames are rejected).
• Wait 1 unable to pass Type-A Frames to the
  Segmentation layer.
• Retransmit 1 one or more Type-A Frames, on a
  particular VC, have been rejected or found
  missing by the FARM.

• (3.1) FARM-B Counter (2 bits)
• Increments once each time a Type-B Frame is
  accepted on a VC.
• (3.2) Reserved Spare (1 bit)
• At present shall be set to value 0.
• (3.3) Report Value N(R) (8 bits)
• Contains next expected frame sequence number V(R)
  (FARM counter) used only for Type-A Frame.

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DATA ROUTING SERVICE

• SEGMENTATION LAYER

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SENDING END SERVICE SPECIFICATION

• Inputs from the layer above
• TC Packets and control instructions.
• Inputs from the layer below
• Information describing the status of transfer of
  Frame through a given VC.

• Outputs to the layer above
• Information describing the status of transfer of
  TC Packets.
• Outputs to the layer below
• TC Segments and control instructions.

• Internal functions
• Assigns individual TC Packets to particular MAPs.
• Breaks or aggregates the TC Packets into pieces
  (data field of Segment).
• Optionally labels each piece with sequence
  control and MAP id.
• Multiplexes TC Segments from different MAPs
  together onto one VC.
• Monitors the process of transferring and knows
  status and availability of each VC.

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RECEIVING END SERVICE SPECIFICATION

• Inputs from the layer above
• Information concerning the ability of the higher
  layer to accept more data.
• Inputs from the layer below
• TC Segments in sequence and complete, without
  omission or duplication.

• Outputs to the layer above
• Reconstructed TC Packets.
• Outputs to the layer below
• Information concerning the ability of this layer
  to accept more data.

• Internal functions
• Receives TC Segments from the Transfer layer,
  delivered on individual VC.
• Sorts Segments associated with individual VCs
  according to their MAP id.
• Determines when all TC Segments associated with a
  particular TC Packet have been received
  correctly.
• Extracts and reconstructs TC Packets passes them
  to the layer above.

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TC SEGMENT FORMAT

• (1.1) Sequence Flags (2 bits)
• 0 1 First Segment of TC Packet on one MAP.
• 0 0 Continuing Segment of TC Packet on one MAP.
• 1 0 Last Segment of TC Packet on one MAP.
• 1 1 No segmentation.
• (1.2) Multiplexer Access Point (MAP) Identifier
  (6 bits)
• This six-bit field enables up to 64 MAP addresses
  to be associated with each VC provided by
  Transfer layer.
• The MAP facility allows user command data from
  different sources to be multiplexed together so
  that they share the communications capacity of
  one VC (no resource monopoly).
• (2) Segment Data Field (up to 1018 bytes)
• Contains all or a portion or an aggregation of TC
  Packets.

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DATA MANAGEMENT SERVICE

• PACKETIZATION LAYER

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SENDING END SERVICE SPECIFICATION (1)

• Inputs from the layer above
• Named sets of transportable TC Application Data
  and control instructions.
• Requests for reports of the status of data and
  for security measures.
• Inputs from the receiving end of the layer
• TLM Packets describing the status of receipt of
  TC Packets.
• Inputs from the layer below
• Reports describing the status of the data routing
  process.

• Outputs to the layer above
• On-demand reports defining the transport status
  of TC Application Data.
• Outputs to the receiving end of the layer
• Control instructions defining the data reassembly
  and forwarding procedures which are to be used,
  including parameters for algorithms which
  implement optional data security measures.
• Outputs to the layer below
• TC Packets, including security encoding of this
  data and control instructions.

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SENDING END SERVICE SPECIFICATION (2)

• Internal functions
• Encapsulates named sets of TC Application Data
  into the data field of TC Packets, adding the
  header of TC Packets.
• Inserts local naming syntax into the TC Packets
  to maintains traceability.
• Encodes the Application Data as required to
  implement data security measures.
• Generates control instructions to the layer below
  and to the receiving end.
• Analyses reports from the receiving end and from
  the layer below.
• Generates on-demand reports to the layer above
  describing the status of end-to-end transport
  of named sets of telecommands.

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RECEIVING END SERVICE SPECIFICATION (1)

• Inputs from the layer above
• Information concerning the ability of the layer
  to accept more data.
• Inputs from the sending end of the layer
• Control instructions defining system conditions
  which must exist in order to reassemble and pass
  the data to the layer above, including security
  measures.
• Inputs from the layer below
• TC Packets and information describing the status
  of the data routing process.

• Outputs to the layer above
• Named sets of TC Application Data and transport
  status information relating to the correctness,
  completeness and sequentially of the TC data.

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RECEIVING END SERVICE SPECIFICATION (2)

• Outputs to the sending end of the layer
• Reports describing the status of receipt of TC
  Packets.
• Outputs to the layer below
• Reports describing the ability of the layer to
  accept more data.

• Internal functions
• Extracts TC Application Data from the data fields
  TC Packets.
• Analyses control instructions from the sending
  end.
• Passes the TC Application Data to the layer
  above.
• Formulates reports to the sending end describing
  the status of receipt and reassembly of
  particular named sets of TC Application Data.
• Formulates reports to the layer below describing
  its ability to accept more data.

38
TC PACKET FORMAT (1)

• (1.1) Version Number (3 bits)
• At present, only Version-1 is defined the bits
  are set to value 000.
• (1.2) Type (1 bit)
• 0 Telemetry Packet 1 Telecommand Packet.
• (1.3) Secondary Header Flag (1 bit)
• Absence (0) or presence (1) of a Secondary
  Header within TC Packet.
• (1.4) Application Process Identifier (11 bits)
• Identifies the individual receiving application
  process in the spacecraft.
• (1.5) Sequence Flags (2 bits)
• 0 1 First Packet of TC Application Data.
• 0 0 Continuing Packet of TC Application Data.
• 1 0 Last Packet of TC Application Data.

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TC PACKET FORMAT (2)

• 1 1 Standalone Packet.

• (1.6) Packet Name or Sequence Count (14 bits)
• If the Packet is independent of other spacecraft,
  naming consists of sequence number otherwise it
  may be internally partitioned to include the File
  Name.
• (1.7) Packet Length (2 bytes)
• Contains the length C, in byte, between the
  first bit of the Secondary Header and the last
  bit of the Packet, expressed as C (nbyte - 1)
• (2) Secondary Header (optional and variable but
  integer number of bytes)
• Provides a means for encoding any auxiliary data
  (at present not defined).
• (3) Application Data (variable)
• Users are free to adopt whatever formatting
  conventions are convenient.
• An error detection code may be included in this
  field to verify the overall integrity of the TC
  Packet.
• Secondary Header Length Application Data Length
  up to 65536 bytes.

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SECURITY MEASURES

• Data protection may be provided by two
  mechanisms
• Physical impracticable in space mission.
• Logical manipulation or interpretation data is
  extremely difficult.

• A system, with logical mechanism, may use first
  technique only or both techniques together
• Encrypted Authentication the sending end
  generates a unique authentication word
  accompanies each clear-text block that is
  transmitted. The receiving end recognises the
  authentication word by performing complementary
  decryption, and send back a status message in
  clear-text to the sending end.
• Data Encryption the command application data are
  transformed to make them unintelligible to an
  unauthorised observer. TC Application Data are
  transformed by applying special algorithms and
  can only be interpreted after processing by a
  complementary process at the receiving end.

41
DATA MANAGEMENT SERVICE

• SYSTEM MANAGEMENT LAYER

42
SENDING END SERVICE SPECIFICATION (1)

• Inputs from the layer above
• Command Directives and their delivery control
  instructions.
• Requests for reports of delivery status.
• Inputs from the receiving end of the layer
• Reports defining the status of receipt of
  commands.
• Inputs from the layer below
• Reports defining the status of end-to-end
  transport of named sets of TC.

• Outputs to the layer above
• Reports describing the delivery status of Command
  Directives.
• Outputs to the receiving end of the layer
• Control instructions defining conditions which
  must exist at time of delivery.
• Outputs to the layer below
• Named sets of TC Application Data and control
  instructions.
• Requests for the status of transport of TC
  Application Data.
• Requests to implement security measures.

43
SENDING END SERVICE SPECIFICATION (2)

• Internal functions
• Translates abstract-syntax user Command
  Directives into correspondingly transportable
  concrete-syntax TC Application Data.
• Prepares TC Application Data for transport,
  including security measures.
• Establishes the TC Sessions by requesting
  physical connection services from lower layers,
  and passing the TC Application Data to the layer
  below.

• Manages and supervises the TC Sessions (e.g., by
  permitting TC Application Data to be suspended,
  resumed, released for delivery and aborted as
  required).
• Generates control instructions to the receiving
  end.
• Analyses reports from the layer below which
  describe the status of transport of TC
  Application Data and creates appropriate recovery
  instructions.
• Formulates on-demand reports to the layer above
  which describe the delivery status of TC
  Application Data.

44
RECEIVING END SERVICE SPECIFICATION (1)

• Inputs from the layer above
• Information concerning the ability of the layer
  to accept more data.
• Inputs from the sending end of the layer
• Session control instructions.
• Inputs from the layer below
• TC Application Data and transport information
  relating to the correctness, completeness and
  sequentially of the received TC Application Data.

• Outputs to the layer above
• User Command Directives.
• Outputs to the sending end of the layer
• Reports defining the status of delivery of
  commands.
• Outputs to the layer below
• Reports defining the ability of the layer to
  accept more data.

45
RECEIVING END SERVICE SPECIFICATION (2)

• Internal functions
• Receives TC Application Data from the layer
  below.
• Receives control instructions from the sending
  end of the layer defining the conditions for
  delivery of TC Application Data to the layer
  above.
• Receives transport status information from the
  layer below and determines when the TC
  Application Data are ready for delivery to the
  layer above.
• Translates concrete-syntax TC Application Data
  back into abstract-syntax commands, if required
  by the layer above.

• Delivers user Command Directives to appropriate
  addresses within the layer above when the
  delivery conditions are satisfied.
• Formulates reports back to the sending end of the
  layer describing the delivery status of user
  commands.
• Formulates reports to the layer below describing
  its ability to accept more data.

46
DATA MANAGEMENT SERVICE

• APPLICATION PROCESS LAYER

47
SENDING END SERVICE SPECIFICATION (1)

• Inputs from the user
• Individual requests for specific command actions
  (in high-level language).
• Instructions defining the delivery and execution
  requirements.
• Inputs from the receiving end of the layer
• Reports describing command execution status.
• Inputs from the layer below
• Reports describing TC system delivery performance.

• Outputs to the user
• Mission information and reports about execution
  of specific command actions.
• Reports status delivery and naming convention
  used to label the commands.
• Outputs to the receiving end of the layer
• Control instructions defining conditions which
  must exist at time of execution.
• Outputs to the layer below
• Multi-user Command Directives and requests for
  their status of delivery.
• Control instructions which select the particular
  system configuration.

48
SENDING END SERVICE SPECIFICATION (2)

• Internal functions
• Maintains a data base of mission information.
• Displays selected information in response to
  queries from individual users.
• Iteratively responds to user requests by
  indicating system resource availability.
• Translates user requests into correspondingly
  Command Directives.

• Provides facilities for the user to suspend,
  resume or abort the transfer of Command
  Directives at any time prior to execution.
• Integrates the command directives from multiple
  users into named sets and communicates the naming
  conventions to the user.
• Organises the named sets of directive into TC
  Sessions and possibly estimates the overall
  effect of TC Sessions by simulating the response
  to commands.

• Generates the control instructions to the layer
  below.
• Generates the control instructions to the
  receiving end.
• Analyses reports from the receiving end and from
  the layer below and formulates on-demand reports
  to the user.

49
RECEIVING END SERVICE SPECIFICATION (1)

• Inputs from the spacecraft system
• Current spacecraft state, resource status, time.
• Inputs from the sending end of the layer
• Control instructions defining conditions which
  must exist in order to execute the named sets of
  Command Directives.
• Inputs from the layer below
• Named sets of Command Directives.

• Outputs to the spacecraft system
• Executed command actions which cause changes in
  spacecraft state.
• Outputs to the sending end of the layer
• Reports describing command execution status.
• Outputs to the layer below
• Reports describing ability of the layer to accept
  more data.

50
RECEIVING END SERVICE SPECIFICATION (2)

• Internal functions
• Receives named sets of executable commands.
• Analyses control instructions from the sending
  end which define the operational conditions that
  must exist at time of execution.
• Execute named sets of commands when operational
  conditions are satisfied.

• Provides the capability for the user to suspend,
  resume or abort named sets of command execution.
• Formulates reports to the sending end describing
  the status of command execution.
• Notifies the layer below its capability to accept
  more data.

51
TELECOMMAND SYSTEM
?
END
?