How the Telemetry System in the Apollo 11 Spacecraft Software Transmitted Data to Earth

The telemetry system in the Apollo 11 spacecraft software is an interrupt-driven down-link subsystem that transmits selected on-board data to ground stations every 20 milliseconds via the DOWN-TELEMETRY PROGRAM.

The Apollo 11 Command Module software, preserved in the chrislgarry/Apollo-11 repository, contains a deterministic telemetry pipeline responsible for streaming critical flight data to Mission Control. This telemetry system in the Apollo 11 spacecraft software operates as a real-time down-link mechanism, packaging erasable memory contents and hardware channel states into formatted transmissions sent through dedicated spacecraft interfaces.

The DODOWNTM Interrupt Handler

At the heart of the telemetry system lies the DODOWNTM interrupt handler in Comanche055/DOWN-TELEMETRY_PROGRAM.agc. This routine triggers every 20 milliseconds when the spacecraft’s telemetry converter generates a "down-telemetry end-pulse."

DODOWNTM    TS      BANKRUPT
            EXTEND
            QXCH    QRUPT          # Save Q (interrupt flag)

            CA      BIT7           # Set word-order code = 1 (temporary)

            EXTEND
            WOR     CHAN13         # Force word-order = 0 for the new list

            TC      DNTMGOTO       # Jump to the appropriate phase

The handler saves the current interrupt state, initializes word-order code handling via CHAN13, and transfers control to the telemetry phase manager. This design ensures that data transmission resumes seamlessly even after system restarts by re-initializing the list pointer DNTMGOTO to the beginning of the current list.

Down-Link List Processing

The telemetry system uses configurable down-link lists defined in Comanche055/DOWNLINK_LISTS.agc to determine which data addresses are transmitted. Each list consists of control words followed by address descriptors (1DNADR, 2DNADR) and channel identifiers (DNCHAN).

The FETCH2WD routine retrieves data from erasable memory based on the current list pointer:

FETCH2WD    CA      DNECADR        # Load current down-link address

            TS      EBANK          # Switch to the target erasable bank

            MASK    LOW8           # Isolate low-order address bits

            TS      L
            CA      DNADRDCR      # Decrement count & ECADR

            ADS     DNECADR
            EXTEND
            INDEX   L
            EBANK=  1400
            DCA     1400          # Fetch two data words

            EBANK=  DNTMBUFF
            TCF     DNTMEXIT      # Send them out

This bank-switching mechanism allows the telemetry program to access any location in the erasable memory space, fetching double-precision words for transmission.

Writing to Telemetry Channels

Once data is fetched, the DNTMEXIT routine writes words to the hardware down-link channels. In Comanche055/DOWN-TELEMETRY_PROGRAM.agc, the system outputs to channels 34 and 35 using the WRITE instruction:

DNTMEXIT    EXTEND
            WRITE   DNTM1          # Word → Channel 34

            CA      L
            WRITE   DNTM2          # Word → Channel 35

            TCF     RESUME         # Return to caller

This pipeline transmits approximately 200 AGC words every 2 seconds (100 double-precision words), maintaining a constant data stream to ground stations throughout the mission.

Word-Order Codes and Data Formatting

The system manages word-order codes through CHAN13 to instruct ground decoders how to recombine double-precision words. These bit-fields, documented in Luminary099/INPUT_OUTPUT_CHANNEL_BIT_DESCRIPTIONS.agc, ensure that high-order and low-order portions of data words are correctly reassembled by Mission Control's receiving equipment.

Ground-Initiated Memory Dumps

The telemetry system supports on-demand erasable-memory dumps via the DNEDUMP routine, triggered by uplink verb 74 (VB74). When ground control sends this command through Comanche055/EXTENDED_VERBS.agc, the software initiates a special dump mode:

; In EXTENDED_VERBS.agc
TC  DNEDUMP    # VB74 – initialise down-telemetry program (dump)

Unlike the periodic telemetry stream, DNEDUMP transmits entire erasable banks following the same list-driven scheme but with expanded address ranges, allowing engineers to retrieve complete memory snapshots for debugging.

Restart Protection and Flags

Telemetry operations are protected against system faults through initialization logic in Comanche055/FRESH_START_AND_RESTART.agc. On fresh start or restart, the system clears telemetry flags and resets the down-link list pointer, ensuring that transmission resumes from a known state rather than corrupting mid-stream data.

Summary

• The DOWN-TELEMETRY PROGRAM operates as a 20-millisecond interrupt service routine triggered by DODOWNTM.
• Data selection occurs via configurable down-link lists that specify erasable memory addresses and hardware channels.
• Transmission occurs through channels 34 and 35, managed by the WRITE instruction in the DNTMEXIT routine.
• Word-order codes in CHAN13 ensure proper reassembly of double-precision words at ground stations.
• Ground control can initiate full memory dumps using VB74 (DNEDUMP) through the extended verbs interface.
• Restart protection in FRESH_START_AND_RESTART.agc ensures telemetry integrity after system resets.

Frequently Asked Questions

How often did the Apollo 11 telemetry system transmit data?

The system transmitted data every 20 milliseconds via the DODOWNTM interrupt, resulting in approximately 200 AGC words (100 double-precision words) being sent to ground stations every 2 seconds throughout the mission.

What hardware channels did the telemetry system use?

According to Comanche055/DOWN-TELEMETRY_PROGRAM.agc, the system wrote data to down-link channels 34 and 35 using the WRITE instruction, while channel 13 managed word-order codes for data formatting.

How was the telemetry system restarted after a system fault?

The FRESH_START_AND_RESTART.agc file contains logic that re-initializes the DNTMGOTO list pointer and clears telemetry flags on fresh start or restart, ensuring the DOWN-TELEMETRY PROGRAM resumes from the beginning of the current down-link list rather than continuing with corrupted state.

What is the difference between regular telemetry and DNEDUMP?

Regular telemetry periodically transmits selected data words based on the active down-link list, while DNEDUMP (activated by uplink verb 74) initiates a complete dump of specified erasable memory banks, transmitting entire memory regions rather than curated telemetry samples.

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