The Command Subsystem

On this page are discussed:

1. The geographic network of the Command Subsystem
2. The Command Subsystem's principle of operation, which includes the type of commands which could be sent
3. Functional overview of the Command Subsystem
4. Overview of the command loads and the real-time commands of the Apollo 11 mission, which includes some explanation about their various purposes.
5. The command initiation procedures and the employed command panels of the control consoles
   

To me, the Command Subsystem, which enabled the mission controllers to control the spacecraft remotely, is one of the most intriguing parts of the Apollo mission control.

From the Mission Control Center (MCC) commands could be sent to the spacecrafts via a network of communication stations. The Goddard Space Flight Center (GSFC) was the hub of this network.

The network was actually comprised of the Manned Space Flight Network (MSFN) and the Deep Space Network (DSN). The MSFN consisted of a network of eleven land-based stations and four ship-based stations. The DSN consisted of three land-based stations; each station had two antenna dishes of 85 ft and 210 ft diameter, respectively. The maximum antenna diameter of an MSFN station was 29 ft. The MSFN could be used for distances up to 60 000 km and was developed to support Earth orbital missions. To conduct moon missions, distances up to 400 000 km had to be covered. So when the distances to the Apollo spacecrafts exceeded 60 000 km, tracking and communication were provided by the DSN stations MAD, HSK and GDS.

The locations of the ship-based stations were determined by the mission-specific tracking requirements in the four critical mission phases near Earth. The purpose of station VAN was to determine trajectory parameters with the required accuracy at the moment of Earth Orbit Injection at the end of the launch phase. The purpose of the stations MER and RED was to determine the state vector (position, velocity and acceleration) of the spacecraft during and just after Trans Lunar Injection. The mission of station HUN was to determine state vectors in the reentry phase.

The hub of the MSFN and the Mission Control Center are marked in red.
The three stations of the Deep Space Network with their 85-foot dish antenna are marked in red lettering.
All stations, except for station HUN, are equipped with Unified S-band (USB) equipment. It was a relatively new technology to use a single carrier frequency in the 2-4 GHz band for voice, TV and data communication and for tracking (range and velocity). The range of the transmit band was between 2.090 and 2.120 GHz and the receive band between 2.270 and 2.300 GHz. To provide communication and tracking on a single carrier signal with a set of subcarrier signals, a bandwidth of 4 MHz was used. Therefore, by using two carrier signals 5 MHz apart, tracking and communications could be provided for two spacecrafts simultaneously.

Diagram based on descriptions in ref.1.

In this diagram is shown that:

1. The command chain consisted of four components: the MCC, GSFC, the remote site and the spacecraft.
2. that the MCC could send three types of command data and that, of the command data type, the execute command request was subdivided into two types:
   1. requests to uplink command loads
   2. requests to uplink real-time command (RTCs)

In this diagram is indicated which controllers had command panels on their consoles to initiate uplinks of command loads or real-time commands (RTCs) to the spacecraft:

• MOCR controllers: BOOSTER, GUIDO, CONTROL and INCO
• CCATS controllers: Command Load Controller and Computer Command Controller
• CCATS controllers in a supporting role: Real-Time Controller and Telemetry Instrumentation Controller
  Their role was related to the formatting and routing of the command data messages.
• RTCC controllers: RTCC Network & Command Process Controller ?

Personal reminder 2
I need to check whether and how the RTCC Network & Command Process Controller could initiate uplinks. The primary responsibility was to generate command loads on request of a MOCR controller. The console of this RTCC controller was equipped with panels, which seems to suggest that uplinks could be initiated.

In this diagram is shown which MOCR controllers were involved in generating command loads and which controllers were required to request uplinks of command loads and RTCs.

Load generation requests
According to ref.11 on page 9-6 the controllers BOOSTER, FIDO, GUIDO and RETRO were required to request the generation of command loads. According to the voice loop configuration as depicted above, the voice loops "RTCC DYN", "REENTRY SUPPORT" and "RTCC CMD NET" were used by these four MOCR controllers to request a so-called command load generation. These three voice loops were monitored by the RTCC Computer Command Controller, who was responsible for generating the command loads.
When they have gone through the generation process, the command loads were transferred to various remote sites, if not to all, on request of the MOCR controller.

Uplink requests
The consoles of the controllers INCO, CONTROL, BOOSTER and GUIDO were equipped with command panels to send requests via CCATS to a remote site to uplink a command load or an RTC to the spacecraft.
It is remarkable, however, that the controllers FIDO and RETRO were required to request a command load generation but that their consoles were not equipped with command panels to request an uplink.
Note 1
I assume that FIDO and RETRO had several options to have an uplink requested:

1. Asking their colleague controllers GUIDO and BOOSTER to request an uplink using their command panels.
2. Based on ref.2 figure 1, the CCATS Command Controller (CCC) could dispatch an uplink request on behalf of a MOCR Controller.
3. Asking the RTCC Computer Command Controller to request an uplink. I assume the RTCC CCC was using the teletype to issue the request via CCATS. I also assume that the RTCC CCC could use voice loop "RTCC CMD NET" to pass the request on to the CCC.

Note 4
Another option for requesting uplinks would have been asking support from the CCATS controllers. I assume that the system engineers were using voice loop "RTC" to issue an upload request. The name "RTC" refers to the fact that the CCATS Real-Time Controller (RTC) was central in this voice loop. The RTC established the command data channel by selecting the console of the applicant and the remote station from which the RTC must be uplinked. I therefore assume that the voice loop "RTC" was monitored by the CCC to be able to act promptly and request an uplink on behalf of the MOCR system engineer. I also assume that this voice loop was monitored by the CCATS Command Load Controller and the RTCC CCC.

1. command loads
   to control the guidance computers LVDC, AGC and LGC.
2. real-time commands (RTCs)
   to control systems and subsystems of the launch vehicle and the spacecrafts from the ground

Based on ref.4, section 3.3.5, tabel III.

106 Different command loads have been distributed; 16 of them were distributed prior to lift-off, and 90 load commands have been distributed during the mission. The content of the load commands depended on the mission circumstances, such as trajectory parameters, for example. So to create a command load the most recent data had to be fed into this creation process. In the bottom table, it can be seen that 58 load commands have been uplinked.

Note 1
These figures might illustrate that it was probably policy to have command loads created in advance to anticipate possible requests during the validity periods of the command loads.
However, command loads could also be created on the spot to have them uplinked as soon as possible. In the list above a distinction between these in situ command loads and the normal command loads has not been made, leaving us clueless about to what extent MCC still had to react to unforeseen events.

Note 2
I have also not been able to find any substantive information about the command loads and the rationale behind the number methodology. It would be interesting to know what kind of command loads were uplinked or were kept ready in the various mission phases, their purpose and for which guidance computers they were intended. I assume that all the pre-mission command loads were intended for the LVDC, and probably most of them were intended for contingencies like the various abort modes and forced staging. The "T-30" command load might have been depending on the latest flight parameters (flight azimuth in relation to expected time of launch, ascent profile optimization in relation to wind conditions ?)

In this overview is shown how many Real-Time Commands (RTCs) were uplinked. By far the most RTCs were uplinked via the three stations of the Deep Space Network (HSK, GDS and MAD).

The global timeline of the Apollo mission provides some context in which the RTCs have been uplinked over the course of nine days.

Personal note
The system configuration of the Saturn Launch vehicle and the spacecrafts could differ per mission. Therefore the RTC inventory might differ per mission. These RTCs were used to flip a particular valve or switch on/off a system component, such as an onboard tape recorder, for example. So the RTC inventory was set when the system configuration was set. The RSCC could be provided with all the RTCs well in advance.
However, I have not been able to find any listing of RTCs containing information about the RTC numbering and their purpose.

Personal reminder
In the overview 17 RTCs seemed to have been uplinked from HAW on 19 July 1969. That surprises me. First, at that date the S-IVB stage, the CSM and the LM were all in the vicinity of the Moon, well beyond the range of the HAW station, so I thought. I need to check on its range. And second, the three DSN stations MAD, HSK and GDS together could provide 100% coverage with regard to communication and tracking for space vehicles beyond 30 000 km from Earth, rendering all other stations redundant.

This pictorial description of how an uplink of a command load was requested is based on descriptions in ref. 1, 4, 11, 12 and 14.

The command panels.
Panel 1: the Control Mode Module (CMM)
Panel 2: the Digital Select Matrix (DSM)
Panel 3: the Digital Readout Module (DRM)

The RTC PBI panels, which enabled the flight controller to uplink a specific RTC with the push of a button, are not depicted in this figure.

In Figure 4 it is shown that the command panels were connected to an encoder to convert a key entry into a digital code.

1. Familiarization Manual
   Mission Control Center Houston
   PHO-FAM001, 30 June 1965
   by Western Development Laboratories, Houston Operations

2. AS-508; MCC / MSFN; Mission Configuration / System Description
   March 1970
   by the Manned Spacecraft Center, Flight Support Division

3. Mission Operational Configuration
   Mission J1, AS-510 / SC122 / LM10, Apollo 15
   PHO - TR155, 15 April 1971
   Manned Spacecraft Center, Houston

4. Network Controller's Mission Report Apollo 11
   Prepared by Flight Support Division
   Manned Spacecraft Center, Houston
   15 August 1969

5. Mission Operation Report
   Apollo 11 (AS-506) Mission
   Report No. M-932-69-11, 24 June 1969
   Prepared by Apollo Program Office
   Office of Manned Space Flight

6. Apollo C-Band Radar Tracking Capability
   Apollo 11 (AS-506) Mission
   by P.R. Schmid
   Goddard Space Flight Center, Greenbelt, Maryland
   15 September 1967

7. Ground Tracking of The Apollo
   Apollo 11 (AS-506) Mission
   by Friedrich O. Vonbun
   Goddard Space Flight Center, Greenbelt, Maryland
   1 January 1965

8. Apollo Digital Command System
   by C.B. Cox
   Goddard Space Flight Center, Greenbelt, Maryland
   Proceedings of the Apollo Unified S-Band Technical Conference
   July 14-15, 1965

9. Apollo Remote Site Display System
   by G.N. Georgeadis
   Goddard Space Flight Center, Greenbelt, Maryland
   Proceedings of the Apollo Unified S-Band Technical Conference
   July 14-15, 1965

10. Command and Communication System
    by B. Reed
    Marshall Space Flight Center, Huntsville, Alabama
    Proceedings of the Apollo Unified S-Band Technical Conference
    July 14-15, 1965

11. Saturn V Flight Manual SA-506 (Apollo 11)
    George C. Marshall Space Flight Center
    MSFC-MAN-506, 25 February 1969
    page 9-5 and 9-6

12. Project Apollo 500 RTCC Operations Support Plan For Mission G
    MSC Internal Note No. 69-FS-2
    Flight Support Division, Flight Software Branch
    Manned Spacecraft Center, Houston, April 1969

13. Apollo 13 mission, EECOM console layout
    https://history.nasa.gov/afj/ap13fj/pdf-hr/a13-eecom-console.pdf
    Prepared by Andy Anderson
    From the Apollo 13 Mission Documents section of

    The Apollo 13 Flight Journal
    by David Woods, Johannes Kemppanen, Alexander Turhanov and Lennox J. Waugh
    NASA History Division

14. Universal CSM Console Handbook
    S/C 106 and subsequent vehicles
    Section D5
    Flight Control Division
    Manned Spacecraft Center, Houston, May 1969