Fig. 4-1

Guidance Measurement Devices

Navigation Hardware

Star Tracker Star Tracker Diagram

Uses line of sight between stars of known position to compute the orbiter exact inertial attitude.  It is then used to align and calibrate the IMU's

IMU's (Inertial Measurement Units)

Senses the vehicle's attitude in inertial space   Measures changes in angle of travel relative to the shuttle's flight path.  Located in avionics bays 1 and 2 and are cooled by the fans located in bay 1.  Requires 5 minute warm up period.

Flight Control Hardware

AA's (Accelerometer Assembly)

Measures linear accelerations along the three primary axis.  Detects changes in the Yaw, Pitch or Roll angles.  Located in avionics bays 1 and 2 and are air cooled same as the IMU's.  Requires 5 minute warm up period.

RGA's (Rate Gyro Assembly)

Measures rate of vehicle attitude change. Detects linear motion along any of the three primary X, Y or Z axis.  Mounted on cold plates located on the aft bulkhead, near the aft avionics bays, below the payload bay floor.  Requires 5 minute warm up period.

 Fig. 4-2

Forward Avionics Equipment

GPC's

All of the General Purpose Computers (GPC's) are located in the mid deck area.  GPC's 1 and 4 are located in Avionics Bay 1.  GPC's 2 and 5 are in Avionics Bay 2 and GPC 3 is located in Avionics Bay 3A.

Two typical GPC's from the shuttle program are shown above.

MTU

The GPC's require a constant steady source of time provided by the Master Timing Unit (MTU).  The MTU is in avionics bay 3B.

MMU's

Before programs are run on the GPC's they are stored in Mass Memory Units (MMU's).  MMU 1 is in Avionics Bay 1 and MMU 2 is in Bay 2.  The MMU's closely resemble the GPC's in size and shape.

PCMMU's

The Pulse Code Modulation Master Unit (PCMMU) gathers the different data representing the shuttles current flight performance for transmission to Mission Control.  PCMMU 1 is in Avionics Bay 1 and PCMMU 2 is in Avionics Bay 2.

MDM's

Multiplexer/Demultiplexer units (MDM's) interface between the GPC's and various hardware systems.  They convert the raw data from sensors into a digital signal the computers can understand and also convert commands from the computers into an analog signal required by some hardware onboard the shuttle.  There are (20) MDM's aboard the orbiter, (13) of the units connect directly to the GPC's and the Data Processing System (DPS).  The additional (7) units send data to the Pulse Code Modulation Master Units (PCMMU's) and are called Operational Instrumentation MDM's or (OIMDM's).  The MDM's are slightly smaller than the MMU's and GPC's.  MDM's are referred to by the following name designations,

• Flight Critical DPS Units:

  • Flight Critical Forward (FF1 - 4)

  • Flight Critical Aft (FA1 - 4)

  • Payload (PL1 & 2)

  • Launch Forward (LF1)

  • Launch Mid (LM1)

  • Launch Aft (LA1)

• Operational Instrumentation Units:

  • Operational Forward (OF1 - 4)

  • Operational Aft (OA1 - 3)

FF1 - 4, PL1 & 2, LF1 and LM1 are located in the forward avionics bays.  FA1 - 4 and LA1 are located in the aft avionics bays.

Computer Programs Run On The GPC's

OPS 1

OPS 101 - Prelaunch from below T-20 minutes

OPS 102 - First stage ascent

OPS 103 - Second stage ascent

OPS 104 - OMS 1

OPS 105 - OMS 2

OPS 106 - Post burn Coast

OPS 2

OPS 201 - Orbit Operations

OPS 202 - On-orbit Burns (firing of the OMS engines)

OPS 3

OPS 301 - Deorbit burn preparation

OPS 302 - De-orbit burn

OPS 303 - De-orbit burn coast

OPS 304 - Entry from entry interface

OPS 305 - TAEM (Terminal Area Energy Management)

OPS 6

OPS 601 - Powered flight RTLS

OPS 602 - RTLS entry (Alpha recovery, pullout phase)

OPS 603 - RTLS entry (TAEM to landing)

OPS 8

OPS 801 - In flight checkout  

Fig. 4-3

Fig. 4-4

State Vector and Altitude

The shuttle's absolute position in space relative to a point on the ground or some other mission dependent coordinate  is known as its "State Vector".  Initially the State Vector can be determined by calibrating the Star Tracker assembly while in orbit.  Once this is established the Star Tracker data is used to calibrate the IMU's

The IMU's compute and save the shuttle's current inertial attitude which is sent to the GPC's where the required Aero Torque to reestablish the shuttle's flight path is then computed based on any shuttle movements that stray off course.  The Star Tracker unit acts as a check and backup to the IMU's during flight.  The State Vector includes the current altitude and the IMU's should keep track of the altitude as the flight progresses.  The shuttle's navigation system also employs a software sensor called "Drag Altitude" which uses an atmospheric model to compute the altitude based on the drag acceleration measured by the IMU's.

There are other methods for checking the shuttle's current altitude during the descent flight phase.  The TACAN system can be used after the shuttle descends to 156,000 feet.  The TACAN uses radio waves to triangulate the shuttles position.  This system relies on direct contact with a TACAN or VORTAC station on the ground and is not longer useful after the altitude reaches 1500 feet.  The shuttle has 2 radar altimeters which operate between 0 and 5000 feet.  The Air Data system onboard the shuttle utilizes Air Data Transducer Assemblies ADTA's which provide data on Angle of Attack (AOA), Mach No., True Air Speed (TAS) and the barometric altitude.  However, the air data system may only be used after the velocity is reduced to Mach 5.

More detailed information for this section can be found in the documents

1. SubSys_Guidance_Navigation.pdf

2. SubSys_Data_Processing.pdf

Fig. 4-5

Fig. 4-6

More detailed information for this section can be found in the documents

1. SubSys_Communications.pdf

Fig. 4-7

To the left are the Lightweight Headset Unit and the Headset Interface Unit that the STS-107 crew would have worn to communicate with each other as well as with Mission Control.

The following communications systems diagrams of the S-Band PM and FM systems are part of an ongoing project to determine why the Columbia was able to transmit at least some data during reentry but no voice or video.  The diagrams in Fig. 4-16 and 4-17 are of the Closed Circuit Television (CCTV) system and how it transmits through the orbiters communications systems.

Fig. 4-16

Fig. 4-17