Martin-Baltimore submitted its initial proposal for the redundant flight control and hydraulic subsystems for the Gemini launch vehicle; on March 1, Martin was authorized to proceed with study and design work. The major change in the flight control system from Titan II missile to Gemini launch vehicle was substitution of the General Electric Mod IIIG radio guidance system (RGS) and Titan I three-axis reference system for the Titan II inertial guidance system. Air Force Space Systems Division issued a letter contract to General Electric Company, Syracuse, New York, for the RGS on June 27. Technical liaison, computer programs, and ground-based computer operation and maintenance were contracted to Burroughs Corporation, Paoli, Pennsylvania, on July 3.

Advanced Technology Laboratories, Inc, Mountain View, California, received a $3.2 million subcontract from McDonnell to provide the horizon sensor system for the Gemini spacecraft. Two horizon sensors, one primary and one standby, were part of the spacecraft's guidance and control system. They scanned, detected, and tracked the infrared radiation gradient between Earth and space (Earth's infrared horizon) to provide reference signals for aligning the inertial platform and error signals to the attitude control and maneuver electronics for controlling the spacecraft's attitude and its pitch and roll axes.

McDonnell awarded a $4.475 million subcontract to the Western Military Division of Motorola, Inc, Scottsdale, Airzona, to design and build the digital command system (DCS) for the Gemini spacecraft. Consisting of a receiver/decoder package and three relay packages, the DCS received digital commands transmitted from ground stations, decoded them and transferred them to the appropriate spacecraft systems. Commands were of two types: real-time commands to control various spacecraft functions and stored program commands to provide data updating the time reference system and the digital computer.

The St. Petersburg, Florida, Aeronautical Division of Minneapolis-Honeywell received an $18 million subcontract from McDonnell to provide the inertial measuring unit (IMU) for the Gemini spacecraft. The IMU was a stabilized inertial platform including an electronic unit and a power supply. Its primary functions were to provide a stable reference for determining spacecraft attitude and to indicate changes in spacecraft velocity.

McDonnell awarded a $26.6 million subcontract to International Business Machines (IBM) Corporation's Space Guidance Center, Owego, New York, to provide the computer system for the Gemini spacecraft. The digital computer was the heart of the spacecraft's guidance and control system; supplementary equipment consisted of the incremental velocity indicator (which visually displayed changes in spacecraft velocity), the manual data insertion unit (for inserting data into, and displaying readouts from, the computer), and the auxiliary computer power unit (to maintain stable computer input voltages). In addition to providing the computer and its associated equipment, IBM was also responsible for integrating the computer with the systems and components it connected with electrically, including the inertial platform, rendezvous radar, time reference system, digital command system, data acquisition system, attitude control and maneuver electronics, the launch vehicle autopilot, console controls and displays, and aerospace ground equipment.

In the opinion of Flight Operations Division's Project Gemini working group: 'One of the biggest problem areas seems to be the on-board computer; exactly what is it going to do; what is its sequence of operation; what does it need from the ground computer complex and how often; exactly how is it used by astronauts; what is the job of the on-board computer for early missions?'

George M Low, Director of Spacecraft and Flight Missions, Office of Manned Space Flight, explained to the House Subcommittee on Manned Space Flight why eight rendezvous missions were planned. 'In developing the rendezvous capability, we must study a number of different possible ways of conducting the rendezvous ..... For example, we can conduct a rendezvous maneuver in Gemini by purely visual or optical means. In this case there will be a flashing light on the target vehicle. The pilot in the spacecraft will look out of his window and he will rendezvous and fly the spacecraft toward the flashing light and perform the docking. This is one extreme of a purely manual system. On the opposite end of the spectrum we have a purely automatic system in which we have a radar, computer, and stabilized platform and, from about 200 or 500 miles out, the spacecraft and the target vehicle can lock on to each other by radar and all maneuvers take place automatically from that point on. We know from our studies on the ground and our simulations that the automatic way is probably the most efficient way of doing it. We would need the least amount of fuel to do it automatically. On the other hand it is also the most complex way. We need more equipment, and more equipment can fail this maneuver so it might not be the most reliable way. The completely visual method is least efficient as far as propellants are concerned, but perhaps the simplest. In between there are many possible combinations of these things. For example, we could use a radar for determining the distance and the relative velocity between the two without determining the relative angle between the two spacecraft and let the man himself determine the relative angle. We feel we must get actual experience in space flight of a number of these possibilities before we can perform the lunar orbit rendezvous for Apollo.'

The first engineering prototype of the onboard computer completed integration testing with the inertial platform at International Business Machines Corporation (IBM) and was delivered to McDonnell. At McDonnell, the computer underwent further tests. Some trouble developed during the initial test, but IBM technicians corrected the condition and the computer successfully passed diagnostic test checks.

The first engineering prototype inertial guidance system underwent integration and compatibility testing with a complete guidance and control system at McDonnell. All spacecraft wiring was found to be compatible with the computer, and the component operated with complete accuracy.

McDonnell warned Gemini Project Office that the capacity of the spacecraft computer was in danger of being exceeded. The original function of the computer had been limited to providing rendezvous and reentry guidance. Other functions were subsequently added, and the computer's spare capacity no longer appeared adequate to handle all of them. McDonnell requested an immediate review of computer requirements. In the meantime, it advised International Business Machines to delete one of the added functions, orbital navigation, from computers for spacecraft Nos. 2 and 3.

The first production version of the inertial guidance system developed for Gemini was delivered to McDonnell. Special tests on the configuration test unit, using spacecraft No. 2 guidance and control equipment, were expected to be completed in January 1964.

The spacecraft computer formal qualification unit completed Predelivery Acceptance Tests (PDA) and was delivered to McDonnell. The flight unit for spacecraft No. 2 was delivered during the first week in May. Later in the month, a complete inrtial guidance system formal integration PDA was completed on spacecraft No. 2 (May 22). The spacecraft No. 3 flight unit completed PDA on June 6.

The Electrical Interface Integrated Validation, confirming compatibility between launch vehicle and spacecraft and checking out redundant circuits connecting the interface, was completed November 9. This was followed by the Joint Guidance and Control Test, completed Novenber 12, which established proper functioning of the secondary guidance system, comprising the spacecraft inertial guidance system and the launch vehicle's secondary flight control system.

A spacecraft computer malfunction caused a hold of the countdown 10 minutes before the scheduled launch of Gemini-Titan 5. While the problem was being investigated, thunderstorms approached the Cape Kennedy area. With the computer problem unresolved and the weather deteriorating rapidly, the mission was scrubbed and rescheduled for August 21. Recycling began with unloading propellants.