SPC03 - Explorer 18 - Digital Data Processor - The First ICs in Space


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Explorer 18, also called IMP-A (Interplanetary Monitoring Platform), was launched on November 26, 1963, as part of NASA's Explorer program. This satellite was designed to study space beyond Earth's atmosphere, focusing on the interplanetary medium, magnetic fields, and cosmic rays. It spun at 22 rotations per minute (RPM) and had several scientific instruments, including a Digital Data Processor (DDP). This processor used integrated circuits, making Explorer 18 one of the first spacecraft to do so. The DDP managed experiments, stored results, and sent data back to Earth using pulse-frequency modulation signals.

Explorer 18's DDP telemetry system was the first operational use of integrated circuits in a spacecraft, even before the Minuteman II missile's D-37C computer and the Apollo Guidance Computer. The DDP stored data from different experiments into accumulators, which periodically sent the data as pulse-frequency modulation (PFM) signals to ground stations. The accumulators could hold a total of 105 bits of data, plus a 15-bit clock. More than half of the transmission cycle was used to send analog signals across 9 of the 16 frames.

The processor used Series 51 chips from Texas Instruments (TI), including the SN510 (a clocked SR latch) and the SN514 (dual 3-input NAND/NOR gates), which were in 10-pin flatpack packages and built using resistor-capacitor-transistor logic (RCTL). The artwork features a Texas Instruments SN514, the same type used in Explorer 18.