Electronic Design

Tiros III Carries New Solid-State Timer

An all-solid-state timer on the newly orbited Tiros III weather satellite represents a significant electronic advance, according to its designers at RCA. Tiros III is making possible studies of the formation of tropical storms over the Caribbean, Atlantic, and Far East waters.

Its completely solid-state timing system, using all-silicon components, is probably the most significant design advance in the satellite's payload. The timing system on earlier Tiros vehicles included mechanical sequencing and stepping switches, and germanium components.

Serious problems are encountered with mechanical components in space environments, according to Walter Morgan, engineer in charge of the Tiros reliability program for RCA, Astro-Electronics Div., Princeton, N.J. Mechanical switches must operate dry in the vacuum environment of space, since any oil on them is sucked away. Also, dust that might settle on contacts cannot be removed. These problems can lead to sticky contacts or cam wheels. All-solid-state circuitry not only eliminates these difficulties but also gives significant savings in power dissipation.

The new system is built around an Incremag clock made by General Time Corp., Stamford, Conn., as was the previous mechanical version. The clock uses a high-precision quartz crystal oscillator (drift within 0.005% over −25° to +85°C) operating at about 18 kc to provide signals for synchronism and timing of events in the satellite.

As an example of power savings achieved, the mechanical system required 2-amp pulses of about 100-msec duration to stepping-switch solenoids every 2 sec. The new system substitutes 100-ma pulses of 10-µsec duration every 2 sec to provide a similar function.

Further power savings (about 40 to 1) were accomplished by using pnp-npn complementary flip-flops. Use of transistors, rather than diodes, in 2- and 3-input AND gates also significantly cuts power dissipation. (Electronic Design, August 2, 1961, p. 12)

This timer appears to be an early predecessor of today's ubiquitous digital timer devices. (Tiros stands for television infrared observation satellite.) Launched on July 12, 1961, Tiros III had an operational life of 230 days. Since the launch of Tiros I in 1958, more than 20 Tiros satellites have been put into orbit. Four of them are in service now, according to the NASA Web site: www.earth.nasa.gov/history/tiros/tiros3.html. The photo shows RCA engineer Glenn Corrington checking instruments for the Tiros III payload.

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