Abstract: A viewport for a welder's protective mask or the like, with a semitransparent window arranged to be opened by mandibular action of the user for inspection of a newly set weld, for example, while leaving both hands free to hold or manipulate other equipment.

Abstract: A virtual secondary surveillance radar (SSR) is provided at any desired location within the coverage area of an actual SSR by receiving the interrogations from the actual SSR and the transponder replies thereto. The received signals are processed to yield information as to the positions of the transponders with respect to the desired location, simulating the performance of an actual SSR at that location.

Abstract: A variable frequency voltage controlled oscillator is tuned to a selected frequency by applying a varying voltage to the control port of the oscillator to sweep the oscillator signal through a frequency range which includes the frequency of a reference frequency source. The value of the varying voltage is stored at the instant the oscillator frequency coincides with the frequency of the reference source. A dc offset voltage is then added to the stored voltage to produce a sum voltage which is substituted for the varying voltage at the control port of the oscillator to tune the oscillator to the selected frequency. The offset voltage is equal to the difference in the control voltages at the selected and reference frequencies and may be determined from the oscillator control voltage characteristic.