Microwave landing systems (MLS) facilitate aircraft approach and landing operations. These systems include so-called precision distance measuring equipment (DME/P), which provides aircraft distance information by measuring total round-trip time between pulse interrogations from an airborne transmitter and replies from a ground transponder. This type of system provides high accuracy ranging information in the severe multipath environment encountered during landing operations.
In operation, the ground transponder in the DME/P generates a time delay upon receipt of an interrogation pulse. After a fixed time, a reply pulse is transmitted by the transponder through use of an RF pulse transmitter. To provide accurate aircraft distance information over the closely-spaced channels in the system (usually located 1 MHz apart), the RF pulse transmitter in the transponder must provide a carefully shaped reply pulse to help conserve the available spectrum.
Typically, the reply pulse has a "cos/cos.sup.2 " envelope with a pulse rise time of 1 microsecond and a linear partial rise time of 250 nanoseconds. The pulse is generated by an RF modulator under the control of a modulation drive voltage. The RF modulator, however, normally produces incidental phase modulation (IPM) during the partial rise time of the pulse. Such modulation causes the upper frequency spectral components of the pulse to increase in amplitude, producing undesirable adjacent channel interference.
There is therefore a need for an improved RF pulse transmitter for use in precision distance measuring equipment, or in any other pulse transmitter where spectrum must be conserved, which includes incidental phase modulation (IPM) correction.