(i) Swept Frequency Mode - 3.3 s of fixed-frequency operation (at 0.12, 0.48, 1.0, 1.95, 4.0, or 9.303 MHz) followed by a quasi-logarithmic sweep from 0.1 to either 10 (normal) or 20 MHz (extended). (Note: during the swept-frequency operation, frequency markers appear as maximum- amplitude signals extending for at least one complete listening period between sounder pulses at the following frequencies on ISIS 1 & 2: 0.1, 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 12.0, 14.0, 16.0, 18.0 and 20.0 MHz. Note: it is possible for one or more of these frequency markers to be missing from the digitial ionogram display due to limitations in screen resolution.)
(ii) D Mode - transmitter ON/OFF for alternate frame pairs operating at frequencies as in (i) above.
(iii) G Mode - transmission of alternate frames of swept and fixed frequencies, as selected in (i) above.
(iv) Mixed Mode - transmission of one of the fixed frequencies while the sounder receiver is sweeping.
(v) Alternate VLF/Sounder Mode - the VLF only can be selected to replace the fixed -frequency sounding.
(vi) AIT Mode - allows for the automatic operation of the sounder system once every three minutes in either swept-frequency mode or mixed mode.
(Note: in the digital ionogram header records, no reference is made to (v) above because we were not aware of any evidence that the mode was ever put into operation and no flag pertaining to this mode could be identified in the pcm data.)