FIRSTLO gives the LO sum for each baseband channel for all mixes other then the one that will be set if FREQ or DOPPLER is specified. For most stations, this is for use by SCHED only; it is not written to the output file. However for stations that use VLBA control files, but are not proper VLBA stations, it will be written in a parameter that passes the information to the logging and correlator systems so that they can figure out the observing frequency. Please see FREQREF for a discussion of required parameters. If a value is not specified for a channel, it will be set to the first. Since most observations use the same firstlo for all channels, it is usually only necessary to specify one.
For the VLBA: For frequencies below 16 GHz this is just the front end synthesizer as specified by SYNTH. For 1 cm, it is the sum of synthesizers 2 and 3. For 7 mm, it is the sum of synthesizer 1 and 3 times synthesizer 3. For 4 mm, it is the sum of synthesizer 1 and 6 times synthesizer 3. At 90cm the value should be -500.0 MHz to make any frequency calculations come out right; that mix is an up-convert.
If FIRSTLO is not specified, SCHED will try to determine it from the frequency catalog. This should be the usual case.