What indication is not valid while transmitting on VHF #1?

When considering the limitations and functioning of VHF communications in aircraft, the standby magnetic compass is not a valid indication transmitted over VHF #1. VHF radios are primarily designed for voice communication and data transmission related to air traffic control and other airborne communications.

The standby magnetic compass, however, serves a distinct purpose in navigation. Its primary function is to give pilots a reliable indication of heading, independent of electrical systems, and it is not utilized for communication. In contrast, indicators such as the turn indicator, altitude indicator, and airspeed indicator, which reflect critical operational data for flight safety and navigation, could potentially be integrated into systems that provide data for display or transmission but are not directly related to the function of VHF communication.

Therefore, the standby magnetic compass does not serve a role in VHF #1 transmissions and is not valid for use in that context.