What is the purpose of the internal receivers on an Inertial+?
Imagine you're using an Inertial+ with an external receiver and the internal primary receiver (secondary antenna not connected). The external receiver gives the system position, velocity, heading and all of the other measurements a GNSS receiver normally would. The internal receiver is used purely for GPS time—which is used to synchronise the inertial measurements with the external receiver data. When using the system in this way, it is possible to just "slap" internal receiver's antenna on the vehicle without measuring its position relative to the INS, because all that is required from that antenna is time. However, we still advised people to put the internal receiver antenna in a specific location and measure out the lever arms. That way, if there is a problem with the external receiver you can post-process the RD file using the internal receiver (instead of the external receiver) to give you position, velocity etc. and you have the correct antenna lever arms.
What is the purpose of the secondary internal receiver on an Inertial+2?
With an Inertial+2, you have the option to use a secondary internal receiver to give you improved heading accuracy at low speeds. For this to work, you measure the lever arms to your external receiver (as normal), but you must rigidly mount the primary and secondary internal receiver antennas to the vehicle. When asked in NAVconfig to enter the antenna separation, it is the distance between these two antennas that you use. The system now actively uses both the primary and secondary antennas to calculate an accurate heading.
In simplified terms, if you configure the Inertial+2 to use a dual antenna setup, then the internal receivers need position information to improve the heading. If they don't have position information, then the heading from the external receiver is used. If you don't configure the dual antenna setup, then the external receiver heading is again used.