Question: In my application I send the RTCA message format (RTCAREF and RTCAOBS) as differential corrections to two rovers (RT-2 and RT-20) in motion. Is it necessary in RTK mode to send the RTCA1 message type?
Answer: No, it is not necessary to send the RTCA1 message type when operating in RTK mode; sending RTCA1 is purely optional. RTCA1 contains the pseudodifferential corrections, which provide "sub-meter" positions at the remote rover. Some users prefer to send RTCA1 in addition to RTCAREF and RTCAOBS so that if the RTK performance is compromised for some reason, the remote rover position type will revert to pseudoDGPS instead of single point. Possible scenarios would include carrier phase cycle slips (under occasional foliage, travelling under overpasses, etc), high multipath environments (urban canyon, etc), or cases when there are only four or five matched satellites between the base and rover, which would form a condition where pseudoDGPS would be possible, but RTK would not (RTK requires at least five, preferably six or more matched satellites). Broadcasting RTCA1 provides another level of possible accuracy between RTK and single point, should strange things happen to the RTK solution.