54 OEM4 Family Installation and Operation User Manual Rev 12Chapter 5 Message FormatsThe chapter discusses the various industry-standard message formats that can be used with yourNovAtel OEM4 family receiver, including RTCA, RTCM, CMR, and NMEA. See Section4.3,Transmitting and Receiving Corrections on Page 46 for more information on using these messageformats for differential operation.5.1 RTCA-Format MessagesThe RTCA (Radio Technical Commission for Aviation Services) Standard is being designed tosupport Differential Global Navigation Satellite System (DGNSS) Special Category I (SCAT-I)precision instrument approaches. The RTCA Standard is in a preliminary state. Described below isNovAtel’s current support for this standard. It is based on “Minimum Aviation System PerformanceStandards DGNSS Instrument Approach System: Special Category I (SCAT-I)” dated August 27,1993 (RTCA/DO-217).NovAtel has defined three proprietary RTCA Standard Type 71 binary-format messages, RTCAOBS,RTCAREF and RTCAEPHEM for base station transmissions. These can be used with either single ordual-frequency NovAtel receivers. The RTCA message format outperforms the RTCM format in thefollowing ways, among others:• a more efficient data structure (lower overhead)• better error detection• allowance for a longer message, if necessaryRTCAREF and RTCAOBS, respectively, correspond to the RTCM Type 3 and Type 59 logs used insingle-frequency-only measurements. Both are NovAtel-proprietary RTCA Standard Type 7 messageswith an ‘N’ primary sub-label.5.1.1 RTCA1This log enables transmission of RTCA Standard format Type 1 messages from the receiver whenoperating as a base station. Before this message can be transmitted, the receiver FIX POSITIONcommand must be set. The RTCA log will be accepted by a receiver operating as a rover station overa COM port after an INTERFACEMODE port RTCA_INTERFACE command is issued.The RTCA Standard for SCAT-I stipulates that the maximum age of differential correction (Type 1)messages accepted by the rover station cannot be greater than 22 seconds. Refer to theDGPSTIMEOUT command in Volume 2 of this manual for information regarding DGPS delaysettings.1.For further information on RTCA Standard messages, you may wish to refer to:Minimum Aviation System Performance Standards - DGNSS Instrument Approach System: Special CategoryI (SCAT-I), Document No. RTCA/DO-217 (April 19,1995); Appendix A, Page 21.
