Interoperability Problem with Topcon Units

Through testing, we have experienced the following issue with Topcon Legacy-E with firmware version MF009JP010008F. In RTK applications, using Topcon and NovAtel combination receivers, users may experience incompatibility due to conflicting manufacture defined settings. According to Topcon documentation, their determination of the sign pertaining to the fractional portion of the time is different, resulting in the Topcon units being incompatible with NovAtel receivers. RTCM18 provides uncorrelated carrier phase measurements and RTCM19 provides uncorrelated pseudorange measurements. The measurements are not corrected by the ephemerides contained in the satellite message. Word 3, the first data word after the header, contains a GPS TIME OF MEASUREMENT field, which is used to increase the resolution of the MODIFIED Z-COUNT in the header. Word 2, the MODIFIED Z-COUNT, is the summation of word3 from both RTCM messages (Z-COUNT = WORD3RTCM18 + WORD3RTCM19), measured in GPS time. The Z-COUNT value is rounded to the nearest whole second (known as integral seconds). If the base station is running on a clock bias (i.e. taking measurements independently), the accuracy of the Z-COUNT value will slightly deviate from the integral seconds.