Message Formats Chapter 5OEM4 Family Installation and Operation User Manual Rev 12 59The base station transmitting the Type 9 corrections must be operating with a high-stability clockto prevent degradation of navigation accuracy due to the unmodeled clock drift that can occurbetween Type 9 messages. For this reason, only OEM4-G2 receivers with an external oscillatorcan generate Type 9 messages. All OEM4 family receivers can accept Type 9 messages.NovAtel recommends a high-stability clock such as the PIEZO Model 2900082 whose 2-sample(Allan) variance meets the following stability requirements:3.24 x 10 -24 s2 /s2 between 0.5 - 2.0 seconds, and1.69 x 10 -22 T s 2/s 2 between 2.0 - 100.0 secondsAn external clock, such as an OCXO, requires approximately 10 minutes to warm up and becomefully stabilized after power is applied; do not broadcast RTCM Type 9 corrections during thiswarm-up period.Structure:(Follows the RTCM Standard SC-104 for a Type 1 message)Type 9 messages contain the following information for a group of three satellites in view at the basestation:• Scale factor• User Differential Range Error• Satellite ID• Pseudorange correction• Range-rate correction• Issue of Data (IOD)5.2.4 RTCM15 Ionospheric CorrectionsRTCM Type 15 messages are designed to support the broadcast of ionospheric delay and rate ofchange measurements for each satellite as determined by the base station receiver. This message isused to improve the ionospheric de-correlation that would otherwise be experienced by a rover at along distance from the base station. This log is designed to work in conjunction with Type 1 messagesusing dual frequency receivers. It is anticipated Type 15 messages will be broadcast every 5-10minutes.Type 15 messages are designed to enable the rover to continuously remove the ionospheric componentfrom received pseudorange corrections. The delay and rate terms are added exactly like Type 1corrections to provide the total ionospheric delay at a given time, and the total ionospheric delay isthen subtracted from the pseudorange corrections. The resulting corrections are then "iono-free". Therover subtracts its measurements (or estimates) of ionospheric delay from its own pseudorangemeasurements and applies the iono-free corrections.Structure:(Follows RTCM standard for Type 15 message)
