## 2 BDS-3 SAIM Monitoring Method
### Enhanced Ephemeris Monitoring Methods  
Until now, BDS-3 SAIM has not been able to monitor ephemeris anomalies in broadcast messages. The ephemeris anomaly monitoring scheme presented below can effectively fill this gap and further strengthen the on-orbit autonomous monitoring capability of BDS-3.  
The generation, update and broadcast mode of the navigation ephemeris message is as follows: the ground operation management center collects the observation data of the navigation satellite's downlink signal, processes the data uniformly, determines the satellite's orbit and calculates the clock error. Next, the long-term navigation ephemeris is injected into the whole network satellite at a fixed time per hour for a period of time in the future. When the satellite needs to broadcast the navigation message, it retrieves the reference time of each group of navigation messages according to the current time, and a combination of the closest reference time and the current time is selected to send to the signal's downlink broadcast module to broadcast to the ground. Since there is little difference between the preceding and the following hour ephemeris, the current hour can be used to check the next hour.  
The ephemeris of a satellite is composed of 18 quasi-Kepler orbital parameters and the ephemeris data age, with a total of 454 bits in each group. The format of the ephemeris is the same as that of the uplink injection at the ground station. The ephemeris mainly includes: issue of data ephemeris, the ephemeris reference time, the orbit type of satellite, the deviation of the semi-major axis contrast to the reference value, the semi-major axis change rate, the difference between the average speed of the satellite and the calculated value, the rate of change of the difference between the average speed of the satellite and the calculated value, mean anomaly at the reference time, the eccentric rate e, the amplitude of the near-earth point, the reference longitude of the ascending anode, the orbital inclination at the reference time, the variable quantity of the ascending node's right ascension rate of change, the change rate of orbital inclination and orbit inclination, the radius, and the latitude amplitude angle of the sine, cosine harmonic correction term, etc.  
It is meaningless to make a simple threshold judgment for each separate parameter of an ephemeris message. The latest array of ephemeris messages should be transformed to obtain a set of one-dimensional results. According to the distribution characteristics of the parameter pairs and the safety requirements of engineering monitoring, a reasonable error threshold is set to judge the rationality of the ephemeris message parameters.  
In order to realize autonomous integrity monitoring of ephemeris, we propose a SAIM enhancement design, which consists of three relatively independent methods. In essence, no matter which method, an additional reference benchmark is needed to compare with the ephemeris received by the satellite at the current time in order to achieve the monitoring of ephemeris. If the difference exceeds the alarm threshold, an alarm would be output. The reference data of the three methods are respectively: (1) the ephemeris extrapolated from the previous cycle; (2) the ephemeris generated by autonomous orbit determination; (3) inter-satellite link distance measurement data. The specific comparison process is shown in Figure 2.  
Figure 3 shows the new SAIM design with additional ephemeris message integrity monitoring. In order to reduce the false alarm probability of ephemeris integrity messages, it is necessary to use a combination of monitoring methods to produce an alarm. SAIM will send an alarm only when the monitoring results of at least two of these monitoring methods exceed the alarm threshold, and the satellite will set the \"SIF\" parameter in the broadcast ephemeris from \"0\" to \"1\" to ensure that the alarm messages are broadcast to the users.