Lecture 3.2 - Types of Ephemeris - Civil Engineering (CE) PDF Download

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Lecture-2 
3.2 Types of Ephemeris 
 
? Ephemerides are a plural of ephemeris or values with which position and velocity of a satellite at any 
time can be determined. There are two types of  ephemerides: 
 
? Broadcast ephemeris 
? Precise ephemeris 
 
? Broadcast Ephemeris: It is a predicted satellite position which is broadcasted in the navigational 
message from the satellite in real time. Monitor Stations (of Control segment) receives the satellite 
tracking data which is used by MCS (Master Control Station) to compute the new orbital parameters 
of the satellite and is transmitted back to the satellite by MCS. The daily update is done and it is 
transmitted with the navigational message. These ephemerides are received by the receiver capable of 
capturing either C/A code or P code. 
 
? Precise Ephemeris: The more accurate satellite positions are obtained by the actual tracking data and 
post-processing is done later on. These are more accurate than the predicted data, as it is based on 
actual tracking of satellite orbits in real-time. The US Department of Defense doesn’t want the precise 
ephemerides to be available for online GPS applications like missile guidance system, other warfare 
operations, therefore does not provide these in real-time. These are available with a delay of about 12-
hrs for ultra rapid to 12- days for most precise services, for civil users. 
 
GPS Satellite Ephemerides information is given in Table 3.1. 
 
Table 3.1 GPS Satellite Ephemerides Information 
 
S. 
No. 
Ephemeris Orbital Accuracy 
(cm) 
Latency 
(approx.) 
Updates Sample 
1.  Broadcast 260 Real-time - Daily 
2.  Predicted 
(Ultra rapid) 
25 Real-time Twice daily 15min/ 15min 
3.  Rapid < 5 to 10 14 to 17 hours Daily 15min/ 5min 
4.  Final < 5 13 days Weekly 15min/ 5min