Extended Ephemeris data

Extended Ephemeris data

Ephemeris data 

Ephemeris data is precise positioning information broadcast by each GPS satellite regarding the satellite's position in the sky. About every 30 seconds the satellite broadcasts very precise data that includes its location and orbit data for the next 2 to 4 hours.  A GPS receiver needs to collect this data for every satellite that it is using to help maintain a location fix.  This can take some time for units that are either coming up for the first time, have been off for more than 2 hours, or have traveled greater than 100 miles since the last time they acquired a fix.  This process can take anywhere from 60 seconds to 20 minutes to obtain a fix.  This can be more difficult in "urban canyons" or if the device is moving while obtaining its first fix 


Extended Ephemeris data 

The CN3 provides customers a way to greatly reduce the "time to first fix" by uploading extended ephemeris data to the handheld at no additional charge to the customer.  Extended ephemeris is basically the entire set of ephemeris data (see above) for all satellites for the next 7 days.  This data is pre-calculated by the GPS radio manufacturer .

http://www.mt-system.ru/sites/default/files/docs/documents/extended%20ephemeris%20-%20english%20v2.0.pdf