Almanac and Ephemeris Data as used by GPS receivers

The satellites broadcast two types of data, Almanac and Ephemeris. Almanac data is course orbital parameters for all SVs. Each SV broadcasts Almanac data for ALL SVs. This Almanac data is not very precise and is considered valid for up to several months. Ephemeris data by comparison is very precise orbital and clock correction for each SV and is necessary for precise positioning. EACH SV broadcasts ONLY its own Ephemeris data. This data is only considered valid for about 30 minutes. The Ephemeris data is broadcast by each SV every 30 seconds. When the GPS is initially turned on after being off for more than 30 minutes, it "looks" for SVs based on where it is based on the almanac and current time. With this information, appropriate SVs can be selected for initial search. When the GPS receiver initially locks onto a SV, the Garmin display then shows "hollow" signal strength bars. At this time, the Ephemeris data has yet to be completely collected. Once the ephemeris data is collected from EACH SV in turn, the associated signal strength bar will turn "solid" black and then the data from that SV is considered valid for navigation. If power is cycled on a GPS unit, and when turned back on, the Ephemeris data is less than 30 minutes old, lock-on will be very quick since the GPS does not have to collect new Ephemeris data. This is called a "warm" start. If it is later than 30 minutes, this is considered a "cold" start and all Ephemeris data will have to be recollected. If the GPS has moved more than a few hundred miles or accurate time is lost, the Almanac data will be invalid and if you are far enough off, none of the SVs that the Almanac thinks should be overhead will be there. In such case, the GPS will have to "sky search" or be reinitialized so it can download a new Almanac and start over. More on Almanac & Ephemeris data….

GPS Data

• The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. SV Clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.
  
  
• Data frames (1500 bits) are sent every thirty seconds. Each frame consists of five subframes.
  
  
• Data bit subframes (300 bits transmitted over six seconds) contain parity bits that allow for data checking and limited error correction.
  
  
• Clock data parameters describe the SV clock and its relationship to GPS time. · Ephemeris data parameters describe SV orbits for short sections of the satellite orbits. Normally, a receiver gathers new ephemeris data each hour, but can use old data for up to four hours without much error. The ephemeris parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the ephemeris parameter set.
  
  
• Almanacs are approximate orbital data parameters for all SVs. The ten-parameter almanacs describe SV orbits over extended periods of time (useful for months in some cases) and a set for all SVs is sent by each SV over a period of 12.5 minutes (at least). Signal acquisition time on receiver start-up can be significantly aided by the availability of current almanacs. The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency (the frequency shift caused by the rate of change in range to the moving SV) of each SV in the constellation.