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One of the first announcements was the addition of a new civilian-use signal, to be transmitted on a frequency other than the L1 frequency used for the coarse/acquisition (C/A) signal. Ultimately, this became the L2C signal, so called because it is broadcast on the L2 frequency.
However, one can often account for most of the discrepancy by the introduction of gravitational time dilation, the slowing down of time near gravitating bodies. In case of the GPS, the receivers are closer to Earth than the satellites, causing the clocks at the altitude of the satellite to be faster by a factor of 5×10 −10, or about +45.8 ...
tech: the GPS signal strength measured at the surface of the Earth. [clarification needed] [5] 10 −16: 2 × 10 −16: −127 dBm biomed: approximate theoretical minimum luminosity detectable by the human eye under perfect conditions 10 −15: femto-(fW) 2.5 × 10 −15: −116 dBm tech: minimum discernible signal at the antenna terminal of a ...
A major subclass is made of geopositioning systems, used for determining an object's position with respect to Earth, i.e., its geographical position; one of the most well-known and commonly used geopositioning systems is the Global Positioning System (GPS) and similar global navigation satellite systems (GNSS).
The United States' Global Positioning System (GPS) consists of up to 32 medium Earth orbit satellites in six different orbital planes. The exact number of satellites varies as older satellites are retired and replaced. Operational since 1978 and globally available since 1994, GPS is the world's most utilized satellite navigation system.
This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival-time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.
Neglecting ionospheric [3] and tropospheric [4] effects, the signal from navigation satellites has a fixed precision. Therefore, the relative satellite-receiver geometry plays a major role in determining the precision of estimated positions and times.
DGPS Reference Station (choke ring antenna)A reference station calculates differential corrections for its own location and time. Users may be up to 200 nautical miles (370 km) from the station, however, and some of the compensated errors vary with space: specifically, satellite ephemeris errors and those introduced by ionospheric and tropospheric distortions.