Final answer:
GNSS satellites send two critical types of information for position determination: precise time signals from onboard atomic clocks and their own location in orbit. This data, along with taking relativity into account, allows GPS devices to calculate a user's precise latitude, longitude, and altitude.
Step-by-step explanation:
The two key types of information sent by GNSS satellites to receivers that allow for the determining of position are precise time signals and the satellite's location in orbit. Each GNSS satellite is equipped with an atomic clock that keeps exact time, which is essential for calculating the time it takes for signals to travel from space to a receiver on Earth. This is extremely important because even a tiny error in time measurement could lead to a considerable error in calculating position due to the high speed at which these signals travel - the speed of light.
Moreover, these satellites transmit data about their specific location in orbit, which is used by GPS devices to effectively triangulate a user's position on Earth. By receiving signals from at least four satellites, a GPS device can determine its user's latitude, longitude, and altitude, allowing for accurate location pinpointing.
To ensure the precision needed for these measurements, the effects of relativity must be taken into account. Specifically, time dilation due to special relativity, as the satellites are moving at high speeds relative to the surface of the Earth, and general relativity, as the time experienced by the satellites in weaker gravitational fields (higher altitudes) is faster compared to that on Earth.