Navigation satellites have become an indispensable part of our daily lives, seamlessly guiding us through unknown terrains and connecting us to the world. In this blog post, we'll delve into the world of navigation satellites, exploring the nuances of these technologies, their functionality, and their impact on our global positioning.
Among the many systems in operation today, two key terms often surface - GPS and GNSS. GPS, or Global Positioning System, is a term commonly used to refer to the American navigation satellite system. However, GNSS, which stands for Global Navigation Satellite System, is a more encompassing term that includes not only the American GPS but also other satellite navigation systems such as Russia's GLONASS, Europe's Galileo, China's BeiDou, and others. While GPS is specific to the American system, GNSS reflects the global nature of satellite navigation systems collectively.
GNSS satellites are in Medium Earth Orbit (MEO), whereas a lot of the objects we’re more familiar with like the International Space Station, are in Low Earth Orbit (LEO) which is up to 2,000 km abouve sea level. Here are a list of altitudes for common space objects:
The ISS orbits at about 340 kilometers.
SpaceX's Starlink satellite constellation for global broadband internet, operates at altitudes ranging from 340 kilometers to 1,200 kilometers.
The Hubble Space Telescope orbits at approximately 595 kilometers.
Earth observation satellites such as Landsat and Sentinel 2 orbit at 705 km and 786 km respectively.
To appreciate the varying altitudes of navigation satellite constellations, let's explore the heights above sea level of the GPS, GLONASS, and Galileo systems.
GLONASS: Approximately 19,130 kilometers above sea level.
GPS: Approximately 20,200 kilometers above sea level.
Galileo: Approximately 23,222 kilometers above sea level.
The altitudes are derived from this source and this source.
The image below puts it in to perspective.
An example of the altitude of the GNSS orbits in relation to other low earth orbit satellites.
At the core of satellite navigation is the transmission of signals encoded with precise timing information. The fundamental principle involves triangulation, where signals from multiple satellites are received by a ground-based receiver. By calculating the time it takes for each signal to reach the receiver, the distance to each satellite can be determined. Using this information, the receiver can then pinpoint its exact location through a process known as trilateration.
This concept relies on the constant speed of light, and it forms the basis of accurate navigation across vast expanses. However, the theory encounters a fascinating twist when considering the impact of relativity on the signals.
While the speed of light is indeed constant, the time it takes for signals to travel from satellites to Earth is influenced by the theory of relativity, as proposed by Albert Einstein. There are two key components of relativity that come into play: time dilation and gravitational time dilation.
Time Dilation: According to the theory of relativity, time moves slower for objects in motion. Since GNSS satellites are in motion, their onboard clocks experience time dilation compared to clocks on the Earth's surface. To maintain accuracy, the satellite clocks are adjusted to account for this time difference.
Gravitational Time Dilation: Gravity also affects the passage of time. In regions with stronger gravitational fields, time moves more slowly. Given that GNSS satellites orbit Earth at a significant distance, their clocks tick slightly faster than those on the Earth's surface. Again, adjustments are made to ensure accurate timekeeping.
Navigation satellites, whether part of GPS, GLONASS, Galileo, or other GNSS constellations, have revolutionized the way we navigate and perceive the world. The intricate dance between satellite signals, the speed of light, and the principles of relativity unveils the complexity of these systems. As we continue to explore the skies and expand our satellite constellations, understanding the subtle nuances of their operation becomes crucial for optimizing their performance and maintaining the accuracy we've come to rely on in our daily lives.
Refer to our previous blog posts on best practices for GPS/GNSS use, choosing the correct GNSS, and selecting the best mobile phone for mapping.