GPS

• U.S. Department of Defence launched the GPS program in 1973.
• First satellite launched in 1978.

Satellite Constellation:

• Modern GPS constellation: 24 satellites in six orbits.
• Each satellite completes two orbits daily.
• Orbits located 20,200 km above Earth.
• Four satellites per orbit ensure global visibility.

Segments of GPS

1. Space Segment: Comprises the 24 satellites in orbit.
2. Control Segment: Global network of ground-based control stations and antennae ensure satellite performance and transmit commands.
   • Master control station: Schriever Air Force Base, Colorado.
   • Alternate control station: Vandenberg Air Force Base, California.
   • Ground antennae in various locations worldwide.
3. User Segment: Involves GPS use across sectors and applications.
   • Sectors: Agriculture, construction, surveying, logistics, telecommunications, power transmission, search and rescue, air travel, meteorology, seismology, military operations.
   • Estimated 6.5 billion GNSS devices globally in 2021, expected to reach 10 billion by 2031.

How GPS Works:

1. Satellite Broadcast:
   • Each GPS satellite broadcasts radio signals.
   • Signals contain location, operational status, and emission time information.
   • Broadcasted at L1 (1,575.42 MHz) and L2 (1,227.6 MHz) frequencies at 50 bits/second.
   • Encoded with code-division multiple access (CDMA) for multiple signals in the same channel.
2. Signal Reception and Calculation:
   • GPS receiver on a device (e.g., smartphone) picks up the signal.
   • Receiver calculates precise distance from the satellite.
   • Distance = Speed of light × Signal’s travel time.
   • Signal’s travel time = Receiver’s clock time minus Signal emission time.
   • Access to signals from four satellites allows accurate triangulation in four dimensions (three of space + one of time) for precise location determination.
3. Triangulation for Location:
   • With signals from four satellites, the receiver triangulates its location accurately.
   • Triangulation involves determining the intersection point of spheres (signal distances) to pinpoint the receiver’s location on Earth.
4. Adjustments for Accuracy:
   • Adjustments are made for accuracy:
     • Satellites’ onboard clocks run 38 microseconds faster due to weaker gravitational potential, following the general theory of relativity.
     • Special theory of relativity accounts for relative velocities of satellites and receivers.

Atomic clock

Accurate timekeeping is paramount for the effective operation of the Global Positioning System (GPS). Satellites in the GPS constellation employ atomic clocks to maintain precise time synchronization, with each clock on board synchronized within a remarkable 10 nanoseconds of each other and with reference clocks on the ground.

• Without adjustments, even a 38-microsecond clock offset can lead to a 10 km error within a day.
• A one-millisecond offset can result in a 300 km error.

The fundamental principle of atomic clocks relies on the quantized energy levels of electrons within an atom, forming a distinct “staircase” of energy steps. When radiation at the resonant frequency, specific to these energy steps, is supplied, electrons absorb it, facilitating their jumps between energy levels.

• This resonant frequency serves as a reliable measure of time, for instance, 50 cycles per second indicating one second has passed. Scientists continually fine-tune the radiation source to the resonant frequency, ensuring accurate and synchronized timekeeping across the GPS satellite constellation, preventing errors that could significantly impact global navigation precision.