Invention of GPS: Roger Easton's navigation system using satellites and passive ranging techniques

Topic

applied sciences

Frequently Asked Questions (FAQ)

What is the main purpose of this navigation system? This navigation system aims to provide highly accurate location information to a navigator, like a ship or airplane, by using signals from satellites. It surpasses the accuracy of traditional celestial navigation methods.
How does this system determine the navigator’s location? The system relies on measuring the distance (or range) between the navigator and at least two satellites with known positions. This is achieved by comparing the phase of signals received from the satellites with reference signals generated locally by the navigator’s equipment. The intersection of the lines of position (LOPs) derived from these range measurements pinpoints the navigator’s location.
What is the main advantage of this system over previous satellite navigation systems? Unlike earlier systems that required the navigator to send out an interrogation signal to the satellite, this system is passive. The navigator only receives signals from the satellites, without transmitting anything back. This has two key benefits: it prevents the navigator’s location from being revealed and eliminates the risk of overloading the satellite’s transponder with multiple simultaneous requests.
How are the signals transmitted by the satellites generated? Each satellite has a highly stable oscillator, like an atomic clock, generating a precise base frequency signal (e.g., 5 MHz). This signal is then processed to produce multiple signals at lower frequencies (e.g., 100 Hz, 1 kHz, 10 kHz). These multiple frequencies modulate a high-frequency carrier signal, which is then broadcast by the satellite.
How does the navigator’s equipment process the received satellite signals? The navigator’s equipment includes a receiver that captures the multi-frequency modulated carrier signal from the satellite. It then separates the various frequency components and removes the carrier. This process is done carefully to avoid introducing any phase shifts to the signals.
What role do the oscillators play in the navigator’s equipment? The navigator’s equipment also has its own highly stable oscillator, similar to the one on the satellite. This oscillator is crucial for generating local reference signals at the same frequencies as the received satellite signals. Importantly, these locally generated signals need to be precisely synchronised, or phase-locked, with the signals from the satellite.
How is the range to a satellite measured? The navigator’s equipment compares the phase of each received frequency component from the satellite with the corresponding locally generated reference signal. The phase difference between the two signals is directly proportional to the signal’s travel time from the satellite to the navigator. This travel time, combined with the speed of light, allows for the calculation of the distance to the satellite.
Why are multiple frequencies used in this system? Employing multiple frequencies helps overcome ambiguity in range measurement. Lower frequencies provide unambiguous range information but with lower accuracy. Higher frequencies offer greater accuracy but suffer from ambiguity due to the cyclical nature of phase measurements. By combining the information from different frequencies, the system achieves both accurate and unambiguous range determination.

Significance

Understanding these findings helps advance our knowledge and inform better decisions. This research represents an important contribution to the field. For the full details, watch the video above and explore the linked resources.

Resources & Further Watching

Read the patent (US 3,789,409): https://patents.google.com/patent/US3789409A/

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