Optimizing GPS with Geosynchronous Satellites

GPS, the Global Positioning System, is a critical technology for navigation and location services. The current GPS constellation consists of 31 satellites, orbiting the Earth at an altitude of approximately 20,000 kilometers in medium Earth orbit (MEO). However, the question arises: what if all satellites were placed in geosynchronous orbit? This article explores the feasibility and implications of such a configuration.

Theoretical Minimal Coverage

Theoretically, at geosynchronous and equatorial orbits, only three satellites would be needed to cover the entire Earth, except for a small circle of about 500 meters in diameter at each pole. This is particularly useful in regions where a very tall antenna could compensate, making up for the theoretical shortcomings in the Arctic Ocean.

Comparison with Current System

While it might seem that the current system consisting of 31 satellites could be reduced to just 31 in geosynchronous orbit, the practicality is questionable. The current GPS satellites complete two orbits per day, whereas satellites in a 24-hour geosynchronous orbit would provide little to no improvement in coverage. In fact, the signals would be weaker due to the increased distance from the Earth's surface. Thus, the current number of 31 satellites may offer a more balanced solution, as reducing the number could result in an actual disadvantage.

Difference Between Geosynchronous and Geostationary Orbits

It is crucial to differentiate between geosynchronous orbit and geostationary orbit. While geosynchronous orbit refers to an orbit that takes the same amount of time to complete as the Earth's rotation, geostationary orbit is a specific type of geosynchronous orbit where the satellite appears stationary above a particular point on the Earth's equator. This is what allows a geostationary satellite to stay in the same spot in the sky.

Challenges with Geostationary Orbits

The question specifically pertains to geosynchronous orbits and not geostationary orbits. It is not possible to achieve global coverage with geostationary orbits because the poles will be permanently out of reach. However, geosynchronous orbits with considerable inclination are perfectly feasible and common, as demonstrated by the orbits of the Japanese QZSS (Quasi-Zenith Satellite System) birds.

Optimal Satellite Placement

A minimum of four satellites in view at all times is required for GNSS coverage. Ideally, about 16 satellites could provide this coverage by being placed in four high-inclination orbits, each with four satellites. This arrangement would ensure that two satellites are visible from any latitude under their orbit at any time. Additionally, two birds from the adjacent orbit could provide coverage near the poles.

The configuration, however, would result in poor accuracy due to the poor geometry. More satellites would help but would still not significantly outperform the current MEO GNSS system. For instance, GPS with about 24 satellites achieves decent coverage, even if it is poor at extreme latitudes. Similarly, an increased number of geosynchronous satellites would significantly reduce signal strength and accuracy due to the expanded distance.

Alternative Configurations

To address the challenges of low-latitude coverage, a configuration with three satellites in each of four high-inclination orbits might be considered. This approach would, however, lead to the exclusion of the southernmost parts of Africa and South America. Alternatively, achieving coverage with 12 geostationary satellites is possible but would result in terrible geometry and, consequently, poor accuracy.

Conclusion

In conclusion, while geosynchronous orbits offer theoretical benefits, the practical challenges and signals' weaker strength make the current satellite constellation more advantageous. The key lies in optimizing the geometry and signal strength for accurate and reliable navigation. Further considerations and advancements in technology may be necessary to achieve better global coverage without compromising on accuracy.