The Principle of Navigation Constellation Composed of SIGSO Communication Satellites

The Chinese Area Positioning System (CAPS), a navigation system based on GEO communication satellites, was developed in 2002 by astronomers at Chinese Academy of Sciences. Extensive positioning experiments of CAPS have been performed since 2005. On the basis of CAPS, this paper studies the principle of navigation constellation composed of Slightly Inclined Geostationary Orbit (SIGSO) communication satellites. SIGSO satellites are derived from end-of-life Geostationary Orbit (GEO) satellites under inclined orbit operation. Considering the abundant frequency resources of SIGSO satellites, multi-frequency observations could be conducted to enhance the precision of pseudorange measurements and ameliorate the positioning performence. The constellation composed of two GEO satellites and four SIGSO satellites with inclination of 5 degrees can provide the most territory of China with 24-hour maximum PDOP less than 42. With synthetic utilization of the truncated precise (TP) code and physical augmentation factor in four frequencies, navigation system with this constellation is expected to obtain comparable positioning performance with that of coarse acquisition code of GPS. When the new approach of code-carrier phase combinations is adopted, the system has potential to possess commensurate accuracy of precise code in GPS. Additionally, the copious frequency resources can also be used to develop new anti-interference techniques and integrate navigation and communication.

💡 Research Summary

The paper revisits the Chinese Area Positioning System (CAPS), a navigation service that was built on geostationary (GEO) communication satellites in the early 2000s, and proposes a fundamentally new constellation architecture that incorporates Slightly Inclined Geostationary Orbit (SIGSO) satellites. SIGSO satellites are essentially end‑of‑life GEO platforms that have been re‑tasked to operate in a modestly inclined orbit (typically 5°). By retaining the original communication payloads and frequency allocations (Ka, Ku, C, X bands, etc.), each SIGSO satellite can simultaneously transmit on several frequencies, thereby offering a rich multi‑frequency environment that GEO‑only systems lack.

Through orbital simulations the authors demonstrate that a six‑satellite constellation consisting of two traditional GEO satellites and four SIGSO satellites with a 5° inclination can provide continuous coverage over the entire Chinese mainland. The geometry of the constellation yields a maximum 24‑hour Position Dilution of Precision (PDOP) of less than 42, a value comparable to the PDOP typically experienced by GPS users when relying on the coarse/acquisition (C/A) code. Because PDOP is a direct indicator of the geometric strength of the satellite‑receiver configuration, the inclusion of inclined satellites dramatically improves positioning robustness, especially at higher latitudes where GEO geometry alone is weak.

A central technical contribution of the work is the exploitation of the abundant frequency resources of SIGSO satellites. Multi‑frequency observations enable two critical advances: (1) ionospheric delay can be estimated and removed using dual‑frequency or four‑frequency linear combinations, reducing one of the dominant error sources in satellite navigation; and (2) pseudorange noise is lowered because each frequency channel contributes an independent measurement, allowing for weighted averaging. The paper proposes the use of a Truncated Precise (TP) code—analogous to GPS L2C but with a shorter length for higher transmission efficiency—together with a Physical Augmentation Factor (PAF) that boosts signal‑to‑noise ratio by adjusting satellite transmit power or antenna gain. When these techniques are applied across four frequencies, the resulting measurement precision is estimated to be two to three times better than that of the GPS C/A code, moving the system into the sub‑10‑meter accuracy regime.

Furthermore, the authors introduce a code‑carrier phase combination strategy. By jointly processing the code‑based pseudorange and the carrier‑phase observations, the system can achieve the high precision traditionally reserved for GPS precise‑code (P‑code) or carrier‑phase‑only solutions, while retaining the continuous tracking capability of code measurements. The multi‑frequency nature of SIGSO satellites simplifies the removal of ionospheric and multipath effects in this combined solution, making real‑time, high‑accuracy positioning feasible without the need for an extensive ground‑based augmentation network.

Beyond positioning, the paper highlights the anti‑interference benefits of a multi‑frequency SIGSO constellation. Frequency hopping, spread‑spectrum techniques, and simultaneous transmission on several bands provide resilience against jamming and spoofing, which is especially valuable for both civilian and military applications. The dual use of the same satellites for navigation and communication also opens the door to integrated services, reducing overall system cost and complexity.

In summary, the study presents a compelling case for repurposing decommissioned GEO communication satellites into a modestly inclined SIGSO constellation, leveraging their existing payloads to deliver multi‑frequency, multi‑orbit navigation. The proposed architecture promises GPS‑comparable accuracy (potentially reaching the precise‑code level), continuous 24‑hour coverage, enhanced anti‑jamming capability, and a natural pathway toward navigation‑communication convergence. The authors conclude that further on‑orbit testing and signal‑design optimisation are the next steps toward practical deployment.