Precise time synchronization of two Milstar communications satellites without ground intervention
✍ Scribed by Camparo, J. C. ;Frueholz, R. P. ;Dubin, A. P.
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 114 KB
- Volume
- 15
- Category
- Article
- ISSN
- 0737-2884
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✦ Synopsis
Satellite navigation and communication systems often require precise synchronization among spacecraft clocks. In the traditional method for achieving synchronization, a ground station makes time-offset measurements to the various spacecraft clocks, and then updates the time and frequency of each satellite as needed. Though straightforward in its implementation, disadvantages to the traditional approach include the large workload placed on the ground station, the need for multiple ground stations to view satellites in different geosynchronous positions, and unaccounted-for delays in atmospheric propagation.
In early 1996 Milstar became the first satellite system to employ crosslinks for precise satellite time synchronization. At that time, the crystal oscillator clock onboard FLT-1, the first Milstar satellite, had its time and frequency tied (i.e., slaved) to the rubidium (Rb) atomic clock carried onboard FLT-2, the second Milstar satellite. The FLT-2 Rb atomic clock was controlled by the ground, while the slaving of FLT-1 to FLT-2 was accomplished without ground intervention: all timing information required by the slaving algorithm was obtained through the FLT-1 to FLT-2 satellite crosslink. In this paper we will first show the timekeeping capabilities of the two satellite clocks when operating independently, which indicate that both clocks are performing well. Then, we will present ground station measurements of FLT-1 and FLT-2 timekeeping that demonstrate satellite synchronization to better than 150 nsec without ground intervention. As satellites are added to the Milstar constellation, crosslink slaving will minimize ground station timekeeping activities, thereby lowering system operating costs.