Current deep spacecraft rely heavily on ground-based navigation and tracking for state measurement. The requirement for long ground navigation passes, coupled with analysis support, produces a large latency for updating a vehicle’s state. As the current infrastructure continues to age and is increasingly utilized, the capability for these observations will also be diminished.
This navigation problem is unique primarily due to the great distances involved and the need for high-accuracy onboard state estimation capability and autonomy. The great distances be tween assets generates a unique signal environment in terms of signal trans mission times, power generation, and computational capability onboard the spacecraft.
Network-based navigation as part of the Multi-spacecraft Autonomous Positioning System provides an integrated navigation and communication capability. This innovation embeds navigation information into all transmissions between assets in the communication network. This allows for onboard autonomous navigation and state estimation through communication with a wide array of assets.
The system embeds specific state information, such as timing, position, and velocity as well as estimated accuracy, into data packets being transmitted by each asset. These are embedded into the message headers that are available as part of the Consultative Committee for Space Data Systems (CCSDS) standard Space Packet standards. Combined with onboard timing information, a spacecraft is able to calculate range to a target by comparing the timing transmission and reception times. Integration with onboard navigation state estimation routines allows for the spacecraft to update its onboard estimation of position, velocity, and time. By observing multiple packets from the same asset, the spacecraft is also able to determine range rate measurements, which provide additional insight into the vehicle state. The utilization of the transmitted estimated accuracy feeds into the measurement sensitivity models, which are then used to account for the uncertainty in the information, and provide for an optimal state update based on the measurement. As the spacecraft continues to perform navigation updates relative to other assets in the network (both in-orbit and ground-based), a Global Positioning System-like updated is achieved.
The unique feature of integrated navigation headers into space communication packets allows for purely digital embedded communication and navigation. This method is independent of the transmission/communication method, as it is embedded into the digital information transferred. The accuracy of the method is dependent on the timing stability and accuracy of the assets in the network, number of assets transmitting navigation headers, and current geometry of assets. Additionally, this functionality can be achieved through characterization of onboard timing systems and updates to Command and Data Handling software. Even with a limited network, the use of this technology reduces the burden on ground-based navigation systems while also improving onboard state estimation capability and accuracy.