TPF-I SWG Report - Exoplanet Exploration Program - NASA

F ORMATION F LYING A L G O R I T H M D E V E L O P M E N T
required. Relative sensing and ISC are the unique hardware models that couple individual spacecraft
simulations. This coupling differentiates a formation simulator from a constellation simulator.
As the number of spacecraft in a formation increases, the overhead required to manage communication and
synchronization between distributed simulation components grows rapidly. A scalable, flexible, and easily
extensible architecture is needed to automate communication and manage connections between distributed
applications. HYDRA automates the connection of distributed simulation elements using a publish–
subscribe, client-server paradigm. As each client application is started, it provides the HYDRA server with
a list of offered and desired services. The HYDRA server commands two clients to form a connection when
they have advertised compatible services. Adding a simulated spacecraft to the formation only requires
starting up another spacecraft client. HYDRA allows the user to override default behaviors at several
layers. While HYDRA is similar to other distributed architectures (such as CORBA and HLA), it was
specifically designed for the needs of high-speed, distributed simulation.
In HYDRA, client applications communicate through connectors that abstract message passing over a
variety of protocols and infrastructure, including Scalable Coherent Interface (SCI) and transmission
control protocol/internet protocol (TCP/IP). Each spacecraft simulation in FAST is a HYDRA client. A
sixth simulation computer functions as the HYDRA server. All clients register with the server over the
Ethernet local-area network (LAN). The majority of inter-process communication in is handled by
HYDRA, including:
1. ISC traffic over the SCI connection,
ii) 2. Uplink, downlink, time coordination, and relative sensor traffic over Ethernet, and
3. Inter-process traffic within a computer. Simulation computer-to-spacecraft computer traffic over the
fiber-optic reflective memory cards is controlled via an interface specification.
Each simulation computer simulates the dynamics for only its associated spacecraft. The spacecraft
dynamics are integrated using the Dynamics Algorithms for Real-Time Simulation (DARTS) software
package (Jain and Rodriguez 1992). DARTS is a multi-platform software library written in C and is based
on spatial operator algebra. It provides efficient numerical algorithms for both rigid-body and flexible-body
dynamics. Spacecraft mass and inertia properties are input to DARTS using a Tool Command Language
(TCL) script. A lightweight interface to DARTS provides external actuator and sensor models access to
force and torque inputs and state outputs. A numerical integrator is used to propagate the system state based
on accelerations computed by DARTS. FAST currently provides either a fixed-step, fourth-order Runge-
Kutta integrator or the variable-step CVODE integrator (Cohen and Hindmarch 1996). The fixed step
integrator provides real-time, deterministic performance while the variable step integrator provides higher
accuracy.
A critical function of HYDRA is the synchronization of the separate spacecraft simulations for relative
sensing (Sohl et al. 2005). Specifically, relative sensing requires synchronization of the dynamics
integrators running on separate computers to provide consistent state information at a given time. A second
crucial function coupled to state synchronization is the simulation of local spacecraft clocks (SCLKs).
Spacecraft clocks are used to time-tag measurements and initiate digital control cycles. As spacecraft
clocks will have different offsets and drifts, control cycles on spacecraft will start at different true times and
will move with respect to one another. In addition, ISC will have jitter and dropouts. These characteristics
must be accurately simulated to evaluate formation robustness.
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