Compared to previous GSFC missions, the Laser Interferometer Space Antenna (LISA) mission represents a significant step-up in flight software (FSW) complexity. Lines between spacecraft and payload become blurred as the LISA science instrument is created via laser links connecting three spacecraft forming approximately an equilateral triangle of side length 5 million kilometers. The science measurement is formed by measuring to extraordinarily high levels of precision the distances separating the three spacecraft via the exchange of laser light. The individual spacecraft maintain their positions in inertial space by following the movements of the proof masses along the sensitive axes of their gravity sensors.
From the standpoint of Bus FSW providing spacecraft attitude and position control, the number of sensors and actuators that must be interrogated and commanded is at least twice the number associated with a more traditional mission. Similarly, the number of control modes is double that of a typical astrophysics mission, as are the number of parameters solved for by the state estimator. These factors also suggest the Bus FSW will need to perform fault detection on at least twice the normal number of telemetry points, with a correspondingly large number of fault correction responses. So it’s clear simply by counting pieces of functionality, LISA’s Bus FSW will be significantly more elaborate than most previous GSFC missions. For the most part, the nature of the processing to be performed in support of these functions will be comparable to what is typically done. Output from sensors & actuators will be converted from raw data to engineering units by polynomial functions often as simple as linear. The command interface to the sensors & actuators also should be fairly straightforward. The control laws implemented within the control modes will be the usual Proportional-Integral-Derivative (PID) ones, although the number of inputs and outputs will be larger for both the control laws and estimators. As with most projects, the specification of fault detection & correction (FDC) currently is less mature than for other onboard functionality. However, the basic logic of the processing is well understood: i.e., there will be distributed fault checking based on limits and tolerances and a table-driven centralized fault correction. There will be counters identifying repeated violations of the limits/tolerances; if a counter limit is tripped, a flag will be set indicating an anomaly has occurred. Probably, most individual detected anomalies will be dealt with on an individual rather than coupled basis, but the details of the correction process cannot be known until all the possible failure mechanisms and scenarios have been thoroughly analyzed; hence, the comment regarding the lower level of maturity.
Furthermore, many of the LISA Bus FSW functions support new technologies with which there has not been a great deal of in-flight experience. LISA will use Micro-Newton thrusters to make the very small corrections to spacecraft attitude and position required to enable the spacecrafts’ to follow their proof masses motions in Drag Free control. Both the Micro-Newton thrusters and gravity sensors are new technology whose concepts will be flight-tested by the LISA Pathfinder mission. Similarly, the control laws enabling Drag Free control are new, developed at GSFC to support LISA Pathfinder and LISA. Although laser metrology is not new, the precision at which LISA will be performing its metrology is quite unique. Over the measurement baseline of 5
accuracy demands of many LISA functions will be a step up from their counterparts on previous GSFC missions.
The high precision/accuracy requirements suggest a spill-over into FSW testing. Not only must algorithms be developed and implemented that enable these severe requirements to be met but, in addition, testing methodologies must be developed that will verify that the requirements have been met. Traditional mission-pointing accuracy requirements are of order arcsecond. Even for the state-of-the art Hubble Space Telescope (HST), milliarcsecond pointing stability is the standard. For LISA, mispointings on the order of milliarcseconds would be enough to disrupt the laser links; an order of magnitude worse might suffice to break the links entirely. Performance of LISA science requires pointing stability to a tenth of a milliarcsecond or better, an order of magnitude more demanding than that for HST. FSW validation will, therefore, probably need to be able to see “errors” to a couple orders of magnitude better: i.e., micro-arcsecond level, which might necessitate the utilization of special software tools capable of identifying variations from “truth” at this level. In other words, simple visual examination of a graphical plot of position and orientation angles vs. time will probably not suffice in determining whether a control law is meeting LISA’s stringent requirements. The data might, instead, need to be processed statistically to determine goodness-of-fit, or to determine if the apparent noise in the errors is truly random. These additional efforts, should they be required, probably will appear largely in the form of extra hours of analytical support for FSW build testing and analyst-created statistical/graphical tools.
Strictly speaking, the LISA mission does not require implementing an onboard capability for formation flying between spacecraft in a constellation. Primarily, what a LISA spacecraft will do is follow its own individual proof masses in Drag Free control independent of the other two spacecraft. This formation flying between a LISA spacecraft and its proof masses must be controlled to within a nanometer or better accuracy. The triangle formation between the three LISA spacecraft is largely maintained by orbit selection, and small variations in triangle vertex angles over time are accommodated via Telescope Articulators that permit modifying the angles between the lasers and Point-ahead Actuators that place the laser light where the target spacecraft will be (i.e., compensate for light travel time). These angle changes are orbit dependent and largely predictable, so a portion of the angle change can be ground calculated and a table can be uplinked to the Bus FSW for use in open-loop commanding. The remaining angle change will be measured via feedback between spacecraft and compensated for via closed-loop commanding.
In other aspects, the LISA Bus FSW complexity is comparable to traditional GSFC missions. Specifically, onboard autonomy has been restricted to functionality required to maintain health & safety and to provide an acceptable level of science efficiency. Currently, there will be no onboard Time Delay Interferometry (TDI) processing of science data, although some diagnostic- oriented processing might be performed to help assess the stability of the laser links. Processing of the science data will be purely a ground responsibility. There will be no onboard autonomous overall management of the LISA Constellation. That also will be a ground responsibility. Had these functions been implemented in FSW, the size and complexity of the LISA Bus FSW would have been greatly increased. GSFC also has had some previous experience developing Bus FSW for an in-house constellation mission. ST-5, which comprised a constellation of three spacecraft, has been launched and is being operated successfully. So on LISA, GSFC will not have to figure
out for the first time how to perform configuration management on the FSW for more than one spacecraft, each with its own unique identification and independent commanding needs.
In the future, many in the science community would like to build on the LISA constellation experience to develop new constellation missions. Astrophysicists have conceptualized many missions that would autonomously formation-fly many individual lensing spacecraft to create a virtual mirror whose light would be integrated at a hub spacecraft. Many missions have been conceptualized by Earth scientists that would autonomously coordinate the data collection efforts of many “smart” instruments distributed on multiple spacecraft to improve data collection efficiency and provide enhanced target of opportunity (TOO) response. As always, budget limitations will dictate which of these exciting mission concepts will become real missions and which will remain on the drawing board.