Adeepakpublishing JoSSonline Pre-Launch .

Sternberg, D. C. et al.LF Guidance, Navigation, and Control SystemThe LF GNC system is summarized in this section.For details, readers are referred to Lai (2018 and2020). The LF spacecraft layout is shown in Figure 2.The XACT-50 unit is the core of LF GNC system. Itconsists of an SRU, IMU, and three RWAs, along withthe GNC flight software (FSW) within one box(shown in Figure 3). Unlike ASTERIA or MarCO(Sternberg, 2019), LF has four BCT-designed CSS toprovide close to 4π steradian coverage: one sensor ismounted to the body /-Y and /-Z axes. These CSSare mounted separately, but connect electronically tothe XACT-50.Figure 2. Final spacecraft layout and coordination system.Figure 3. XACT-50 integrated GNC system.The LF GNC FSW has two control modes, SafeMode (SFM), pointing the solar panels to the Sun; andFine-Pointing Mode (FPM), which points the spacecraft in a commanded attitude. The system powers onCopyright A. Deepak Publishing. All rights reserved.in SFM, performing sun search autonomously until thespacecraft reaches the sun-pointing attitude that pointsthe spacecraft Z axis toward the Sun for maximumpower available from the solar arrays. In this mode,the CSS and IMU are used to establish spacecraft attitude, while the RWAs orient the spacecraft to point towards the Sun. A 0.2 deg/sec rotisserie spin is commanded along the sun vector to increase spacecraft stability, improve thermal balancing, and help balancemomentum build-up.While in SFM, the spacecraft can be commandedto enter FPM for cruise and scientific activities. In thismode, the SRU and IMU determine spacecraft attitudewith measured data processed by a Kalman filter.Upon entering this mode, GNC FSW autonomouslyperforms a lost-in-space star identification algorithmwithin a few seconds. Once the correct attitude is determined, the FSW then changes to the tracking mode.When the SRU’s field of view (FOV) is obstructed orintruded by a bright object, the IMU is used to propagate attitude until the SRU can regain attitudeknowledge. In nominal operation, the RWAs are alsoused for attitude control in FPM.The LF spacecraft does not have direct commandor telemetry lines to its propulsion system, though itprovides 5 V and 12 V power directly to the propulsionsystem. All the propulsion system commands and telemetry go through the XACT-50 unit. Therefore, it iscritical to verify the proper interface connectivity between the spacecraft computer, the XACT-50, and thepropulsion unit early in the spacecraft developmentstage. To verify this interface, an EDU was used fortesting at BCT during the XACT-50 development anddelivered to JPL for use in the ATB. This ATB testingis described in a later section of this paper. There wasno flight-like testing of the propulsion system once itwas integrated with the rest of the spacecraft. Instead,a set of simulations was performed to assess the performance of the propulsion system.The CAD model of the propulsion system is shownin Figure 4, including four thrusters and a propellanttank. The four thrusters are canted at 12 for three-dimensional control, and are used for dumping accumulated RWA momentum and performing ΔV maneuvers. Each thruster is capable of operating in continuous burn and pulsed modes.JoSS, Vol. 10, No. 1, p. 962

Pre-Launch Testing of the Lunar Flashlight (LF) CubeSat GNC SystemFigure 4. Bottom view of spacecraft showing propulsion module andsun sensor.GNC Hardware Validation Test at Small Satellite Dynamics Testbed3.1. Creating a Test Sequence for the LF GNCSystemLF GNC testing used many of the testing philosophies used by larger spacecraft that are described inPong (2019). Unlike on the MarCO and ASTERIAmissions, for example, the LF GNC testing processwas designed to include dynamic tests instead of juststatic tests. Lessons learned from both large and smallmissions were applied in the development of the LFGNC testing process.When the XACT-50 was delivered to JPL, it wasnecessary to test the unit before ATB integration. Thistesting ensured that it met the functional requirementsfor its successful operation within the spacecraft andthat its key functions were exercised in a relevant environment. Three sets of tests were performed on theEDU and FM XACT-50 units as listed in Table 1, toprovide an assessment of the functionality of each major element of the XACT-50. The three sets of tests,grouped by the color-coding, were scoped to provideTable 1. XACT-50 Hardware Test MatrixCopyright A. Deepak Publishing. All rights reserved.assurance that both units would function as expected,with the specific tests on each unit tailored to the hardware contained within each XACT-50 unit. Phasingtests provide assurance that the hardware and softwareboth are able to operate and are able to match the conventions for coordinate frames defined by the LF project. Software mode tests were included to build uponthe phasing tests, as these closed-loop tests assess thefunctionality of the controllers themselves. Lastly, thejitter and imbalance testing was performed to assessthe one of the dominant noise sources that will existonboard the spacecraft. Unlike larger missions, the LFtesting was able to be performed with the entire attitude control system mounted to an environmental simulator in a dynamically relevant physical environment,entirely separate from the rest of the spacecraft. Additionally, because the XACT-50 is a complete vendorsupplied unit, all attitude determination and controlsoftware had already been loaded onto the hardwareand tested by the vendor. Consequently, the testingperformed at JPL was of a system that had already undergone basic testing by the vendor.The EDU XACT-50 and FM XACT-50 units havedifferent GNC hardware components. While they havethe same internal software structure, the FSW delivered with the EDU XACT-50 was not the final software version, which had not yet been completed.While the FM XACT-50 is a complete unit with all ofthe flight sensors and actuators, including four CSS formounting to the exterior of the spacecraft (each CSSconsists of four diodes each), the EDU XACT-50 doesnot have the SRU and has only one CSS. The softwaredifference between the two units is centered on a setof physical property information that is stored inmemory tables within the XACT-50 units; becausethese tests are not performance tests, this software difference does not play a role in assessing the functionality of the hardware and two main software modes.The green-highlighted tests in Table 1 were performed together as the first set to assess the EDU’sfunctionality through: a RWA phasing test (determining that commands to the reaction wheels yield the expected rotation directions), an IMU phasing test (determining that the estimated rotations are in the expected directions and at the commanded rates), and aCSS phasing test (to ensure the diode counts from aJoSS, Vol. 10, No. 1, p. 963

Sternberg, D. C. et al.sun simulator match expectations). Additionally, thisset included a fine-pointing operation test (to ensurethat the XACT-50 can attempt to maintain a specificspacecraft attitude), and a sun-pointing operation test(to ensure that the XACT-50 can attempt to find andpoint spacecraft to the sun simulator).The same set of tests was then performed on theFM XACT-50 (the yellow-highlighted set in Table 1),though a test of the SRU phasing (to determine if theSRU could identify the rate and direction of star fieldmotions) was added to test this sensor. Importantly,that the FPM performance will be different betweenthe EDU and FM sets of testing campaigns: the lack ofSRU in the EDU limits the fine-pointing test becausewithout the external reference sensor, it cannot compensate for gyroscopic drift.Lastly, a RWA jitter test was performed on theEDU only to characterize this newly developed 50mNm RWA (brown test in Table 1). It therefore provided a first order assessment of the jitter that will beseen on flight, since the FM and EDU XACT-50 unitsare not structurally identical. Unlike many larger missions, a detailed jitter model is typically not availablefor CubeSats with limited budget for use in determining a quantifiable effect on the science return from thespacecraft’s instrumentation.The aforementioned set of tests is described in therest of this section. Section 3.2 describes the commonset of documentation that accompanies each test aspart of the overall rigorous testing scheme used by theLF project, while Section 3.3 describes the testing venues used for each of the tests. Section 3.4 describeswhy the LF FM XACT-50 requires different test configurations than the EDU. Section 3.5 provides moredetails about the tests conducted on the EDU, whileSection 3.6 provides the details for the EDU jitter testsand Section 3.7 provides data from the FM tests.3.2. Documentation and Reviews for Each TestThe LF GNC system testing process was the firsteffort at JPL to apply a large-scale spacecraft GNC approach to how the test planning, documentation, andreviews were conducted for the development of a CubeSat project’s GNC system. One of the most criticaldocuments that was developed for each type of testCopyright A. Deepak Publishing. All rights reserved.was a test procedure document. Developed after extensive table-top planning and a test plan review with aparticular focus on ensuring a complete functional assessment of each hardware element and softwaremode, a test procedure was prepared to span a set ofrelated tests (such as all EDU tests being aggregatedinto one test procedure, since they were all performedserially and in one testing phase). While other JPL CubeSat projects had test procedure documents as well,the LF procedures were the most thoroughly vetted todate. Dedicated, independent reviews were performedby project and line management as well as projectquality assurance prior to a formal procedure review.A final test readiness review was also performed priorto each test. No such process existed at JPL for subsystem testing on CubeSat missions prior to LF.For LF, the result of the preliminary work for eachtest procedure was a document that contained all of theinformation necessary for an engineer not on the project to understand the scope and technical justificationfor each test, as well as to be able to complete eachstep in the procedure without any additional training.To reach this level of procedural detail, the test procedures include: detailed photos and drawings of thehardware in the to-be-tested configuration with instructions on how to assemble that configuration; setsof test objectives with pass/fail criteria in relation toproject requirements and telemetry to be analyzed;handling and storage requirements; necessary trainingfor all operators; step-by-step procedures for runningeach test; and a set of unplanned test termination andtroubleshooting steps for the operator to use in theevent that a test needs to be aborted. All of this information provides the test conductor sufficient background and operational guidance to perform each testsafely while ensuring that the test data products are acquired as expected.After each set of tests were completed, two keydocuments were created. The first was a reviewpresentation describing the analyzed test results andany anomalies that were encountered during the testing. This test result review served as the main venuefor presenting the results of each test and evaluatingthe success of the testing, the current state of the project, and any issues that arose with the test equipment.All issues were also reported, as is standard for all JPLJoSS, Vol. 10, No. 1, p. 964

Pre-Launch Testing of the Lunar Flashlight (LF) CubeSat GNC Systemprojects, in a dedicated problem reporting system toolso that root causes can be tracked and all affiliated documents surrounding the issue are saved for access byany future user of the hardware or software in question. Further, a test report document was provided toboth project and line management with a set of lessonslearned was prepared to capture operational considerations for testing and integrating the unit under test andground support equipment later in the project development cycle. The data from these tests was not used forperformance validation, but the assurance that each element functions as expected is invaluable throughoutthe subsequent requirement verification and validationprocess. The data and as-run procedures are saved inthe project’s data repository for access by anyone onthe project, and like all JPL projects, any unexpectedresults are saved in a dedicated JPL system for accessby other projects that use similar hardware, software,or processes. The project’s systems engineers are ableto complete concept of operations designs, missiontimeline analyses, and maneuver sequences, amongstothers, using the outputs from these tests as well.3.3. Testing VenuesAll of the acceptance and functionality tests exceptfor the RWA jitter test were performed in the SSDT.The SSDT was founded at JPL in 2014 to reduce risksassociated with attitude control and to centralize thetesting infrastructure needs for small satellites. The facility maintains the spacecraft hardware and testingenvironments necessary to support dynamics testingwith a variety of SmallSat sensors, actuators, and software. In particular, the SSDT has two separate airbearing environments: a spherical air bearing withthree rotational degrees of freedom (DOFs), and a planar air bearing with one rotational and two translational degrees of freedom (Sternberg, 2018). The planar air bearing can operate either on a 0.6 m 0.6 mgranite table in the SSDT or the much larger (7.3 m x8.5 m) flat floor of JPL’s Formation Control Testbed(FCT) (Scharf et al., Parts I and II, 2010). The facilityspace in the SSDT allows other equipment to be included as sensors and stimulators as well. The LF project made use of a 1kW lamp as a sun simulator, forexample.Copyright A. Deepak Publishing. All rights reserved.The SSDT testing of LF’s EDU and FM XACT-50units was performed on the spherical air bearing to assess the XACTs’ performance on all three rotationaldegrees of freedom. The SSDT’s spherical air bearingcan accommodate a wide range of test payloads, andwas configured to provide a neutrally stable platformcapable of powering the XACT-50 unit under test,send commands, log telemetry, and provide sensor inputs as necessary without any cables or tethers constraining the motion of the hardware. Coarse and finebalance weights were used to achieve a minimal separation between the center of mass and center of rotation of the air bearing, since any offset leads to gravityinduced disturbance torques that would not be presentin the space environment.Jitter testing was performed at night in a basementlaboratory at JPL to minimize the impact from sourcesof disturbance vibration from the environment, such aselevator or car motion. This testing was performed ona large isolator upon which the XACT-50 EDU wasmounted. After sending a range of known RWA speed,commands, the resulting vibrations were then recorded.Connected to the XACT-50 units for all groundtesting was a separate unit from BCT called the Realtime Dynamics Processor (RDP). The RDP acts bothas an environmental simulator that can inject simulated sensor measurements based on a real-time simulation of the spacecraft’s position and attitude overtime, while also capturing telemetry and allowing theuser to send commands to the unit. The RDP thereforewas instrumental in the testing process, since it allowed the sensors and actuators to respond as if theywere in the flight environment.3.4. Test Setup for Engineering Design UnitTestingThis section describes the test setup used for theLF EDU hardware in the SSDT, with the EDU testingprocess providing an example of how the test setupsupported the overall LF testing campaign. The EDUtesting was designed for generating and proofing theprocedures required for testing the FM, and the overallset of EDU tests proceeded as planned, with test planJoSS, Vol. 10, No. 1, p. 965

Sternberg, D. C. et al.and procedure improvements folded into the FM testing process. Importantly, no tests in this section directly verified the mission’s GNC requirements.The test configuration on the SSDT spherical airbearing is shown in Figure 5, with the bulk of the setupbeing reused for FM testing. The solar simulator shining onto the air bearing is shown in Figure 6; at thisdistance, the CSS are able to identify the lamp as thedominant illumination source without risking thermaldamage from the 1 kW halogen lamp. Further, a StarField Simulator (SFS) developed at the SSDT was incorporated into the testing (Filipe, 2017). The SFS displays an image of a star field on a small high-definitionscreen, with the image being sent via HDMI cablefrom a dedicated SFS computer.The star field is computed using the current orientation of the XACT-50 unit as determined by a separateground truth gyroscope; the SFS computer determinesthe correct star field to be displayed for a given spacecraft attitude. Filipe (2017) provides a much more indepth assessment of the performance capabilities ofthe SFS and how the system functions as ground testequipment; importantly, the high-definition screen isable to produce brightness and centroid data graphically to a level of accuracy such that the XACT-50’salgorithms are the limiting factor in achieving finepointing. A baffle is used to prevent stray light fromentering the XACT-50’s optics, and a collimator lensis used to ensure that the incident light from the SRUbehaves as if the XACT-50 were viewing far awaystars. This complete setup is shown in Figure 7. Although the SFS was not used as part of the control loopfor EDU testing because of there not being a SRU inside the EDU XACT-50, the hardware was included inthe test configuration to maintain close mass propertycommonality with the FM test configuration. Insteadof the SFS for the EDU tests, the RDP was used togenerate attitude knowledge for XACT-50 FSW during the FPM test, since the RDP provided simulatedSRU data.Figure 5. Top-down view of SSDT spherical air bearing system.Figure 6. SSDT solar simulator setup.Copyright A. Deepak Publishing. All rights reserved.Figure 7. SSDT star field simulator, with screen, baffle, and collimator lens as black structures to the right of the silver XACT-50 and redRDP.JoSS, Vol. 10, No. 1, p. 966

Pre-Launch Testing of the Lunar Flashlight (LF) CubeSat GNC SystemThe overall electrical integration that was used forthe EDU and FM testing is shown in Figure 8, thoughthere are some differences between the connections, asdescribed in Section 3.5. The XACT-50 unit, with itsconnected CSS, interfaces with a single connection toanother device. For this test configuration, the otherdevice is the RDP, which receives power from theSSDT spherical air bearing’s power distribution unit(PDU), which itself draws from an on-board Li-Ionbattery pack. The EDU does not have connections forthe second through the fourth CSS. Data from the RDPis sent over an Ethernet connection to a Quantum Pluscomputer, which serves as an on-board telemetry andcommanding computer. The Quantum Plus is a Windows 10 computer that runs the data-logging softwareand allows for commands to be sent to the XACT-50from the SSDT’s ground station computer. The PDUon the spherical air bearing also powers the SFS system, which relies on the spherical air bearing’s KVH1760 gyroscope, Atlas single board computer, and theSFS screen, hood, and lens assembly. A separate laptop is used for designing the SFS initialization files;this laptop is not mounted to the spherical air bearingfor testing. All of the cabling was tightly secured toprevent center-of-mass changes during dynamic motions, which would create unwanted disturbance torques on the system. At the start of the testing process,coarse balancing with large masses were attached tothe spherical air bearing, and trim masses of washerson threaded rods were used to fine balance the complete system before each test in the event that cablesshifted.3.5. Unique Flight Model Test Configurat

both time and monetary costs to flight projects. Vali-dation and verification tests are critical, since once launched, there are limited opportunities for in-flight adjustments, and even small anomalies may lead to mission failure. Rigorous testing requires extensive planning and costs, which can place a significant bur-