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Mission / Spacecraft Analysis and Design 7

2 Mission / Spacecraft Analysis and Design

Rosetta approach to Steins © ESA

J. Eickhoff, Onboard Computers, Onboard Software and Satellite Operations, Springer Aerospace Technology, © Springer-Verlag Berlin Heidelberg 2012


2.1 Phases and Tasks in Spacecraft Development

The following figure shows the phase breakdown of spacecraft development. Listed in addition are also the main tasks to be performed within each phase. Figure 2.2 depicts additionally the prescribed review milestones according to ECSS-E-M30A.

Evaluation of mission and compliant payloaddesign solutions

Phase 0/A

Production, assembly,integration and test

Phase C/D

Design refinement,design verification

Phase B/C Phase E

●Definition of mission objectives and constraints

●Definition of a mission baseline and alternatives / variants

●Analysis of minimum requirements

●Documentation

●Payload requirements analysis

●Definition of alternative payload concepts

●Analysis of resulting spacecraft / orbit / trajectory requirements and constraints

●Standardized documentation

●System design refinement and design verification

●Development and verification of system and equipment specifications

●Functional algorithm design and performance verification

●Design support regarding interfaces and budgets

●Subcontracting of component manufacture

●Detailed design of components and system layout

●EGSE development and test

●Onboard software development and verification

●Development and validation of test procedures

●Unit and subsystem tests

●Software verification

●System integration and tests

●Validation regarding operational and functional performance

●Development and verification of flight procedures

●Ground segment validation

●Operator training●Launch●In orbit commissioning

●Payload calibration●Performance evaluation

●Prime contractor provides trouble shooting support for spacecraft

Spacecraftoperations

Conceptualizationof mission, payload andspacecraft design

Figure 2.1: Tasks in Spacecraft Development Phases.

● PRR Preliminary Requirements Review● SRR System Requirements Review● PDR Preliminary Design Review● CDR Critical Design Review● QR Qualification Review● FAR Flight Acceptance Review

0+A B C D E F

Mission

RequirementsDefinition

Design Definition

Verification &Qualification

Production

Operation

FAR

QR

CDR

PDRSRR

PRRMDR

Launch

PSP Layer 0

PSP Layer nRequirements

DefinitionVerification

Production

Phases

Task

s

Deorbiting

Figure 2.2: Spacecraft development phases and reviews. Source © ECSS-M30A

Phases and Tasks in Spacecraft Development 9

Mission analysis is already performed in the early phases 0/A of a project. From these analysis phases result the requirements towards the space and ground segment of the mission which are further refined in phase B up to the PDR review. The system design – also concerning OBCs, OBSW and Operations Concept – start after SRR. Thus over phases A-C up to CDR the following elements must be defined:● S/C payloads and their functions● S/C orbit / trajectories / maneuvers● S/C operational modes● Required S/C AOCS and platform subsystems● Used onboard equipment and according design● Ground / space link equipment● Onboard functions for system and equipment monitoring and control● Autonomous functions – e.g. for the “Launch and Early Orbit Phase”, (LEOP),

timeline execution● FDIR functions, Safe Mode handling etc.● Test functions● Identification of functions being realized in hardware respectively in software

All these are essential drivers for OBC and OBSW design, the spacecraft's top level and subsystem design as well as for the spacecraft operations concept.

2.2 Phase A – Mission Analysis

Mission analysis serves for determining the optimum orbit w.r.t. ● payload mission product quality,● required target revisit times,● possible ground station contacts

for mission product downlinks and ground servicing.

Resulting from these are requirements towards● mission product data storage

aboard,● onboard timelines / autonomy,● data transmission link budgets.

From this elementary assessment follows the definition of ● characteristics of payload instru-

ments,● operational orbit and LEOP orbit /

trajectory conceptualization,● S/C geometrical concept:◊ Body mounted solar array, (SA), deployable SA, deployable antennas,◊ deployable booms parts,◊ etc.

Figure 2.3: Example: LEOP orbit ground tracks and station visibility. © Astrium GmbH


Next follows the conceptual requirements definition and technology selection for the main functional components such as ● AOCS subsystem sensors / actuators, ● power subsystem equipment,● thermal subsystem equipment,● data handling subsystem equipment.

And finally come the first definitions on● elementary PL modes,● elementary S/C modes,● plus non functional design data such as budgets (mass, power).

The following shall be the first of four consecutive figures restating and sketching out from top to bottom for each development phase the subsequent level of growing design detail.

Table 2.1: Phase A design perimeter.

2.3 Phase B – Spacecraft Design Definition

Phase B serves as first complete design definition on system level. This includes a number of detailed analyses in various fields. Without claiming completeness of the list the most prominent ones shall be cited including their subtasks. One is the refinement of of the orbit definition, which includes● nominal operations orbit,● transfer orbits / trajectories including LEOP trajectories,● orbit control maneuvers and● de-orbiting / re-orbiting after end of life.

Closely associated with the orbits, maneuvers and trajectories is the definition of the spacecraft's operational modes in nominal and failure conditions. The figure below depicts an example of a spacecraft level mode diagram. It includes notation of

Phase B – Spacecraft Design Definition 11

possible transitions be-tween spacecraft modes and identification of transition triggers respec-tively, and required com-manding to invoke the according mode transi-tion. At this level detailed telecommands are ob-viously not yet defined. However these identified modes are already of relevance as they are to be controlled later by the onboard software.The next step of design refinement in phase B concerns the elaboration of a complete satellite product tree with all main physical and functional elements, i.e. including onboard software as a product tree item and eventually any software included for satellite instruments to be developed or software for subsystem controllers. Figure 2.5 shows an example excerpt from such a product tree at the phase B development stage.

Figure 2.5: Phase B product tree example. © Astrium GmbH

Figure 2.4: Satellite modes and transitions. © Astrium GmbH


Next after the completion of the spacecraft product tree is the identification of the individual types of equipment to be used for the mission – i.e. the selection to use star tracker X from supplier Y. In the ideal process this selection is foreseen to be made already at the end of phase B. In real projects however the situation may arise that certain selected equipment has not yet reached the required quali-fication level. In such cases multiple alternative solutions must be kept under con-sideration. For those units where dedicated equipment already could be selected via the supplier documentation then automatically the equipment modes, transitions, telecommands and telemetry becomes available.

Another step in phase B is a first allocation of such equipment operational modes to the nominal and non-nominal spacecraft modes respectively. This identifies mode statuses for the diverse equipment to be switched by the OBSW during spacecraft mode transitions plus possible unit A/B redundancy configurations.

Figure 2.7: Equipment operational modes versus spacecraft modes. © Astrium GmbH

Figure 2.6: Equipment mode diagram example.

PCDU::BatteryBypass_LogicalOperation

entry:MilBusBypassStatus = 1BypassMilBusOffSel . = 1BypassSelection = Parking

Armed

entry:MilBusBypassStatus = 0BypassMilBusOffSel . = 0bypassFired = 0BypassSelection = output

Disarmed

[MilBusCmdBypassFire == 1 || MilBusCmdBypassOff == 1]

[hlcBypassArmingOn == 1]

Output selection and firing executed via Mil1553 command

entry:MilBusBypassStatus = 1BypassMilBusOffSel . = 0BypassSelection = output

Selected

[MilBusCmdSelection == output]

entry:MilBusBypassStatus = 0BypassMilBusOffSel . = 0BypassSelection = Parking

Parked

[MilBusCmdPark == 1] [MilBusCmdBypassOff == 1]

Phase B – Spacecraft Design Definition 13

With this information becoming available a first definition of variable sets – so-called data pools – for the OBSW can be defined, namely the definition of ● variables to be managed via spacecraft telecommands and telemetry,● equipment onboard command and telemetry parameters,● and the complementary set of data bus interface variables to be managed.

Table 2.2: Phase B design perimeter.

In phase B of the S/C development the OBSW architectural design already starts and the subsequent stages are incrementally defined as OBSW is usually developed in a stepwise approach. Concerning the large amount of design refinements performed in the next phase C only those shall be followed further which concern the onboard computers, the software and the S/C operations from ground respectively.


2.4 Phase C – Spacecraft Design Refinement

The first step in phase C is the freeze of the product tree and completing the selection of suppliers for onboard equipment. These final decisions then allow● the completion of interface definitions between onboard equipment (hardware,

signal types / levels and data protocols),● the design consolidation for interfaces between OBC and onboard equipment ◊ either implemented via data buses or ◊ as low level line interfaces via a so-called “Remote Interface Unit”, (RIU)

connected to the core OBC.1

● Furthermore the design for so-called “High Priority Command”, (HPC), interfaces can be finalized. Such HPC lines are commandable from ground even when the OBSW has problems or is down for emergency reconfiguration.

● And with the consolidation of the electrical and data handling design via RIU finally the onboard software variable sets (“data pools”) can be refined for ◊ ground/space TC/TM,◊ for the core OBC,◊ for data handled via RIU and ◊ for TC/TM data of onboard equipment like sensors / actuators /

instruments.

Table 2.3: Phase C design perimeter.

1 Such a RIU in most cases is connected via a data bus to the OBC and provides all required types of low level interfaces like analog, serial, bi-level, pulse for control of simple equipment like heaters, simple sensors etc.

Phase C – Spacecraft Design Refinement 15

After phase C the following design information has been collected:

● Mission concept including orbit, transfer orbits and maneuvers

● Spacecraft product tree● Spacecraft budgets● Spacecraft modes and transitions● Selected equipment types from

dedicated suppliers● Allocation of equipment modes to

spacecraft modes● Equipment modes, interface types● OBC Equipment bus interfaces

● OBC to RIU interfaces● RIU to equipment interfaces● High priority command interfaces● Data pool definitions for ◊ ground / space telecommand /

telemetry,◊ onboard communication and◊ OBC internal onboard software

data pool for OBC internal algorithms

During phase C thus significant design input for the OBSW is consolidated and during this phase the OBSW development is enhanced to detailed design and coding as well as verification of first versions. The detailed roadmap is project specific.

2.5 Phase D – Spacecraft Flight Model Production

In phase C the design of the spacecraft was completed and Engineering Models of the diverse equipment on board (including instruments and payloads) were developed and qualified. Phase D thereafter is devoted to the production of the S/C Flight Model. At the beginning of this phase procurement for all flight models of the required equipment and of the spacecraft structure and flight harness is performed by the S/C prime contractor. During the assembly, integration and test, (AIT), program they subsequently are assembled.

2.5.1 Launcher Selection

Another important step at the beginning of phase D, after project CDR is the final selection of the launcher since for at least most conventional Earth Observation and science satellites missions multiple launcher options exist. During previous design phases the S/C design has deliberately been formulated for compatibility with the 2-3 most likely carriers. The primary selection of a potential launcher which is performed during phase B already evaluates parameters like

● mass to orbit● suitability for according orbit depending on inclination, escape velocity, and

launcher upper stage reignition requirements● overall launcher ΔV

The final selection in phase D then is mainly driven by launch slot availability, cost and status of launcher qualification for new types. The following figures show a


Plesetsk

Plesetzk:

62.70° N40.35° E

Figure 2.8: Rokot launcher and launch site Plesetzk. © DLR

typical example for com-peting launcher systems for Earth Observation satellites of the 1000 kg class at orbit altitude of approximately 700km.

With the final selection of the launcher already implicitly a number of operational edge conditions are frozen, namely the required interfaces between operations center and launch site, the first ground contact times and some required antenna stations.

This directly leads over to the topic of engineering the launch and early orbit phase in detail.

2.5.2 Launch and Early Orbit Phase Engineering

Launch and early orbit phase engineering implies the detailed development of the automated sequences on board the satellite from separation detection. These include

Kourou:

5,23° N52,79° W Figure 2.9: VEGA Launcher and launch site Kourou. © ESA and DLR

Phase D – Spacecraft Flight Model Production 17

● the OBC taking over control of the S/C after being deployed by the launcher's upper stage,

● automatic position and attitude / rotational rate detection,● automated rate damping,● automatic deployment (antennas and solar panels),● to establishment of ground station contact.

Such sequences are subject to tests in S/C assembly phase prior to launch and will be treated in more detail in part IV of this book.

Figure 2.10: Launch sequence and satellites deployment in orbit. © DLR

2.5.3 Onboard Software and Hardware Design Freeze

The final design freezes at the beginning of phase D after CDR comprise definition of● operationally used unit redundancies and redundancy configurations (not all

combinations are usually foreseen for operational use),● the applied line interconnection redundancies,● secondary functions like equipment mode commands, reconfiguration

functions, low level “Failure Detection, Isolation and Recovery”, (FDIR),● final consolidation of data protocols and bus access sequences,● finalization of FDIR concept and last but not least● functions for S/C AOCS in orbit characterization and the complements for

payload instruments characterization.


Table 2.4: Phase D design perimeter.

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