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Most NASA GN&C projects ollow a tra-

ditional process: Domain experts and

analysts speciy the behavior o the core

algorithms in detailed requirements docu-

ments. Following CDR, these documents

are handed o to the ight soware engi-

neers or implementation into the ormal

ight soware. Producing the specication

document alone oen requires years o e-

ort, and because coding can begin only

aer the spec is complete, it can be years

beore there is any code to test.

Te design and development o the

GN&C ight algorithms or Orion is a part-

nership between NASA, Lockheed Martin,

and other contractors. Model-Based Design

has helped these organizations work on

both the GN&C algorithm and ight so-

ware development concurrently. Simulink

models serve as an executable specication

rom which ight soware is automatically

generated. As a result, the domain experts

the GN&C analystswork directly with the

executable algorithm models rather than

with documents that must then be inter-

preted by soware developers (Figure 1).

Merging the design and analysis environ-

By Scott Tamblyn and Joel Henry, NASA, and John Rapp, Lockheed Martin

When the guidance, navigation, and control (GN&C) system for the Orion

crew vehicle undergoes Critical Design Review (CDR), more than 90% o the

ight sotware will already be developeda frst or NASA on a project o thisscope and complexity. This achievement is due in large part to a new develop-

ment approach using Model-Based Design.

Accelerating NASA GN&C FlightSoftware Development

MATLAB Digest

ment with the ight soware development

environment enables the joint team to iden-

tiy and resolve problems earlier and has

the potential to reduce overall development

time by a year or more.

Products Used

Simulink

Embedded Coder

Simulink Verification and Validation

Figure 1. Schematic comparing our traditional design process and the approach we used to developOrion GN&C software. Image courtesy NASA.

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Laying the Groundwork for theNew Approach

While Lockheed Martin was already amil-

iar with Model-Based Design, this approachrepresented a paradigm shi or many

NASA engineers and contractors. o prepare

the ground, lead GN&C and ight soware

engineers collaborated in detailing the

benets o the new development process,

and explained how it would change the roles

and responsibilities o the GN&C analysts

and ight soware engineers.

Modeling standards were developed

to enable the approximately 100 engineers

rom multiple organizations working on

the GN&C algorithms to develop consis-

tent models, understand each others work,

and collaborate efciently. Tese standards

ensure that all models are clear and

readableparticularly important or a

large team using the models as documenta-

tion. Te standards were based on lessons

learned in earlier projects and on

MathWorks Automotive Advisory Board

(MAAB) guidelines.

Developing and IntegratingGN&C Algorithms

Te rst step in developing the GN&C

system architecture was the creation o

the Empty-Box Architecture (EBA).

Te EBA contains approximately 100 unc-

tional modules, or computer soware units

(CSUs).

Te engineer or subteam responsible ora CSU assembles a model comprising one

hundred or more Simulink library blocks

and components. Because the CSUs are

specied as Model Reerence blocks, each

unit can be thoroughly simulated on the

desktop beore it is handed o to the ight

soware team. Te engineer then uses

Simulink Verication and Validation and

the Simulink Model Advisor tool to veriy

that the model meets the modeling stan-

dards. Te engineer also generates code to

ensure that there are no problems in themodel that would preclude code generation.

When delivering the CSU, the engineer pro-

vides both the Simulink model and the test

inputs and expected test outputs rom the

unit tests.

o validate the overall GN&C soware,

as well, the team runs the higher-level

model composed o all unctional units in

closed-loop simulations to urther debug

and test the algorithms. In these situations,

NASA relies on rick, a high-delity, six-

degree-o-reedom simulation inrastruc-

ture that NASA has been rening or more

than 20 years. Te simulation environment

includes mathematical models o the vehi-cles sensors (such as inertial measurement

units and star trackers) and eectors (such

as reaction control systems), as well as aero-

dynamics, gravity, and the space environ-

ment.

Te higher-level model is linked to the

rick environment via a socket connec-

tion. Tis setup enables engineers to debug

their CSU algorithms in the ull-eatured

Simulink environment while runningclosed-loop simulations. For example,

Orion

Image courtesy NASA.

Designed for deep space missions, NASAs Orion crew exploration

vehicle will carry a new generation of astronauts far beyond low Earth

orbit to multiple destinations throughout our solar system, such as as-

teroids, the moon, and eventually, Mars. Orion will replace the space

shuttle as the agencys principal vehicle for human space exploration.

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engineers can simulate the atmospheric

entry phase o ight and use scopes in

Simulink to observe signals in their own

CSU or anywhere in the EBA.

Generating Code withEmbedded Coder

Because the C++ ight code is automatically

generated rom the Simulink models, most

ight soware coding will be completed

beore CDR. In addition to saving time

and reducing risk, code generation with

Embedded Coder provides three advan-

tages at this stage o the program: First, it

enables us to veriy that the code that will

ultimately get deployed aboard the target

vehicle can be generated and that it pro-

duces the same results as the Simulink

simulations o the source models. Second,

code generation enables engineers who are

accustomed to writing their own code to

inspect the generated C++ code and even

debug directly in the code. Tird, it enables

analysts to dramatically realize closed-loop

run-time perormance by embedding thegenerated code directly into the rick simu-

lation inrastructure.

Simulink is ideal or running closed-loop

simulations because its interactive, visual

environment helps engineers identiy and

resolve deects quickly. For ull analytical

verication testing, however, simulation

speed is a more important consideration

because o the need to run hundreds o

thousands o Monte Carlo simulations or a

range o scenarios.

Closed-loop simulations employing the

generated code embedded in rick per-

orm approximately 10 times aster than

real time. As a result, a ull 10-day Orion

mission can be simulated in just one day.

Engineers have already conducted entry-

stage simulations using both approaches

using the Simulink model to drive rick,

and embedding the generated code into

rick. When compared, the results o the

two simulations were ound to be bit-or-bittrue; they matched down to the lowest level.

Blazing a Trail

Tis GN&C project is blazing a new

trail or NASA in many ways. Simu-

link and Embedded Coder are

enabling a large team o domain experts

rom NASA, Lockheed, and other con-

tractors to develop algorithms or com-

plicated trajectories and scenarios, run

simulations within a legacy simulation

environment, and generate ight soware

code that will ultimately be deployed on

the vehicle.

On long-term projects like this one, it is

not uncommon or agency requirements

and priorities to shi. With the algorithms

captured in models rather than in low-level

code, the engineers are well positioned to

take the project in whatever direction it

needs to go.

Learn More Orion Crew Exploration Vehicle

nasa.gov/orion

Orion GN&C: MATLAB andSimulink Standardsmathworks.com/orion_gnc

NASAs X-43A Scramjet AchievesRecord-Breaking Mach 10 Speedmathworks.com/nasa-scramjet

91876v00 04/11

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are registered trademarks o The MathWorks, Inc. Seewww.mathworks.com/trademarks or a list o additionaltrademarks. Other product or brand names may be trade-marks or registered trademarks o their respective holders.

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