Trevor M. Jones, Mark E. McNelis, Lucas D. Staab, James C. Akers,

and Vicente J. Suarez

Glenn Research Center, Cleveland, Ohio

CoNNeCT Antenna Positioning System Dynamic 

Simulator Modal Model Correlation

NASA/TM—2012-217747

October 2012
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Trevor M. Jones, Mark E. McNelis, Lucas D. Staab, James C. Akers,

and Vicente J. Suarez

Glenn Research Center, Cleveland, Ohio

CoNNeCT Antenna Positioning System Dynamic 

Simulator Modal Model Correlation

NASA/TM—2012-217747

October 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Spacecraft and Launch Vehicle Dynamic Environments Workshop

sponsored by the Aerospace Corporation

El Segundo, California, June 19–21, 2012
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217747 1 

CoNNeCT Antenna Positioning System Dynamic Simulator 

Modal Model Correlation 

Trevor M. Jones, Mark E. McNelis, Lucas D. Staab, James C. Akers, and Vicente J. Suarez 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

1.0 Introduction 

1.1 Background 

The National Aeronautics and Space Administration (NASA) developed an on-orbit, adaptable, 

Software Defined Radios (SDR)/Space Telecommunications Radio System (STRS)-based testbed facility 

to conduct a suite of experiments to advance technologies, reduce risk, and enable future mission 

capabilities on the International Space Station (ISS). The Communications, Navigation, and Networking 

reConfigurable Testbed (CoNNeCT) Project will provide NASA, industry, other Government agencies, 

and academic partners the opportunity to develop and field communications, navigation, and networking 

technologies in both the laboratory and space environment based on reconfigurable, software-defined 

radio platforms and the STRS Architecture. The CoNNeCT Payload Operations Nomenclature is “SCAN 

Testbed,” and this nomenclature will be used in all ISS integration, safety, verification, and operations 

documentation. The SCAN Testbed (payload) is a Flight Releasable Attachment Mechanism (FRAM) 

based payload that will launch aboard the Japanese H-II Transfer Vehicle (HTV) Multipurpose Exposed 

Pallet (EP-MP) to the International Space Station (ISS), and will be transferred to the Express Logistics 

Carrier 3 (ELC3) via Extravehicular Robotics (EVR). The SCAN Testbed will operate on-orbit for a 

minimum of two years. 

One major subsystem of the CoNNeCT system is the Antenna Pointing System (APS). The APS is 

attached to the top of the CoNNeCT payload (Fig. 1). System-level protoflight random vibration testing 

of CoNNeCT was required. Due to the APS flight system’s lengthy development schedule, the flight APS 

hardware was not available at the time of the CoNNeCT system-level protoflight random vibration test. 

Previous random vibration analysis has shown that the dynamics of the APS has a large effect on the 

loading seen by other subsystems during random vibration input. Because of this, a dynamic APS mass 

simulator was designed, fabricated, and used during the CoNNeCT system level protoflight random 

vibration test. 

1.2 Purpose of Model Correlation 

The APS simulator modal survey was conducted to validate the APS dynamic simulator finite 

element model (FEM). A modal survey was performed experimentally using a modal hammer to identify 

the frequencies, damping values, and mode shapes for the primary resonant modes of the assembly. The 

APS simulator FEM was correlated by updating the model to represent the test mode shapes. 

The test-correlated APS dynamic simulator FEM was analytically integrated with the CoNNeCT 

system protoflight FEM in the test configuration. A random vibration analysis of the CoNNeCT system 

protoflight FEM was conducted later and verified the APS dynamic simulator behaves similarly to the 

FEM of the APS actual flight hardware. Once verified, the APS dynamic simulator hardware was 

accepted for use during the CoNNeCT system test. 
NASA/TM

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M—2012-21774

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M—2012-21774

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M—2012-21774

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NASA/TM—2012-217747 

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NASA/TM—2012-217747 7 

6.0 Test Results 

A modal survey using a modal hammer was conducted on the CoNNeCT APS dynamic simulator 

from November 22 to 24, 2010, at the NASA Glenn Research Center Structural Dynamics Laboratory 

(SDL). The CoNNeCT APS Dynamic Simulator and its fixture were installed on the SDL fixed base 

modal floor. 

The pretest analysis predicted the frequency of the target mode to be 116.7 Hz (dominated by global 

Z-direction oil canning of the fixture plate). The initial modal survey test found the frequency of the target 

mode to be 161.5 Hz, which is a 44.8 Hz difference and 27.7 percent higher than the FEM prediction. 

Also, not surprisingly, the FEM and test mode shapes had significant differences. Hence a model 

correlation starting at this assembly model level was judged not to be feasible. Therefore, the modal 

survey and model correlation scope was expanded to include a component (i.e., modal fixture only) and a 

subassembly (i.e., modal fixture and APS dynamic simulator with both antennas removed). In order to 

expedite the modal testing, the order the modal tests were performed coincided with the assigned 

configuration numbering. 

Configuration 1: Modal fixture and fully assembled APS dynamic simulator 

Configuration 2: Modal fixture and APS dynamic simulator with both antennas removed 

Configuration 3: Modal fixture only 

The model correlation effort was performed in the reverse order to allow a building block approach, 

which made this task manageable. 

Time history data was processed into frequency response functions (FRF’s), coherence, and auto 

spectra. Data quality checks were performed to ensure good quality test data. The initial data quality 

check was the visual inspection of the time histories. The next data quality check was the inspection of 

FRF’s, which showed there was minimal measurement noise in the frequency range of interest (100 to 

300 Hz) and well defined resonance peaks. Move response mode shapes, extracted well below the first 

FRF resonance peaks, provided a final data quality check of the instrumentation and the data acquisition 

system. Figures 5 to 7 show FRF plots for the three configurations tested. The plots include the responses 

of all the in-line accelerometers. Time-domain polyreference was the method used for mode extraction. 

The residual fits looked very good. A multivariate mode indicator function (MMIF) was used to aid in the 

modal extraction. The MMIF lined up well with the FRF resonance peaks of the modes of interest. 

Figure 8 is a stability plot showing the pole estimates with the MMIF overlaid for Configuration 1. Modal 

extraction was relatively straight forward for this test and did not require any manual “tweaking” of the 

extracted modal parameters. Table 4 lists the extracted modes and mode shape descriptions for each of the 

three configurations. 

 
NASA/TM—2012-217747 

Figure 5.—

Figure 6.—

Configuration

Configuration

8 

 1 in-line accel

 2 in-line accel

lerometers FRF

lerometers FRF

 plot. 

 plot. 
NASA/TM—2012-217747 

Figure 7.—

Figure 8.—Sta

Configuration

bility diagram 

9 

 3 in-line accel

with MMIF ove

lerometers FRF

rlaid for config

 plot. 

uration 1. 
NASA/TM—2012-217747 10 

TABLE 4.—TEST MODES AND MODE SHAPE DESCRIPTIONS 

Configuration 1: Fully Assembled APS Dynamic simulator

Mode # Freq (Hz) Mode Shape Description

1 114.8 HGA Bending X Direction

2 121.6 HGA Bending Y Direction

3 161.5 Global Z Direction Bending HGA Out of Phase

4 212.7 APS Arm Bending About Y

5 224.3 MGA X Direction Mode

6 237.9 Actuator Moving in X/Y Direction

Configuration 2: APS Dynamic simulator With HGA and MGA Removed

Mode # Freq (Hz) Mode Shape Description

1 181.4 Panel Mode Z APS in Phase

2 237.1 Actuator Moving in X/Y Direction

3 336.8 APS Arm Bending About Y

Configuration 3: APS Dynamic simulator Removed ‐ Fixture Only

Mode # Freq (Hz) Mode Shape Description

1 209.2 Panel Mode Z (One Node)

2 379.0 Panel Mode Z (Two Nodes in Y)

3 469.5 Panel Mode Z (Two Nodes in X)

7.0 Model Correlation 

7.1 Correlation Methodology 

The APS Dynamic Simulator correlation methodology was based on a building block approach using 

three configurations (correlation steps). The three correlation steps correspond with the three test 

configurations. The correlation was conducted in reverse order with respect to the order the modal tests 

were performed. The three correlation steps were as follows: 

Step 1 was the correlation of the FEM of the fixture base plate by itself, without the APS dynamic 

simulator, to test configuration 3 test data. The goal here was to get the dynamics of the fixture base plate 

by itself correct. 

Step 2 involved adding the APS dynamic simulator, but without the antennas, to the correlated fixture 

base plate FEM from Step 1 and correlating this model to test configuration 2 test data. 

Step 3 involved correlating the full FEM, which incorporated the correlated components from Step 2, 

to test configuration 1 test data. Figure 9 shows the FEM for each of the three model correlation steps. 
NASA/TM

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M—2012-21774

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NASA/TM—2012-217747 

F

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NASA/TM—2012-217747 13 

7.3 Step 2: APS With HGA and MGA Removed Correlation 

With the fixture base plate FEM well correlated, the second step was to begin correlation of the APS 

dynamic simulator hardware. The FEM to be correlated in Step 2 added the APS dynamic simulator 

hardware with the two antennas removed (HGA and MGA) to the correlated base plate FEM from Step 1. 

A modal test was conducted on the APS Dynamic simulator assembly in this configuration, and three test 

modes were extracted. The comparison of the analytical and test modal frequencies is given in Table 6. 

The modal effective mass (MEF) table for the Step 2 baseline and correlated FEM is also given in 

Table 6. A visual inspection of the FEM versus test mode shapes (using the TDM) was performed for 

each of the three modes. Based on visual inspection, the three test modes appear to match with the first 

three modes of the FEM. FEM Mode 1 has the highest effective mass at 33 percent in the Z-direction. The 

shape of Mode 1 also seems to compare well with the target mode identified in the pretest analysis and 

can be described as the global panel mode in the Z-direction. Because this is the mode of interest, the 

correlation effort for Step 2 was focused on closely matching the frequency of Mode 1 to the test results. 

For mode 1 the FEM prediction was 156.77 Hz and the modal survey test showed the same mode to be 

181.36 Hz, which is 13.56 percent higher, necessitating modification of the FEM. 

In order to increase the FEM mode 1 modal frequency, several correlation parameters were considered 

to stiffen the model: the thickness of the parts, the material properties used, the mass, and the boundary 

condition connecting various components. As with the base plate, a micrometer was used to measure 

thicknesses of the APS dynamic simulator hardware and they agree well with the FEM properties. The FEM 

material properties were also verified to be correct. Table 2 compares the FEM components weights to those 

of the actual hardware. The weight differences for the APS base and APS arm frame assembly are about 5 

and 25 percent, respectively. The FEM contained some non-structural mass which was adjusted to help 

bring the FEM into closer alignment with the actual weights of the hardware. 

With the first three correlation parameters exhausted, the remaining parameter was the boundary 

condition between the various components. The interface between the fixture base plate and the APS 

dynamic simulator base was considered first (Fig. 12). Additional RBE2 elements (constrained in all 

degrees of freedom) were added to the FEM in an attempt to more accurately represent that interface. 

Approximately thirty RBE2 elements were added. The elements connect nodes on the fixture base plate to 

nodes on the APS dynamic simulator base. This change did increase the resulting frequency of mode 1. 

The next boundary condition examined was the kinematic pins connecting the APS arm frame to the APS 

base (Fig. 12). The pins in the model had rotational degrees of freedom released. It was determined that 

due to a stiff connection and possible friction at these locations the connection would be better modeled 

as fixed in translation as well as in rotation. This change was implemented in the FEM. The final 

boundary condition taken into consideration was the interface between the actuators and the L-bracket. 

Additional RBE2 elements (constrained in all degrees of freedom) were added here to account for the 

relatively thick flange at those joints. Eight spider RBE2 elements were added around the circumference 

of the two joints between the existing RBE2 elements. The existing RBE 2 elements are at the fastener 

locations. With all these changes implemented collectively, the frequency of Mode 1 increased to 

184.81 Hz, which was less than 2 percent from the test modal frequency. Subsequently, the percent error 

for Mode 2 and 3 were also decreased. Table 6 contains the updated effective mass table for the correlated 

APS dynamic simulator without antennas model. Visual inspection the FEM mode shapes and test modes, 

visualized using the TDM, showed good agreement. With the frequencies and mode shapes matching 

well, the APS dynamic simulator without antennas is considered to be correlated. 
NASA/TM

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M—2012-21774

Figure 1

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embled config

ction and is al

ble for the co

mode modal 

higher. Hence

elected as sec

cies for these 

14 

with HGA and 

MGA REMOVE

Dynamic Si

ully completed

p 3 was the co

A modal test w

guration, and 

lso the target 

orrelated mode

frequency is 

e the FEM wa

ondary mode

secondary m

MGA removed

ED EFFECTIVE

imulator Co

d, the final st

orrelated mod

was conducte

six test mode

mode for cor

el from Step 

141.29 Hz wh

as judged to b

es for correlat

modes were hi

d; model updat

E MASS TABLE

orrelation 

tep was to cor

del from Step 

ed on the APS

es were extrac

rrelation base

2 with the un

hile the test m

be not correlat

tion. Similar t

gher than the

tes. 

ES 

rrelate the ful

2 with the 

S Dynamic 

cted. Mode 3

ed on the prete

ncorrelated 

modal frequen

ted and neede

to Mode 3, th

e correspondin

l APS 

 was 

est 

ncy is 

ed to 

he 

ng 
NASA/TM

Once 

thickness 

antennas t

and HGA

HGA mat

hardware 

the hardw

correspon

RBE2 elem

The updat

the two se

Table

the correla

5 percen

accomplis

values be 

secondary

for the tar

mode. Fig

shows goo

visualizat

considere

M—2012-21774

TAB

TA

again, severa

of the parts, th

to the rest of t

. The material

tched their cor

as compared 

ware and FEM 

nding FEM mo

ments were ad

ted model app

econdary mod

e 9 is a cross-o

ated FEM. Th

nt between tes

shed. The prim

greater than 0

y mode shape

rget and two s

gures 13 to 15

od agreement

ion. Since all

d to be correl

47 

BLE 7.—FULL A

ABLE 8.—COMP

l parameters w

he material pr

the hardware. 

l properties of

rresponding F

to the fully as

is less than 1

ode showed ex

dded to the FE

pears to correl

des match the t

orthogonality

he frequency 

st data and the

mary mode sh

0.9 for the tar

correlation g

secondary mo

5 compare the

t between them

l correlation g

lated. 

APS DYNAMIC

PONENT WEIG

FEM VERSU

were consider

roperties used

A micromete

f MGA and H

FEM weights. 

ssembly FEM

percent. Anim

xaggerated FE

EM at the con

late very well 

test results wi

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model correla

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hape model co

rget and two s

goal was for th

odes. This sec

e test and corr

m. The test m

goals have bee

15 

C SIMULATOR

GHTS TEST AR

US CORRELATE

red in order to

d, the mass, an

er was used to 

HGA were also

Table 8 summ

M. Table 8 show

mations of the

EM antenna d

nnection betw

with the test d

ith less than 2

of the first thr

ation goal wa

model for the 

orrelation goa

secondary mo

he magnitude

condary corre

relation FEM 

mode shapes h

en met, the fu

R EFFECTIVE M

RTICLE VERSU

ED FEM 

o increase FEM

nd the bounda

 verify the thi

o verified. Th

marizes the co

ws that the ov

e mode shape

displacement. 

een the antenn

data; the frequ

2 percent error

hree test mode

as a frequency

target and tw

al was for the

odes; which w

e of the off-di

elation goal w

 mode shapes

have been bac

ull APS dynam

MASS TABLES 

US BASELINE 

M modal freq

ary condition c

ickness prope

he weights of t

omponent wei

verall weight d

s for test mod

Because of th

nas and the A

uency of the t

r, shown in Ta

es to the first t

y difference o

wo secondary m

e magnitude o

was accomplis

iagonal terms 

was accomplis

s for the first 

ck-expanded t

mic simulator

quencies: the 

connecting th

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the MGA and

ights of the 

difference bet

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APS arm assem

target mode an

able 7. 

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modes; which

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be less than 0

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to aid with 

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d

tween 

l

mbly. 

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from 

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0.1 

arget 

and 
NASA/TM

1

2

Test Modes

3

M—2012-21774

TABLE 

Freq (Hz) 11

114.83 0

121.58 0

161.54 0

47 

9.—CROSS OR

Figure 13.—

Figure 14.—

1 2

16.95 121.5

0.99 0.13

0.09 0.99

0.04 0.07

Analysis M

RTHOGONALIT

—APS HGA loc

—APS HGA loc

3

55 163.88

3 0.03

9 0.05

7 0.99

Modes

16 

TY TEST MOD

cal X-direction b

cal Y-direction b

%Freq Diff Mo

1.85 AP

0.03 AP

1.45 Glo

ES VERSUS AN

bending mode 

bending mode 

ode Shape De

PS HGA Local X

PS HGA Local Y

obal Z Plate Be

NALYSIS MOD

shape. 

shape. 

scription

X-Direction Ben

Y-Direction Ben

ending HGA/AP

DES 

ding Mode

nding Mode

S out of phase
NASA/TM

The A

criteria. T

had a freq

table (Tab

primary ta

1. Mark 

“Fluid

NASA

2. MSC 

3. Matth

Feb 2

4. Dr. Pe

M—2012-21774

Fig

APS dynamic 

Three modes w

quency differe

ble 9) contain

arget mode. 

E. McNelis, 

ds and Combu

A/TM—2005

Software Cor

hew Kaplan “A

010. 

eter Avitabile

47 

gure 15.—Glob

simulator FE

were selected 

ence of less th

ned diagonal v

Vicente J. Su

ustion Facility

5-213795, Ma

rporation, “M

Attune Increa

e, Dr. John M

bal Z plate bend

8.0

EM was succe

as target mod

han 5 percent

values greater

uarez, Timoth

y: Combustio

ay 2005. 

MSC.Nastran 2

asing Harmon

Mottershead, “M

17 

ding HGA/APS

Conclusio

essfully correl

des, one prim

comparing te

r than 0.9 and

Referenc

hy L. Sullivan

on Integrated 

2005 Quick R

ny: An Introdu

Model Correl

S out of phase m

ons 

lated based on

mary and two s

est to analysis

d off diagonal 

ces 

n, Kim D. Otte

Rack Modal 

Reference Gu

uction to Attu

lation and Up

mode shape. 

n the ISS mod

secondary. A

s. The final cr

 values less th

en, and James

Model Corre

uide,” 2005. 

une v2.0,” AT

pdating,” Apri

del correlatio

ll selected mo

ross-orthogon

han 0.1 for th

s C. Akers 

lation,” 

TA Engineeri

il 2000. 

on 

odes 

nally 

he 

ing 
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CoNNeCT Antenna Positioning System Dynamic Simulator Modal Model Correlation 

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The National Aeronautics and Space Administration (NASA) developed an on-orbit, adaptable, Software Defined Radios (SDR)/Space 

Telecommunications Radio System (STRS)-based testbed facility to conduct a suite of experiments to advance technologies, reduce risk, 

and enable future mission capabilities on the International Space Station (ISS). The Communications, Navigation, and Networking

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opportunity to develop and field communications, navigation, and networking technologies in both the laboratory and space environment 

based on reconfigurable, software-defined radio platforms and the STRS Architecture. The CoNNeCT Payload Operations Nomenclature is 

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Logistics Carrier 3 (ELC3) via Extravehicular Robotics (EVR). The SCAN Testbed will operate on-orbit for a minimum of two years. 

15. SUBJECT TERMS

Structural dynamics; Structural dynamic response 

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