THE DESIGN OF G2-SAT OPERATION SCENARIO
FOR DATA ACQUISITION AND TRANSMISSION
Ery Fltrianingsih

Mechatronlcs Division, Indonesian Institute of Aeronautics and Space, LAPAN
Email: ery_fitrianingslh@yahoo.com

ABSTRACT
G2-SAT mission concept is designed according to its payload requirements. G2SAT will carry four channels imager as its payload, to support food security program.
Results from orbit analysis are combined with payload specification data, ground
station accesses and by implementing mission constraints to create the mission
concept. This paper will present t h e design of operation scenario t h a t c a n be applied on
INASAT-1 as a part of mission concept. Operation scenarios will define how d a t a is
acquired a n d transmitted from t h e satellite to ground stations so t h a t its mission c a n
be fulfilled.
Keywords: G2-SAT, Mission concept, Mission design
ABSTRAK
Konsep misi satelit LAPAN generasi II (G2-SAT) dirancang berdasarkan m u a t a n
yang a k a n dibawa oleh G2-SAT yaitu sistem imager dengan empat kanal sebagai alat
u n t u k menghasilkan citra yang dapat dimanfaatkan u n t u k program k e t a h a n a n
pangan. Perancangan konsep ini dilakukan dengan menggabungkan hasil analisa orbit,
d a t a spesifikasi m u a t a n yang telah diolah, a k s e s satelit ke stasiun bumi serta dengan
menerapkan b a t a s a n - b a t a s a n misi. Paper ini m e m b a h a s tentang perancangan skenario
operasi yang dapat diterapkan pada G2-SAT sebagai bagian dari s e b u a h konsep misi.
Skenario operasi menjelaskan bagaimana d a t a / c i t r a di ambil oleh satelit d a n
dikirimkan dari satelit ke stasiun bumi sehingga tujuan misi yang direncanakan dapat
dicapai.
Kata k u n c i : C2-SAT, Mission concept, Mission design
1

INTRODUCTION

Satellite is a system consists of
several subsystems, t h e satellite b u s a n d
payload. Those subsystems; structure,
power, communication, onboard d a t a
handling, attitude and orbit determination
a n d control a n d thermal control are
interdependent to each other creating a
satellite b u s . The duty of satellite b u s is
supporting the payload while the payload
does t h e mission.
The process of designing a
satellite begins with designing a mission
concept to answer mission requirements
and constraints. In the mission concept,
the relations between subsystems are
defined. Mission concept also consists of
t h e operation scenario, which describe
92

how the satellite will operate during its
mission.
In the mission concept
of
operation, it is defined how t h e satellite
communicate with the ground station,
how the payload do its mission, how t h e
mission is control, etc. For a satellite
with an imager as its payload, like G2SAT, it is defined in the mission concept,
how the imaging system captures an
image a n d how the data is delivered.
Mission
concept
design
is
influenced by a satellite orbit, as well as
its ground stations location and the
imager specifications. It is necessary to
conduct through analysis of satellite
orbit, its access to ground stations and
image-data parameters to design a
concept of operation.

• This research aims to design
some operation scenarios as a part of
mission concept for G2-SAT. The operation
scenario will describe t h e scanning
process (capturing images) a n d sending
images to ground stations as a p a r t of
mission concept. Those scenarios will be
created based on analysis result of
orbital parameters a n d ground tracks,
ground station access a n d image (data)
related parameters.
2

METHODOLOGY

Following successful launch of
LAPAN-TUBSAT, LAPAN is initiating a
new satellite program in 2005. The
satellite, which is a remote sensing
satellite, h a s a mission to support
national food security program which
become one of priorities of Indonesian
government.
In order to support food security
program, the satellite should be able to
estimate crop planting and harvesting
area, identify crop growth stage, identify
vegetation type, identify crop condition
and estimate l a n d ' productivity. G2-SAT
is a satellite concept proposed to fulfill
the mission.
To
perform
those
functions
(estimating crop area, growth stage, etc),
the
satellite
must
satisfy
certain
requirements. As stated in the mission
definition document (Mission Analysis
and Design Team, 2006), the mission
requires t h a t G2-SAT imager capable of
providing images with 10 to 30 m spatial
resolution, able to capture images of t h e
whole area of Indonesia in one month
and have four channels in RGB and NIR
bands. It is also required that images a r e
taken
under
constant
illumination
condition for analysis purpose.
An imager that h a s four channels
in red, green, blue and near infrared
frequency is chosen based on the mission
requirements above. It has been analyzed
that the imager is capable to provide an
approximately 22 m spatial resolution
and 221 km swath width in nadir
pointing orientation (Payload Analysis

and Study Team, 2007). The imager
consists of two linier CCD, each contains
of 3 lines but only four will be u s e d .
Each CCD consists of 10200 pixels with
scan rate of 411 Hz. This payload system
also h a s its own memory unit to stored
images (Sun Space, 2007).
G2-SAT orbit is also selected
regarding to its mission requirements
a n d constraints too. G2-SAT will be
orbited in low altitude (Low Earth Orbit)
with Sun Synchronous Polar Orbit
(SSPO) as orbit type. SSPO is necessary
to get a constant illumination condition
during scanning process due to its
constant local time of ascending/
descending node (Wertz, J a m e s R. a n d
Larson, Wiley J., 1999).
Piggyback l a u n c h strategy as a
mission constraints are also taken into
consideration in designing G2-SAT orbit.
This strategy is a common solution for a
small satellite program to reduce the
overall cost of the program. But as a
consequence, t h e mission designer h a s
no freedom to choose the satellite orbit
as desired. It m e a n s orbital parameters
(altitude and inclination) will follow the
main payload of the launcher.
In G2-SAT case, it h a s been
analyzed t h a t t h e most suitable launcher
is Long March 4B which scheduled to
l a u n c h satellites to 778 km altitude in
2011 (G2-SAT Launch System Team,
2007).
Having
the
orbital
altitude
decided, other orbital parameters are
now can be obtained u s i n g Keplerian
orbit equations. Keplerian orbit a r e often
u s e d to describe the motion of a body
orbiting a central body. In Keplerian
orbits, only two bodies are taken into
considerations.
Keplerian orbital equations are
known as 2-body equation of motion.
Effect of third body, for example gravity
of s u n or moon in t h e case of earth
orbiting satellite is ignored. Earth gravity
is a s s u m e d to be the only force acting on
it. Figure 2-1 shows the definition of
93

parameters in 2-body equation of motion
(Wertz, J a m e s R. and Larson, Wiley J.,
1999).
In Figure 2 - 1 , orbital orientations
are defined regarding to inertial frame (X,
Y, Z). This frame is a s s u m e d fix in space.
Ascending node is the node where the
orbit cross equator in ascend/descend
motion. Inclination can be defined as the
tilting angle of the orbit from earth
pole.

ju9 is geocentric gravitational c o n s t a n t
After estimating orbital parameters,
the payload data p a r a m e t e r s c a n be
obtained from the following equations.
Scanned lines in the service area
Lines=Access

durationxLine_rate

(2-8)

Scanned pixels in the service area
Pixel=LinesxPixel_per_lines

(2-9)

Scan time in sec
Ts=LenglhxVg

(2-10)

Data size in bits
Data

Figure 2 - 1 : Definition of Keplerian orbital
parameters
Keplerian orbital equations to
calculate orbital parameters for circular
SSPO orbit analysis are:

_

sue=PixelxQuantization

(2-11)

Equations 2-8 to 2-11
are
parameters needed for analytical purpose
to create the operation scenario. Lines in
equation 2-8 refers to total scanned line
in one p a s s when the satellite is inside
the service area. Pixel in equation 2-9
refers to total pixel of the image taken in
one p a s s . Scan time in equation 2-10 is
the time needed to scan the service area.
Line rate, quantization a n d pixel rate are
parameters provided from the imager
specification.
The complete step in designing
G2-SAT mission concept is shown in
Figure 2-2.

Figure 2-2: Steps in designing G2-SAT
mission concept
As shown in Figure 2-2, the
design of mission concept is started by
defining
mission
requirements
and
constraints t h e n followed by establishing
payload specifications. In G2-SAT case,
the first three processes had been done
previously while the fourth a n d the fifth
step are conducted in this research.
94

The purpose of doing orbit
analysis is to predict the ground track of
G2-SAT orbit and estimate orbital
parameters. Orbital parameters are
determined
using
Keplerian
orbit
equations. By predicting the ground
track via simulation, mission designer
can predict the performance of the
satellite and propose some operation
scenario.
After conducting orbit analysis,
the next analysis as c a n be seen in
Figure 2-2 is access analysis between
satellite and ground station. Access
duration is obtained as a simulation
result using STK software.
Once the access analysis is done,
the payload data parameters: data rates,
number of pixels, n u m b e r of scenes per
orbit, etc can be estimated using equation
2-8 to 2 - 1 1 .
The final step of designing
mission concept is defining a strategy,
when and how G2-SAT will do its
missions. Several assumptions need to
be taken in order to conduct the
analysis.
2.1 Service Area
Service area is defined as a region
where the satellite does its mission. In
G2-SAT case, the mission consists of two
main tasks: capturing images a n d
sending those images to ground stations.
Service area of G2-SAT is assumed
limited to Indonesian boundary which
lies from 6° North Latitude to 11° South
Latitude and 95° to 141° East Longitude.
This assumption is considered
valid since the mission itself does not
required to provide the data for other
countries. The definition of G2-SAT
service area within Indonesian boundary
is shown in Figure 2-3. On Figure 2-3,
G2-SAT service area is described by the
yellow box which m e a n s t h a t G2-SAT
will operate only inside this box. The
green line is the track where G2-SAT will
scan or capture images.

Figure 2-3: Definition of G2-SAT service
area
2.2 Ground Stations
After conducting orbit analysis,
the next step in designing mission
concept is investigating the coverage of
ground stations. Three ground stations,
all located in Indonesia, are a s s u m e d to
be utilized to communicate with G2-SAT.
One ground station which located in
Biak is utilized for Telemetry, Tracking
and Control (TTC), hence to control the
satellite. Another one, located in ParePare is utilized for downloading data.
While the last one, located in Rumpin
can be utilized for both TTC a n d data.
Location of each ground stations
are Biak; 1,15° S, 136.05° E, Altitude:
100m, Rumpin; 6.2168° S, 106.3261° E,
Altitude 100 m, Pare-pare; 4.0083° S
119.618° E, Altitude 100m. TTC ground
stations operated in S-band frequency
while d a t a ground stations operated in
X-bands frequency. Elevation angle of all
ground stations are a s s u m e to be 5°.
3

RESULT AND ANALYSIS

3.1 Orbit Analysis
Orbital parameters of G2-SAT are
presented in Table 3-1. These parameters
are calculated using equation 2-1 - 2-6
for 778 km orbital altitude, which is the
expected altitude given by G2-SAT
launcher.

95

Table 3 - 1 : ORBITAL PARAMETERS OF
G2-SAT AT 778 KM ALTITUDE
Parameter
Altitude, h
semi major axis, a
orbital period, P
Mean motion, n
satellite velocity, v
inclination, i
•

•

Value
778 k m
7156.14 km
100.41
minutes
14.34 revs/day
7.463 k m / s
98.51 deg

i

eccentricity, e

0

From Table 3 - 1 , G2-SAT orbital
period at 778 km altitude is expected to
be 100.41 minutes. It m e a n s G2-SAT will
need 100.41 minutes to complete one
revolution a r o u n d the earth. Orbit
revolutions per day are expected to be
14.34 revolutions per day. It m e a n s G2SAT circling a r o u n d the earth 14.34
times a day. The (SSPO) inclination is
98.51 deg m e a n s G2-SAT orbit tilted
8.51 deg from earth poles. G2-SAT
eccentricity is 0 which is expected to be
an approximate value since it is
a s s u m e d t h a t the orbit is circular and
only earth gravity is taken into account
(Keplerian orbit).
Parameters in Table 3-1 are u s e d
to do the simulation of the ground track
of G2-SAT orbit. In Figure 3 . 1 , the
simulation result using software STK 8.0
is presented. The simulation result is
G2-SAT's predicted ground track which
is propagated for two days. Keplerian
orbit in this simulation is corrected by
selecting J2 propagator to represent the
effect of earth oblateness.

descending paths only. The brown lines
are the ground tracks of G2-SAT orbit.
Simulation results of ascending p a t h s do
not shown here because it is almost
similar to the descending p a t h s ; differ
only in the direction which is opposite to
one another. Four brown lines cross
Indonesia every two days in Figure 3 - 1 .
It m e a n s G2-SAT p a s s Indonesia two
times a day for descending p a t h and the
s a m e h a p p e n to ascending p a t h . Total
p a s s a day is four times.
It can be seen in Figure 3 - 1 , G2SAT ground track drifting westward. This
phenomenon occurs as a result of earth
rotation from west to east. Longitude
shift between two neighboring orbits is
25.103° or a b o u t 2794.417 km at
equator. The ground track of G2-SAT
orbits on day one is represented by the
yellow n u m b e r s while green n u m b e r s
represent the ground track on day 2.
Since the imager field of view is
221 km as required by the mission, there
will be a lot of gaps in the images. Other
simulations were done by adding
propagation days to check how many
days needed so t h a t images of Indonesia
are taken without gaps. Figure 3-2
shows ground tracks of G2-SAT orbits
for 13-days propagation.

Figure 3-2: Ground tracks of G2-SAT
orbits for 13-days propagation

Figure 3 - 1 : Ground track of G2-SAT orbit
for two days propagation

is
96

The simulation result in Figure 3-1
showing G2-SAT ground track in

It is shown in Figure 3-2 that
after 13 days propagation, the distance
of two orbits decreased to 607 km.
Another interesting result from Figure 3-2
is that after 13 days, the ground track
repeating the p a t h of day 1 with a slight

drift.

More simulation was done by
adding more propagation days to see in
how many days will the distance between
orbits is close to 221 km. The result is
presented in Figure 3-3 which shows
ground track simulation for 30 days
propagation.

Figure 3-4: Coverage of G2-SAT ground
stations

Figure 3-3: One month propagation of
G2-SAT orbits
Figure 3-3 shows t h a t for one
month orbit propagation, the maximum
distance between two neighbors' orbits is
approximately 222.7 km. This value is
closed to payload swath width parameter
required at this altitude which is 221
km. By a s s u m i n g t h a t G2-SAT is nadir
pointing when capturing images, full
coverage of Indonesia without any gap
will be achieved in one month.
3.2 Ground Station A c c e s s Analysis
Figure 3-4 shows coverage of all
G2-Sat ground stations. As mention
before in chapter 2.2, TTC ground
stations are located in Biak and Rumpin.
It means only from these two ground
stations G2-SAT will be able to
controlled,
downlink telemetry and
commanded.
Data ground stations,
utilized for downloading image data are
located in Pare-Pare and Rumpin. Only
from these two ground stations, image
data will be downlinked from G2-SAT.

Figure 3-4 show the region where
G2-SAT can communicate with the
ground stations which represented by
four dashed circles. The blue circle is the
region where G2-SAT can communicate
with Biak (TTC) ground stations. The
pink circle represents the region where
G2-SAT can down link image d a t a to
Pare-Pare ground station.
Two orange circles with different
size represent the region where G2-SAT
can communicate with Rumpin ground
station. Down linked data can only be
done in the smaller region while
telemetry and control can be done in the
bigger region. The real time process can
be done in the smaller region since only
in this area, images can be down linked.
Maximum access duration which
defines how long a satellite can
communicate with ground station is
estimated using reports from STK
simulations. For G2-SAT, maximum
access duration is divided into two
categories: access duration to TTC
ground stations and access duration to
data ground station. STK reports for
access s u m m a r y from
G2-SAT to
Rumpin (TTC ground station) are
presented in Table 3-2.

97

Table 3-2: ACCESS REPORT FOR S-BAND
16 «ir 200714:55:11
16 Mr 2007 11:55:11
&te'ilite-G2SAT-Sefsor-SJ*id-To-facni:y-RWK-Sensor-TTC_aitf2: Access SMHry Seport

3 . 3 Image-Data Parameters Analysis
Image-data parameters of G2-SAT
are presented in Table 3-4. Parameter
values are estimated values calculated
using equation 2-7 - 2-11 except for
n u m b e r of pixels, line rate a n d
quantization which are obtained from
imager specifications.
Table 3-4: IMAGE-DATA PARAMETERS
OF G2-SAT AT 778 KM
ALTITUDE
Parameter

As can be seen in Table 3-2,
maximum access duration from G2-SAT
to TTC ground station is approximately
248 second or 4.13 minutes. During this
time, G2-SAT can be control by command
from ground station a n d be able to
downlink the telemetry data.
STK reports for access s u m m a r y
from G2-SAT to Rumpin (X-Band Ground
Station) are presented in Table 3-3.
Table 3-3: ACCESS REPORT FOR X-BAND

MSN
Sitellite-e23AT-Sensor-XJuid-To-Fjcility-J!imit-Sensor-SJWTA2:

Kces Swary Report

From Table 3-3, it c a n be seen
t h a t maximum access duration from G2SAT
to
TTC
ground
station
is
approximately 167.3 second or 2.8
minutes which is relatively short. It
m e a n s this is maximum duration for G2SAT to downlink images to ground
stations. It is become important to
optimize this duration for d a t a downlink
by designing the good scenario.
98

Pixel
Line rate
Quantization
Ground velocity
Total scan length in the
service area
Total scan lines in the
service area

Value
10200
411 Hz
8 bits/Pixel
6.653 k m / s
1907.083 km
117809 lines

Total number of pixel in 1201.65 Mpix
the service area
Total scan time
4.78 minutes
Total data size in the 9.613 Gb
service area
Parameters in Table 3-4 describe
the image data taken inside the service
area. From the results in Table 3-4,
when G2-SAT passes the service area, it
s c a n s 117809 lines which is divided into
four channels. Data sized for the whole
service area t h a t needs to be sent to
ground is 9.613 GBits.
It mentioned before that realtime
process can only be done in a certain
region around data ground station, so
image-data parameters in this area should
also be estimated. These parameters are
presented in Table 3-5. Assumption
taken when calculating parameters in
Table 3-5 is that data rate for
downlinking the data is 50 Mbps.

Table 3-5: IMAGE-DATA PARAMETERS
OF G2-SAT DURING ACCESS
TO GROUND STATION

Figure 3-5a: Semi
realtime
during daytime

It is shown in Table 3-5 t h a t
during access to ground stations which
is about 2.8 minutes, G2-SAT collects
9612 Mbits data. 1402.710 Mbits can be
sent realtime, while the remaining
8210.5 Mbits d a t a h a s to be stored in
the memory units. The time needed to
send the stored data is 164.2 sec.
Because the maximum access duration
is 167 sec, all d a t a in the memory u n i t
will be transfer completely in one p a s s .
3.4 Mission Concept
All information gather from orbit,
ground station access and image-data
parameters analysis are combined to
create scenarios for G2-SAT to do its
mission, hence taking and sending
images. G2-SAT p a s s Indonesia four
times a day, two p a s s at day time and
two p a s s at night. The image should be
taken at daytime p a s s , since the imager
is optical imager t h a t needs sunlight.
Based on the analysis, there are
two operation scenarios which are
possible to be implemented on G2-SAT;
the stored and forward operation and
semi real-time operation. Store and
forward scenario is illustrated in Figure
3-5a and 3-5b while Semi realtime
scenario is illustrated in Figure 3-6a and
3-6b.

scenario

Figure 3-5b: Semi real time concept of
operation during night time
On store a n d forward scenario,
G2-SAT will scan (take images) the earth
when it enters the service area during
daytime. In Figure 3-6a, the service area
is inside the brown border. When G2SAT reach ground station coverage, one
channel will be down linked. The
remaining data will be stored in the
memory units inside the payload system.
Data communication system will be off
during this process. When G2-SAT
p a s s e s its ground stations other at night
(right side), stored d a t a in the memory
unit will be down linked.
Semi real-time scenario m e a n s
the real-time process where image is
taken a n d immediately sent to ground
only happen at some p a r t of the service
area. Outside the service area, data will
be stored. The scanning process itself
h a p p e n s when G2-SAT passes service
area as in store and forward scenario.
99

Regarding to the payload specification,
only one channel can be direct linking.
The illustration of this process is shown
in Figure 3-6b.

possible to be implemented on LAPAN's
satellite program. Although each concept
will drive a different result on the satellite
configuration, it should give a big
advantage if the concept implemented at
LAPAN's satellite program.
5

ACKNOWLEDGEMENT

Biggest gratitude to Dr. Masno
Ginting for advices a n d suggestions to
improve the quality of my paper, G2-SAT
team for your hardwork, B u s t a n u l Arifin,
Eriko N. Nassser and Dian Yudhistira for
all the supports.

Figure 3-6a: Store and forward scenario
during daytime

4

CONCLUSION

Two scenarios had been designed
as a p a r t of G2-SAT mission concept;
semi realtime operation and store and
forward operation. These scenarios are
resulted from detail analyses of G2-SAT
orbit; ground stations access and imagedata parameters. Both scenarios are

100

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Mission Analysis a n d Design Team,
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Payload Analysis and Study Team, 2007.
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Proposal
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and
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S u n Space, 2007. Proposal of LAPAN - G2
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South Africa.
Wertz, J a m e s R. and Larson, Wiley J.,
1999. Space Mission Analysis and
Design, Kluwer Academic Publishers,
USA.
G2-SAT Launch System Team, 2007. G2SAT Preliminary Concept; Launch
System
Version
2.0,
LAPAN,
Indonesia.
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