SSLV, EOS-02, ISRO, Liftoff, Satish Dhawan

EOS-07 & Others | SSLV

Lift-Off Time
February 10, 2023 – 03:48 UTC | 09:48 IST
Mission Name
EOS-7 & Others, an Indian Earth-observation satellite, and other rideshare payloads
Launch Provider
(What rocket company launched it?)
Indian Space Research Organization (ISRO) through its commercial branch NewSpace India Limited (NSIL)
(Who paid for this?)
U R Rao Satellite Centre (URSC) a specialized institution belonging to ISRO
Antaris Space
Space Kidz India
Small Satellite Launch Vehicle (SSLV)
Launch Location
First Launch Pad at the Satish Dhawan Space Centre, in Sriharikota, India
Payload mass
175.3 kg (~390 lb), all payloads
Where did the spacecraft go?
A circular low Earth-orbit at 450 km (~280 mi) altitude and 37.2 degrees inclination
Did they attempt to recover the first stage?
No, this is not a capability of the SSLV
Where did the first stage land?
It will crash into the Bay of Bengal, southeast of the launch complex
Did they attempt to recover the fairings?
No, this is not a capability of the SSLV
Were these fairings new?
This was the:
-2nd SSLV’s mission overall
-1st SSLV’s mission in 2023
-85th ISRO’s mission overall
-1st ISRO’s mission in 2023
-21st orbital launch attempt in 2023
Where to watch
Official livestream

How Did It Go?

For ISRO’s SSLV rocket, this was the return to flight mission — EOS-07, its second demonstration launch (SSLV-D2) — after the failure suffered during the first demo (SSLV-D1). Consequently, the Indian space agency readied the First Launch Pad at the Satish Dhawan Space Centre, in Sriharikota, India. From this site, the vehicle took to the skies carrying payloads both for ISRO, and other commercial clients, i.e. Antaris Space and Space Kidz India. After lifting off, and achieving all flight milestones, the upper stage successfully placed three satellites into their operational orbits, all of them at an altitude of 450 km (~280 mi) and an inclination of 37.2 degrees.

What Is EOS-07?

Named Earth Observation Satellite 07 (EOS-07), the ISRO had the URSC develop and manufacture it. With the latter being a part of the former, this specialized center built all of the previous Indian satellites designated EOS. In this particular flight, the main payload seems to be a repeat of the one lost in the first flight of the SSLV: the EOS-02. It is possible to arrive at such a conclusion by comparing images shared by the space agency, although they have not formally confirmed it.

See the following illustrations (zoomable) of the upper stage of this rocket and the payloads carried in each flight. You will notice the general shape of both EOS satellites is basically identical.

SSLV-D1 payloads
(credit: ISRO)
SSLV-D2 payloads
(credit: ISRO)

According to the description of the EOS-02 spacecraft, EOS-07 should be an experimental remote sensing satellite. Its dimensions would be 55.2x60x60 cm (~22x23x23 in), with 156.3 kg (~345 lb) of mass — this has been specifically reported for the present flight. Two solar arrays would provide the spacecraft with 357 W of power. On top of its structure, the satellite features the imaging system with a metallic primary mirror. Two different sensors would allow for observations in mid-wavelength and long-wavelength infrared with a resolution of 6 m (~20 ft).

EOS-07’s primary mission objective would be to gather data for Geographic Information Systems (GIS) applications, such as cartography, regulation of use of coastal land, urban and rural management, and many more.

SSLV, SSLV-D1, AzaadiSAT, EOS, EOS-02, VTM, Velocity Trimming Module
SSLV’s upper stage, the VTM, integrated with EOS-02 and AzaadiSAT before the SSLV-D1 flight (credit: ISRO)

In a more recent report, however, the mm-Wave Humidity Sounder (MHS) instrument and a Spectrum Monitoring Payload (SMP) appear as components of EOS-07. Similarly, there are new mission objectives listed, which are related to developing apt hardware for rapid turnaround times, and hosting new kinds of payloads.

EOS-07’s Bus

The EOS-02 sat and, by extension following our reasoning of a repeat mission, the EOS-07 sat are based on the bus developed for the Indian Microsatellite-1 (IMS-1). This spacecraft rode atop a PSLV rocket back on April 28, 2008. IMS-1’s bus masses 70 kg (~150 lb) and could host payloads of up to 30 kg (~70 lb). These specifications clearly got upgraded, as the whole Earth-observation sat of the present mission is approximately 50 % more massive. Nevertheless, we will provide further description of the bus in an illustrative manner.

IMS-1, EOS-07, bus
EOS-07’s bus with stowed solar arrays (credit: ISRO)

Its structure consists of aluminum honeycomb panels and internal frames forming a cube-like body. On its lower part, there is the launch vehicle-spacecraft interface. The opposite face is where the payloads are attached. Electrical power flows from both solar panels, which are 0.81x 0.72 m ( ~2.6×2.4 ft), into a Li-ion battery with a capacity of 10.5 Ah — EOS-07, 27.2 Ah.

For attitude and orbit control, i.e. correctly pointing and positioning the satellite, Sun sensors and star sensors provide the pointing. Once this is done, magnetic torquers and micro reaction wheels rotate the spacecraft as desired. GPS receivers are capable of letting the sat know where it is with a precision of 30 m or less. A single tank with a capacity of 8 liters carries 3.5 kg (~7.5 lb) of hydrazine for orbit correction by means of one 1.00 N (~0.2 lbf) thruster.

Thermal control makes use of passive elements like multilayer insulation (MLI), thermal paints, and surface reflectors. However, heaters are also used where necessary.

ISRO’s EOS Satellites

Previously, there have been a number of these EOS satellites launched by ISRO. We present them in the following table, though it’s worth noting that there is the EOS-05 (GISAT-2) in the planning, slated for a later time in 2023.

NameLaunch VehicleLaunch DateStatus/Orbit
EOS-01 (RISAT-2BR2)PSLVNovember 7, 2020 – 09:41 UTCActive/~578 km x 36.9°
EOS-02 (Microsat 2A)SSLVAugust 07, 2022 – 03:48 UTCFailed/~356 km x 37.2°
EOS-03 (GISAT-1)GSLV Mk IIAugust 12, 2021 – 00:13 UTCFailed/GEO
EOS-04 (RISAT-1A)PSLVFebruary 14, 2022 – 00:29 UTCActive/~535 km x 97.5°
EOS-06 (Oceansat-3)PSLVNovember 26, 2022 – 06:26 UTCActive/~743 km x 98.4°
EOS-07SSLVFebruary 10, 2023 – 03:48 UTCPlanned/450 km x 37.2°

What Are The Other Payloads?


A California-based company — Antaris Space — designed and managed the manufacturing of JANUS-1, a 6U CubeSat massing 10.2 kg (~23 lb). By using their design software, this nanosat went from concept to launch readiness in about ten months. What is more, the company was capable of reducing costs by 75 %. And, as if that was not good enough, a virtual twin of this satellite is already operating in a computational simulation.

Once the physical spacecraft is already in orbit, cloud-based software allows either the company or the client to take charge of its operation. In this mission, a number of clients will carry out tasks involving advanced experimental laser communications, internet of things (IoT) communications, machine learning (ML), and radio communications.


In this spirit, this organization designed 75 very small experiments, so students can assemble and learn from them. All of these payloads, each one massing about 50 g (~0.1 lb), are integrated into a 8U CubeSat with a mass of 8.7 kg (~19 lb). This nanosat, a repeat spacecraft of the AzaadiSAT lost during the SSLV-D1 flight, hosts UHF-VHF transponders for amateur radio operators relaying voice and data, diode-based devices for in-orbit ionizing radiation measuring, a long range transponder, and a selfie camera. Space Kidz developed the ground segment to receive the data from space.

Public schools of an economically weak background are the target for this project, in which 75 schools for girls were selected to participate, and from each of them ten students from 8th to 12th grade. This initiative started in 2022, to commemorate the 75th anniversary of the country’s independence. By means of this project, Space Kidz India is trying to encourage women in STEM, while providing them at a young age with a minimum, basic understanding of space knowledge.

AzaadiSAT, AzaadiSAT-2, SSLV, EOS
AzaadiSAT mounted on the VTM prior to the SSLV-D1 flight (credit: ISRO)

What Happened To The SSLV-D1 Rocket?

As we already mentioned, the SSLV-D1 flight did not reach the expected orbit. That is, aiming at a circular low-Earth orbit of 356 km (~220 mi) in altitude and 37.2 degrees inclination, the satellites were dropped on an elliptical 75.66×360.56 km (~47×224 mi) at an inclination of 36.56 degrees.

Understanding Facts And Reasons

A shortfall of velocity was the cause of such an unstable orbit — particularly because of the low perigee — from which the spacecraft soon reentered the atmosphere. In order to understand why the satellites lacked the last ~56 m/s (~180 ft/s) of acceleration, it is necessary to grasp how the SSLV travels to a specific orbit.

The first SSLV rises carrying the EOS-02 satellite (credit: ISRO)

When the rocket is standing on the pad, it knows precisely where it is, but after lift-off there are different methods contributing to letting it know its trajectory. One of them is the inertial navigation system (INS) and its sensors: 6 gyros, to measure changes in rotational velocities, and 6 accelerometers, for the same, but for linear velocities. These feed a flight computer, which for every instant calculates where the vehicle is, and how it is moving. For safety, if one of these sensors starts providing bad values, the faulty one is isolated. In this way, the degraded measurements will not confuse the computer and the launcher will continue nominally.

If all accelerometers were to fail at once, then the rocket would enter the mission salvage mode, following a predetermined path. With no feedback from the sensors, it is as if the vehicle was flying blindly. Such a scenario took place at second stage separation from the third. For longer than expected, a vibration more intense than expected, and coming from separation hardware, caused the accelerometers to get saturated. This triggered the mentioned mode, dooming the mission. Nevertheless, data retrieved later showed after the anomalous behavior sensors continued working fine.

The Path Ahead

Taking the failure cause into consideration, corrective measures were taken. In the first place, the separation system was removed: a circular below that expanded cutting the rivets and imprinting separation velocity. In its stead, a simpler Marman clamp and springs should introduce less vibration. Additionally, the rules for isolating sensors have been modified to offer a better response. Besides these fixes, a study and redesign of key structural regions of the rocket should prevent it from being heavily affected by similar vibrations.

In reaction to future incidents, the vehicle will utilize navigation signals from the Indian Regional Navigation Satellite System. Even if everything fails and the SSLV ends up having to “fly blindly” again, the upper stage will now be an active part of the salvage mode. This should increase the probabilities of accelerating enough to remain in orbit.

Mission Timeline For EOS-07 — SSLV

[km (mi)]
SS1 Ignition0.0
SS2 Ignition123.794.0 (  ~58)
SS1 Separation124.094.0 (  ~58)
S2C Separation129.0103.0 (  ~64)
SPLF Separation158.4149.0 (  ~93)
SS2 Separation384.2423.0 (~263)
SS3 Ignition394.0429.0 (~267)
SS3 Separation674.9450.0 (~280)
VTM Ignition450.0450.0 (~280)
EOS-07 Separation785.1450.0 (~280)
Janus-1 Separation880.1450.0 (~280)
AzaadiSAT-2 Separation900.1450.0 (~280)
Approximate timeline according to ISRO
Visualization of EOS-07’s mission milestones (credit: ISRO)

  What Is The SSLV?

Stacking of SSLV’s first stage segments (credit: ISRO)

ISRO developed the Small Satellite Launch Vehicle, or SSLV, to be a 4-stage, small-lift rocket capable of orbiting mini-, micro-, and nanosatellites. This Indian launcher was designed with the “launch on demand” basis in mind, while offering low-cost access to space. Furthermore, it aims to feature low turn-around times, capability of integrating multiple payloads, and needing minimal launch infrastructure. The First Launch Pad at the Satish Dhawan Space Centre, in India, serves as its operational base.

Lift-off mass [kg (lb)]120,000 (~270,000)
Height [m (ft)]34 (~110)
Diameter [m (ft)]2.0 (~4.4)
LEO mass [kg (lb)],
500 km altitude
500 (~1,100)
SSLV’s general specifications

Stages Of The SSLV

Three of its stages are solid rocket motors, all of them using hydroxyl-terminated polybutadiene (HTPB), which is the correct designation of a substance used to bind both fuel and oxidizer. Typically, these are aluminum powder and ammonium perchlorate (AP) respectively. Conversely, the fourth stage presents 16 engines burning Monomethylhydrazine (MMH) as fuel, and a solution of nitric oxide (NO) in dinitrogen tetroxide/nitrogen dioxide (N2O4 and NO2) as the oxidizer: MON-3.

Each of the solid rocket motors is designated SS, probably standing for “solid stage.” VTM means Velocity Trimming Module, and acts as a kick stage precisely injecting the payload into the required orbit.

[m (ft)]
[m (ft)]
Propellant mass
[kg (lb)]
[kN (kipf)]
Burn time
SS122.5 (~74)2.0 (~6.5)87,000 (~193,300)2,496.00 (~560)114.6
SS23.2 (  ~7)2.0 (~6.5)7,700 (  ~17,100)234.20 (  ~50)124.0
SS32.8 (  ~6)1.7 (~5.6)4,500 (  ~10,000)160.00 (  ~35)103.9
VTM2.0 (~6.5)50 (       ~110)0.05 (~0.01)
SSLV’s stages compared specifications

Inside The Payload Fairing

In order to offer launch versatility, ISRO provides many different configurations for payload disposition, allowing for rideshare missions.

The many payload configurations available on the SSLV (credit: ISRO)

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