PSLV Orbital Experiment Module

PSLV Orbital Experiment Platform (POEM) also known as PSLV Stage 4 Orbital Platform (PS4-OP) is an orbital micro-gravity test bed based on spent fourth stage of PSLV. By adding modular subsystems for power generation, communication and stabilization like photovoltaic cells, Telemetry and Telecommand (TT&C) package, attitude control system, data storage etc to the PSLV fourth stage, it can function as a satellite bus. This augmented stage can then host payloads for up to six months while in orbit, making it useful for qualifying components, gaining space heritage and conduct experiments in micro-gravity conditions. Usually the fourth stage of PSLV is discarded after deployment of satellite and remains in orbit for a significant duration in a passive state as a piece of space debris.

Objective
POEM or PS4-OP was conceived by VSSC/ISRO to help Indian academia and start-ups by providing a low cost platform with essential subsystems to support their payloads hence lowering the barriers of reaching orbit. POEM off-loads the burden of designing, qualifying, procuring a satellite bus, setting up a ground station etc. which reduces the development time and costs associated with using space-grade components thus allowing entities to just focus on payload.

History
PSLV fourth stage or PS4 has often been used to carry non-separable payloads like AAM on PSLV-C8, Rubin 9.1/Rubin 9.2 on PSLV-C14 and mRESINS on PSLV-C21 etc. but such payloads could be supported only for a very short duration as PS4 lacked the capacity to generate power and maintain attitude stability. So it was proposed to augment PS4 with modular subsystems to convert it into a long duration orbital platform after completion of primary mission.

In 2017 as an early demonstration on PSLV-C37/Cartosat-2D and PSLV-C38/Cartosat-2E campaigns, PS4 was kept operational and monitored for over ten orbits after completion of its primary mission of deploying spacecraft. Fourth stage on PSLV-C38 hosted IDEA (Ionization Density and Electric field Analyzer) as a non-separable payload by Space Physics Laboratory of VSSC, two other non-separable payloads were mAMP and 'Earth Pointing Platform'.

In January 2019, PSLV-C44/Microsat-R became the first campaign where PS4 functioned as an independent orbital platform for short duration as the provisions for on-board power generation capacity were not yet made. It hosted a 1U cubesat called KalamSAT-V2 as a non-separable payload by Space Kidz India based on Interorbital Systems kit. Later in April, the fourth stage on PSLV-C45/EMISAT had its own power generation capaciity as it was augmented with an array of fixed solar cells wrapped around the propellant tank of PS4 generating about 200 watts of power. Three payloads hosted on PS4-OP were, Advanced Retarding Potential Analyzer for Ionospheric Studies (ARIS 101F) by IIST, experimental AIS payload by ISRO and AISAT by Satellize. To function as an orbital platform, fourth stage was put in spin-stabilized mode using its RCS thrusters. PS4-OP on this campaign had mission life of 3 months and AISAT payload onboard was operational for almost a year.

After these initial testing campaigns, ISRO issued an Announcement of Opportunity in June 2019 to carry out in-orbit scientific experiments on PS4 orbital platform. And in following years POEM-1 aboard PSLV-C53/DS-EO campaign in June 2022 and POEM-2 aboard PSLV-C55/TeLEOS-2 campaign in April 2023 were launched. Both of these orbital platforms had mature configurations utilizing 3-axis stabilization and on-board power generation capacity.

In July 2023, Indian National Space Promotion and Authorisation Centre (IN-SPACe) issued an announcement of opportunity inviting payloads to be hosted aboard upcoming POEM campaigns tentatively scheduled between November 2023 and June 2024.

On 1 January 2024, POEM-3 was launched along with XpoSat on PSLV-C58 hosting ten payloads. The stage was lowered from a 650 km (circular) injection orbit of XpoSAT to a 350 km circular orbit for POEM-3 operations. POEM-3 re-entered atmosphere on 21 March 2024 over the Pacific Ocean after having accomplished it's objectives by February and thus leaving behind no space-debris from the launch.

Platform capabilities
Specifications and capabilities of PSLV fourth stage based orbital platform which may improve over time.


 * Mission life: Up to six months depending on left-over Helium pressurant in fourth stage.
 * Maximum mass of payload: 30 kg
 * Maximum size: About 3U, depends on spare area available for a particular mission and mounting location.
 * Maximum power of payload: 200 to 500 watts
 * Power bus: 28 Volts
 * Maximum data rate: 1 Mbps
 * Pointing accuracy:
 * During sunlit period: within ±1°
 * During eclipse: ±5°
 * Slew rate within 0.5° per second during stabilized regime.
 * Data storage: 1GB
 * TT&C: S-band
 * Downlink at ISRO ground station
 * Suitable for experiments requiring large Inertia and structure.

Missions
List of missions where POEM has been utilized in its matured configuration.

POEM-1
In June 2022, the PSLV-C53/DS-EO campaign had its PS4-OP referred as PSLV Orbital Experiment Module (POEM) and it was first PSLV fourth stage based orbital platform to be actively stabilized using eight Helium based cold gas thrusters after the primary mission and stage passivization. POEM-1 (COSPAR ID:2022-072E, SATCAT no. :52939) had additional sensors and navigational aids (4 Sun sensors, Magnetometer, MRGPD and NavIC). It generated about 150 watts of power using non-deployable, fixed solar cells wrapped around PS4 tank. Six non-separable payloads facilitated by NSIL and IN-SPACe were hosted aboard.
 * 1) DSOD-1U Small satellite deployer by Dhruva Space.
 * 2) ROBI (ROBust Integrating proton fluence metre) by Digantara Research and Technologies
 * 3) Software Defined Radio based Telemetry Multi-Media Transmitter (SDRT-MTx)
 * 4) UHF Transmitter
 * 5) OP-VIS - Configured with one GVIS and two cameras
 * 6) POEM also carried preamble to the Constitution of India bearing Indian flag.

POEM-2
In April 2023, on PSLV-C55/TeLEOS-2 campaign, seven non separable payloads were hosted by POEM. POEM-2 had flexible solar arrays wrapped around PS4 propellant tank that generated about 500 watts of power upon deployment. POEM-2 was again 3-axis stabilized by eight Helium based 1N cold gas thrusters (OPACS) developed by LPSC that utilize left-over PS4 pressurant. Expected mission life of POEM-2 (COSPAR ID:2023-057A, SATCAT no. :56308) was of one month. Payloads hosted on POEM-2 are:
 * 1) ARIS-2 (Advanced Retarding potential analyser for Ionosphere Studies) by IIST for ionosphere studies in Low Earth Orbit.
 * 2) PSLV-in-orbitaL OBC and Thermals (PiLOT):
 * 3) Starberry-Sense: A low-cost Raspberry Pi Zero based star sensor by Indian Institute of Astrophysics (IIA) for cubesats.
 * 4) ARKA200: Xenon based Hall-effect thruster (HET) by Bellatrix Aerospace.
 * 5) DSOD-3U: Cubesat deployer by Dhruva Space.
 * 6) DSOD-6U: Cubesat deployer by Dhruva Space.
 * 7) DSOL-Transceiver: Satellite Orbiter Link in S & X bands.

POEM-3
On PSLV-C58/XPoSat campaign, POEM-3 hosted ten payloads weighing ~145 kg cumulatively. PSLV fourth stage was lowered to 350 km orbit at 9.6° inclination after deploying XPoSat to reach the POEM-3 operational orbit. POEM-3 will nominally be operational for a period of one month. For power generation and storage it will again have flexible solar panels in conjunction with 50Ah Li-ion battery and will be three-axis stabilized. Payloads hosted on POEM-3 (COSPAR ID:2024-001A, SATCAT no. :58694) are following, seven of them facilitated by IN-SPACe and three are by ISRO,


 * 1) Radiation Shielding Experimental Module (RSEM): Experimental payload by TakeMe2Space to evaluate effectiveness of Tantalum coating for radiation shielding.
 * 2) Women Engineered Satellite (WESAT): Payload by LBS Institute of Technology for Women to compare and measure UV radiation in space and on Earth's surface in real-time.
 * 3) BeliefSat-0: Amateur Band UHF to VHF FM voice repeater, and VHF APRS Digipeater satellite by K. J. Somaiya Institute of Technology.
 * 4) Green Impulse TrAnsmitter (GITA): Green bipropellant CubeSat propulsion unit by Inspecity Space Labs Pvt. Ltd.
 * 5) Launching Expeditions for Aspiring Technologies-Technology Demonstrator (LEAP-TD):  P-30 nanosatellite platform subsystems validation by Dhruva Space.
 * 6) RUDRA 0.3 HPGP: Green Monopropellant Thruster by Bellatrix Aerospace Pvt. Ltd.
 * 7) ARKA200: Xenon based Hall-effect thruster (HET) by Bellatrix Aerospace.
 * 8) Dust Experiment (DEX): Interplanetary dust count measurement by Physical Research Laboratory.
 * 9) Fuel cell Power System (FCPS): Demonstration of 100 watt fuel cell power system by VSSC.
 * 10) Si based High Energy cell: Demonstration of Silicon based High Energy cells by VSSC.

Future payloads
List of payloads proposed to be hosted aboard POEM on a future flight.


 * Quantum Entanglement Studies in Space (QuantESS) by SAC/PRL.
 * Atomic Oxygen Sensor (ATOXS) by SPL/VSSC
 * PlasDEM (Plasmaspheric electron Density Measurements)
 * Near Ultraviolet Transient Surveyor (NUTS)
 * Smart Space Robot Experiment
 * Potential experiments related to space robotics, proximity operations etc.