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TES (Technology Experiment Satellite)

TES is an ISRO-developed experimental satellite in LEO to demonstrate and validate in-orbit technologies that could be used in the future satellites of ISRO (Indian Space Research Organization). A main goal is to carry out high-resolution panchromatic imaging. However, the imagery is only being made available for Indian government and defense uses (test of an independent indigenous military photoreconnaissance capability; however, there is no commercial sale of TES imagery). The TES mission is classified.

Some of the technologies that are being demonstrated in TES are: 1)

• Attitude and orbit control system

• High torque reaction wheels

• A new reaction control system with optimized thrusters and a single propellant tank

• A light-weight spacecraft structure

• Solid-state recorder (SSR) built indigenously

• X-band phased array antenna capable of directional narrow-beam downloading of data

• Improved satellite positioning system (SPS)

• Miniaturized TT&C and power system

• Two-mirror-on-axis camera optics


Figure 1: Artist's view of the TES spacecraft (image credit: ISRO)


The spacecraft is three-axis stabilized, based on the IRS series bus design, with a mass of 1108 kg at launch. TES is an agile satellite featuring a body-pointing capability in any direction (cross-track or along-track). The spacecraft design life is 3 years.

EPS (Electric Power Subsystem): The solar cells are laid out in series and parallel configuration on the solar panel to form strings; nine such strings are configured for each bus. The dual bus configuration with the solar array strings (series and parallel connection of solar cells to achieve required voltage and current) distributed on all the panels provide power during the sunlit phase of the orbit for housekeeping loads, payload and battery charging.

Use of two NiCd (Nickel Cadmium) batteries, each of 28 cells providing a capacity of 24 Ah. The bus voltage is 42 V. A TCR (Taper Charge Regulator), connected to each bus provides bus regulation, battery charging and battery over charge protection.

To protect the battery from over discharging, two types of protection are provided in the power system, of which one may be selected.

- The first level of protection will monitor the voltage across group of four cells and if at any time the voltage goes below 4 V the battery is isolated from the bus by autonomous opening of emergency relay which disconnects all the loads from battery.

- The second level of protection monitors the battery voltage and if during discharge the total battery voltage goes below 28 V, opening of emergency relay isolates the corresponding battery. 2)


Figure 2: Normal configuration of the EPS (image credit: ISTRAC)


Figure 3: Solar panel and battery configuration of TES (image credit: ISTRAC)

Launch: A launch of TES took place on Oct. 22, 2001 on the PSLV-C3 vehicle from SHAR (Sriharikota High Altitude Range), India. - TES was the primary payload on this flight along with PROBA (ESA) and BIRD (DLR), the latter two as secondary payloads. 3) 4) 5)

Immediately after orbit injection, the two solar panels of the satellite were automatically deployed and the satellite was put in three-axes stabilization mode using reaction wheels as per plan.

Orbit: Near-circular sun-synchronous orbit, altitude = 572 km, inclination =97.8º, period = 96 min, LTDN (Local Time on Descending Node) at 10:30 hours. The revisit capability of this satellite which would normally be around 20 days could be reduced to 3 days by the satellite tilting technology.

RF communications: X-band data (imagery) transmission.

The spacecraft is being operated by ISTRAC (ISRO Telemetry Tracking and Command Network) in Bangalore, India.


Mission status:

• The TES spacecraft is still operational in 2012 in its 11th year on orbit; however, it is only being operated for a few minutes daily. 6)

- In March 2012, Space Daily is reporting: “With 11 remote sensing/earth observation satellites orbiting in the space, India is a world leader in the remote sensing data market. The 11 satellites are TES, Resourcesat 1, Cartosat- 1, -2, -2A and -2B, IMS-1, RISAT-2, OceanSat-2, ResourceSat-2, and Megha-Tropiques.” 7)

• The TES spacecraft is operational in 2011 (in its 10th year on orbit). 8)

• The spacecraft is still operational in the spring of 2010. 9)

• While all the new technologies worked properly, the EPS batteries started showing up signs of degradation, six months from the launch. Subsequently solar array strings started disconnecting from the power bus creating a low power generation condition. It was presumed that the mission might end prematurely before the expected design life of 3 years. However, the spacecraft operations team at SCC (Satellite Control Center) of ISTRAC/ISRO employed many innovative techniques after careful observation of the the battery behavior and extended the mission life by more than 8 years. The satellite is still active (in 2010) with its camera performing excellently and its imagery is being used for many cartographic applications (Ref 2).

Eventually, the SSR (Solid State Recorder) operational support had to be suspended to save the power for the most essential spacecraft functions.

• Although ISRO referred to the TES spacecraft as “civilian use consistent with our security concerns," defense sources in India referred to TES in 2002 as a spy satellite. TES imaging capabilities were tested on and successfully relayed high-quality images of the war in Afghanistan and of Pakistani troop movements along the border.

• In the aftermath of 9/11/2001 terrorist attack on the World Trade Center in New York, the TES data has helped the United States with high resolution imagery for its counter terrorism operations.

• The launch of TES made India the second country in the world after the United States (Ikonos spacecraft) that can commercially offer images with a 1 m spatial resolution.

• Shortly after its injection into orbit by PSLV, the two solar panels of the satellite were automatically deployed and the satellite was put into a 3-axes stabilization mode using reaction wheels.


Sensor complement: (HRPIC)

HRPIC (High Resolution Panchromatic Imaging Camera):

TES features a panchromatic camera, developed and built by Elbit/ElOp (Electro-Optics Industries Ltd.) of Rehovot, Israel (PIC is similar to the camera flown on EROS-A1).

HRPIC is capable of producing imagery of ≤ 1 m spatial resolution. The camera optics feature an aperture diameter of 550 mm and are based on a two-mirror on-axis optics and focal plane configuration. Asynchronous pushbroom scanning is provided for panchromatic imagery in the spectral range of 0.5 - 0.9 µm. In an alternate synchronous mode, it has a step-and-stare capability (also referred to as “spotlight”) to look at the same ground spot for longer durations.

The payload data of TES are being received, processed, distributed and archived by NRSA (National Remote Sensing Agency) of Hyderabad. TES represents a major technological advancement for India, in particular in the field of Earth Observation. TES Imagery is only available to the Indian national security agencies.


Operation strategies after battery degradation

Six months after the launch, one of the battery cells started showing a degraded performance which resulted in the opening of the ER (Emergency Relay). Nominally, the EODV (End Of Charge Voltage) for a cell would be around 1.5 V, and the EODV (End Of Discharge Voltage) would be more than 1.25 V. Nominally, the voltage across the group of four cells should vary between 5.0 V (EODV) and 6.0 V (EOCV).

Since the cell voltage had degraded with nominal discharge, the problem of maintaining the battery posed challenge for the mission. The cells are connected in series and there was no provision to isolate individual cell or battery. If the emergency relay is left in open condition, then the degraded battery would discharge at a very low rate to support the few essential loads in eclipse; however, in the sunlit phase of the orbit, the battery gets overcharged due to higher power availability from the first two strings. The NiCd batteries are designed to take 10 % overcharge. Charging more than this would result in oxygen liberation which fails to recombine with the active material, leading to temperature rise of the entire battery and eventually the loss of capacity of the other cells.

Since both the batteries are mounted on a common base plate, a rise in temperature of one battery would increase the temperature of the other battery resulting in degraded performance of the good battery. If the nominal discharge was allowed for the degraded battery, then the cell voltage would go below 0 V resulting in cell reversal.

Emergency relay operations strategy:

In view of isolating the battery during eclipse to avoid higher discharge on the degraded battery, thereby avoiding cell reversal while at the same time avoiding overcharging in the sunlit phase of the orbit, was the only way to sustain the operations. To realize this, it was essential to open the emergency relays by command just before eclipse entry that would ensure low discharge on the degraded battery (essential loads were supported by the degraded battery), and close the emergency relays immediately after the eclipse exit to bring back the loads on the problematic bus which would be more than the power generated by first two solar array strings, thereby avoiding overcharging. In this scenario the healthy bus takes over the load of the problematic bus in the eclipse phase of the orbit. To sustain the above requirements, these operations were to be carried out for all 15 orbits per day.

The on-board system is configured to operate a command either in real-time or in a time-tagged memory storage mode. The commands to be operated in time-tagged mode had to be tagged with a delay time so that the command will activate at the user-defined time in the non-visible regions. - The emergency relay 'open' at eclipse entry and emergency relay 'close' at eclipse exit was commanded from the ground in a time-tagged mode for all 15 orbits per day. These operations isolated the degraded battery from the bus in eclipse (single bus operation) and also avoided overcharging of the battery in the sunlit phase. Figure 4 shows the power schematic when the emergency relay is opened by command and the BPR (Bus Parallel Relay) closes automatically so that the Bus-A takes the full load.


Figure 4: EPS configuration with the Bus-B emergency relay open (image credit: ISTRAC)

In all, the TES spacecraft has given a great learning opportunity to the satellite operations team - as well to the battery designers and the solar array designers.

1) V., Raghuvanshi, S. Magnuson, “India develops Spy Satellite,” Space News, July 24, 2000, pp. 4 and 34

2) B. N. Ramakrishna, P. Ekambaram, M. Pitchaimani, S. K. Shivakumar, “Circumventing the Power Contingency in Technology Experiment Satellite,” Proceedings of the SpaceOps 2010 Conference, Huntsville, ALA, USA, April 25-30, 2010, paper: AIAA 2010-1960

3) Technology Experiment Satellite (TES),” ISRO, URL:


5) “India launches satellite trio aboard one PSLV rocket,” Spaceflight Now, Oct. 22, 2001, URL:

6) Information provided by A. S. Kiran Kumar of ISRO/SAC, Ahmedabad, India

7) Venkatachari Jagannathan, “India's spy satellite to be launched in April,” Space Daily, March 13, 2012, URL:

8) “ISRO To Launch Remote Sensing Resourcesat In February,” The Hindu, January 31, 2011, URL:


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.