EROS-A (Earth Remote Observation System-A)
EROS-A (of Ofeq-3 heritage) is a high-resolution commercial imaging satellite of ImageSat International N.V. (ISI) headquartered at Limassol, Cyprus, and incorporated in the Netherlands Antilles. The spacecraft was designed and built by Israeli Aircraft Industries Ltd. (IAI). ISI is majority-owned by IAI, established in 1997.
The overall objective is to launch and operate a constellation of high-resolution commercial satellites, primarily for intelligence and national security applications and to serve a global customer base. The spaceborne remote sensing technology for the EROS family was approved by the government of Israel in Oct. 1996. 1) 2)
• The option to acquire exclusive imaging rights over a defined footprint or the ability to acquire imagery on an exclusive basis.
• Rapid delivery of imagery to the customer, either through direct downlink, electronic transfer or courier.
• Provision of low-cost imagery products to support applications in many fields.
Background on the EROS program:
EROS is a program of ImageSat International, N.V., formerly WIS (West Indian Space) Ltd., Cayman Islands. The name change took place in June 2000. ImageSat was re-incorporated in Curacao, Netherlands Antilles. ImageSat's ownership includes: Israel Aircraft Industries (IAI) of Tel-Aviv (owned by the Israeli government), Elbit Systems Ltd. of Haifa, and private investors from Europe and the United States.
• EROS-B was launched on April 25, 2006 on a Start-1 launch vehicle from the Svobodny Cosmodrome in eastern Siberia
• A third satellite, EROS C, will offer comparable panchromatic resolution to EROS B, as well as multispectral resolution of 2.8 m GSD. A date for the launch of EROS-C has not been set so far (2010).
The EROS-A satellite structure consists of low-mass composite material with passive thermal control, it is three-axis stabilized. Attitude control and navigation is performed using horizon sensors, sun sensors, gyros and magnetometer, four reaction wheels and thrusters. The pointing accuracy is <0.1º in all three axes, attitude stabilization is < 40 µrad/s, the jitter is < 0.2 µrad.
S/C mass = 260 kg (178 kg S/C bus, 42 kg of instrument mass and 30 kg of hydrazine), solar panel power (silicon array, fixed panels) = 450 W (EOL), plus 14 Ah NiCd batteries for eclipse operations, the bus power consumption at imaging session is 300 W. S/C design life = 4 years, however the estimated operational life of the satellite is ten years. 6)
Figure 1: Line drawing of the EROS-A spacecraft (image credit: IAI)
RF communications: Imagery is transmitted in X-band at a rate of 70 Mbit/s (RF downlink) to the ground receiving stations, using a 1.5 W transmitter and one of the two existing two-axis gimbaled directional antennas. The EROS satellites are monitored/operated in S-Band (TT&C) via a single ground control station (GCS), located at IAI/MBT in Israel (3 to 4 passes per day and per satellite are in station visibility). The S-band data rate is either 2.5 or 15 kbit/s selectable by the GCS.
ImageSat has a global network of ground segment infrastructure, for real-time image data acquisition. This network is comprised of the ImageSat Central Ground Control Station, a network of EROS-compatible Ground Receiving Stations on 5 continents and EROS-compatible Ground Control Stations based at exclusive customers' premises (see SOP Program).
The EROS A satellite has limited availability of onboard source data storage, as the satellite was designed to cater to customers acquiring real-time, exclusive imaging and download rights over a defined geographic footprint, in view of their own EROS-compatible GRS (Ground Receiving Stations).
Operational capabilities/services primarily include:
• Satellite Operating Partner (SOP) Program. This service provides a dedicated regional satellite with local customer tasking. SOP receiving ground stations are able to plan, to generate and to transmit imaging commands to the satellite and to download imagery in real-time.
• PAS (Priority Acquisition Service) Program. The service provides highest priority tasking of the EROS satellite, in areas not previously acquired by SOP Customers.
• Non-exclusive acquisitions. ImageSat sells EROS imagery on a non-exclusive basis to customers for civilian applications, such as mapping, disaster planning and monitoring, environmental management, homeland security and border control, and a range of development-related projects.
Table 1: Performance characteristics of the EROS-A downlink
Figure 2: Photo of the EROS-A spacecraft in deployed configuration (image credit: ISI)
Launch: EROS-A was launched on a Russian Start-1 launcher on Dec. 5, 2000 from the Svobodny Cosmodrome in eastern Siberia.
Orbit for EROS-A: Circular sun-synchronous orbit, altitude 480 km, inclination = 97.3º, period = 94.7 minutes, local time of descending node at 10:00 hours. The revisit capability at at latitude of 10º within a 15º cone is within 10.5 days. A revisit period/satellite at latitude of 10º is 4.5 days within a 30º cone, and only 2.5 days within a 45º cone.
Status of mission:
• The EROS-A spacecraft and its payload are operating nominally in 2014.
Table 2: EROS satellites at a glance 7)
• The EROS-A spacecraft and its payload are operating nominally in 2012.
In May 2011, ImageSat and RapidEye formed a partnership. The combination of RapidEye's constellation and the EROS satellites allows ImageSat to offer colorized data with great quality for a wide range of applications. 8)
• The EROS-A spacecraft and all subsystems are operating nominally in 2009. Operations are expected to last at least until 2010.
Figure 3: Flood damage in Franklin, Virginia, USA, acquired with EROS-A on Oct. 12, 2006 (image credit: ImageSat)
• In Dec. 2004, the ScanEx R&D (Research & Development) Center, Moscow, Russia signed a license agreement with the ImageSat International N.V. on the EROS-A data reception at the UniScan™ stations in Russia. On May 28, 2006 an amendment on the EROS-B data reception was signed. 9) 10)
Figure 4: Image of Banda Aceh, Indonesia -after the Tsunami impact, acquired with EROS-A on Dec. 30, 2004 (image credit: ImageSat)
Figure 5: Image of Banda Aceh, Indonesia -before the Tsunami impact, acquired with EROS-A on July 15, 2002 (image credit: ImageSat)
Sensor complement: (PIC)
PIC (Panchromatic Imaging Camera):
PIC was developed and built by Elbit Systems/ElOp (Electro-Optics Industries Ltd.) of Rehovot, Israel. PIC is a body-mounted instrument, featuring the CCD pushbroom technology. PIC uses a Cassegrain telescope with an aperture of 30 cm in diameter and a focal length of 3.45 m. FOV = 1.5º. The CCD detector array provides 7490 pixels per line (2 CCD line arrays). The scan speed is between 30 - 750 line/s, this corresponds to 18 to 1.3 ms of integration time, respectively. Asynchronous pushbroom scanning is provided for panchromatic imagery in the spectral range of 0.5 - 0.9 µm. 11)
Figure 6: Illustration of the PIC instrument (image credit: IAI)
The ground sampling distance (GSD) is 1.9 m, the swath width is 14 km (from a 500 km orbital altitude). The data is quantized at 12 bits (samples) and transmitted at 11 bits. SNR < 2/2,048 gray levels after quantization. Proper observations require sunlight conditions with a sun-over-the-horizon angle of >20º. Instrument mass = 31 kg (includes telemetry unit), power = 44 W (average).
A body-pointing technique is employed for instrument pointing (i.e., the entire S/C is pointed into the desired direction), permitting a field of regard (FOR) of ±45º from nadir into any direction (this translates for instance to a maximum ground cross-track distance of 960 km for oblique target observations). The physical pointing of the S/C is provided by the four reaction wheels (the maximum angular rate of 1.8º/s). The ability to point and shoot the camera also allows for stereo imaging during the same orbit (along-track imaging) ), as well as triplets (a stereo pair with a hyper-sampled image in between) and two stereo pairs in a single pass.
Table 3: Observation options for image scenes of a satellite pass
Figure 7: Schematic view of the EROS-A spacecraft pointing capability (image credit: IAI)
Asynchronous imaging mode technique:
Conventional remote sensing (imaging) satellites are being designed to scan in a synchronous mode, which means that the scanning velocity of the satellite's camera equals the satellite's ground speed. For a circular orbit, this ground speed is a constant number that only depends on the satellite's orbital altitude. In such a synchronous mode, the satellite's camera is typically designed to acquire images along the satellite ground track or parallel to it through the use of mirrors or other mechanical devices.
The EROS-A satellite allows for pushbroom scanning in a non-synchronous (or asynchronous) mode. Non-synchronous imaging implies that the ground scanning velocity is different than the satellite's ground velocity, and can be adjusted and optimized to the prevailing light conditions of the target area. The pointing agility of the spacecraft permits:
• Spotlight imaging support: In this mode the imaging is fixed (concentrated) to a particular target region. In this case, the imaging forward velocity is much lower than the satellite velocity, the satellite actually bends further backwards as the satellite moves forwards, enabling its detectors to dwell the necessary time (to improve the 'integration time' ) over each imaging area. In this way the sensor is able to collect more light, thereby improving contrast and SNR (Signal-to-Noise Ratio) for higher quality imagery.
Obviously, the imagery of the EROS-A PIC instrument requires orthorectification in a processing step to account and compensate for geometric distortions, due to its variable observational scene geometries (changing scanning angles) - generated by the maneuverability of the spacecraft.
Figure 8: Schematic view of the asynchronous imaging mode
1) Note: OFEQ-1 was launched in September 1988, OFEQ-2 in April 1990; both had a lifetime of 6 months. Ofeq-3 was launched April 15, 1995 (the satellite is still operational as of 2002)
2) A. Klein, “Flight Operations Engineering for the Earth Resource Spacecraft Eros-A,” Space Mission Operations and Ground Data Systems - SpaceOps '96, Proceedings of the Fourth International Symposium, Sept. 16-20, 1996, Munich, Germany,. Edited by T.-D. Guyenne. ESA SP-394, URL: http://adsabs.harvard.edu/full/1996ESASP.394.1051K
3) B. Opall, “ImageSat Initiates Production of New Craft,” Space News, Aug. 13, p. 3 and p. 28
4) Information provided by P. Rosenbaum of IAI
6) Fred Ortenberg, “Israel in Space - Twenty Years of Exploration (1988-2008),” book, 2009, printed at Technion Press, Haifa, Israel, ISBN: 987-965-555-457-1
7) ImageSat International, “Where are we now ???,” Proceedings of the 51st Session of Scientific & Technical Subcommittee of UNCOPUOS, Vienna, Austria, Feb. 11-22, 2014, URL: http://www.unoosa.org/pdf/pres/stsc2014/tech-58E.pdf
10) Vladimir Gershenzon, Olga Gershenzon, Marina Sergeeva, Vitaly Ippolitov, “Remote Sensing Center at a University for Real-time Acquisition of High Resolution Optical and Radar Imagery,” URL: http://www.lars.cl/biblioteca/LARS_Gershenzon_Gershenzon.pdf
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.