Abid (satellite)
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Abid (Arabic: عابد, Worshipper or Devout) is the code name of a series of five radar imaging reconnaissance satellites operated by the Riysian Air Force Intelligence Directorate (Mukhabarat) and the Riysian Air Force. All five are believed to have been constructed by the Meshaal Rocketry Institute, with the first being launched on March 22nd, 2008, and the final one being orbited on September 31st, 2009.
The Abid constellation is the first of its kind in Riysa, and has been surrounded with significantly more secrecy than other official satellite missions, with the exception of Riysa's SIGINT satellites. It is considered a high-revisit constellation, . Though the
On the basis of the Abid's spacecraft bus, a "civilianized" platform called the Meshaal M1000 was created by Meshaal for a variety of remote sensing and environmental/scientific missions, minus the classified electronics and redundancies. Much of the information on the basic structure of the Abid are based off of it.
History
It is believed that the development of the Abid satellites started some time in the late 1990s, with the Riysian Mukhabarat was known to have a long-standing interest in space-based radar reconnaissance; inspired by the successes of both commercial and military SAR satellites, and wanting to compete with the increasing number of .
Design
Spacecraft
Structure
Abid spacecraft are built by the Meshaal Rocketry Institute (MS Meshaal) design office, sharing a standardized bus 3.30 meters long and 1.34 meters wide. The Abid bus is built around a central tubular structure made up of a carbon fiber (CFRP) honeycomb sandwich, with external panels and structures instead made of aluminium alloy; it is notably the first Riysian spacecraft to make its primary structure out of CFRP. It is an agile bus and highly reconfigurable, and is also notable for being the first Meshaal spacecraft bus created entirely through computer-assisted design (CAD).
Each spacecraft is three-axis stabilized, with control provided by reaction wheels and magnetorquers, and attitude/orbit sensing provided by a sun and two star sensors, laser gyros, and a GNSS receiver. This gives it high agility, extending the viewing angle of the spacecraft and allowing for excellent precision when pointing at an imaging target. It is believed that control moment gyroscopes (CMGs) can be fitted instead of reaction wheels to provide greater agility, as it is an option on the M1000, but no evidence indicates that any of the five current Abid units use them.
The mass at launch of each Abid spacecraft is around 1197 kg, with around 350 kg belonging to the payload. The design life is no less than 10 years, with a goal of 12.5, but with enough consumables to last potentially up to 20 years. The long design life, which was a strict requirement during its development, is achieved through high redundancy and hardening across all of the satellite's systems as well as a generous supply of consumables. This special redundant design concept is not fully present in the civilian M1000, as its details are considered very sensitive.
Power
Electrical power is provided by two folding solar arrays with a total area of 20.6 m2 and mass of around 85 kg, providing an estimated maximum of 4.0 kW of power at end of life (EOL) and 4.8 kW of power at beginning of life (BOL). Batteries are lithium-ion, and according to official requirements store enough power for 30 minutes of continuous operation per orbit.
Propulsion
Like most Riysian satellites, the Abid satellites are believed to use a bipropellant N2O4/WS-50 propulsion system rather monopropellant. A total of 113 kg of propellant is believed to be carried on-board, which would be enough for around 20 years of operations.
Electronics and Communications
Telemetry, tracking, and command (TT&C) is provided in the S-band, with a nadir (Earth-facing) and a zenith hemispheric antenna, while payload data downlink is in two X-band channels, with a directional antenna. Data rates are an estimated 4 kbps for the uplink, up to 512 kbps for the TT&C downlink, and up to 310 Mbit/s for the payload downlink. An inter-satellite link is also provided in the S-band for the low-rate sharing of TT&C communications, presumably using the zenith antenna; this link is used to maintain tasking and tracking of satellites when out of range of a ground station, by using other satellites in the constellation as a relay.
In addition to standard satellite-ground communications and the inter-satellite link, the Abid satellites are believed to be able to use the Murshid data relay satellites, though all Abid spacecraft were orbited before it. For this, the zenith antenna would be used for TT&C communications in both directions, and a Ka-band directional antenna would be used for transmitting payload data. This functionality would've likely been uploaded to each satellite as a software update some time after the Murshid became operational.
Generated payload data is stored in a solid state memory recorder, believed to have 272 Gbits of capacity at EOL. Recorded data can be set to any one of multiple modes; the default mode is that the data is downlinked when in range of a relay or a ground station, then deleted. Alternatively, the data can be set to be stored without immediate downlinking, stored and a copy downlinked, or if the satellite is in range of a relay or ground station during imaging, then it is believed that the data can be passed on in near-real time. Data is losslessly compressed onboard prior to transmission to reduce the bandwidth needed.
Due to the military nature of these satellites, both downlinks and the uplink are encrypted, and it is probable that the data is encrypted at rest. TT&C communications
Sensor Complement
RF-8M
The main feature of the Abid is its RM-8F Suhail (Canopus) S-band synthetic aperture radar, designed and developed by the General Radio-Electronics Institute (MARE) of Jamraya, Riysa. It is a multi-mode, programmable, polarimetric, high-resolution system using a folding parabolic mesh antenna fed by a phased array, with an antenna area of around 42 square meters (7 x 6 m) and a wide suspected field of regard of 20° to 59.5°. Its center frequency and peak power are classified, but the former is believed to be around 3.13 GHz to prevent interference with the S-band TT&C link, and the power supply and imaging requirements indicate that the peak power is at least 10 kW.
Development of the Suhail radar was the focal point of this project, representing a series of engineering decisions and trade-offs to meet its stringent requirements. Though active phased array/AESA radars like what was being flown on most new SAR satellites were originally considered, it was decided instead to use a traditional parabolic antenna, but with a phased array feed instead of a single feed horn. By using a parabolic reflector, the mass and complexity of the antenna is significantly reduced compared to an AESA of the same aperture size, and the radar can be dynamically pointed along the cross-track direction towards either side of the satellite. The addition of the phased array feed in turn allows for some electronic beam steering and multi-beam capability, like on a larger AESA system. It is not publicly known why the S-band was chosen as its operating frequency, though S-band radar provides good penetration of weather and vegetation, with better resolution than the more commonly used L-band.
Imaging modes of the radar are largely classified, but it has at least two . Given its performance and spacecraft altitude, it should be able to take any of these image modes in a field of regard of around 1,300 kilometers in the cross track direction, or 650 kilometers on either side.
Ground Segment and Control
3 m downlink
Orbit and Performance
The Abid satellites operate in a circular orbit at a nominal altitude of 637 kilometers, in two orbital planes of inclinations 57° and 68°, with a period of 97.5 minutes and a repeat cycle of around 14 days. Both planes are "frozen" orbits, minimizing the number of reboosts needed to maintain the orbit and thus potentially increasing its mission lifespan. Three satellites orbit in the 57 degree plane, while two orbit in the 68 degree plane. These orbits appear to have been selected for maximum coverage and a high revisit frequency over the areas of the world most important to Riysian national interests, namely Europe and the Middle East, as well as parts of North America.
Based on the estimated performance of the Abid spacecraft and their orbits, the whole constellation is estimated to be able to revisit any spot on the Earth every 2.4 to 3 hours on average and less than 12 hours at most, with more time in between revisits required around the poles and latitudes closer to the equator. Individually, each spacecraft is estimated to have an average revisit time of 12 to 15 hours, allowing for at least daily revisits over most of the planet.