South Korea’s Arirang-5 satellite was launched on Thursday atop a Russian Dnepr rocket. The launch took place at 14:39 UTC (20:39 local time) from Site 370/13 at the Dombarovsky launch site near Russia’s border with Kazakhstan.
Arirang-5, or KOMPSat-5, is the fourth satellite in South Korea’s Arirang program. Arirang is also known as the Korea Multi-Purpose Satellite, or KOMPSAT, program.
Arirang-1, which was launched in December 1999 aboard an American Taurus rocket, carried optical and multispectral imaging payloads. The satellite operated until January 2008, when contact with the spacecraft was lost.
A second Arirang spacecraft equipped with a multispectral imaging payload, Arirang-2, was orbited by a Rokot/Briz-KM in July 2006. Arirang-3, with an optical imaging payload, was orbited by a Japanese H-IIA rocket last May, as a secondary payload to the Shizuku, or GCOM-W, satellite.
It will be supplemented by Arirang-3A, with an infrared imaging system, which is expected to be launched by another Dnepr in the near future. The sequence skips over Arirang-4, as the number four is considered unlucky in Korea.
Arirang-5 is a radar imaging satellite, the first to be developed and operated by South Korea. Developed by KARI, with participation from the South Korean aerospace industry, Arirang-5 is based around the satellite bus developed for Arirang-2.
It has a fuelled mass of approximately 1,400 kilograms (3,100 lb), and is expected to function for at least five years. The spacecraft will be powered by a pair of deployable solar arrays, generating upwards of 1,400 watts. These will charge lithium ion batteries, with a capacity of around 100 amp-hours.
Data will be relayed to the ground via an x-band downlink, providing a data rate in the region of 310 megabits per second. A lower data-rate s-band transceiver will be used to relay telemetry and commands between the satellite and its ground stations.
The primary instrument aboard Arirang-5 is the Corea SAR Instrument, or COSI, which uses Synthetic Aperture Radar to produce images of the Earth. COSI uses X-band radio signals, with a frequency of around 9.66 gigahertz, to achieve this, with a maximum ground resolution of one meter (3 feet). The SAR instrument has a mass of 520 kilograms (1,150 lb)
COSI is able to produce images at different resolutions, as a trade-off against the size of the area imaged. In its standard operating mode, it will produce images with a three-meter (10 foot) resolution of a 30 kilometer (19 mile) swath.
If optimized for maximum resolution, the swath width decreases to 5 kilometers (3 miles), while at the maximum swath of 100 kilometers (62 miles), the satellite’s resolution is only 20 meters (66 feet).
In order to return useful data, COSI must be calibrated properly, and the satellite’s orbit must be determined exactly. As part of calibration, several reflector arrays have been placed in a remote area of Mongolia, which will produce strong signals when radar signals are reflected off them.
The Atmosphere Occultation and Precision Orbit Determination payload, or AOPOD, contains two instruments to return data on Arirang-5’s orbit. The Integrated GPS Occultation Receiver, or IGOR, is a 4.2-kilogram (9.3 pound) instrument which monitors the L1 and L2 signals broadcast by US Global Positioning System satellites.
In addition to using GPS signals to determine the satellite’s orbit, IGOR will allow studies to be made into how the signals are occulted as the pass through Earth’s atmosphere. AOPOD’s other instrument is a laser retroreflector array, consisting of four prisms allowing the satellite’s altitude to be determined from the ground by means of reflecting a laser off the satellite.
Arirang-5 will operate in a circular sun-synchronous orbit at an altitude of 550 kilometres (340 statute miles, 300 nautical miles) and an inclination of 97.6 degrees, giving it an orbital period of 95.8 minutes and a revisit period of 28 days. With a local time of ascending node of 06:00, the satellite will make dawn and dusk passes over the Earth’s surface.
Thursday’s launch marked the eighteenth flight of a Dnepr rocket, which is based on decommissioned R-36MUTTH missiles. Known by Western observers as the SS-18 Mod.4 or “Satan” during the Cold War, the missile has the GRAU index 15A18, and is identified in treaties as the RS-20B. The R-36MUTTH entered service with the Soviet Union’s strategic missile forces in 1979.
Following the introduction of the R-36M2 in 1988, and reductions in nuclear armaments, R-36MUTTH missiles began to be retired in the 1990s, with some being converted to launch satellites. The last R-36MUTTH was scheduled to be removed from service in 2009; however it is unclear whether this has happened yet. Over 300 missiles were produced.
The Dnepr was developed as a way to use surplus missiles to place satellites into orbit, and is one of several conversions which Russia developed to use retired missiles – along with the Start, Volna, Shtil, Rokot and Strela. To date, it has only been used to launch commercial payloads, with these launches conducted by Kosmotras.
The Dnepr features minimal modifications to the R-36 missile with the post-boost stage, which was designed to allow for precision targeting of the nuclear warheads, instead being used to inject the payload into orbit. The first and second stages of the missile are unmodified.
Dnepr’s first stage is powered by four RD-263 engines, with the second stage powered by an RD-0255, and an RD-869 providing thrust for the third stage.
All three stages are fuelled by a mixture of unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide; storable hypergolic propellants which would allow an R-36 missile to sit in its silo fully-fuelled for long periods, allowing a faster response in the event of nuclear war, since launch would not need to wait until the missiles had been fuelled.
Due to their toxicity and corrosive, volatile nature, hypergolic fuels have largely been replaced by solid propellant in more modern missiles.
Dnepr rockets are launched from underground silos originally built for R-36 missiles. Most launches to date have occurred from a silo at Site 109/95 at the Baikonur Cosmodrome; however Sites 370/11 and 370/13 at the Dombarovsky missile base have also been used.
Thursday’s launch was from 370/13 at Dombarovsky.
Dombarovsky is located near the town of Yasny in south-west Russia, close to the border with Kazakhstan. It is a few hundred miles almost due north of the Dnepr’s other launch site at Baikonur. An air force base, during the Cold War the site was home to two squadrons of interceptor aircraft and a contingent of nuclear-armed R-36 missiles.
Sixty four R-36 silos were built at Dombarovsky, some of which still contain missiles on alert – in 2012 Russia still had 55 R-36M2 missiles in service, split between Dombarovsky and another base at Uzhur, each armed with ten warheads.
The launch of Arirang-5 was the sixth orbital launch from Dombarovsky, all of which have been of Dnepr rockets.
The first, on 12 July 2007, carried the Genesis 1 prototype inflatable space station into orbit, with a follow-up mission, Genesis 2, launching in June 2007. Both of these launches were made from Site 370/11, after which launches switched to Site 370/13.
The most recent orbital launch from Dombarovsky was the last Dnepr launch, in August 2011.
R-36 silos are not designed to withstand a rocket igniting within the silo, so the missile is ejected from the silo by a hot gas generator, which separates from the missile once it clears the silo.
Per the planned launch sequence, first stage ignition occurs several seconds after launch, at an altitude of around 20 meters (66 feet). The first stage will burn for approximately 98 seconds before burning out and separating; ignition of the second stage will occur six seconds later.
During the 168-second burn of the second stage, the outer payload fairing will separate from the rocket. The inner fairing, or Gas Dynamic Shield, will remain attached to protect the payload from the third stage’s exhaust.
The fairing is modified from the original R-36 fairing; for the Arirang-5 launch it has been stretched to accommodate the larger payload. This stretched fairing was previously used on the launches of Germany’s TerraSAR-X and TanDEM-X radar imaging satellites in 2008 and 2010 respectively, and ESA’s CryoSat-2 in 2010.
Following the end of second stage flight, the third stage will separate. Because of its heritage as a post-boost stage, the third stage has an unusual configuration for an orbital launch system. Its engine’s four chambers are oriented towards the nose of the rocket, in the same direction as the payload hence the need for the Gas Dynamic Shield.
Following separation from the second stage, the third stage turns through 180 degrees and flies backwards, with the payload behind it.
Seconds before reaching the target orbit, the gas dynamic shield will be jettisoned, with spacecraft separation occurring shortly afterwards whilst the third stage is still firing. The third stage burn will continue past spacecraft separation, to avoid cluttering the spacecraft’s operational orbit with the spent stage.
Thursday’s launch was the first Dnepr launch in two years. The next launch is scheduled for November, carrying DubaiSat-2 and twenty six secondary payloads.
Before that, the next planned launch of a former Soviet rocket is scheduled for 31 August, when Israel’s Amos-4 communications satellite will be deployed by a Zenit-3SLB.
The launch of Airiang-5 is the forty-fourth or forty-fifth orbital launch attempt of 2013 – depending on the accuracy of reports of a rumored Iranian launch failure in February.