China back in action with dual Compass launch

by Rui C. Barbosa

China has launched a new pair of navigation satellites today to upgrade its satellite navigation constellation. The launch of the Compass-M5 and Compass-M6 satellites took place at 19:10UTC on Tuesday from the LC2 launch complex of the Xichang Satellite Launch Center, using the powerful Long March-3B/E (Chang Zheng-3B/E).

Chinese Launch:

Developed from the DFH-3B satellite platform, the Compass-M satellites are deployed in 21500~24100 km, 55 degrees inclination intermediate circular orbits.

The first Compass-M (31115 2007-011A) satellite was launched at 2011UTC on April 13th, 2007 by the Chang Zheng-3A (Y13) from the LC3 launch complex of the Xichang Satellite Launch Center.

China has developed two models for Compass-M satellites. The two satellites that where launched today are based on DFH-3B bus and are equipped with an apogee propulsion system for final orbit insertion.

The second model is not equipped with an apogee propulsion system and is completely different from DFH-3 bus. Still under development, this model will not fly until the second construction phase of the Compass constellation starts.

The Compass Navigation Satellite System (CNSS) is China’s second-generation satellite navigation system approved by the Chinese government in 2004, and is capable of providing continuous, real-time passive 3D geo-spatial positioning and speed measurement.

The system is used initially to provide high-accuracy positioning services for users in China and its neighboring regions, covering an area of about 120 degrees longitude in the Northern Hemisphere using five Compass-G, 5 Compass-IGSO and four Compass-M satellites.

The long-term goal is to develop a global navigation satellite network similar to the GPS and GLONASS by 2020, eventually consisting a constellation of 35 vehicles, including 27 MEO (21,500 km orbits) satellites, 3 IGSO satellites (inclined at 55 degrees) and five GSO satellites.

The system will consist of two types of services: a civilian service that will give an accuracy of 10 meters in the user position, 0.2 m/s on the user velocity and 10 nanoseconds in time accuracy; and the military and authorized user’s service, providing higher accuracies.

The first phase of the project will see the coverage of the Chinese territory but in the future the Compass constellation will cover the entire globe. The satellites transmit signals on the: 1195.14-1219.14MHz, 1256.52-1280.52MHz, 1559.05-1563.15MHz and 1587.69-1591.79MHz, carrier frequencies.

The previous double Beidou-2 ‘Compass’ launch took place on April 29th, 2011, when the Chang Zheng-3B/E (Y14) orbited the ‘Compass-M3’ (38250 2012-018A) and ‘Compass-M4’ (38251 2012-018B) satellites.

DFH-3B is an updated version of DFH-3 bus, a communications satellite bus whose capability is between high and medium ones. It adopts hexahedral structure, consisting of propulsion, service and communication modules, communication antennas and solar arrays and adopts 3-axis stabilized attitude control. Its dimensions are 2200mm × 2000mm × 3100mm, and its mass is 3,800 kg with a payload mass of 400 kg to 450 kg.

This satellite bus is applicable to communications and navigation satellites and deep space probes through adaptive modification.

China’s 12th successful orbital launch in 2012 was also the 167th successful Chinese orbital launch, the 167th launch of a Chang Zheng launch vehicle, the 7th launch from Xichang in 2012 and the 75nd successful orbital launch from Xichang.

Launch vehicle:

This was the second dual launch for the CZ-3B Chang Zheng-3B launch vehicle. For this launch, CZ-3B was using the 3700Z fairing. This fairing has an external diameter of 3,700 mm, a total height of 10,796 mm, and is used for dual launch with the LM-3B launch vehicle with the satellite encapsulated in the BS3.

The satellite is mated to the Payload Adapter (PLA) and encapsulated in the fairing, prior to being shipped to the launch pad, completing the assembly when it is mated to the launch vehicle.

This fairing can be used with the 1194 and 1194A Payload Adapter interfaces. For this fairing – like others used on the CZ-3B – the longitudinal release mechanism uses notched bolts, explosive cord, expanding hose, and two explosive bolts. All these parts jointly perform the release functions.

Developed from the Chang Zheng-3A, the Chang Zheng-3B is the most powerful launch vehicle on the Chinese space launch fleet.

The CZ-3B features enlarged launch propellant tanks, better computer systems, a larger 4.2 meter diameter payload fairing and the addition of four strap-on boosters in the core stage that give an additional help in the first phase of the launch.

The rocket is capable of launching a 11,200 kg satellite to a low Earth orbit or a 5,100 kg cargo to a geosynchronous transfer orbit. The rocket has a total length of 54.84 meters and a core diameter of 3.35 meters.

Each of the four boosters has a 15.326 meter length with a 2.25 meter diameter, consuming 37,700 kg of N2O4 / UDMH. Equipped with a YF-25 engine capable of a ground thrust of 740.4 kN and a ground specific impulse of 2,556.2 Ns/kg.

The first stage has a 23.272 meter length with a 3.35 meter diameter, consuming 171,800 kg of N2O4 / UDMH. Equipped with a YF-21C engine capable of a ground thrust of 2,961.6 kN and a ground specific impulse of 2,556.5 Ns/kg.

The second stage has a 19.92 meter length with a 3.35 meter diameter, consuming 49,400 kg of N2O4 / UDMH. Equipped with a YF-24E cluster engine with a main engine capable of a vacuum thrust of 742 kN and a vernier engine with a vacuum thrust of 47.1 kN (specific impulses of 2,922.6 Ns/kg and 2,910.5 Ns/kg, respectively).

The third stage has a 12.375 meter length with a 3.0 meter diameter, consuming 18,200 kg of LH2 / LOX. Equipped with a YF-75 engine capable of a vacuum thrust of 167.17 kN and a specific impulse in vacuum of 4,295 Ns/kg.

In recent years, the CZ-3B/E (Enhanced Version) launch vehicle was developed on the basis of CZ-3B, increasing the GTO capacity up to 5,500kg. The CZ-3B/E has nearly the same configurations with CZ-3B except its enlarged core stage and boosters.

On May 14, 2007, the first flight of CZ-3B/E was performed successfully, accurately sending the NigcomSat-1 into pre-determined orbit. With the GTO launch capability of 5,500kg, CZ-3B/E is dedicated for launching heavy GEO communications satellite.

The CZ-3B and CZ-3B/E launch vehicles are comprised of the vehicle structure, propulsion system, control system, measurement system (telemetry system and tracking & range safety system), propellant management and reaction control system, propellant utilization system, separation system and auxiliary system, etc.

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The third stage includes the payload adapter, the vehicle equipment bay (VEB) and the cryogenic propellant tanks and engines. The payload adapter mates the satellite to CZ-3B and bears the mechanical loads. This cargo adapter can be one of the international standard interfaces designated as 937B, 1194 or 1194A.

The CZ-3B/E is now the standard version used for China.

The Xichang Satellite Launch Centre is situated in the Sichuan Province, south-western China and is the country’s launch site for geosynchronous orbital launches.

Equipped with two launch pads (LC2 and LC3), the centre has a dedicated railway and highway lead directly to the launch site.

The Command and Control Centre is located seven kilometers south-west of the launch pad, providing flight and safety control during launch rehearsal and launch. The CZ-3B launch pad is located at 28.25 deg. N – 102.02 deg. E and at an elevation of 1,825 meters.

Other facilities on the Xichang Satellite Launch Centre are the Launch Control Centre, propellant fuelling systems, communications systems for launch command, telephone and data communications for users, and support equipment for meteorological monitoring and forecasting.

The first launch from Xichang took place at 12:25UTC on January 29, 1984, when the CZ-3 Chang Zheng-3 (CZ3-1) was launched the Shiyan Weixing (14670 1984-008A) communications satellite into orbit.

(Images via

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