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Friday, August 1, 2014

Processing to Launch a GPS Satellite

As reported by Pocket GPS WorldWe all know and use GPS technology in some form every day of our lives, but have you stopped to think how this technology came into being, and how it is maintained? The theory of navigation goes back a long way and and is very closely linked to accurate timing. The more accurate the time the more accurate your location can be determined. 

Back in 1730 John Harrison created a marine clock to compete for the £20,000 Longitude Prize, a competition to find the most accurate timepiece to enable mariners to navigate across the seas. Accurate time combined with a sextant and a chart enabled the early navigators to determine with reasonable precision where on the planet they were.

Jump forward nearly 300 years and the same principles of precise timing and triangulation are still being used, this time using GPS Satellites, atomic clocks and computer algorithms. This, of course, is all transparent to us, we just see a moving dot on a map showing us where we are.

How does this all work? Rather than explain this again we have an excellent article describing How does the Global Positioning System work?. This article explains how a GPS satellite is prepared for launch into orbit.

The GPS IIF Satellite
The life of a GPS satellite starts at a planning committee meeting in the United States Department of Defense. There a requirement is identified for a new feature or the replacement of ageing systems. This is then processed through a number of departments for planning, budget, and functionality design prior to a primary contractor being selected to produce the spacecraft. In the case of the GPS Block IIF satellites the contractor was Boeing.
The GPS IIF satnav satellite
Boeing manufacturing plant at El Segundo Credit: Boeing

Boeing have had a lot of experience in GPS satellite manufacture having build over 40 of the 66 GPS spacecraft. Normal satellite construction takes place on an individual basis as there is seldom a requirement to build more then one or two of the same satellite. With the Block IIF spacecraft Boeing had a contract to build 12 so decided to introduce a production line based on the tried and trusted principles of their aircraft production lines. The concept was to increase quality and reduce time to build for each satellite. The video below gives an insight into the Boeing process:

Video credit: Boeing

Using this process Boeing have now built all the 12 spacecraft that have been contracted and are storing the remaining satellites until they are ready to be integrated into the launch vehicle.

The Boeing production plant is a El Segundo in California and the rocket launching the satellite is on the other side of the USA in Cape Canaveral Florida. The GPS Satellite is transported on a USAF C-17 air-lifter. The Cape Canaveral AFS has a landing strip called the ‘Skid Strip’ within the station large enough to handle the heaviest of transports. 

On arrival at Cape Canaveral the GPS IIF is transported to Area 59, the Navstar Processing Facility, for fueling and testing. Although the satellite has been fully checked out in California final tests are made to ensure that the transporting of the spacecraft has not altered any of the sensitive components and systems. This will include the checking of the control systems and the actually user navigational signals themselves. Once the GPS IIF has been cleared then the on-board batteries are installed and the propellant is loaded. The propellant allows the GPS satellite to be maneuvered when it is in orbit and separated from the rocket.
The Navstar Processing Facility CCAFS
The Navstar Processing Facility CCAFS Credit: Boeing

The final task in the Navstar Processing Facility is the installation of the Payload Adapter. This enables the Satellite to be attached to the top of the rocket securely but also contains the pyrotechnics to separate the satellite from the second stage before flying freely. All operations up to this point are common and apply to the GPS satellite irrespective of the rocket that will launch it into orbit. The Payload Adapter is where things start to get specific to the rocket.
Encapsulating the GPS in the payload fairing
Encapsulating the GPS in the payload fairing Credit: ULA

The Launch Vehicle
The atlas V rocketThe GPS IIF satellites can be launched on either a Delta IV or an Atlas V rocket. Both rockets are built by United Launch Alliance (ULA) and are launched from separate launch complexes at Cape Canaveral Air Force Station in Florida. As GPS IIF-7 is to be launched on an Atlas V rocket we will continue looking at the processing for that booster.

The Atlas V complex is SLC-41 and it the most northernmost active launch pad at Cape Canaveral. This launch complex holds all the infrastructure to ready and launch the rocket, including fuels, communications and power. Just outside the Launch Complex is the Vertical Integration Facility where the various parts of the rocket are assembled.

The Atlas V rocket is a two stage rocket that can have additional solid rocket boosters (SRBs) attached to give more thrust at launch. For the GPS IIF satellites the SRBs are not required as the payload is sufficiently light enabling the main core booster to lift the rocket without assistance. 

The main parts of the Atlas V rocket are:

The main or common core booster. This is the first stage of the rocket and will be ignited just before liftoff. This is 3.8 metres in diameter and 32.5 metres high. The booster is mainly structurally stable fuel tank constructed from isogrid aluminium barrels. This stage has twin tanks for the RP-1 (a highly refined form of Kerosene) and LOX (super cold liquid oxygen) propellants. The engine on the first stage is an Russian made RD-180 with twin combustion chambers. Also contained within the structure of the first stage is a high pressure helium tank. The helium is used to pressurise the propellant tanks just before launch.

The Centaur second stage is connected to the common core booster by an interstage adapter. This adapter contains the pyrotechnics which are used to separate the components during the launch. The Centaur is 3.1 meters in diameter and 12.7 meters high. This is powered by a single Pratt and Whitney RD-10A engine fueled by LOX and LH2 (super cold liquid hydrogen). This is commonly referred to as cryogenic fuel. As these tanks contain very cold liquid fuels the tanks are very heavily insulated to prevent the liquids boiling both during the countdown and due to the friction heat generated by the launch. On the top of the Centaur is the Centaur Forward Adapter which provides the avionics for the rocket along with other systems providing telemetry and control. This provides the avionics for both stages of the rocket.

Sitting on top of the Centaur Forwards Adapter is the Payload Adapter this connects the GPS II satellite to the rocket. The Payload Adapter can vary dependent on the spacecraft being attached.

Finally the GPS IIF satellite is surrounded by the Payload Fairing. The payload fairing is constructed in two sections designed to be explosively jettisoned during the launch when the atmospheric pressures are no longer a threat to the GPS payload. In the Atlas V 401 configuration the payload fairing is 4 meters in diameter. 

The components for the rocket are manufactured at a number of sites across America and in Russia. 

The main first stage (called the core booster) is manufactured in Decatur, Alabama and is transported to Cape Canaveral by ship called the Mariner. The Mariner sails down the Mississippi River, round the Florida Keys and into the Cape Canaveral AFS via Port Canaveral. 
The Atlas V unloading from Mariner at CCAFS
The Atlas V unloading from Mariner at CCAFS Credit: AmericaSpace / Jeffrey Soulliere

The same is true for the Centaur second stage. It is likely that the Mariner will make separate trips for each stage, although it can carry two main core boosters. On arriving both stages are processed at the Atlas Spaceflight Operations center which also houses the Launch Control Center and the Mission Director’s Center.

The Pratt & Whitney RL-10A second stage engine is manufactured in West Palm Beach in Florida, whilst the RD-180 main core booster engine is made in Khimki, Russia.

The Payload Faring, and various adapter plates are fabricated in Harlingen Texas and will be shipped to the Cape via truck, each section of the payload fairing having its own vehicle.

Preparing for Launch
With all the components delivered to Cape Canaveral the final assembly of the rocket can take place. In the case of the Atlas V this is done in the VIF (Vertical Integration Facility) about 400 meters from the launch complex.

The common core booster is the first to arrive at the VIF this is transported on a specially designed transport from the Atlas Spaceflight Operations Center to the VIF where it is hauled into the upright position on top of a Mobile Launch Platform (MLP). The MLP provides hard wired services to the rocket during construction and throughout the launch countdown. It is designed to move the assembled rocket out to the launch pad via a pair of engines which also tow mobile support services, more of which later.
Raising the Atlas V common core booster
Raising the Atlas V common core booster Credit: NASA

Next to arrive at the VIF is the inter-stage adapter. This allows the Centaur upper stage to be connected the the common core booster and also to separate the two stages in flight.

The Centaur upper stage is now delivered. This arrives on a lorry in the horizontal position. It is lifted off the lorry and then elevated into the vertical position. Now it is winched up and carefully lined up with the top of the common core booster where it is attached to the inter-stage adapter. The avionics linkages are now connected and tested.
Lifting the Atlas V Centaur upper stage
Lifting the Atlas V Centaur upper stage Credit: ULA

The final item to arrive is the GPS IIF satellite. It will already have been powered, fueled and encapsulated within the payload fairing. This is transported from the Navstar Processing Facility in the upright position so just needs to be lifted to the top of the stack and the payload adapter plate will be connected to the top of the Centaur Forward Adapter.
Mating the GPS Payload to the Atlas V
Mating the GPS Payload to the Atlas V Credit: ULA

Ready for Launch
With the payload connected to the top of the Atlas V rocket the GPS satellite is nearly ready for launch. The next process in the chain is to move the rocket to the launch pad and start the final preparations for launch. I will cover this in the next article: Launching the GPS IIF satellite.