June 19, 2013

Ground Operations

Now that we've had a look at the basics of systems engineering and an overview of the systems aboard a spacecraft we can look more closely at how a spacecraft comes together and is used.  We'll consider both the pre-launch part of ground operations, integration and test, and the post-launch, mission operations.

A mission goes through 8 phases from beginning to end:
  1. Conceptual Design - the mission is described and the mission requirements and constraints developed.  Results are summarized in a document called the Conceptual Design Review (CoDR)
  2. Preliminary Design - system requirements and constraints are derived and the basic subsystem selections are made.  Results summarized in Preliminary Design Review (PDR).
  3. Detail Design - subsystems are designed. Critical Design Review (CDR).
  4. Fabrication and Test - components are purchased and assembled and tested to ensure they work together and perform. Pre-Environmental Review (PER).
  5. Vehicle Integration - spacecraft is assembled.  Pre-Ship Review (PSR).
  6. Environmental Test - vehicle is subjected to tests to ensure it can survive mission conditions.  Launch Readiness Review (LRR).
  7. Launch - spacecraft is lifted from the ground into space.  Launch operations leads directly into mission operations
  8. Mission Operations - mission begins.  Begins with final check to that vehicle is in proper orbit and all systems have survived launch.
In the previous lesson we looked at the first two phases.  Here we're considering two related topics: mission operations and integration and test.  Integration and test covers phases 4 through 6.  A later lesson will consider launch vehicles and launch operations.  Detail design is a broad topic that will take much of the remainder, though each subsystem design goes through three basic phases:
  1. Engineering Design - subsystem components are selected and their configuration is determined.
  2. Package Design/Layout - specific placement of components and surrounding structure and mounting system is determined.
  3. Fabrication and Test - subsystem is assembled and tested.

Integration and test is the final step in spacecraft construction, where components are fabricated and assembled.  This, of course, comprises two parts: integration and verification.  Integration is the process of assembling subsystems and systems into a vehicle and ensuring they perform properly.  Verification is the process of determining that an element will meet requirements and operate properly under mission operating conditions.
Performance testing is a verification that the unit under test will operate, or perform, properly under use.  A subset of performance testing is functional testing, where the unit's suitability is determined by varying only a few important criteria.  For example during performance testing an analog to digital converter may be made to run through its full operating spectrum, while functional testing would choose three representative points.  Functional tests can often save time and money but may not reveal all possible problems and often rely more heavily on simulation and analysis.
Environmental testing occurs later.  Performance tests ensure a component does what it was designed to do.  Environmental tests ensure that the component will survive the conditions that it is to be exposed to.
Some of the tests that must be performed before a spacecraft can be installed in the launch vehicle:
  • safe to mate - test that all components receive proper voltage prior to mating any flight components.
  • electromagnetic compatibility - components must be compatible with each other.
  • ground station compatibility - ensure ground control station can interface with spacecraft and with mission control equipment.
  • alignment - any self-contained optical or mechanical equipment must be checked for alignment at ambient and operating conditions.
  • deployment - deployment tests must be performed both before and after exposure to mechanical stresses from launch.
  • radiation - since ambient radiation is more intense outside the earth's atmosphere (particularly passing through the Van Allen belts) it is necessary to simulate this environment to ensure electronic components will survive.
  • leakage - hermetically sealed components must be checked for leaks that may be caused by changes in operating conditions.
  • ambient baseline electrical - a test is at ambient temperature, pressure, and humidity to determine the baseline voltage for other tests.
  • initial magnetic field - test for stray magnetic fields that may interact with sensitive magnetic field measuring equipment.
  • shock and acoustics -
  • vibration - each component is subjected to vibration from a shaker table to simulate launch conditions.
  • temperature - spacecraft temperature can vary dramatically so it is necessary to subject the vehicle not only to the possible extremes, but also to cycles between them.
  • thermal vacuum - test not only that components can survive the vacuum of space, but also for possible hot spots caused by electrical equipment where heat has nowhere to dissipate.
  • mass properties - components and vehicle are weighed and placed on a center of gravity table to find their c/g and moment of inertia.
Obviously this is a lot of testing for one vehicle to go through.  So we'll often run through a number of different models.  Engineering mock-ups are non-flight ready test articles.  Mock-ups are used for operational tests and to experiment with configuration.  These are things like breadboards and small-scale models, used to demonstrate the components will work and integrate properly.  Test models are full-scale articles built as the final flight system will be.  These are used for final environmental testing and final qualification tests.
Once all the testing is over the time comes to finally assemble all the systems into a vehicle.  Although the specific integration is vehicle dependent, there is a generally preferred order:
  1. Structure - obviously it is necessary to have a skeleton on which to build before anything can be attached.
  2. Propulsion - vehicles with onboard engines should have these installed first, since other components can be fit around fuel tanks and lines.
  3. Electrical Power - so that other systems can be tested as they are installed the power sources and power distribution should be installed next.
  4. Command and Data Handling - computer systems follow the power system since the remaining systems all interface directly with the computer.
  5. Communications - with computer systems in place the communications system is installed and communication checks made.
  6. Attitude and Orbit Determination and Control - guidance systems follow communications.
  7. Payload - the payload is always installed last.
Because of launch and flight risks it's usually a good idea to have more than one flight ready vehicle, a flight spare in addition to the flight model.  Having a flight spare on hand can save a great deal if something goes wrong with the first launch, since the delay is much shorter.  For limited window missions a flight spare is almost essential.
For missions where more than one flight vehicle is necessary the solution is simple.  By always having one more vehicle than necessary you always have a flight spare.  If nothing goes wrong then the spare is graduated to the flight model and you build a new spare.
When the vehicle is unique, however, having a spare means spending twice as much as necessary on integration if nothing goes wrong.  Many missions save money and time by reusing test models as flight spares, or in some cases even as the sole flight model.
An example of the occasionally fuzzy line between test model and flight model can be seen in the first three space shuttles.  On each shuttle is a tail number beginning with the designation OV, for Orbital Vehicle.  But if you look at the shuttles in construction order you'll see that that Columbia, the first flight ready shuttle, has the tail number OV-102, which would suggest that it is the second shuttle.  Before Columbia came OV-101, Enterprise, which was used for configuration, approach, and landing tests.  Its tail number would suggest, though, that it was a flight model (numbers above 100 are for flight models, under 100 for test models, 100 is not used).  NASA originally intended Enterprise to be refit into a flight ready shuttle after testing was finished.  However there were enough changes between Enterprise and Columbia that they decided it was more practical to build a new shuttle instead.
So NASA commissioned the space shuttle Challenger, OV-99.  Again the number seems confusing.  Although Challenger was the third shuttle it has a lower tail number, and even though it was flight ready its number suggests it was a test article.  In fact it was.  OV-99 began its life as structural test article STA-099, before construction finished on Columbia.  STA-099 survived its testing admirably and had an identical skeleton to Columbia, so it was used as the frame of the new shuttle.

Once the spacecraft is in orbit control shifts from launch operations (a topic we'll look at later) to mission operations.  For a NASA mission this shift is a handover from ground control in Cape Canaveral to mission control in Huston.
The mission operations center (MOC) is responsible for ensuring mission success once the spacecraft is in orbit.  All communications and monitoring is supervised by the MOC team, who also takes care of mission planning, flight simulation, and performance assessment.  MOC also has engineering teams to provide whatever engineering support needed by either the mission operations or payload teams require.  Mission control must have all relevant engineering and operations documentation:
  • concept of operations (CONOPS) - a basic overview of how the mission is to be performed and run.
  • flight rules - list of flight and user constraints.
  • standard and contingency procedures - step by step instructions for normal and emergency situations.
  • work plans - future work to be done by the spacecraft or future instructions to be sent to it.
  • operating logs - record of previous communications and status updates from the vehicle.
Communication with the spacecraft can take place either directly between the vehicle and the MOC or through a relay network:
The general term for information sent back from the spacecraft is telemetry.  When telemetry is sent directly from the spacecraft to a ground station it is called downlink, when it passes through a relay it is called a return link.  Instructions sent to the spacecraft are called commands.  Commands sent directly to the vehicle are on an uplink, sent through a relay the connection is called a forward link.
Tracking networks monitor the spacecraft position and offer the ability to communicate with it throughout its orbit.  Without the use of tracking stations and relay satellites we must wait until the spacecraft is directly above mission control to communicate with it.
NASA and the Department of Defense operate their own tracking and relay networks.  NASA operates the Spaceflight Tracking and Data Network (STDN) for near earth spacecraft and the Deep Space Network (DSN) monitors interplanetary probes.  The DoD operates the ground based Space Surveillance Network (SSN) which monitors objects in orbit through radar stations and Air Force Satellite Control (AFSCN) controls all US military satellites.  The ESA is currently developing its own European Data Relay System (EDRS).
A few commercial entities operate their own satellite constellations, which can serve as relay systems, but most space ventures rely on the STDN or another tracking network if they need to communicate or monitor their vehicle when it is out of touch with their own MOC.  Use of a tracking network must be booked well in advance and is costly.  For that reason few programs opt for real time satellite control and instead try to develop autonomous operation.
An autonomous spacecraft has its normal and contingency procedures pre-loaded and receives occasional updates from mission control when errors are found or mission parameters change.  Although the spacecraft hardware is more complicated and software development much more difficult this can drastically minimize the need for tracking network use, which is far more costly.

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