Guidance, Navigation, and Control Sensors: GPS, GLONASS,

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Guidance, Navigation, and Control SensorsGPS,
GLONASS, RGAs

• Timothy S. Monk

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What is GNC, and why is it important?

• The International Space Station is a
  multi-billion dollar object orbiting the Earth at
  about 7 kilometers per second

• Humans maintain a constant presence aboard the
  ISS

• Keeping track of the stations movement around
  the Earth is essential, and the ability to
  correct or alter its orbit is necessary

• GNC sensors and software work together to
  determine the position and attitude of the
  spacecraft, and they relay the information to the
  proper control devices for adjustments if needed

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GNC configuration onboard the ISS

• There are two systems the US GNC system and the
  Russian Motion Control System

• Guidance is generally a Russian function and is
  used to tell the station what route to follow
  from point A to point B

• Navigation uses information from both the Russian
  segment and the US segment to determine the state
  and attitude of the ISS. It also has a Pointing
  and Support function to pass navigation
  information to other systems aboard the ISS

• Control is the implementation of the route
  determined by the Guidance subsystem. It includes
  translational and rotational control of the ISS.
  It utilizes hardware and software located in both
  the Russian and US segments

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Navigation Subsytem

• The US segment uses the Global Positioning System
  (GPS) and Rate Gyro Assemblies (RGAs) for state
  and attitude determination

• The Russian segment uses the Global Navigational
  Satellite System (GLONASS), as well as, several
  other types of sensors

• GPS and GLONASS are very similar to one another.
  Some similarities include
• 24 satellite constellations
• 3 satellites needed for triangulation, 1
  additional for timing
• 2 channels of service Channel of Standard
  Accuracy and the Channel of High Accuracy

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How do GPS and GLONASS work?

• Both systems have 24 satellite constellations
  that orbit the Earth at an altitude of 10,900
  nautical miles (Geosynchronous orbit)

• These satellites broadcast their position and the
  exact time that they are in that position on a
  continuous basis

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• A receiver intercepts the signal broadcast from
  the individual satellites in GEO

• One satellite creates a sphere, upon the surface
  of which the receiver must me located

• Another satellite creates a second sphere, which
  intersects the first sphere to reduce the
  location to a circle

• A third satellite intersects the circle in two
  locations. One of these will prove to be
  illogical, leaving a specific point as the
  location

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The US Segment

• 4 GPS Antenna Assemblies
• Located on the S0 Truss
• Receive the GPS Course Acquisition (C/A) codes at
  a frequency of 1575.42 Mhz
• Function is to detect the incoming signal, filter
  out noise, amplify the required signal, and
  splits the signal so it can be sent to two SIGI
  R/Ps
• Only 2 of the 4 assemblies can be used as the
  master Antenna Assembly for attitude determination

• For state determination, at least one antenna
  assembly must be receiving data from 4 GPS
  satellites simultaneously

• For attitude determination, at least three
  antenna assemblies must be receiving data from 4
  GPS satellites simultaneously

• Not only a single fault tolerant system, but more
  accurate with the fourth antenna assembly

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Attitude Determination with 3 GPS Antennas

• With the known baseline between the Master
  Antenna and two other antennae on the Z0 truss,
  an interferometry technique is used to determine
  the attitude of the ISS
• The technique is measuring the displacement of
  the phase angle of the carrier waves as they
  reach the different antennae
• GNC software uses the displacement angle between
  each slave and the master antenna for attitude
  determination

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• 2 SIGI (Space Integrated GPS/INS)
  Receiver/Processors (R/Ps)
• Honeywell Space Integrate GPS/Inertial Navigation
  System R/Ps
• Located in the Destiny Lab Module
• Once the required signal is received, state and
  attitude can be determined
• Monitors the overall health of the GPS system

• Processes the signal received into a state vector
  estimate, an attitude estimate, a time tag, and
  data quality information. The state vector
  estimate consists of 3 position elements, 3
  velocity elements, and a time element

• Information is sent to GNC MDMs for use by the
  State Determination and Attitude Determination
  software

• 12 channel availability with the ability to lock
  onto a maximum of six GPS satellites at any given
  time

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• 2 RGAs (Rate Gyro Assemblies)
• Located on the S0 Truss
• Uses three Ring Laser Gyros (RLGs) to determine
  attitude rate
• Attitude rate is used to propagate attitude
  through periods of time when 4 GPS satellites can
  not be acquired
• Mounted skewed in relation to one another to
  allow for fault detection
• Only 1 is powered on at a time, except for
  critical events such as rendezvous

• Cathode sends out a laser which is split into two
  directions. These 2 beams are reflected upward to
  a detector. One travels directly to the detector,
  while another is made parallel to the other by
  reflecting it through a prism

• The two beams are rejoined, and the shift in
  frequency is directly proportional to the
  attitude change rate along that axis

• Three RLGs are mounted orthogonal to each other
  to account for movement in the three directions
  of inertial space

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The Russian Segment

• 4 GLONASS Receiving Antennas (ACH 2401-1)
• Located on the Zvezda Service Module
• Receive signals from the GLONASS constellation
  satellites to determine state and attitude of ISS
• One antenna is needed for absolute and relative
  navigation measurements
• Angular measurements require three antennas
• GLONASS receiver can receive signals from GPS
  satellites to determine state, but can not use
  this information to determine attitude

• 2 signal receivers (PHC ACH 2404-2)
• Located in the Zvezda Service Module
• Includes 2 receiver sets each, which receive the
  data from the antennas, processes it, and
  calculates absolute navigation values
• Also includes 2 secondary power sources (27
  volts) and interface processors on the 1553 bus
  (and the redundant PC-422 channel)

• The Russian segment also has its own version of
  the Rate Gyroscope Assemblies, called the GIVUS
  assembly. This is provided redundancy by another
  system known as the ORT assembly

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2 systems, 1 spacecraft

• US GNC system is the primary system
• The Russian Terminal Computer and US GNC MDM can
  exchange estimates through 1553 bus (PC 422
  channel)

• Of course, 1 priority redundancy
• Several layers of redundancy within each system
• Russian system provides a redundant system for
  the US GNC subsystem
• Since they are designed differently, similar
  failures are very unlikely

• Accuracy verification
• Russian system compares state, attitude, and
  attitude rate estimates to US estimates for
  verification and fault detection

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Questions?