GNC Sensors Monk

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

Guidance, Navigation, and Control Sensors: GPS, GLONASS, & RGAs Timothy S. Monk

What is GNC, and why is it important?: 

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 station’s 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

GNC configuration onboard the ISS: 

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

Navigation Subsytem: 

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

How do GPS and GLONASS work?: 

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

Slide6: 

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

The US Segment: 

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

Attitude Determination with 3 GPS Antennas: 

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

Slide9: 

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

Slide10: 

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

The Russian Segment: 

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

2 systems, 1 spacecraft: 

2 systems, 1 spacecraft 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 US GNC system is the primary system The Russian Terminal Computer and US GNC MDM can exchange estimates through 1553 bus (PC 422 channel) Accuracy verification Russian system compares state, attitude, and attitude rate estimates to US estimates for verification and fault detection

Questions?: 

Questions?