The International Space Station (ISS) has been under construction since 1998, and will be equiped with a GPS/INS integrated system for navigation and attitude determination. The SIGI sensor is intended to be the primary navigation and attitude determination source for the ISS. The new sensor, SIGI, was demonstrated on-orbit for the first time in the SOAR demonstration on the Space Shuttle Atlantis in May 2000. Through this successful SOAR demonstration, the suitability of the SIGI sensor for the space operation was proved. The navigation and attitude determination system including the SIGI sensor will be fully operational on the ISS after the Central Truss Segment, on which GPS antenna array is installed, is assembled in January 2002. 

Numerous proximity operations near the ISS have been and will be performed. Orbiters will continuously carry astronauts and modules required for the assembly, and the Crew Return Vehicle (CRV) is planned for emergency evacuation. The Automatic Transfer Vehicle (ATV) of the European Space Agency (ESA) will perform regular reboosting and refueling of the ISS. Other vehicles such as Progress, Soyuz and H-II Transfer Vehicle will fly for crew transport, logistics, and resupply. Relative positioning rather than absolute positioning is the major concern for these operations. Better relative accuracy will provide fewer correction burns, thus saving consumables and other operational resources. 

One of the most pressing current technical issue is the design of a new system for autonmous relative navigation in the proximity of the ISS. Even though the rendezvous of two spacecraft has been accomplished since the days of the Apollo program, current methods of relative navigation for the rendezvous mission requires manual operation of relative sensors such as radar or laser rangers and, as a consequence, is subject to the possibility of human error in the proximity operation and docking phases . The risks of failure during this mission phase were demonstrated as recently as 1997 when a Progress resupply module collides into the Mir solar array, causing significant damage to the station and temporary loss of electric power. 

GPS-based relative navigation also has not been viewed as the sole navigation system for the autonomous rendezvous mission because of blockage or corruption of GPS signals as the chaser approaches the target vehicle. In this study, an integrated system of GPS/INS is investigated for the possibility of a fully autonomous relative navigation system. 

The GPS-based attitude determination for the ISS will be the first space implementation for a regular operation even though there have been several space applications for test purpose. Prior to the actual operation in the ISS, the SOAR demonstration on the Shuttle provided not only the on-board attitude determination performance of the SIGI but also oportunities to improve the performance. 

Differently from the terrestrial applications, wide ranges of attitude variation should be taken into account in spacecraft applications. A cold start initialization algorithm was necessary to determine the attitude in any orientation because existing algorithms have an operational limitation in initializing the GPS-based attitude determination system which is appropriate for the terrestrial applications. 

The GPS/INS integrated system for attitude determination must be a great benefit because it provides not only more accurate attitude solutions but also reliable solutions for a certain period of time even without sufficient number of GPS satellites in view for attitude determination. The flight data will be utilized for comparing the attitude estimation performance with two different GPS/INS integration algorithms.

Additional effort to mitigate the error sources due to line bias and multipath effect with these flight data will be interesting. Especially for cases such as ISS where there is difficulty in completing a preflight self-survey, and the cases where some variations in line bias are expected for any reason during long term operation, preflight self-survey to estimate the line bias is impossible or is not desirable.

In the mid 1990's, NASA committed to use a GPS receiver as the primary means of navigation and attitude determination on the ISS and the CRV. Under this program, Trimble Force-19 terrestrial model GPS receiver was modified by NASA engineers for space operation and attitude determination, and became a part of the Space Integrated GPS/INS (SIGI) sensor. The SIGI sensor, integrated by Honeywell Inc., is composed of the modified Force-19 GPS receiver, GG1320 digital Ring Laser Gyros, QA2000 accelerometers, system processor board and a DC power supply. It provides three output: both navigation and attitude solutions from the GPS/INS EKF, navigation solutions from the stand-alone GPS EKF, and GPS-based point attitude solutions. 

The origianl 12 channels in the Force-19 GPS receiver have been split in the new design so that 6 channels are dedicated to C/A code tracking for navigation, and 6 channels are multiplexed across 4 antenna inputs to provide differential carrier phase measurements. The additional microprocessor was added for space operation and attitude determination tasks. The software was modified for high doppler tracking, and an orbit propagater was added. The attitude determination algorithm was added based on heritage software from the TANS Vector. The INS package will not be used on the ISS in favor of the more accurate Rate Gyro Assemblies (RGAs) available on the ISS while the complete GPS/INS capability will be utilized on the CRV.

The SIGI was demonstrated on-orbit for the first time in the SIGI Orbital Attitude Readiness (SOAR)(I) experiment during the STS-101 mission of the Space Shuttle Atlantis in May 2000. An additional experiment, SOAR(II), was performed during the STS-106 mission in September 2000. Only the SOAR(I) data has been used for this study. The nominal attitude of the Space Shuttle is level flight with the nose backward in the local horizontal (LVLH) frame prior to docking with ISS (predock mode) and it is 41.5 deg nose-up for the ISS attitude mode. The antenna array in the ISS is pitched down by 41.5 deg to reduce the shadowing and multipath reflection of the GPS signal from the vertical solar array tower. The CRV mode is an attitude hold mode either in the LVLH frame or an inertial frame. The GPS antenna mounting structure (GAMS) is installed in the payload bay of the Shuttle. Four patch antennas were mounted on multipath-suppressing choke rings in the corners and two star trackers were included to provide accurate attitude solutions as good as 15 arcseconds (1 s). For the star trackers, the Ball CT-633 was the primary sensor and the Caltrac was the backup sensor. The attitude solutions from the star trackers were used as truth in this study. Two payload and general support computers (PGSC) were located in the Spacehab module to communicate with other equipment through the interface panel.