EGS / Ajisai
EGS (Experimental Geodetic Satellite) / Ajisai
EGS is a Japanese (JAXA, formerly NASDA) passive satellite for a geodetic mission. The primary objective was performance confirmation of the H-1 launch vehicle. Objectives of EGS: provision of long-range geodetic applications aimed at rectifying Japan's domestic geodetic triangular network, determining the exact position of many Japanese islands and establishing Japan's geodetic point of origin. The EGS nickname is Ajisai (Japanese for Hydrangea).
JAXA and Kawasaki Heavy Industries developed the spacecraft. EGS is a passive spherical satellite of 2.15 m diameter and a mass of 685.2 kg, carrying 318 mirrors and 120 laser retroreflector assemblies (1436 corner cube reflectors) for precise satellite laser ranging (SLR) measurements from ground-based laser ranging stations. The reflectors and mirrors on the satellite sphere reflect laser beams back to the source, regardless of the angle of incidence (solid Earth studies, crustal movements, plate tectonics). The reflection of solar light is used to determine the direction to the satellite from an observation site.
Figure 1: EGS with attachment ring in the TNSC service tower (image credit: JAXA)
The body of the satellite is a hollow sphere made of glass-fiber-reinforced plastics. The surface is covered with corner cube reflectors (CCRs) and solar light reflectors. Twelve CCRs form a set of laser retroreflectors (LRRs), and 120 LRRs are distributed almost uniformly over the surface. The remaining surface is covered with 318 solar reflectors. The base of the mirrors is made of an aluminum alloy and the surface has a protective coating of silicon oxide. 1) 2) 3)
Launch: A launch of EGS took place on Aug. 12, 1986 from TNSC (Tanegashima Space Center), Japan, with the H-1 launch vehicle. The secondary payload on this flight was JAS-1 (Japanese Amateur radio Satellite), also referred to as FUJI-1 with a mass of 50 kg.
Orbit: near-circular orbit with a perigee of 1490 km, eccentricity = 0.001, inclination = 50º, period = 116 minutes.
Observation method of EGS: The basic concept for geodetic use of the EGS is to simultaneously determine directions through photography and measure distances by laser ranging. The principle is to determine the relative geodetic location of two observation sites by simultaneously measuring the distance and direction to the EGS at a known station (named the base station) and an unknown station (primary station) of which the precise geodetic location is unknown. The distance from a site to the EGS is measured by SLR. To determine the location of a number of unknown stations around the base station, at least one Transportable SLR (TSLR) is used.
The laser ranging surveys are conducted by the `Geophysical Survey Institute' of the Ministry of Construction, and the `Hydrography Department of the Maritime Safety Agency', Ministry of Transport.
Table 1: Main parameters of the EGS/Ajisai spacecraft
• The life expectancy of the passive spacecraft EGS/Ajisai is many decades providing the service for SLR (Satellite Laser Ranging) from the ground segment.
• In 2011, the Ajisai spacecraft was 25 years on orbit. For this occasion, the project team received the Tsuboi Prize of the Geodetic Society of Japan (Group Prize). The group consists of members from JAXA, the National Institute of Information and Communications Technology (NICT), the Japan Coast Guard (JCG), Geospatial Information Authority of Japan (GSI), and Hitotsubashi University. 4)
The major achievements of Ajisai are:
- Contributions to updating a nautical chart of Japan's neighboring waters and determining locations of islets (JCG's original objectives)
- Communications and positioning experiments and technological developments (NICT, Hitotsubashi University)
- Research on satellite rotation (GSI)
- Acquired technology for accurate orbit determination by laser ranging (JAXA)
- Decided the 36th to 50th gravity field model (world research institutes, such as JPL (Jet Propulsion Laboratory), and DLR (German Aerospace Center)
- Contributed to construct the International Terrestrial Reference Frames (ITRF)
- Determined the absolute value of the velocity field for plate tectonics motion thanks to the long-term motion analysis of 25 years by the Ajisai mission
- Recently verified that the core inside the Earth is vibrating through the analysis combined with the LAGEOS-1/-2 . The result of the verified fact was incorporated into the new earth rotation model (IERS2010).
• The passive spacecraft is in its nominal orbit as of 2008 providing SLR services to ground stations (national and international) equipped with SLR (Satellite Laser Ranging) instrumentation. The expected lifetime of EGS is estimated to be on the order of many decades. Satellite laser ranging to Ajisai is used for precision orbit determination (POD) of various spacecraft; it is also used to improve Earth's gravity field.
In a wider context, EGS is part of a global network of spacecraft (LAGEOS, Etalon, Starlette, EGS, Envisat, CHAMP, GRACE, ALOS, GIOVE, ICESat, ETS-VIII, etc.) that are equipped with corner cube reflectors providing their ranging services to a community called ILRS (International Laser Ranging Service) which administers and coordinates these services and maintains a large database in support of fundamental physics and many other applications. 5)
Figure 2: The image puts the Ajisai spacecraft into a proper perspective regarding size (image credit: JAXA)
1) M. Sasaki, H. Hashimoto, “Launch and Observation of the Experimental Geodetic Satellite of Japan,” IEEE Transactions on Geoscience and Remote Sensing, Volume 25, No. 5, Sept. 1987
5) M. R. Pearlman, J. J. Degnan, J. M. Bosworth, “The International Laser Ranging Service,” Advances in Space Research, Vol. 30, No. 2, pp. 135-143, July 2002, URL: http://ilrs.gsfc.nasa.gov/docs/cospar_paper_warsaw_final.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.