Experimental Geodetic Payload
Encyclopedia
The Experimental Geodetic Payload is a Japanese satellite
sponsored by NASDA, and launched in 1986 on the first flight of the H-I rocket. After launch, the satellite was renamed Ajisai (the Japanese name for the Hydrangea
plant), but it is most commonly known by the acronym EGP. Some sources identify the satellite as the Experimental Geodetic Satellite, or EGS.
on August 12, 1986 at 20:45 GMT (August 13, 05:45 local time) . The launch vehicle was the first H-I rocket. After launch, the spacecraft was given the International Designator
1986-061A and Satellite Catalog Number
16908.
and medium earth orbit
. The orbital period
is 116 minutes, and the orbital inclination
is 50 degrees .
. The primary objectives were improving Japan's geodetic triangulation network, determining the position of remote islands, and integrating Japan's geodetic network with geodetic systems used in other parts of the world .
There are two modes of operation. A ground-based laser can be reflected off the satellite, and light's return time measured to determine the round-trip distance . Alternatively, when the satellite is in sunlight, it can be photographed against the background stars .
In both cases, since the satellite's orbit is precisely known, information about the ground-based observer's position can be calculated. EGP's orbital altitude is high enough that atmospheric drag has no significant effect on the spacecraft trajectory . This is advantageous for maintaining the stable orbit required for geodesy.
EGP's flashes are visible in binoculars if the observer is on the nighttime side of the planet, and the satellite is in sunlight while its orbital trajectory takes it above the observer's horizon. These conditions are often met in the hours after sunset, and the hours before sunrise. When EGP enters the Earth's shadow, the stream of flashes abruptly ceases.
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
sponsored by NASDA, and launched in 1986 on the first flight of the H-I rocket. After launch, the satellite was renamed Ajisai (the Japanese name for the Hydrangea
Hydrangea
Hydrangea is a genus of about 70 to 75 species of flowering plants native to southern and eastern Asia and North and South America. By far the greatest species diversity is in eastern Asia, notably China, Japan, and Korea...
plant), but it is most commonly known by the acronym EGP. Some sources identify the satellite as the Experimental Geodetic Satellite, or EGS.
History
EGP was launched from the Tanegashima Space CenterTanegashima Space Center
The is one of Japan's space development facilities. It is located on Tanegashima, an island located 115 km south of Kyūshū. It was established in 1969 when the National Space Development Agency of Japan was formed...
on August 12, 1986 at 20:45 GMT (August 13, 05:45 local time) . The launch vehicle was the first H-I rocket. After launch, the spacecraft was given the International Designator
International Designator
The International Designator, also known as COSPAR designation, and in the United States as NSSDC ID, is an international naming convention for satellites...
1986-061A and Satellite Catalog Number
Satellite Catalog Number
The Satellite Catalog Number is a sequential 5-digit number assigned by USSPACECOM to all Earth orbiting satellites in order of identification. Before USSPACECOM, the catalog was maintained by NORAD...
16908.
Structure
EGP is entirely passive, and operates by reflecting sunlight or ground-based lasers . The satellite is a 685-kg hollow sphere with a diameter of 2.15 meters, and the surface is covered with 318 mirrors for reflecting sunlight and 1436 corner reflectors for reflecting laser beams . The mirrors are 10x10 inches, and the corner reflectors are one inch in diameter and grouped into 120 laser reflection assemblies .Orbit
EGP is in a nearly circular orbit at an altitude of approximately 1488 km, close to the (not firmly defined) boundary between low earth orbitLow Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
and medium earth orbit
Medium Earth Orbit
Medium Earth orbit , sometimes called intermediate circular orbit , is the region of space around the Earth above low Earth orbit and below geostationary orbit ....
. The orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
is 116 minutes, and the orbital inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
is 50 degrees .
Mission
EGP's mission is geodesyGeodesy
Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...
. The primary objectives were improving Japan's geodetic triangulation network, determining the position of remote islands, and integrating Japan's geodetic network with geodetic systems used in other parts of the world .
There are two modes of operation. A ground-based laser can be reflected off the satellite, and light's return time measured to determine the round-trip distance . Alternatively, when the satellite is in sunlight, it can be photographed against the background stars .
In both cases, since the satellite's orbit is precisely known, information about the ground-based observer's position can be calculated. EGP's orbital altitude is high enough that atmospheric drag has no significant effect on the spacecraft trajectory . This is advantageous for maintaining the stable orbit required for geodesy.
Visual Appearance
There is a flash produced when the sun's reflection from one of the satellite's mirrors crosses an observer's position on the earth. Due to the satellite's rotation, and the changing geometry as the satellite moves along its orbit, EGP produces several of these flashes per second. EGP can take up to 18 minutes to cross the sky.EGP's flashes are visible in binoculars if the observer is on the nighttime side of the planet, and the satellite is in sunlight while its orbital trajectory takes it above the observer's horizon. These conditions are often met in the hours after sunset, and the hours before sunrise. When EGP enters the Earth's shadow, the stream of flashes abruptly ceases.
View from Earth
The mirrors are used to reflect laser beams and are invisible to amateur observers, but the mirrors are spectacular. They are designed to reflect sunlight so the satellite can be photographed by ground stations for precise geodetic surveying measurements. The glints are probably in the third magnitude range but are visible to the naked eye only in very dark skies under good conditions. The brief flashes are too short to be noticed by the naked eye. In binoculars EGP resembles the strobe of an airplane but the flash pattern is more complex than a strobe light. Because of the extremely high orbital altitude of 1,500 kilometers, EGP is often visible closer to midnight than other satellites, and can frequently be seen on as many as four orbits during a single overnight observing session.External links
- Heavens Above is a website that displays satellite orbits and calculates satellite visibility. This page at Heavens Above shows the current orbital position of EGP.
- The Use of Videography in Tracking Earth Satellites includes a time exposure photograph showing a series of EGP flashes against the background stars (page 48). The file is large and mostly unrelated to EGP, but this image is a good illustration of the satellite's visual appearance from the ground.