======================================================================== Visual Satellite Observing F A Q Chapter-13 Glossary of Terms ======================================================================== Launch Sites Note: When known, latitude and longitude of launch sites are included in this glossary. Latitude is given as decimal degrees North (N) or South (S). Longitude is given as decimal degrees East (E) or West (W). The values are in parentheses following the launch site name, separated by a slash (/). Additional information about launch sites can be found in Chapter 9 of this FAQ and at the URL: http://solar.rtd.edu/~mwade/sites/lauindex.htm ======================================================================== 2-line See Two-Line Elements Altitude There are two common meanings: (1) The angle above or below the observer's local horizon. (2) Distance of an orbiting body above the earth. Argument of Perigee The angle measured from the ascending node northward until the perigee is reached. Letter designator is "w" or "omega". Ariane European Space Agency (ESA) launch vehicle. The Ariane 4 is the primary rocket, with the Ariane 5 in initial testing as of late 1997. Ascending Node The point where a satellite's orbit crosses the equator, north-bound. May be expressed as longitude or right-ascension. Ascending Node Precession The change in a satellite's Right Ascension of the Ascending Node (RAAN) caused by the Earth's equatorial bulge. Astrodynamics The study of the orbital motion of artificial satellites. Atlas A United States launch vehicle. Azimuth Angular position in degrees measured eastward from due north. North is 0 or 360 degrees, east is 90 degrees, south is 180 degrees, west is 270 degrees. Baykonur Cosmodrome (45.6N/63.4E) A Russian launch complex in Kazakhstan. Also known as Tyuratum. The space age started here with the launch of Sputnik in 1957. Celestial Mechanics The study of the orbital motion of the planets, comets, moons and other natural objects. Centaur An upper stage used to move payloads from low earth orbit to geosynchronous and similar orbits. COBE COsmic Background Explorer. Launched by the United States in 1989, it measures the cosmic microwave background. Combined Radio Frequency/Optical Surveillance System (CROSS) Comsat Communications satellite. Conestoga A solid fueled booster made by EER Systems Corporation. Cosmos A generic name (and sequential number) given to many Russian satellites. Also a rocket used to place military satellites in medium earth orbits. COSPAR The COmittee on SPAce Research was established by the International Council of Scientific Unions (ICSU) in October 1958 to continue the cooperative programs of rocket and satellite research undertaken during the International Geophysical Year of 1957-1958. It is an interdisciplinary scientific organization concerned with international progress in all areas of scientific research carried out with space vehicles, rockets, and balloons. Additional information on COSPAR is at URL: http://cospar.itodys.jussieu.fr Declination A celestial coordinate that measures the angle north or south of the celestial equator. Zero degrees is on the celestial equator, and 90 degrees North is the celestial north pole. Delta United States launch vehicle used for commercial and military satellites. DMSP Defense Meteorological Support Program - a series of U.S. military LEO weather satellites DSCS Defense Satellite Communications System ELINT Electronic intelligence. Eccentricity Describes the shape of a conic section; abbreviated by the letter "e". Circles have eccentricity of zero. Ellipses have eccentricity greater than zero and less than one. Hyperbolas have eccentricity greater than one. EGP A laser/visual geodetic satellite launched in 1986. Elevation There are two common meanings: (1) Observer's distance above mean sea level. (2) Angular distance above the observer's local horizon; 0 degrees is the horizon and 90 degrees is directly overhead (zenith). Elset Orbital element set. Data used to describe a satellite's orbit. A common format used by satellite prediction software is known as two-line elsets or TLEs. Epoch Time associated with an element set. Usually expressed as YYDDD.DDDDDDDD where YY is the year, DDD.DDDDDDDD is the day of the year and fractional part of that day. The day and time are referenced to Universal Time. European Space Agency (ESA) The governing organization for European space activities. Flash Period The period between successive maxima or minima of brightness of a rotating satellite as seen by an observer. Geosynchronous Earth Orbit (GEO) An orbit with a period of one sidereal day. This equates to an altitude of 19,323 nautical miles. Geostationary Orbit (GSO) A subset of GEO orbits, in which the inclination is nearly zero so that the satellite stays over the same spot on the Earth. Global Positioning System (GPS) Global navigation system fielded by the United States. It consists of 21 satellites in mid-earth orbit (MEO). GLONASS Global navigation system fielded by Russia, which is similar to GPS. Ground-based Electro-Optical Deep Space Surveillance (GEODDS) Optical satellite tracking systems located in Cicero, New Mexico, United States; Maui, Hawaii, United States; Choe Jong San, South Korea; Diego Garcia, Indian Ocean. More information about GEODDS is at the URL: http://www.satobs.org/seesat/Jan-1996/0057.html H2 A Japanese two-stage rocket operated by National Space Development Agency (NASDA). High-Earth Orbit (HEO) Any orbit with altitude greater than geosynchronous. IMINT Imaging intelligence Inclination The angle of an orbital plane relative to the Earth's equator. For "i" equal zero, the orbit stays over the Earth's equator. For "i" equal 90 degrees, the orbit goes over the north and south poles. Inertial Upper Stage (IUS) A two-stage, solid fuel upper rocket stage used to propel satellites from low-earth orbit (LEO) to geosynchronous and interplanetary orbits. Institute of Space and Astronautical Science (ISAS) The Japanese organization responsible for space-related activities. Integrated Apogee Boost System (IABS) A single stage, liquid-fueled vehicle used to circularize DSCS satellites at geosynchronous altitude. International Designation System used to catalogue orbiting objects that originated on Earth. Format is YYYY-NNNPPP, where YYYY denotes year of launch, NNN denotes serial number of launch, and PPP is an alphabetic string denoting a specific object resulting from a launch. Often abbreviated as YYNNNPPP. Letters "I" and "O" are not used. For example, letter "A" denotes the first piece. Normally letters are assigned so that payloads are listed first, followed by rocket bodies, and finally debris. If the number of pieces exceeds 24, then two letters are used, e.g. AA denotes the 25th piece from a launch. IOD format A commonly used format to report positional observations, named after the software for which it was originally developed: Interactive Orbit Determination. See http://www.satobs.org/position/IODformat.html ISS International Space Station J1 A three-stage Japanese rocket operated by the National Space Development Agency (NASDA). Jiuquan (40.7N/100.0E) A Chinese launch complex in the Gobi desert. It supports the CZ-2 rockets. Kagoshima Space Center (31.2N/131.1E) A Japanese launch facility operated by Institute of Space and Astronautical Science (ISAS). Kapustin Yar (48.4N/56.5E) A Russian launch complex. It is no longer used. The last launch from that site was in 1987. KeyHole satellites Aka KH-11. U.S. optical imaging intelligence satellites. Kennedy Space Center (KSC) (28.5N/80.5W) United States launch complex in Florida. It is used to launch the Space Shuttle and most of the expendable launch vehicles. Orbits from KSC are typically less than about 59 degrees inclination. KSC is part of Cape Canaveral. Kourou Launch Site (5.2N/52.8W) Located in French Guyana, this European Space Agency site launches the Ariane rockets. Lacrosse satellites A series of U.S. imaging intelligence satellites that employ synthetic aperture radar, enabling them to produce images in darkness and through cloud cover. See http://www.satobs.org/spysat.html Latitude Angular distance of a point on the Earth measured north or south of the equator. Latitude is expressed as 0 to 90 degrees North or South. Lockheed Martin Launch Vehicle (LMLV) A new launcher consisting of a Castor 120 first stage, an Orbus 21D second stage, and an OAMS hydrazine third stage. Longitude Angular distance of a point on the Earth measured eastward. The zero point runs along a line from the North Pole through Greenwich, England to the South Pole. It is expressed as degrees from 0 to 360. Another, more common, convention is to express longitude as 0 to 180 degrees East or West. Long March China's launch vehicle. Models are designated CZ-1 to CZ-4. Low-Earth Orbit (LEO) Roughly speaking, an orbit below about 2000 km altitude. Low Inclination Orbit An orbit with an inclination less than 45 degrees. M5 A three-stage, solid fueled, rocket operated by Japan's Institute of Space and Astronautical Science (ISAS). Magnitude An indication of relative brightness of celestial objects. Smaller numbers are brighter. Faintest naked eye magnitude is about +6; the moon is -12. Maui Optical Tracking and Identification System (MOTIF) Mean Motion The number of revolutions made by a satellite per unit of time, commonly expressed as revolutions per day. Mid-Earth Orbit (MEO) Also known as semi-synchronous orbit. Examples include GPS satellites at altitudes of 10,900 nautical miles. Mir The Russian space station, which was launched in 1986, and de-orbited in 2001. Molniya A Russian launch vehicle as well as a class of satellites in highly eccentric orbits. Molniya Orbit A highly eccentric orbit with an inclination near 63.4 degrees. The perigee stays in the same place at this inclination. (See Perigee Precession.) National Aeronautics and Space Administration (NASA) United States space agency, formed in October 1958. National Space Development Agency (NASDA) Japan's lead organization for commercial launch activities. Nodal Regression A change in a satellite's Right Ascension of the Ascending Node (RAAN) caused by gravitational variations due to the Earth's oblateness. NOSS Naval Ocean Surveillance System. Formations of two or three satellites used to locate and track ships at sea by detecting their radio transmissions and analyzing them using the TDOA (time-difference-of-arrival) technique. See: http://www.satobs.org/noss.html Orbital Information Group (OIG) NASA Goddard Space Flight Center's Orbital Information Group (OIG) was the primary distributor of U.S. Department of Defense (DoD) 2-line orbital elements and related data, until 2005 Jan 03, when it was replaced by Space-Track.org. OIG permanently ceased operations on 2005 Mar 31. Orbital Period Duration of one revolution of an orbiting object about its parent object. Commonly expressed in minutes in the case of Earth satellites. Palmachim Launch Site (31.0N/35.0E) Israeli site for Shavit satellite launches that have to go due west over the Mediterranean Sea, which places the satellite in a highly retrograde orbit. Payload Assist Module (PAM) A solid fueled rocket used to boost satellites into higher orbits. It is used on the Space Shuttle, Delta, and Atlas vehicles. Pegasus An unmanned United States launch vehicle that is launched from an aircraft. Perigee An orbit's closest point to the Earth. Perigee Precession The shifting of a satellite's perigee point around the path of the orbit, due to Earth's irregular gravitational force. This occurs for all satellites except those that are in a Molniya orbit. Perturbation Any force that causes a satellite to deviate from its Keplerian orbital path. Phase Angle The sun-satellite-observer angle (with the satellite at the vertex). When the phase angle is close to zero, the satellite is fully illuminated. Another convention is to define phase angle as the satellite-observer-sun angle (with the observer at the vertex). Using this second convention, when the phase angle close to 180 degrees, the satellite is fully illuminated. Plesetsk (62.8N/40.3E) A Russian launch complex. Polar Orbit An orbit with an inclination of 90 degrees. (Loosely speaking, an orbit with an inclination near 90 degrees.) Polar Satellite Launch Vehicle (PSLV) India's only operational launch vehicle. Positional Observation An observation of a satellite's position in the sky. Consists of at least the date. time, right-ascension and declination. May include visual magnitude and optical behaviour. Photometric Period of Artificial Satellites (PPAS) A computerized database of satellite flash behaviour. Prograde Orbit An orbit with an inclination less than 90 degrees. Proton A Russian space launch vehicle. Different models use three or four stages. Retrograde Orbit An orbit with an inclination greater than 90 degrees. Right Ascension The celestial coordinate used to measure the angle of longitude on the on the celestial sphere. Zero degrees right ascension is the position of the Sun during the vernal equinox. Normally expressed in units of hours, minutes and seconds, on the basis of 15 deg per hour. Right Ascension of the Ascending Node (RAAN) The angle, measured along the equator, from the Vernal Equinox to the ascending node of the satellite's orbit. Satellite Catalog Number A unique identifier assigned by the Unites States Strategic Command (USSTRATCOM) to every artificial object placed into orbit around the Earth or Sun, or which attains escape velocity from the solar system. Many acronyms are used because of their brevity and past history of use. These acronyms include NORAD (North American Air Defense) and NSSC (NORAD Space Surveillance Center). The catalog number identifier started with 00001 for Sputnik 1 Rocket, 00002 for Sputnik 1, and by late 2005 is greater than 28000. SDP4 "Deep space" version of Satellite General Perturbations #4, an orbit propagation model compatible with USSTRATCOM's 2-line (aka TLE) orbital elements. Applies to deep space orbits, which SDP4 defines as having an orbital period greater than 225 minutes. SDP4 is a standard model in use today. Semi-Major Axis One half the longest linear dimension of an ellipse. SGP Satellite General Perturbations - an orbit propagation model compatible with USSTRATCOM's 2-line (aka TLE) orbital elements. Replaced by SGP4. SGP4 Satellite General Perturbations #4 - an orbit propagation model compatible with USSTRATCOM's 2-line (aka TLE) orbital elements. Applies to low earth orbits, which SGP4 defines as having an orbital period less than or equal to 225 minutes. SGP4 is the standard model in use today. Shavit Israel's three-stage rocket. SIGINT Signals intelligence Soyuz A Russian manned launch vehicle. It is also used for remote sensing and science missions. Space Surveillance Network (SSN) A network of tracking and detection stations to keep track of objects orbiting the Earth. It is operated by United States Strategic Command (USSTRATCOM). Space Shuttle United States manned, reusable spacecraft. Space Track Space-Track.org is the primary distributor of U.S. Department of Defense (DoD) 2-line orbital elements and related data, replacing NASA Goddard Space Flight Center's Orbital Information Group (OIG), which permanently ceased operations on 2005 Mar 31. Located at http://www.space-track.org SPACEWARN Bulletin Summary of space launches and related satellite information. It is published by the National Aeronautics and Space Administration (NASA) National Space Science/World Data Center. Spin Stabilized A method of maintaining attitude control by spinning all or a portion of a satellite, similar in principle to that of a spinning gyroscope. Sriharikota Launching Range (SHAR) India's launch site. State Vector Position and velocity of an orbiting body at a specific point in time. Sun-Synchronous Orbit A special orbit used by remote sensing satellites such that the orbit regresses at a rate that allows the satellite to cross a point at the same sun angle each day. Svobodniy (51.5N/138.5E) Russian launch center formerly used for the SS-19 ICBM, now planned for replacing Baykonur in Kazakstan. Synodic Effect The change in a satellite's flash pattern due to changing illumination as it moves relative to the observer. Tanegashima Space Center (30.2N/131.0E) A Japanese launch site operated by National Space Development Agency (NASDA). Currently, Japan's H2 and J1 rockets are launched from this site. Taiyuan (37.5N/112.6E) A Chinese launch site supporting the CZ-3 and CZ-4 rockets. Sun-synchronous satellites are launched from this site. Taurus A hybrid launch vehicle consisting of the Peacekeeper first stage and a wingless version of the Pegasus. TEARR Acronym for Time, Elevation, Azimuth, Range and Range Rate data. This position data collected on satellites is used to determine their orbits. Titan United States launch vehicle. Retired from service in 2005. TLE See Two-Line Elements TOPEX Launched in 1992, it performs radar altimetry of the world's oceans. Transfer Orbit An intermediate orbit between a parking orbit and the final operational orbit. Tsiklon A Russian launch vehicle. Two and three stage versions launch radar and electronic intelligence satellites. Two-Line Elements A standard USSTRATCOM data format that defines a satellites orbit in terms of classical Keplerian parameters. Commonly abbreviated as "2-line" or TLE. USSPACECOM United States Space Command. USSTRATCOM United States Strategic Command Univeral Time (UT) The standard time reference system corresponding to the time in Greenwich, England. It is also referred to as Greenwich Mean Time (GMT) and Zulu time. Vandenberg Air Force Base (VAFB) (34.7N/120.4W) United States launch site for polar launches. Satellite inclinations from VAFB are in the 56 to 104 degree range. Wallops Island (37.9N/75.3W) A launch site in Virginia, United States. It is primarily used for sounding rockets. Offshore, the facility is used for Pegasus airborne launches. Xichang (28.2N/102.0E) A Chinese launch site supporting CZ-3 and CZ-2E rockets. Zenit A Russian two or three stage launch vehicle. Zenith The point directly overhead the observer. ======================================================================== This FAQ was written by members of the SeeSat-L mailing list, which is devoted to visual satellite observation. Members of this group also maintain a World Wide Web site. The home page can be found at the URL: http://www.satobs.org/ The information on the VSOHP web site is much more dynamic than that found in this FAQ. For example, the VSOHP site contains current satellite visibility and decay predictions, as well as information about current and upcoming Space Shuttle missions and Mir dockings. The VSOHP site also contains many images, equations, and data/program files that could not be included in this FAQ while maintaining its plain text form. This FAQ and the VSOHP web site are maintained asynchronously, but an effort is made to synchronize information contents as much as possible. The material in this FAQ chapter was last updated in January 2006. ========================================================================