OrbView-1 (formerly Microlab-1)
Background on OrbView satellite series of ORBIMAGE: OrbView® is the name of an imaging satellite series of Orbital Image Corporation (ORBIMAGE) of Dulles, VA, an affiliate of Orbital Sciences Corporation (OSC). The objective of this commercial satellite series is to acquire affordable high-quality imagery of the Earth for a variety of customers that include local governments, telecommunication companies, architects, civil engineers, real estate managers, farmers and environmental monitoring agencies.
ORBIMAGE renamed its satellites in 1997, affecting S/C in development and in operational service. A renaming list is provided to avoid confusion depending on what references are cited of the available literature. ORBIMAGE is employing a uniquely integrated global system of imaging satellites, ground stations and Internet-based sales channels to collect, process and distribute imagery products at affordable cost. Operational services of these spacecraft is provided by SOC (Satellite Operations Center) of ORBIMAGE at Dulles, VA.
Figure 1: ORBIMAGE change of S/C naming
OrbView-1 is an imaging microsatellite, developed, owned, managed and operated by Orbital Imaging Corporation (ORBIMAGE) of Dulles, VA. ORBIMAGE is a commercial provider of Earth imagery acquired from a family of imaging satellites that it owns and operates. The OrbView-1 satellite includes two government sponsored sensor systems: OTD (Optical Transient Detector) a lightning imaging system, and GPS/MET (GPS/Meteorology), an atmospheric measurement system by radio occultation techniques.
OrbView-1 is gravity-gradient stabilized and nadir pointing microsatellite with two solar panels of 96 cm in diameter. The S/C shape and bus structure in orbit resembles a butterfly, a cylinder of 1.04 m diameter and 38 cm in height, whose covers open like the wings of a bird; the gravity gradient boom points toward the Earth's center. The bus is referred to as MicroStar.
Attitude is controlled using three torque rods, controlled by the ACS (Attitude Control Subsystem) processor; attitude is measured by the following sensors: six sun, two Earth, and one magnetometer. In addition, a Trimble TANS Vector GPS receiver with 4 antennas is being used for attitude determination. The nominal pointing accuracy of OrbView-1 pointing knowledge is ±2º (1σ), depending on the modeling accuracy of the Earth's magnetic field. The short-term pointing stability is 0.10º/s. The design of OrbView-1 spacecraft maximizes solar array illumination by a combination of yaw steering and solar array pitch rotation in order to maintain the solar panels fully exposed to the sun at all times except eclipse.
Launch: An air launch of OrbView-1 took place on April 3, 1995 by a Pegasus vehicle of OSC, carried aloft by an L-1011 aircraft flying out of Vandenberg AFB, CA. In addition to Orbview-1 (formerly Microlab-1), OSC launched its commercial communication satellites Orbcomm FM1 and FM2.
Orbit: circular non-sun-synchronous orbit, altitude = 734-747 km, inclination = 70º, period=100 minutes.
RF communications: S-band data transmission for TT&C and science data.
Figure 2: The OrbView-1 S/C model (image credit OrbImage)
Figure 3: Illustration of the deployed OrbView-1 spacecraft (image credit: OrbImage)
Mission status: The OrbView-1 satellite exceeded it's planned design life of 2 years in orbit and successfully completed it's mission in April 2000 (mission of 5 years). OrbView-1 has provided NASA with information in support of its atmospheric research program for five years.
Sensor complement: (OTD, GPS/MET)
The primary payload of the OrbView-1 spacecraft is the OTD (Optical Transient Detector), a lightning instrument package provided by NASA/MSFC. The secondary payload on OrbView-1 is GPS/MET (GPS/Meteorology), an instrument provided by UCAR (University Consortium for Atmospheric Research, with collaboration by JPL, Stanford University, and the University of Arizona) which uses GPS signals to measure Earth atmospheric properties. Science data processing of OTD and GPS/MET is done at MSFC and UCAR, respectively.
OTD (Optical Transient Detector):
OTD is a NASA technology demonstration instrument developed at MSFC (Marshall Space Flight Center) in Huntsville, ALA, PI: H. J. Christian (development of OTD started in 1993). Objectives: observation of the global distribution of lightning, leading to the formation of a climatic database of the spatial and temporal distribution of thunderstorms and lightning. The goal is to better understand thunderstorm activity on a global scale. The OTD instrument is a prototype engineering model for LIS (Lightning Imaging Sensor), which was launched in November 1997 on TRMM (Tropical Rainfall Measuring Mission).
OTD is a staring imager for the detection of lightning over a large region of the Earth's surface with storm scale resolution; it marks the time of occurrence and location of the lightning and measures its radiant energy. Both intracloud and cloud-to-ground discharges can be detected during daytime and nighttime conditions. OTD images a scene much like a television camera; however, daytime detection of highly transient lightning sources against a bright cloud-top background makes actual data handling and processing much more involved than that required by a simple imager. The OTD instrument is actually the LIS engineering model of TRMM. 4) 5) 6) 7) 8) 9)
Figure 4: Illustration of the OTD instrument (image credit: NASA/MSFC)
OTD is composed of six major subsystems: an imaging system, a focal plane assembly (including a CCD array detector of 128 x 128 pixels, preamplifiers, and multiplexers), a Real-Time Event Processor (RTEP) and background remover, an event processor and formatter, power supply, and interface electronics. The imaging system is a simple telescope consisting of a beam expander, an interference filter, and reimaging optics. The filter is narrow band (8.4Å) and is centered about a prominent neutral oxygen emission triplet in the lightning spectrum to optimize SNR in the presence of a bright solar-lit cloud top.
OTD parameters: view direction: nadir; FOV = 100º x 100º; spatial resolution = 10 km; temporal resolution = 2 ms; wavelength = 777.4 nm; sensor mass = 2 kg; power = 3 W. The two modules with electronics units have a mass of about 18 kg. The combination of the wide field-of-view lens and the altitude of the orbit allows OTD to observe a large area of the Earth simultaneously, equivalent to 1300 km x 1300 km. “Flashes” are determined by comparing the luminance of adjoining frames of OTD optical data. If the difference is more than a specified threshold value, an “event” is recorded.
Figure 5: The OTD sensor (and zoom of lens system) on the Orbview-1 microsatellite (image credit: NASA)
GPS/MET (Global Positioning System / Meteorology) instrument:
GPS/MET is a proof-of-concept demonstration instrument of UCAR/JPL (PIs: Randalph Ware, Michael Exner, and Christian Rocken). The GPS/MET program is sponsored by NSF (National Science Foundation), FAA (Federal Aviation Agency), NOAA, and NASA. Other participants are the GPS/MET program are: UNAVCO (University NAVSTAR Consortium), NCAR (National Center for Atmospheric Research), both based in Boulder, CO, and the University of Arizona. 10) 11) 12) 13) 14) 15) 16) 17) 18) 19)
Objective: demonstration of limb sounding of the atmosphere by using the radio occultation measurement technique provided by the signals of the GPS constellation. GPS/MET is a modified commercial high-precision GPS receiver (TurboRogue heritage of Allen Osborne Associates Inc. of Westlake Village, CA) with the capability of simultaneously receiving GPS signals on L1 and L2 frequencies by tracking of up to eight GPS satellites. The receiver can track both C/A and P codes and also implements a codeless carrier recovery technique. The accurate measurement in the change of carrier phase permits to determine the atmospheric refractive index as a function of altitude. Pressure and temperature profiles can then be derived using the gas law. All observables are sampled 50 times per second. A single low-gain antenna (a standard `patch' on a ground plane) points roughly in the anti-velocity direction.
High resolution atmospheric soundings can be retrieved when the radio path between the LEO GPS receiver and one GPS satellite traverses the Earth's atmosphere. When the path of the GPS signal begins to transect the mesopause at about 85 km altitude, it is sufficiently retarded so that a detectable delay in the order of 1 mm in the dual frequency carrier phase observations is obtained by the LEO GPS receiver. As the signal path descends through successively denser layers of the atmosphere, the delay increases to approximately 1 km at the Earth's surface. Thus the atmosphere creates a unique signal with over six orders of magnitude in dynamic range. A single LEO GPS receiver can observe about 200 such occultations per day. The key observables are atmospheric temperature and moisture distributions. GPS/MET measurement resolutions:
• Precision of phase measurements: order of 1 mm
• Velocity accuracy: 10-4 m/s
• Position accuracy: 1 m (vertical: <1 km; horizontal: <200 km)
GPS/MET temperature retrieval accuracies of better than 1K have been demonstrated in the altitude range of 10-30 km during the proof-of-concept phase of the mission with vertical resolution of better than 1 km.
Figure 6: Schematic occultation monitoring configuration for a S/C in LEO
Radio occultation principle:
Fundamentally, the technique relies on the simple fact that a planet's atmosphere acts much like a spherical lens, bending and slowing propagation of microwave signals passing through it tangent to the surface. The lens effect results from decreasing atmospheric density with altitude. If the positions of transmitting and receiving satellites are precisely known, then the atmospheric delay can be measured precisely, the time derivative of which (Doppler) can be inverted to give atmospheric density versus altitude. The limb sounding technique is a horizon-looking (or edge-looking i.e. outer edge of the apparent disk of a celestial body) observation technique that uses a distant object [(for occultation sounding) sun, star, or a sensor on another satellite in a different Earth orbit as a source to observe the signal on its path through the atmosphere that is essentially tangential to the Earth's surface. Refractive occultations: They occur because density gradients in the atmosphere lead to refraction of the incoming radiation, causing it to follow curved paths through the atmosphere. Relative measurements of the degree to which the path of the incoming radiation is changed provide the bulk of atmospheric properties (density, pressure, temperature).
Accomplishments of OrbView-1 observations:
Two pioneering instruments were developed and flown leading toward significant improvements in operational weather forecasting. The radio occultation technique was first proposed/developed at the Stanford University Center for Radar Astronomy (SUCRA) for studies of planetary atmospheres in 1962.
• OTD provided the world's first spaceborne broad-area intracloud and cloud-to-ground lightning imagery. The instrument was capable of detecting, locating and measuring the intensity of lightning events in the daytime as well as during the nighttime. The lightning imagery of OTD provided nearly global coverage every two days with a swath width of 1,300 km. OTD collected a five-year record of lightning observations between April 1995 and April 2000.
• The GPS/MET instrument of UCAR/JPL introduced successfully a new era of atmospheric sounding (profiling) technology by using radio occultation measurements of the signals of the GPS constellation. UCAR/JPL are in particular credited for their open data policy that enabled international experimentation and cooperation, fostering in turn new projects. The GPS/MET instrument provided observations during the years 1995 - 1997.
1) Information provided by Mark Pastrone of ORBIMAGE
2) Information provided by G. Moody and D. Finn of OSC, and by W. J. Koshak of NASA/MSFC
3) S. Kennison, “Orbview-1 Autonomous Mission Operations,” Proceedings of the 11th AIAA/USU Conference on Small Satellites, Sept. 1997. Logan, UT, USA
5) H. J. Christian, “Optical Detection of Lightning from Space,” Proceedings of the 11 th International Conference on Atmospheric Electricity, Guntersville, Alabama, June 7-11, 1999, pp. 715-718
6) R. J. Blakeslee, K. T. Driscoll, D. E. Buechler, D. J. Boccippio, W. J. Boeck, H. J. Christian, S. J. Goodman, J. M. Hall, W. J. Koshak, D. A. Mach, M. F. Stewart, “Diurnal Lightning Distribution as Observed by the Optical Transient Detector (OTD),” Proceedings of the 11th International Conference on Atmospheric Electricity, Guntersville, Alabama, June 7-11, 1999, pp. 742-745.
8) H. J. Christian, K. T. Driscoll, S. J. Goodman, R. J. Blakeslee, D. A. Mach, D. E. Buechler; “The Optical Transient Detector (OTD),” Proceedings of the 10th International Conference on Atmospheric Electricity; Osaka, Japan; June 10-14, 1996; pp 368-371
9) D. J. Boccippio, W. Koshak, R. Blakeslee, K. Driscoll, D. Mach, D. Buechler, W. Boeck, H. J. Christian, S. J. Goodman, “The Optical Transient Detector (OTD): Instrument Characteristics and Cross-Sensor Validation,” Journal of Atmospheric and Oceanic Technology, Vol. 17, Issue 4, April 2000, pp. 441-458, URL: http://journals.ametsoc.org/doi/pdf/10.1175/1520-0426%282000%29017%3C0441%3ATOTDOI%3E2.0.CO%3B2
10) W. G. Melbourne, E. S. Davis, C. B. Duncan, G. A. Hajj, et al., “The application of spaceborne GPS to atmospheric limb sounding and global change monitoring.” JPL Publication 94-18, April 1994, 147 pp.
11) “GPS/MET, Preliminary Report,” July 1995, URL: http://www.cosmic.ucar.edu/gpsmet/over/septsumm_top.html
12) E. R. Kursinski, et al., “Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System,” Journal of Geophysical Research, Vol. 102, No. D19, Oct. 20, 1997,pp. 23,429-23,465
13) S. S. Leroy, “The Measurement of Geopotential Heights by GPS Radio Occultation. Journal of Geophysical
14) R. Ware, M. Exner, D. Feng, M Gorbunov, K. Hardy, B. Herman, Y. Kuo, T. Meehan, W. Melborne, C. Rocken, W. Schreiner, S. Sokolovskiy, F. Solheim, X. Zou, R. Anthes, S. Businger, K. Trenberth “GPS sounding of the atmosphere from low earth orbit: preliminary results,” Bulletin of the American Meteorological Society (BAMS), Vol. 77, No. 1, 1996,. pp. 19-40, URL: http://journals.ametsoc.org/doi/pdf/10.1175/1520-0477%281996%29077%3C0019%3AGSOTAF%3E2.0.CO%3B2
16) G. Hajj, E. R. Kursinski, W. Bertiger, S. Leroy, L. Romans, J. T. Schofield, “Sensing the atmosphere from a low-earth orbiter tracking GPS: early results and lessons from the GPS/MET experiment,” Proceedings of the ION GPS-95, Institute of Navigation, Palm Springs, CA, USA., Sept. 12-15, 1995, pp. 1167-1174.
17) C. Rocken, R. Anthes, M. Exner, D. Hunt, S. Sokolovskiy, et al., “Analysis and validation of GPS/MET data in the neutral atmosphere,” Journal of Geophysical Research, Vol. 102(D25), 1997, pp. 29,849-29,866
18) Y.-H. Kuo, X. Zou, S. J. Chen, W. Huang, et al., “A GPS/MET Sounding through an Intense Upper-Level Front,” Bulletin of the American Meteorological Society (BAMS), Vol. 79, No. 4, April 1998, pp. 617-626
19) Derek D. Feng, Benjamin M. Herman, “Remotely Sensing the Earth’s Atmosphere Using the Global Positioning
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.