OrbView-2 (renamed from SeaStar in 1997)

OrbView-2 is an imaging satellite, developed, owned, managed and operated by Orbital Imaging Corporation (OrbImage) of Dulles, VA. The overall objective of the OrbView-2/SeaWiFS mission is to provide quantitative data on global ocean bio-optical properties to the Earth science community. The OrbView-2 satellite includes the SeaWiFS (Sea-Viewing Wide Field-of-View Sensor) imaging system, an 8-band multispectral imaging instrument with a one kilometer spatial resolution.

Spacecraft:

The spacecraft design is based on PegaStar, a multi-purpose bus of OSC. The platform combines functions common to both satellites and launch vehicles, including guidance and control, power, communications, and data systems. The S/C is three-axis stabilized (0.5º with 0.08º knowledge) using three orthogonal magnetic torque rods for roll and yaw control and two momentum wheels for pitch stabilization. Attitude is sensed with three 2-axis sun sensors, two scanning horizon sensors, and two 2-axis magnetometers. Redundant GPS receivers are used for orbit determination. The propulsion system consists of two subsystems, a reaction control system and a hydrazine propulsion system. Nominal design life = 5 years. S/C mass = 390 kg. A hydrazine propulsion system using four thrusters is used for orbit raising and orbit maintenance. Four deployed solar panels with zenith-facing cells and two body-mounted side-facing solar panels produce 165 W orbit-average power after 5 years. ^{1) 2) 3) 4) 5)}

Figure 1: Illustration of the OrbView-2 spacecraft

The OrbView-2 satellite operations as well as the commercial data processing and distribution functions are performed by ORBIMAGE. The processing and distribution of data for research applications is handled by the SeaWiFS Project at NASA/GSFC; the SeaWiFS mission is part of NASA's ESE (Earth Science Enterprise) program.

Application: Ocean-color data, ocean biology and ecology, phytoplankton concentrations and growth, pollution, algae blooms, etc. The data may help scientists to understand the role of ocean plant life in the Earth's carbon cycle.

Orbit: Sun-synchronous polar circular orbit, altitude = 705 km, inclination =98.2º, equator crossing time at local noon (12:00 hours on descending node), successive orbit equatorial crossing longitude = -24.721º, period = 98.2 minutes, orbital repeat time = 16 days (233 orbits).

Launch: The OrbView-2 S/C was air-launched on August 1, 1997 by a Pegasus XL rocket (also of OSC) from Vandenberg AFB, CA.

RF communications: On-board storage of sensor data is provided. With two dumps/day, this allows global observations at reduced spatial resolution to be recorded, and a limited amount (about 20 min/day) of high-resolution Local Area Coverage (LAC) data. The real-time LAC data stream is merged with S/C health and instrument telemetry at a rate of 665.4 kbit/s; this is transmitted at L-band with a frequency of 1702.56 MHz.

To increase the coverage of LAC data, NASA encourages the operation of HRPT (High Resolution Picture Transmission) stations by the user community throughout the world. These HRPT stations can collect real time LAC data via direct broadcast whenever the spacecraft is in view. The other telemetry stream consists of stored GAC and selected LAC data, along with S/C health and instrument telemetry, at 2.0 Mbit/s; this is transmitted at S-band with a frequency of 2275.5 MHz. ⁶⁾

Table 1: Requirements for a SeaWiFS LAC downlink ground station HRPT

Figure 2: Artist's rendition of the deployed OrbView-2 spacecraft (image credit: NASA)

Mission status: The OrbView-2 spacecraft and its payload are operational as of 2007 (completing its 10th year on orbit in August 2007).

• OrbView-2 continues to perform exceedingly well on orbit, exceeding its design life by far. The contract with NASA has been extended to provide further coverage to the worldwide data user community.

• In Dec. 2002, NASA extended the original 5-year contract with OrbImage for continued service provision.

Sensor complement:

SeaWiFS (Sea-Viewing Wide Field-of-View Sensor) built by Raytheon SBRC of Goleta, CA. Objective: measurement of reflected visible sunlight from the ocean surface for the derivation of ocean-color data. The SeaWiFS sensor may be regarded as a next generation instrument of CZCS (Nimbus-7 sensor which ceased operations in 1986, after an eight-year mission). The SeaWiFS instrument employs scanning mechanisms to drive an off-axis (afocal) folded-optics scanning telescope and a rotating half-angle mirror that is phase-synchronized with, and rotating at half the speed of, the folded telescope. The scanning telescope design minimizes the polarization sensitivity and, correspondingly, maximizes the radiometric fidelity of its ocean color measurements.

The SeaWiFS scanning concept employs a 360º uni-directional cross-track rotating telescope scanner. A small counter-rotating flat scan mirror between the telescope and the focal planes eliminates image rotation which would otherwise occur as the telescope rotates. The absence of FOV rotation permits the use of a multichannel, time delay integration (TDI) processing in each of the eight spectral bands to achieve the required SNR. Incoming scene radiation is collected by the folded telescope and reflected onto the rotating half-angle mirror. The collected radiation is then relayed through dichroic beam splitters to separate the radiation into four wavelength intervals (each wavelength encompassing two of the eight SeaWiFS spectral bands). The radiation in the four separate wavelength intervals is directed by four corresponding aft-optics assemblies through two separate spectral bandpass filters that further separate the radiation into the eight required SeaWiFS spectral bands. The aft-optics also image each of the resultant bands of radiation onto four silicon detectors that are aligned in the scan direction. The detected signals are then amplified for TDI processing in the electronics module. ^{7) 8)}

Table 2: Parameters of the multispectral imager SeaWiFS

The off-axis telescope (with an aperture of 7.6 cm; f/2) rotates at six revolutions per second (or 360 rpm) in the cross-track direction (for HRPT format compatibility) to provide contiguous scan coverage. Data quantization = 10 bit; spatial resolution = 1.6 mrad (1.13 km at nadir); an active scan angle of ±58.3º is used. A scanner tilt mechanism enables the entire sensor to be oriented in the along-track direction to )20º, 0º, or -20º to avoid specular sun reflection (sun glint from the sea surface).

Table 3: SeaWiFS spectral performance summary

Instrument calibration: SeaWiFS uses short-period solar calibration (for a few orbits) and long-term lunar calibration (for a few months and longer). Solar calibration employs a solar radiation diffuser and an input port located in a fixed position outside of the 58.3º SeaWIFS scene-scan interval. Lunar calibration is accomplished by a S/C maneuver to view the moon during the nighttime portion of an orbit. Of particular importance to the postlaunch validation period are field measurements from MOBY (Marine Optical Buoy, located off the coast of Lanai, Hawaii. ^{9) 10) 11) 12)}

Figure 3: Schematic illustration of the SeaWiFS sensor (image credit: NASA)

To protect the commercial interests of the builder and operator of the mission, ORBIMAGE, SeaWiFS real time LAC data are encrypted. All data users require a license from OSC (a decryptor is furnished with the license). Commercial data users pay a fee for their license, research licences are issued at no cost, but are subject to certain restrictions. Commercial stations requesting HRPT stations for research data use must obtain prior approval from NASA (SeaWiFS Project Office at GSFC).

Table 4: SeaWiFS sensor/transmission characteristics

Figure 4: Illustration of the SeaWiFS instrument (image credit: NASA)

2) "OSC Reviews Seastar Design," Space News, Oct. 28 - Nov. 3, 1991, p. 22

3) P. R. Leygraaf, "OrbView-2 (SeaStar) Flight Operations and Data Delivery," Proceedings of the 11th AIAA/USU Conference on Small Satellites, Sept. 15-18, 1997, Logan, UT

4) http://oceancolor.gsfc.nasa.gov/SeaWiFS/BACKGROUND/

5) http://www.orbimage.com/corp/orbimage_system/ov2/index.html

6) "Roles and Responsibilities of HRPT Stations for SeaWiFS," SeaWiFS Project Office, GSFC, Dec. 19, 1991

7) "System Concept for Wide-Field-Of View Observations of Ocean Phenomena from Space," NASA-NOAA-Eosat publication, 1987

8) H. v.d. Piepen, V. Amman, R. Doerffer, "Remote Sensing of Substances in Water," GeoJournal 24.1, pp. 24-27, 1991 (May) by Kluwer Academic Publishers

9) G. Valenti, "Sea-viewing Wide Field-of-view Sensor," The Earth Observer, March/April 1998, Vol 10, pp. 20-22

10) Note: the reflected radiance is related to the concentration of chlorophyll and other plant pigments present, since chlorophyll is a green pigment and the color of the water changes from blue to green as the concentration of chlorophyll increases. If the concentration of chlorophyll is known, the amount of phytoplankton, or `ocean color,' may be calculated.

11) SeaWiFS Project Home Page at http://seawifs.gsfc.nasa.gov/SEAWIFS.html

12) http://oceancolor.gsfc.nasa.gov/SeaWiFS/SEASTAR/SPACECRAFT.html