OrbView-4 is an imaging minisatellite owned and operated by OrbImage of Dulles, VA. The S/C includes two imaging instruments, OHRIS and OHIS (OrbView Hyperspectral Imaging System). The satellite was designed and built by OSC (Orbital Sciences Corporation). 1)
Figure 1: Line drawing of the OrbView-4 spacecraft (image credit: OSC)
OrbView-4 uses the LEOStar spacecraft bus of OSC. The spacecraft structure (body) consists of a six-sided cylinder of 102 cm in diameter and 152 cm in length (first use of the LeoStar-2 bus).
Orbview-4 is three-axis stabilized; attitude is sensed by star trackers, sun sensors, a magnetometer, and a gyro; actuation is provided by four reaction wheels (3 are always active, one is a cold spare) and thrusters. The attitude system provides a real-time pointing accuracy of better than 100 arcseconds. The post-processed pointing accuracy allows pixels to be located to within 12 m for certain product types.
ADCS (Attitude Determination and Control Subsystem): ADCS uses data from the two star trackers and the IRU to estimate the inertial-to-body attitude quaternion and body rates for input to the attitude control system. Data from both star trackers is used in normal operation, but the system is also configured for single-tracker operation in the event of a data loss from either of them. The attitude determination logic is implemented as a conventional Kalman filter updated with star tracker data at 1 Hz. Between filter updates the attitude quaternion is propagated with gyro data at 10 Hz.
The four logic components are identified in Figure 3, a functional diagram of the attitude determination system, along with their software execution rates. In addition to using data from the star trackers and IRU, the logic also uses data from the GOODS (GPS Onboard Orbit Determination Software) to correct the tracker data for velocity aberration.
Figure 2: ADCS functional block diagram (image credit: OSC)
Figure 3: Functional diagram of the ADCS (image credit: OSC)
A three-axis magnetometer and three magnetic torquer bars are used for momentum control. An agile body-pointing technique, using reaction wheels, is employed for instrument pointing, i.e., the entire S/C is pointed into the desired direction, permitting a field of regard of ±50º into any direction. A GPS receiver provides orbit determination and all on-board timing services.
Electrical power is provided by six solar panels, mounted in a star configuration on the cylinder base. In addition, NiH batteries serve as a power source for eclipse operations.
Figure 4: Illustration of the OrbView-4 satellite (image credit: OrbImage)
Launch: The OrbView-4 satellite was launched on a Taurus vehicle on September 21 2001 (along with QuikTOMS of NASA as a secondary payload) from the VAFB launch site. Unfortunately, a launch failure was experienced which destroyed both spacecraft, representing a great loss to all involved. 6)
RF communications: An on-board recorder of 32 Gbit capacity provides storage for non-contact periods. RF communications of the imagery is provided in X-band. The downlink data rate is 150 Mbit/s. Mission operations are provided by OrbImage via its Dulles-based operations center and two uplink/downlink ground stations located in Point Barrow Alaska and Dulles, Virginia. Data distribution is provided by OrbImage and its worldwide Regional Distributors.
Orbit: Sun-synchronous circular orbit, altitude = 470 km, inclination = 97.3º, equator crossing on descending node 10:30 AM. The large off-nadir slewing capability of Orbview-4 offers a worst-case revisit time of three days.
OHIS (OrbView Hyperspectral Imaging System). The objective is to demonstrate the utility of hyperspectral technology for the support of various applications. The instrument is a body-mounted optomechanical instrument (whiskbroom scanning imager), built by Northrop Grumman. The spectral range of the instrument is 0.4 - 2.5 μm. Up to 200 contiguous spectral bands can be defined within the spectral region. A typical scene size is 5 km (swath width) x 20 km (length) with a spatial resolution of 8 m (GSD) at nadir. The data quantization is at 12 bit. No data compression is employed.
Table 1: Characteristics of the OHIS instrument
Note: OHIS had originally an additional MWIR band in the wavelength range 3.0 - 5.0 μm, with HgCdTe detectors (80 x 640), a band spacing of 25 nm, an operating temperature of 90 K, and 80 bands. This option was cancelled in 2000.
OHRIS (OrbView High Resolution Imaging System) of OrbImage designed and built by Northrop Grumman. The instrument is identical to OHRIS flown on OrbView-3, the description is provided under OrbView-3.
Both instruments can be operated in parallel.
1) Information provided by Mark Pastrone of OrbImage
2) “LEOStar™-2 Spacecraft Bus,” Orbital, URL: http://www.orbital.com/newsinfo/publications/leostar_fact.pdf
3) W. Ferster, “Orbimage Restructuring Delays OrbView-4 Launch,” Space News, Aug. 27, p. 3 and p. 27
5) Dewey Adams, Dominick Bruno, Piyush Shah, Brian S. Keller, “Precision Control, Knowledge and Orbit Determination on a Small Spacecraft Bus - The OrbView-4 Attitude Control System,” Proceedings of the 12th AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 31-Sept. 3, 1998, paper: SSC98-IX-3, URL: http://microsat.sm.bmstu.ru/e-library/Algorithms/CommonDesign/sscix3.pdf
6) J. Ray, “Taurus rocket fails to achieve Earth orbit OrbView-4, QuikTOMS satellites lost,” Spaceflight Now, Sept. 21, 2001, URL: http://www.spaceflightnow.com/taurus/t6/010921launch.html
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.