WorldView-2 (WV2) is a commercial imaging satellite of DigitalGlobe Inc. of Longmont, CO, USA (follow-on spacecraft to WorldView-1). The overall objective is to meet the growing commercial demand for high-resolution satellite imagery (0.46 cm Pan, 1.8 m MS at nadir - representing one of the highest available spaceborne resolutions on the market).
In the fall of 2003, DigitalGlobe had received a contract from NGA (National Geospatial-Intelligence Agency) of Washington DC to provide high-resolution imagery from the next-generation commercial imaging satellites. The contract award was made within NGA's NextView program. The NGA requirements called for imagery with a spatial resolution of 0.5 m panchromatic and 2 m MS (Multispectral) data.1)
Figure 1: Illustration of the WorldView-2 spacecraft (image credit: DigitalGlobe)
Like its Worldview-1 predecessor, the WorldView-2 spacecraft is being manufactured at BATC (Ball Aerospace and Technologies Corporation) of Boulder, CO which was awarded a contract in late 2006. BATC is providing its BCP 5000 (Ball Commercial Platform 5000) spacecraft bus for WorldView-2 and will integrate the remote sensing instrument onto the bus ( with WorldView-2, a larger imaging payload is being mounted onto the same spacecraft bus as that used for WorldView-1). A new vibration isolation system is being used on WorldView-2 for the payload to control jitter induced by the spacecraft. The BCP-5000 bus provides state-of-the-art power, stability, agility, data storage and data transmission (over the BCP-2000 bus). 2) 3) 4)
The spacecraft is 3-axis stabilized. The ADCS (Attitude Determination and Control Subsystem) employs star trackers, SIRUTM (Space Inertial Reference Unit- scalable) of Northrop Grumman, and GPS for attitude sensing, and CMGs as actuators for highly responsive pointing control. A spacecraft body-pointing range of ±40º about nadir is provided corresponding to a FOR (Field of Regard) of 1355 km in cross-track. An instantaneous geolocation accuracy of ≤ 500 m is provided at any start and stop of an imaging sequence. With its improved agility, WorldView-2 acts like a paintbrush, sweeping back and forth to collect very large areas of multispectral imagery in a single pass. WorldView-2 alone has a collection capacity of 975,000 km2/day. The combination of WorldView-2’s increased agility and high altitude (770 km) enables it to typically revisit any place on Earth in 1.1 days.
Figure 2: View of the WV110 instrument (left) and the S/C bus BCP-5000 (right), image credit: DigitalGlobe
The QuAD (Quiet Array Drive) motion control technology of Starsys Inc. is being used to articulate the solar arrays. The low disturbance implementation permits imaging observations to be conducted in parallel to the array articulation task.
A single-board BAE Systems RAD750 radiation-hardened computer manages the data processing command and control functions for WorldView-2.
Table 1: Overview of some spacecraft parameters
Figure 3: Common spacecraft bus of WorldView-1 and -2 (image credit: DigitalGlobe)
The advanced CMGs (Control Moment Gyroscopes) provided by Ball Aerospace for WorldView-2, as well as for DigitalGlobe's WorldView-1, afford the satellites the flexibility to capture more imagery than ever before. This high-performance technology provides acceleration up to 10 times that of other attitude control actuators and improves both maneuvering and targeting capability, while reducing slew time from over 60 seconds to only 9 seconds to cover 300 km. This means WorldView-2 will be able to rapidly swing precisely from one target to another, allowing extensive imaging of many targets, as well as stereo, in a single orbital pass.
BATC used the M-95 CMG configuration for the WorldView-1 and WorldView-2 spacecraft providing a torque of up to 6.1 Nm. The M-95 CMG configuration has a total mass of 261.6 kg (including isolation mounts and electronics) and a power consumption of 220 W. 5)
Figure 4: Photo of the BATC M-95 CMG four-wheel pyramid configuration (image credit: BATC)
Figure 5: Photo of the WorldView-2 spacecraft with the WV110 instrument on top in the clean room of BATC (image credit: DigitalGlobe)
CTIF (Command Interface Formatter Module): CTIF was developed at SwRI (Southwest Research Institute), San Antonio, TX. On WorldView-2, a redundant pair of CTIF modules provides complete uplink and downlink telemetry processing. The CTIF modules represent a continuation of SwRI's long track record of providing highly reliable spaceflight electronics supporting CCSDS's (Consultative Committee for Space Data Systems) command and telemetry protocols. The CTIF module is unique in that it provides significant hardware capabilities to offload traditional command and telemetry processing from the main spacecraft computer and to provide those core capabilities even if the main computer should go offline. 6)
RF communications: The command data are in S-band at 2 or 64 kbit/s. The housekeeping telemetry and tracking is being done in X-band at 4, 16, or 32 kbit/s of real-time data, or 524 kbit/s of stored data. The imagery is downlinked in X-band at 800 Mbit/s (dual polarization). The spacecraft provides a data storage capacity of 2.2 Tbit in solid state memory with EDAC (Error Detection and Correction). A total of 331 Gbit of imagery per orbit may be collected.
In addition, direct (real-time) downlinks to customer sites are available using the same high-speed 800 Mbit/s X-band link.
Launch: The WorldView-2 spacecraft was launched on October 8, 2009 on a Delta 7920 vehicle of ULA (United Launch Alliance) from VAFB, CA. ULA provided the services for this mission on behalf of BLS (Boeing Launch Services). 7) 8)
Orbit: Sun-synchronous nearly circular orbit, altitude = 770 km, inclination = 97.8º, period = 100.2 minutes, LTDN (Local Time on Descending Node) is at 10:30 hours.
• The WorldView-2 spacecraft and its payload are operating nominally in 2013. - The image of Figure 6 was featured on the Earth from Space video program (Earth observation image of the week) of ESA on Sept. 26, 2013. 9)
Figure 6: WorldView-2 image of Athens, Greece acquired on January 4, 2013 (image credit: European Space Imaging/DigitalGlobe)
Legend to Figure 6: The image shows the center city of Athens, the capital and largest city of Greece. Near the center of the image is the famous Acropolis of Athens – standing high about the surrounding city as evident by the shadow to the north. Nearby to the southeast is the Temple of Zeus, with shadows from the remaining standing columns stretching across the grass. Further northeast are the National Gardens surrounding the Zappeion building. At the upper left corner of the gardens is the Greek parliament building, overlooking Syntagma Square. In the lower-right corner one can see the large, white marble Panathenaic Stadium, originally built for the athletics part of the Panathenaic Games – in honor of the goddess Athena – it has been rebuilt, enlarged, excavated and refurbished over the centuries. In 1896, it hosted the first modern Olympic Games, which saw over 240 athletes from 14 nations.
• The WorldView-2 spacecraft and its payload are operating nominally in 2012. 10)
Figure 7: This WorldView-2 image depicts the Space Shuttle Atlantis in Merritt Island, Florida, as part of its historic final mission in July 2011 (image credit: DigitalGlobe) 11)
• The spacecraft and its payload are operating nominally in 2011. Figure 8 is an example of the spacecraft's event monitoring capability. DigitalGlobe is supporting this crisis/event monitoring service on a regular basis. 12) 13)
Figure 8: WorldView-2 image of the Fukushima Daiichi Nuclear Power Plant, Japan, March 14, 2011, (image credit: DigitalGlobe)
Table 3: Orbit lowering considerations for the WorldView-2 mission 16)
Figure 9: WorldView-2 image of the catastrophic mudslide in Zhouqu County in Gansu, China, observed on Aug. 10, 2010 (image credit: DigitalGlobe)
Legend to Figure 9: Around midnight on Aug. 8, 2010, a violent surge of loosened earth roared down mountain slopes and slammed into quietly sleeping neighborhoods in Zhouqu County in Gansu, China. The catastrophic mudslides - the deadliest in decades according to state media - buried some areas under as much as 7 m of suffocating sludge. 1,765 people died. Property damages totaled an estimated $759 million.
• For the Haiti earthquake disaster which occurred on January 12, 2010, DigitalGlobe is providing a free-of-charge high-resolution imagery service to aid the extensive relief and recovery efforts.
Figure 10: WorldView-2 image of Rio de Janeiro, Brazil acquired on January 19, 2010 (image credit: DigitalGlobe)
Figure 11: Sample image of WorldView-2 of a Texas scene taken on Oct. 19, 2009 only 11 days after launch (image credit: DigitalGlobe)
Figure 12: Sample image of Al Wakrah Port, Qatar observed on April 5, 2009 (image credit: DigitalGlobe)
Figure 13: WorldView-2 image of Aitutaki Lagoon (Cook Islands, South Pacific Ocean) observed on Nov. 23, 2009 (image credit: DigitalGlobe) 22)
• The DigitalGlobe ground station received a downlink signal confirming that the satellite successfully separated from its launch vehicle and automatically initialized its onboard processors. WorldView-2 is currently undergoing a calibration and check-out period. DigitalGlobe expects imagery products and services from WorldView-2 to be commercially available in approximately 90 days. 23)
Sensor complement: (WV110)
WV110 (WorldView-110 camera):
WV110 was designed and developed at ITT Corporation's Space Systems Division of Rochester, NY. The objective of the WV110 instrument is to provide high-resolution panchromatic as well as 8-band multispectral imagery for enhanced mapping and monitoring applications (including stereo imagery due to rapid retargeting capability). 24) 25)
In September 2008, BATC started with the integration of the WV110 camera. On Feb. 24, 2009 the WV110 camera had been integrated into the WorldView-2 spacecraft and system-level testing has commenced. 26) 27) 28) 29)
Table 4: Parameter specification of the GIS instrument
Table 5: Specification of spectral bands for WorldView-1 and WorldView-2 imagers
Table 6: Overview and parameter comparison of DigitalGlobe spacecraft 30)
WorldView-2 is the first commercial satellite to carry a very high spatial resolution 8-band multispectral sensor. Focal planes on the WV2 sensors are enhancements over those used on QuickBird (QB). In addition to overall increased agility, the WV110 focal plane carried by WV2 has a total of one panchromatic and eight multispectral bands with center wavelengths at 425 (coastal blue), 480 (blue), 545 (green), 605 (yellow), 660 (red), 725 (red edge), 835 (NIR1), and 950 (NIR2) nm, respectively. 31)
The new spectral dimensions in WV2 (coastal blue, yellow, rededge, NIR2) target costal and vegetation land cover types with applications in plant species identification, mapping of vegetation stress and crop types, mapping of benthic habitats, wetlands, coast water quality, and bathymetry. The addition of yellow and red edge bands fills important gaps in the spectrum that relate to our ability to capture vegetation phenomenology. The coastal and NIR2 bands extend the spectral coverage to wavelengths where there is increased divergence among the spectral response of vegetation types and many man-made materials. Overall, the broader and continuous coverage, along with sharper multispectral channels provide the potential for more robust modeling and discrimination of spectral signatures.
Figure 14: Illustration of spectral regions of the DigitalGlobe spacecraft (image credit: DigitalGlobe)
Table 7: Focal plane operating modes of WV110
Note: the PAN and MS line rates stated in Table 7 are at pixel level, not at detector level.
The focal plane is comprised of fifty panchromatic staggered DSAs (Detector Sub-Arrays), and two sets of ten MS, staggered DSAs, as shown in Figure 15. The two sets of staggered MS arrays are positioned on either side of the Pan array, one for the MS1 bands (MS1: NIR1, Red, Green, Blue), and the other for the MS2 bands (MS2: RedEdge, Yellow, Coastal, NIR2) . Each DSA contains four parallel rows of detectors, each with a different color filter. For each DSA, the individual bands are collected by a separate readout register. The Pan array uses two separate readout registers for each of its fifty DSAs. Each readout register has its own analog-to-digital converter.
The WorldView-2 spectral radiance response is defined as the ratio of the number of photo-electrons measured by the system, to the spectral radiance [W m-2 sr-1 µm-1] at a particular wavelength present at the entrance to the telescope aperture. It includes not only raw detector quantum efficiency, but also transmission losses due to the telescope optics and MS filters. The spectral radiance response for each band is normalized by dividing by the maximum response value for that band to arrive at a relative spectral radiance response. These curves for the WorldView-2 panchromatic and MS bands are shown in Figure 16.
Figure 16: Relative spectral radiance response of the WV-110 instrument (image credit: Digital Globe)
Figure 17: Illustration of the DigitalGlobe imaging constellation (image credit: DigitalGlobe)
1) “Worldview-2 Earth Observation Satellite, USA,” URL: http://www.aerospace-technology.com/projects/worldview-2/
5) R. C. Hopkins, C.L. Johnson, C. Kouveliotou, D. Jones, M. Baysinger, T. Bedsole, C.D. Maples, P. J. Benfield, M. Turner, P. Capizzo, L. Fabisinski, L. Hornsby, K. Thompson, J. H. Miernik, T. Percy, “Xenia Mission: Spacecraft Design Concept,” NASA/MSFC, NASA/TM—2009–216270, Dec. 2009, p. 16, URL: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100019148_2010019690.pdf
6) “Southwest Research Institute (SwRI) 2009 News Release: WorldView-2 satellite features key processing electronics from SwRI,” SwRI, Oct. 8, 2009, URL: http://www.swri.org/9what/releases/2009/WorldView2.htm
7) “United Launch Alliance Successfully Launches WorldView-2 Mission for DigitalGlobe & Boeing Launch Services,” Oct. 8, 2009, URL: http://www.prnewswire.com/news-releases/united-launch-alliance-successfully-launches-worldview-2-mission-for-dig
9) “Earth from Space: City of Knowledge,” ESA, Sept. 26, 2013, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Earth_from_Space_City_of_knowledge
10) “DigitalGlobe Incorporated Satellite and Aerial Program Update,” Proceedings of the 11th Annual JACIE (Joint Agency Commercial Imagery Evaluation ) Workshop, Fairfax, Va, USA, April 17-19, 2012, URL: http://calval.cr.usgs.gov/wordpress/wp-content/uploads/Thomassie_DigitalGlobe_JACIE_4_17_121.pdf
11) “WorldView-2 Satellite Images,” Satellite Imaging Corporation, URL: http://www.satimagingcorp.com/gallery-worldview-2.html
12) Brett P. Thomassie, “DigitalGlobe Systems and Products Overview,” 10th Annual JACIE ( Joint Agency Commercial Imagery Evaluation) Workshop, March 29-31, 2011, Boulder CO, USA, URL: http://calval.cr.usgs.gov/JACIE_files/JACIE11/Presentations/WedPM/405_Thomassie_J43.pdf
13) Jianwei Tao, Wenxian Yu, “A Preliminary Study on Imaging Time Difference Among Bands of Worldview-2 and Its Potential Applications,” 10th Annual JACIE ( Joint Agency Commercial Imagery Evaluation) Workshop, March 29-31, 2011, Boulder CO, USA, URL: http://calval.cr.usgs.gov/JACIE_files/JACIE11/Presentations/WedPM/315_Tao_JACIE_11.137.pdf
14) Byron Smiley, “Geolocation Accuracy Topics Relevant to DigitalGlobe’s Satellite Constellation,” 10th Annual JACIE ( Joint Agency Commercial Imagery Evaluation) Workshop, March 29-31, 2011, Boulder CO, USA, URL: http://calval.cr.usgs.gov/JACIE_files/JACIE11/Presentations/WedPM/430_Smiley_JACIE_.12126.pdf
15) Paul C. Brennan, “Geolocation Accuracy Monitoring of High Resolution Commercial Imagery,” 10th Annual JACIE ( Joint Agency Commercial Imagery Evaluation) Workshop, March 29-31, 2011, Boulder CO, USA, URL: http://calval.cr.usgs.gov/JACIE_files/JACIE11/Presentations/WedPM/115_Bresnahan_JAC.008.pdf
16) Michael A. Taverna, “DigitalGlobe To Change WorldView-2 Orbit,” Aviation Week, Sept. 20, 2010, URL: http://www.aviationweek.com/aw/generic/story_generic.jsp?channel=space&id=news/asd/2010/09/17/01.xml&headline=DigitalGlobe%20To%20Change%View-2%20Orbit
17) Information provided by Charles P. Herring of DitigalGlobe Inc., Longmont, CO
18) “New Dimension in High Resolution Imagery,” NRSC (National Remote Sensing Center) User Interaction Workshop, Hyderabad, India, Feb. 3-4, 2010
19) Philip Cheng, Chuck Chaapel, “WorldView-2 Satellite,” GEO Informatics, June 2010, URL: http://www.pcigeomatics.com/pdfs/GeoInformatics_WorldView-2.pdf
20) “DigitalGlobe's WorldView-2 Reaches Full Operational Capability on Schedule,” DigitalGlobe, January 4, 2010
21) DigitalGlobe's WorldView-2 Declared fully operational,” Space News, January 11, 2010, p. 8
22) “Coastal Applications Using WorldView-2, Proceedings of JACIE 2010 (Joint Agengy Commercial Imagery Evaluation) Workshop, Fairfax, VA, USA, March 16-18, 2010, URL: http://calval.cr.usgs.gov/JACIE_files/JACIE10/Posters/Thomassie_Brett_4band_NIR.pdf
23) DigitalGlobe Announces the Successful Launch of WorldView-2,” GEOICT Oct. 8, 2009, URL: http://geoict.yorku.ca/news/digitalglobe-announces-the-successful-launch-of
24) “The Benefits of the 8 Spectral Bands of WorldView-2,” WorldView, Aug. 2009, URL: http://worldview2.digitalglobe.com/docs/WorldView-2_8-Band_Applications_Whitepaper.pdf
26) “Ball Aerospace Begins Integration of WorldView-2 Imaging Instrument,” BATC, Sept. 3, 2008, URL: http://www.pressreleasepoint.com/ball-aerospace-begins-integration-worldview2-imaging-instrument
27) Todd Updike, Chris Comp, “Radiometric Use of WorldView-2 Imagery,” DigitalGlobe Technical Note, Nov. 1, 2010
28) D. Poli, E. Angiuli, F. Remondino, “Radiometric and Geometric Analysis of WorldView-2 Stereo Scenes,” ISPRS, Commission I, WG I/4, 2010, URL: http://www.isprs.org/proceedings/XXXVIII/part1/03/03_04_Paper_188.pdf
29) “WorldView-2 Satellite Sensor,” Satellite Imaging Corporation, URL: http://www.satimagingcorp.com/satellite-sensors/worldview-2.html
30) Brett Thomasie, “DigitalGlobe update,” Proceedings of JACIE 2007 (Joint Agency Commercial Imagery Evaluation), Fairfax, VA, USA, March 20-22, 2007, URL: http://calval.cr.usgs.gov/JACIE_files/JACIE07/Files/19Thomas.pdf
31) G. Marchisio, F. Pacifici, C. Padwick, “On the relative predictive value of the new spectral bands in the WorldView-2 sensor,” Proceedings of IGARSS (IEEE International Geoscience and Remote Sensing Symposium) 2010, Honolulu, HI, USA, July 25-30, 2010
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.