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WorldView-2

Spacecraft     Launch    Mission Status     Sensor Complement    References

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)

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Figure 1: Illustration of the WorldView-2 spacecraft (image credit: DigitalGlobe)

Spacecraft:

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.

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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.

S/C bus type

BCP-5000

S/C stabilization

- 3-axis stabilized using star trackers and solid-state IRU for sensing
- CMG (Control Moment Gyro) assembly for actuation (providing high S/C agility)
- S/C pointing at 3.5º/s, acceleration of 1.5º s-2, slewing of 300 km in 9 s

Pointing accuracy

- Accuracy: <500 m at image start and stop
- Geolocation accuracy on ground: 4.6-10.7 m without GCP
- Geolocation accuracy on ground: 2.0 m with GCP (Ground Control Point, 3σ)

FOR (Field of Regard)

1355 km in cross-track (nominally ±40º off-nadir body pointing capability
for stereo imaging support and event monitoring)

S/C bus size

4.3 m (height) x 2.5 m (diameter), 7.1 m span width (deployed)

S/C launch mass, power

2800 kg, 3.2 kW (EOL, 100 Ah NiH2 battery)

Mission design life

7.25 years

Onboard data storage

2.2 Tbit in solid-state memory with EDAC (Error Detection and Correction)

Table 1: Overview of some spacecraft parameters

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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)

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Figure 4: Photo of the BATC M-95 CMG four-wheel pyramid configuration (image credit: BATC)

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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 = 767 km, inclination = 97.8º, period = 100.2 minutes, LTDN (Local Time on Descending Node) is at 10:30 hours.

 


 

Mission status:

• The WorldView-2 spacecraft is operational in 2017 in its 8th year on orbit. The image of Figure 6 was provided by DigitalGlobe on Feb. 3, 2017. 9) 10)

- DigitalGlobe released the WorldView-2 image to commemorate the milestone of "WorldView-4 operations start" — showing how the ULA (United Launch Alliance) Atlas 5 blasted off with WorldView-4 on Nov. 11, 2016 from Vandenberg Air Force Base, California.

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Figure 6: The launch of WorldView-4 on Nov. 11, 2016 as seen from space by WorldView-2. When captured, WorldView-2 was 637 km NE of the VAFB (Vandenberg Air Force Base) & 38º off-nadir (image credit: DigitalGlobe)

• July 19, 2016: According to the USAF (US Air Force), a DigitalGlobe imagery satellite was involved in what has been described as a debris-causing event. DigitalGlobe released an hours-old photo of Oakland, CA on July 19 (Figure 7), hoping to squash questions about the operability of one of its high-resolution imagery satellites after the U.S. Air Force said it had been part of a debris-causing event earlier in the day. -WorldView-2 offers 46 cm resolution imagery to both government and commercial customers. 11)

- JSpOC (Joint Space Operations Center), which is the Defense Department's nerve center for space operations and tracks space objects from Vandenberg Air Force Base in California, tweeted on July 19 that it had identified a debris-causing event related DigitalGlobe's WorldView-2 satellite. — As a result, the JSpOC is tracking eight pieces of debris related to the incident. An estimated time of the event was not immediately available.

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Figure 7: An image of downtown Oakland captured on July 19, 2016 by DigitalGlobe's WorldView-2 satellite (image credit: DigitalGlobe)

• December 4, 2015: Mount Etna showed its colors on 3 December erupting again just after 2 UTC for the first time since May 2015. The spectacular upwards jet of lava from the Voragine crater lasted for 50 minutes sending an ash plume more than 3 km high. The fountain of lava is said to have reached 1 km above the crater. This was the biggest eruption since September 1999 when the volcano shot volcanic material more than 12 km high. 12)

- Etna is the tallest and largest volcano of Europe and has four summit craters. Its eruptions occur both at the top and from its sides down to a few hundred meters above the sea-level. It is one of the most active volcanoes on the Earth so is continuously monitored by the Etna Observatory based in Sicily.

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Figure 8: WorldView-2 satellite captured the fumarole plumes of smoke and gas of Mount Etna that continue to spout from the crater shortly after the eruption on December 3, 2015.

• April 28, 2015: DigitalGlobe, Inc. today announced the general availability of its Basemap +Vivid product for the entire African continent. Keeping pace with the rapid evolution of mapping technology, this is the first time that a complete, consistent satellite imagery base layer with 50 cm ground resolution has been available for Africa. 13)

- Many parts of Africa have never been mapped at this resolution from space, and never before has there been a complete imagery base map of Africa with this resolution from space, and never before has there been a complete imagery base map of Africa with this level of detail. While 50 cm satellite imagery for Africa has been available since 2008, it previously only covered smaller areas of interest.

- This Africa base map strives to maximize consistency and completeness of the imagery, aligning to DigitalGlobe's A3C quality program. Whether zoomed out to view an entire country, or zoomed in all the way down to view local vegetation, dwellings, and infrastructure, the imagery looks the way a user expects the Earth's surface to look. This uniformity helps local governments or global development agencies to build out maps and value-added information layers such as road vectors or population polygons for the people they serve. For web-enabled mapping platforms and location-based applications, users will stay immersed in their experience and not be distracted by inconsistencies in the imagery.

- "This is an important accomplishment in response to requests from our customers to create a verifiable, authoritative base map covering the entire continent," said Hyune Hand, DigitalGlobe's Senior Vice President for Product Marketing and Management. "Demand continues to grow, fueled by both regional projects and programs that require consistent quality coverage of an entire country."

- DigitalGlobe was in a unique position to develop this product with the world's most advanced constellation of commercial imaging satellites and six years' worth of sub-50 cm archive imagery. Not only does DigitalGlobe have the largest and most comprehensive archive of commercial satellite imagery for source data, but the research and development teams have invented and patented algorithms to process imagery at an unmatched speed and scale. Now prospective imagery users do not have to tackle the challenges of building a large imagery mosaic - such as inconsistencies between images, misalignment, visible seam lines, color imbalances, seasonality, haze, and cloud cover - to have a country-wide imagery layer on which to build the next generation of maps and applications.

• Feb. 26, 2015: DigitalGlobe today announced the signing of a MOU (Memorandum of Understanding) with the United Nations. Under the MOU, DigitalGlobe and UNOOSA ( United Nations Office for Outer Space Affairs) will take stock of their combined expertise in the use of Earth observation technologies for economic, social, and scientific development and improved decision-making, particularly in developing countries. 14)

- "DigitalGlobe is thrilled to partner with UNOOSA, the United Nations body that promotes international cooperation in the peaceful uses of outer space," said Jeffrey R. Tarr, DigitalGlobe President and Chief Executive Officer. "The arrangement provides an ideal platform to explore how high resolution satellite imagery and geospatial analytics can be more efficiently and effectively shared across the entire United Nations System, thus propelling us toward our purpose of Seeing a better world™."

- UNOOSA and DigitalGlobe will work to develop an online platform to provide easy access to imagery catalogues as well as data and analytical services specifically tailored for the needs of the United Nations. Under the agreement, DigitalGlobe will provide advisory services on remote sensing imagery and geospatial analytics, working with UNOOSA to advance and accelerate adoption of geospatial and satellite imagery-based analytics across the entire United Nations System. The cooperation will also extend to DigitalGlobe's participation in relevant UNOOSA-supported events and activities, including those of the United Nations Platform for Space-based Information for Disaster Management and Emergency Response (UN-SPIDER) and of the United Nations Geographic Information Working Group (UNGIWG).

• Dec. 18, 2014: DigitalGlobe has reported that, as a result of its annual satellite life review, the company will extend the useful lives of two of its satellites: WorldView-1 and WorldView-2. WorldView-1 will be extended by 2.5 years to 13 years, a 24% lifespan improvement, and WorldView-2 will be extended two years to 13 years, an 18% lifespan improvement. 15) 16)

- WorldView-1, which was originally expected to reach its end of life in the second quarter of 2018, is now expected to reach end of life in the fourth quarter of 2020.

- WorldView-2, which was originally expected to reach its end of life in the fourth quarter of 2020, is now expected to reach end of life in the fourth quarter of 2022.

In June 2014, DigitalGlobe announced that it received notice from the U.S. Department of Commerce (DOC) on its application to allow the company to sell its highest-quality and industry-leading commercial satellite imagery. Effective immediately, DigitalGlobe will be permitted to offer customers the highest resolution imagery available from their current constellation. Additionally, the updated approvals will permit DigitalGlobe to sell imagery to all of its customers at up to 0.25 m panchromatic and 1.0 m multispectral ground sample distance (GSD) beginning six months after its next satellite WorldView-3 is operational. WorldView-3 is scheduled to launch, August 13 , 2014 from VAFB (Vandenberg Air Force Base), CA. 17) 18)

- Two of DigitalGlobe's satellites, GeoEye-1 and WorldView-2 , collect imagery sharper than 0.50 m, and all customers will have access to that imagery at the highest native resolution. WorldView-3 will provide even higher resolution at 0.31 m, and the GeoEye-2 satellite, which is substantially complete, will capture similarly sharp images when it is launched to replace a satellite currently in service or as an expansion to the constellation once warranted by market demand.

Parameter

Ikonos-2

QuickBird-2

WorldView-1

GeoEye-1

WorldView-2

WorldView-3

GeoEye-2

Spectral characteristics

Pan+4 MS

Pan+4 MS

Pan

Pan+4 MS

Pan+8 MS

Pan+8 MS +8 SWIR

Pan+4 MS

Panchromatic resolution (nadir)

0.82 m

0.55 m

0.50 m

0.41 m

0.31 m

0.31 m

0.31 m

Multispectral resolution (nadir)

3.28 m

2.20 m

N/A

1.64 m

1.84 m

1.24 m

1.24 m

Accuracy specification (nadir)

9 m CE90

23 m CE90

4 m CE90

3 m CE90

3.5 m CE90

3.5 m CE90

3.5 m CE90

Onboard storage capacity

80 Gbit

128 Gbit

2199 Gbit

1000 Gbit

2199 Gbit

2199 Gbit

3000 Gbit

Collection capacity/day

150,000 km2

160,000 km2

1,500,000 km2

350,000 km2

1,200,000 km2

680,000 km2

680,000 km2

Launch date

Sept. 24, 1999

Oct. 18, 2001

Sept. 18, 2007

Sept. 06, 2008

Oct. 08, 2009

Aug. 13, 2014

2016

Table 2: Overview of the DigitalGlobe high-resolution imaging constellation (Ref. 17)

• The WorldView-2 spacecraft and its payload are operating nominally in 2014, in its 5th year on orbit.

• November 2013: Typhoon Haiyan, also known as Typhoon Yolanda in the Philippines, was a powerful tropical cyclone that devastated portions of Southeast Asia, particularly the Philippines, on November 8, 2013. It is the deadliest Philippine typhoon on record, killing at least 6,268 people in that country alone. Haiyan is also the strongest storm recorded at landfall, and unofficially the strongest typhoon ever recorded in terms of wind speed. 19)

There was widespread devastation from the storm surge in Tacloban City especially in San Jose, with many buildings being destroyed, trees knocked over or broken, and cars piled up. The low-lying areas on the eastern side of Tacloban city were hardest hit, with some areas completely washed away (90%of the city had been destroyed) . Flooding also extended for 1 km inland on the east coast of the province. The WorldView-2 spacecraft observed the region on Nov. 13, 2013 (Figure 9).

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Figure 9: WorldView-2 image of Tacloban City observed on Nov. 13, 2013 (image credit: CRISP, DigitalGlobe)

• The WorldView-2 spacecraft and its payload are operating nominally in 2013. - The image of Figure 10 was featured on the Earth from Space video program (Earth observation image of the week) of ESA on Sept. 26, 2013. 20)

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Figure 10: WorldView-2 image of Athens, Greece acquired on January 4, 2013 (image credit: European Space Imaging/DigitalGlobe)

Legend to Figure 10: 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. 21)

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Figure 11: 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) 22)

• The spacecraft and its payload are operating nominally in 2011. Figure 12 is an example of the spacecraft's event monitoring capability. DigitalGlobe is supporting this crisis/event monitoring service on a regular basis. 23) 24)

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Figure 12: WorldView-2 image of the Fukushima Daiichi Nuclear Power Plant, Japan, March 14, 2011, (image credit: DigitalGlobe)

Absolute geolocation accuracy conclusions

• The monoscopic geolocation accuracy goals are met by all three satellites

- QB02 (QuickBird-2) has CCAP (Civil and Commercial Applications Project) metrics between 13 to 21 m at nadir, < 23 m

- WV01 (WorldView-1) has CCAP metrics between 3.6 to 4.9 m at nadir, < 6.5 m

- WV02 (WorldView-2) has CCAP metrics between 2.4 to 3.5 m at nadir, < 6.5 m

• WV02 (WorldView-2) stereo geolocation accuracy components also within 6.5 m

- Horizontal CCAP metrics [CE90 (Circular Error of 90%)] between 2.6 and 4.6 m

- Vertical CCAP metrics (LE90s) between 3.1 to 4.5 m

Relative geolocation accuracy conclusions

• PAN camera has a time independent relative geolocation accuracy

- 0.59 ± 0.08 m (1σ)

- 1.28 ± 0.2 PAN pixels (1σ)

• All nine spectral bands have time independent band-to-band registration

- MS-MS (same band group) had some 90% under 0.5 PAN pixel

- PAN-MS had all 90% under 1 PAN pixel

- MS-MS (different band group) had some 90% just over 1 PAN pixel

Table 3: Geolocation accuracies of DigitalGlobe's Satellite Constellation 25) 26)

• In September 2010, Aviation Week reported that DigitalGlobe would lower the orbital altitude of the WorldView-2 spacecraft from 770 km to 680 km on request from NGA (National Geospatial-Intelligence Agency), resulting in a GSD (Ground Sample Distance) of 41 cm at nadir (from previously 46 cm at an altitude of 770 km). The lowering maneuver was planned to be implemented for late September. 2010. 27)

- However, as of April 2011, this lowering of the orbit didn't occur so far. Further considerations by the project came to the conclusion that the global coverage and revisit capability would certainly be affected by this move - a gain in GSD would entail a reduction in swath width (the original swath width at 770 km altitude is 16.4 km at nadir).

- For the commercial customer base, the resolution of their imagery would not be affected by the lowering of the orbit - it will remain constant at 50 cm, the resolution limit as prescribed by the U.S. Government licensing code.

- A potential second shift to an orbital altitude of 496 km, contemplated after September 2013, would bring the resolution down to 30 cm for NGA.

NGA, as the main sponsor of the WorldView-2 mission, has a Service Level Agreement with DigitalGlobe which specifies the agreed-upon targets relating to the volume, quality and delivery speed of the data.

Table 4: Orbit lowering considerations for the WorldView-2 mission 27)

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Figure 13: WorldView-2 image of the catastrophic mudslide in Zhouqu County in Gansu, China, observed on Aug. 10, 2010 (image credit: DigitalGlobe)

Legend to Figure 13: 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.

• The WorldView-2 spacecraft and its payload are operating nominally in 2010. 28) 29) 30)

• 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.

• On January 4, 2010, DigitalGlobe reported that its spacecraft WorldView-2 has achieved full operational capability and that its imagery is now commercially available. 31) 32)

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Figure 14: WorldView-2 image of Rio de Janeiro, Brazil acquired on January 19, 2010 (image credit: DigitalGlobe)

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Figure 15: Sample image of WorldView-2 of a Texas scene taken on Oct. 19, 2009 only 11 days after launch (image credit: DigitalGlobe)

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Figure 16: Sample image of Al Wakrah Port, Qatar observed on April 5, 2009 (image credit: DigitalGlobe)

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Figure 17: WorldView-2 image of Aitutaki Lagoon (Cook Islands, South Pacific Ocean) observed on Nov. 23, 2009 (image credit: DigitalGlobe) 33)

• 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. 34)

 


 

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). 35) 36)

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. 37) 38) 39) 40)

Imager type

Pushbroom imager (or a line scan imaging system)

Imaging mode

Panchromatic (Pan)

Multispectral (MS) 8 bands
(4 standard + 4 additional colors)

Spectral range

450-800 nm

400-450 nm (coastal blue)
450-510 nm (blue)
510-580 nm (green)
585-625 nm (yellow)
630-690 nm (red)
705-745 nm (red edge)
770-895 nm (NIR1)
860-1040 nm (NIR2)

Spatial resolution at nadir

0.46 m GSD (0.52 m at 20º off-nadir)

1.8 m GSD (2.4 m at 20º off-nadir)

Swath width

16.4 km (multiple adjoining paths can be imaged in a target area in a single orbit pass due to S/C agility)

Detectors

Pan: Si CCD array (8 µm pixel size) with a row of > 35,000 detectors
MS: Si CCD 4 arrays (32 µm pixel size) with a row of > 9,300 detectors

Data quantization

11 bit

Geolocation accuracy of imagery

≤ 3 m (using a GPS receiver, a gyroscope and a star tracker) without any GCP (Ground Control Points)

Optics

TMA telescope with an aperture diameter of 1.1 m, focal length = 13.3 m, f/12

TDI (Time Delay Integration)

6 selectable levels from 8 to 64 in Pan and MS

FOV (Field of View)

> 1.28º

Instrument size

3 m tall

Table 5: Parameter specification of the GIS instrument

Spectral band

Center wavelength (nm)

Minimum lower band edge (nm)

Maximum upper band edge (nm)

Pan (WorldView-1) imager

650

400

900

Pan (WorldView-2) imager

625

447

808

MS1 (NIR1)

831

765

901

MS2 (red)

659

630

690

MS3 (green)

546

506

586

MS4 (blue)

478

442

515

MS5 (red edge)

724

699

749

MS6 (yellow)

608

584

632

MS7 (coastal blue)

427

396

458

MS8 (NIR2)

908

856

1043

Table 6: Specification of spectral bands for WorldView-1 and WorldView-2 imagers

Parameter / Spacecraft

QuickBird-2 (QB)

WorldView-1

WorldView-2

Launch date

Oct. 21, 2001

Sept. 18. 2007

Oct. 08, 2009

Orbital altitude (SSO)

450 km

450 km

770 km

Spacecraft mass at launch

931 kg

2500 kg

2800 kg

Spacecraft bus size

3 m x 1.6 m Ø

3.6 m x 2.5 m Ø

4.3 m x 2.5 m Ø

Spacecraft bus type

BCP-2000

BCP-5000

BCP-5000

Solar array span

5.2 m

7.1 m

7.1 m

Spacecraft power

1.14 kW (EOL) single junction GaAs cells

3.2 kW (EOL) triple junction GaAs cells

3.2 kW (EOL) triple junction GaAs cells

Battery

40 Ah NiH2

100 Ah NiH2

100 Ah NiH2

Attitude actuation

Reaction wheels

CMG assembly

CMG assembly

S/C body pointing capability

±30º (nominal in any direction)

±40º (nominal in any direction)

±40º (nominal in any direction)

Onboard propulsion

4 x 4.4 N hydrazine thrusters

Yes

Yes

Spacecraft design life

5 years

7.25 years

7.25 years

RF Wideband downlink

320 Mbit/s

800 Mbit/s

800 Mbit/s

Onboard data storage

128 Gbit

2.2 Tbit

2.2 Tbit

 

 

 

 

Payload (builder)

BHRC60 (BATC)

WV60 (ITT)

WV110 (ITT)

Telescope aperture

60 cm Ø

60 cm Ø

110 cm Ø

Swath width

16.5 km

16.4 km

16.4 km

Pan resolution at nadir

0.61 cm

50 cm

46 cm

MS resolution at nadir

2.4 m

-

1.8 m (8 bands)

Monoscopic area coverage

1 x

> 4 x

> 4 x

Single pass mono coverage

1 strip of 350 km

1 strip of 650 km
1 area of 60 km x 110 km

1 strip of 650 km
1 area of 96 km x 110 km

Single pass stereo coverage

Single scene (<10º off nadir track)

3 strip x 55 km
2 strip x 110 km
1 strip x 220 km

3 strip x 55 km
2 strip x 110 km
1 strip x 220 km

Table 7: Overview and parameter comparison of DigitalGlobe spacecraft 41)

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. 42)

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.

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Figure 18: Illustration of spectral regions of the DigitalGlobe spacecraft (image credit: DigitalGlobe)

Mode

PAN line rate
(lines/s)

MS line rate
(lines/s)

Aggregation

TDI rates

Cmpression levels (bpp)

A

24,000

N/A

PAN= 1 x 1

PAN= 8, 16, 32, 48, 56, 64

2.75, 2.4

B

24,000

3,000

PAN= 1 x1
MS= 1 x 2

PAN= 8, 16, 32, 48, 56, 64
MS= 3, 6, 10, 14, 18, 21, 24

PAN: 2.75, 2.4
MS: 4.3, 3.2, 2.4

C

20,000

5,000

PAN= 1 x1
MS= 1 x 1

PAN= 8, 16, 32, 48, 56, 64
MS= 3, 6, 10, 14, 18, 21, 24

PAN: 2.75, 2.4
MS: 4.3, 3.2, 2.4

Table 8: Focal plane operating modes of WV110

Note: the PAN and MS line rates stated in Table 8 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 19. 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.

WorldView2_Auto2

Figure 19: Schematic layout of the focal plane (image credit: DigitalGlobe, Ref. 38)

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 20.

WorldView2_Auto1

Figure 20: Relative spectral radiance response of the WV-110 instrument (image credit: Digital Globe)

WorldView2_Auto0

Figure 21: Illustration of the DigitalGlobe imaging constellation (image credit: DigitalGlobe)

 


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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 (herb.kramer@gmx.net).

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