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WorldView-4: Another feather in DigitalGlobe`s cap

by Pubali Mukherjee

The global leader in earth imagery information: DigitalGlobe (DG) is all set to complete its constellation of very high resolution satellite with the launch of WorldView-4(WV-4) in September 2016. With WV-4 in orbit the satellite imagery giant will add another feather in its cap. The satellite is built upon its predecessor WV-3 with the same spatial resolution of 30 centimetres, the important factor to note is that this one will have an increased revisit time which is expected to give an extra niche in tracking the changes in this ever changing planet, moreover the imagery of WV-4 has the advantage of being available to Non-US government agencies in a way that WV-3 was not.

WorldView-4 previously known as GeoEye-2 is scheduled to be launched by DG in September 2016.

Following table explains the Design and specifications of WorldView-4:

Orbit Altitude: 617 km
Type: SunSync, 10:30 am descending Node
Period: 97 min.
Life Estimated Service Life: 10 to 12 years
Spacecraft Size
and Aperture
Size: 5.3 m (17.7 ft.) tall x 2.5 m (8 ft.) across 7.9 m (26 ft.) across deployed solar arrays
Aperture: 1.1m
Sensor Bands Panchromatic: 450 – 800 nm
4 Multispectral:
Red: 655 – 690 nm
Green: 510 – 580 nm
Blue: 450 – 510 nm
Near-IR: 780 – 920 nm
Sensor Resolution(GSD, Ground Sample Distance, geometric mean) Panchromatic Nadir: 0.31 m20° Off-Nadir: 0.34 m56° Off-Nadir: 1.00 m65° (earth limb): 3.51 mMultispectral Nadir: 1.24 m20° Off-Nadir: 1.38 m56° Off-Nadir: 4.00 m65° (earth limb): 14.00 m
Dynamic Range  11-bits per pixel 
Swath Width  At nadir: 13.1 km 
Attitude Determination and Control  Type: 3-axis StabilizedActuators: Control Moment Gyros (CMGs)Sensors: Star trackers, precision IRU, GPS
Retargeting Agility  Time to Slew 200 km: 10.6 sec 
Onboard Storage  3200 Gb solid state with EDAC 
Communications  Image & Ancillary Data: 800 Mbps X-bandHousekeeping: 120 kbps real time, X-bandCommand: 64 kbps S-band
Max Contiguous AreaCollected in a Single Pass(30° off-nadir angle) Mono: 66.5 km x 112 km (5 strips)Stereo: 26.6 km x 112 km (2 pairs)
Revisit Frequency(at 40°N Latitude) 1 m GSD: < 1.0 dayTotal constellation >4.5 accesses/day
Geolocation Accuracy(CE90) Predicted <4 m CE90 without ground control 

680,000 km2 per day

                                           Constellation of DigitalGlobe(Image courtesy:Google Images)

The important factor which led to the launch of WV-4 is the aim of adding increased capacity reserved for Direct Access partners (DAP) and other Non U.S Governmental business. DAP government alliances like Saudi Arabia will have access to the most capable and technologically advanced electro-optical satellite constellation in the world. With increasing Geo-political tension in the Middle East, 30cm imagery will definitely provide a helping hand.  Earlier National Intelligence Agency of United states Government had reserved a substantial amount of the company high resolution imagery but with DAP and other customers seeking high resolution imagery DG moved forward with the launch of WV-4.Organization principle of the DigitalGlobe customers is “Location and Time”, people want to know where particular object is precisely and for that 30 cm resolution is a sweet spot for DigitalGlobe. Although the resolution restriction imposed by the federal government was uplifted in 2014, DigitalGlobe pushed the launch of WV-4 to increase its business revenues and serve a new era of information and insight about of extremely dynamic planet.WV-4 is expected to provide DigitalGlobe with near monopoly in the high resolution commercial imagery market in U.S.

DigitalGlobe has a smaller constellation when it comes to other satellite launching organizations based in the Silicon Valley.WV-4 has the same features as its predecessor WV-3 but free from the legal obligations like its precursors. Addition of another satellite to its small constellation will empower the satellite imagery giant to provide its customers with cutting edge technology timely collection of imagery as the satellites knows where to look and how to look with their enhanced agility and aperture features. With enhanced agility the satellites can capture high quality resolution images within seconds, for example WV-3 can retarget from San Francisco to Las Vegas within 15 Seconds. Off nadir viewing is a very important aspect of DigitalGlobe constellation in terms of responding to disasters. World is a big place where numerous changes are happening every second so it is not only important to know where to look , it is equally important to know how fast the satellite can sweep the target and refresh the images. Following the launch of WV-4 this process is expected to be done in less than half the time relative to a notational constellation of a dozen of one meter class satellites. DG is of the opinion that it is not about the cost or size of the tool but rather how much it can deliver per unit cost.

                                                                        Imaging capability of DigitalGlobe satellite.(Image courtesy:DigitalGlobe)

Above is an example of fast sweeping ability of DG constellation.one of DG`s customer needed to image the whole Taiwan as soon as possible. Target was imaged with 15 image strips within 5 minutes. (Image courtesy: DigitalGlobe)

As important it is to know how quickly a satellite constellation can image a single city it is equally important to know how fast it can refresh the image regularly, let us consider how much time would a satellite constellation take to image world`s 5000 most populous cities. The following graphic explains the ability of DG`s constellation compared to other satellite constellation.

                       Image Courtesy:DigitalGlobe

                                                                Fukushima Nuclear reactor, before the explosion captured by DigitalGlobe(Image Courtesy:DigitalGlobe)

Fukushima Nuclear reactor, after the explosion captured by DigitalGlobe(Image Courtesy:DigitalGlobe)

Enhanced pointing agility enabled DG to capture the image of Fukushima reactor 3 both one minute before and 3 minutes after the explosion on march 14, 2011.WV-1 imaged the plant at a 34.5 degrees off nadir angle and WV-2 imaged the same  4 minutes at an off nadir angle of 32.7 degrees.

Ever since the blue marble image of our planet by Apollo mission in 1971 a lot has changed in terms of space technology and at the same time there has been an unprecedented increase in natural disasters and economic crisis. Thus optimization of limited resources is of utmost importance and that is exactly where high resolution satellite imagery comes in the scenario, where DigitalGlobe with 3o cm imagery definitely seems to have an upper hand.
Following Video of Sydney Opera,Australia explains the enhanced ability of DigitalGlobe satellites to track the minute changes.SydneyOpera captured by DigitalGlobe

SATPALDA is a privately owned company and a leading provider of satellite imagery and GeoSpatial services to the user community. Established in 2002, SATPALDA has successfully completed wide range of photogrammetric and Remote Sensing Projects.

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