The next-generation commercial imaging satellite of DigitalGlobe Inc. (Longmont, CO, USA) is called WorldView-1, a successor of QuickBird-2 (launch Oct. 18, 2001 - and fully operational as of 2008). In Oct. 2003, DigitalGlobe was awarded a sizeable contract by NGA (National Geospatial-Intelligence Agency) of Washington DC, formerly NIMA (National Imaging and Mapping Agency), to provide high-resolution imagery from the next-generation commercial imaging satellites.
The NGA requirements call for imagery with a spatial resolution of 0.5 m panchromatic and 2 m MS (Multispectral) data. The contract award was made within NGA's NextView program, designed to give the US commercial imaging satellite operators the financing to build their satellites for high-resolution imaging. The WorldView mission is intended to provide imaging services to NGA as well as to the commercial customer base of DigitalGlobe.
BATC (Ball Aerospace and Technologies Corporation) of Boulder, CO, is the prime contractor and integrator of the spacecraft, providing the S/C bus (Ball Commercial Platform BCP-5000) and a WorldView-60 camera. A new feature of the WorldView spacecraft are CMG (Control Moment Gyroscopes) actuators for precise and highly responsive pointing control. The BCP-5000 bus provides increased power, stability, agility, data storage and transmission (over the BCP-2000 bus) as the demand for Earth remote-sensing information becomes more comprehensive. 1) 2) 3)
The S/C is 3-axis stabilized. The ADCS (Attitude Determination and Control Subsystem) employs star trackers, IRU (Inertial Reference Unit) and GPS for attitude sensing, and CMGs as actuators. A S/C body-pointing range of ±40º about nadir is provided corresponding to a FOR (Field of Regard) of 775 km in cross-track. An instantaneous pointing accuracy of ≤ 500 m is provided at any start and stop of an imaging sequence. On the ground, the geolocation accuracy of the imagery is 5.8 to 7.6 m without GPCs (Ground Control Points) and 2 m with GPCs (3σ). The agile S/C provides retargeting at a rate of 4.5º/s with an acceleration of 2.5º/s2; it takes 9 s to slew the S/C over a ground distance of 300 km. 4) 5)
S/C power of 3.2 kW (EOL) is provided by the triple-junction solar panels; the battery capacity is 100 Ah. The solar arrays are being articulated into the sun (normal pointing into the sun). The rotational drive assemblies and drive control electronics, referred to as QuAD (Quiet Array Drive), are being provided by Starsys, a subsidiary of SpaceDev, Poway, CA. Unlike traditional stepper motor solar array drives, the QuAD technology provides low disturbance actuation, allowing spacecraft images to be captured at the same time that the solar arrays are being pointed. Two BAE Systems RAD750 radiation-hardened single-board computers are managing the command and control functions onboard the WorldView-1 satellite ( the RAD750 is a licensed radiation-hardened version of the IBM PowerPC 750).
The S/C bus has dimensions of 3.6 m (high) and 2.5 m in diameter, the span of the deployed panels measures 7.1 m. WorldView-1 has a launch mass of 2500 kg. The mission design life is 7.25 years.
Figure 1: Illustration of the WorldView-1 spacecraft (image credit: DigitalGlobe)
Table 1: Overview of some spacecraft parameters
Orbit: Sun-synchronous circular orbit, altitude = 496 km (nominal), inclination = 97.2º. The equator crossing time is at 10:30 hours on a descending node. The period is 94.6 minutes. Note: While the low-altitude orbit selection offers better spatial resolutions than a higher one, it requires also more frequent reboosts to maintain the low orbit due to atmospheric drag influences.
RF communications: The command data are in S-band at 2 or 64 kbit/s. The housekeeping telemetry and tracking is 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 S/C 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.
Figure 2: Integration of the WorlrdView-1 spacecraft at BATC (image credit: DigitalGlobe)
Figure 3: Photo of the WorldView-1 spacecraft after performance testing (image credit: BATC)
• The WorldView-1 spacecraft and its payload are operating nominally in 2012.
Legend to Figure 4: Snapshot of the shipwreck from space. The luxury liner is visible through a scattered cloud layer, as it lies half-submerged off the coast of Giglio, Italy (located off the cost of Tuscany). The ocean liner hit a reef on January 13 and tipped over. The Costa Concordia was carrying about 3,200 passengers and a crew of 1,000 when it ran aground, according to news reports. The accident killed 11 people, with more than two dozen others still missing (as of January 18, 2012).
• The spacecraft and its payload are operating nominally in 2010. EOL (End of Life) is expected in 2018. 9)
• On January 12, 2010, an earthquake of magnitude 7.0 struck Haiti, in particular the capital city Port-au-Prince. On January 13, 2010, WorldView-1 collected a high off-nadir image of Port-au-Prince and the surrounding area. DigitalGlobe provided this information to the relief organizations.
• In early January 2008, DigitalGlobe announced the general availability of WorldView-1 imagery to its customers after all check-out aspects of the spacecraft and its payload had been completed. WorldView-1 had reached FOC (Full Operational Capability). 10)
• NGA has acquired approximately 60% of the global capacity on WorldView-1, with requirements spread worldwide. Most data collected for NGA will be available for purchase out of the archive.
• DigitalGlobe delivered its first sample set of high-resolution images on Oct. 15, 2007 and began supplying imagery to the National Geospatial-Intelligence Agency (NGA) on Nov. 26, 2007 (the S/C reached already full operating capacity on Nov. 17. 2007).
Figure 5: Constellation (QuickBird + WorldView-1) average revisit at 20 off nadir (image credit: DigitalGlobe)
Sensor complement: (WV60)
WV60 (WorldView-60 camera):
WV60 was designed and developed by BATC. The objective is to provide highly detailed imagery for a wide spectrum of applications such as precise map creation, change detection and in-depth image analysis (note that imagery must be re-sampled to 50 cm for non-US Government customers).
Table 2: Performance parameters of the WorldView-60 camera
WV60 consists of the following elements: Optical subsystem, FPU (Focal Plane Unit) and the DPU (Digital Processing Unit), with FPU and DPU designed and custom-built by ITT Space Systems Division, formerly Kodak Remote Sensing Systems of Rochester, New York.
The optical subsystem, mounted on an optical bench (with sunshield and internal baffling to suppress stray light), is of Ball design (telescope aperture of 60 cm diameter, lightweight structure, focal length of 8.8 m, f/14.7, the telescope mass is 138 kg, telescope size: 115 cm x 141 cm x 195 cm), providing a FOV (Field of View) of 2.12º, obtained with an unobscured off-axis three-mirror-anastigmatic (TMA) optical form. A fourth mirror is used to fold the light bundle for compact telescope packaging.
The pushbroom imager is rigidly aligned with the S/C axis, providing a nominal body-pointing capability of ±40º into the along-track and cross-track directions (45º max). WV60 may also be used for stereo imaging by slewing the S/C fore and aft. The TDI feature provides a capability for low-light observations of imagery. The on-board processor provides real-time radiometric/geometric calibration and image compression for all imaging data. The focal plane array and the compression technique ADPCM (Adaptive Differential Pulse Code Modulation) employed are provided by Kodak. Instrument mass = 380 kg, instrument power = 250 W.
DigitalGlobe's imagery may be used for a number of mapping and planning activities within the defense, intelligence, and commercial markets around the world.
Table 3: Specification of spectral bands for WorldView-1 and WorldView-2 imagers
Background on the selection of the two cameras for the new spacecraft series: DigitalGlobe placed 2 separate instruments under contract with its vendors.
• To satisfy the NextView requirements with the shortest schedule and lowest risk possible, the baseline design is the WV60 telescope configuration of QuickBird heritage; however, with next-generation focal plane and electronics. The WorldView-1 instrument, WV60, provides 0.5 m panchromatic only data. The instrument name results from the telescope aperture size of 60 cm diameter, hence WV60.
• The WorldView-2 instrument, WV110 (WorldView-110 camera) with a telescope aperture of 110 cm, will provide 0.5m panchromatic and 2 m MS imagery in 8 bands (from a higher orbit). WV110 is a new instrument design. The WorldView-2 launch is planned for late 2008.
The reason is that the WorldView-1 instrument was available far sooner than the WV110 camera of the WorldView-2 spacecraft. NGA agreed to this arrangement and opted for an earlier possible launch of WorldView-1.
Figure 7: View of the WorldView-1 spacecraft resulting from Figure 6 (image credit: DigitalGlobe, Eurimage)
1) Ball Aerospace Wins WorldView 2 Contract,” Jan. 2, 2007, URL: http://www.ballaerospace.com/page.jsp?page=30&id=55
3) “Introducing WorldView: DigitalGlobe’s Next Generation System,” April 11, 2007, URL: http://www.auricht.com/.../WorldView_Satellites.pdf
4) Information provided by Charles P. Herring of DigitalGlobe Inc., Longmont, CO, USA
5) A. Celentano, “WorldView-1 & -2 Latest status,” BARSC (British Association of Remote Sensing Companies) Workshop, June 12, 2008, London, UK
8) “Satellite Spots Costa Concordia Shipwreck From Space,” January 18, 2012, URL: http://www.space.com/14273-satellite-photo-costa-concordia-cruise-shipwreck.html
9) Information provided by Charles P. Herring of DitigalGlobe Inc., Longmont, CO
10) “DigitalGlobe's WorldView-1 Reaches Full Operational Capability With the National Geospatial-Intelligence Agency,” Jan. 3, 2008, URL: http://www.prnewswire.com/news-releases/digitalglobes-worldview-1-reaches-full-operational-capability-with-the-
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.