QuickBird-2

QuickBird-2 is an imaging satellite of DigitalGlobe Inc. (formerly EarthWatch) of Longmont, CO, USA, offering commercial imagery at 0.61 m (PAN) and at 2.8 m (MS) resolution. As of 2004, this represents the highest resolution satellite imagery, along with the highest location accuracy available in the commercial market. The DigitalGlobe system has been developed to facilitate the collection and archival of high quality Earth imagery data and to provide an easy-to-use and flexible distribution system. ^{1) 2)}

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

Spacecraft:

QuickBird-2 uses the BCP 2000 (Ball Commercial Platform 2000) satellite bus design. ³⁾The spacecraft is 3-axis stabilized. The ADCS (Attitude Determination and Control subsystem) uses two star trackers, redundant IRUs (Inertial Reference Units), sun sensors and magnetometers for attitude sensing. Attitude control is provided by low-vibration reaction wheels (0.68 Nm, 20 Nms), three torque rods, and four hydrazine thrusters. Position knowledge is provided by redundant GPS receivers. The pointing accuracy is ±0.016º (3 sigma steady state in all three axes; the attitude pointing knowledge is ±0.0008º (3σ steady state in all three axes), geolocation knowledge < 15 m (3σ) after ground processing.

S/C design life of 5 years. S/C mass = 931 kg at launch. The BCP 2000 configuration uses a simple panel-post aluminum honeycomb structure. The total bus mass (wet) is 641 kg. Two solar panels (GaAs/Ge cells), each of 3.2 m² area and single axis drive, provide a S/C power of 1500 W. The NiH₂ battery provides energy of 40 Ah for ecliptic operations. The spacecraft bus has a height of 3.04 m. ⁴⁾

Launch: A successful launch of QuickBird-2 took place on Oct. 18, 2001 on a Delta-2 vehicle of Boeing from VAFB (Vandenberg Air Force Base), CA, USA. ⁵⁾

Orbit: Sun-synchronous circular orbit, altitude = 450 km, inclination = 98º, average revisit time of 1-3.5 days.

Note: In 2001 the QuickBird project of DigitalGlobe opted for a fairly low orbital altitude of 450 km (from the previous 600 km altitude level) to obtain a higher ground spatial resolution of the imagery; however, at the expense of swath width. Naturally, the low orbit of 450 km requires more orbit raising maneuvers due to the increased drag influence of the atmosphere. The argument went that the S/C carries enough fuel to adjust the lower orbit more frequently over the operational lifetime of the mission.

RF communications: An onboard image data storage capability of 128 Gbit is provided in solid-state memory. The downlink of all imaging data is provided in X-band at data rates of up to 320 Mbit/s to ground receiving stations in the USA, Europe and Asia. The real-time X-band data channel is PCM/PSK/PM modulated, while the playback data channel is PCM/PM modulated. The TT&C functions are provided in S-band at data rates of 4-16 kbit/s in downlink and 2 kbit/s in uplink. The realtime narrow-band data is downlinked at these rates on the subcarrier, while the stored engineering data is downlinked on the carrier at a rate of 256 kbit/s.

Table 1: Performance features of QuickBird-2

Mission status: QuickBird-2 is fully operational and fully supports the imaging mission as of 2007. There are no operational constraints. Expected life until late 2009. ⁶⁾

• DigitalGlobe was granted a license by the DOC (Department of Commerce)/NOAA in December 2000 to operate a 0.5 m resolution satellite system. The company was able to modify its plans for QuickBird-2 to increase the resolution of the satellite from the originally planned 1 m resolution imaging system to a 61 cm system by adjusting the orbit in which the satellite is flown. The result is that the panchromatic resolution is increased from 1 m to 0.61 m and multispectral is increased from 4 m to 2.8 m resolution. ⁷⁾

Sensor complement:

BGIS 2000 (Ball Global Imaging System 2000). BGIS is a BATC-developed imager - the instrument-bus combination is called BGIS 2000. The bus itself is called BCP 2000. The camera instrument of BGIS-2000 is called BHRC 60 (Ball High Resolution Camera 60). BHRC 60 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 Kodak (same, except for size, as that used on IKONOS). BHRC 60 has a design life of > 5 years, achieved with a redundant architecture. Instrument mass = 380 kg, instrument power = 250 W silicon and 430 W for GaAs (orbital average).

• 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 enlarged FOV of the BHRC 60 instrument offers a ground swath of 15 km at 400 km orbital altitude or 34 km at 900 km altitude with a GSD varying between 0.5-1.5 m, respectively.

• The detector subsystem employs the pushbroom imaging technique. The CCD detector array features 27,568 pixels in the cross-track direction for the panchromatic band and 6892 pixels for each of the four multispectral bands. The spectral ranges of the multispectral BGIS bands correspond to the first four bands of the ETM+ instrument on Landsat-7, the PAN band is also identical to PAN on ETM+.

• The TDI (Time Delay Integration) concept is employed for PAN imagery. TDI levels of 10, 13, 18, 24, and 32 are available and selectable. The TDI arrays prevent exposure saturation while maximizing the SNR over a wide range of angles and Earth albedos.

Table 2: Performance parameters of the BGIS 2000 instrument

The instrument provides high-resolution panchromatic and multispectral imagery simultaneously. The pushbroom imager is rigidly aligned with the S/C axis, providing a nominal body-pointing capability of ±30º into the along-track and cross-track directions (45º max). The panchromatic and multispectral image scenes are coincident. BGIS 2000 may also be used for stereo imaging by slewing the S/C fore and aft. 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 employed ADPCM (Adaptive Differential Pulse Code Modulation) are provided by Kodak. ⁸⁾

Figure 2: Functional block diagram of the BHRC 60 detector subsystem

The QuickBird satellite early history

The following description gives some background of the various spacecraft and instruments of the QuickBird satellite family. Prior to this successful launch QuickBird-2, there were two launches of company imaging satellites, namely EarlyBird and QuickBird-1.

EarlyBird

In 1993, the US Department of Commerce granted DigitalGlobe's predecessor, WorldView Imaging Corporation (WorldView), the first US license allowing a private enterprise to build and operate a satellite system to gather high spatial resolution digital imagery of the earth for commercial sale. This enabled WorldView to design its first spacecraft, EarlyBird, to collect 3 m resolution panchromatic and 15 m multispectral imagery.

EarlyBird was an imaging spacecraft designed and built by EarthWatch Inc. (EarthWatch was formed in 1995 by Ball Aerospace and WorldView), along with its major partners: CTA Inc. of McLean, VA; Hitachi Ltd. of Tokyo, Japan; and Telespazio of Rome, Italy.

The EarlyBird S/C was three-axis stabilized; the attitude was sensed by a star tracker, position knowledge by GPS receiver; design life = 3 years, 5 years of on-board fuel; S/C mass = 310 kg; payload mass = 150 kg; power = 90 W; on-board storage capability of a 16 Gbit solid-state recorder.

During the period 1996 and 1997, EarthWatch developed its order processing and manufacturing systems, ground infrastructure, and constructed the EarlyBird satellite.

Figure 3: The EarlyBird spacecraft

Launch: A launch of EarlyBird-1 took place on December 24, 1997 with a Start-1 launch vehicle from the Svobodny Cosmodrome in Eastern Russia (note: the Start-1 rocket is based on the SS20 and SS25 intercontinental ballistic missiles with proven launch performance). Although EarlyBird was launched successfully, the satellite failed on orbit four days later due to a problem with the onboard power system. Despite extensive efforts, EarthWatch was unable to regain communications with the satellite. EarthWatch controllers lost contact with the S/C on Dec. 28, 1997, stopping the commencement of any operations.

Orbit: Sun-synchronous polar orbit, altitude = 470 km, inclination = 97.3º, period = 94 min, 10:30 AM equator crossing, descending node (1:30 PM equator crossing for the second satellite), repeat cycle = 20 days (max), revisit time = 1.5 - 2.5 days (with a two satellite configuration).

Data: The downlink of imaging data was encrypted and provided in X-band at data rates of 25 Mbit/s to EarthWatch-owned ground receiving stations in the USA, Europe and Asia. The TT&C up- and downlinks were in UHF-band. The objective of EarthWatch was to become the first global supplier of commercial high-resolution imagery and related geographic information products by creating and maintaining a so-called "DigitalGlobe" product database (the master archive, a gateway, and the satellite control center are located in Longmont, CO). A number of customized service options were developed with product delivery times ranging from 30 minutes to 48 hours after acquisition. ⁹⁾

Sensor complement:

EBP (EarlyBird Panchromatic), and EBM (EarlyBird Multispectral), built by EarthWatch, same concept design as on the Clark S/C of NASA, but with enhanced capabilities. The instrument provides high-resolution panchromatic and multispectral imagery simultaneously. A staring focal plane array detector technology (Kodak designed and built) was employed featuring a gimbaled mirror design (see Figure 4) with a 30º pointing capability from nadir into any direction. The fast steering mirror permits exposures of a single frame or of a matrix of images (point and shoot). The panchromatic image scene was always contained within the larger matrix of the multispectral image scene, in a fixed location. However, by taking a multispectral image and then moving the gimbal around to take additional multiple exposures, this permitted to cover the area of the multispectral image with panchromatic images, or there was the ability to collect a panchromatic image in any desired location within a multispectral image. The cameras could also be used for stereo imaging by slewing the gimbal mirror fore and aft in the S/C flight direction. The on-board processor provided real-time radiometric/geometric calibration and image compression for all imaging data.

Table 3: Performance characteristics of the EarlyBird imagery

Figure 4: The EBP and EBM optical design concept

QuickBird-1

After the failure of EarlyBird, a new-generation satellite, QuickBird-1, was developed by Ball Aerospace (funding by DigitalGlobe) with the objective to provide commercial imagery at 1 m (PAN) and at 4 m (MS) resolution. The QuickBird-1 spacecraft and instrument design is practically identical with the design of QuickBird-2.

From a technical point of view, the high-resolution imaging capability of QuickBird-1 required a change in instrument technology from a staring-array design (of EarlyBird) to a pushbroom/large-telescope technique, resulting in a new spacecraft design.

Launch: A launch of QuickBird-1 took place on Nov. 20, 2000 on a Cosmos-3M vehicle from Plesetsk, Russia. Unfortunately the QuickBird-1 failed to reach orbit. No contact could be established with the spacecraft ending in a loss of the mission.

Orbit: Circular orbit (non-sun-synchronous), altitude = 600 km, inclination = 66º, the equator crossing time is variable, average revisit time = 1 to 5 days depending on latitude.

Figure 5: Illustration of QuickBird configured with BGIS-2000

2) http://www.digitalglobe.com/

3) "Ball Commercial Platform 2000 (BCP 2000)," Technical Description, Jan. 2000, provided by Tom Miers of BATC

4) http://www.ballaerospace.com/media/nr04_20_99.html

5) http://www.digitalglobe.com/company/pdfs/QuickBird%20Spacecraft.pdf

6) Information provided by Charles P. Herring of DitigalGlobe Inc., Longmont, CO

7) http://www.spaceandtech.com/digest/sd2001-01/sd2001-01-013.shtml

8) "Ball High Resolution Camera 60 (BHRC 60), Technical Description, Jan. 2000, provided by Tom Miers of BATC

9) Information provided by D. B. Gerull, R. N. Herring, and B. Wientzen of EarthWatch, Longmont, CO.