Ikonos-2

Ikonos-2 is a commercial imaging satellite of GeoEye (formerly of Space Imaging Inc. of Thornton, CO), providing high-resolution imagery on a commercial basis. With Ikonos-2, a new era of 1 m spatial resolution imagery began for spaceborne instruments in the field of civil Earth observation. The Ikonos satellite system was built by LMMS (Lockheed Martin Missiles & Space) with HQ in Sunnyvale, CA, USA.

Some background on the Ikonos program:

• CRSS (Commercial Remote Sensing System) was a remote sensing imaging satellite project of Lockheed Martin that started in 1991. In 1994 a new company was formed for this venture, namely Space Imaging Inc. with the following partners: LMMS: (space segment, satellite operations, and tasking of ground segment), Raytheon/E-Systems (Garland TX: communications, image processing and customer service center). Eastman Kodak Co. of Rochester, NY designed and built the digital camera/sensor. The overall objective was to offer commercial high-resolution (1 m GSD panchromatic and 4 m GSD multispectral) imagery with excellent location knowledge in near real-time and offline.

• In Aug. 1995, Space Imaging was awarded a license by the FCC (Federal Communications Commission) to construct, launch and operate a commercial remote sensing satellite system comprised of two satellites. In 1997 the CRSS satellite was renamed by Space Imaging to "Ikonos-1," supposedly a variant of the Greek word `eikon' (icon), meaning "image." Note: Space Imaging acquired EOSAT (a joint venture of Lockheed Martin and Hughes Aircraft) in 1995. The new company was subsequently renamed into: Space Imaging EOSAT. Eventually, it became simply: Space Imaging Inc. ^{1) 2)}

• Launch: A launch of Ikonos-1 took place on April 27, 1999 from VAFB aboard an Athena 2 launcher. Unfortunately, the rocket's nose cone failed to separate as planned at 4 minutes, 27 seconds into the flight - resulting in a complete loss of the satellite. With the protective shroud still attached, the rocket's upper stage and satellite did not have enough speed to reach a stable orbit around Earth. The vehicle then reentered the atmosphere over the South Pacific Ocean.

• Launch of Ikonos-2: The spacecraft was launched successfully on Sept. 24, 1999 from VAFB, CA aboard an Athena 2 launcher of Lockheed Martin. Space Imaging began to sell Ikonos-2 imagery on January 1 2000.

• In 2000, Space Imaging was awarded one of the most coveted prizes for technological achievement in the space industry - "The Industry Innovation Award in the Technology Category" - from the Society of Space Professionals International, due to the excellent performance of Ikonos-2.

• As of Sept. 2005, Lockheed Martin and Raytheon, the parent companies of Space Imaging, have agreed to sell Space Imaging to OrbImage Inc. of Dulles, VA.

• As of Jan. 12, 2006, the commercial imaging company GeoEye was established (merger completed), made up of former Orbimage of Dulles VA, and of Space Imaging of Thornton, CO (Orbimage acquired Space Imaging and gave the merged company the new name of GeoEye). The new company has HQs in Dulles, VA. - Hence, the Ikonos-2 spacecraft is now owned and operated by GeoEye (the company operates in 2007 also the imaging missions OrbView-2 and OrbView-3, with the GeoEye-1 launch scheduled for fall 2007). ^{3) 4)}

Spacecraft:

Ikonos-2 is a 3-axis stabilized spacecraft, using the LM900 satellite bus system (also referred to as Block-1). The LM900 spacecraft design was based on the CRSS imaging bus. The attitude is measured by two star trackers and a sun sensor and controlled by four reaction wheels (actuators); location knowledge is provided by a GPS receiver. The spacecraft provides precision pointing on an ultra-stable highly agile platform. The spacecraft features a body-pointing technique permitting a field of regard (FOR) of ±30º into any direction. This provides excellent observation capabilities. The design life is 7 years; S/C body size=1.83 m x 1.57 m (hexagonal configuration); S/C mass = 817 kg; power = 1.5 kW provided by 3 solar panels.

Figure 1: Photo at integration of the Ikonos-2 spacecraft at LMMS, Colorado Springs, CO (image credit: NASA) ⁵⁾

Figure 2: The Ikonos spacecraft (image credit: Space Imaging Inc.)

Launch: Ikonos-2, (identical to Ikonos-1 and built in parallel to Ikonos-1), was launched successfully on Sept. 24, 1999 from VAFB, CA aboard an Athena 2 launcher of Lockheed Martin. On Oct. 12, 1999, Space Imaging released the world's first high-resolution commercial satellite image of the Earth - a 1 m resolution black-and-white image of Washington, D.C. (see Figure 4).

RF communications: The downlink of all imaging data is in X-band (8345.968-8346.032 MHz) at a rate of 320 Mbit/s to dedicated ground stations located around the world (on-board data recording capacity is 64 Gbit in solid-state memory). The TT&C function is provided in S-band (2025-2110 MHz, uplink of tasking and command data at 2 kbit/s, downlink of housekeeping data and metadata at 32 kbit/s). ⁶⁾

The spacecraft operations of Ikonos-2 is unique among the current commercial imaging satellites in that it allows each international affiliate to operate its own ground station(s). These ground stations are assigned blocks of time on the satellite during which they can directly task Ikonos, and immediately receive the downlinked imagery for which they tasked. In addition to virtually instant data receipt, this allows each affiliate to make the best use of local weather data. However, this capability is only available when the ground station is in contact with Ikonos-2.

Figure 3: Artist's conception of the Ikonos-2 Block-1 spacecraft (image credit: Space Imaging Inc.)

Orbit: Sun-synchronous near-polar circular orbit, altitude = 681, inclination = 98.1º, period = 98 min, repeat cycle = 14 days (max), revisit cycle = 1-3 days (for observations at 40º latitude or higher). The local equator crossing time is at 10:30 AM on the descending node.

Mission status: Ikonos-2 is operating nominally as of 2007 - and there are no signs of data deterioration or component failure. Current estimates of GeoEye are that Ikonos-2 will last until 2008 (and possibly longer).

• The company GeoEye distributes Ikonos-2 imagery under the trade name of CARTERRA (same name as was used by Space Imaging).

• The Ikonos-2 spacecraft became operational in December 1999.

Sensor complement:

OSA (Optical Sensor Assembly), designed and custom-built by Kodak Co. of Rochester, NY (Space Imaging owns the design of OSA). The instrument features a Cassegrain-type telescope with a 70 cm diameter primary mirror, a 10 m focal length (folded optics design). The OTA (Optical Telescope Assembly) captures imagery across a swath of 11-13 km, it uses five mirrors to reflect the imagery to the imaging sensor arrays at the back end of the telescope. Three of the mirrors are powered (curved), and are of TMA (Three Mirror Anastigmatic) design. Note: TMA refers to lenses that are able to form approximately point images of target (object) points. The other two mirrors are flat, and serve to `fold' or bounce the imagery across the width of the telescope.

Pushbroom detector technology (a large focal plane detector array, generation of 6500 lines/s of panchromatic image data) is employed. Simultaneous imaging in panchromatic and multispectral modes is provided. The pixel size on the detector array is 12 µm for the panchromatic (PAN), and 48 µm for the multispectral (MS) detectors. The MS bands correspond to those of TM on Landsat in the visible range of the spectrum. The instrument light level is governed by a 70 cm aperture and a choice of 10, 13, 18, 24, or 32 TDI (Time Delay Integration) stages for panchromatic (gray-scale) imaging. The detector array offers a cumulative exposure concept for panchromatic imaging.

On-board electronics provide low-loss data compression of the original 11-bit data using ADPCM (Adaptive Differential Pulse Code Modulation). - The OSA instrument design features lightweight materials and advanced manufacturing techniques. The mass of the primary mirror was reduced by cutting a honeycomb pattern into its core using abrasive waterjet technology, and fusing thin mirror plates to each face.

Table 1: OSA instrument layout

A body-pointing technique with antenna gimbals and reaction wheels is employed for instrument pointing (the entire S/C is pointed into the desired direction), permitting a field of regard of ±30º into any direction. The angular slew rate is sufficient to perform both wide-area monoscopic and same-pass stereo collections. The location knowledge accuracy of the imagery is 2 m horizontal (relative) i.e. with ground control points, and 12 m (absolute), i.e. without the use of ground control points. Smooth scanning is provided with accurate gyros, low disturbance torques (smooth antenna gimbals and reaction wheels), and a rigid high-frequency structure of the satellite.

The S/C may also be rotated about its imaging axis for proper (broadside) detector array orientation. This technique permits, for instance, the full-swath imaging of a particular feature of interest on the Earth's surface, such as a coastline, which traverses under some angle through the in-track direction.

Table 2: Some performance parameters of the OSA instrument

Instrument calibration: The agile pointing capability of Ikonos is being utilized for instrument calibration. Solar, lunar and stellar scenes serve as radiometric instrument calibration sources. The ecliptic portion of the orbit is being used for stellar calibration. Absolute calibration of the Ikonos sensors is performed by comparing the total digital numbers found in the stellar image, to the absolute in-band spectral radiance of several radiometrically characterized stars. The radiometric calibration provides relative and absolute corrections for detector channel responsivity differences. ⁷⁾

Since launch, the Ikonos-2 satellite has undergone a series of geometric calibrations. The Ikonos geometric sensor model includes the interior orientation parameters, i.e. the Field Angle Map (FAM), and the elements of the exterior orientation parameter set, namely the interlock angles. The initial values of the interlock angles and the FAM parameters were determined by pre-launch measurements. They were subsequently refined by a series of on-orbit geometric calibrations.

Ground image data processing provides geocoding along with image compensation algorithms [misregistration, image motion, radiometric correction, MTF (modulation transfer function) compensation, etc.]. Space Imaging Inc. introduced a global archive (of digital imagery and services) under the trade name CARTERRA, which in turn is made up of regional archives, operated by regional partners. A great variety of image products and services are provided. Standard products are:

• Radiometrically corrected images

• Geometrically corrected images

• Orthorectified images and mosaics

• Digital terrain model (DTM) data

• Multispectral images

Figure 4: First Ikonos image of Washington D. C. with the Washington Monument (image credit: Space Imaging Inc.)

Kodak Model 1000 Camera System

As of July 1999, Kodak/C&GS (Commercial & Government Systems) is offering a "Model 1000TM camera system" of OSA camera heritage as an off-the-shelf product - at a 30% discount, deliverable within 24 months of order placement. This Model 1000 system design is owned by Kodak, containing some design changes with respect to OSA (reduced telescope aperture and instrument mass to fit onto minisatellites). ^{8) 9) 10)}

The Model 1000 camera system consists of the following elements: OTU (Optical Telescope Unit), FPU (Focal Plane Unit), DPU (Digital Processing Unit), PSU (Power Supply Unit), and CU (Cabling Unit). The total mass of the system is <100 kg.

• The OTU is an all-reflective three mirror anastigmatic design with two flat fold mirrors to decrease package volume (Korsch TMA telescope design). The optical components are made from high quality, low thermal expansion glass substrates. The metering and mounting structures are made from low thermal expansion materials to match the expansion properties of the glass components. OTU has a mass of 45 kg, the power consumption is 15 W.

• The FPU includes the PAN and MS detectors and A/D converters. Timing and command signals are received from the DPU, power is received from the PSU. The mass of FPU is 16 kg, power = 85 W.

• The DPU generates the timing for the sensor electronics via a master clock. DPU accepts S/C commands over a standard 1553 bus and routes the information to the FPU and PSU. The DPU compresses the 11 bit digitized image data to about 2.5 bits/pixel using the Kodak proprietary algorithms of ADPCM (Advanced Differential Pulse Code Modulation). The DPU can format data for interface with an on-board storage unit and data downlink. DPU mass = 14 kg, power = 130 W.

• The PSU filters, regulates and generates the unregulated S/C power to the DPU and PSU. Mass = 8 kg, power 75 W. There is full redundancy.

• The CU provides the cabling between the various electronic boxes. Mass = 5kg, power = 10W.

Table 3: Specification of the Model 1000 camera system

Figure 5: Illustration of Kodak's Model 1000 camera system

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

3) M. O'Connell, "GeoEye Overview GIS Capabilities for Civil Government Agencies," 2006, URL: http://www.nesdis.noaa.gov/space/library/workshops/2006-10-19/oconnell.ppt

4) http://www.geoeye.com/corporate/constellation.htm#IKONOS

5) http://rst.gsfc.nasa.gov/Intro/Part2_26f.html

6) W. Martin, "Satellite image collection optimization," Optical Engineering, Vol. 41, No 9, Sept. 2001, pp.2083-2087

7) H. S. Bowen, "Absolute Radiometric Calibration of the Ikonos Sensor Using Radiometrically Characterized Stellar Sources," Pecora 15/Land Satellite Information IV Conference, ISPRS Commission I Mid-term Symposium/FIEOS (Future Intelligent Earth Observing Satellites), Nov. 10-14, 2002, Denver, CO

8) Information provided by Michael J. Richardson of Eastman Kodak Company, Rochester, NY

9) "Kodak Introduces 1-Meter-Resolution Remote Sensing Camera In An Off-The-Shelf, Fixed Price Configuration," Kodak press release of July 19, 1999

10) T. Delaney, "Satellite Imagery in Land Development Applications," EOM, Oct. 1999, pp. 47-48