Visualisation technical High resolution satellite imaging systems – an overview by Dr.-Ing Karsten Jacobsen, Hannover University, Germany
More and more high and very high resolution optical space sensors are becoming available. Synthetic Aperture Radar (SAR) sensors with ground sampling distance (GSD) of up to 1 m are being announced for the near future. The various systems are well not always well known, and this paper discusses the systems available for topographic mapping.
ith the higher resolution Radar has the advantage of penetrating TDI-sensors and unrestricted access to clouds, so mapping is possible also in The optical space sensors are located Wimages taken by satellites, a rain-forest areas. competition between aerial images and in a flying altitude corresponding to a Within a few years there will also be an space data exists, starting for a map speed of approximately 7 km/s for the scale 1:5000. alternative available between satellite nadir point. So for a GSD of 1 m, only images and aerial images, coming from 1,4 ms exposure time is available. This Based on experience, optical images high altitude long endurance (HALE) is a not sufficient integration time for should have a ground sampling unmanned aerial vehicles (UAV) with an the generation of an acceptable image distance (GSD) of approximately operating altitude in the range of quality, and for this reason, some of 0,05 mm up to 0,1 mm in the map 20 km. the very high resolution space sensors scale, corresponding to a map scale of are equipped with time delay and Images are however not accessible 1:20000 up to 1:10000 for a GSD of integration (TDI) sensors. The TDI- from all systems - while they may not 1 m. GSD is the distance of the centre sensors used in space are CCD-arrays be classified, sometimes no distribution of neighbouring pixels projected on with small dimensions in the flight channels exist and it is difficult to order the ground. Because of over- or under- direction. The charge generated by the images. sampling, the GSD is not identical to the energy reflected from the ground the projected size of a pixel, but for the Details of imaging sensors is shifted with the speed of the image user, the GSD appears as pixel size on motion to the next CCD-element, and the ground. An over- or under-sampling View direction more charge can be added to the only influences the image contrast, charge collected by the first CCD- The first imaging satellites had a which may also be caused by the element. So a larger charge is summed fixed view direction in relation to the atmosphere. up over several CCD-elements. There orbit. Only by panoramic cameras, are some limits due to inclined view Mapping today involves data acquisition scanning from one side to the other, directions and vibrations, so in most for geo-information systems (GIS). In was the swath width enlarged. For cases the energy is summed up over a GIS, the positions are available with stereoscopic coverage, a combination 13 CCD-elements. IKONOS, QuickBird their national coordinates, so by simple of cameras with different longitudinal and OrbView-3 are equipped with TDI- theory, a GIS is independent of the view directions was used, as in the sensors while EROS-A and the Indian map scale, but the information content CORONA 4 serious and later the MOMS, TES do not have these, and they have corresponds to a publishing scale. In no ASTER, SPOT 5 HRS and Cartosat-1 to increase the integration time by case is the full information available in systems. With SPOT, the change of the permanent rotation of the satellite a GIS - for a large presentation scale view direction across the orbit came the generalisation starts with the size of through a steerable mirror. IRS-1C and building extensions which are included, -1D have the possibility of rotating while for small scales the full effect of the whole panchromatic camera in generalisation is required. So for large relation to the satellite. This requires presentation scales, more details have fuel, and so it has not been used very to be identified in the images, while often. IKONOS, launched in 1999, for smaller scales a larger GSD may was the first civilian reconnaissance be sufficient. If the GSD exceeds 5 m, satellite, with flexible view direction. not all details, usually shown in the Such satellites are equipped with high corresponding publishing scale, can be torque reaction wheels for all axes. If identified. these reaction wheels are slowed down Not only do optical images have to be or accelerated, a moment will go to taken into account, because in the near the satellite and it rotates. No fuel is Fig. 1: Increase of integration time with future high resolution synthetic aperture required for this, only electric energy factor b/a by continuous change of view radar (SAR) images will be available. from the solar panels. direction.
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have a radiometric resolution up to 11 bit, corresponding to 2048 different grey values. Usually there is not a good distribution of grey values over the whole histogram, so the important part can be optimised for presentation with the 8 bit grey values of a computer screen. The Fig. 2: Arrangement of CCD-lines in focal Fig. 3: Staggered CCD-lines. higher radiometric resolution includes plane. Above: panchromatic; below: multi-spectral. the advantage of an optimal use of the grey values in extreme cases such as bright roofs alongside a shadow. during imaging (see Fig. 1). Also, physical pixel size projected to the Also, for 11 bit-sensors, there are QuickBird uses this technique because ground is 5 m, and based on staggered some limits. If sunlight is reflected the sensor originally was planned for CCD-lines, the supermode has 2,5 m by a glass roof directly to the sensor, the same flying altitude as IKONOS, GSD. In theory this corresponds to the over-saturation will occur and the but with the allowance of a smaller information contents of an image with generated electrons will flow to the GSD, the flying height was reduced, 3 m GSD. neighbouring CCD-elements, and the resulting in a smaller pixel size. The Multi spectral information read-out will be influenced over a short sampling rate of 6900 lines/s could time. The over-saturation (Fig. 4) does not be increased, and this has to be Sensors usable for topographic mapping not cause problems, but the human compensated by change of the view are sensitive at the visible and near operator should know about it to avoid direction during imaging, but with a infrared (NIR) spectral range. The blue a misinterpretation of the objects. significantly smaller factor compared to range with a wavelength of 420 - 520 nm is not used by all sensors because EROS-A and TES. Direct sensor orientation of the higher atmospheric scatter effect, CCD-configuration which reduces the contrast. In most The satellites are equipped with a cases the multi-spectral information positioning system such as GPS, Most of the sensors do not have just is collected with a larger GSD like the gyroscopes and star sensors. So one CCD-line but a combination of panchromatic. With so-called pan- without control points, the geo-location shorter CCD-lines or small CCD-arrays. sharpening, the lower resolution multi- can be determined. For example The CCD lines are shifted with respect spectral information can be merged to each other - see Fig. 2. IKONOS can determine the imaged with the higher resolution panchromatic positions with a standard deviation of The merging of sub-images achieved to achieve a higher resolution colour approximately 4 m. Often, with national by the panchromatic CCD-lines belongs image. This pan-sharpening uses the datum that are not well known, further characteristic of the human eye which to the inner orientation, and the user problems exist. notice it. Usually the matching accuracy is more sensitive to grey values than of the corresponding sub-images is in to colour. A linear relation of 4 between Imaging satellites the lower sub-pixel range, so that the panchromatic and colour GSD is common. Imaging satellites were first used geometry of the mosaiced image does for military reconnaissance. So, 20 not show any influence. This may be The panchromatic range does not months after the launch of SPUTNIK different for the larger offset of the correspond to the original definition in October 1957, the US tests with the colour CCD-lines. Stationary objects - the visible spectral range. Often CORONA system started in 1959. For are fused without any problems during the blue range is cut off and the NIR reconnaissance, the USA used film up the pan-sharpening process. In theory, is added to the spectral range of to 1963, while the Soviet Union and only in extreme mountainous areas approximately 500 to 900 nm. later Russia made the last satellite can unimportant effects be seen. This photo flight in 2000. The historical is different for moving objects - the Imaging problems images were declassified by the USA in time delay of the colour against the Modern CCD-sensors used in space panchromatic image causes different locations in the intensity and the colour. The different colour bands are following the intensity. This effect is unimportant for mapping, because only stationary objects are used.
Staggered CCD-lines
The ground resolution can be improved by staggered CCD-lines (Fig. 3). Two CCD-lines are used, shifted by half a pixel with respect to each other, so more detail can be seen in the generated images. Because of the over- sampling, the information content does not correspond to the linear doubling Fig. 4: Over-saturation. Left: ASTER; Right: IKONOS. if the information. For SPOT 5 the
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1995, and Russia is also now selling the System launch GSD [m] swath [km] remarks old images. pan/MS o For civilian or dual use (i.e. use for SPOT 1 France 1986 10/20 60 +/-27 across orbit SPOT 2 1990 military and civilian applications), SPOT 3 1993 digital imaging from space started with SPOT 4 1998 Landsat 1 in 1972, but the GSD was SPOT 5 France 2002 5/10 60 +/-27o staggered 2.5 120 not sufficient for mapping purposes. o fore HRS 5*10 +/-23 This changed with the French SPOT +/-23o after satellite, first launched in 1986. With JERS-1 Japan 1992 OPS 18 75 + SAR the stereoscopic possibilities and MOMS 02 Germany 1993 4.5/13.5 37/78 nadir + 21.5o fore 10 m GSD, this system was used + 21.5o after for the generation and updating MOMS-2P Germany 1996 6/18 48/100 like MOMS 02 of topographic maps up to a scale IRS-1C + 1D India 1995 5.7/23 70/142 + / - 26o across 1:50 000 but not with the same 1997 information content of traditional maps IRS P6 India resoiurcesat 2003 5.7 MS 23/70 + / - 26o across in this scale. This has been improved KOMPSAT-1 South Korea 1999 6.6 pan 17 + / - 45o across with the Indian IRS-1 C in 1995 having CBERS-1 + 2 1999 20 113 + / - 31o across a GSD of 5,7 m. The next big step China + Brazil 2003 came with the very high resolution Terra USA / 1999 15, 30, 90 60 nadir + 24o aft IKONOS in 1999. Today there is a ASTER Japan all MS large variety of imaging sensors in IKONOS-2 USA SpaceImage 1999 0.82/3.24 11 free view direction, TDI space, including more economical small EROS A1 Israel Imagesat 2000 1.8 pan 12.6 free view direction satellite systems operated by a growing TES India 2001 1 pan 15 free view direction number of countries. QuickBird-2 USA DigitalGlobe 2002 0.62/2.48 17 free view direction, TDI
With the advent of synthetic aperture OrbView-2 USA OrbImage 2003 1/4 8 free view dir., TDI radar (SAR), imaging radar systems are FORMOSAT-2 Taiwan 2004 2/8 24 free view dir., TDI available which are independent of the IRS-P5 Cartosat-1 India 2005 2.5 pan 30 -5o, +26o in orbit cloud coverage. Because of the speckle and quite different imaging conditions, Table 1: Larger optical space sensors. SAR images cannot be compared directly with optical images having the there is no more justification for such a seconds. A similar solution is also same GSD. So up to now SAR has only classification, but some governmental available for ASTER and Cartosat 1. The been used in tropical rainforest areas organisations like to maintain their use of three view directions, has been for the generation of topographic maps, "importance" by such restrictions. used by MOMS before. but this may change with the advent of Space images can be used by private The very high resolution IKONOS, very high resolution SAR satellites. companies for the generation of the QuickBird, OrbView and EROS A1 different photogrammetric products, Several space missions failed, especially systems are operated by private even when some countries still try to in the beginning. Thus, of the 10 companies. However, without financial restrict it. launches of CORONA KH-1, only one support through military contracts, they was successful, while for the CORONA There is a general tendency in the would not survive, so in reality they KH-4A, only three of 52 missions failed. development of high resolution optical are of dual use. The use is dominated The highest ground resolution was space sensors: the resolution is by the military, but the free capacity possible with panoramic cameras like improving and the new systems have a is commercially available. There are the CORONA series without KH-5, and flexible view direction. By using reaction some restrictions - the images from US the similar Russian KVR-1000. Also, for wheels, the satellites can change companies are not released within 24 military reconnaissance, a stereoscopic attitude in a controlled manner, quickly hours of collection, and for EROS A1 coverage was important, so the and precisely enough to generate images, the military has priority of data CORONA 4-series was equipped with images, even during the rotation. This collection. two convergent mounted cameras. The has advantages over the change of view The very high resolution EROS A1 and Soviet Union preferred frame cameras direction across the orbit direction, in TES systems are not equipped with for getting the 3rd dimension – for that a stereoscopic coverage can be TDI-sensors, so they have to enlarge example they used the KVR-1000 in generated within some seconds, while the exposure time by continuous combination with the TK-350. for the view direction across the orbit, change of the view direction (see the second image has to be taken from Space images achieved a growing Fig. 1). This has only a limited influence market share in photogrammetry. They another orbit under optimal conditions to the radiometric and geometric image have been established as a complete the following day. If this is not possible, quality, but it reduces the imaging and partial replacement of aerial problems can be caused for automatic capacity. images. They can be used in remote image matching. SPOT Image reacted locations and “no-fly” zones. In several to this problem with an additional HRS- SPOT 5 with the super mode and countries, aerial images are classified, sensor on SPOTS, allowing viewing OrbView are improving the GSD and commercialisation is complicated or with two optics 23° forward and 23° by staggered linear CCDs. An edge impossible. Because of the availability backward, and providing stereoscopic analysis of both image types lead to of very high resolution space imagery, coverage within approximately 100 a GSD identical to nominal resolution.
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System Launch GSD [m] Swath Remarks Radar is an active system, independent pan/MS [km] of sunlight. However, for SAR images,
IRS Cartosat-2, India 2005 1 pan 10 free view direction the GSD of 10 m and larger is not sufficient for topographic mapping, ALOS, Japan 2005 2.5 / 10 35 / 70 24o nadir +24o in orbit and are thus only used for tropical COMPSAT 2 South Korea 2005 1 / 4 15 free view direction rain forests. But in interferometric Resurs DK1 Russia 2005 1 / 2.5-3.5 28 free view direction constellation (InSAR), SAR image Monitor-E Russia 2005 8 / 20 94 / 160 free view direction combinations can be used for the
EROS B Israel 2005 0.7 pan 14 free view dir., TDI generation of height models. So ERS- 1 and ERS-2 have been operated EROS C Israel 2009 0.7 / 2.8 11 free view dir., TDI for a period of approximately one RazakSat Malaysia 2009 2.5 / 5 20 free view direction, year in a tandem constellation for inclin. 7o DEM generation. The Shuttle Radar CBERS 2B China, Brazil 2005/2006 2.5 / 20 +/- 32o across Topography Mission (SRTM) has CBERS-3+4 China, Brazil 2008 5 / 20 60 / 120 +/- 32o across generated a homogenous and qualified WorldView 1 DigitalGlobe 2006 0.2 / free view dir., TDI DEM covering the earth from 56° OrbView 5 OrbImage 2006 0.41 / 1.64 15 free view dir., TDI southern latitude up to 60,25° northern latitude. THEOS Thailand 2007 2 / 15 free view dir., TDI Pleiades 1 + 2 France 2008/2009 0.7 / 2.8 20 free view dir., TDI A higher number of optical satellite KOMPSAT-3 South Korea 2009 0.7 / 2.8 free view dir., TDI systems have been announced (Tables 2 and 3). The proposed launch date Table 2: Announced larger optical space sensors is often delayed, and some systems may disappear, or the launch may fail. The specification of the systems may But the analysed images have been which are largely used only by the change, and in some cases they are edge enhanced like most of the space countries that own them. A strong not fixed or published yet. It is also no images, and this is leading to over- position in this field is held by Surrey longer so time consuming to assemble optimistic results. Satellite Technologies (SSTL). SSTL qualified reconnaissance satellites, so made the UOSAT12 and a group of Access to the images is well organised additional systems may be announced. satellites belonging to the disaster by commercial companies, and SPOT There are some general trends - the monitoring constellation (DMC). In Image and India are also using a GSD is getting smaller; weight is the case of natural disasters, the DMC network of commercial distributors. reducing; TDI will become standard; satellites are cooperating to generate For the FORMOSAT-2, SPOT Image got very high resolution SAR sensors will images from the affected area as fast the exclusive distribution rights. The come; and dual-use reduces expenses as possible. The satellite constellation ASTER images are available as web- and enables commercial use. Nearly all guarantees a daily coverage of the based images for a handling fee. Also, satellites will be equipped with reaction earth by images using the Landsat-ETM Japan has solved the distribution of the wheels, leading to high agility and bands 2, 3 and 4. SSTL is using off- less detailed JERS-1 image like German free view direction. ALOS is designed the-shelf components, and so today the images, via the DLR. For KOMPSAT and especially for 3D-mapping based on price for a satellite system including CBERS, distribution is more difficult, but three cameras having the view in orbit launch and ground station, may be in still possible. direction for the generation of stereo the range of US$10-million. The small models. Initially, the required technical satellites do have a free view direction. knowledge and the access to the They are partially equipped with CCD- Within the USA NextView program, required components limited the arrays instead of a CCD-line. contracts have been placed with manufacturing of imaging satellites to DigitalGlobe and OrbImage for Optical images can only be taken under just few countries. But today the major operating satellites with at least 50 cm cloud free conditions and with sufficient components, and also whole systems, GSD in the panchromatic range. The sunlight. So all systems listed in Table 1 can be ordered. The FORMOSAT-2 GSD for nadir view will be smaller, and the small satellites previously satellite was made by the European but the USA is restricting the GSD to mentioned, do have sun-synchronous EADS ASTRIUM. Similar cooperation orbits with imaging between 9h30 and at least 50 cm, so the commercially exists for smaller satellites. Launch 23h00 local (German) time - the day available images will be limited to this. is also not a problem, and there is time with the best viewing conditions. More and more countries are entering strong competition. Because of their This is different for radar satellites. the field of commercial very high lower price, Russia is dominating the resolution optical systems. A GSD of at launches, followed by the USA, China, Europe, the private Sea Launch, and
India. System Launch GSD [m] pan/MS Swath [km] Remarks
With the reduced size and weight DMC China 2005 4/32 600 DMC of electronic components, imaging TopSat UK BNSC 2005 2.5/5 10/15 free view dir., TDI satellites can be smaller today, leading to compact satellites with a weight X-Sat Singapore 2006 10 MS 50 below 200 kg. These systems are not RapidEye Germany 2007 6.5 MS 78 free view direction, included in Table 1 because of the commercial 5 satell. limited access to the collected images, Table 3: Announced high resolution small optical space sensors.
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least 1 m will be supported by the USA, System Launch GSD [m] Swath Remarks India, Israel, France, South Korea and pan/MS [km] Russia. In addition, for GSD of up to SAR-X Cosmo-Skymed-1, Italy 2006 1-several 10-few X-band 2,5 m, Malaysia, China, Brazil, Thailand 110th hundreds 3.1 cm and the UK are involved, and military RADARSAT-2, Canada 2006 3-100 20-500 C-band 5.6cm, full systems are not involved here. polarisation TerraSAR-X Germany ppp 2006 1/3/16 10/30/100 X-band 3.1 cm Again, the small satellites are listed RISAT India 2006 3-50 10-240 C-band separately because distribution channels are often missing. Most of the small Surveyor SAR, China 2007 10/25 100/250 C-band 5 satelites optical satellites are assembled by SSTL and also Rapid Eye. Rapid Eye will be a Table 4: Announced SAR space sensors; ppp = private publuc partnership system of five small satellites, mainly for use by high tech agriculture. It will be the first commercial system outside the area of dual-use. flight of its HALE UAV Pegasus for 2006. technology. More and more off-the- The solar powered Pegasus is designed shelf components can be used, and With SAR-X, Cosmo-Skymed-1 and for continuous operation over several in addition, satellites can be ordered Terra SAR-X are two radar systems months (Biesemann et al. 2005). It from different manufacturers. In the with a GSD of 1 m that have been will operate at a height of 20 km, and Tables given, a total of 22 countries are announced. SAR images with such a overnight will go down to 16 km, and mentioned. resolution can be used for mapping then rise again the next morning. This purposes. The information contents With the improving ground sampling altitude is above aeronautic control, of the 1 m SAR-images may be in the distance, space images are coming into avoiding safety problems. In a partially range of optical images with 2 m GSD. autonomous flight it can be directed competition with aerial images, and SAR-images do have the big advantage to the area for imaging. It will carry a because of the classification of aerial of being independent of cloud coverage digital camera with four spectral bands photos in some countries, space images - this is important for regions having and 12000 pixels, and have a GSD of have become even more important only few cloud free days. In addition, 20 cm. This will later be extended to 10 than in countries without restrictions. with RADARSAT-2 and RISAT, two spectral bands and 30 000 pixels, and In the near future, with the HALE UAV systems with a GSD of 3 m will come. SAR and LIDAR may be included. there will be a stronger overlap of Terra SAR-X will be operated under applications. Very high resolution space Conclusion a private public partnership between images are not only a supplement or competition to aerial photos, they will the German Aerospace Center DLR The conditions for mapping with also be used for new applications. and EADS ASTRIUM. SAR-X Cosmo- high and very high space images are Skymed-1 will be operated by Italy improved permanently. More and more Acknowledgement in cooperation with France within the sensors are entering the field, leading dual use ORFEO program. Under this to better coverage and the possibility This paper was published in PFG, cooperation, France will operate two of selecting the optimal solution. journal of the Deutchen Gesellschaft very high resolution optical Pleiades The image databases are becoming für Photogrammetrie, Fernkunding systems, and Italy four SAR-systems. more and more complete, allowing und Geoinformation (DGPF), Issue 6, 2005, pp 487 – 496, and is The Tandem X project is under fast access corresponding to the republished with kind permission. PFG investigation which will include a second needs. Competition between different is published by E. Schweizerbart’sche TerraSARX in tandem configuration distributors has improved the order Verlagsbuchhandlung (Nägele u. for the generation of digital elevation conditions, and in part is leading to Obermiller), Science Publishers, models with better than 12 m DEM reduced prices. On the other hand, the point spacing, and a vertical accuracy of high expenses for the large systems Stuttgart, Germany. 2 m and better. For the SAR-X Cosmo- cannot be covered by civilian projects. References Skymed-1, the so-called Cartwheel Without dual-use, private companies [1] Baudoin, A, 2004: Beyond SPOT5: is being studied, and it could include in this field cannot survive. This may Pleiades, Part of the French-Italian one active SAR-satellite together with change with the increasing capacity of Program ORFEO. - ISPRS Istanbul 3 to 4 passive micro-satellites for the small satellites. IntArchPhRS, Vol XXXV, B1:260-267. generation of DEMs with a standard In addition to optical images, SAR [2] Biesemann, J, Everaerts, J. & deviation of 1 m. also has to be taken into account. Lewychyj, N., 2005: Pegasus: Remoe Sensing from a Haleuav, ASPRS With the very high resolution of the Hale UAV annual convention Baltimore, 2005. announced systems, SAR is becoming - on CD. High altitude long endurance (HALE) important for mapping purposes. With unmanned aerial vehicles (UAV) may [3] Mcdonald (editor), 1997: Corona InSAR, accurate digital surface models Between the Sun & the Earth. - 440 provide competition to space and aerial can be generated. Only in cities and pp., ASPRS, Bethesda, Md, USA. images. Up to now UAVs have mainly mountainous areas do InSAR and SAR been used for military reconnaissance, Contact Dr.-Ing Karsten Jacobsen, still have some problems. but it seems these will also lead to Hannover University, civilian applications. The Belgian The operation of high resolution [email protected] Flemish Institute for Technological satellites is no longer restricted to Research, VITO, plans the first test only a few countries with advanced
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