3.3 Sensors
3.3.2 Basic principles of different types of satellite and airborne
Classical photogrammetry uses aerial photography with a central perspective geometry (Figure 3.4) for the acquisition of three dimensional information for cartographic purposes.
Chapter 3. Data Acquisition
Lens
A
1
Figure 3.4 Frame-type image of a photographic or video camera
The central projection is important because it is the basis of the image formation of a frame in many sensors. Space cameras using this type of geometry include the KFA 1000, Metric Camera Zeiss, KATE 200, Large Format Camera Itek. Table 3.1 gives some details of the characteristics of these cameras. Research has been carried out concerning the suitability of these satellite sensors for mapping. Jacobsen (1986) assessed the accuracy of the bundle block adjustment with the Metric Camera (MC) in Spacelab and the Large Format Camera (LFC). Togliatti (1988) reviewed the investigations performed with LFC images and concludes that they have proved to be of unmatched spatial resolution and geometric stability. The geometry of stereo mapping photographs, whether taken from aircraft or satellite is well known and widely documented.
The satellite offers the unique advantages over aircraft of much greater stability and uniform velocity (Colvocoresses, 1982). The vast majority of data provided by satellite sensors is in digital form. Most often the sensors are mechanical-rotational scanners (Figure 3.5) (Landsat MSS, Thematic Mapper, Meteosat) or linear arrays (Figure 3.6) (SPOT, MOMS). Table 3.2 lists the characteristics of some satellite sensors.
Table 3.1 Space camera systems (modified from Torlegard(1991) and Dowman (1985)) S a te llite S en so r H eig h t (K m ) F o ca l len g th F orm at (m m ) Ground r e so lu tio n (m ) S ter eo
Cosmos 1 KFA-1000 200 1000 300x300 5-10 Yes
Cosmos 1 Kate 200 200 200 180x180 15-30 " Cosmos 2 M K ^ 180-450 300 180x180 5-8 " Manned Orbit Station Kate 140 300 140 180x180 50 " Manned Orbit Station MKF-6 300 125 56x81 20-50 " ESA Spacelab STS 17 MC Zeiss RMK 30/23 250 305 240x230 30 " Shuttle STS14 / STS 17 LFC-Itek 300 305 240x480 10 " Cylindrical Image Surface Planar Image Surface
Figure 3.5 Continuous strip images from an optical-mechanical scanner
Figure 3.6 Continuous strip image from a linear pusbroom scanner
Chapter 3. Data Acquisition
Table 3.2 General features of the Lands at, SPOT and MOMS02/PRIRODA
programmes (after UCL, 1995)
Landstat
programme SPOT program m e MOMS program me No. of spacecraft in
se r ie s 2 3 1
D esign lifetim e 3 years 3 years 2 years
Launch date 4; 16 July 1982
5: 1 March 1984
1: 22 Feb. 1986 2: 21 Jan. 1990 3: 26 Sep. 1993
Nov 1995
Country / Operator USA/EOSAT-NOAA ESA/Spot Image Russia / Germany
Prime contrator GE Astro MMS KB Salyut
Spacecraft launch
m ass 1941 Kg 1900 Kg 20 000 Kg
Body dim ensions 4 m (L) 2 X 2 X 3.5m
S tab ilisation type 3-axis 3-axis 3-axis
O rbit Heliosynchronous: 805Km; 98.2^; 99mins Heliosy nchronous : 832Km; 98.?0; lOlmins EO: 360-400Km; 91.60 lOmins
R e-visit interval 16 days (equator) 3.7 days 3-17 days
The Landsat programme comprises two imaging instruments, the Multispectral Scanner System (MSS) and the Thematic Mapper (TM). The former acquires information for mapping coast features, roads and urban areas and for vegetation studies. TM has seven bands so that more detailed information such as thermal mapping, plant species differentiation, snow/cloud differentiation and biomass surveys can be provided.
The SPOT programme mission is based on the two imaging instruments High Resolution Visible Detectors (HRV) using pusbroom scanning. The pushbroom sensor consists of a CCD linear array of detectors in the focal plane of the sensor oriented perpendicular to the flight path. The linear array camera records the terrain along the orbit path as a series of successive lines or strips that are a function of the spacecraft velocity and time (Welch, 1980). The precise geometric position and high sensitivity of the detectors of these sensors together with the fact that no moving optics are used constitute the main advantages of these scanning sensors (Thompson, 1979). This type of imagery has been intensively studied in order to obtain useful geometric information, 1:50 000 maps have been created using SPOT data.
The Modular Opto-electronic Multispectral Stereo Scanner (MOMS) programme started in the mid-70s. The first generation (MOMSOl) flew twice (1983/1984)
providing about 1000 image scenes for exploitation. In 1993 the MOMS02 comprised a new concept of a combined stereo/multispectral sensor and was oriented towards present photogrammetric and thematic applications. The main characteristics of this sensor include: three-line stereo capability, along-track stereo capability, high resolution imagery and the simultaneous recording of multispectral and stereo images.
There is increasing interest in the use of radar sensors for mapping at larger scales. The main reasons for this include their all-weather capabilities and the possibility of using various polarisations for interpretation (Torlegard, 1991). Airborne radar systems make use of an antenna fixed below the aircraft and pointed to the side. These systems are called Side Looking Radar (SLR) or Side Looking Airborne Radar (SLAR) and produce continuous strips of imagery representing very large ground areas located adjacent to the aircraft flight line. SLAR was first developed for military reconnaissance in the early 1950s while radar remote sensing from space only began with the launch of Seas at in 1978 (Lillesand and Kieffer, 1994). The operating and geometric considerations for SLAR systems also apply to radar systems from space. However, because only synthetic aperture radar systems are used for spaceborne radar the term SAR (Synthetic Aperture Radar) is used to refer to side looking radar spaceborne systems. SAR imagery geometry is fundamentally different from both photography and scanner imagery geometry, the difference being that SAR is a distance rather than an angle measuring system. The basic operating principle of a SAR system is shown in Figure 3.7. The radar image depends on the reflective properties of the area sensed and it is independent of natural lighting conditions.
orbit sensor
Figure 3.7 Geometry corresponding to a side-looking imaging radar, where R and S are the antenna footprint dimensions
Chapter 3. Data Acquisition
Besides geometric correction of images for registration with existing maps in projects concerning radar imagery interpretation, radar is not used much for mapping. (Dowman, 1984). Radar systems have been extensively used in fields such as geological interpretation, forestry and land use (Trevett, 1986). Radar imaging sensors are included into several recent programs such as the Earth Remote Sensing Satellite (ERS), the Japanese Earth Resources Satellite (JERS-1) and the Canadian Radar Satellite (RADARSAT). Table 3.3 lists some general features of these three programs.
Table 3.3 General features of the ERS, JERS-1 and RAD ATS AT programmes (after UCL, 1995)
ERS program me JERS-1 program me R A D A R S A Tprogram m e No. o f spacecraft in
se r ie s 2 1 2
D esign lifetim e 3 years 2 years 5 years
Launch date and v e h ic le s
1: 17 July 1991 2: early 1995
11 Feb 1992 (H-1) liSept. 1995
2: not yet defined
Country / Operator ESA Japan/NASDA-MITI Canadian Space Agency / Radarsat International
Prim e contrator Domier Mitsubishi Spar Aerospace
Spacecraft launch
m ass 2400 Kg 1400 Kg 3155 Kg
Body dim ensions
10 X Im SAR antenna
X 3.5m solar array; 11.9
X 2.4m SAR
1.6 X 15m SAR
antenna
S tab ilisation type 3-axis 3-axis 3-axis
O rbit Heliosynchronous; SOOKm; 98.50; lOOmins Heliosynchronous: 568Km; 98.5^ Heliosynchronous: 792Km; 98.5^; lOlmins
R e-visit interval 3, 17, 35, 176 days 44 days 17 days
Until now the ERS-1 has been very successful, more specifically image to image interferometry using the SAR can provide DEMs of 30m resolution and an accuracy of 5-10m in height. The ERS-1 satellite was launched in July to observe the oceans for scientific purposes. The JERS-1 SAR sensor is for high resolution monitoring of land use and type, glacier extent, snow cover, ocean current and waves. The RADARS AT collects data on ice-covered regions of Canada, oceans and rain forests.
The geometric characteristics of video frame scanners are substantially different from optical based sensing devices such as photographic cameras, optical-mechanical line scanners, linear array (pushbroom) scanners and video or CCD areal array
cameras (Amin and Petrie, 1994). Video frame scanners scan the object in two directions simultaneously using a pair of mirrors operating at right angles to each other (Figure 3.8). This results in a discrete frame image with a spherical surface, where all points are shown as being equidistant from a single perspective centre. However, due to the platform motion effects that occur during the time taken to scan a complete frame this cannot be strictly true.
Image Surface
lens
Figure 3.8 Frame-type image from a video frame scanner
Still video cameras are able to take and store images without being connected to a computer, making them extremely mobile. They are suitable tools for close range applications. Kersten and Maas (1994) compared the Kodak DCS200 with a conventional film based small format camera Leica R5. Peipe and Schneider (1994) investigated the Canon ION RC560 and the Kodak DCS200 carrying out tests using photogrammetric camera calibration methods. Table 3.4 shows the technical details of the video cameras Kodak DCS200 and Canon ION RC560.
Table 3.4 Technical details of the video cameras Kodak DCS200 and Canon ION RC560 (adopted from Peipe and Schneider, 1994 and Kersten and Maas, 1994)
Kodak DCS200 Canon ION RC560 S en so r
P ixel size
760x552 Interline CCD sensor 1 1 x 1 1 urn
1524x1012 Full frame sensor 9 x 9 um
L en s (focal length)
8 - 4 mm zoom wide angle converter (5.2 mm)
Nikon lenses
e.g. 28mm (std. version) or 15mm
Internal memory 2” Mini floppy disk for 25 full frame or 50 half frames images 80 MByte hard disk for 50 images of 1.5 MByte S iz e 148 X 125 X 46.5 mm 170 X 114 X 208 mm
Chapter 3. Data Acquisition