Patent Application
20080063270
Generally, the present invention is directed to the determination of ground location information associated with at least one pixel of an image acquired by an imaging system aboard a satellite or other remote sensing platform. The process involved in producing the ground location information includes (a) obtaining one or more images (reference images) of areas covering points whose locations are precisely known, (b) predicting the locations of these points using time varying position, attitude, and distortion information available for the imaging system, (c) comparing the predicted locations with the known locations using a data fitting algorithm to derive one or more compensation factors, (d) interpolating or extrapolating the compensation factor(s) to other instants in time, and then (e) applying the compensation factor(s) to one or more other images (target images) of areas which are not covering points with the precisely known locations of the reference images. The process can be applied to a target image that does not overlap the reference image, and may also be applied to a target image that does overlap the reference image.
Having generally described the process for producing the image and ground location information, an embodiment of the process is described in greater detail. Referring to
As illustrated in
When the satellite 100 is acquiring images of the surface of the earth 104, the associated ground location of any particular image pixel(s) may be calculated based on information related to the state of the imaging system, including the position of the system, attitude of the system, and distortion information, as will be described in more detail below. The ground location may be calculated in terms of latitude, longitude, and elevation, or in terms of any other applicable coordinate system. It is often desirable to have knowledge of the location of one or more features associated with an image from such a satellite, and, furthermore, to have a relatively accurate knowledge of the location of each image pixel. Images collected from the satellite may be used in commercial and non-commercial applications. The number of applications for which an image 112 may be useful increases with higher resolution of the imaging system, and is further increased when the ground location of one or more pixels contained in the image 112 is known to higher accuracy.
Referring now to
The attitude measurement system 120 is used in determining attitude information for the imaging system 140. In one embodiment, the attitude measurement system 120 includes one or more gyroscopes that measure angular rate and one or more star trackers that obtain images of various celestial bodies. The location of the celestial bodies within the images obtained by the star trackers is used to determine the attitude of the imaging system 140. The star trackers, in an embodiment, are placed to provide roll, pitch, and yaw orientation information for a reference coordinate system fixed to the imaging system 140. Similarly as described above with respect to the position measurement system 116, the star trackers of the attitude measurement system operate to obtain images at periodic intervals. The attitude of the imaging system 140 can, and often does, change between these periodic intervals. For example, in one embodiment, the star trackers collect images at a rate of about 10 Hz, although the frequency may be increased or decreased. In this embodiment, the imaging system 140 operates to obtain images at line rates between 7 kHz and 24 kHz, although these frequencies may also be increased or decreased. In any event, the imaging system 140 generally operates at a higher rate than the star trackers, resulting in numerous ground image pixels being acquired between successive attitude measurements from the star trackers. The attitude of the imaging system 140 for time periods between successive images of the star trackers is determined using star tracker information along with additional information, such as angular rate information from the gyroscopes, to predict the attitude of the imaging system 140. The gyroscopes are used to detect the angular rates of the imaging system 140, with this information used to adjust the attitude information for the imaging system 140. The attitude measurement system 120, also has limitations on the accuracy of information provided, resulting in errors in the predicted attitude of the imaging system 140. While this error is generally small, it is often a relatively significant contributor to uncertainty in ground location associated with pixels in the ground image.
The thermal measurement system 124 is used in determining thermal characteristics of the imaging system 140. Thermal characteristics are used, in this embodiment, to compensate for thermal distortion in the imaging system 140. As is well understood, a source of error when determining ground location associated with an image collected by such a satellite-based imaging system 140 is distortion in the imaging system. Thermal variations monitored by the thermal measurement system 124 are used in this embodiment to compensate for distortions in the imaging system 140. Such thermal variations occur, for example, when the satellite 100, or portions of the satellite 100, move in or out of sunlight due to shadows cast by the earth or other portions of the satellite 100. The difference in energy received at the components of the imaging system 140 results in the components being heated, thereby resulting in distortion of the imaging system 140 and/or changes in the alignments between the imaging system 140 and the position and attitude measurement systems 116 and 120. Such energy changes may occur when, for example, a solar panel of the satellite 100 changes orientation relative to the satellite body and results in the imaging system components being subject to additional radiation from the sun. In addition to reflections from component parts of the satellite 100, and to the satellite 100 moving into and out of the earth's shadow, the reflected energy from the earth itself may cause thermal variations in the imaging system 140. For example, if the portion of the earth which is reflecting light to the imaging system 140 is particularly cloudy, more energy is received at the satellite 100 relative to the energy received over a non-cloudy area, thus resulting in additional thermal distortions. The thermal measurement system 124 monitors changing thermal characteristics, and this information is used to compensate for such thermal distortions. The thermal measurement system 124, has limitations on the accuracy of information provided, resulting in errors in the thermal compensation of the imaging system 140 of the satellite 100. While this error is generally relatively small, when used in determining the ground location of pixels within an image that includes a portion of the surface of the earth, this error also contributes to uncertainty in ground location.
In addition to thermal distortions from the imaging system 140, atmospheric distortions may also be present which increase the error of the imaging system 140. Such atmospheric distortions may be caused by a variety of sources within the atmosphere associated with the area being imaged, including heating, water vapor, pollutants, and a relatively high or low concentration of aerosols, to name a few. The image distortions resulting from these atmospheric distortions are a further component of error when determining ground location information associated with an area being imaged by the imaging system 140. Furthermore, in addition to the errors in position, attitude, and distortion information, the velocity at which the satellite 100 travels results in relativistic distortions in information received. In one embodiment, the satellite 100 travels at a velocity of about seven and one-half kilometers per second. At this velocity relativistic considerations, while relatively small, are nonetheless present and in one embodiment images collected at the satellite 100 are compensated to reflect such considerations. Although this compensation is performed to a relatively high degree of accuracy, some error still is present as a result of the relativistic changes. While this error is generally small, it is often a relatively significant contributor to uncertainty in ground location associated with pixels in the ground image.
The added error of the position measurement system 116, the attitude measurement system 120, thermal measurement system 124, atmospheric distortion, and relativistic changes result in ground location calculations having a degree of uncertainty which, in one embodiment, is about 20 meters. While this uncertainty is relatively small for typical satellite imaging systems, further reduction of this uncertainty would increase the utility of the ground images for a large number of users, and also enable the images to be used in a larger number of applications.
The transmit/receive circuitry 128 in this embodiment includes well known components for communications with the satellite 100 and ground stations and/or other satellites. The satellite 100 generally receives command information related to controlling the positioning of the satellite 100 and the pointing of the imaging system 140, various transmit/receive antennas, and/or solar panels. The satellite 100 generally transmits image data along with satellite information from the position measurement system 116, attitude measurement system 120, thermal measurement system 124, and other information used for the monitoring and control of the satellite system 100.
The movement system 132 contains a number of momentum devices and thrust devices. The momentum devices are utilized in control of the satellite 100 by providing inertial attitude control, as is well understood in the art. As is also understood in the art, satellite positions are controlled by thrust devices mounted on the satellite that operate to position the satellite 100 in various orbital positions. The movement system may be used to change the satellite position and to compensate for various perturbations that result from a number of environmental factors such as solar array or antenna movement, atmospheric drag, solar radiation pressure, gravity gradient effects, or other external or internal forces.
The satellite system 100 also contains a power system 136. The power system may be any power system used in generating power for a satellite. In one embodiment, the power system includes solar panels (not shown) having a plurality of solar cells which operate to generate electricity from light received at the solar panels. The solar panels are connected to the remainder of the power system, which includes a battery, a power regulator, a power supply, and circuitry which operates to change the relative orientation of the solar panels with respect to the satellite system 100 in order to enhance power output from the solar panels by maintaining proper alignment with the sun.
The imaging system 140, as mentioned above, is used to collect images that include all or a portion of the surface of the earth. The imaging system 140, in one embodiment, utilizes a pushbroom type imager operating to collect lines of pixels at an adjustable frequency between 7 kHz and 24 kHz. The imaging system 140 may include a plurality of imagers which operate to collect images in different wavelength bands. In one embodiment the imaging system 140 includes imagers for red, green, blue, and near infrared bands. The images collected from these bands may be combined in order to produce a color image of visible light reflected from the surface being imaged. Similarly, the images from any one band, or combination of bands, may be utilized to obtain various types of information related to the imaged surface, such as agricultural information, air quality information, and the like. While four bands of imagery are described, more or fewer bands of imagery are collected in other embodiments. For example, infrared and ultraviolet imagery may be collected, depending upon the applications for which the images will be used. In one embodiment, the imaging system 140 includes imagers comprising an array of CCD pixels, each pixel capable of acquiring up to 2048 levels of brightness and then representing this brightness with 11 bits of data for each pixel in the image.
Referring now to
Following the determination of the position, attitude, and distortion information, the predicted pixel location of at least one predetermined ground point is calculated, according to block 212. In one embodiment, this predicted pixel location is determined using the position of the imaging system, attitude of the imaging system, and distortion of the imaging system to calculate a ground location of at least one pixel from the image. Specifically the position provides the location of the imaging system above the earth's surface, the attitude provides the direction from which the imaging system is collecting images, and the distortion provides the amount by which the light rays are skewed from what they would be if there were no thermal, atmospheric, or relativistic effects. The position of the imaging system, along with the direction in which the imaging system is pointed, and the effects of distortion on the imaging system result in a theoretical location on the earth's surface which produced the light received by the imaging system. This theoretical location is then further adjusted based on surface features of the location on the earth's surface, such as mountainous terrain. This additional calculation is made, and the predicted pixel location is produced.
Following the determination of the predicted pixel location of each predetermined ground point in the reference image, a compensation factor is calculated for one or more of the position, attitude, and distortion information based on a comparison between the predicted pixel location of each predetermined ground point in the reference image and the actual pixel location of each predetermined ground point, as indicated at block 216. The calculation of the compensation factor(s) will be described in more detail below.
Following the calculation of the compensation factor(s), the ground location of at least one pixel in other images collected by the imaging system may be computed using the compensated attitude, position, and/or distortion information. In the embodiment of
It is noted that the order in which the operational steps are described with respect to
Referring now to
Referring again to
The compensation factor is determined, in one embodiment, by solving a set of equations having variables related to position of the imaging system, attitude of the imaging system, distortion of the imaging system, and the ground location of images acquired by the imaging system. In one embodiment, where imaging system attitude is compensated, the position of the imaging system determined at block 200 in
In one embodiment, triangulation is used to compute the compensated imaging system attitude. Triangulation, in this embodiment, is performed using a state-space estimation approach. The state-space approach to the triangulation may utilize least squares, least squares utilizing apriori information, or stochastic or Bayesian estimation such as a Kalman filter. In an embodiment utilizing a basic least squares approach, it is assumed that the position is correct, the distortion is correct, and that the ground location associated with a pixel in the reference image corresponding to a predetermined ground point is correct. The attitude is then solved for and utilized as the compensation factor.
In another embodiment, a least squares approach utilizing a priori information is utilized to determine the compensation factor. In this embodiment, the imaging system position, attitude, distortion and pixel location of the predetermined ground point, along with a priori covariance information related to each of these factors are utilized in calculating the compensation factor. In this embodiment, all of the factors may be compensated, with the amount of compensation to each parameter controlled by their respective covariances. Covariance is a measure of uncertainty, and may be represented by a covariance matrix. For example, a 3×3 covariance matrix may be used for position of the imaging system, with elements in the matrix corresponding to the in-track, cross-rack, and radial distance position of the imaging system. The 3×3 matrix includes diagonal elements that are the variance of the position error for each axis of position information, and the off-diagonal elements are correlation factors between position errors for each element. Other covariance matrices may be generated for imaging system attitude information, distortion information, and the predetermined ground point location.
Using least squares or Kalman filter with a priori covariances, compensations are generated for each parameter. In addition, covariances associated with each parameter are also produced. Hence, the a posteriori covariance of the improved attitude, for example, is known using the covariance associated with the attitude corrections.
As described above, in one embodiment two or more reference images are collected and utilized to calculate the compensation factor. In this embodiment, triangulation (via the methods described above) is performed on each image independently to determine compensation factors for each. These compensation factors are then combined for use in determining ground locations associated with images collected in which ground location is determined without using predetermined ground points. The compensation factors may be combined using methods such as interpolation, polynomial fit, simple averaging, covariance-weighted averaging, etc. Alternatively a single triangulation (using the same methods described above) is performed on all the images together, resulting in a global compensation factor that would apply to the entire span of orbit within the appropriate timeframe. This global compensation factor could be applied to any image without using predetermined ground points.
While the position parameters described above are assumed to be correct, or to have a small covariance, when determining compensated imaging system attitude information, other alternatives may also be used. In the above-described embodiment, imaging system attitude is selected because, in this embodiment, the imaging system attitude is the primary source of uncertainty. By reducing the primary source of uncertainty, the accuracy of the ground locations associated with other images which do not overlap ground control points is increased. In other embodiments, where imaging system attitude is not the primary source of uncertainty, other parameters may be compensated as appropriate.
As discussed previously, in one embodiment multiple reference images are collected from a particular orbit of the imaging system. In this embodiment, as illustrated in
Similarly, the actual locations of the predetermined ground points contained in the second reference image 408 are compared with predicted locations of predetermined ground points contained in the second reference image 408. A second compensation factor is determined based on the difference between the predicted predetermined ground point location and the actual predetermined ground point locations. A combination of the first and second compensation factors, as described above, may then be utilized to determine the ground locations for one or more pixels in each of the target images 412, 416.
The imaging system of the satellite may be controlled to acquire the various images in any order. For example, the satellite may acquire the third and fourth images 412, 416, and then acquire the first and second reference images 404, 408. In one embodiment, the images are acquired in the following order: the first reference image 404 is acquired, followed by the third image 412, followed by the fourth image 416, and finally the second reference image 408 is acquired. In this example, the compensation factor for the third and fourth image 412, 416 is calculated according to a least squares fit of the first and second compensation factors. If the images were acquired in a different order, it would be straightforward, and well within the capabilities of one of ordinary skill in the art, to calculate compensation factors for the third and fourth images 412, 416 utilizing similar techniques.
As mentioned previously, the satellite transmits collected images to at least one ground station located on the earth. The ground station is situated such that the satellite may communicate with the ground station for a portion of an orbit. The images received at a ground station may be analyzed at the ground station to determine location information for the pixels in the images, with this information sent to a user or to a data center (hereinafter referred to as a receiver). Alternatively, the raw data received from the satellite at the ground station may be sent from the ground station to a receiver directly without any processing to determine ground location information associated with images. The raw data, which includes information related to position, attitude, and distortion of the imaging system may then be analyzed to determine images containing predetermined ground points. Using the predetermined ground points in those images, along with other information as described above, the ground locations for pixels in other images may be calculated. In one embodiment, the image(s) are transmitted to the receiver by conveying the images over the Internet. Typically, an image is conveyed in a compressed format. Once received, the receiver is able to produce an image of the earth location along with ground location information associated with the image. It is also possible to convey the image(s) to the receiver in other ways. For instance, the image(s) can be recorded on a magnetic disk, CD, tape or other recording medium and mailed to the receiver. If needed the recording medium can also include the satellite position, attitude, and distortion information. It is also possible to simply produce a hard copy of an image and then mail the hardcopy the receiver. The hard copy can also be faxed or otherwise electronically sent to the receiver.
While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention.
This application claims priority to U.S. Provisional Patent Application No. 60/521,729 filed Jun. 25, 2004, entitled “METHOD AND APPARATUS FOR DETERMINING A LOCATION ASSOCIATED WITH AN IMAGE,” which is incorporated by reference herein in its entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US05/22961 | 6/24/2005 | WO | 00 | 12/26/2006 |
| Number | Date | Country | |
|---|---|---|---|
| 60521729 | Jun 2004 | US |
