2.1

Geometric calibration of AVNIR-2

The band-to-band registration is important because if it accuracy is not sufficient the color composite image runs in level 1B2 (L1B2) standard product. The band-to-band registration is defined to adjust bands 1, 2, and 4 to band 3 that is as the base band into the geometric sensor model. The current parameters which are used to generate standard product and related to band-to-band registration have been updated on December 2006.

Fig. 1 shows the relative geometric error for each band of AVNIR-2 compared with band 3 that shows the band-to-band registration. The images used this evaluation were acquired from April to December 2007 with various pointing angles from +/-41.5 degree and cubic convolution (CC) methods was applied for resampling method in Fig. 1. The plots show scene averaged relative geometric error for band 1, 2 and 4, respectively. Left graphs of Fig. 1 show X (pixel) direction and the right show Y (line) direction. These geometric errors were calculated that the special feature points were automatically identified from images, than compared between each band and band 3 by the least square matching technique. They show +/-0.2 pixels from Fig. 1 (b) in both X and Y directions, which meet to previous results that have been reported for 0 degree pointing angle with CC resampling method [3]. Therefore, accuracy of band-to-band registration is keeping with our expectation.

Fig. 1.

Evaluation of band-to-band registration as averaged relative geometric error of AVNIR-2 for cubic convolution resampling (left: X direction, and right: Y direction, the images were acquired from April to December 2007).

Regarding absolute geometric calibration of AVNIR-2, Fig. 2 shows results of geometric correction accuracy evaluation using ground control points (GCPs) as a function of (a) pointing angle, and (b) observation date as time trend since October 2006. The attitude coordinate of ALOS is defined at origin of the star tracker (STT) coordinate. Through the calibration of STT itself, its coordinate is changing during recurrent of the satellite due to change the thermal condition. Therefore, STT alignment parameter has been updated to adjust this disturbance on March 22, 2007. This adjustment is caused by about 20 meter gap in Y (line) direction of AVNIR-2 imageries that can be seen in Fig. 2 (a), where x plots show scene averaged absolute geometric error in Y direction, and square plots show them in X (pixel) direction, and color indicates different observation period of the image i.e. blue plots show the evaluated image were acquired before March 22, 2007, on the other hand pink and green show after it. The lines show regression curves of each x plots (Y direction). The difference between curves of blue and pink is about two pixels that are corresponding to 20m on the ground. Actually, blue plots are corresponds to previous calibration results [3]. At that time, sensor alignment of AVNIR-2 has been evaluated using acquired imageries over whole pointing angle ranges of +/-44 degree, and parameters were updated. Currently, geometric accuracies are becoming better for +/-22 deg. pointing angle ranges due to STT parameter change. In terms of time trend of geometric accuracy of AVNIR-2, significant changes could not see in Y direction before and after March 22, 2007 from Fig. 2 (b) that is also shows results of whole pointing angle ranges. However, small trend with observation date may be found in Y direction. Therefore, we have a plan to update the alignment parameter of AVNIR-2 on September 2008.

Fig. 2.

Evaluation of geometric correction accuracy of AVNIR-2.

2.2

Geometric calibration of PRISM

PRISM consists of multiple CCD units and each unit has approximately 5,000 detectors. The nadir-looking radiometer of PRISM has six CCD units, which can be covered 70km observation swath width with 2.5m spatial resolution. The forward- and backward-looking radiometers have eight CCD units to observe same area with nadir’s one even the earth rotating on its axis due to time gaps between those observations (about 46 seconds each). The boundary between each CCD unit is overlapped 32 pixels. Thus, these 32 pixels can be obtained image of same areas theoretically. One of parameters of sensor model is a relative alignment between CCD units called CCD alignment. The relative geometric calibration (i.e. self calibration) of PRISM is carried out to evaluate and update CCD alignments parameters. The latest version of it was released on June 2007 as version 2 [5]. The absolute geometric calibration is done by evaluating the pointing alignment parameters, which are calculated by the Precision Pointing and Geolocation Determination System (PPDS) that is a ground processing system to achieve determinations of precise attitude and pointing vectors for each PRISM radiometer [6]. The geometric correction accuracy shows the overall geometric accuracy, and could be calculated from comparison between the actual geographical coordination (longitude, latitude and height above ellipsoid) of GCP that could be identified on the images and the calculated one by using image coordinate (pixel, line) of GCP with transformation coefficients that are derived from sensor model at the system correction and include in the product [5].

Fig. 3 shows geometric residuals in X (pixel) and Y (line) directions as a function of pixel number for nadirlooking radiometer as example. The geometric residuals of each boundary between CCD units were refined with the sub-pixel image matching on overlapping 32 pixels data with latest version of CCD alignment. Table 1 summarizes the standard deviation of accumulated exterior orientation results on image space for nadir-, forward- and backward-looking radiometers, respectively. The accuracies of CCD alignments i.e. interior orientation parameters are keeping the sub-pixel level from 0.6 to 0.7 pixels from latest version. It looks very good and cannot see accuracy degradation from latest evaluation.

Fig. 3.

Evaluation of relative CCD alignments of PRISM nadir-looking radiometer using latest version of alignment parameters (left: X direction, and right: Y direction).

Table 1.

Standard deviations of geometric residuals on image space (pixel).

Regarding to absolute geometric calibration, ALOS is achieving the precise attitude determination function based on STT data as well as the precise orbit determination using dual frequencies GPS receiver (GPSR) with post ground processing [6]. To improve geometric absolute accuracy of PRISM, the pointing alignment parameters that are used into the processing of standard products, have to estimate precisely including variations of recurrent as well as time dependency. The current pointing alignment parameters are version 15 that released on July 25, 2008. We will update them about each two months to keep the accuracy if accuracy degradation is confirmed.

Addition to the pointing alignment parameters, we are also trying to generate the sensor alignment model, which is mainly using to confirm the accuracy of the pointing alignment as well as to create the high level products of PRISM at JAXA. The sensor alignment model consist of two time scales i.e. short term variation parameter “s” that is a normalized value of the observing time from satellite eclipse to orbit recurrent period (98.7 minutes), and long term variation parameter “d” that is depends on the date of observation from January 1, 2006 as a definition. The errors of PRISM sensor alignment model were expressed in Euler angles (roll, pitch and yaw) for the analysis and it was confirmed that yaw angle could be negligible. The 2nd degree Fourier series was applied to estimate the short term trend, and the linear model was applied to the long term trend as the current alignment change models from the various preliminary experiments. 70, 95 and 92 scenes for forward-, nadir-, and backward-looking image were sampled from 42 different GCP sites in the period from Mar. 2007 to Jan. 2008 for the latest model fittings. Fig. 4 shows the results of short and long term variation models fitting for nadir-looking radiometer, and rolling and pitching error angles. The standard deviation of fitting residual for each radiometer is shown in Table 2.

Fig. 4.

Evaluation of sensor alignment model of PRISM nadir-looking radiometer.

Table 2.

PRISM sensor alignment model fitting residual.

Fig. 5 shows results of geometric correction accuracy evaluation of PRISM nadir looking radiometer both (a) scene averaged geometric error in X and Y directions and (b) its standard deviation since April 2007. X-axis of Fig. 5 indicates observation date of evaluated image. Each color shows evaluation date, which is corresponding to update PRISM pointing alignment parameter. Previous results had been presented [5]. It looks a little bit scattered the averaged geometric accuracy of both X and Y directions from Fig. 5(a). However, many plots of them are less than +/-10 meters. This means that most of PRISM images have +/- 10 meters of geolocation determination accuracy without GCPs. In this evaluation, we were used more than 270 scenes with 3,300 GCPs as check points that are located in the world. On the other hand, relative geometric error that shown as standard deviation of geometric error in Fig. 5 (b) is almost less than 2 meter that consists of the result of Fig. 3. Therefore, relative calibration is worked very well as I described above.

Fig. 5.

Time trend of geometric correction accuracy of PRISM nadir-looking radiometer since April 2007.