Aerospace and Electronic Systems Magazine June 2017 - 27
Cohenour, Rovito, and van Graas
The resolution target line spacing using (1) and R0 from (2) is
given in Table 1. For convenience, the spacing is given in terms of
line pairs per meter, and meters per line pair.
MEASURING IMAGE RESOLUTION
Projecting the image requires remapping the pixels from the
source (raw) image to the destination image. The mapping is not
one to one so some interpolation must be performed. In addition,
the projected image may, as in this case, be compressed to reduce disk space, as well as the bandwidth needed to transmit the
image. The loss of resolution for frame 100 is shown in Figure 5.
In this frame, the loss of resolution is from an index of -3 in the
raw image to an index of -5 in the NITF image, or from 1.21
lp/m to 0.96 lp/m.
Determining the resolution is done manually, and based on the
following rules:
Figure 5.
Resolution chart at WPAFB based on NITF image (left), and raw image
(right).
1. Both the horizontal bars, and the vertical bars must be visible
for the index.
2. The bars are viewed in isolation.
3. When viewed in isolation, the orientation of the bars, horizontal or vertical, must be clear.
This is shown in Figure 6. Here the bars from the raw image of
Figure 5 are isolated. The orientation of the bars in "B" cannot be
determined. The bars in "C" are vertical. The bars in "A" are also
vertical. The three subimages in Figure 6 are from the raw image
in Figure 5, at indices -3, -2, -4. The subfigures have been scaled
to the same size.
This procedure is repeated for each NITF image, and each projected raw image in a sample of 40 frames from the data set. The
40 frames of data are selected to maximize the angular diversity of
the aircraft over two orbits, as detailed in [8]-[12].
Figure 6.
Resolution target bars shown in isolation.
The results are tabulated in Table 2. For the NITF images the
mean resolution index is -4.25. Using (1) this is a resolution of 1.05
lp/m. The projected raw images have an average index of -2.8, or
1.24 lp/m.
IMAGE COMPRESSION
The raw image data set considered here is 3.53 GB for 40 frames of
data, while the NITF image data set is 1.58 GB. This is a compres-
Table 2.
Resolution of NITF and Projected Raw Images
NITF
Idx
Raw
Idx
Frame
NITF
Idx
Raw
Idx
Frame
NITF
Idx
Raw
Idx
Frame
NITF
Idx
Raw
Idx
100
-5
-3
222
-5
-3
359
-5
-3
479
-4
-5
112
-4
-2
236
-4
-2
371
-5
-2
493
-4
-2
124
-3
-3
251
-4
-3
382
-4
-2
508
-4
-2
135
-5
-3
265
-3
-3
394
-5
-3
522
-4
-3
147
-4
-5
280
-3
-3
405
-5
-3
537
-4
-2
158
-4
-3
295
-3
-3
417
-5
-5
552
-4
-3
170
-5
-2
309
-5
-2
428
-5
-3
566
-4
-2
182
-3
-2
323
-4
-3
440
-4
-2
581
-5
-2
195
-5
-2
336
-4
-3
453
-4
-3
594
-4
-3
208
-3
-3
349
-6
-2
466
-6
-4
608
-3
-3
-
-
-
-
-
-
-
-
-
Mean
-4.25
-2.8
Frame
JUNE 2017
IEEE A&E SYSTEMS MAGAZINE
27
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