Stickler Posted April 6 Posted April 6 (edited) As seen in the attached .trks and .acmis, the optical sight slant range of a reference aircraft underreads by around 350-400 ft in 20,000 ft (HEAT/RADAR), AIR-GND mode. Specifically, comparing the slant ranges (SR) indicated on the optical sight in the "air-gnd_ranges.trk" with the corresponding .acmi, the following results are obtained: Time stamp SR (.trk) SR (.acmi) 57,954 18000 18343 61,355 15000 15354 64,752 12000 12352 68,158 9000 9350 71,472 6000 6423 It is unlikely that this is caused by the radar not looking through the center of the the pipper, since in 6667 ft (GUN) mode, the optical sight's range bar is very accurate as shown in the other .trk and .acmi. The below errors are likely measuring errors or caused by range bar inertia. Time stamp SR (.trk) SR (.acmi) 73,644 6000 5925 74,738 5000 4945 75,876 4000 3945 77,014 3000 2933 78,094 2000 1975 It is noteworthy that the (apparently miscalibrated) 20,000 ft mode nevertheless seems to perfectly match the range bar as situated on the radar screen. For example, when the optical sight erroneously displays 18000 ft, the range bar is positioned exactly on the 3 nm position. Also of note is that while the 6667 ft (GUN) mode is very accurate relative to the actual position of the aircraft, a target shown at 6000 ft slant range in the optical sight will be noticeably short of the 1 nm range on the radar screen. Lastly, even though the VISIDENT indicator in the back seat should not work in AIR-GND mode, currently its indications seem to very closely match the optical sight's indications in GUN mode (and therefore actual ranges). It would be interesting to know if the miscalibration in 20,000 ft mode is purely cosmetic or if it translates to wrong slant ranges being fed into the WRCS at pickle for DT and DL. The only vague hypothesis I have is that the issue might be related to the difference between a radar mile (6000 ft) and an actual nautical mile (6076.12 ft). This post is loosely related to that one. air-ground_ranges_track.acmi air-ground_ranges_gun_track.acmi air-gnd_ranges.trk air-gnd_ranges_gun.trk Edited April 6 by Stickler 1
Stickler Posted April 23 Author Posted April 23 I'm reasonable sure the cause for this problem is that the B-mode ranges are miscalibrated. Observe the below screenshot which was created by superimposing two screenshots taken at the exact same time in active pause, once in PPI and once in B. The radar shows ships that were placed at exactly 1 through 9 nm slant range from the aircraft. Note how regardless of the radar display mode selected, the radar returns are located at the same spot on the scope. However, whereas the PPI scale perfectly matches the actual distance of the returns, the returns are noticeably short of the B scale at 4 nm and less. Since the optical sight's range bar seems to be calibrated to match the B scale, it will show the targets at smaller slant ranges than they actually are. 1
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