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[Logged] Day/night terrain-following altitudes

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[Logged] Day/night terrain-following altitudes - 5/1/2021 2:07:24 PM   


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So I was testing navigation through an IADS I built and I started looking at the minimum AGL terrain following (TF) flight for different types of aircraft after reading a couple of earlier threads about low level penetration. I haven't dug too deep into the DB but I can identify two different special attributes for aircraft that reduce the maximum TF altitude and allow day/night TF. These are Terrain Following (found it on the F-111) and Terrain Navigation (found it on the B-52 B-2 and Su-34). Weather and proficiency seem to affect day but not night minimum AGL, and numbers are different over the sea; I'll neglect these for now. From my tests, min altitude TF AGL with Regular pilots in perfect weather is as follows:

MiG-21/F-16C+ Block 52 Sniper/F-35A day/night: 400/1000 ft AGL
F-111F day/night: 200/200 ft AGL
B-52/B-2 day/night 300/300 ft AGL

Now let's turn to USAF F-16, F-15E, and F-35 operating procedures.

F-16 Combat Aircraft Fundamentals MCH 11-F16) suggests that day and properly-equipped night F-16 operating altitudes are lower than those in the sim. See Chapter 10 on page 263 and continuing for details of LANTIRN night operations and in particular the entry for the LANTIRN TFR VLC mode on page 273-274.


Enter VLC by selecting 100' SCP on the TFR page. Initial commands will be to 200' until the terrain data criteria is met for 100' SCP. If the terrain does not continue to meet system requirements for 100', the SCP automatically bumps up to 200' and the jet climbs to 200'. A DMS forward anytime you are at 100' changes the SCP to 200' and climbs the jet. VLC is normally used in high threat, relatively flat terrain, where maneuvering is minimal.

This suggests that LANTIRN navigation-equipped F-16 pilots were able to fly down to 200 feet routinely and 100 feet in flat areas. Corroboration is provided by the LANTIRN information here:


The TFR significantly enhances the aircraft's chances to survive on the modern battlefield, since it not only allows the pilot to automatically avoid the terrain but also enables to evade air defense systems by maneuvering in the horizontal plane. The radar can be linked directly to the F-16's autopilot to automatically maintain a preset altitude down to 100 feet while flying over virtually any kind of terrain. I(t) has five modes five modes: Normal, Weather, ECCM, Low Probability of Intercept (LPI), and Very Low Clearance (VLC).

Retired pilots also state that LANTIRN-equipped F-16 Block 40/42s trained at altitudes of 200 feet on initial instructor-led flights.


Retired Colonel Dave Martin, an early LANTIRN test pilot, says the forward-looking infrared technology was so good that when they later added night vision goggles, the goggles just added weight without any real benefit. He recalls that on his first flight he had a hard time trusting the terrain-following guidance, but his instructor coaxed him down to 200 feet while screeching along at almost 575 mph. “That was the first time I’d imagined doing something like that in an airplane at night,” he says. “After some practice, it just became second nature.”

Although similar information on the F-15E and F-35 is classified, we do have declassified USAF low altitude training (LOWAT) aircrew training standards for the F-16, F-15E, and F-35A. These all go in three qualification stages with operations from 1000-500 feet, 500-300 feet, and 300-100 feet AGL.

But, you might say, the LANTIRN navigation pod had a specialized TF radar. Can aircraft lacking the navigation pod still perform low level radar TF flight? My suspicion is that they can perform passive database TF if they have appropriate NVD or FLIR and active radar TF if they have an AESA radar alongside that. My justification for this is based on an F-16E/F slide deck where the F-16E's AN/APG-80 early AESA radar is described as being able to provide simultaneous interleaved radar TF, SAR map, and air to air tracks. Active radar TF is shown to maintain 100 feet above water in normal test conditions and passive database TF is shown to maintain 200 feet above rough terrain and 100 feet above normal terrain.

I suggest the following:

- Allow fighter aircraft to travel in the 200'-400' AGL band at up to Military speed during the day. Consider allowing the min altitude to drop as low as 100' if the local slope is below a certain angle and the local ground cover is favorable.

- Add Terrain Following 200' to strike aircraft with AESA radars and advanced FLIR pods or integrated IRSTs. This would provide another motivation for the modern strike aircraft AESA radars like the AN/APG-81, AN/APG-82(V)1, and AN/APG-83 SABR, and explain the removal of the TFR from later generations of targeting pods. The AN/ASQ-236 Dragon Eye pod replaces the LANTIRN navigation pod on non-AESA F-15Es and F-16s.

- Consider reducing the minimum night AGL altitude for 4.5-5 gen aircraft with advanced FLIR/IRST alone to 400' AGL based on the availability of passive database TF when GPS is available.

- Consider modeling radar TF emissions during Terrain Following flight and disallowing radar TF during passive EMCON. The F-16 manual above describes different LANTIRN radar TF settings, some of which are specifically focused on LPI operation. This would balance the effectiveness of low level flight in avoiding LOS targeting.

< Message edited by WSBot -- 5/3/2021 4:33:34 PM >
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RE: Day/night terrain-following altitudes - 5/1/2021 6:18:19 PM   

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Good stuff!

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< Message edited by Blast33 -- 5/1/2021 6:22:48 PM >

(in reply to DESRON420)
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RE: Day/night terrain-following altitudes - 5/1/2021 7:04:10 PM   


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Related discussion (also might have saved you some testing) that might be of interest:

(in reply to DESRON420)
Post #: 3
RE: Day/night terrain-following altitudes - 5/1/2021 9:40:00 PM   


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Haha thanks! Great thread. I didn't search carefully enough, I would have posted this as a reply if I'd seen it before. I'm glad that I could add more detail on routine planning for combat operations at low altitude. :)

The testing was not onerous at all, I was actually testing detection differences on different stealth aircraft and noticed that the B-2's Min Altitude was a lot lower than the F-22 and F-35's, which seemed a little peculiar, so I dug in further and eventually realized that the explicit presence of a specialized terrain following radar system on the aircraft was the heuristic used to assign the 6001/6002 designation and the lower TF altitude. At that point the F-16E slide deck was the important connection to realize that part of the reason for the wave of AESA radar modernization in Western strike fighters might involve integrating the radar part of TF onto the aircraft and getting rid of the pod.

As I alluded to before I think it would be interesting to get rid of the hard land/sea dichotomy and instead take into account the average ground cover and slope variation in a 5-10 nm radius around the aircraft. If ground cover is something predictable like sea, plains, croplands, steppe, tundra etc and slope does not vary too much in the local region then aircraft can fly more towards 100'. In mixed forest or rugged terrain they would go up to 200' and if they entered built up areas they might ascend to 500' or 1000' to avoid endangering civilians.

One more thought on the NATO nuclear sharing program and the B61-12. A fighter's ability to perform a low altitude toss delivery is very important to this strategically and politically important program. In an age now ending this mission was carried out by F-16s and Tornados. The US, Belgium, Italy, and the Netherlands (and until recently Turkey!) have all chosen the F-35 to carry on this mission, while Germany is seriously considering the F/A-18E/F as a Tornado replacement. This suggests that the F-35 and Super Hornet can both carry out low level toss bombing missions and therefore that they can achieve the ultra-low altitude arrival at the target typical of this mission. For the Super Hornet in particular this would be essential as it is not stealthy enough to avoid detection at long range. Perhaps the Greens will save Germany quite a bit of money by withdrawing from the nuclear sharing program and removing the need for a new platform!

< Message edited by DESRON420 -- 5/1/2021 10:30:18 PM >

(in reply to KnightHawk75)
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RE: Day/night terrain-following altitudes - 5/3/2021 4:32:15 PM   


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(in reply to DESRON420)
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