Setting SPI for JDAM: Why Laser?

[jeproject]

What is the benifit of lasing the target with the TGP when setting an SPI to drop a JDAM on? Is it for range finding so it can see the target in 3d sticking out from it's background?

 

OT :I realise I'm posting a lot of questions and the answers are all out there somewhere but I'm trying to learn as much as I can before my week off work so I can get really stuck in. I keep find videos/tutorials where people say "yeah and this is, urmm well it doesn't matter what this is, just press these keys" but it matters to me. I find these kinds of "tutorials" very frustrating. The JDAM videos in the EagleWiki are a bit beyond me yet, I haven't started with the CDU yet. I just want to drop one JDAM on one target.

Edited November 14, 2013 by jeproject

[EchoMasterMind]

As far as I understand, the laser guides the bomb is useful if the target moves, as it will adjust its path to still hit the target. Unlike GPS-guided JDAM that will hit the place were the target were at the point of dropping the bomb.

[jeproject]

This isn't quite what I was talking about. I've seen people using GPS/INS munitions and when they set their SPI they enable the laser.

 

One youtube video said "you sometimes want to enable the laser" : http://youtu.be/5g1_euMP8q4?t=2m43s

 

This post says the same thing : (2nd post in thread) http://simhq.com/forum/ubbthreads.php/topics/3171145/How_to_use_weapons_Picture_gui.html#Post3171145

 

"Step by step guide on how to deploy GBU-38 (JDAM) - a GPS/INS guided 500 pound 'smart' bomb." says....

 

"Press NWS (INSERT) (turns laser on) and Press "TMS FORWARD LONG (UP + LCtrl) " to mark / create a way-point for your sensor (creates SPI). Press again NWS (INSERT) to turn off laser."

 

Are they just misinformed or is there a reason to do this?

Edited November 14, 2013 by jeproject

[Hansolo]

Check out this by ExcessiveHeadspace:

 

Might be the explanation you are looking for.

 

Cheers

Hans

[Evil.Bonsai]

Yes, it is to mark the target and not the ground it sits on. Your TGP know where the ground is but doesn't know there's a target there unless you use the laser. Gives a slightly more accurate fix for targeting/marking

[quyes]

I can't find the graphics someone had done. Without using the laser to measure the distance your sensors will continue in a straight line from your aircraft until it hits the ground. Where this line hits the ground is where the SPI is setup. Depending on your angle from the target relative to the ground this can result in your SPI not being right on target. If your target is on the edge of the cliff with the cliff behind them relative to your aircraft your SPI will actually designate on the other side of the valley.

 

By using the laser to measure the distance to the target when setting the SPI, the sensors will put the SPI on the ground right under the measured distance. I'll see if I can find the illustrations when I get home.

[jeproject]

Thanks guys. It's clever how a lot of what the a10s systems can do is automatic e.g. laser is on so tgp uses that information without any further action required.

[NonWonderDog]

This has always seemed slightly strange to me, but for a different reason.

 

Lasers have divergence. High-powered military targeting lasers tend to have LOTS of divergence by laser standards, up to several mils (Russian tank rangefinder specs are particularly shocking). "First return" vs. "Last return" matters for lasers almost as much as it does for radars. Not only that, but laser rangefinders have some amount of uncertainty on their clocks, and that leads to an uncertainty in the range result.

 

Now in DCS we have perfect GPS elevation data and perfectly collimated lasers controlled by perfect clocks, so using the laser will let you get the range to a tank's side rather than the range to the ground under the tank. But that just seems like a gross over-estimation of the capabilities of both systems. It seems like in real life you might want to use the laser to make up for any inaccuracies in the GPS data, but unless the tank is silhouetted against a ridge the CEP has got to be more than the difference between the side of the tank and the ground.

 

But then again, the accuracies of these things are almost certainly classified. I'm sure the laser in a Litening or Sniper pod is nothing like the one on a T-72, and probably significantly better than the one on an Abrams; I'll just probably never know by how much.

[jeproject]

I think the never knowing is balanced out by being able to make a cup of tea a few mins after you get shot down.

[quyes]

HMA beat me to it. The graphics are in that video.

[jeproject]

HMA beat me to it. The graphics are in that video.

 

Well I read all the responses at once so at least I had a heads up on what to expect before I even watched the video. When multiple people resond to a quesation it all builds towards a very clear picture of what's going on.

[Hekktor]

laser isn't laser...

well of course it is BUT one time you only use it for ACCURATE RANGING to get the best target coordinate possible (tank on hill seen from flat angle and some basic trigonometry -> tank is on slight another position than ground 'behind' him... as described in above mentioned well done tutorials) to be used with your JDAM.

and than you use your laser no ONLY for ranging but ALSO for GUIDING your LGB's. Thats when you really need to 'illuminate' your target all the way down to impact of your bomb...

 

something else? Ok you can switch over to IR and point to a target for your buddy to find it or the like...

 

Happy Kills,

Hek