n00b Altimeter JDAM INS Question

[Chaffee]

Where do JDAMS get their altitude data?

The reason: All is generally well for this n00b (nailing those CAT III recoveries, etc.) Hot-start JDAMS? Awesome...

Did a full cold start the other day. Loaded JDAMS. Got an interesting grouping of 4/4 perfect misses. (Sorry, no track. Likely repeatable though).

Did a process review: Realized I fudged the altimeter setting (just entered 29.92... ish), then decided to be meticulous about the JDAM stuff (alt-left F10 click and binding the UFC to the num pad is glorious).

Thus, my question, stated another way: could a mis-set altimeter mess up my JDAMs? Either directly, or indirectly by messing with the INS?

If true, honestly that's an amazing detail with one caveat...

In my search for answers, I noted some major reports over the last few years of some well-documented anomalous altimeter behavior on various airframes. Also noted a recent well-documented experiment on carrier-launch variation between hot and cold starting the F/A-18C.

So, also, might there be something deeper here RE: INS alignment/altimeter cold starting in this airframe?

Anyway. That's a lot. May run a controlled test and see if I can dig something out. Depends on whether I simply botched my JDAMs by fudging the barometric setting. 

Thanks in advance for your responses.

Edited February 22, 2024 by Chaffee

[Ivandrov]

The Hornet grabs elevation data from a terrain database. The Navy specifically has one for precision targeting, but I don't remember what it is called.

 

It can probably get elevation data from several other sources as well. Including GPS.

 

I tried making my baro hideously wrong and still got a perfect hit. Both TPOD des and Radar des.

Edited February 22, 2024 by Ivandrov

[Chaffee]

1 hour ago, Ivandrov said:

The Hornet grabs elevation data from a terrain database. The Navy specifically has one for precision targeting, but I don't remember what it is called.

 

It can probably get elevation data from several other sources as well. Including GPS.

 

I tried making my baro hideously wrong and still got a perfect hit. Both TPOD des and Radar des.

 

Thanks, mate. This is useful info.

[Phantom711]

@Chaffee

There are multiple opportunities to <profanity> things up with JDAMs, but the altimeter setting should not play a role in that.

So what exactly did you do? TOO or PP?

If you did PP, then a wrong coordinate format would be an obvious error...orr not entering the correct elevation data of the target. For TOO there are also several possibilities.

[Chaffee]

3 hours ago, Phantom711 said:

@Chaffee

There are multiple opportunities to <profanity> things up with JDAMs, but the altimeter setting should not play a role in that.

So what exactly did you do? TOO or PP?

If you did PP, then a wrong coordinate format would be an obvious error...orr not entering the correct elevation data of the target. For TOO there are also several possibilities.

PP. Passed all the usual cross-checks that you mentioned.

Could, of course, be bad luck. JDAMs do miss. Good to know it's not my altimeter sloppiness. It was an interesting pattern. I'm possibly reading too much into it.

[Ivandrov]

4 hours ago, Chaffee said:

PP. Passed all the usual cross-checks that you mentioned.

Could, of course, be bad luck. JDAMs do miss. Good to know it's not my altimeter sloppiness. It was an interesting pattern. I'm possibly reading too much into it.

Yeah, PP would completlely bypass anything that could possibly be wrong with the Jet, as you are directly inserting coordinates and elevation into the JDAM. The JDAM has its own GPS/INS systems.

[QuiGon]

6 hours ago, Ivandrov said:

Yeah, PP would completlely bypass anything that could possibly be wrong with the Jet, as you are directly inserting coordinates and elevation into the JDAM. The JDAM has its own GPS/INS systems.

The JDAM's INS is being aligned based on the aircraft's INS though. Now if GPS is available that shouldn't matter, but in GPS restricted scenarios this is a different story.

[EmJay22]

On 2/22/2024 at 12:26 AM, Chaffee said:

(alt-left F10 click and binding the UFC to the num pad is glorious).

What is this tricksie devilry?  I can't make it work or find any info.

[Ivandrov]

4 hours ago, EmJay22 said:

What is this tricksie devilry?  I can't make it work or find any info.

It's just grabbing coordinates and elevation of targets off of the F10 map using the Lalt + click and inputting those values into each pylon in PP mode.

[EmJay22]

13 hours ago, Ivandrov said:

It's just grabbing coordinates and elevation of targets off of the F10 map using the Lalt + click and inputting those values into each pylon in PP mode.

Oh cool. That is a great to "copy" the coordinates. I don't see how to "paste" or "enter" them. 

[QuiGon]

1 hour ago, EmJay22 said:

Oh cool. That is a great to "copy" the coordinates. I don't see how to "paste" or "enter" them. 

You can switch back into the cockpit and the coordinate window stays open, so you can just look at it while typing the coords into your system.

[EmJay22]

3 minutes ago, QuiGon said:

You can switch back into the cockpit and the coordinate window stays open, so you can just look at it while typing the coords into your system.

Oh... very cool. I will give that a try. 

[Chaffee]

On 3/1/2024 at 5:57 AM, EmJay22 said:

Oh... very cool. I will give that a try. 

This is the way to happiness.