JDAM/JSOW elevation in AGL

I saw NineLine/Wags' posts in the bug forum about JDAM elevation being entered in feet above ground level. I would just like to know... how does this work? How does the bomb then know the actual location of the target vertically above (or below) sea level)? Additionally why does it say 'elevation' (a term usually used for an altitude above sea level) and WGS (World Geodetic System, which defines sea level for GPS)?

Maybe I dont really understand your question. The JSOW uses the coordinates you input including elevation and is then guided by on board GPS.

"How does the bomb then know the actual location of the target vertically above (or below) sea level)?"

Because you told it so when you entered in the coordinates/elevation

Maybe I dont really understand your question. The JSOW uses the coordinates you input including elevation and is then guided by on board GPS.

 

"How does the bomb then know the actual location of the target vertically above (or below) sea level)?"

 

Because you told it so when you entered in the coordinates/elevation

Aye - however, ED says the elevation you enter is actually above ground level (AGL), not ASL, hence my confusion.

Maybe they're saying that although it's entered as ASL, you always have to pick AGL+1 as a minimum. So at sea level, you'd put 1ft ASL. If it's in a mountain at 1000ft ASL, you'd enter 1001ft. That's the only way I can make sense of it - I've been dropping JDAMs accurately with hundreds of feet 'elevation' entered and it's hitting tanks on the ground at the coordinates.

I went and read the closed bug thread you mentioned. Isee what your asking now. Im also curious, because if they are using AGL as the elevation then how would you ever attack a target location below sea level? Is it just the way the f10 map provides coordinates?

Right now the elevation entry doesn't seem to do anything so long as it's set > 0.

For a target located at 3,000 feet MSL, I set one bomb at MSL, I've set one at 1 foot (assuming ASL), and a third at 5,000 feet (deliberately wrong). All bombs impacted within twenty feet of each other.

I'd assume this is WIP, because this can not be how the weapon functions in reality. I find it difficult to believe the F/A-18 has a terrain database accurate enough at the resolution required for that sort of accuracy.

I think it is because GPS relies on digital information. A point in 3 dimensional space. Hence you need digital location that can be reference from your AC’s current position. Not like when you are bombing with conventional bomb that you have to use analog altimeter, analog speed, etc.

Same way a regular GPS receiver knows your altitude, it does not care if you are on a plane or mountain

I think it is because GPS relies on digital information. A point in 3 dimensional space. Hence you need digital location that can be reference from your AC’s current position. Not like when you are bombing with conventional bomb that you have to use analog altimeter, analog speed, etc.

This seems more like an explanation for why it would be ASL rather than AGL. Your plane knows it's altitude in feet ASL (unless you're below 5kft AGL and have the radalt on).

I stand corrected GPS does not need to know it's relation to you're AC. It knows where it is in space, it just need a point to fly to. I think it does not care what the altitude is base on atmospheric pressure it just need a point in space, which is translated to distances from the source of gps signal from space.

ASL is like analog way of figuring out your altitude. why go through all that when your goal is to get distances from gps signal sources.

reading back the original post. it says "elevation" not altidute. it's is more like typing in an address for the bomb. lol

The point is the GPS won't know where you're talking about vertically if you give it AGL. The AGL relies entirely on the elevation above sea level of the ground above which you are measuring the elevation AGL.

you are over thinking it. it needs elevation, it does not care if it is above ground or under the see. it needs to figure out how far is the target from the gps source from "space"

you're target is on the ground therefore you do not need to know how high it is from the sea, much less getting the current atmospheric pressure to figure that out. the elevation above ground more the 0 feet would be if you need to place the bomb on top of a building.

Yes, precisely, hence why it would make sense to give it an elevation above / below MSL. I don't see how a height AGL alone would even give it any meaningful information.

you get a measuring stick that is the easiest way to get AGL, I know top of my head is 5'2" any where I go. MSL is for altitude, so everyone can fly at certain altitude regardless of the terrain. which will need specific atmospheric pressure reading at sea level per station which covers an area. you cannot ask that information from the airfield that you are bombing

or you can reverse the process, ask clearance to land on the airfield you are bombing, ask it's altitude MSL, then dial your altimeter to that said altitude.

That's true, you'd need correct pressure for an accurate barometric altitude, however you could just use a "wrong" altitude, as long as the bomb is using the same incorrect pressure.

I still do not get how the bomb knows where the target is vertically if the entry is in height above ground. I don't see how it would know where the ground actually is.

it does not care where the ground is. it just fly to that 3(lat,long,elev) dimensional point in space that you put. thanus can snap the earth to ash and it will still fly to that point(assuming atmosphere/gravity/etc has not change). it comes down to how far is that point from the satellite source from space.

it does not care where the ground is. it just fly to that 3(lat,long,elev) dimensional point in space that you put. thanus can snap the earth to ash and it will still fly to that point(assuming atmosphere/gravity/etc has not change). it comes down to how far is that point from the satellite source from space.

I'm not sure what you're trying to argue to be honest - if the elevation is entered as AGL, then it *has* to care where the ground is, otherwise it'll have no idea where to go. If I enter the elevation as '1' at a point on top of Everest, and then '1' at a point in the valley next to it, how does the bomb know where to fly? One is going to be tens of thousands of feet higher than the other.

it does not care where the ground is. it just fly to that 3(lat,long,elev) dimensional point in space that you put. thanus can snap the earth to ash and it will still fly to that point(assuming atmosphere/gravity/etc has not change). it comes down to how far is that point from the satellite source from space.

But "1ft AGL" does not provide that, as the ground can be closer or farther away from the satelites.

The absolute distance is needed. And that would be provided by giving an elevation in relation to a fixed reference (i.e. known by GPS). That would be, so far my understanding goes, the geoid - a mathematically model of the earth surface, for example "WGS", wich we also already saw in the JDAM data page.

you are over thinking it. it needs elevation, it does not care if it is above ground or under the see. it needs to figure out how far is the target from the gps source from "space"

you're target is on the ground therefore you do not need to know how high it is from the sea, much less getting the current atmospheric pressure to figure that out. the elevation above ground more the 0 feet would be if you need to place the bomb on top of a building.

Actually, you are under-thinking it. Altitude AGL makes no sense for a GPS guided bomb, since the bomb has no idea where the ground is below it. JDAMs, JSOWs and all other GPS guided weapons use HAE, Height Above Ellipsoid, which gives you the height referenced to the WGS84-ellipsoid. Altitude in MSL can also be input, but this is converted to HAE by the avionics. HAE input is always preferred, as it avoids potential errors from rounding or truncation when converting between the two.

OK, sure, the physical ground doesn't technically matter. Let me rephrase: I don't see how it would know where the three dimensional point actually is with respect to the elevation.

EDIT: posted this before I saw the previous 3 posts. But yeah, like they said.

Actually, you are under-thinking it. Altitude AGL makes no sense for a GPS guided bomb, since the bomb has no idea where the ground is below it. JDAMs, JSOWs and all other GPS guided weapons use HAE, Height Above Ellipsoid, which gives you the height referenced to the WGS84-ellipsoid. Altitude in MSL can also be input, but this is converted to HAE by the avionics. HAE input is always preferred, as it avoids potential errors from rounding or truncation when converting between the two.

well we agree on something, but going back to the under thinking, all you explained was ways of inputting a location which will then be converted to a point in space.

go pull up a map. point your finger on it and you will see that the earth has been mapped with lat. long. and elev. a fitbit can even know where it was at.

now one ask about bombing a highest point of the mountain? there would only be 1 point consisting of (lat,long,alt) that location, you may edit the elevation but you only wanna go higher to account for the building if there is. you do not wanna target below ground(negative)

now one ask about bombing a highest point of the mountain? there would only be 1 point consisting of (lat,long,alt) that location, you may edit the elevation but you only wanna go higher to account for the building if there is. you do not wanna target below ground(negative)

The elevation is relative to the geoid that was choosen for projecting the earth surface to a flat map. And that elevation is cruical for our 3D coordinates. It would therefore make no sense to just say "1 ft above ground" as that says nothing about the absolute position of that point (i.e. THAT is what we are looking for: the elevation of the ground!).

The elevation is relative to the geoid that was choosen for projecting the earth surface to a flat map. And that elevation is cruical for our 3D coordinates. It would therefore make no sense to just say "1 ft above ground" as that says nothing about the absolute position of that point (i.e. THAT is what we are looking for: the elevation of the ground!).

that 1' above ground on your house is always going to be 1' above ground in that particular location of the house. the map has been mapped long time ago. and that "hemorrhoid" :lol: of the earth has been incorporated figuring out GPS coordinates. what is a bad idea is incorporating MSL to the equation because you cannot accurately get that value specially in enemy air space. like I said you cannot ask the airbase you're about to bomb what their barometric pressure at the moment