another newbie with a few questions

[159th_Viper]

Here's a short track of Inflight Alignment and takeoff from Batumi.....

 

There's nothing even closely resembling INS alignment, in-flight or otherwise.

 

 

..There's nothing in the flight manual that says they require a 4 minute alignment. If that's the case, fine....

 

That is indeed the case and I would have thought it's obvious what with the JDAM being an INS-Guided weapon. Accordingly, if the INS data is unavailable/corrupted/incomplete it stands to reason that JDAM selection will return the relevant error, ie Alignment Unsatisfactory.

 

In any event, the manual is clear: INS Ground alignment takes anything up to 5 min and In-Flight alignment 5 -10 minutes.

 

 

Utilize the stock check-lists, alternatively those provided by members - will eliminate errors such as these.

[Hoozin]

...

I will happily admit you're right here.

 

Was a fun discussion at least.

Edited September 15, 2011 by Hoozin

[Eddie]

As Viper said Hoozin, there was no alignment at all in your track, hence the problems.

 

I've took control of your track and did a quick demo start up for ya, going pretty much by the 'book' (as accurate as possible without actually getting the 'book' out). As you'll see it takes a bit longer than yours did. But the when it comes to these things the phrase 'less haste, more speed' very much applies.

Cold Start.trk

[Druid_]

They can, and they do. However the accuracy of their alignment is only as good as data they are provided by the parent aircraft's nav system. So in other words, poor EGI alignment = poor GBU alignment = insufficient GBU alignment for release.

 

And for the record, JDAMs are not GPS guided, they are INS guided with GPS updates to the INS (yes there is a difference).

OK so we are happy that the GBUs take their position & flight data from the aircraft at time of release. So, this information is taken from the EGI which is being supplied by the GPS and therefore very accurate, so no poor EGI alignment there.

 

So if we do not do a full EGI align on the ground (scramble start) then why is this affecting the GBUs? When you also imply the GBUs position is updated by its own GPS then the previous points made make little sense.

 

The only thing that I can see as a problem is the ATT mode(*) of the GBUs INS. It is supplying flight data as well as position (GPS updated) data. Why this is tied in to the aircrafts time to full align I think is the question to ask here.

 

(*) There are various stages in an INS alignment, the first involves orientation with the Earths axis which for most laser gyros only takes 3 minutes.

[Eddie]

The weapon has to know where it is to start with, a GPS fix can only tell it its position on the earth's surface, not its position in 3d space.

 

GPS cannot and does not provide enough data alone, you still need data from the INS portion of the EGI. Now while a fast alignment is enough to allow you to fly the aircraft, it isn't enough for JDAM. Think of it this way, a 10m error in your INS is of no consequence to you when flying, a 10m error in a weapons INS is a potentially a BIG problem.

 

You seem to be forgetting that and aircraft/JDAM needs to know more than simply its position over the earth's surface. Heading, pitch, roll, yaw, elevation and speed are all important.

Edited September 16, 2011 by Eddie

[Druid_]

The weapon has to know where it is to start with, a GPS fix can only tell it its position on the earth's surface, not its position in 3d space.

Incorrect, unless you are receiving only 2 satellites, GPS will give you a 3D triangular fix (a.m.s.l) that is several times more accurate than any INS. In fact INS alone cannot provide you with a 3D position in space which is why the ADC provides speed and altitude data to the GBU. ALso I doubt an INS exists that is accurate to 10m even after 30 mins of flight.

 

You are fixing on positional error alignment of the INS, I think you may have misunderstood my previous post. Regardless of 'INS DRIFT' the onboard GPS will take care of it. Both aircraft and GBU. Thats what they are there for.

 

You seem to be forgetting that and aircraft/JDAM needs to know more than simply its position over the earth's surface. Heading, pitch, roll, yaw, elevation and speed are all important.

 

Not at all see prev post.

The only thing that I can see as a problem is the ATT mode(*) of the GBUs INS. It is supplying flight data as well as position (GPS updated) data. Why this is tied in to the aircrafts time to full align I think is the question to ask here.

 

(*) There are various stages in an INS alignment, the first involves orientation with the Earths axis which for most laser gyros only takes 3 minutes.

ATT stands for ATTitude. A GPS can provide Heading, pitch, roll, yaw, elevation and groundspeed although I believe some of these are fed from the aircrafts ADC.

Edited September 16, 2011 by Druid_

[Eddie]

You're assuming the aircraft/weapon nav system is more intelligent than it is.

 

The fact of the matter is that YES the sim is modelled as per the real aircraft as far as this goes, apart from the retry caption/status which is not implemented at present, the SMEs involved in the project have ensured that is the case.

Edited September 16, 2011 by Eddie

[Druid_]

ok, understood, was just interested in knowing why.

 

Is it possible to do an inflight alignment of the GBU? I have tried to no avail. Also btw there is no mention of ALN UNS in the manual that I could find.

[159th_Viper]

Is it possible to do an inflight alignment of the GBU? I have tried to no avail....

 

It's my understanding that the In-Flight alignment is merely a 'realignment' in-flight as a result of a borked ground alignment. As such not sure whether the in-flight alignment will be of a sufficient fidelity to satisfy JDAM deployment.

 

Also btw there is no mention of ALN UNS in the manual that I could find.

 

New feature introduced in 1.1.0.9 - manual needs to be adjusted to reflect.

[normntaz]

I don't know if I'm totally following this, but even if the JDAM has GPS in perfect working order, and is perfectly aligned it still might not hit the target if the Aircraft's nav system isn't also aligned properly. When you create a SPI on your TADS or with the TGP, the coordinates of that SPI are only going to be as good as the aircraft's alignment state. The bomb may happily go to the coordinates it is given, but those coordinates may not be trusted.

 

I think that a quick launch scenario where you have to get in the air quickly would be better served by using an LGB.

 

</Pure Uneducated Speculation>

[Eddie]

ok, understood, was just interested in knowing why.

 

Put simply the GPS side of the nav system doesn't tell the weapon/aircraft where it is as such, it's just an error correction/prevention system. Military aircraft in general are far less reliant on GPS than their civilian counterparts, GPS can be jammed, INS cannot. Not only that, but also consider than in all but the newest military aircraft the GPS part of the nav system have been essentially strapped on to existing INS systems and therefore don't have the same integration that newer systems might have.

 

Is it possible to do an inflight alignment of the GBU? I have tried to no avail. Also btw there is no mention of ALN UNS in the manual that I could find.

 

An in-flight alignment is a 'get you home' function. If you need to do an in-flight alignment (due to loss of power etc) your combat mission is over. Even a sub 4 minute ground alignment is better than a 10 minute in-flight alignment.

[Druid_]

I am talking of the GBU aligning in flight and not the aircraft. The aircraft in-flight alignment works fine & I've had no problems there. THe EGI knows to take position data from the GPS.