F15E Radar question.

[Bill Silvey]

Hey all...

I'm an enthusiast of the game Janes F15 (a fairly high fidelity F15E sim;
old but still great) and I was wondering why the A-G radar mode works the
way it does...

Essentially, to get a patch-map of the target area, the target area can't be
in the "bore" of a sixty-degree arc directly in front of the aircraft. It
has to be within the port or starboard thirty degree arcs, and cannot
be...well, it's hard to describe but if you know what I'm talking about this
should make sense:

When the radar is set to A-G mode, and you're trying to make a patch map, as
I said the cursor cannot be directly in boresight, nor can you bring the
cursor 2/3rds of the way down the MFD. Attempting to do either gets you a
GIMBAL LIMIT error and the radar won't scan the area you tried to snap.

What this means is that if you want to map the target area, you must
approach it offset so that it is in the ten-to-eleven o'clock position or
the one-to-three o'clock position; once you've built the patch map you can
then pick out targets on the map and enter attack mode.

So, why is the radar so limited in it's coverage? The blind spot directly
in front of the radar is particularly puzzling. I mean, if I'm in A-A, the
whole antenna does TWS or of course Boresight modes or whatever.

--
http://www.delversdungeon.dragonsfoot.org
Remove the X's in my email address to respond.
"Damn you Silvey, and your endless fortunes." - Stephen Weir
I hate furries.

[Simon Robbins]

"Bill Silvey" > wrote in message
...
> Hey all...
>
> I'm an enthusiast of the game Janes F15 (a fairly high fidelity F15E sim;
> old but still great) and I was wondering why the A-G radar mode works the
> way it does...
<snip>

It's been a long time since I played F15, but... are you sure the radar
isn't painting the area in the front, and you're not getting confused by
trying to slave the weapon seeker from the radar? That might be limited by
the location of the weapon on the aircraft or the laser designator pod.

Si

[Joey Bishop]

"Bill Silvey" > wrote
>
> Essentially, to get a patch-map of the target area, the target area can't be
> in the "bore" of a sixty-degree arc directly in front of the aircraft. It
> has to be within the port or starboard thirty degree arcs, and cannot
> be...well, it's hard to describe but if you know what I'm talking about this
> should make sense:

I've never worked on a Synthetic Aperture Radar (SAR), but I think the
theory is, that you are simulating a long antenna by flying along-track and
gaining very fine resolution:

http://www.sandia.gov/radar/whatis.html

Probably to save computer processing time, the F-15E radar gives you
only a quadrant that is valuable to targeting a weapon (ahead and off
boresight).

The stuff straight ahead (boresight) is of little use in simulating a big
antenna.

Just a Guess...

[patrick savoie]

Actually it is because radars need to take mutliple beam sweeps to map the
ground. Everytime it hits the same patch of ground the angle between the
aircraft (actually the radar antenna) changes a little compared to the
target area (ground) and this slight change is used to make the calculations
as to terrain shape.

If the radar attempt to map the area in front of the aircraft then the angle
is too small for accurate calculations and directly in front of the nose of
the aircraft (called boresight) then the radar simply returns distance since
calculation are not possible

Any book on radar principals of operations could give you the exact details
but it makes for some heavy reading.

Before anyone flames me on my simplified explanation .. I kept it as simple
as possible so that it can be understandable .. and I'm writing this late at
night after my shift at work .. which happens to have included CF-18 radar
repairs.

Pat

"Bill Silvey" > a écrit dans le message de
...
> Hey all...
>
> I'm an enthusiast of the game Janes F15 (a fairly high fidelity F15E sim;
> old but still great) and I was wondering why the A-G radar mode works the
> way it does...
>
> Essentially, to get a patch-map of the target area, the target area can't
be
> in the "bore" of a sixty-degree arc directly in front of the aircraft. It
> has to be within the port or starboard thirty degree arcs, and cannot
> be...well, it's hard to describe but if you know what I'm talking about
this
> should make sense:
>
> When the radar is set to A-G mode, and you're trying to make a patch map,
as
> I said the cursor cannot be directly in boresight, nor can you bring the
> cursor 2/3rds of the way down the MFD. Attempting to do either gets you a
> GIMBAL LIMIT error and the radar won't scan the area you tried to snap.
>
> What this means is that if you want to map the target area, you must
> approach it offset so that it is in the ten-to-eleven o'clock position or
> the one-to-three o'clock position; once you've built the patch map you can
> then pick out targets on the map and enter attack mode.
>
> So, why is the radar so limited in it's coverage? The blind spot directly
> in front of the radar is particularly puzzling. I mean, if I'm in A-A,
the
> whole antenna does TWS or of course Boresight modes or whatever.
>
> --
> http://www.delversdungeon.dragonsfoot.org
> Remove the X's in my email address to respond.
> "Damn you Silvey, and your endless fortunes." - Stephen Weir
> I hate furries.
>
>

[Paul Hirose]

Bill Silvey wrote:
>
> So, why is the radar so limited in it's coverage? The blind spot directly
> in front of the radar is particularly puzzling. I mean, if I'm in A-A, the
> whole antenna does TWS or of course Boresight modes or whatever.


When a Doppler ground mapping technique is being employed the
direction of your line of sight matters a lot. The difference in
Doppler between targets A and B is very small. On the other hand, C
and D are in the position of max Doppler difference and are much
easier for the radar to resolve. The size of this difference is not
the only factor involved, but it's very important.

->- A
B



C D

For instance, the B-1B Offensive Systems manual says, "The HI-RES
ground map processing uses Synthetic Aperture Radar (SAR) techniques
and as such must look off the flight vector by [deleted] degrees or
more to get a good resolution map. A HI-RES map can be taken near the
flight vector, but the quality will be very poor."

Problems also occur at steep lookdown angles. The signal processing
becomes sensitive to altitude errors for such targets. The B-1B manual
has an elaborate discussion of all the factors involved in getting a
top quality ground map.

Often, sophisticated radars will have a "real beam" mode which
functions much like an old fashioned pulsed radar. The B-1B does: "The
primary use for Real Beam ground maps is to provide mapping capability
near the velocity vector (aircraft track) where HI-RES maps cannot be
made, and for large area mapping."

--

Paul Hirose >
retired USAF radar maintainer

[Joey Bishop]

"Paul Hirose" > wrote
>
> When a Doppler ground mapping technique is being employed the
> direction of your line of sight matters a lot. The difference in
> Doppler between targets A and B is very small. On the other hand, C
> and D are in the position of max Doppler difference and are much
> easier for the radar to resolve. The size of this difference is not
> the only factor involved, but it's very important.
>
> ->- A
> B
>
>
>
> C D

Targets C and D are known as "tangential targets." That is, their Doppler
is getting close to zero. The benefit of SAR is not really in the Doppler. It
is in the long look at the target. Targets straight ahead would be like looking
off the side of your parabolic antenna. What you want is a parabolic
antenna that is 5 miles wide:

->->->->->->->->->

C D

Now you can crunch the target data and simulate say an effective 1 degree
beam with a physical 3 degree beam (which at 5 miles can cover a whole
city block, while a 1 degree beam can focus on buildings).