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I follow you...Imagine a plate sitting on a table. That is your antenna dish. Now imagine there's a small depression in the exact center of the plate in which the point of a tall slender top is spinning. Gyroscopic rigidity in space keeps the top perpendicular to the plate. That is your rotating feedhorn.
So the higher the error-signal, the more the antenna adjusts until there is none?Now imagine you can suspend the laws of physics enough to pick up this plate/spinning top assembly and bolt it into an articulated gimbal structure on the front end of your aircraft. This gimbal has powerful little motors that drive the dish left-right-up-a-notch, left-right-up-a-notch, and so forth until it reaches the upper gimbal limit, then repeats the process down to the lower limit and back to level. Since a feedhorn isn't as symmetrical as a top, the beam it projects from the dish at any point in its rotation is not symmetrical around the central axis of the dish. Rotation of the feedhorn causes this asymmetrical beam to form a very slender cone of signal centered on the axis of the dish, strongest on axis and weaker towards the edges. This oprovides the signal strength comparisons that generate the "pointing error" signals used to drive the antenna to put its axis "on target". Once the antenna is "on target", the azimuth and elevation are known and return time provides range.
Because it eventually pings the ground...Those are some of the limitations, to be sure. And your look down angle is also limited by your altitude. Without MTI (and these early radars did not have MTI) looking down will show you the ground out at the end of the range trace. The more you look down, for a given aircraft altitude, the shorter you maximum range to track or detect another aircraft will be.
Oh, I figured that if an aircraft was a sufficient distance from the ground, it would be distinct from it, if close enough, it would blend in.I think you are using resolution cell incorrectly though. The resolution cell is the time occupied by the radar pulse. It is a moving space defined in depth by the duration of the pulse and in width and height by the beamwidth of the radar. The resolution cell is the area where a radar can detect a target is present but cannot resolve the number of targets present in the cell. And it has nothing to do with maximum range.
Which is a negative feedback loop...Errors are how the radar knows where the target is. Yes, you want to keep the errors as small as possible, but to do this you must first find the errors, so you can counter or null them. Every track has errors, no track is perfect, so you want to make the errors as small as possible.
The Conical scan is how the radar, in this case, derives the errors to know what direction to drive the antenna, so as to null the errors.
Boy, that's a tongue-twister and I'd probably have to draw it out to really make sense of it (some things I'm better with visually, some things audibly). Something like a decade or more back, I remember something similar in nature being mentioned about navigation systems.Kind of like this, only substitute target track for missile:
View: https://www.youtube.com/watch?v=F4Dvc1NrZJI
Oh, okayThe Palmer scan is not better at anything, it is simply the combination of two different scan types at the same time. In this specific application the Conical scan is required to track, the Raster is required to search. When the radar has both types of scans active at the same time it is called a Palmer scan. The radar cannot both scan and track at the same time, it is one or the other.
I think I got it...So while searching, or scanning a large volume of space, it is using a Raster scan, it just so happens the beam also has a Conical scan at the same time. While tracking the radar stops the Raster scan, stares at the target, and uses its Conical scan to derive tracking errors.
Could a palmer scan simply stop once it covered a certain zone that would radiate the plane? It sounds silly, but from a purely academic standpoint, it is interesting...For a bidirectional Raster scan, which by definition changes direction for every bar of the raster, you must overcome that inertia, no matter how small or large it may be. For a Helical scan there is no need to overcome anything except friction, the antenna just goes around and around in the same direction at the same rate, stepping up at some point in each revolution. A unidirectional Raster may do the same thing as a Helical, and have no inertia to overcome, or it might stop and retrace, having to overcome inertia each stop.
I'm not sure I really grasp this...No, the portion of the antenna mechanism that rotates the antenna and grossly moves the beam around in space in elevation and azimuth is unrelated to the skewed feed that provides the Conical scan.
Just the feed (which I assume is the pointy thing at the middle of the dish) spins round and round and the dish sits still? Up to this point, I thought the whole dish spun 'round and 'round.A fixed feed antenna, like the SCR-720, just stares (with its beam) straight ahead of the dish. The device the antenna is mounted on (often called a pedestal) then moves the beam around in space, in the case of the SCR-720 in a Helical fashion (360 degrees of rotation, step up, 360 degrees of rotation, step up, etc), in the case of the APG-1/2 in a Raster.
Completely separate from that is the Conical scan. It rotates the beam of the antenna in small circles out in front of the antenna, independent of how the pedestal moves the dish. To do this it is intentionally skewed, mechanically, so that the beam does not go out directly ahead of, and on axis with, the dish. It goes slightly off center of where the dish is looking.
The whole dish spins in this set-up...More often when you want to allow both Conical scan (a spinning off boresight axis beam) and on boresight axis fixed beam operation you don't use a mechanically skewed feed assembly. Instead you wobble the dish itself to create the circular beam motion about the boresight axis. This would result in a nutating beam.
This looks a lot like triangulation, but Token mentioned something about measuring errors through amplitude and phase changes (which I assume to be constructive/destructive interference effects).
A tracking radar will produce the elevation and azimuth errors by either amplitude changes or phase changes, depending on the system design. I am pretty sure the APG-1/2 used amplitude changes.
Oh, I figured that if an aircraft was a sufficient distance from the ground, it would be distinct from it, if close enough, it would blend in.
Could a palmer scan simply stop once it covered a certain zone that would radiate the plane? It sounds silly, but from a purely academic standpoint, it is interesting...
I'm not sure I really grasp this...
Just the feed (which I assume is the pointy thing at the middle of the dish) spins round and round and the dish sits still? Up to this point, I thought the whole dish spun 'round and 'round.
The whole dish spins in this set-up...
I would have thought that the gun laying radar would be shorter range than the search radar, so you would need both?
The SCR-720 did this, with its Helical scan, by spinning the dish structure around and around, continuously, in azimuth, or the X axis, with steps in the elevation, or Y axis.
The APG-1/2 apparently used a bidirectional Raster scan (however I am not 100% sure of that), which requires you to stop at each end of azimuth, or X axis, motion and reverse direction, stepping up or down in the elevation, or Y axis, at each stopping point. The APG-1/2 could have used a Helical scan also, however what little information I can find seems to say it did not.
If I am reading you correctly, you are saying the dish spins on its axis 360° around its vertical axis (ie when viewed from above)?
If so, are you sure about that?
To me it looks like the dish is mounted on forks sitting a 360 degree rotating pedestal, so yes helical scan.
If I am reading you correctly, you are saying the dish spins on its axis 360° around its vertical axis (ie when viewed from above)?
If so, are you sure about that?
http://pwencycl.kgbudge.com/images/S/SCR-720_airborne_radar.jpg
https://www.ibiblio.org/hyperwar/USN/ref/NightFighterRadars/images/P-61Antenna.gif
It looks to me that movement was limited to +/- 90° from straight ahead.
Which would mean it scanned in a similar way to that described below?
No this system had two separate antennas, one in the nose and one in the tail. I think it might have had two separate R/T units too, but I don't remember that or the aircraft type either.I wondered if that was the Wellington AEW.
Token, I remember somewhere back in the mists of time reading of a radar system that had two helical scan antennas, one forward and one aft, that rotated in sync with each other, with a PPI scope displaying the forward sweep, then the aft sweep in one seamless rotating beam for 360 degree coverage. I think it might have been some early version of AEW or AWACS. Does this ring a bell with you? Just curious.
I don't recall any that had two separate antennas, with separate drives, in sync doing this, or any using a helical scan. However I have seen and worked with a few that where somewhat similar to what you describe. All the ones I can recall used a circular scan, not a helical, and most of these have been back to back mounted antennas on a single barbet / pedestal.
Some used the back beam as an identical sweep to the front beam, just 180 degrees out, resulting in twice the hits on a given target in a given time period. Some used the back beam for other information, such as altitude.
But I will look through a few references and see if I can find anything similar to what you describe.
T!
No this system had two separate antennas, one in the nose and one in the tail. I think it might have had two separate R/T units too, but I don't remember that or the aircraft type either.
Cheers,
Wes
What's a phase-change? I assume it means interference effects...Token said:I said either amplitude or phase changes, not amplitude and phase changes.
I think I grasp what you're sayingIt is by comparison of the amplitudes of the target when the Conical scan is in varying locations that does this. If the target is stronger when the conical scan is pointing right and weaker when pointed left then the antenna boresight needs to be driven to the right until the target is equal in amplitude when the Con scan is at both left and right.
Well, I was thinking if the range of the target plane was far enough from the ground that it would appear as a different target than the ground, much like how two planes flying very close appear as one, but once they get far enough...Assuming you are talking about the target aircraft and that sidelobe power does not cover the target (the lower sidelobe will have a shorter path, slant range, to ground), then yes.
Okay, that worksInstead of mechanically stopping the moving antenna (for the Raster) and moving feed (for the beam) it would be simpler to just inhibit the transmitter in areas it might get self reflections. This is what the SCR-720, with its continuous helical scan which pointed back towards the aircraft a lot of the time, did.
Gotcha...Yes, in the case of the SCR-720 and the APG-1/2 the feed is that thing pointing out of the center of the parabolic dish.
OkSeparate out the two functions and tasks in your head. There is a Search function, and there is a Track function. They require different techniques and must be addressed individually. In the specific case we are talking about the Raster scan exists for search, the Conical scan exists for track.
When talking about spinning and turning it can get confusing. Lets call azimuth, right and left, the X axis, call elevation, up and down, the Y axis, and call range the Z axis.
For the search requirement you need to grossly move the beam in space to cover a relatively large volume. In this case you do it by moving the entire antenna. The dish "points" in both elevation, Y axis, and azimuth, X axis, to cover the desired volume of space. The SCR-720 did this, with its Helical scan, by spinning the dish structure around and around, continuously, in azimuth, or the X axis, with steps in the elevation, or Y axis. The APG-1/2 apparently used a bidirectional Raster scan (however I am not 100% sure of that), which requires you to stop at each end of azimuth, or X axis, motion and reverse direction, stepping up or down in the elevation, or Y axis, at each stopping point. The APG-1/2 could have used a Helical scan also, however what little information I can find seems to say it did not.
For the track function you need to finely move the beam in space, just enough to generate track errors so you can center the target. In the case of the Conical scan specific to the APG-1/2 you do this by spinning the mechanically offset feed assembly along the Z axis (other radars may use different techniques to generate the Conical scan). Errors generated by this process then move the antenna boresight, the centerline of the entire dish assembly, in both the X and Y axis to null the errors.
Blind fire tail guns such as "Village Inn" tail radar used on the Lancaster would I think have not been a success. This is because the same technology that made this radar and the equivalent in the B29 possible also made a bigger, more accurate longer range radar in the fighter possible.
You need to brush up on your basic electronics, especially wave propagation. It can't be explained simply and unconfusingly without a lot of illustrations, which I can't do on this phone. Essentially what it means is the return wave is not perfectly in sync with the transmission wave. But there's a lot more to it than that. Check it out.What's a phase-change? I assume it means interference effects.
Not sure that the Village Inn system could be termed "blind fire".
And the supposition that the system would not work because of advances in enemy fighter radar is, IMO, false.
The range of the turret mgs remained the same - and was similar to the guns of the attacking aircraft. If the attacking fighter had weapons with greater effective range then the radar may have allowed it to stay out of range of the defender's guns - with or without Village Inn gun laying radar.
The advantage of the Village Inn system is that it calculated most of the parameters for the gunner, so the likelihood of a hit was enhanced.
The B-29 did not use a Gun Laying Radar system - at least not in production versions. It had remotely operated turrets with computing gun sights.
B-29's certainly had a GE remote control gunnery system complete with computers that required range from the stedometric range finder (worked of wing span) to not only calculate ballistics but also compensate for parallax error. Some also received an AN/APG-8 or 15 tail radar for blind fire as B-29B's were operating at night over Japan and many only had a tail turret.