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Author Topic: HGA re-vamp  (Read 1309 times)
eddievhfan1984
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« on: January 11, 2017, 07:30:18 PM »

So I'm doing some work on the 2016 side of things, before we try migrating code to each stage's own vehicle. I'm trying to work out how to do HGA auto pointing.

I thought about trying to directly model the real-world tracking method (most likely phase-cancellation caused by the circular polarization of the S-band signal), but that's kinda overdoing it a bit. So a thought—if we already can determine the angle to the radio sources to simulate the signal fall-off, we can just use that as the basis for auto pointing, right?
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indy91
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« Reply #1 on: January 11, 2017, 07:38:00 PM »

Yeah, I can work out and implement the geometry of the four parabolic dishes. They are each offset 10° from the centerline of the antenna. And I guess the signal strength from these four is then used in the electronics to determine the direction of the signal for the auto mode, right? You can then handle what the HGA electronics are doing. Of course if you want to do this geometry stuff yourself, feel free. Happy
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rcflyinghokie
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« Reply #2 on: January 11, 2017, 07:44:22 PM »

This reminds me actually (I meant to ask and I know it's an easy change) but did Apollo 9 use a HGA even though it remained in LEO?  If not, how do I remove it from the mission?
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indy91
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« Reply #3 on: January 11, 2017, 07:53:38 PM »

Looks like one to me: https://www.flickr.com/photos/projectapolloarchive/21963814312/in/album-72157659522900305/ In the Apollo 7 launch scenario there is a special parameter, "NOHGA 1" or something like that.

Fun fact, ASTP also carried a HGA and used it for communication with a geosynchronous satellite.
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eddievhfan1984
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« Reply #4 on: January 11, 2017, 08:00:51 PM »

Yeah, I can work out and implement the geometry of the four parabolic dishes. They are each offset 10° from the centerline of the antenna. And I guess the signal strength from these four is then used in the electronics to determine the direction of the signal for the auto mode, right? You can then handle what the HGA electronics are doing. Of course if you want to do this geometry stuff yourself, feel free. Happy

Based on what I read, I thought the dishes where all along the HGA boresight, and it was the feedhorns which were offset from the boresight. But maybe I read it wrong.

Basically, a lot of the logic is there, but we need to just move the pointing logic AFTER the signal strength logic, instead of how it is now. It also means not storing signal strength on the last timestep for comparison (which is what I thought it was originally gonna be). So we just need to decompose the HGA response pattern into that of the individual dishes.

Also, do we have B-axis pointing enabled? Since I know we control A and C only in manual, the AUTO and REACQ modes permit B axis rotation to allow more continuous tracking.
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indy91
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« Reply #5 on: January 12, 2017, 01:15:03 AM »


Based on what I read, I thought the dishes where all along the HGA boresight, and it was the feedhorns which were offset from the boresight. But maybe I read it wrong.

No, I think you are right. The Systems Handbook has a helpful schematic, although it doesn't go into quite as much detail as for other systems. So the HGA does conical scanning and no separate signal strength calculation is required for the separate antennas?

Quote
Also, do we have B-axis pointing enabled? Since I know we control A and C only in manual, the AUTO and REACQ modes permit B axis rotation to allow more continuous tracking.

B-axis is not implemented yet. The manual commanded pitch and yaw commands are not even translated into the correct A- and C-axis for the gimbal drives yet. I'll look into it.
« Last Edit: January 12, 2017, 01:20:43 AM by indy91 » Logged
eddievhfan1984
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« Reply #6 on: January 12, 2017, 05:51:50 PM »

Not quite. Because of the circular polarization of the S-band signal, the signal strength is more or less equal in all the HGA transceiver components. However, they do show phase differences, which is used to derive aiming, making it more like a monopulse tracker. However, until Orbiter 2024 includes radio propagation logic, I don't think phase modeling is practical. LMAO

But that leads to another problem. The distance from one antenna focus to a adjacent one is about 10 feet or so, right? At a range of 250,000 miles, that's an angular offset of about 7.6 nanoradians. That's well within each antenna's individual lobe size. So even if you simulated signal fall off, you couldn't get enough of a gain loss to do signal strength tracking.
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dseagrav
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« Reply #7 on: January 14, 2017, 10:11:29 AM »

It should be good enough to just say "If the station is within X degrees conical of the beam and the distance is acceptable, we can track it, otherwise we can't", possibly applying a random lossage factor at the outer edges of the cone to give it some apparent realism. That's what I had planned to model for AOS/LOS purposes anyway.
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