CSM VHF Ranging

[Thespacer]

Hi

Use the latest build (1597), I’m having great difficulty getting the CSM VHF Ranging to work in Apollo 11, just after undocking and DOI. I follow the steps to activate VHF ranging per the Apollo 11 Operations Checklist F10-20, and am running P20 (V87 gets me the tracker light on the DSKY). The LM VHFA XMTR switch is in the ranging position. Using prior builds last year, I do not recall having any such issues whatsoever. Has anything changed in the last few builds or can someone verify if they can get it to work? I can supply a scn if needed.

 

[n72.75]

I did some refactoring with this system last year, October I think. I tested it pretty thoroughly though, if you could post a scenario I'd be glad to help you out.

The only thing I can think of off the top of my head without seeing it yet, would be to try switching vhf antennas.

 

[Thespacer]

Thanks n27.75, I've attached one scn. I'll try switching antennae around in the meantime. Greatly appreciate your assistance (as well as your efforts in general to expand the sim).

 

[n72.75]

Hey. Congrats. You found a bug.

I just submitted a pull request with a fix for it. Nothing will change in the scenario side of things, so you can pick right back up where you left off and the scenario you you send me will have VHF ranging working, when the fix is merged.

If you want a quick workaround, you can switch the LEM PCM to high bit-rate, and that will allow VHF ranging to work until the fix is merged and a new build is released.

Thanks for testing and helping us make NASSP better.

 

[Thespacer]

Hey. Congrats. You found a bug.

I just submitted a pull request with a fix for it. Nothing will change in the scenario side of things, so you can pick right back up where you left off and the scenario you you send me will have VHF ranging working, when the fix is merged.

If you want a quick workaround, you can switch the LEM PCM to high bit-rate, and that will allow VHF ranging to work until the fix is merged and a new build is released.

Thanks for testing and helping us make NASSP better.

Thanks! Happy to help. I tried the quick workaround and it indeed worked, although I note, once I get ranging to work, the ranging does not appear to care greatly which position the PCM switch is in. But more curiously, I reverted to an earlier scn I saved from the same flight - just after the Sep burn - and I confess I am still unable to get ranging to work, regardless of the position of the LM PCM switch. It is rather odd. I have attached that scn, if it might help indicate if I'm making an error or if there is something underlying.

(Edit: the EMS rotary switch was accidentally set to display Delta V, but even in VHF RNG it displays 0)

(Edit 2: I did some more testing with the scn attached below. I proceed with the flight plan in preparation for DOI, and all the way until approx T -15mins from DOI the ranging doesn't work; then, inexplicably to me, at about 15 mins to go, the ranging starts to work (at a range of about 1.9NM). I did not manipulate any of the switches that are relevant to ranging in either the CSM or LM at that time. Is there a minimum range at which VHF ranging works? Am I missing something?)

 

[n72.75]

Hey, I saw you on IRC, this morning but I didn't catch you in time. I haven't forgotten about you just had a few other things to look at with NASSP, and then life got in the way for a bit.

I'll take a look at it soon.

 

[Thespacer]

Thanks n72.75, no rush at all, appreciate your support!
I did some more testing of other situations - namely lunar ascent - and can indicate the following: as TIG approaches and the CSM is basically overhead, I noticed that VHF ranging kicks in about 60nm, goes to about 57 (obviously equivalent to altitude above the landing site), and then rises, but cuts out abruptly at about 62nm, thereafter displaying 0.

I also cannot get VHF ranging to work while in P20 after LM insertion, while in P32 and awaiting CSI.

Hope that helps!

 

[Thespacer]

Just to close this out, the latest builds appears to have cured the problem I noted above. My thanks to n72.72 and the team.

For those who are curious, I went through lunar liftoff to rendezvous on Apollo 11. The full 3-crew procedure requires the CSM to, among other things, take VHF and sextant marks, while in P20, to update its knowledge of the LM state vector. (Note that, in Orbiter, the LM/LM’s tracking light is only visible from about 108NM, far less than real life, I understand).

While it appears you can successfully rendezvous without doing anything at all in the CSM, there is a certain level of satisfaction in completing just about all of the CSM’s tasks as well, which, if nothing else, gives you a level of comfort that the rendezvous is proceeding accurately. Indeed, this is really what the CSM was doing, acting as the backup active vehicle. And observing the LM/LM tracking light right in the middle of the sextant from about CDH onwards, is probably the best proof you have that the rendezvous is going well.

When it came round to TPF, I activated V89 in the CSM (to track the LM through the COAS), and, again, seeing the LM just about right in the middle of the cross-hairs is quite a rewarding sight after such a long and complicated journey.

So, well-done to the developers again.