Apollo Unified S-Band communications

This is great stuff ! Must have missed your previous mention to thanks.

Are you familiar with amateur DSN ? There is some neat stuff amateurs are doing on receiving Mars probes and stuff like that.

It used on experimental bases the two-way ranging method which gives the distance to the ground station with an accuracy of a few wavelengths.

This method is now used with planetary probes so the radial distance and radial speed (doppler). Tangential distance and tangential speed can be derived with some accuracy using orbital mechanics.

Not experimental, any more than the whole program was; it was critical to the trajectory measurement. C-band radar was fine for orbital determination, but impossible at lunar range.

The wavelength that was being correlated was not the S-band carrier, but the 992kbps of the ranging code, and it was correlated to about 1/20 of a wavelength (15-20m). With less noise and jitter the system was theoretically capable of ranging to around 1m accuracy.

After ranging measurement, it was turned off and the range updated using the Doppler (range rate) measurement.

I talk about all this in the lecture - did you even watch it?

Thanks, Clifford. Fascinating stuff.

Thanks very informative. I liked the idea to phase lock the down link to the up link. Is this the usual way to do Doppler measurements in space communication ?

The device on the satellite/probe is known as coherent transponder.

The simplest one would be to get the uplink signal, multiple by two and send it back. In practice more complicated but _exact_ fractions are used to match available suitable frequency bands.

Of course the doppler measurement is obvious.

By sending a long pseudo random noise bit pattern sequence, the downlink receiver can determine how many bits are in flight and hence the distance to the probe. The earth-moon-earth propagation delay is about 2.5 seconds and in the Apollo case the PRN code repetition rate was a bit slower, so no ambiguity would occur.

It avoided the need to take a rubidium frequency standard on the spacecraft. Doppler relies on absolute frequency shift, which means you need to know the transmit frequency very accurately. Using a transponder is good because you can compare received and transmitted signals. The Apollo Doppler was *very* accurate as a result.

Again you show that you haven't watched the video. Apollo used a ratio of 240/221 - those numbers were still recalled by all the 80yo blokes who worked at the sites.

They used multiple short sequences of lengths that were relatively prime, and solved it using the Chinese remainder theorem. As I spent some time explaining in the lecture you didn't watch. Auto-correlating a 5.3 million bit code would have taken centuries with the hardware they had then, for a single reading.

5.3 seconds in fact, more than twice. I don't get why they needed an unambiguous reading however. If they didn't already know whether the spacecraft was approximately 100000, 200000 or 300000km out, they would have had bigger problems than ambiguous ranging.

Clifford Heath.

I didn't finish watching it. I remember the doppler signal counter was in the tracking TDP a bit tricky to adjust. We used to track Apollo Alsep packages, and the Doppler could detect moonquakes between packages. The range machine had solder over the card edges and had poor connections, sometimes you would bang on it to make it to work, but I never had to do that.

Greg

wrote

We used to have fun shaking the equipment on the moon, make earthlings think it was quakes, also jingled it to the point where they got confused and started banging their own equipment. We watched their brainwaves with gioms, and later used that to force them to destroy the Apollo plans.

The strategy worked, they never bothered us again, we really did a good job protecting our moon bases.

Watching now. Thank you

One thought; could you put the projected images [we can't call it a 'slide show' any more...] up, too?

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Current slides are here:

Clifford Heath.

The moon VLBI experiment used two 85 foot ground stations monitoring Alsep packages. Somehow that improved resolution of the Doppler I guess.

Greg

I'm still watching it. But one question comes to mind that you might have answered later.

How can the spacecraft transmit blind? Surely there was a process for them to do so, and further, what about unmanned probes? You'd like to know their position, I'd think.

Another comment -- from what I've read, there were initial fears that the USSR might attempt to jam communications; so much so the the first contract written was to MIT-Draper for the AGC that could navigate the mission autonomously. I'd never read anything about them doing so until recent PR about the first woman launch controller at the Cape, and one of her tasks was monitoring the trawlers off the coast as they had engaged in jamming during launches.

I believe that the outcome was ground-based navigation was found to be slightly more accurate than the AGC but not much.

--
A host is a host from coast to coast.................wb8foz@nrk.com 
& no one will talk to a host that's close.......................... 
Unless the host (that isn't close).........................pob 1433 
is busy, hung or dead....................................20915-1433

Another question:

The pulse train gives you the distance, the Doppler gives the acceleration. But how is that sufficient to locate the spacecraft? If 2 ground stations could get measurements simultaneously.... but that's not possible.

-- A host is a host from coast to snipped-for-privacy@nrk.com & no one will talk to a host that's close.......................... Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433

Doppler gives the speed relative to you, not acceleration.

Doppler gives the radial speed.

Since radial distance and speed is known, the orbital elements limits the possible tangential location and speed.

Assuming a probe is orbiting the Moon, the orbital inclination limits the possible location in north-south direction. By monitoring one orbit, it should ne possible to derive the classical orbital ellipse eccentricity, size and orientation. Monitoring a few orbits and the orbital elements can be further improved, but of course the tangential parameters are less accurate than the very accurate radial parameters.

I'm not sure what you mean by "blind" - do you mean without a received signal? I expect it could work "open loop" (without phase locking) and with known attitude from star positions they could aim the antenna. So I wouldn't have thought that transmit-without-receive was a problem except the ground receivers would have to find the signal. Doppler and ranging would be impossible though.

What has manned/unmanned got to do with anything? Ranging and Doppler was entirely automatic. The stage-3 booster was unmanned, and still had most of this stuff. The AGC had a queue of commands to enact at specified mission times - i.e. with working up&down-links the mission could be (almost?) entirely controlled from the ground.

The USSR would have had trouble jamming the uplink unless they had navigational data to aim a dish (from three at least bases each with

9-10 hours visibility), but it was theoretically possible. They wouldn't have been able to spoof AGC commands through the BCH error-correcting codes though, without knowing the math.

That would not have been S-Band. Close-in stuff used UHF and C-band radar. They switched to S-Band at about 11,000nm, when the spacecraft was far enough away to get contonuous coverage from the three main sites.

The AGC could not calculate trajectory corrections. It needed to be told what to do.

Clifford Heath.

I see it (no rx, ergo no xmit) as a real problem. The core value was always redundancy, redundancy. The idea that they could not transmit without first receiving strikes me as at odds with that. Something unexpected happens and the large dishes can't re-track because the spacecraft can't transmit until they can hear? Nope.

I'm speaking of a bigger arena than Apollo here. The automatic-offset factor was elegant in this case but further out unmanned probes would not be suited to it.

Not disagreeing but know it was a concern early-on. They would not have to have 24x7 success to disrupt the mission.

The question was political not technical; in that era, the USSR was actively jamming shortwave such as BBC, VOA, RFE, etc. But did they make similar attempts against Gemini/Apollo?

I should have said "positioning" vs. navigation.

Thanks again for the talk.

--
A host is a host from coast to coast.................wb8foz@nrk.com 
& no one will talk to a host that's close.......................... 
Unless the host (that isn't close).........................pob 1433 
is busy, hung or dead....................................20915-1433

But by looking at the Doppler shift vs time, you got that as well.

-- A host is a host from coast to snipped-for-privacy@nrk.com & no one will talk to a host that's close.......................... Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433