karn@petrus.UUCP (05/17/85)
This is a question that's bugged me for some time. Those of us who are space junkies have gotten used to listening to space-to-ground audio links that are, shall we say, somewhat less than "telephone quality". While the overall intelligibility has gotten better over the years, it is still worse than I would expect. In the case of the space shuttle, which uses digital transmission (delta mod) for its primary (non-UHF) audio links, I had assumed that the noise you hear when an astronaut opens his mike must be due to things like ventilating fans on board (after all, air doesn't move by convection without gravity). However, when Owen Garriott made his famous DX-pedition, using the same headset connected to a 2m FM handie-talkie, his audio quality was absolutely clear -- virtually no transmitted noise or distortion. If I hadn't already been familiar with his voice from things like interviews and press conferences, I would have suspected a hoax. So where does all the noise come from in the NASA chain? Do the orbiter avionics use large amounts of speech compression or clipping, or what? Should we hams offer to replace all of their communications gear? Phil
jcp@BRL.ARPA (Joe Pistritto) (05/17/85)
Some of the noise on the air to ground is due to marginal conditions getting into the ground stations. (On the last STS, I could tell how close to LOS/AOS the shuttle was by the signal/noise ratio on the air to ground). Some ground stations seem to be very good (TDRS is pretty good, the Indonesia/Australia ground stations are pretty bad). A lot of this is due to the telephone links from the remote sites to Goddard, which are highly variable it seems. -JCP-
wmartin@brl-tgr.ARPA (Will Martin ) (05/18/85)
A good question, and an opening for a topic I had been meaning to post for some time now, and always forgot to... What has the radio equipment available to the astronauts been, over the history of the manned-spaceflight program, and especially now, in the Shuttle? As a longtime DXer, I have always wondered what it would be like to have radios in orbit, and be able to tune around and see what I get. Can the astronauts do this? I would expect that you'd get some shortwave and medium wave signals -- the portion that punched through the ionosphere and wasn't reflected, and that part radiated straight up, which doesn't get reflected, as you pass over those antennae. Higher frequencies would go right through the ionosphere, of course, but I'd expect to get a mixture of everything on the hemisphere transmitting on that particular frequency (with the closer transmitters dominating, so it would constantly change as the receiver orbits). So, do the astronauts now have general-coverage receivers that they can tune, or is all the radio gear fixed-frequency and dedicated to the mission? What about video reception? Do they have PAL and SECAM and NTSC equipment to look at TV signals from all over the world as they orbit? I like to think of getting a mix of every US station on channel 5 at the same time... surrealistic TV DX! The latest episode of the PBS "Spaceflight" series mentioned how deadly bored the Gemini astronauts were during scheduled sleeptimes (when they weren't sleepy) and during the last part of long-endurance missions. I DX at times like those -- did they have the chance to? What about antennae? Spacecraft being metal cans, it probably is necessary to get some sort of antennae outside the craft and the signal fed through. Are there spare or unused antennae that can be used for recreational radio reception? What about HF -- are there reeled-out longwire antennae on the Shuttle for HF, or is that portion of the spectrum ignored? (Could you receive LF or VLF from orbit, or are those all ground-hugging signals?) Any NASA types out there who have info on the the radio-related aspects of the space program, please post info on this! Inquiring minds want to know! Regards, Will Martin USENET: seismo!brl-bmd!wmartin or ARPA/MILNET: wmartin@almsa-1.ARPA
Taylor.Wbst@Xerox.ARPA (05/21/85)
WILL, I'VE WONDERED THE SAME THING. IT WOULD CERTAINLY BE A FASCINATING STUDY--JUST TUNING ACROSS THE FULL FREQUENCY SPECTRUM WITH DIFFERENT CONDITIONS OF DAYLIGHT, SUN SPOTS, ETC. I'LL BET THERE ARE SOME SURPRIZES OUT THERE! CHEERS! JIM (W2OZH)
Taylor.Wbst@Xerox.ARPA (05/22/85)
WILL, I AGREE!-- IT WOULD CERTAINLY BE AN INTERESTING EXPERIMENT! I'LL BET THERE ARE SOME SUPRISES IN STORE FOR THE FIRST TO DO THIS ALL-BAND MONITORING. CHEERS! JIM (W2OZH)
karn@petrus.UUCP (05/25/85)
Regarding my own question, I recently had the chance to put it to an astronaut (Ron Parise, WA4SIR). He said he had wondered the same thing, and didn't really know the reason for certain. We came to a consensus on a couple of possibilities, which I paraphrase here: 1. Pickup from noise sources on board (Owen Garriott might have just found a quiet spot when he operated 2mFM). The orbiter is filled with blowers, fans, and is quite noisy. These might have been amplified by compressors in the regular speech processors (see #4). 2. The air-to-ground links are digital only to the TDRS ground station. From there they are relayed in analog form over a concatenation of satellite and landline links which can each add their share of noise. 3. Noise in the wireless headset links used on board. 4. Large amounts of compression and/or clipping in the speech processors, combined with incorrect gain settings and astronauts talking away from their microphones. Regarding the sub-topic of monitoring the HF bands from orbit, I suspect that conditions on the lower frequencies would be rather poor. We hams have already had experiences with sending signals through the ionosphere with the Oscar and RS Mode A downlinks on 10m, and the HF beacons experiment on UoSAT-Oscar-9. In each case, if the band in question is "open", you'll have a hard time penetrating the ionosphere for earth/satellite communications. Even when a low frequency band, e.g., 80m, is closed, it's because of high ionospheric absorption so again you're going to have a rough time. For example, whenever 10 meters is open, Mode A reception when the satellite is just above the horizon is very poor and in fact you're just as likely to hear the satellite when it's below the horizon. DXing the VHF/UHF bands and above is likely to be more productive, since you can see everything in line of sight and the ionosphere rarely interferes. I remember a story a while ago (I forget where) which listed a few UHF TV stations which were receivable on the moon. The criteria wasn't signal strength, it was lack of other interfering assignments to the same channel. I remembered this story because one of the stations cited was WMPB-TV on channel 67 in Owings Mills, MD, where I used to spend college summers working as a broadcast technician. Phil
cjl@ecsvax.UUCP (Charles Lord) (05/30/85)
my only question is- "who did the TVDXpedition to the moon to find out?" Charles Lord WD4CHW cjl@ecsvax.UUCP