ISS Communication - delay?
Hello everyone,
I've been interested in space exploration and astronomy for some years now. Recently while I was in a discussion with someone on the subject of the International Space Station, he brought up the idea that there is a 7 to 10 second delay in the live transmission. Now he couldn't remember the source of this number himself either, and I don't know what to make of this. But it did get me thinking about what kind of delays a listener would encounter tuning into the ISS audio transmission.
This apparently being a forum where amateur radio listeners come together to converse about the ISS, I thought this would be a good place to ask some questions about the subject.
Tuning into the live ISS transmissions, what kind of delays do you experience? Is there any significant delay between NASA and the ISS when the two are communicating, and if so, what causes these delays?
Thanks in advance :)
- BertL
Note: I have just joined this forum after browsing around on the website for a bit, so I might be completely mistaken on what this forum is about. Also, I have no idea what the etiquette and rules of this forum are. If I am doing something wrong, please let me know. I'd hate to be a bad guest.
digital vs analog delay
There is a delay on the digital transmissions from 3-10 seconds depending on the number of links and digital encoding/decoding delays. The higher the bandwith (video) the longer the delay.
The delay for a non digital transmission directly received by a ground station is next to nothing.
Kenneth - N5VHO
http://spaceflight.nasa.gov/station/reference/radio/
Support ARISS http://www.amsat-na.com/store/donation.php (select "Human Spaceflight (ARISS))
Delay added by nasa TV
Something else to consider: If the poster's info abt delay came from an unknown source, his friend may have heard abt the typical 7 second delay WRT its being added intentionally to most live broadcast in the TV Radio industry- giving them time to use a "dump button" to avoid certain language or events from going out live.
I believe, (not withstanding all the techinical reasons mentioned already by others) that NASA TV and the originating signals used for rebroadcast on C band satellite, internet, cable TV etc, are all being delayed intensionally by AT LEAST 7 seconds. I seem to recall this started either around the Challanger Explosion, or when unidentiifed obejcts were seen on NASA TV and it started a big UFO debate. Plus possible sesitive military ops etc..
If it is simply a question of delay of the DIRECT over-the-air reception from the ISS on orbit, directly to YOUR antenna/receiver, then read the info in the other replies. But as there is uncertainty abt the posters delay info, and no one else mentions the intensional NASA re-broadcast delay, I thought I'd reply for further comments.
PS
Dont forgot that even on C-band satellite, you are watching /hearing the UPLINK being retransmitted to you on your Downlink trasponder frequency. This gives NASA the oppertunity to delay all they want. I beleive this is the signal used for rebroadcast on Ham repeaters and shortwave radio by the Godard Space Centre Ham Radio Club. Same for Internet etc.
Comparing a directly recvd signal to one you are listening to at the exact same time via ShortWave, internet, or Nasa TV etc, would be fun and to confirm the length of that delay. A confirmable, direct simplex signal from ISS would be best. Such as an EVA direct simplex transmission (not via the repeater) . Any good Earth Sations want to try, or already know the number?
DELAY
This effect is well known to us satellite operators. It's called doopler effect and to correct this lack of signal from 7 to 10 seconds you'll have to correct the frequency of the radio.
There are some computer programs that make this correction automatically and are very helpful in contact. The best known and used by us is the Ham Radio de Luxe that you find on the Internet for free.
Another problem of loss of signal from satellites are antenna. A vertical antenna rejects a horizontal antenna and the satellite signal passes through the atmosphere from the Earth horizontally or vertically. To overcome the problem are correct for satellite antennas with circular polarization. There are several companies that manufacture these circular polarization antennas and there are several pages on the Internet that explain how to build them.
Building antennas and understanding the biases you got much knowledge in this area of electronics.
73´s
MAURICIO BERALDO PY4MAB
py4mab@amsat.org
BRAZIL
Re: Delay
Thank you very much for your answer, it pointed me to a lot of info on the influence of the Doppler effect when communicating with the ISS. I never expected the influence to be as big as it is.
One thing that is unclear to me though, is the influence of the Doppler shift on transmission delays. (Note: I'm pretty clueless when it comes to more technical stuff; pretty much the only things I know about radio frequencies and communication is what I learned in high school.) I'm aware that the answer to this question is probably more complicated, depending on the elevation on the ISS's orbital pass, but I'll ask it anyways.
How long would it take for the ISS's radio transmissions to get to an Earth-based receiver? Knowing that radio waves travel at the speed of light and the ISS is only a few thousand kilometers away (when it is above the horizon) for an observer, a figure of 7-10 seconds for this seems a quite out of place.
DELAY ON ISS FREQUENCIES
For you to understand a little about the effect doopler you just watch a moving train. When the train passes you by sounding the whistle sound travels through the air. While the train is already there in front you will hear the sound of the whistle for a few moments.
Talking about the international space station it travels through space at more than 12 thousand kilometers of speed and more than 300 kilometers high.
As the speed of the ISS is very high speed that ends up causing the same effect as the train whistle. On the train doopler the effect is felt in the sound of the whistle, as the ISS doopler the effect is felt in the frequency of Radio.
I live in South America I use the following frequencies to talk with the astronauts on board
144.490 Uplink
144.800 Downlink
If I stay in frequency without modifying it I lose the signal from the ISS. To correct this effect and not lose the signal I use 3 frequencies that are configured as follows:
chanel 1: 145.805
chanel 2: 145.800
chanel 3: 145.795
When the ISS is coming up on Brazil I will be on channel 1. When she is on the Brazil I will be on channel 2. And when the ISS is falling on the horizon I'll be on channel 3. So I'll compensate the effect doopler.
The VHF has a feature to maintain reliable contacts they receive little influence of the atmospheric layers and you will get contact with ground stations over 200 miles away. As the ISS is 300 miles away you get a perfect contact, without noise because the satellites for ham radio operators, including the International Space Station, are the low elevation.
Here's where you'll see on Youtube a short video I made with all the contacts I had with the astronauts aboard the ISS. Among them are Sergey Krikalov, Leroy Chao. Mike Fincke, William MacCarthur and end the sound of Radio Packet ISS. For these contacts, I used a vertical antenna brand Comet GP9 model with 50 watts of output. This is not the ideal antenna for satellites.
http://www.youtube.com/watch?v=jCVy2foqmHk
MAURICIO BERALDO PY4MAB
BRAZIL
Zero Contribution to Propagation-Delay from Doppler-Effect
comparing apples to oranges .. there may well be a displacement in optimum frequency for earth to ISS communication as a consequence of Doppler Effect. There is no question there is .. a matter of doing the math .. which is pretty straight-forward as far as 'orbital communication maths' are concerned.
As for time-of-flight delay .. consider Ground Control (Houston) communication with lunar expeditions - the moon - as assuredly further than the ISS. There was a 1.5 second round-trip delay .. 3/4 of-a-second either way.
FYI... the ISS's perigee is 190.5 nautical miles (352.8km). Its apogee is 191.3 nautical miles (354.2km). Space shuttle missions have (had) a general-service apogee of about 520mi (1000km).
The time-of-flight contribution to propagation-delay at this distance (round-trip or otherwise) is measured in micro to mili-seconds. The same propagation delay as found in placing a phone call between say Boston and New York .. except less : )
Any (and all) noticeable delay in ISS-to-ground communication is digital-compression and 'packet handling' in nature - as correctly stated by N5VHO.
The Doppler discussion is interesting in it's own right - but not necessarily on-the-mark re: original question. To add to it we can state that if we confidently know the ISS transmission frequency and confidently know our reception frequency (and we know the speed of light) we can accurately assess the ISS velocity (along our line-of-sight). Which is what Mauricio is intimating (along with interesting antennae/atmospheric-polarization considerations/discussion.)
Trust this as a reasonable sizing-up-of-matters.
Tango Tango Out! : )
It also depends on *which*
It also depends on *which* communication media the OP's friend is talking to. If it's NASA TV, IIRC, that signal gets relayed from the TDRS satellites in geosynchronous orbit some 22,500 miles up. From there, it goes to Houston, then from Houston back up to the Clark belt again for public distribution on NASA TV, Channel 212 of Dish network, etc, etc, etc, and then back to Earth for your viewing pleasure. This can be a factor also, but video buffering, encoding, and decoding is in general a larger one.
ISS vs apollo
Earth to ISS transmission delay is 7-8 seconds 280 miles away
Apollo hoax missions had a 0-1.3 seconds transmission delay
240,000 miles away
Earth to ISS
Earth to ISS is nano seconds along a direct path like amateur radio.
Earth to ground station to satellite to ISS and return is about 2-3 seconds via analog and 3-6 seconds via digital.