Transmission to Mars and beyond?

60-80 foot dishes, mostly. One is at Goldstone, AZ, but they have receiving sites around the world so they can get data when it is opportune for the mission hardware to send. I think most of this has now moved to Ku band, 12-18 GHz or even higher, like Ka band (18-24 GHz).

The moon is no problem, hams can bounce signals off it with home made equipment. They still get a little data from the Voyager probe, which is now outside the solar system. That takes a really huge antenna like Arecibo, and a data rate of a couple bits a second.

Jon

AFAIK, bit rate to locations like Mars (various), Jupiter (Gallileo) and Saturn (Cassini) are in the 5kB/s range, from high gain antennas (parabolic something or other), in the GHz range, and in the tens of watts range (vacuum tubes are still used, actually -- TWTs). Dunno what modulation they use -- I would guess BPSK, or four point PSK or QAM, since these tend to be the best for weak or noisy channels.

Picking up really distant signals is cool. They physically record signals from radio observatories around the world, correlate the datasets by atomic time, adjusting for Earth's rotation with respect to the target location with Doppler compensation, and even relativisitic corrections due to the Earth's gravity, rotation, and who knows what else. Finally, multiple signals are correlated to find the peak, which rises magically out of the noise.

With big antenna arrays (VLB for instance), the SNR can be less than -40dB. It's pretty awesome.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"Ramon F. Herrera"  wrote in message 
news:1eba6db4-6ae7-421e-b382-8aa9b0179116@w8g2000vbx.googlegroups.com...
>
> Can somebody please explain or point to info sources? This is only for
> my curiosity, awakened by the upcoming Mars lander.
>
> How large are the earthbound antennas? Is it only one or they connect
> together across continents to have a large "area"??
>
> I suppose that as the vehicle goes away the bit rate has to diminish.
> What is the bit rate to Mars? The Moon? Jupiter? What is the farthest
> transmission possible?
>
> How about frequencies? They are probably reserved so nobody on Earth
> uses them?
>
> TIA,
>
> -Ramon

Look up the Viking..

Try here:

bye delo

"Ramon F. Herrera" ha scritto nel messaggio news: snipped-for-privacy@w8g2000vbx.googlegroups.com...

"Ramon F. Herrera"

** Voyager 1 was launched in 1977, it is now over 16 hours away by radio.

It sends out the odd " SMS " and gets one back too.

.... Phil

This is a reasonable introduction to the JPL deep space network which normally handles telemetry from space probes. Short on details though.

No they do operate sequentially around the globe though but to obtain full signal continuity and keep the data flowing. Dishes look to be about the same size as VLA main antennas.

Arecibo is much bigger and can transmit a sufficiently powerful short bursts to do precise radar ranging of the inner planets. And nowadays also to image a few asteriods that come close to Earth:

That is a much harder question than it sounds. I am guessing but their datarate now is about the same as one channel of terrestrial digital TV ~2-3Mbps. It has been better than 100kbps since about 1977.

The transmitters in Voyager are getting weaker through radioactive decay and distance but our sophistication in signal detection from noise and antenna design is getting better all the time. If we lose contact then knowing their trajectory it is still possible that the SKA will be able to reacquire them again when it comes onstream.

I can't vouch for its accuracy but this looks like a reasonable review.

Not sure what they use for space probes but I'd hazard a guess it isn't too far outside the protected radio astronomy frequencies - certainly the big radio astronomy dishes sometimes get used with minor tweaks to the receivers and back end.

For important missions like Russia probes to Venus Jodrell Bank was well equipped to eavesdrop on their telemetry and famously Londoners were the first in the world to see pictures from the surface of Venus. This sort of signal intercept of Russian space probes by the UK persisted until at least the mid 80's (maybe still does).

--
Regards,
Martin Brown

It is a very asymmetric datarate. The command channel to the spacecraft is very slow as you say with enormous effort at error correctability to give the feeble small antenna on the space probe a chance of detecting it. The effective uplink speed can be as low as 1kbps (which is pretty tedious if you want to alter the firmware whilst in flight).

But the downstream link is quite fast since the big dish can grab a decent amount of signal, and power consumption, size and weight is no object on the receiver electronics and signal processing on Earth.

It was already good enough for Fax data transmission when Luna 9 landed on the moon in 1966 - UK Daily Mail published the moon pictures first! (leading to a minor diplomatic incident)

I think the latest probes are expected to reach a datarates >2 Mbps. (roughly one DTV channels worth of bandwidth)

I think for most of the downlinks any one dish will do to listen (except possibly for probes that are at the edge of the solar system).

--
Regards,
Martin Brown

"Martin Brown"

** Really ?????

The uplink has nearly unlimited power capacity available and an antenna with massive directional gain.

** Just as it can concentrate the beam of a massive one.

** Makes no sense to me.

You blowing it right out your arse??

... Phil

The uplink is not usually a problem, transmitter power and directivity (gain) is easily available on ground based equipment. 1 kbit/s should be plenty, with flight time in years.

The downlink is far more complicated due to the limited power budget at the spacecraft. In addition to use large antenna gains at the spacecraft, some means are required to stabilize the craft in order to use a directional antenna and hence antenna gain.

The first pictures showing craters on Mars were taken by Mariner IV in the flyby in 1965. The 21 pictures were stored on tape and sent back to earth a few days later at 8.33 bits/s.

The available power for the transmitter is severely limited in outer planetary probes, thus limiting the speed pictures can be sent to earth. in order to use a big (and hence highly directional) antenna at the probe, extremely good attitude control is needed.

This was just a basic telephoto system over some voice grade (3 kHz channel).

The mars station at goldstone handles much of deep space operations. I think it's currently 230 feet diameter. S band was the prime band used,

2275 MHz for Apollo missions, usually handled by prime 65 foot dish antennas, goldstone Apollo, Spain, Australia. Other dishes located at goldstone, with most stations shielded by mountains. I used to work at Apollo, before earth orbiting satellites were monitored by orbiting relays, downlinked to white sands. We at Apollo used to monitor the moon, as well as sun orbiting, Helios. That was a bugger to tune in.

Greg

and

what

=20

=20

I'll bet that was the net rate. Maybe over 10 times higher with all the ecc. The original spec for CDs had enough ecc to end up recording approximately 2.5 bits for each bit of music recovered. You can = calculate the gross bit rate from the disc if you want.

=20

ECC in the early 1960's ????

They used time diversity, i.e. the tape was replayed twice and the recordings were compared on earth.

The 8.33 bit/s was the gross data rate at Mars distance, including telemetry, 33.33 bit/s was used closer to Earth.

The first playback occurred from July 15 to 24, i.e 10 days.

On the tape, there was 21.1 pictures x 200x200 pixels x 6 bits/pixel or about 5 Mbit, thus the average download speed was about 5.9 bits/s, thus about 1 pixel/s.

If by ECC, you meant the Hamming Code, it was invented in 1950 so the timing isn't wrong. It wouldn't be used because it's not useful in a communications channel where dropout (long runs of missing data) is common. HC is only useful where the failure mode is single-bit flips, like memory subsystems. OTOH, ECC refers to a class of codes, not just the Hamming Code. So, why the objection ECC in the 1960s?

I assume by "inventing" you are referring to some mathematical paper ?

But how many systems actually used Hamming codes in the 1960's ? The first large scale use of Hamming codes that I am aware of was in Teletext message headers to store on decimal digit (magazine, page or row number) in one 8 bit byte in the mid 1970's.

When the space probe is not in the galactic plane, the background is just "cold" (2.7 K) sky, thus the receiving station noise is determined to the preamplifier noise (apparently cryogenic masers in those days), This noise is predominantly white noise, so Hamming code would in fact be usable.

Complexity of generation. I have no idea, what logic circuits were used on Mariner IV, possibly RTL (was it space qualified in those days?), but more likely discrete transistors. Could you justify the extra complexity (including additional solder joints) ? It should be noted that in the early 1960's, (when the probe was designed), there was not much information about the interplanetary space outside the van Allen radiation belts.

Described the generalized Hamming codes, sure.

In the 60's, loads. All, at least high-end mainframes had (Hamming) ECC memory. The implementation is trivial.

Good grief, mainframes used it at least a decade before that. Even the PDP-11s I had in the mid-70s had ECC memory.

Come on! It's just XORs. Again, ECC does not imply a Hamming code.

It was programmed logic (a processor), any ECC is just a little code.

I was referring to (wireless) data communication system, not much digital communication systems (besides telephony PCM) in those days.

The core memory modules from DEC for the PDP-11/70 had parity only. The add-on modules from Intel using 8 kib (partially working 16 kib) DRAM chips used ECC, since DRAMs in those days suffered from random bit flips.

Later on, it was discovered that the package material used in those days emitted alpha particles, which caused bit flips in the storage cells. Replacing the packing material and the problem nearly disappeared and these days ECC memory is mainly used in server applications.

But the command channel in extremis has to work with that on the Earth and only the low gain almost omnidirectional antenna on the spacecraft. If the high gain antenna loses pointing that is all there is.

Across the high gain antenna they do have afster uplink capability but the whole system is fundamentally optimised for downlink speed.

But that doesn't help if the other end is no longer pointing its own high gain antenna towards Earth. Accurate high gain antenna pointing is a big problem when course correction burns occur etc.

No you are.

I posted a review of downlink speed against date in an adjacent reply which also states the command channel link speed is typically kbps.

--
Regards,
Martin Brown

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Yes. ECC dates back to the 1850s. Not common before 1950 where it = became useful for mag tape.

Time diversity is a EC technique.

OK.