AMSAT (a.k.a AMSAT-NA) is a good place to start if you are interested in how amateur radio has found itself participating in the technologies of the 21st and 22nd centuries. They have a page here where you can find material that will introduce amateur satellites. Another resource is Project Oscar.
My interest in amateur satellites is mostly digital in nature. Being a child of the space age (Yuri Gagarin orbited our planet the same year I was born) I can't help but have more than a passing fancy with the techniques and technology of space flight. It would have been great to be a HAM in 1957 when Sputnik (Sputnik - wikipedia) went beeping around the globe. I remember the moon landings of the Apollo era (Apollo-wikipedia).
Unfortunately it took many years before I really did anything about such interests. It wasn't until I got my amateur radio license in 1997 that I began to dig into the stuff I had always wondered about.
One of the aspects of digital information from spacecraft I find so absorbing is the Pioneer 10 and Pioneer 11 spacecraft, in the form of the so called Pioneer Anomaly. But there are many other spacecraft which have radioed back data which have revolutionized our understanding of our place in the cosmos, and more importantly how we effect our planet. The Viking landers on Mars, Voyager 1, Voyager 2, Cassini-Huygens, Pathfinder, the Mars Exploration Rovers, Pheonix, Hubble, COBE, WMAP, Chandra, Spitzer, Keppler, the Venera series... the list is long, and fortunately getting longer.
But how do they get all the information back from space craft?
Pick up the phone and call?
E-mail via the internet?
No it's a whole lot more complicated than those... yet simpler as well. Simple in concept at any rate. Far more difficult to do.
Fortunately there were, and still are, some fairly simple satellites in orbit, which can help, those who are interested, appreciate what Rososmos (decedent of the Soviet space program, Russian Federal Space Agency - wikipedia), NASA, ESA, SpaceX, ArianeSpace, JAXA... and many others are doing.
Such satellites are open and free for normal folks to try their luck, and sometimes their patience, at pulling telemetry from a spacecraft.
Here are some amateur satellites which supposedly have some digital capability.
The above list is by no means exhaustive. It is just a quick survey of the satellites listed on AMSAT-NA's operational satellites page. And the status and the list of satellites changes often...
I have collected telemetry, and some BBS data from AO-51. I have also collected data from GeneSat-1. With my very primitive antenna, and antenna pointing system, I haven't heard much from the others. Sometimes I would hear bursts of data but the audio level was never enough above the noise floor such that the modem could do much with it.
I harvested enough data from GeneSat-1 that I wrote a program to decode the telemetry data. The program was written under OSX, and ran on my G3 Macintosh computer. That was great fun.
There are many utilities out on the web that can be used on AO-51.
A good place to start for info on the OSCARs is the Amateur Satellite Resource Guide. For digital satellites "The AMSAT-NA Digital Satellite Guide", compilation from 1994 (AMSAT-NA books) is a bit dated but serves as a good measure of width and breadth of digital modes on the OSCARs.
One of the limitations of the current generation of PacSats is their limited transmitter power. Most of them are Cubesats / Nanosats / Picosats. So called because of their diminutive size, and low transmitting power (1 watt or less). The satellites are usually several hundred kilometers above the earths surface, so the signals have a long way to go, especially as the satellites appear above the horizon. CO-57 for instance can be almost 9,400 km away from my QTH when it first pops up into view. AO-51 can be 11,800 km away when it first creeps above the horizon. These are long distances for radio waves to traverse at such low power.
As a crude estimate, suppose you receive 1 watt of power per square meter from an omni directional antenna on a satellite 1 meter away. Now move that spacecraft away 10 meters. Your antenna will now receive only 10 milliwatts per square meter of power or one hundredth the power it received when it was only one meter away. At a full kilometer the amount of power at your antenna would be 1 microwatt per square meter. That is 0.000 001 watts of power over a one meter square area. At 10,000 km the amount of power available per square meter drops to 10 fW (fempto watts) or 10.0e-12 watts. The drop in power verses distance, from an omni directional point source, is referred to as the inverse square law of electromagnetic radiation.
Whatever it is called it seems like a small amount of power. However, most modern amateur radio receivers can respond to signals as low as 0.8 fW (fempto watts) at their antenna input port. That is 0.000 000 000 000 000 8 watts, presuming FM (FM-wikipedia) detection. This presumes the background noise is below this power level. If the noise power is at or above this level, your receiver is not likely capable of pulling the signal out of the noise. At least not without help.
Seems like a run of the mill amateur radio station should be able to cope with that. It's not that simple. Detection limit specifications are usually given in ÁV (micro volts), across a presumed resistance of 50 ohms. Our calculations have been in watts, and watts per square meter. How are these two sets of units reconciled?
We are making a prediction of how much power is available at a remote site (someone on the ground with a receiver) based on a point source (an orbiting satellite) a long way away. There has to be a way to take whatever power we have from the remote site, and get it into the radio. There are devices to do this we call them antennas, and feed lines. The antenna collects the available power, within the limits of its design and construction, the feed line brings it to the port on the radio. The combination of these two devices is presumed to do so efficiently. However, there are always losses, so not everything that you would expect to be available at the antenna, gets to the radio.
Another question is how can a typical antenna, made from a piece or pieces of wire constitute a meaningful amount of receiving area, since the power density we have calculated is in watts per square meter? After all a length of wire does not cover much in the way of area.
All antennas, including an antenna made of a single piece of wire, have a characteristic aperture. This aperture expresses the antennas ability to collect power from the available power density (watts per square meter) around it. Although related to physical size, an antenna's characteristic aperture is not completely determined by its physical size.
For the time being we are mostly concerned with how watts per square meter compares to the radio's detection limit, given in micro volts.