Satellite Operations
A good place to start a review of satellite operations is with the hardware. An amateur satellite typically is described as “cubesat.” These are built by radio clubs and university groups. Cubesats get launched as an add-on to commercial rocket payloads. The Artemis 1 mission even took a few cubesats to the moon.
NASA’s Artemis 1 loaded with cubesats
Cubesats have various types of antenna. Simpler versions use dipoles constructed of metal of cut-up tape measures. Others use a circularly polarized antenna.That minimizes the effects of spin modulation and Faraday rotation.
These cubesats typically have a linear transponder that can relay different types of signals. Those include:
- FM and CW
- SSB and slow-scan TV
- PSK and packet data
All of these are correct when asked about linear transponder signals on the exam.
One type of transponder used is an inverting linear transponder. The satellite processes the upload or “uplink” signal sent from Earth. On an inverting transponder the signal is mixed with a local oscillator signal and the difference product is transmitted.
When a satellite is using an inverting linear transponder, the following occurs:
- Doppler shift is reduced because the uplink and downlink shifts are in opposite directions
- Signal position in the band is reversed and
- Upper sideband on the uplink becomes lower sideband on the downlink, and vice versa
All of these are correct for linear transponder signals as well.
Cubesats are small, many can fit in a large hand. They are not designed to put out high-power RF signals. Effective-radiated power to a satellite that uses a linear transponder should be limited. This will avoid reducing the downlink power to all other users. 5 watts with a Yagi works well for many users.
Each amateur radio satellite has a mode. The mode indicates the satellite’s uplink and downlink frequency bands. The letters in a satellite’s mode designator specify the uplink and downlink frequency ranges. These are standard across the communications industry. For instance, the terms “L band” and “S band” specify the 23-centimeter and 13-centimeter bands.
Here’s a list of bands and their mode designators. You might recognize V for VHF and U for UHF. If you want to see an up-to-date list of active amateur satellites, visit AMSAT.org/status. This is a great crowdsourced tool that will tell you what’s active right now.
Some amateur radio satellites have a function called “digital store and forward.” This gives the ability to hold digital messages in the satellite for later download. Store-and-forward is normally used on low-Earth orbiting digital satellites. It’s a function that allows you to relay messages around the world. Think of it as dropping a short message to someone on another continent.
When you are ready to contact a satellite, you’ll need to know where it is in the sky. Keplerian elements are parameters that define the orbit of a satellite.
You probably are familiar with TV satellites that are geostationary and appear to stay in one position in the sky. Most amateur satellites have a low-earth elliptical orbit. That means they pass over irregularly. That’s where tools like satellite trackers come in handy. On their elliptical orbit, the satellite may pass overhead from different directions. An ascending pass for an amateur satellite is from south to north.
This diagram details ascending and descending passes.
Let’s use the International Space Station as an example. Instead of being in a fixed point above Earth, it passes overhead every 90 minutes or so. You can use a service like AMSAT’s pass predictions to see when it will be over your part of the world. Their tool usually has the latest Keplarian elements update.
Should you be in a position to be the control operator of a satellite, you need to be aware of the issue of latency. That’s the delay between when a control operator sends a command, and then sees the corresponding change in the transmitted signal.
Fast-scan and Slow-scan Television
While it’s called amateur radio and not amateur video, there are ways to send images via ham radio. We’re going to take a look at three methods. In a moment we’ll cover sending one picture using slow-scan TV. Let’s start with fast-scan television, which is full video transmission.
In the 1940’s the NTSC standard defined what we know as “analog TV.” Fast-scan television uses that pre-digital standard. With NTSC, pictures are sent as frames. 525 horizontal lines make up a fast-scan frame. Fast-scan has a way to manage bandwidth. It uses an interlaced scanning pattern by scanning odd-numbered lines in one field and even-numbered lines in the next.
The 70-cm band has frequencies allocated for transmitting fast-scan video. They are around 420-430 MHz. These frequencies are transmitting on channels shared with cable TV. So, older analog TV receivers can be used for fast-scan TV operations. If you want to watch possible fast-scan TV in your area, you’ll need an older TV that can tune analog cable channels 57 to 60.
There are a few more details about fast scan or NTSC TV you’ll want to know. Vestigial sideband modulation is amplitude modulation in which one complete sideband and a portion of the other are transmitted. In analog fast-scan TV transmissions, vestigial sideband reduces bandwidth while allowing for simple video detector circuitry.
Check out this graphic for all the elements that go into sending and receiving a fast-scan TV signal.
In addition to fast-scan TV, hams are experimenting with digital video transmissions. Amateur television DVB-T signals usually use QAM and QPSK modulation. To ensure the video gets through as expected the encoding includes error correction. A digital TV coding rate of 3/4 means 25% of the data sent is forward error correction data.
Many hams are not ready to set up their own home TV transmitters. But, they want to be able to send pictures from point to point using their radio gear. This is where slow-scan television, or SSTV, comes in.
Some differences between slow-scan and fast-scan include:
- color lines are sent sequentially when color information is sent in analog SSTV and
- Picture brightness is encoded using a tone frequency in analog SSTV.
To identify the SSTV mode being used, Vertical Interval Signaling, or VIS, code, is sent as part of the transmission. Also sent are specific tone frequencies to signal SSTV software to begin a new picture. No special equipment is usually required. An SSB receiver can be used to receive and decode SSTV using the Digital Radio Mondiale (DRM) protocol. It’s not just HF either. Amateur Radio on the International Space Station (ARISS) does SSTV events from the ISS you can capture with a handheld and smartphone.
