Teaching and Learning Activities

What Happens When Things Move Fast in Space – Doppler
  • A common way of explaining the Doppler effect is to have students imagine that they are listening to an ambulance driving towards them with the siren operating. As the ambulance towards you, the frequency of the sound increases. As it passes you, the frequency will drop.
  • Low earth orbit satellites travel so fast that they can orbit the earth in 90 minutes. As the satellite rises up over the horizon, its frequency will rise by up to 10 khz.
  • As it passes overhead and moves towards the horizon, the frequency will fall by up to 10kHz.
  • These changes in frequency pose a problem for communications. In most cases, both the transmitting and receiving stations need to continuously retune their radios as the satellite passes.
  • Not all satellites orbit the earth so quickly. Geosynchronous satellites are placed at a height that allows the satellite to orbit at the same speed as the Earth. This means that the satellite appears to be over the same point on the earth at all times. Geosynchronous satellites are used to transmit television. The fact that they are geosynchronous means that the receiving and transmitting antennas do not have to move.

Interesting Satellite Facts

Designing and building a satellite is far harder than it looks. Here are some reasons why.
  • Space is a very hostile environment for equipment. When the satellite faces the sun it is heated to very high temperatures. The side of the satellite which faces away from the sun is freezing cold. This means that if a satellite did not spin, it would essentially cook all of the components on one side while freezing the components on the other side.
  • One solution is to spin the satellite. But remember, with no atmosphere and reduced gravity, one sustained push would not only spin the satellite, but spin it so fast that it could mechanically fail. Satellites are carefully designed to spin at a predetermined rate.
  • Amateur satellites are solar powered. What happens when the satellite moves into darkness. On any given pass, the satellite will be in darkness for approximately half of the time. This presents a real challenge for the satellite designers who have to make sure that the satellite can safely recharge itself during each pass it is exposed to the sun.
  • Satellites have to be constructed in such a way as to ensure that not even the smallest part can become detached. With the speed a satellite is travelling, even a small component could cause serious damage if it strikes other.
  • Satellites have to be self sufficient. From the time the satellite is loaded onto the launch vehicle, the satellite has to be able to look after itself. In the design and construction of satellites, nothing is left to chance. Remember, there are no service facilities available in space for amateur satellites.
  • Satellites are equipped with telemetry systems. This allows important data concerning the operation of the satellite to be sent to earth for analysis. The controlling ground station can instruct the satellite to change its mode of operation if required.

Satellite Predictions

  • Finding a satellite in space is not as easy as it looks. Compared to the size of space, satellites are tiny and they move very fast.
  • Amateur satellites often carry an identification beacon. By checking the frequency of the beacon, it is possible to identify and locate a satellite.
  • For station operators, hearing the beacon of a satellite means that the satellite is in the correct position to use.
  • To predict the path of a satellite, you need to have some information available. You must know your latitude, longitude and altitude. The altitude is important because it determines how much of the sky you can see from your location.
  • Once you have this information visit AMSAT satellite prediction page
  • www.amsat.org/amsat-new/tools/predict/
  • For the more mathematically inclined, there are a number of programs available to calculate the location of satellites. These programs require up to date keplerian elements for each satellite to be available.
  • www.amsat.org/amsat-new/tools/keps.php


The Station

The station has two multiband transceivers that provides coverage form 3,5 Mhz to 1.2 Ghz. The transceivers are capable of transmitting in FM, AM, SSB, digital and CW (Morse Code).
The main satellite transceiver – Kenwood TS2000 is capable of transmitting and receiving at the same time. This is an important feature for satellite communication since the operator needs to be able to hear their own signal through the satellite.

The secondary transceiver is a physically smaller unit but houses all of the features of a sophisticated radio station.
The Yaesu FT-857 provides coverage on all amateur bands between 1.6 Mhz and 430Mhz in all available modes. The unit incorporates the latest digital audio processing that operates to identify noise in signals and is particularly useful in hearing very weak signals. While the transceiver is not capable of simultaneous reception and transmission, it is satellite capable.

<<insert FT-857 Brochure (PDF) or the image here>>

A small FM only transceiver is available for local communications and repeater access. With even a simple device like this, international communications fro students is possible via the IRLP system.

The broad frequency range allows students to compare the characteristics of each frequency of communication with each particular mode.
The station is capable of carrying out long range communications over the High Frequency spectrum in much the same way that communications would have been carried out half a century ago. The main difference is that the station receivers are considerably more sensitive and selective than those older value units of yesteryear.
We have the option of turning down the sensitivity of the receivers so that they sound like equipment of the last generation.

The station’s equipment also has the advantage of utilizing the latest digital noise reduction and filtering technology. This system digitizes the audio signal and significantly reduces the effects of naturally occurring noise.
Did you know?
That the sun is a giant transmitter which generates strong signals in the High Frequency bands


Antennas

Dipole
 A dipole is the simplest type of radio antenna. It is often used as a reference point to compare measurements with other antennas. If an antenna was said to have a gain of 3db, it would mean that it would perform twiceas good as a dipole.


Collinear
The gain of an antenna can be improved by stacking dipoles on top of each other. Collinear antennas have the advantage of providing better performance that a dipole but still remaining omnidirectional. This means that the antenna can transmit and receive in all directions.
The radio station uses VHF and UHF collinear and is capable of producing 8db of gain at UHF.
The benefits of a collinear antenna also extend to transmission. The gain of the antenna will make the station appear to be operating a more powerful transmitter.
Collinear antennas are not well suited to space communication because they only work well when a signal is received form the side of the antenna. Since orbiting satellites travel overhead, this type of antenna will perform poorly under these circumstances.


Yagi
Yagi antennas are multiple element devices that allow a signal to be transmitted and received in one direction. Concentrating the energy in one direction increases the strength of the signal, causing the antenna to exhibit gain.
Most people would have seen a television antenna. Most of these antennas are yagis.
Our radio station uses two yagi antennas mounted together to send and receive signals to satellites. Because of the directional nature of the antennas, it is necessary to rotate the antennas in order to maintain communication. You will notice that the elements on the antennas are different sizes. The antenna with the shortest elements if designed for UHF. Increasing the number of elements improves the gain of the antenna.


Amplifier
The station employs a masthead amplifier located at the base of the UHF yagi antenna. The purpose of the antenna is to amplify the signal received to overcome any losses in the coaxial cable. While it might be easier to use the amplifier at ground level, the problem would be that the signal would already be too weak to be heard by the time it made it through the co-axial cable. The masthead amplifier is capable of increasing the signal by 20db.


Calculating Signal Loss

How much signal is actually lost between an earth radio station and a satellite?
The earth radio station and the satellite can be considered to be a communications system. As signals are moved around the system, their strength is either increased when they are amplified or decreased. If sufficient signal reaches the earth, it would be possible to access the satellite.
Changes in the power of a signals is measured in decibels. We use the known gain and losses to calculate the overall performance of the system.
A large amount of signal loss occurs because of the distance between the transmitter and receiver is very large. Low earth orbit satellites are approximately 600Km above the earth. This results in a loss of over 140db.
At UHF and above, considerable losses are encountered as the signals pass through the co-axial cable. The type and length of cable, greatly affect the amount of signal available to receive.
In the radio station, we used low loss co-axial cable to minimise signal losses.
The amount of signal that will be received from a satellite can be calculated by using the spreadsheet.

It should be noted that satellites have very strict power budgets. This means that only a small amount of power is available to operate the on board radio transmitters. Most amateur satellites only transmit approx 1 Watt of radio energy. This is equivalent to a small hand held radio.


Teacher Reflections

The project provided challenges and learning opportunities for all of the students and staff members involved.
For the students, this was their first glimpse at the technical world of communications. The station allows such a broad range of activities to take place that there are interesting and meaningful work available for students of all ages.
For staff was a very steep learning curve. Although one of the staff members has experience with radio communications however space communication was a new and very challenging field.
Building the station presented a multitude of unforseen issues. Much of the communications equipment was very specialised, and most of is was not readily available in Australia. At the time the station went into operation, we were still adding equipment and expanding its capabilities. I believe that this will continue into next year.
The professional development provided as part of the program was excellent. Working with the Centre for learning Innovation allowed us to undertake aspects of the project which would not normally have been possible.