My First Overseas Contact Through International Space Station!
Today evening I was lucky to have my first ever overseas contact through the Amateur Radio on International Space Station. It was a pass of International Space Station starting from around 6 pm local time. Before the pass I had set my Moxon Yagi for satellite operations towards the middle of the pass so that I could hear stations well when the satellite is near its closest approach. During the second half of the satellite pass I heard another ham from Mumbai contacting a ham from Thailand with call sign HS8MOM.
Soon after, I heard the overseas ham giving a CQ call, which is a general call for any radio amateur operator. Immediately I called him back and was quite surprised to receive his reply, from a distance of 2575 km! You can hear the reply here. The call signs are being expanded in phonetic code, which is the usual practice in amateur radio during first contact, especially in overseas contacts. My call sign VU2JO can be heard as Victor Uniform Two Juliet Oscar. Signal report is expressed as 5,9, meaning a good signal.
Calls through the Amateur Radio on International Space Station are sent on the nominal uplink frequency of 145.990 MHz in the VHF or Very High Frequency band. Reply is received on the nominal downlink frequency of 437.800 MHz in the UHF or Ultra High Frequency band. As the satellite is moving fast, Doppler correction for the apparent change in received and transmitted frequency has to be made for a successful contact. Ideally the antenna has to be pointed in the exact direction (azimuth) and elevation of the satellite for best results. As I do not have an antenna rotator, I have to be satisfied with an occasional contact when the satellite is in the direction of my fixed antenna.
Amateur Radio on the International Space Station receives your call, extracts the audio and sends it down on the downlink frequency for the other Earth station to receive. In that way, it is acting as an FM radio repeater. That is how large distances can be covered. Normally terrestrial communication on VHF and UHF bands have a line-of-sight range of around 40 km till the curvature of the Earth sends out the signal to outer space. The satellite repeater which is located high up relays the signal for you to reach longer distances as the over 2500 km in my first ever overseas contact through a satellite.
A full duplex dual band radio is needed for amateur radio satellite communication. Full duplex radio is one which can transmit in one band and simultaneously receive on another band. Usual radios cut off the reception during transmit time to prevent the receiver being overloaded by the transmitted signal. Good input filters are needed in dual band radios for preventing the transmitted signal from overloading the front end of the receiver. Otherwise they cannot receive on one band while transmitting on another band simultaneously. Even though the signals are on two bands, as both are moving through the same feedline, interference will occur if good front end filters are not there, to prevent the transmitted signal from reaching the receiver side of the radio. Alternatively, two radios, one for transmission and another for reception, with two antennas and feedlines, kept at a physical distance, can be used.
Though I was using a dual bander base station radio inside my room and a fixed antenna on my balcony connected to the radio with a 10m cable, most satellite operators use hand held radios and directional antennas. They operate outdoors and continuously point their antennas in the direction of the satellite pass using mobile phone applications which show them the azimuth and elevation continuously. More sophisticated operators use base station radio along with computer software which can automatically rotate the antenna in the direction of the satellite. Automatic satellite tracking along with Doppler corrections make the weak signals from the satellite much more clearer. But that is a very expensive group of equipments to possess and very few hams have it!