|Which of the following techniques is normally used by low Earth orbiting digital satellites to relay messages around the world?|
|C. Multi-satellite relaying|
|D. Node hopping|
E2D - Operating methods: VHF and UHF digital modes and procedures; APRS; EME procedures, meteor scatter procedures
One of the most commonly misunderstood concepts in digital communications is the baud. A baud is not equal to a bit per second, except for very simple systems. Rather, the definition of baud is the number of data symbols transmitted per second, and a data symbol may represent multiple bits.
While modern Internet digital communication techniques can send and receive digital data at megabit per second (Mbps) speeds, digital communications over amateur radio links is much slower.
Under clear communications conditions, 300-baud packet is the digital communication mode that has the fastest data throughput. (E2D09)
In the past ten years or so, the number of digital modes has just exploded. JT65 is one example. The type of modulation used for JT65 contacts is multi-tone AFSK. (E2D13) One advantage of using JT65 coding is the ability to decode signals which have a very low signal to noise ratio. (E2D14)
JT65 is a digital mode especially useful for EME communications. (E2D03) JT65 improves EME communications because it can decode signals many dB below the noise floor using FEC.(E2D12)
One of the reasons that JT65 is such an effective method of establishing EME contacts is because it uses time synchronous transmissions alternately from each station. (E2D06) JT65 software organizes the timing of contacts by alternating transmissions at 1 minute intervals. (E2E03)
One of the most popular digital modes is the Automatic Packet Reporting System, or APRS. Most APRS operation takes place on the 2 m band around 144.39 MHz.
AX.25 is the digital protocol used by APRS. (E2D07) AX.25 is more commonly known as packet radio. Unnumbered Information is the type of packet frame used to transmit APRS beacon data. (E2D08)
APRS stations can be used to help support a public service communications activity. An APRS station with a GPS unit can automatically transmit information to show a mobile station's position during the event. (E2D10) Latitude and longitude are used by the APRS network to communicate your location. (E2D11)
Digital modes can also be used to make meteor scatter contacts. FSK441 is a digital mode especially designed for use for meteor scatter signals. (E2D01) All of these choices are correct when talking about good techniques for making meteor scatter contacts: (E2D02)
- 15 second timed transmission sequences with stations alternating based on location • Use of high speed CW or digital modes
- Short transmission with rapidly repeated call signs and signal reports
Satellite digital communications
Amateurs that enjoy satellite communications also use digital modes. For example, store-and-forward is a technique normally used by low Earth orbiting digital satellites to relay messages around the world. (E2D05) The purpose of digital store-and-forward functions on an Amateur Radio satellite is to store digital messages in the satellite for later download by other stations. (E2D04)
Thanks to KB6NU DAN ROMANCHIK