FSK441, introduced in 2001, is designed to support communication using streaks of radio-reflecting ions created in the ionosphere by the trails of meteors entering the Earth's atmosphere.

The bursts of signal created by such trails are commonly referred to as “pings”, due to their characteristic sound. Such pings may be as short as a tenth of a second and carry enough information to complete at least one stage of a contact.

FSK441 employs multi-frequency shift keying using four tones, at a data rate of 441 baud. Because of the choice of character codes in the protocol, it is self-synchronizing and does not require an explicit synchronization tone. FSK441 is generally used on the 2-meter and 70-centimeter amateur bands.

Contacts may be made at almost any time (that is, a meteor shower is not required to be in progress) at distances of up to 1400 miles (2250 km).

Silly hint: Think of a shooting meteor sounding like "fffiiiisssk!"

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In digital electronics the baud is a unit of measurement that denotes the number of bits that can be transmitted per second. For example, if a modem is rated at 9600 baud it is capable of transmitting data at a rate of 9600 bits per second.

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The above explanation is confusing baud with bps (bits per second). Actually, baud rate is symbols per second. Each symbol could represent more than 1 bit. For example, if each symbol could represent 4 bits, then a 9600 baud transmission could carry 4 x 9600 bits per second or 38,400bps.

To accomplish this, these symbols could be one of sixteen possible tones, or one of 16 signal phases, or combinations thereof. Each would occupy one symbol (or baud) timeslot but it would "transport" the information content of 4 bits (16 values) over the link.

This is how dialup modems were able to exceed the theoretical maximum of 2400 bps on an analog voice-grade line - they turned the "bits" into symbols that represented multiple bits. They only transmitted 2400 of these symbols per second, but achieved data rates of up to 64Kbps.

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JT65 was designed as a digital protocol for extremely weak signals. It is of particular use in Earth-Moon-Earth (EME) communications on the VHF bands. Error-correction and other features allow communication at signal strengths that are well below the audible range.

[Memory aid: In the US's space quest, 1965 was a landmark year: the first spacewalk occurred.]

MSK144 is also a digital mode available in WSJT-X, but it is for meteor scatter communications, not EME.

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With a satellite in low earth orbit, the transmit and receive stations may not have a line of sight path to the satellite at the same time. Store and forward allows the transmission to be stored for a while until the receiving station can be “seen” by the satellite.

HINT: Store-and-forward implies a delay; so messages are downloaded later.

Other Answers

• ...upload ... software - doesn't forward anything.
• ...delay download of telemetry... - nothing is initially sent to store; telemetry is initiated in the satellite.
• ...relay messages between satellites - unfortunately this is a store-and-forward use-case, but doesn't apply to HAM satellites since it requires a larger constellation of satellites to work in low earth orbit.

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APRS (Automatic Packet Reporting System) is a packet radio/software system using the AX.25 protocol that allows any number of stations to exchange data without them having to be concerned with maintaining point to point links. APRS facilitates the reporting of location and status for any number of contributing stations.

144.39 MHz is the commonly used frequency in the 2-meter band.

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All APRS (Automatic Packet Reporting System) information uses AX.25 UI (Unnumbered Information) frames because each single packet needs to be heard by everyone. When an APRS station broadcasts its location it does not know if it has been heard and does not need a acknowledgement frame. The use of UI frames means that the network is “unreliable” but lost packets are not as critical as in normal packet radio communications.

Hint: APRS and AX.25 both have 4 characters

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APRS packets transmit data in the AX.25 protocol, the Amateur variant of the X.25 networking protocol. There are a number of AX.25 packet modes, typically designed for communication links, but APRS sends occasional one-off packets.

Connect and Disconnect packet types would be used to establish, create, and end a formal comms link, and an Acknowledgment packet would be used to confirm receipt of another packet.

APRS packets effectively throw packets into the RF spectrum and hope that somebody receives them, so there is no formal communications link. Therefore, the Unnumbered Information packet type is used, which is designed for this one-off kind of data transmission.

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The RTTY option refers to a 45 baud rate which is slower than the 300 baud packet rate and is therefore an incorrect option.

AMTOR is an error correcting RTTY that relies on frequent handshaking to verify each small group of characters. This handshaking takes up a significant amount of time and reduces the maximum data throughput, which is in the order of 100 baud.

PSK31 is a digital narrow band mode that works at 31.25 baud. The data throughput is obviously lower than 300 baud and so this is an incorrect option.

300-baud packet relies on error checking of relatively large packets of data compared to AMTOR which means that the data overhead is small. This is the fastest mode under good signal conditions.

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Coordinators who are organizing a search party like to use GPS reporting so they know where the searchers are and know which areas have been searched. If the searchers find what they are looking for they can use the GPS fix and locate where the missing person was found. Instead of come to the big tree and down a hill to the right.

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Test hint: APRS location has nothing to do with time and frequency or radio direction finding spectrum analysis. You can also rule out polar coordinates because that is used on graphs.

APRS uses latitude and longitude coordinates to indicate location. (See APRS Positions/objects/items on Wikipedia)

Latitude and Longitude are virtual lines running at 90° to each other around the Earth. Latitude lines run horizontally East and West. Longitude lines run vertical North and South. The lines cut up the surface of the Earth into sections. Latitude lines are 69 miles to each degree and longitude lines are 69 miles to each degree as well.

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JT65 is a digital mode that is capable of detecting and decoding signals that are very weak. The best way to address this question may be by elimination: JT65 is a digial mode and neither controls the receiver nor controls the antenna. Therefore, its ability to decode signals below the noise floor is the answer by elimination.

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