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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Meteor scatter is a technique where radio signals are bounced off the ionized trail left behind a meteor as it passes into and through the atmosphere.

The presence of a meteor at any given time is not readily predictable, so alternating timeslots are often used to give enough time for a meteor trail to appear, while also making sure stations are not all transmitting in the same timeslot.

When a meteor trail does appear, its signal reflecting properties often only last for a few seconds. High-speed CW or digital modes are used to ensure that several complete transmissions can occur during that short time period.

Meteor trails can also be "shimmery," resulting in signals fading in and out, even over the short time required for a high-speed mode to transmit a callsign. Ensuring important data is repeated many times greatly improves the chances of completing a contact.

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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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Store-and-forward means the message is stored and later sent when the satellite is in contact with the appropriate ground station.

Because low-earth satellites can only see a relatively small portion of the earth at once, instantly repeating the message (digipeating) is not a practical method to convey messages long distances.

Unfortunately, there is presently no large network of amateur LEO satellites that would allow instant relaying of messages between multiple satellites.

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Time-synchronous transmissions alternating between stations is used by a few different modes, including both JT65 and FT8. They require a synchronized clock for both the sender and receiver and each station spends one time period transmitting, then one time period receiving.

This allows them to pack in a lot of error correction for a very slow data transfer rate which makes it ideal for situations where your signal may be marginal or cut in and out a bit.

With JT65, for example, the time period is one minute. You transmit for one minute and then receive for one minute. (Technically it runs 46.8 seconds of each minute). Having the same clock time on each side is required for this to work so that you know how long to transmit and what part of the cycle you are in.

FT8 is a similar mode which uses 15 second time periods; it doesn't have as much error correction as JT65, but still can get 13 character messages through where other modes would fail.

Both of these modes are used both for EME (Earth - Moon - Earth) and other types of usually long distance contacts.

Memory tip: you have to time the moon.

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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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Biggest number wins, unsurprisingly, so the answer is 300 baud packet.

Each mode lists its baud rate (45, 31, 300), other than AMTOR which is 100 baud. And all four modes have 1 bit per symbol which means that is also the bit rate. More bits per second = faster data throughput.

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The primary way APRS (Automatic Packet Reporting System) is used is to transmit position-- either automatically at intervals (this is called "beaconing") or on demand.

This is used recreationally, but it's also useful in public service communications. Volunteers can have their current position automatically shown on a map.

This can make it easy to track race or parade progress, know the position of volunteers roving an event, etc.

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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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Earth-Moon-Earth communications involve extremely long paths with extremely weak signals. High amounts of forward error correction (FEC) and narrow signals allow JT65 to punch through despite being potentially too faint to even hear by ear.

[STUDY AID: We were FLOORED when man walked on the moon. --- Only one answer has the word floor in it.]

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JT65 uses many tones with a slow symbol rate (baud) in order to get a very high level of sensitivity.

Multi-tone FSK (MFSK) modulation is too specialized to be directly supported by the vast majority of transceivers. Instead the modulation is done at audio frequency on a computer and fed into the transceiver in upper sideband (USB) mode to elevate it to radio frequency. This technique is called Audio FSK or AFSK.

Cheat: Modulation is Multi tone, frequency, amplitude etc etc...

Other cheat: JT65 is Just Tone...

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JT65 [...] is intended for extremely weak but slowly varying signals, such as those found on troposcatter or Earth-Moon-Earth (EME, or "moonbounce") paths. It can detect signals several dB underneath the noise floor [...] –Wikipedia

Note that the 65 doesn't have anything to do with the bandwidth, it's because the encoded messages are transmitted using MFSK with 65 tones.

Hint: mode in question, decode in answer.

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