From the MSK144 section of the WSJTX user manual:

Meteor scatter QSOs can be made any time on the VHF bands at distances up to about 2100 km (1300 miles).

APRS is designed for sending packet data, like location, text, or weather data. WSPR is designed for testing propagation. Hellschreiber is for keyboard-to-keyboard messaging.

Memory aid: MSK = Meteor Scatter (K)ommunications.

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FT8 and FT4 exchanges are only the received signal strength in dBm and Grid Square. The GRID SQUARE information replaces Signal-to-Noise ratio exchanged in other modes.

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Earth to moon and back to earth (essentially an echo off the moon, or "EME" for short), is not only a long path, but for voice communication requires long boom Yagi antennas and lots of power. The mode JT65 is designed for EME communications, and is part of the WSJT-X software suite. The "JT" refers to Nobel laureate Joe Taylor, and "65" refers to the 65 tones used in the mode.

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http://www.aprs.org/

Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time digital communications of information of immediate value in the local area. In addition, all data is ingested into the APRS Internet System (APRS-IS) and distributed globally for immediate access. Shared information contains global coordinates, altitude, speed, heading, text messages, alerts, announcements, and bulletins. The most visible aspect of APRS is its capability of map display. Anyone may place any object or information on the map, and it is distributed to all maps of all users in the local RF network, or to anyone monitoring the area via the Internet. Any station, radio, or object broadcasting GPS coordinates to the APRS system can be automatically tracked over time.

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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.

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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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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...

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The packet path “WIDE3-1” is used in the Automatic Packet Reporting System (APRS), which is a digital communications protocol for exchanging information among a large number of stations covering a large area, most commonly used in amateur radio.

When you see a path like “WIDE3-1,” it indicates a request for the packet to be repeated by digipeaters. The “3” specifies the number of digipeaters that should repeat the packet, and the “1” indicates how many times each digipeater should repeat it.

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The P in APRS stands for Packet

Think "P"

The P for Packet in APRS needs a Packet digiPeater.

Pick the answer with two Ps.

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