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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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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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.

Hint: mode in question, decode in answer.

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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...

Other cheat: JT65 is Just Tone...

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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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