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World Wide HF-APRS - See your Position in the Internet

The opposite table shows HF digipeaters that can be used for digital data connections and as HF-APRS gateways. All listed HF-APRS gateways forward position information received on their respective HF channels automatically to the Internet. Just check it out: transmit your position data over shortwave to an HF-APRS gateway and see it for example on the following link: http://www.db0anf.de


Robust Packet-Radio (RPR)

Up to now Packet-Radio over shortwave has been basically a non-starter, it has even been heavily criticized because of the low effective throughput and repeats. AX.25 is for shortwave not an ideal protocol, but with automatic FRack-setting and a small MAXFrame value the protocol should, however, function much better on a shortwave channel than has previously been the case generally.

One cannot of course expect an asynchrone protocol to reach the same efficiency as a tight synchrone ARQ protocol (e.g. PACTOR), but for some applications a multi-user service, with very uncritical transmit/receive switching, as well as almost zero power holding up a connection when no data passing, brings a real advantage that outweighs the lower data throughput.

What finally are the reasons that up to now HF-PR works so poorly, and apart from "forwarding" is hardly ever used? One finds a simple answer: The current modulation type for HF-PR namely uncoded 300 Bd FSK is really unsuitable for normal HF channels. The symbols are much too short even with moderate "Multi-Path effect" ("delay spread") to work. Additionally, because no sort of error correction code is used, even short troughs or "static" will destroy a many seconds long Packet. Just one missing bit leads to a repeat of the whole packet.

To help cure this problem, SCS has developed a new class of robust modulations types especially for Packet-Radio. As a special feature for all the variants of this "Robust PR", a completely new synchronizations algorithm with "catch" properties that were not possible before has been realized. Frequency deviations of ±250 Hz are immediately recognized and without any loss of sensitivity compensated, and this with signals that are buried deep in the noise. Because of this it's possible to remove a tuning display. One can say with good conscience that this is "Plug and Play" for shortwave.

The currently available "Robust PR" modulation types have the following properties:

Bandwidth:500 Hz @ -30 dB
Modulation:Pulse-Shaped OFDM (BPSK, QPSK), similar to PACTOR-III
Average Throughput:200 or 600 Bit/sec
Crestfaktor:3.0 or 4.2 dB
Delay-Spread:up to ±8 msec is tolerated
Coding:High performance convolutional code, "full-frame interleaved", rate/2 or rate3/4

Note:

To use the following features you need the current Firmware for the SCS DSP-TNC:

Legende:

Recommendation: For transmitting position data with the Tracker/DSP TNC, we suggest always to use the frequencies as shown in the list with the respective sideband. The position data can then be transmitted either only in RPR, or in RPR and FSK alternately (%AH = 1). In both operating conditions all physical channels are then automatically set in the correct way.

(In case of an alternating transmission, i.e. %AH = 1, the Tracker automatically uses %F = 2000 Hz, in order to set the correct interval of 500 Hz between RPR and FSK channels without any user intervention.)

With gateways offering RPR and FSK 300 on one channel simultaneously, it is assumed that the center audio frequency of the FSK demodulator (%F-parameter) is 500 Hz higher than the center audio frequency of the RPR demodulator. The space between the center frequencies of a simultaneous FSK/RPR channel pair is always 500 Hz.

Basically, gateways receiving RPR and FSK300 simultaneously can also be reached in FSK300 with the %F standard setting of the Tracker (center frequency of 1700 Hz) in LSB mode.

In this case, if LSB is actually used, 3.7 kHz have to be added to the figure shown in USB dial frequency listings. In case of an LSB channel, 0.3 kHz have to be deducted from the listed frequency. Gateways shown in the list as 10147.3 kHz USB can hence be reached in LSB mode with the standard setting of the Tracker (%F = 1700 Hz, %AH = 0) on the standard dial frequency of 10151.0 kHz. Gateways listed as 14103.3 kHz LSB, can be reached in LSB with the default setting of the Tracker (%F = 1700, %AH = 0) on the standard dial frequency of 14103.0 kHz.

In case of alternating RPR and FSK transmissions (%AH = 1), the frequencies shown in the list and the respective side bands have to be programmed. For example, the dial frequencies of 10151.0 kHz LSB or 14103.0 kHz USB MUST NOT be set, as neither the RPR, nor the FSK channel would be reached correctly.