RUDAK A condensation of an article appearing in ASR #126/127 by Peter Guelzow, DB2OS, Deputy RUDAK Project Leader April 27, 1986 (translated by Don Moe, KE6MN/DJ0HC) First, a short review of the sense and purpose of RUDAK: "RUDAK" stands for "Regenerative Umsetzer fuer Digitale Amateur Kommunikation" (in English: Regenerating Transponder for Digital Amateur Communications). It is comparable to a so-called digipeater (Digital Repeater). Digipeaters are terrestrial relay stations for packet radio. They relay digital information between two stations in case there is no direct path between them. The transmission occurs in packets (therefore the name packet radio). The actual information is subsumed within a frame which contains the callsigns of the sending and receiving stations and an entire set of additional entries necessary for forwarding and error handling. In this way packet radio makes possible virtually 100 percent error-free information transmission. The exact contents of these packets was internationally established using the so-called AX.25 protocol. Similarly to analog transponders, it seems desirable to install such a digipeater at the highest possible location with a large coverage area, e.g., aboard a satellite in earth orbit. Thanks to the highly elliptical orbit of Phase 3C, RUDAK should eventually enable the interconnection of several local area nets in addition to point-to-point contacts between radio amateurs across the entire world. Naturally a relay station with such a large coverage area has to contend with a series of difficulties. For example, the problem of multiple uncoordinated access or the selection of optimal modulation techniques are only two of among many that could be mentioned. These and other problems are to be researched primarily with the help of RUDAK with the goal of developing suitable techniques and protocols which will benefit future projects. Details about the RUDAK hardware: The RUDAK hardware consists of 25 integrated circuits and only two discrete transistors. The entire circuitry was realized using CMOS technology so power consumption is only 300 milliwatts. The heart of the RUDAK processor is the CMOS version of the 6502 CPU which is clocked at 800 kHz. For storage of the RAM-resident system software and data, 56KB of static CMOS RAM chips are provided. This concept itself gives RUDAK greater flexibility in case, for example, the entire RUDAK software has to be updated due to changes in the protocol as has already been practiced with OSCAR-10's IHU. A single 2KB fusible link CMOS PROM is used to load the IPS system via the command link after power-on. Additionally, the boot PROM contains various programs which will perform tests of the entire hardware in the RUDAK processor while in orbit. To communicate with the outside world, the RUDAK processor has various parallel and serial input/output ports. One serial line and one 8 bit parallel port with the appropriate control lines are used for communication with the IHU. In the start-up phase, these paths are used to transfer diverse command and diagnosis instructions. Later, using this same path, RUDAK can receive current telemetry data which can be processed further. The IHU can also use a portion of the RUDAK memory as virtual memory in which to store larger quantities of data, e.g., RTTY/PSK bulletins. The capacity of the 16KB RAM in the IHU is already totally used. Normal operation with ground stations is handled by the RUDAK packet port. One send and one receive channel are available. The heart of this port is the CMOS version of the Z80-SIO, a universal chip which supports the AX.25 protocol in addition to asynchronous and synchronous operation. An independent receiver in the Mode-L transponder is provided for the RUDAK uplink on 1269.675 MHz. The demodulator converts the 2400 bps biphase PSK signal into a clean digital signal for the RUDAK processor. Thanks to the sweep circuit in the demodulator, the uplink signals only have to be in the capture window within plus/minus 7.5 kHz of the center frequency. On the downlink side, the output data modulates the RUDAK beacon transmitter in the L-transponder on 435.675 MHz using BPSK at a data rate of 400 bps; the same as for the general beacon of OSCAR-10. Experimentally, the rate can be increased to 1200 bps using NRZI modulation. A short note regarding the modulation methods used: The 2400 bps uplink and the 400 bps downlink signals are generated using the AMSAT standard just the same as for the general beacon of AO-10. In the AMSAT standard, the data bits are transmitted differentially, i.e., a logical "0" is sent when there is no change in two successive bits, whereas a logical "1" is sent for a change between bits. Additionally the clock signal is combined with this data stream. Due to this trick and the differential encoding, the design of the decoder is significantly simplified. Unfortunately another standard has established itself internationally in which the assignment of the logical levels is exactly reversed. In the NRZI standard, a logical "1" is transmitted when there is no change between bits. If the bit clock is also combined with the data, the signal is then called "NRZIC". In order to reduce the confusion as much as possible, it was decided to adopt the previous AMSAT standard for RUDAK. In the case of the 1200 bps downlink option, the NRZI standard was chosen, and, in contrast to the AMSAT technique, the clock signal is not combined with the data, since to do so would exceed the bandwidth of the SSB receiver. Requirements for the ground stations: In the initial stages, RUDAK will emulate the existing digipeater functions as they are defined the AX.25 protocol version 2. No mailbox operation is planned presently although various other messages such as bulletins, orbital data, telemetry values and user instructions can be cyclically transmitted when no uplink signals are being digipeated. New ground stations can take their time in adjusting their receiving equipment. Additionally, a robot-type operation is planned in which the ground stations "connect" to the satellite and are assigned a consecutive number. In a fashion similar to the RS satellites, a RUDAK command station could later download the list and send out QSL cards. It is also hoped that an overview of packet radio activity world-wide could be thereby obtained. Should a suitable link-layer level 3 protocol subsequently become available, it could possibly be implemented. For the majority of the terminal node controllers, e.g., TAPR TNC-1, AEA PKT-1 or Heath HD-4040, the only software modification required is an updated EPROM to handle a hardware bug in the WD1933/35 HDLC controller. Otherwise only a PSK modem for 400/2400 bps has to be connected to the external modem jack in the TNC. Other TNC's such as the Kantronics "Packet Communicator" or various software solutions are unfortunately not suitable due to the software and/or hardware restrictions. The TNC must be capable of operating full-duplex at different transmit/receive baud rates and support the connection of an external modem. Besides the normal equipment, a so-called "RUDAK User Interface" is required. This is under development by the RUDAK group and AMSAT-DL. The RUDAK User Interface consists of an up-converter which translates a 2m signal to 24cm and modulates the carrier with 2400 bps BPSK and the "AMSAT-AFREG" which is the BPSK demodulator for the 400 bps downlink. Additionally various buffers and controls for switching the different signal paths and a power supply are needed. The various schematics, especially for the AMSAT-AFREG and the upconverter, will be published by AMSAT-DL after the design is completed. On the RF side of the ground stations, the 400 bps downlink signal on 435.675 MHz should provide a signal strength of 12dB Eb/No to an antenna with 10 dBi gain. For the uplink on 1296.675 MHz, 12 watts (11 dBW) into a 15 dBi antenna should be sufficient.