SAREX Packet Robot Operation Since it has been a few years since AMSAT has published Tom Clark's article with details of the SAREX packet tnc firmware operation, and there still seems to be some confusion and misconceptions, perhaps a re- iteration is in order. First of all, the SAREX working group has made the decision that a ground station's confirmed reception of a downlink QSO# frame addressed to them is sufficient for a 2-way QSL card. However, a complete SAREX packet robot worked contact consists of the following events in the sequence listed: 1. Ground station transmits a connect request (i.e. C W5RRR-1). 2. Onboard SAREX robot station receives the connect request, transmits an acknowledgment ( frame), and adds the callsign, date, and time to the heard list if it is not a duplicate within the current heard buffer of the last 35 calls. If the callsign is a duplicate, the corresponding date and time of that entry in the heard list is updated. 3. Ground station receives the ack and sees a *** CONNECTED message. 4. Onboard station transmits the QSO# frame. 5. Ground station receives the QSO# frame and transmits an acknowledgment frame. 6. Onboard station receives the ack of the QSO# frame and adds the callsign to the worked list if it is not a duplicate within the current buffer of the last 625 worked calls. 7. Onboard station transmits a disconnect frame. 8. Ground station receives and acknowledges the disconnect frame and displays *** DISCONNECTED. Every two minutes, the normal robot beacon is transmitted, which consists of the following three frames: 1. QRZ: The current buffer of the last 35 calls on the heard list. 2. QSL: The last 15 calls on the worked list, with their QSO numbers. 3. SAREX: Standard I.D. message. Every connect request heard is acknowledged and assigned a new sequential QSO#, whether or not it is from a station previously recorded. However, as mentioned in step 6 above, only the first QSO# acknowledged is recorded. In a majority of cases, this sequence proceeds through step 5, when the ground station sees a QSO# frame, but goes no farther because of weak signals, or qrm from other ground stations on the uplink. The onboard retry parameter is set to 3, so steps 4 and 5 are allowed to repeat only 3 times before retrying out if no ack is heard from the ground station, and that QSO# is not recorded in the worked list. If an incomplete or reject frame is heard onboard from the ground station, a "free" re-transmission of the QSO# frame is made which does not count against the 3 allowable retries (standard AX.25 protocol). On STS-59, there were 5030 connect requests acknowledged and QSO numbers transmitted, but only 567 of those completed the contact sequence through step 6 to make the worked list. The only ways ground stations have of knowing for sure that they have been added to the worked list are: 1. Receiving an automatic disconnect (step 8). 2. Receiving a QSL beacon containing their callsign. If neither of the two above events is observed, steps 6 and 7 MAY still have occurred, but the ground station will not know whether they have or not. However, as mentioned earlier, submission of your received QSO# frame(s) in this instance is sufficient confirmation for a QSL card. Also, in some cases, a station may notice that their QSO# in the QSL beacon will be higher than what they saw sent to them. This can be due to several things, including: 1. An incomplete sequence (through step 5) which was aborted and restarted. 2. Monitoring downlink packets while the connect attempt is in progress. This allows you to see packets sent to other stations while you are trying to connect; however, you may also see a QSO# frame that was transmitted to you before your tnc heard the ack to it's connect request (step 4 occurring before step 3). Since your tnc is not yet in connected mode, it will continue to transmit connect requests. If the QSO# frame has not been acknowledged after 3 retries, that QSO# is abandoned. However, most ground stations use a retry limit of 10 or more, so their connect attempt will continue. When step 3 finally does occur, it will be assigned the next QSO# in sequence. I have personally observed local stations continuing to transmit connect requests while the robot is sending them QSO# frames. It is highly recommended that you set your tnc to display all frames received, in both connected and disconnected modes, and use a frame ack time of at least 4 seconds (i.e. for an AEA tnc, MONITOR 6, MCON 6, FRACK 4). The FRACK 4 or larger is important because the SAREX robot operates in full duplex mode (FULLDUP ON), with a 3 second response delay. The 3 second delay starts when a frame is received, and it will transmit a reply 3 seconds later, ignoring the DCD (frequency busy) signal. If you are the first frame received in the 3 second response interval, and you are using FRACK 3 (typical of most terrestrial operation), the onboard station will "double" with you at the same time you are transmitting, because it does not wait for a clear frequency, and you will not hear the downlink. This "SAREX unique" full duplex mode is necessary because of the heavy uplink traffic of connect requests. In a majority of cases, it would never transmit at all during an entire U.S. pass if it was operating with the DCD holdoff active. Gil Carman, WA5NOM NASA Johnson Space Center