Telebridge Station Design
The telebridge system is used when direct contacts are not practical. This is often due to the Shuttle's orbit being unfavorable for a site, the experience level at the school, or the mismatches between the school's and the crew's schedules. The Telebridge System is an international network of amateur radio stations, called telebridge stations, strategically located around the world. A telebridge station links a school group to the crew using a ground-based telephone conferencing system provided by Darome Telecommunications, Inc. This system is much like the one NASA used up until the early 1980s for communications with the Mercury, Gemini, Apollo, Skylab, and early Shuttle flights, before the Tracking and Data Relay Satellite System (TDRSS) became operational.
In a telebridge contact, your school will be linked to the telebridge ground station assigned to you via telephone. The station assigned to you will depend on the date and time of your group's scheduled contact. The telephone line at your school should be patched to a radio in a back room, or office, or classroom, out of sight of the students and audience. That radio then communicates with a radio in the assembly area to establish a simulated direct link, as well as maintain the "look and feel" of a direct amateur radio contact. Figure 7B-1, SAREX Telebridge Overview illustrates the end-to-end Telebridge system.
Figure 7B-1, SAREX Telebridge Overview
The station requirements for a telebridge contact are simpler than those for a direct contact station. The "hard part" is done for you by the telebridge station, and your technical burden is reduced considerably lightened. The simplest station is a speaker-phone patched into a public address (PA) or other audio distribution system. This reduces the technical requirements to one of audio distribution, and getting two phone lines (one for the telebridge, one for auxiliary & backup) to the place where the contact will take place. However, a speaker-phone eliminates the amateur radio content, making the event essentially nothing more than a telephone call. Therefore, speaker-phones are used only in extreme, special cases, as determined by the SAREX Working Group.
Figure 7B-1, Telebridge Station Diagram, is a functional block diagram of the typical, recommended telebridge station configuration. You should use it as the basic design of your station for a telebridge contact.
This design uses a local radio, out of sight from the student assembly, to link to the telebridge telephone line via a standard manual phone patch. The students then use a another transceiver located in the assembly area, in full view of the media and audience, to talk to this link radio. This retains the amateur radio content, gives the students the experience of talking to a space traveller over the radio, and affords your group the greatest control over the entire situation in case of interference. Low power levels should be used to minimize the chance that your communications link will be found and either interfered with, or "hijacked" by an unauthorized station. To further protect against this slight risk, your team should consider the use of a duplex channel for the link between the two radios, and have several sets of channels available, preprogramming into the radios. Duplex channels should use a different frequency offset than the standard offset used by repeaters on the band where the link channels are located.
Because of past experiences with both intentional and unintentional interference, the use of a repeater as your group's link to the telebridge is no longer an accepted design by the SAREX Working Group. Repeaters may be used to retransmit the contact for others in the community to listen to, but there must be no possibility for input to the bridge coming from the repeater either through its primary or control links.
The telephone conferencing portion of the telebridge system is graciously donated by Darome Telecommunications, of Chicago, IL. During a contact, an AMSAT Bridge Moderator is in constant contact with the technical staff at Darome, working behind the scenes to assure that everything is going smoothly. A Bridge Moderator, usually an AMSAT member, but sometimes a veteran astronaut who has used SAREX from space, will be on the line with the school groups to coordinate activity between the crew, school groups, and ground stations. Frequently, the moderator adds to the event by sharing his or her experiences and talking about amateur radio, the space program, and other topics. Other groups may also be tied-in to the system during your contact, in a "listen only" mode, so that they may use the event in their classrooms. Ask the Bridge Moderator prior to your contact to determine if there are any other school groups listening in on your contact. It would be a positive move on your school's part to mention any other schools, and to say hello to them!
This section provides guidelines and details for selecting and configuring the radio station components to support a telebridge contact. You should perform a site survey of the school, the assembly location, and meet with the school officials and teachers involved with the effort to determine the exact requirements of the station. As is always the case with on-site engineering, the layout and facilities available to you will affect the design, and often present challenges that you must engineer around or consider in your plans. These local (on-site) challenges are beyond the scope of this Guide, and you must rely on your own, and your AMSAT Technical Mentor's experience and judgement to assist you with them.
After you have performed your site survey's, and discussed the school's educational goals with their representatives, meet with your own team and develop a draft technical plan and station block diagram. This diagram should show the actual layout of the equipment and the interconnections between them. Be sure to document the primary and backup telephone locations, the phone numbers associated with each, and the confidential local frequency pairs in use between the student and the bridge-link radios (if known at the time). When this is complete, submit it to your AMSAT Technical Mentor for review and comment. After the comments are received, finalize your plan and send it to your Technical Mentor for his/her records. This should be completed with sufficient lead time to acquire any necessary components or support items. If additional information, such as frequencies and phone numbers, are not available at plan finalization time, or if design changes become necessary (such as radio makes/models, etc.) after the plan has been finalized, make sure that you update your plan and distribute the update revisions to your team and your AMSAT Technical Mentor. Keeping your AMSAT Mentor in the loop the entire time assures that you have the benefit of his/her experience during the process.
Use high-quality, proven 2 meter or 70 cm FM transceivers. Be sure that they are in excellent condition with no problems. You will want to have one transceiver in the back room where the telephone to the telebridge will be located, and the other out in the assembly area for the students to talk over. For appearances, you may want to put your highest-end transceiver model with the students since these will be visible to the cameras and the public.
70 cm radios are preferable for the link between your assembly area and the telebridge link radio in the back room. You will have far less to contend with in terms of interference problems. Low power, under 1 watt, is usually more than sufficient; however, depending on the materials in the walls of the building, and the distance between the two radios, you should experiment to make sure that the audio quality is good, and the signals are full-quieting. You want to use the lowest amount of power possible for the transmitters, to reduce the possibility of accidental discovery and intentional interference. Whatever frequency you choose for your simulated link, keep it confidential to your team members, and then, only on a need-to-know basis.
While simplex links between the two radios can be used, if your area is prone to interference problems, you should consider using duplex frequencies. The offset of these frequencies should be different than the standard offset used by repeaters on the band where the link channels are located. Also, you should have several sets of duplex frequency pairs identified, in case of interference problems. This implies that the transceivers used must have no less than 10 memories, and be capable of non-standard splits.
Some radios, especially earlier variable-offset designs, do not transmit well if a significant offset is used. In these radios, as the offset between the transmit and receive frequencies increases, transmitter performance is increasingly impacted. You will need to experiment with your radios to see if any of them fall into this category, and to determine the appropriate local operating frequencies.
Finally, you should plan for a backup link in case of problems with your primary radios. This would consist of two additional radios along side the primary set, each programmed with the same link frequencies as the primary set. That way, either side of the link can switch to the backup radio without the other end needing to be informed.
Since the all radio communications is taking place within a short distance, simple magnetic-mount mobile antennas can be used. These should be of quality construction, and tuned for the frequencies in use. Placement of the antennas is likely to be indoors, so be sure to locate them away from people, and position them for good reception. Since these antennas normally use the body of a car or other vehicle as a ground plane, a sheet of hobby steel or a larger metal object might be necessary to achieve good performance. Use a sheet large enough to achieve a good ground plane and place the antenna on the sheet of metal using the magnetic mount.
Use a good-quality phone patch with high-quality audio. AC power is preferable to battery power. If you use a battery powered phone patch, make sure that you have plenty of spare fresh batteries on hand. Test the patch with your primary and backup telebridge link radios and make sure that there are no impedance matching problems. If there are, you must either modify the patch or obtain a different one that will interface correctly to the radios. Again, its a good idea to have two phone patches, one for the primary radio, and one for the backup radio.
This is often the one item required that isn't on-hand when you start your design. Its often also the one component that most groups have the least insight on how to obtain. While not an endorsement of any particular vendor, one solution is the MFJ Telepatch II (MFJ-624D), offered by MFJ Enterprises, Inc. of Starkeville, MS. It is externally powered by 12 VDC, provides the necessary functionality, complies with FCC Part 68, and is readily obtainable at nearly all amateur radio dealers. The price for a new one is approximately $100.
Develop a testing checklist that includes the following tests at a minimum:
This check list should be used during the testing of individual station elements and when the station is built. On the day of the contact, approximately 1 to 2 hours prior to the scheduled time, repeat the complete checklist, as appropriate.