The Echo FAQ
These questions and answers about AMSAT-Echo have been prepared by Rick Hambly, W2GPS, with contributions from Paul Williamson, KB5MU (questions), Jim White, WD0E, and Lyle Johnson, KK7P. Please contact Rick if you have additional questions.
The following questions refer to the AMSAT Echo article in the AMSAT Proceedings for AGM 2003, held October 16-19, 2003 in Toronto Canada. For copies of the Proceedings call the AMSAT-NA office at +1(301)589-6062.
1. Echo "can be configured for simultaneous voice and data." But to what extent does the power budget support that?
The satellite should generate up to 18 watts when in sunlight. Most of this will be available to the transmitters as the spacecraft consumes less than 1 W. At 7.5 watts of RF output the transmitters should be about 45% efficient (class AB amplifiers) which comes to 16.6 watts/transmitter or 33.3 watts. We can support that load for 10-15 minutes of activation periods on all orbits and still fully charge the battery. The battery capacity is 35 Wh so even transmitting in eclipse on every orbit with both transmitters near full power that will yield about 20% battery depth of discharge.
It must be noted that we won't know the actual power available till we get on orbit and characterize things. Also, Echo will not start out with any sort of scheduling or orbit determination software on board, so turning things on/off or changing modes automatically will be down the road a ways.
2. How's the uplink budget on 10m? on 23cm?
I don't know but we will find out soon. They share the same receiver. Keep in mind that that when looking down on a "hot" Earth the background noise will dominate the link performance.
When used on L-Band the receiver's IF filter will not be matched to the data rates above 9600 baud so it will take plenty of signal from the ground to overcome that. Thus, the normal modes will be 9.6kbps data and FM voice, and those modes will require the ground station to chase the Doppler to stay in the passband. I expect L-band will be for advanced operators with computer control.
3. What is a "Mary" turnstile? Can you provide a reference?
The Mary turnstile is four quarter wave whips mounted at the corners and canted towards the center at about 53 degrees forming a pyramid. They replace the Martha turnstile and have the advantage that they can be phased to operate either RHCP or LHCP. Mary also adds some coverage to the back of the satellite at -8dBic while Martha was -14 to -20dBic. To the front Mary has a gain of +2dBic.
3a. Where do the names "Mary" and "Martha" come from? Are these generally recognized names or something internal?
The UHF satellite antennas known as "Martha" and "Mary" got their names from the antenna designer and have no special meaning.
4. The two UHF transmitters have opposite circular polarity. Is that a desired feature (and why?), or an unfortunate consequence of the hybrid design?
The new Mary turnstile antenna and matching hybrid combiner were designed to provide RHCP for one transmitter and LHCP for the other. This was a conscious decision to provide the maximum isolation between the transmitters.
4a. The answer doesn't really address the question I was trying to ask. Let me rephrase. If it were not for the hybrid design and the need for isolation between the two transmitters, would there be any advantage to having opposite circular polarizations on the two transmitters? It seems to me it would be more convenient for ground stations if both transmitters used the same polarization. If that's misguided, I'd like to understand why. If not, it would be nice to clarify that the tradeoff between groundstation convenience and spacecraft engineering was made knowingly.
There is no advantage to the ground station to having the two transmitters using opposite polarities. While it might be convenient for the ground user if both transmitters were RHCP that would have required an excessively complex, large and heavy hybrid combiner that would have compromised the mission objectives and put us above the maximum weight restriction.
Echo has two UHF transmitters feeding into one hybrid combiner with four output ports, one for each whip in the Mary array. We have been using this type of configuration since the 1990 Microsats. In order to provide isolation between the transmitter ports there is a phase shift in the hybrid. This method allows the hybrid combiner to be very small and light, has no moving parts or electronics, is temperature stable, and is very efficient in terms of insertion losses.
The issue of RHCP vs. LHCP often comes up. This is one of those design tradeoffs where it is overwhelmingly more beneficial to keep the mass, size and complexity of the antenna matching/combining in the satellite low and take the minor hit of sense switching on the ground.
5. In Figure 9, the link budget is shown for ground antennas of "0 dB". Is that dBi, dBd, dBiC, or what?
dBd, I think. This question has prompted an update to the tools used to evaluate link budgets and future charts will be more precise.
6. In Figure 9, what is assumed about the spacecraft attitude? Does the link margin shown in Figure 9 assume that the spacecraft is perfectly nadir-pointing? If so, that's unrealistic. The current crop of magnetically-stabilized Microsats experiences significant very-slow fading due to attitude changes. At my latitude, I often have to switch polarizations to optimize signal strength on the AO-16 downlink. If the link margin doesn't take this into account, it is overly optimistic (like many other link budgets published pre-launch by AMSAT).
Echo will be pointed approximately towards the ground over the northern hemisphere and away from the ground over the southern hemisphere similar to the other Microsats. The dynamics of this can be adjusted somewhat by the active magnetic attitude control system (the "torquer") but Echo will generally point in the same way as all its predecessors.
The Mary antenna has the advantage of maintaining its polarization over a wide range of offpointing angles so it is expected that ground stations will not need to change polarity during a pass. Also, the link margin in Figure 9 assumed a linear whip on the ground such as would be used by a mobile or HT station and they won't notice this effect.
The attitude will be just like the 1990 Microsats. See the paper by Jim White WD0E in the Journal or Symposium proceedings from 1990, or at http://coloradosatellite.com/Papers/Motion.html. Incidentally, the orbit and space planning program STK now has this stabilization mode built in and Jim is hoping to have a short movie (mpg) of two satellite orbits using it on a web site soon.
The capability of the torquer is limited by the alignment of the B field. Jim's paper shows that alignment. The torquer can be magnetized to a variable degree and the poles can be switched. It can also be nulled out (turned off). While it's theoretically possible to pulse them or run them at various strengths and polarity to achieve a variety of attitude modes the lack of attitude determination sensors on Echo makes this practically very difficult. The practical application is to 'flip' the satellite so the downlink is optimized for one hemisphere or the other. It takes about a minute to reverse the magnetic polarity and it should take perhaps two to four orbits to take effect (remains to be seen). The operations committee will have to decide when and if to do this.
7. It seems that the 57k6 uplink conflicts with the PSK31 mode, since both use the multiband receiver. This could be avoided by using the transponder's L-band Rx for the 57k6 uplink. I guess this is not done because of some difficulty in fitting a wideband filter to that receiver. Is that correct?
There is no dedicated L-band receiver on Echo. This misconception was corrected with the new block diagram that was included in the PowerPoint presentation for AGM 2003. To further clarify, all uplink frequencies except 2-meter VHF will be received with the multi-band tunable receiver (SQRX).
8.What happened to the Digital Voice Recorder (DVR)?
The DVR was not finished in time to meet the launch shedule and so has been deleted from Echo\'s features. We hope to include the DVR on a future mission.
9. Can the torquer rod in the attitude control system be precharged before launch, so if the charging module fails to work on orbit the spacecraft will still have attitude control? Is it planned to do so?
The torquer rod will be precharged before launch.
10. What happens to attitude if the attitude control charger fails? Will the torquer rod retain a charge indefinitely?
Yes, the torquer rod will retain its charge for decades as long as it stays under reasonable temperatures (100?C or lower).
11. Is there a mechanism that spins the spacecraft? Is it black and white paint on the turnstile again? If so, do we have better paints this time?
Spacecraft spin is by means of black and silver tape on the edge of the body of the spacecraft (not on the turnstile). The tape is kapton impregnated with carbon and aluminum to make black and silver.
This is the same concept used on the Microsats but the coatings with differing a/e ratios are on the body rather than the antennas. The antennas are piano wire whips rather than tape measure so there is no surface to coat. The body tape has been used quite successfully on several satellites.
12. Have Echo's operating frequencies been chosen yet?
ECHO's Standard Operating Modes are:
- 435.225 MHz FM Voice Downlink,
- 145.920 MHz FM Voice Uplink + 67 Hz PL tone
Digital 9600 bps, AX.25, PACSAT Protocol mailbox
- 435.150 MHz FM Downlink and Telemetry in the digital stream
- 145.860 MHz FM Uplink
13. Are the VHF receivers crystal-controlled or agile?
They are crystal controlled.
14. Have VHF channels been selected yet? What are they?
Yes. 145.860, 145.880 and 145.920. Each of these has a second frequency that can be used if we need to move for some reason. Those frequencies will be published only if we actually plan to use them in the future. The fourth receiver will be dedicated to command and control and its frequency will not be published.
15. Does the IFC have access to real-time uplink signal strength information? Would it be possible (given the software effort) to provide power control feedback to the users? Think LEILA for FM.
Because of the hard limiting in FM receivers it is very hard to derive meaningful signal strength data. There is no plan to implement a LEILA style function in Echo.
The only RSSI telemetry available is from the SQRX. There is currently no plan to include it in the telemetry downlink. If someone can think of a good reason to report it, or a more interesting application it would be considered.
16. Is the audio from the various receivers handled in analog or digital in the IFC?
17. Is it possible to sum the audio from multiple receivers into a single downlink?
No for voice. For digital uplinks, the IFC always sees all the uplink packets on all the VHF receivers. Among other things that is a critical command/control feature. It also makes it possible to command the satellite even when everything else is in voice mode.
18. The original Microsat BCR relied in part on load management to maintain the health of the battery. Can the Echo BCR keep the batteries healthy with all transmitters off for extended periods of time? (as might be required if the spacecraft is retired from service someday)
19. Are there specs on frequency stability of the various radios? Oscillator drift on AO-40 is a problem for automatic frequency compensation, which will be far more critical on Echo.
Yes. 4ppm over -10C to +50C.
20. On the IFC you mention "agile" modulators and demodulators. Does this mean DSP-based, or something else? If DSP-based, what is the status of the software for those DSP functions?
There is no DSP on Echo. Agile refers to the ability to control the clock, filters, deviation and DC offset. With those controls we can achieve a wide variety of data rates.