The digital mode is not dominant as Eagle is to provide simultaneous analog and digital service. I don't see that using U as the primary uplink and and making L the secondary uplink for the linear transponder is a big problem. P3E designers made the same choice by having the L antenna work only near apogee.
The digital service will be used by those AMSAT members interested in it and we think that it will attract more members to pay for these satellites. The intent is to support 3 digital transponder bit rates - approximately 50 bps, 4800 bps and 256 kbps -- as described in the San Diego meeting document on EaglePedia. 4800 bps allows for digital voice uplinks and downlinks over 75% of the orbit with smaller antennas and less power than we need now for SSB. No one could come up with a modulation scheme (even SSB) that would eliminate the potental for interference on L band as the power spectral density of the uplink signal is so much higher than the Galileo downlink signal. This is why another band was recommended as the primary digital uplink.
As far as I can tell, AMSAT has put the linear transponders that the majority of its members wanted on each phase 3 satellite. Adding digital services that the rest of the world is using seems likely to attract more members than continuing with the same old strategy.
73,
John KD6OZH
----- Original Message ----- From: "Bruce Rahn" [email protected] To: "John B. Stephensen" [email protected] Cc: [email protected]; "AMSAT BB" [email protected] Sent: Friday, September 22, 2006 21:27 UTC Subject: Re: [amsat-bb] Re: Galileo interference on L band
John B. Stephensen wrote:
Part of the concern about using L as the primary digital uplink is the fact that the ground stations will be high duty cycle emitters. BPSK has a very low crest factor and one of the uses for a 256 kbps link is streaming video, so it will be very much like an ATV repeater. Given the equatorial orbit, Eagle will also be closer to the horizon than previous amateur HEOs.
Even a restriction similar to the one in place for U uplinks in areas of the U.S. (1 kW EIRP) would make high-speed uplinks unavailable.
John,
Thank you for bringing this point to my attention...through my neglect of things I have lost track of the dominance the digital mode has taken in this project. You are correct that this signal format is a high duty cycle one. My thoughts of where Eagle was heading have been more aligned with the desires of the membership expressed in the survey results presented in the September/October 2004 issue of "The AMSAT Journal".
I'm going to ask some hypothetical questions here which I really don't expect you or anyone to answer. They are more food for thought than anything else.
- As part of the system engineering process, were other bit rates and
modulation schemes considered which would mitigate potential interference problems?
- What percentage of the user base (AMSAT-NA members) would be
disenfranchised if digital video were eliminated because of its high duty cycle requirements and the potential for causing interference to other spectrum users?
- In the aforementioned survey results, the surveyed members indicated
their highest preference was for analog modes followed in second place by digital. Has the user mindset shifted to digital over analog? If not, or unknown, are the spacecraft resources being fairly partitioned and allocated to support analog users? What percentage of the user community will be using digital video and text messaging?
In a private exchange with Mr. Sanford, I expressed my concern that the user community was not being represented by a strong 'user advocate' at critical design meetings. 'Designers are not users and users are not designers' but both camps must be fairly represented to achieve harmony and consensus between the two. Bringing a strong 'user advocate' into the design process would be a win-win situation for both the user community as well as the design community. Users would feel someone is directly addressing their operational concerns and the 'user advocate' could be the one defending decisions rather than occupying the time of the designers in addressing these concerns. I believe in the 20 plus years I have been an AMSAT member history has demonstrated that the 'if we build it they will come' approach has not worked well. Had it been successful, the organization would have more resources in terms of members and dollars than we could deal with.
Respectfully -- Bruce
-- Bruce Rahn
Wisdom has two parts:
- having a lot to say; and
- not saying it!