MERL – TR2003-035 – Very Low-Cost Sensing and Communication Using Bidirectional LEDs

A novel microprocessor interface circuit is described which can alternately emit and detect light using only an LED, two digital I/O pins and a single current limiting resistor. This technique is first applied to create a smart illumination system that uses a single LED as both light source and sensor. We then present several devices that use an LED as a generic wireless serial data port. An important implication of this work is that every LED connected to a microprocessor can be thought of as a wireless two-way communication port. We present this technology as a solution to the “last centimeter problem'', because it permits disparate devices to communicate with each other simply and cheaply with minimal design modification.

MERL – TR2003-035 – Very Low-Cost Sensing and Communication Using Bidirectional LEDs.

My tussle on amsat-bb

Today’s example comes from the amsat-bb mailing list. I am a member of AMSAT and a subscriber to the amsat-bb mailing list because it’s an interesting subject that I desire to learn more about and that I enjoy sharing with my fellow hams. But AMSAT is seemingly in a struggle to save itself from extinction, and it seems to be gnawing off its own limbs while doing so. An all-too-typical example came in today. Mike Rupprecht, DK3WN and member of the COMPASS-1 cubesat program wrote a simple, concise message:

Dear all,

we are looking for someone to assist us in commanding of our cubesat
COMPASS-1 at the West Coast.

If someone (or more) is interested in, please contact me. Thanks!

73, on behalf of the COMPASS team,


This was a simple, concise plea from help from the amateur committee. You’d think that we as amateurs would be eager to jump at the chance to help. After all, how many of us volunteer to help put up antennas, or fix equipment, or help one another to prepare for license exams, or even help with genuine emergencies, all without any kind of reward beyond our own feeling that we are doing something good.

Here’s one of the first responses that Mike’s plea received:

Hi Mike, DK3WN

COMPASS-1 is not an amateur satellite

If the COMPASS team is looking for someone (or more) radioamateurs 
all around the world in assisting with command and telemetry using our
amateur ground stations built  using our amateur money in my opinion the
COMPASS-1 scientific organization should donate contributions to AMSAT
in order to buid our communication satellites.

Is the COMPASS-1 scientific organization in Germany donating contributions
to AMSAT-DL to build P3-E or not ?

Tank you for you answere

73" de

i8CVS Domenico

First of all, as a matter of practicality, University cubesat programs such as COMPASS-1 are typically funded by grants. The grant money is typically given to perform a given piece of research, and in the case of cubesats, to build, launch and deploy a specific payload. The teams cannot simply give such money away to other programs, no matter how worthwhile or deserving, simply because you or I would like them to. It would be an improper use of their grant money. They simply don’t have that kind of discretion. Even if 100% of their team wanted to split their grant money with P3-E, they couldn’t.

Second of all, cubesat programs are by definition low budget projects. They are low mass, low power objects launched into low earth orbit by begging space on other platforms. A typical cubesat is a 4 inch cube that weighs a kilogram, and is launched into an orbit with an apogee of around 650km. By comparison, P3-E will have an orbital mass of 90kg in a HEO with an apogee of 44,000km. Domenico’s comments make it seem as if they have cash to spare: the reverse is actually the true. A typical cubesat program grant might pay for the satellite, but control stations are typically underfunded if they are funded at all. These cubesat programs are running on very tight budgets, which is one of the reasons why they sometimes seek the cooperation of radio amateurs to gather telemetry, or, in the case of COMPASS-1 to assist with ground control operations. Even if they did have cash to spare, and the discretion to donate it to P3-E, the cost of launching P3-E compared to launching a cubesat is so great that no meaningful fraction of the P3-E’s launch costs could ever be gained by extracting pennies from cubesat launches. We might as well just try funding the launch with bake sales.

Third, one might reasonably ask: why don’t university programs try to work with us to get our satellites launched? Why can’t we compete successfully for the kind of grant money that these cubesat programs tap into? The answer is similar to the answer above: because we don’t offer them anything of any real interest relative to the cost that we would incur to launch P3-E. The cost of launching every cube sat for the last 5 years will not come close to the cost of launching P3-E, and P3-E isn’t carrying any payload that granting agencies are interested in flying anyway.

So, at best Domenico’s criticism is pointless. They can’t do what they ask. Even if they could, it would be a pittance compared to the real costs of launching the satellite that he (and frankly I) would like to see launched.

But it goes well beyond just saying something that is pointless. Domenico is essentially asking for a quid-pro-quo of money in exchange for services. In the United States, all radio amateurs are governed by Part 97, and one of the fundamental principles of this set of regulations is that we are amateurs, and we are prohibited from providing communications for a pecuniary interest. Domenico’s suggestion is that we do precisely that: that we aren’t interested in helping unless they provide us with a financial incentive to do so. It’s a violation of regulations for us to behave this way.

But even more than that: it’s beneath our character as radio amateurs. We have the motto: “When all else fails, amateur radio works.” This isn’t just a statement of technology, it’s a commitment on our part as volunteers to help each other. It’s what we do, and not just when there are lives to be saved. Amateur radio works because generous, thoughtful people give of themselves to make the world a better place. Mike reached out to the ham community for help. Domenico asked for a five spot. That’s not what we are supposed to be about.

I must admit that I lost it when I read Domenico’s letter. I called him a jackass, and while I must admit that might be a bit too harsh, he certainly is acting like a jackass. Nothing he said will help the amateur radio service. It was a pointless, useless attempt to get money from a stone that has no money to give. I also think it was petty and not in the spirit of our service.

Some people disagree or claim that my criticism was uncalled for. They also seemed to use the excuse that Domenico is entitled to his opinion, and that means I should apologize. I wonder why none of them seemed to defend my own entitlement to my own opinion. It’s always easy to defend opinions that you agree with I suppose. But in any case, no apology will be forthcoming.

Mike offered some lucky radio amateurs the opportunity to serve science, to work with a team dedicated to building relationships radio amateurs, and to operate as the control station for an satellite on amateur frequencies. My own resources are too meager to be of any practical use to him, but I suspect he’ll find some with both the equipment and the desire to help.

Addendum: Here is a link to the IARU webpage which outlines the requirements that a satellite needs to go through in order to use amateur radio frequencies. The COMPASS-1 cubesat was coordinated and its use of amateur frequencies was authorized by the International Amateur Radio Union.

Addendum2: While scanning the list of downlink frequencies for cubesats to determine their potential for interference to birds with linear transponders, I came up with this list:

Satellite         No.    Uplink       Downlink     Beacon   Mode          Callsign
CO-66 (SEEDS-II)  32791     .         437.485      437.485  FM,CW,Talker  JQ1YGU
COMPASS-1         32787     .         437.405         .     1200bps AFSK  DP0COM
COMPASS-1         32787     .            .         437.275  CW            DP0COM
CUTE1.7+APDII     32785 1267.600      437.475         .     9600bps GMSK  JQ1YTC
CO-65  (APDII)    32785     .         437.475         .     1200bps AFSK  JQ1YTC
CO-65  (APDII)    32785     .            .         437.275  CW            JQ1YTC
PolySat CP4       31132     .         437.325      437.323  1200bps SSB   N6CP
CO-58 (XI-V)      28895     .            .         437.345  1200bps AFSK  JQ1YGW
CO-58 (XI-V)      28895     .            .         437.345  1200bps AFSK  JQ1YGW
CO-57 (XI-IV)     27848     .         437.490         .     1200bps AFSK  JQ1YCW
CO-57 (XI-IV)     27848     .            .         436.8475 CW
CO-56 (CUTE1.7)   28941     .         437.505      437.382  1200bps AFSK  JQ1YPC
CO-56 (CUTE1.7)   28941 1268.500      437.505      437.382  9600bps GMSK
CO-55 (CUTE-I)    27844     .         437.470         .     1200bps AFSK  JQ1YCY
CO-55 (CUTE-I)    27844     .            .         436.8375 CW

Frequencies cribbed from this link..

An astute observer will note that in virtually every case, these cubesats are using the top 1Mhz of the 435-438Mhz allocation that is internationally recognized as the satellite subband on 70cm. Currently no transponders operate in this section of the band. VO-52 has uplinks on 435.230-435.200. DO-64 has uplinks on 435.570-530. FO-29 has the downlink on 435.900-800. And AO-7? It’s not even in the subband, with an uplink on 432.125-175. The simple fact is that the chance for interference between cubesats and any existing or even forseeable launch is exceedingly low. I was going to run a calculation to demonstrate that even if their frequencies did overlap, the chances of them actually being in the passband of a linear bird was essentially zero, but given that they don’t overlap in frequency at all, the odds aren’t just essentially zero, but are in fact precisely zero.