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LightSail Radio Update: New Answers, New Questions

Posted by Jason Davis

03-09-2014 13:47 CDT

Topics: Planetary Society Projects, mission status, LightSail

It's been two weeks since a set of recurring radio problems scrubbed LightSail-A's day-in-the-life test. LightSail-A is the twin spacecraft of LightSail-B, The Planetary Society's solar sailing CubeSat scheduled to ride a SpaceX Falcon Heavy rocket into orbit in 2016. LightSail-A may go on a test mission in 2015 aboard an Atlas V, but a lot of troubleshooting and testing needs to happen before then.

As I wrote in my last update, LightSail-A's radio system is drawing too much power from the spacecraft's battery. A normal power draw is 2 to 5 watts, but in some instances, the team has seen the radio pull up to 12 watts. Additionally, not enough of that power is being funneled out the spacecraft's antenna in the form of RF signals. A normal power output is 1 watt, but the signal has been registering at just a fourth of that. Confounding the situation further is BenchSat, LightSail's test unit, which also has a low RF output—but not an excessive power draw. 

I previously said the team was zeroing in on an antenna configuration mismatch as a possible source of the problem. Since then, Alex Diaz and Riki Munakata have been working closely with Tyvak Nano-Satellite Systems to test LightSail's antenna and see if it is properly matched to the spacecraft's radio system. Tyvak concluded that while the antenna is indeed mismatched, it shouldn't be causing such an excessive power draw and low signal output. Diaz is now working with Tyvak to refine the antenna's configuration, which will ultimately improve the spacecraft's communications system. So while this line of troubleshooting didn't find a smoking gun for the larger radio issues, it revealed and corrected a slight performance problem.

BenchSat

Ecliptic Enterprises Corporation

BenchSat
BenchSat replicates LightSail's components on an acrylic board, making them easier to access than when they are bundled in CubeSat form.

After the scrubbed Aug. 20 day-in-the-life test, the team took LightSail-A back to Ecliptic Enterprises Corporation in Pasadena. There, Diaz and Munakata made sure they could replicate what they had seen at Cal Poly. The results were confirmed, so they next decided to try installing LightSail-A's radio in BenchSat.

It’s not terribly difficult to swap out LightSail's radio. The spacecraft has two main circuit boards: an avionics board and a Payload Interface Board (PIB). The avionics board has two daughterboards—smaller boards that are fed by the avionics board. One daughterboard houses the connector for LightSail's battery. The other has a connector for the radio system. So, swapping out the radio means unplugging it from the second daughterboard.

And that’s what the team did. They removed LightSail-A’s radio and plugged it into BenchSat. Remember: BenchSat's radio system was also transmitting at just one-fourth of a watt, but unlike LightSail-A, it wasn't drawing excessive power. When LightSail-A's radio was plugged into BenchSat, nothing changed. BenchSat still transmitted at a fourth of a watt, with no out-of-the-ordinary power draw.

De-integrated LightSail

Stellar Exploration / Annotations by Jason Davis

De-integrated LightSail
The individual components of LightSail's avionics section can be seen in this photo, taken during an earlier de-integration of the spacecraft.

Next, Diaz and Munakata transported LightSail-A and BenchSat to Tyvak, determined to find some answers. They began by removing the spacecraft's avionics board, allowing them to begin swapping and testing even more components. For one of their first tests, they disconnected LightSail-A's battery pack and connected the system board and radio to an external power supply.

They ran a transmission test. And everything worked properly.

It was a surprising result, so they swapped LightSail's radio with BenchSat's radio. Everything still worked. They tried again with another radio spare. Everything worked again. In all cases, just 2 to 5 watts of power were being drawn into the radio system, and a full watt was being transmitted.

So, what's going on here? Right now, the team isn't sure, but at the very least, they've found a configuration in which the radio system works correctly. Is there a problem with the spacecraft's battery system? More testing is on the way.

 
See other posts from September 2014

 

Or read more blog entries about: Planetary Society Projects, mission status, LightSail

Comments:

Adolf Schaller: 09/04/2014 12:11 CDT

All the best to you in figuring it out. Nothing that is truly worthwhile is easy, and everything that is deemed worth doing is hard. Carry on! IT WILL WORK IN THE END!

OE4KMC: 09/04/2014 04:05 CDT

Hi LightSail Team, To present myself, I am the radio systems wrangler of the ISEE-3 Reboot project (the guy who helped doing the impssible in 6 weeks). From what I read up there you have serious show stopper problem in your overall design. It seems that you already killed your PA by high SWR. THIS MUST NOT HAPPEN: your PA must be "full circle proof" as a radio engineer would call that. The cause be the following: 1. parametric oscillations caused by resonances in the feeder cable. Cure: add a low impedance (high C, low L) paralell resonant circuit directly at the output. 2. Radiation feedback. From what I see on the photos there isn't much shielding in your setup. So it can happen that your antenna manages to radiate into the transmitter and causes wild oscillations. 3. Your whole TX strip is only conditionally stable. If this is the case, the only thing to do is to ask your TX supplier to redesign their circuit to make it unconditionally stable. 4. Your "measuring tape" antenna, in other words, a quarter wave rod antenna, is not a good choice because it uses the s/c structure as a counterweight. This means that you get a lot of rf going this way and that. This could interfere with the analog and sensor circuitry of your system, rendering their indications spurious and worthless (EMI!!!). 5. The other way round your digital circuitry is generating a lot of rf noise that can interfere with your command signal at the receiver and render it insensitive. There is more to be said, but not in 1000 characters. Contact me by email if you want to know more. 55 es 73 de Karl-Max, OE4KMC

Jason Davis: 09/05/2014 11:58 CDT

Hi OE4KMC, Thanks for your suggestions. I'm going to pass this along to the engineers, along with your offer to help. We appreciate it! Jason

tomasek: 09/12/2014 11:11 CDT

The explanation #4 above is not valid -- the quarter-wave antenna is designed to use the actual solar sail as a ground plane, not a satellite structure. The sail spans >10 lambda, thus almost perfect ground plane.

OE4KMC: 09/17/2014 04:29 CDT

Hi tomasek, What you say has a couple of problems: 1. What do you do as long as the sail is not deplyed yet ? What do you use as a counterweight ? 2. How is your rod antenna attached to the light sail ? You need to mount your antenna way off the rim of the sail. In order to get a good rf connection you have to use a disk which is attached smootly to the light sail. There is also the question of feeder line length. With the kind of coax you use factor in 3 dB/m of line loss. 3. Mantle wave blockers are your friends ! Wind your coax with a few turns through a high mu ferrite core. This way you prevent rf propagation on the cable shielding. Place this device close to the antenna, 4. Frankly, rod antennas are a pain in the ass. Half skeleton slot antennas are much better. In your case, I'd make a half skeleton slot out of two measuring tapes. The BIG advantage is that not only do they not need a counterweght, they radiate at the end and not on the base. 5. Always avoid making things part of any rf circuit that are not specifically designed for that purpose. You will have way less headaches that way. 6. To avoid another TX PA blowout, connect a circulator between TX out and T/R switch. For 1 W you can take tiny SMD devices. A good source is Cobham plc. ( They took over Tekelec Airtronic ). 7 email: oe4kmc at hamradio dot si 8. 70cm is my favourite band. If I know a band, then sure it is this one. 55 es 73 de OE4KMC

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