Dennis Wingo: We have returned from our foray (probably not the last) at Arecibo. It was a wonderful trip, the people down there are marvelous, and Arecibo is the crown jewel of American radio astronomy. Did you know that the Chinese are building a bigger one, modeled after ours? The NSF keeps trying to cut their budget but this is truly an amazing asset to the American people and the world and it should be supported!! I want to thank Mike Nolan, Phil Perillat, Dana Whitlow, Victor Negron, and all the great crew down there.
Technical Progress, Contact Made!
Commanding The Spacecraft
As we put out in our project very brief note on Friday, we have successfully contacted the bird! At the time of the contact we had Morehead State University Space Science Center’s 21 meter dish, the 20 meter dish at Bochum Radio Observatory in Germany, and the SETI Allen Array all listening. This was not without problems. The spacecraft has two transponders, which are oddly enough called transponder A and Transponder B. Transponder B is normally the engineering telemetry transponder and transponder A is the ranging transponder. The final state of the spacecraft before was to have both of the transponders transmitters active and that is what people around the world have been tracking. However, the spacecraft is set up with a lot of redundancy so you can use either transponder A or B to send telemetry or range.
We tried several times to command the spacecraft’s B transponder at 2041.9479 MHz into the mode where it normally sends engineering telemetry, which is our first task. It did not work. We tried several variations of the proper commands and we tried several operational approaches such as scanning across the receive transponder band to make sure that there was not some offset that we did not know about or that the receive frequency had drifted over the years. Nothing worked. Then we tried the same process on transponder A and BINGO, telemetry! Well not really telemetry itself, but modulation from the output of the telemetry system. The initial command was just to turn engineering telemetry on at 512 bits/second. This was successful. Figure one, generated by Achim Volhardt from AMSAT DL and Bochum, shows the modeled spectrum for a 512 bits/sec telemetry rate, which is what we commanded:
Figure 1: Simulated (Solid Line) vs Received Spectrum from ISEE-3/ICE A Transponder
The second image, figure 2, shows a waterfall plot that clearly shows the sidebands of the expected signal, as recorded from our Ettus Research USRP N 210 receiver as processed through GNU radio:
Figure 2: Waterfall Plot of Signal Showing Sidebands Click on image to enlarge.
This provided our initial verification that we indeed has successfully commanded the spacecraft into engineering telemetry mode. We later also, working through the A transponder’s receiver we commanded through the B transponder’s command decoder to output engineering telemetry through transponder B’s transmitter. Thus we have verified so far the following systems on the spacecraft.
1. Transponder A receiver
2. Transponder A’s Command Decoder and Data Handling Unit
3. Transponder B’s Command Decoder and Data Handling Unit
We reviewed some of our documents and found that neither of the ISEE-3/ICE receivers had met their specification in testing. The specification was for -120 dbm sensitivity. However, we found that receiver A was tested at about -114 dbm, and Receiver B at -111 dbm. A difference of 6 and 9 db gain respectively. Working the numbers backward with our 400 watt power amplifier and the gain of the Arecibo dish, we found that we were marginally ok with the A receiver and probably slightly short with the A receiver, calculating in radiated power vs db Eb/No. Normally NASA and the Deep Space Network (DSN) uses transmitters in the tens of kilowatts range. Since we could neither acquire one that big in the time that we had, or could afford to buy one, this had driven our decision to use Arecibo rather than a larger power amplifier at a smaller dish.
Our biggest disappointment was that we then tried to command the spacecraft into 64 bits/second mode, which was a mode that is much more complicated to set up and we did not get working successfully during the limited time that the spacecraft is visible from Arecibo. We need to do this so that the smaller dishes at Morehead State and Bochum will have a positive signal margin so that we can record several hours of data.
THEN WE HAD AN EARTHQUAKE. As many know who follow our social media, on Thursday after our end to end systems test we had an earthquake. I was on the central part of the Arecibo dish, 450 feet in the air with Dana and Anthony, another engineer at the site when this happened. We had just chatted about how observations could be affected by vibrations in the dome structure as it translates during an observation and then that happened! The azimuth tracking system, which is the curved structure on the underside of the top part of the dish, was slewing while we were there as well as the dome. We were sitting in a safe area when everything started shaking. I was doing a video at the time but stopped it to hold on during the shaking!
Demodulating and Decoding the Received Signal
The first miracle was to command the spacecraft. The second is to understand what it says. Figure 3 shows a scope plot of the resulting data and clock plotted in time:
Figure 3: Clock and Data Recovery from Demodulator Click on image to enlarge.
If you look at the bottom of this figure you can see 1s an zero’s, the bits that come out of this process. Now our guys are super exited about this and yesterday morning (Saturday) Austin Epps sent out an email based on the first set of bits that Balint got out of the demodulator: I searched for the synch bits ‘11111010111100110011010000000000 per the SIRD document. That string was found in two locations…starting at bit 575 and again at bit 2623. Note that the two locations are 2048 bits apart, exactly as expected.
We got our synchronization bits, which provides the framing indicator for a frame of data, out of the demodulator! Not to be outdone, (actually everyone is collaborating and working beautifully together), our new volunteer, a very old hand at demodulating satellite data, Phil Karn, jumped on the data that Balint provided and we have the following fully processed first frame of data! Gentlemen, feast your eyes on this:
7c 02 02 02 02 02 02 02 7c 02 02 02 02 02 02 02
7c 00 02 00 00 f2 00 00 7c 02 02 79 a0 00 00 00
7c 00 02 33 c8 02 4d 02 7c 4b 02 76 00 00 00 00
7c 02 02 53 01 02 39 02 7c 44 02 00 b1 49 00 00
7c 00 02 5a 00 19 5c 64 7c 4b 02 0e a0 00 00 00
7c 0e 02 4b 47 63 91 1d 7c 42 02 4d 36 00 00 00
7c 45 02 44 4e 8a 89 02 7c ce 02 50 a4 00 00 00
7c 48 02 32 4b b5 d2 ad 7c 33 02 12 fc 81 9f be
This is my very first Viterbi-decoded frame of ISSE-3 telemetry, extracted from the first frame of the recording I received this morning. Note that it ends with the 12 fc 81 9f be sequence, the 3-byte encoder dump sequence 12fc81 followed by the 2-byte sync sequence 9fbe.
I forced the Viterbi decoder to end in the state 819fbe so those last three bytes could not have been anything else, regardless of what was received. HOWEVER, the 12 fc decoded just before that actually came from the received symbol stream, and since that matches the values given in the documentation this is a strong indication of correct decoding.
As another indication of correct decoding, I re-encoded the Viterbi-decoded data and compared the encoded symbols to the raw symbols received from the spacecraft. There were no errors. None. I think we have it. Now I just need to polish this off so it’s useful.
Fantastic work! Phil later processed our first day’s data dump from the spacecraft and we received 49 full frames of data at a bit rate of 512 bits/second. Until the very end there were no errors on the downlink, and only then when the spacecraft was going beyond the horizon for Arecibo. These are the milestones related to commanding and receiving data from the spacecraft that have been achieved:
1. Successful commanding multiple times of ISEE-3/ICE
2. Received engineering telemetry from both data multiplexing units on the spacecraft.
3. Successful demodulation on the ground of the received data, through the output of bits.
4. Verification of good data at 512 bits/sec, including frame synchronization, correct number of bits/frame, and with no errors, showing a very strong 30+ db link margin through Arecibo.
These milestones alone would be praiseworthy but there is more!
The Trajectory Problem
One of the major problems that we have, that has to be solved, is to update the range to the spacecraft so that its position, velocity, and trajectory into the Earth Moon system can be properly plotted so that we can then plot a course, and fire the engines for a course maneuver so that the spacecraft does a lunar flyby at the proper altitude (around 50 km) on August 10, 2014. The last trajectory solution that we have from the DSN is from 2001 and it is this one that is provided by NASA JPL in its Horizons prediction program that everyone has been using.
The problem is that this solution has been shown to be inaccurate when we are using the extremely narrow beam width (~2 arc minutes), at Arecibo. When plotted out to the approximate distance of the spacecraft, 2 arc minutes is only about 16,800 km wide, meaning that if the spacecraft is more than about half this distance (assuming that we point exactly at the right location), then the spacecraft falls outside of this beam and the signal vanishes. This is except for the fact that the Arecibo telescope is so powerful that the minor lobes of the main beam are still more powerful than most smaller telescopes and that was how we were receiving the spacecraft in the first few days at Arecibo.
Then, Phil Perillat who handles the hard problems in operating the telescope at Arecibo, performed a search to lock on the main beam of the spacecraft. He was able to do this, and our signal level was over 50 db above the noise, a very strong signal for transponder A, and a bit less, around 45 db for transponder B. Phil continued to do this almost every day when we could get time on the extremely busy telescope.
There is a huge side benefit to this technique. In the scientific community for asteroid research radar and optical sighting of Near Earth Objects is used as the only means to determine orbits. This community has gotten extremely good at this method. The spacecraft engineering community uses coherent transponders (which is what transponder A is on ISEE-3/ICE) to lock on and do a two way ranging to allow the engineers to calculate a good orbit ephemeris for a spacecraft.
Amazing Accuracy of the 1986 Trajectory
Using the data from Phil’s daily targeting of the spacecraft, Mike Loucks at Space Exploration Engineering, along with the folks at Applied Defense, and then further verified by Johns Hopkins APL, we have narrowed down the location of the spacecraft. It turns out that it is far closer to the Moon than the JPL Horizons propagated trajectory, and very near being on the course intended for it by the ICE trajectory team in 1986! This is shown in figure 4:
Figure 4: Newly Plotted ISEE-3/ICE Course Compared to Horizons and JPL 2001 Trajectory Click on image to enlarge.
The Blue Circle is the orbit of the Moon with the Moon’s location show at the right side of the circle (August 10, 2014 location). The Yellow Horizons trajectory is shown intersecting the Moon’s orbit but no where near the Moon on that date. The white line was a re-propigation of the JPL Horizons orbit in Systems Tool Kit (Satellite Tool Kit). The dark blue trajectory is the intended trajectory of the ICE navigation team in 1986. The red/green trajectory is the plotted trajectory based on Phil Perillat’s pointing data from the Arecibo telescope!
Consider this, the spacecraft has completed almost 27 orbits of the sun since the last trajectory maneuver. That is 24.87 billion kilometers. They are off course by less than 30,000 km. I can’t even come up with an analogy to how darn good that is!! That is almost 1 part in ten million accuracy! We need to confirm this with a DSN ranging, but if this holds, the fuel needed to accomplish the trajectory change is only about 5.8 meters/sec, or less than 10% of what we thought last week!
We truly stand on the shoulders of steely eyed missile men giants…
What is Next
If we can maneuver the spacecraft by June 17th we get the very small delta V number for the maneuver above. However, this starts to climb rapidly as the spacecraft gets closer to the moon. Also we cannot at this time rule out a lunar impact. It is imperative that we get a ranging pass as soon as possible. We also need time to not only evaluate the health of the spacecraft, but to test the systems, the catalyst bed heaters for the propulsion system, the valve heaters, analyze the rest of the propulsion, power, and attitude control system as rapidly as possible. This will be a lot of commanding so we have to move into high gear next week.
This is a very fluid situation and we have made amazing progress, thanks to the support of those who believed in us in our crowd funding and the support of our NASA sponsors at NASA Ames and NASA headquarters. More to come soon!!