By Dennis Wingo
This update comes 18 days after we started our Rockethub (http://rkthb.co/42228) project to raise $125k to allow us to attempt to contact, evaluate, and command the International Sun-Earth Explorer-3 (ISEE-3) spacecraft to fire its engines in such a way as to return it to Earth orbit after a swingby of the Moon on August 10th, 2014. If you want to know all of the details, please read my previous posting on this subject here:
Today I want to give a discuss some of the technical issues and hurdles that we face in bring this spacecraft back into a stable Earth orbit. I am leaving out the experiments for the time being as we have to focus on the engineering required before we get to that part.
First the Good News
The good news is that the team that did the ISEE-3/ICE mission really planned ahead. In 1986 a series of maneuvers were done in three stages that targeted a flyby of the Moon at an altitude of 63 kilometers over the lunar surface on August 10, 2014 at around 8:30 PM UTC. The first series of firings, with a total delta v of 1.51 meters/sec were done on February 27, 1986 when the spacecraft was near perihelion (closest point to the sun), to target the general vicinity of the Earth. The second series of firings were done on April 7th 1986 to do a plane change to place the spacecraft on an intersect path to the Moon of ~38 meters/second. The third firing sequence was used to target the desired perilune target plane aim point (to the nearest kilometer of course!) with a 0.4 meter/sec burn that gave the lunar flyby distance of 63 kilometers. Figures 1,2 and 3 show the resulting trajectory for these maneuvers:
Figure 1 shows the ICE/ISEE-3 trajectory relative to a fixed inertial frame from April of 1986 through the August 10 2014 lunar flyby.
This graphic, from Mike Loucks at Space Exploration Engineering, shows the flyby at ~15 km. This was from just plugging the 1986 ephemeris into the modern Satellite Tool Kit application and running it. It is shown here just as a general representation of the flyby.
This graphic, from the 1987 ICE Handbook shows the trajectory if everything went perfect, with no further maneuvering required. For those who look at this with an engineering eye, the dark lines in the middle is the orbit of the moon around the earth. There are a lot of out of plane orbits here that require course corrections to fix. The lines to the side of the orbits are tick marks for days. This is where the bad starts in that no matter how brilliant these guys were, they could not completely foresee the influences on the spacecraft from 1986 through now. Also, the orbit chosen does not really, after the last ranging contact in 1999 and 2008, match and the orbit that the spacecraft is actually in, will result in the spacecraft not being permanently captured in Earth orbit. This is shown by a new plot by David Dunham, who did a lot of the previous work and is now on our team:
The above was the good part. The targeting of the ISEE-3/ICE for its various comet rendezvous and return trajectory to the Earth is a tour deforce in understanding orbital dynamics and Dr. Farquhar and his team did a magnificent job. However, there was only so much that computers of that era could do, and even if they had been perfect, in 28 years one has to expect that some course changes have to happen to put the vehicle on target to be captured into Earth orbit. To recap from my previous post, figure 4 and 5 show what will happen if we do nothing vs what we want to do:
This is obviously not desirable. The desired (baseline for our project) orbit is shown in Figure 5:
So, the goal of our project is to achieve the orbit that is in figure 5. However, the trajectory of the spacecraft must be adjusted and it must be adjusted soon. The current orbit is a nearly circular (eccentricity ~0.054) with an inclination relative to the ecliptic plane (the plane of the Earth and Sun projected on a two dimensional plane) of 0.06 degrees. The perihelion (closest point to the sun) is 0.926 AU (astronomical unit or the average distance between the Earth and Sun of 146.6 million km) or 135.75 million kilometers [81.75 million miles]). The aphelion (farthest point from the sun) is 1.033AU or 151.44 million km [91.196 million miles]. The orbital period is about 354 days. So now, after 30 years the alignment of the orbit of ICE/ISEE-3 at aphelion lines up with the Earth’s orbit. Figure six shows the decreasing range of the spacecraft at this time relative to the Earth:
As of May 1, 2014 ICE/ISEE-3 will be at .16 AU or 23.46 million km [14.1 million miles). This is important as the closer the spacecraft gets to the Earth, the more energy it takes to nudge the course in the right direction for the right altitude lunar flyby that will result in capture into Earth orbit. This can be seen in the figure 7 plot of delta v versus time done by our team member David Dunham:
There is approximately 150 meters/sec of dV left on the spacecraft in its hydrazine fuel system. As can readily been seen the amount of energy needed as the range gets closer increases rapidly. Table 1, also by David and a researcher at the University of Arizona, shows the problem in tabular form:
As you can readily see, as the spacecraft gets closer, the more energy needed to turn the spacecraft into the right trajectory. Since there is only about 150 m/s of dV left (uncertain to at least 5%), then sometime very soon after July 1st it is game over. However, it is worse than that in that we would like to have fuel left over in case of further maneuvers. This includes more lunar flybys to trim the orbit or to have enough fuel to operate in Earth orbit for a while at the L1 point. Thus time is of the essence!!!
So, we have a very hard deadline looming. The problem has been that the people at NASA and the retirees that wanted to do this were unrealistic about the chances of obtaining funding. With NASA getting ready to shut down operational missions there was no way that a 36 year old spacecraft coming back to the Earth would have any priority. On April 10th of this year NASA headquarters in a teleconference with senior leadership in the Science Mission Directorate confirmed this to us and the people that wanted to do this mission with NASA funding. Thus, four days later our effort was born.
The problem is that all of this should have started months ago! However, just as we learned in our project to recover the Lunar Orbiter and Nimbus I, II, and III tapes from the 1960′s, modern technology can do things now that would have cost millions for NASA to do even ten years ago. There are several technical problems that have to be solved and we are going to focus on them today and where we are at in the process.
Talking to and Commanding the Spacecraft
First we have to know what language the ICE/ISEE-3 spacecraft speaks in order to understand the formats of the data for commanding and telemetry. Fortunately, the spacecraft does not have a computer!! It has a sequencer that looks pretty much like a printer looks like to a computer. In the days before NASA put computers on spacecraft the overhead for the operations team was huge as each individual command had to be developed on the ground, sequenced, and then sent to the spacecraft. An acknowledgement, in the form of telemetry indicated that the command had been accepted and executed. What we have to do is to recreate that entire system.
Since this particular problem has a very strong ITAR flavor (International Traffic in Arms Regulation), we are doing this part in house with some help of some volunteers that have come on board (we will be paying them through the critical part of this activities). We have to first figure out what the command structure is like for the engineering systems on the spacecraft (we are not bothering with the experiments yet) and figure out how to do that using modern computers. We are making a lot of progress there but we at this time have insufficient documentation. We are working with NASA to develop a Space Act Agreement that will give us access to this documentation. We have enough to develop the command screens and we have an approximation of the data that we need to send. However, there are (as of today) some gaps that we have to fill. We continue to get documents from former engineers that worked the ISEE-3 mission so we are confident that we will have what we need.
Our initial tasks for commanding the spacecraft are as follows:
1. Turn Engineering Telemetry Mode on.
This mode commands the spacecraft to transmit telemetry regarding the functions of the onboard systems, including the attitude determination and control system, the power system, and the propulsion system. This will help us evaluate the health of the spacecraft.
2. Two Way Ranging
This command will place the ranging transponder in a mode that allows ground stations to measure the distance (range) to the spacecraft and back. We have to use multiple stations and use triangulation to get a better fix on the position. We will have a team that takes this range data and then turns that into an updated position in space of the spacecraft. Then the flight dynamics guys (Dr. Farquhar and David Dunham), will update the required energy to put the spacecraft into a proper Earth orbit. Then….
3. Engine Firing
Back in the day, NASA had a program called ICEMAN that worked with a couple of programs called GMAT and GMAS. These were all old mainframe programs that took the data and the developed a firing solution for the thrusters on the spacecraft. This is an area where we don’t have the programming information for the old way so we going back to first principles of the spacecraft and re-deriving what we need to fire the thrusters. We are working with Space Exploration Engineers (that did the NASA LADEE mission trajectories) to help us verify and validate what we are doing so that we can close in on a firing solution that looks good.
So this is what we are going to do. To get there we have to solve the problem of how to read the telemetry, properly format commands, send the commands, and verify that the commands were executed. We are developing the screens and the process to do this now. More as this develops.
Another big problem to solve is to redevelop the demodulator and modulator for talking to the spacecraft. If this was even ten years ago it would be very difficult and expensive to do. However, with the continuing advance in software technology for embedded computer systems we can now develop a demodulator in software and run it on some hardware that interfaces to the transmitters and receivers on the spacecraft.
We are working diligently with Ettus Research on the development of the modulator/demodulator. This basically formats the digital data that comes from the command generation software (also under development) and turns it into a waveform that the S Band transmitter can transmit to the spacecraft and that can then be demodulated on the spacecraft and sent to the command sequencer. Note that there is no computer on the spacecraft and thus the commands are directly executed. There are a lot of details related to this that we are glossing over for now in the interest of brevity.
When we command the spacecraft into engineering telemetry mode, the spacecraft transmits a signal back. The signal is received, also on S-Band and then down converted to a signal that can turn the analog waveforms into bits that can then be received by our telemetry system software (also under development). We will report back more on this later but suffice to say that the folks at Ettus are the best in the business and we will have this part of the system functioning soon.
Another huge issue is redeveloping a telemetry display system. When ISEE-3/ICE was operational this was comprised of several computers with old style displays for command line or numerical telemetry. Today we have off the shelf software that can take the bits output from the demodulator and turn them into graphical displays and switch positions. This is a gross simplification in that we have to be able to take the incoming data, verify that the parity is right, then slice and dice all of the bits at their right addresses to push them to the right displays. We are working with some people at National Instruments as well as working to develop our own displays for certain functions. Following are the three spacecraft subsystems that we are focused on for the critical thruster firing.
- Propulsion System
- Attitude Determination and Control System
- Power System
These are the screens that will be redeveloped and then when we command the spacecraft into engineering telemetry mode we will be able to verify its last known state, evaluate the condition of the spacecraft, and test a few of the subsystems without hitting the propulsion system and turning it on. We are obviously going to be very careful about that one!
Another and possibly our biggest issue is ranging. What ranging is, is to send a signal to the spacecraft, have it returned, and measure the flight time to a very high order of accuracy (a few to tens of nanoseconds over a two and a half minute period). We also really need to do this at multiple stations so that we can get differential flight times that can then be used to create a triangular baseline so that we can do a much better job at determining the spacecraft’s current position. We know it to then tens to hundreds of kilometers level. However, we need to know it much better than that in order to know exactly how much to fire the spacecraft thrusters for the lunar flyby at the right altitude. NASA makes this look easy. One time I read that the Voyager spacecraft’s position was accurate to a few kilometers out to Neptune! This is an incredible feat of navigation that no one else in the world can do. We are trying to replicate this at a much shorter range (about 23 million kilometers). We know what to do, but we don’t have this one fully solved yet.
The above are the biggies right now but another aspect is to keep all the parties in the loop, get our team of mostly volunteers working together, and to coordinate all the things that have to be done. Part of the problem is that to read and absorb the documents takes time, and then to write formal documentation in a manner that should be done also takes time, and time is the one thing that we don’t have! This is probably the biggest challenge overall and we have been bringing a couple of new team members that are local on board to help and start to get a handle on everything. We also have to scare up resources, in people, in hardware, in time. All of this takes time, time that is a very precious resource right now.
There is also the paperwork that we have to do with NASA. We are currently working out a Space Act Agreement with NASA that will allow us to collaborate with them on a no exchange of funds basis. This will give us access to the documents that we need and other things that as soon as the Space Act Agreement is signed, we will tell you about. We will be sharing data about the spacecraft, its performance, and the condition and operation of the experiments with them in return for some things from them.
We have a lot of balls in the air right now, from ordering transmitters, to coordinating ground stations and engineers, to getting documents from NASA, getting the Space Act Agreement signed, to getting the team organized and working as a team. All of this while the crowd funding effort is still going forward and without being able to access the money from it!!
More next Monday, but for all of the Crowd Funding supporters, we are working very diligently to meet our deadline for the firing of the thrusters and so far we don’t see anything that makes this impossible! Just darn difficult!
I would like to thank the following companies people at this time for their support.
Keith Cowing, Media, and all around gadfly of making things happen and my Co-lead on this project.
Tim Reyes, Flight Operations
Cameron Woodman, Flight Operations
Karl-Marx Wagner, Transmitters and communications
Patrick Barthelow, Transmitter and antenna support.
My Wife Nikki!
Mike Loucks, Space Exploration Engineering
Mark Maxwell, Space artist extraordinary!
Leonard Garcia, overall great guy doing this on his own time at NASA
Dr. Robert Farquhar, former ISEE-3 Mission Design lead
David Dunham, Flight Dynamics
I also want to thank my internal team of Austin Epps and Marco Colleluori who are working some of the very difficult problems with telemetry.
and many others!!