It has been a bit since my last update. Tomorrow I hope to have a more thorough one for you all but this one here should be of interest, especially for the more technically inclined. We had a pass from the NASA Deep Space Network (DSN) that was to be used to improve our knowledge of the position of ISEE-3 in space. This is not an easy task, though NASA’s DSN team makes it look easy. Every measurement, in on way or another, depends on accurately measuring the time it takes for a signal to either come from the spacecraft (passive mode), or a two way round trip signal, (ranging mode) or the most accurate means, called coherent ranging where the spacecraft locks to a signal from the ground, and matches its phase coherently. The ISEE-3 spacecraft, even thought it is of a 1975 design vintage has this ability. However, things did not go as planned last week. Here is what I posted on our http://www.spacecollege.org site and this was also sent to our NASA sponsors.
The DSN pass on June 18th that went from 1:45 to 2:45 Pacific Daylight time was not a success. Here is a recap of the pass activity. The DSN pass started at 1:45 pm PDT. Here is a graphic of the pass through a very nice DSN Now web app:
The pass began with a +/- 3 KHz sweep across frequencies representing the input frequency of transponder A (2090.66) MHz + the Doppler offset + an additional 11.25 KHz that came from our most recent command session. The additional offset is due to thermal and or aging issues with the spacecraft transponder. The sweep is done with a carrier only, no modulation, to get the receiver on the spacecraft to lock to the DSN transmitted signal. The output of transponder A will start to vary in a 240/221 relationship when the carrier is locked. Then ranging can occur. The sweep was unsuccessful in establishing a coherent lock. The sweep rate was 60 Hz/sec. This conforms to the procedure used in 1985 by the DSN for the spacecraft during the ICE comet encounter.
A second sweep with a bandwidth of +/-6 KHz was initiated at the same sweep rate. This was also unsuccessful. It should have been successful as a total sweep of 12 KHz encompasses more than the offsets that we have successfully used to command the spacecraft. After this did not work, the DSN ops team then did a sweep at +/- 20 KHz with the center frequency set at our Doppler + 11.25 KHz offset. This surely should have worked in obtaining a lock, but it did not. Due to the time involved to do the sweep this exhausted our available time on the Goldstone dish, and thus we completed the pass without any indication of lock from the spacecraft.
Troubleshooting the Pass
The configuration of the Goldstone system was using the DSS-24 34 meter dish, running 10 kilowatts of power. Table 1 gives the estimated link margin:
Looking at the link margin, it is evident that the link was not the problem.
Failed Subsystem or Procedural Issue?
There are two possibilities for why the ranging failed, when we know for certain that transponder A is functional for commanding the spacecraft. The first is that the ranging function on the transponder has failed. The second is that a procedural error regarding commanding the communications system into coherent mode is at fault. Thus by eliminating the DSN link as an issue and looking at what could be the problem otherwise we have the beginning of a fault tree established for an investigation.
The possibility that the ranging function has failed, or was not functional was taken into account by our team. The spacecraft has two transponders, A, and B, and both are capable of ranging. There are some issues with lower gain on transponder B’s antenna that make it less desirable but with the robust margin from Goldstone seen in figure 2 this is not a problem. On our 6-15-14 Arecibo pass we placed transponder B into coherent ranging mode as well as transponder A. However, due to time constraints the DSN did not have time to attempt ranging to the B transponder. Also, due to lack of licensing to transmit to the B transponder from Arecibo, we have been unable to verify the functionality of that receiver. Thus one possibility would be to range to transponder B for our next DSN pass on 06-25-14. However, this does not address the issue of what the problem might be for transponder A.
Our success with transponder A has not been 100% in sending commands to it. The receiver input frequency seems to be drifting toward a higher frequency over time. We do not know at this time whether that is a random drift or one that is predictable. We overcome that when we do our commanding by sweeping the expected frequency plus Doppler plus an offset when we send commands. Since our first commanding session the offset frequency has appeared to drift to higher frequencies each time. This is not completely unexpected as we found test data that indicated a positive curve for transponder A. If this were the issue the +/- 20 KHz sweep would have locked the receiver. While this does not verify that the ranging mode is not functional, it does narrow the possibilities.
With the incomplete documentation at our disposal it is not unlikely that we have issues in operational procedure that preclude the coherent function of transponder A from working. On top of this problem is the legal issue that we can’t just use transponder B because we have been unable to obtain a license to transmit on the frequencies for that system from Arecibo. We have no schematics or vendor documentation on the transponder. We do have test data and we know the specifications and requirements that it had to meet. Worst of all is that as far as we can determine from the digital subcom that provides verification of communications operation, it is not functional, through either the Data Handling Unit (DHU) A or the redundant DHU B. This is not that surprising in that the DHU’s have absorbed more than five times the radiation that they were designed to handle. The only way that we have to verify a function for the communications system is to command that function and then see if it works in the intended way.
We actually were able to verify that the coherent mode is working and to validate the operational issue. Figure 2 shows our Eureka moment for coherent ranging:
The above graphic, produced by Phil Perillat from Arecibo, shows that our command bits for coherency did go up and did push the transponder into coherent mode, shown by the 18 kHz frequency jump by the downlink. There was a 1.6 second lag afterwards and we send the ranging command again, which stabilized the carrier at the offset frequency. After the end of that command, which at 256 bits/sec took 2.4 seconds, and after an additional 1.6 seconds, the downlink shifts back to its base frequency. This was our first solid indication, on June 15th that the ranging transponder coherent mode was working.
We had tried ranging through the transponder on the previous pass we had on June 9th. We did not think that this had been successful but after looking at some of the analyses by Phil Perillat we noticed that the ranging mode had indeed worked on that day as well. Figure 3 shows this:
In looking at this spectrum we were able to completely verify that the coherent ranging function with tone had worked. The spectrum above shows this. We verified this by looking at a document with test data on it from the spacecraft acceptance test. This is shown in figure 4:
The difference between the signal in figure 3 and figure 4 is that the first is shown in the frequency domain, and the second in the time domain. With the correlation between the acceptance test data in figure 4and the waterfall plot in figure 3 we pondered why the DSN pass did not work.
Even though we have incomplete information, we still have a lot. In our discussion related to the issue we recalled that somewhere in a document it was stated that the spacecraft had to be commanded into coherent mode and that if the carrier dropped for more than three seconds it would automatically drop out of that mode. After searching this document was found and here is what it said:
This provides verification of why we were able to get into coherent mode and the DSN was not able to also do so.
Final Verification of Procedural Issue
Friday June 20th we were going to do the propulsion system test and spin up maneuver. However, one of our pass/fail criterion was real time telemetry and reliable commanding. Neither of these criterion were met and thus we cancelled that activity early in the pass. This gave the team time to focus on operationally testing transponder A’s receive system and to retest the coherent mode to determine whether or not we could command that action and record the results. This we were able to do and figure 6 is our final evidence of Coherent mode operation:
What we were doing, as shown in the waterfall plot above, is that we were sending dummy commands to the spacecraft in order to get the command counter to increment. This was a test of the command counter but we also send a coherent mode command. The difference between figure 2 and figure 6 is that we changed our procedure slightly and sent commands one after another without allowing the carrier to drop, which maintained receiver lock on the spacecraft. This can be seen in the shifting of the waterfall plot and at the end the three seconds of drift before the carrier snaps back to the baseline frequency.
- Coherent Mode Operation
Coherent mode is operating in a manner consistent with the original acceptance test report.
- DSN Failure to Go into Coherent Mode
The failure of the DSN frequency sweep to lock the spacecraft into coherent mode is due to the lack of a command to go into coherent mode before the sweep. The sweep method must be coupled with a command for coherency for coherent mode to be entered.
The ISEE-3 team can provide to the DSN a packaged coherency command that can be broadcast to the spacecraft as the frequency sweep happens. At this time our best estimate for frequency offset is Doppler +10-15 KHz. This is due to aging of the transponder on the spacecraft. After lock is achieved ranging tones should be sent without dropping the carrier. Discussion with the DSN should be centered around how our command can be integrated with DSN operations.
 DSN_Operations_Plan_1985.pdf (page 5-8)
 1978-02_compatibility_test_report_isee-c_flight_model (page 162)