Final Phase - Ulysses
The odyssey continues ...
This is a series of status reports on the current phase of the Ulysses mission which is characterised by the following operational constraints:
- Data (science & engineering) is only available in real-time during a ground station pass;
- S-band only for telecommunications;
- Downlink data rates of 256 (at lower antenna elevations say less than ~30°) and 512 bps;
- Fuel bleed (~1.2g) every two hours to prevent hydrazine from freezing in the fuel line.
These reports were sent by e-mail from the Ulysses Mission Operations Manager to the Ulysses community. They are reproduced verbatim below in reverse chronological order with the exception of the e-mail headers.
Date: Thu, 20 Feb 2009 11:45:38 -0800 From: Nigel Angold Subject: Ulysses Status Update 5 Dear Ulysses colleagues, Yesterday was mission day 6712 and we surpassed 400 days of S-band mission operations. Given that we thought the spacecraft would only survive a few months after the X-band transmitter failure on 15 January 2008, that's pretty good going! The last month or so has seen a dramatic increase in data return. This is due in part to a request by NASA HQ for additional DSN coverage and also due to the fact that we can record and play back data again on board the spacecraft. That's possible because the spacecraft-Earth distance is low enough to support a 1024 bps telemetry data rate at the moment (this situation will last until sometime in mid-March). I've attached a plot of our weekly data return percentages which clearly shows the recent improvements. As far as the hydrazine is concerned, it's obviously not frozen yet, but there can't be very much left. Our estimate is that we have almost no fuel left even using our best-case estimates. However, it's very difficult to get an exact figure of fuel usage over the mission given that we have had about 3 years of closed-loop conscan operations to control nutation when the spacecraft fired the thruster autonomously. During those periods, we had to estimate the number of pulses fired by monitoring the increase in catalyst bed temperature after each period of thruster activity which is not the easiest thing to do. So the bad news is that we don't have an exact estimate of how much fuel is left but the good news is that it's still above zero! We hope that the data returned is continuing to excite you as the solar activity slowly begins to increase. Best regards, Nigel
Date: Thu, 11 Dec 2008 18:39:48 -0800 From: Nigel Angold Subject: Ulysses Status Update 4 Dear Ulysses colleagues, It has been a couple of months since my last status report. That's because nothing much has changed. We are continuing with our S-band science mission and typically we are now tracking once a day for about 2 to 4 hours. We don't want to increase pass durations much more than that because the temperature of the TWTA radiator panel (close to the cold hydrazine pipework) now falls rapidly when we switch the S-band transmitter on. But we are looking to increase our tracking time by taking two short passes per day separated by enough time for the radiator panel temperature to rise again. The data we have been getting recently has been of very good quality. The spacecraft-Earth distance has been decreasing and the link margin has increased to a point where we don't need to drop to 256 bps very often. In fact, we are close to being able to support 1024 bps which would enable us to get data from the tape recorder again. We're keeping a very close eye on the downlink SNR and we'll try 1k data again if we think the data quality won't be degraded. No promises though. There's another benefit from the low Earth range when coupled with the fairly low Earth drift rate that we're experiencing this month. If the hydrazine froze or ran out tomorrow, we could continue to acquire telemetry for around 20 days before the High Gain Antenna offpointing was too great to support even 128 bps. That's compared to about 5 days if the fuel had frozen back in May or June this year. However, this is a short-lived effect and by February next year, we'll only have 8 days of data before the end. Any estimate I make at this point about when the fuel will freeze or run out would be purely guesswork - the spacecraft has surprised even the most optimistic among us. But it's beginning to look like those Ulysses 1990-2008 polo shirts were a little premature. At least they have "the Odyssey continues ..." on the sleeve! I hope I don't have to send out another status report this year, so I wish you all a Merry Christmas and a Happy New Solar Cycle (if it ever decides to show up :-)) Best regards, Nigel
Date: Mon, 06 Oct 2008 10:00:37 -0700 From: Nigel Angold Subject: Ulysses comes of age: 18 with a T-shirt ! Dear Ulysses colleagues, I'm sitting here at JPL today proudly wearing my Ulysses Launch Team shirt. I'm proud because the mission has exceeded all my expectations and is still delivering valuable science data during this unusual period of Solar inactivity eighteen years after launch. And also because, despite nearly two decades of Southern California excesses, I can still fit into that 1990 T-shirt ! We survived the scare of a couple of weeks ago when the sun sensors switched themselves off leaving us without manouevres for a number of hours. And, since we did not freeze fuel during that anomaly, we have reduced the number of pulses in our fuel bleeding operations to conserve fuel. However, fuel bleeding is still performed every two hours. Nobody is more surprised than I am that the hydrazine is still flowing. All our thermal modelling tells us that we should be well and truly frozen by now but our model has always suffered from the fact that we have been operating the spacecraft for many years in ways never imagined or tested before launch. That, and the lack of thermal telemetry from crucial regions of the spacecraft interior have made it difficult to predict temperatures for many important parts of the fuel system. However, I have never been happier to be wrong. Happy eighteenth, everyone. Best regards, Nigel
Date: Mon, 22 Sep 2008 11:40:25 -0700 From: Nigel Angold Subject: Ulysses Status Update Dear Ulysses colleagues, The spacecraft behaved nominally over the weekend and the hydrazine is not frozen. We are in the process of trying to piece together what may have caused the sun sensors to switch off. In addition, we will try to determine what effect the blind commanding had from the limited data we have available. Instrument teams should be aware that, between 263.02.41.04 and 264.04.40.33 SCET, the spacecraft was generating 2 sun reference pulses per spin which may affect science data processing. Contingency commands have been prepared for each pass in case the latching valve closes again. This will minimize any interruption of fuel bleeding. Best regards, Nigel
Date: Fri, 19 Sep 2008 23:51:58 -0700 From: Nigel Angold Subject: Ulysses Status Update Dear Ulysses colleagues, At the beginning of tonight's pass, inspection of the command counter verified that commands transmitted via DSS-54 had been received but commands transmitted from DSS-66 had not. DSS-54 commands were: - CONJ program reloaded to execute fuel bleeding every 2 hours with the spin fail detect logic disabled - LV 1/2 close - LV 1 open - MS-1 off - MS-1 Standby on (switches on redundant optical sensor) - Real-time fuel bleed manouevre. However, at AOS tonight, no MS sensor was on and LV 1 & 2 were closed. The on-board spin rate was still 9.9 rpm. So either the commands did not execute successfully (even though they were received) or MS-1 and LV 1 switched again sometime after the commands were executed. In addition, the following out of limits parameters were observed (the same as observed for anomaly ULY-065 on 17 June 2008): - S095 CTU P RTUQ = OFF - S069 AC CTU/RTU S = NEG Commands transmitted during tonight's pass were successfully executed (and verified a round trip light time later): - MS-1 off A - MS-1 off B - MS-1 standby on - the MS standby sensor came on and the spin rate went back in limits to 4.85 rpm Manoeuvre registers were loaded for a realtime fuel bleed with Oblique + Meridian (OB+ME) sun sensor selection to allow for correct spin rate calculation without MS-1: - LV 1/2 -1 off - LV 1-1 on - LV 1 opened - Execute manoeuvre - fuel bleeding took place and cat bed temps increased as expected - Disable manoeuvre - Deschedule CONJ - Reload CONJ for fuel bleeding with Oblique + Meridian (OB+ME) sun sensor selection - Fuel bleeding executed correctly at 06:00 SCET. MS-1 Standby remained on and LV 1 remained open. So, it is not known why the MS-1 standby sensor was off and LV1 closed at the beginning of the pass. Nor are we certain how many (if any) fuel bleeding manoeuvres took place out of pass. But the good news is that it looks like the fuel is not frozen. Commands for the next Earth pointing manoeuvre will be loaded shortly and the situation will be monitored carefully over the weekend. Fuel bleeding should take place every 2 hours. Best regards, Nigel
Date: Fri, 19 Sep 2008 02:08:18 -0700 From: Nigel Angold Subject: Ulysses Status Dear Ulysses colleagues, At the beginning of our last Ulysses pass (DOY 263, 03:20 UTC), telemetry indicated that the spin rate was ~9.9 rpm. The spin fail detect (set for 5 +- 0.2 rpm) showed "failed" and the latching valves in the hydrazine lines were closed. Further investigations revealed that sun sensors MS-1 and XBS-2 were off. We don't know why the sun sensors switched off but a chain of events will have occurred when they did: 1) Without the Meridian Slit sensor data, the on-board spin rate is calculated erroneously due to double-crossings. In fact, the spin rate is calculated as twice the actual spin rate of ~4.85 rpm. So we are confident that the spacecraft is not actually spinning at 9.9 rpm. 2) The spin fail detect program sensed that the spin rate was outside the failure deadbands and closed the latching valves to prevent thruster firings. 3) Any subsequent manoeuvres, whether loaded in the time tag buffer, in the CONJ program or simply commanded in real-time have not taken place. Since the received signal strength appears to be nominal, it can be assumed that the last Earth-pointing manoeuvre at 262.21:20 UTC executed successfully and the sun sensor switch-off occurred after that. However, it is not known if any fuel bleeding has occurred since then. Consequently, there has been a significant increase in risk of fuel freezing and the initial recovery actions have focused on reestablishing fuel bleeding. Additional uplink time from DSS-54 was obtained (uplink in the blind because the spacecraft S-band transmitter was off) and the on-board CONJ program was reloaded to execute fuel bleeding every 2 hours with the spin fail detect logic disabled. Further commands were sent to open Latching Valve 1, select the redundant MS sun sensor and perform a real-time fuel bleed manoeuvre. None of these commands will be verified until the next downlink pass. We will try to obtain an additional 70m pass during the local daytime tomorrow. Keep your fingers crossed. Best regards, Nigel
Date: Tue, 09 Sep 2008 15:07:56 -0700 From: Nigel Angold Subject: Ulysses Status Dear Ulysses colleagues, Well, August came and went and spacecraft operations continue. However, we now have to reduce the telemetry bit rate to 256 bps for low elevation portions of our 70m tracks. As reported in early August, a further X-band switch on attempt failed and we do not feel that approach is worth trying again. We will try a more aggressive switching scenario (involving Main Bus Undervoltage) at the first signs of fuel freezing. We estimate that we have approximately 0.85 kg fuel remaining but there is an uncertainty of +0.6/-0.7 kg. So with our current rate of fuel usage (~0.5 kg per month), we expect to run out sometime between the end of September and December. We could use less fuel during our 2-hourly fuel bleeding activities, but that would increase the risk of freezing significantly. Hopefully, we will be able to continue the S-band mission on a day-by-day basis until the fuel freezes or runs out. And with a bit of luck, we'll encounter the slow speed solar wind before then. Best regards, Nigel
Date: Fri, 08 Aug 2008 08:29:47 -0700 From: Nigel Angold Subject: Ulysses Status Dear Ulysses colleagues, We had another unsuccessful attempt to switch the X-band on a few minutes ago. This time, we switched the LV on three times for a period of 3 minutes to heat the filament. Each time, we switched it off before the automatic HV on command could execute. Then we sent a fourth LV on command, timed to arrive in the middle of a series of 37 time-tagged HV on commands. All of the HV on commands were given the same time-tag so they would execute in a rapid sequence 125 ms apart. So when the LV command executed on board, an HV on command executed between 0 and 125 ms later. The result of this was an immediate switch off of the LV i.e. the same as we have seen for every switch-on attempt since the anomaly occurred in January. It seems that we are not able to "fool" the protection logic by switching the HV immediately after the LV on. I am not sure if we will repeat this procedure. However, we will plan to perform a more "aggressive" switch-on involving the use of a Main Bus undervoltage to limit the inrush current during HV switch-on. This will most likely take place at the end of August. In the meantime, we are still getting real-time data at 512 or 256 bps. Best regards, Nigel
Date: Wed, 06 Aug 2008 15:55:35 -0700 From: Nigel Angold Subject: Ulysses Status Dear Ulysses colleagues, Another month has passed and the hydrazine has not frozen yet. Fuel bleeding every 2 hours continues to keep the hydrazine moving through the pipes. Earth-pointing manoeuvres are interleaved, when required. Now that we are into August, we'll try to switch the X-band this Friday morning by switching the HV on immediately after the LV on (as suggested by Dave McComas). We'll perform some filament heating before that with multiple LV on/off cycles. Testing of the redundant on-board receiver which is connected to the front low gain antenna (LGA-F) went well. We validated our ability to command the spacecraft when the HGA is not pointing towards Earth in preparation for when we are no longer able to Earth-point due to fuel freezing. The link margin is now reduced to a level where 512 bps is marginal at low elevations via the 70m antennas. Although we would like to avoid low elevation tracking, antenna time is difficult to obtain. We will be dropping to 256 bps for low elevation tracks from now on. Best regards, Nigel
Date: Wed, 09 Jul 2008 16:13:49 -0700 From: Nigel Angold Subject: Ulysses Status Dear Ulysses friends and colleagues, As you are aware, the proposed July 1st Ulysses operations end date has come and gone. And as a result of our fuel bleeding and other operations strategies, we have managed to avoid freezing the hydrazine so far! Now we are continuing operations on a day-to-day basis until the fuel freezes. That includes fuel bleeding every 2 hours to keep the hydrazine moving through the pipes and Earth-pointing manoeuvres interleaved, when required. When we see the fuel freezing, we will switch off the S-band and some instruments for a couple of days in an effort to thaw the hydrazine. After that, we'll try to switch on the X-band once again using some more radical (and hence more risky) procedures. If the fuel has not frozen by the end of July, we'll try some benign X-band switching in early August and plan for radical switching in late August. What will happen in the unlikely event that we re-establish our X-band downlink? Well, obviously we'll leave it on!!! And at the end of August we should have enough fuel remaining to continue for a number of months (assuming that we stop the fuel bleeding). The short portion of pipe that is currently close to freezing will warm up but other areas of pipework will cool down and they will get close to freezing during the last quarter of 2008. As for funding for operations beyond August, that's another question. This coming weekend, we will be testing the redundant on-board receiver which is connected to the front low gain antenna (LGA-F). This is to validate our ability to command the spacecraft when the HGA is not pointing towards Earth e.g. if no manoeuvres have been performed for several days due to frozen fuel. I know some of the instrument teams have held wakes to celebrate the fantastic journey that we have been on all these years. However, I make no apology for the fact that Ulysses is not dead yet. I hope that in the coming weeks we get to see the transition from fast to slow solar wind. Very best regards, Nigel