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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.

Status report: 20-Feb-2009

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,

Status report: 11-Dec-2008

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,

18th anniversary of launch, 6-Oct-2008:

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,

Update on recovery from Sun sensor anomaly, 22-Sep-2008:

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 and 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,

Recovery from Sun sensor anomaly, 20-Sep-2008:

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):

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,

Spacecraft Sun sensor anomaly, 19-Sep-2008:

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,

Status report 9-Sep-2008:

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,

Status report 8-Aug-2008:

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,

Status report 6-Aug-2008:

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

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,

Status report 9-Jul-2008:

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,