Data Return Plots - Ulysses
For most of its mission, Ulysses has been passing through regions of space that no other spacecraft has visited. This is due to its unique orbit out of the ecliptic plane and over the poles of the Sun. Data continuity has therefore been of prime importance since the spacecraft was launched in October 1990. An 18 year plot of the Ulysses data return broken down into the various bit rates is shown above.
Ulysses can provide four scientific real-time downlink data rates (128, 256, 512 & 1024 bps) and two engineering rates (64 & 1024 bps). During normal operations, approximately 16 hours of 512 bps and 8 hours of 1024 bps scientific data are returned each day. Note that the maximum data rate that can be recorded by the on-board tape recorders is 512 bps.
There have been a number of periods during the mission when the data return been significantly different from the standard requirements. These are listed below.
Post-launch Spacecraft Checkout
Activities during the first two weeks after launch on 6 October 1990 focused on assessment of the spacecraft platform health and performing a number of attitude manoeuvres. The science instruments were, for the most part, switched off. An engineering data format (1024 bps) was therefore used except for brief periods during tape recorder checkout.
Following this was a month-long period of continuous tracking as the experiments were turned on one by one and checked out. Since the tape recorders were not in use, a scientific data rate of 1024 bps was maintained for most of this time. Some brief periods of engineering data were required, however, following the appearance of spacecraft nutation when the axial boom was deployed.
Continuous tracking was extended into December to monitor the nutation anomaly which coincidentally resulted in increased 1024 bps scientific data return. At the end of December, 24 hour coverage was also required during the First Opposition period.
Early Routine Operations
It had been assumed before launch that an eight hour pass per day would be sufficient to return 8 hours of 1024 bps data. Once Ulysses was flying, however, it became obvious that there were significant periods (particularly at the beginning of a track) when 512 bps was required during each track to ensure that data continuity was maintained. Hence there was not enough time to play back recorded data using a bit rate which would also provide the required 1024 bps real-time data.
The resulting reduction in science data return was tremendous. The Science Working Team therefore passed a resolution at its meeting in Heidelberg, Germany in April 1991 urging the two agencies to find a way to restore the expected data return. This was quickly achieved by increasing the Ulysses tracking requirements to 10 hours per day.
First Conjunction was the only period in the mission when continuous data return was not maintained. During this time, the Radio Science Solar Corona experiment required both an X-band and an S-band downlink. To provide enough power for S-band transmitter switch-on, the tape recorders were switched off from 5 August to 5 September 1991.
For a day either side of Conjunction, the data rate was also reduced to 128 bps and even 64 bps. This was because the interference to the downlink as it passed through the Sun's corona prevented higher bit rates from being received.
Jupiter Fly-by and Second Opposition
The Ulysses scientific instruments were expected to require peak power as they encountered the Jovian environment. This, coupled with the need for an S-band downlink to perform the Io Plasma Torus Radio Science experiment, ruled out the use of the on-board tape recorders during the Jupiter fly-by.
As a result, continuous tracking was required for a period of 8 days either side of closest approach on 8 February 1992. Indeed, dual station coverage was provided for four days around closest approach to ensure reception of this valuable data set. 1024 bps scientific data was obtained for the whole of the planetary encounter.
Second Opposition (27 February 1992) was the prime period for the Radio Science Gravitational Wave experiment. A requirement of 24 hour coverage for 28 days around opposition resulted in continuous tracking from the Jupiter fly-by period until 18 March. Once again, 1024 bps scientific data was returned throughout.
Joint Gravitational Wave experiment
A joint Gravitational Wave experiment was performed in March/April 1993 involving simultaneous observations from the Ulysses, Galileo and Mars Observer spacecraft. The increased tracking time over a period of 3 weeks resulted in higher percentages of 1024 bps scientific data.
DSS 61 bearing failure
In early June 1993, the bearing of the DSN Standard 34 meter antenna in Madrid (DSS 61) failed. The resulting repairs lasted until mid-August. Even with additional coverage provided by the emergency use of the Weilheim 30 meter antenna in Germany, the Ulysses project lost a significant amount of tracking time. During this period, the rate at which the data was recorded on board the spacecraft was often reduced from 512 bps to 256 bps.
The return of the mission-threatening nutation for two periods in 1994 and 1995 demanded special operating techniques to keep it under control. As a result, continuous uplink to the spacecraft and round-the-clock monitoring of spacecraft telemetry were required.
A fortunate consequence of this full-time coverage was that most data returned during the periods August 1994 - February 1995 and April - September 1995 were at 1024 bps. 512 bps data were returned only when DSN tracking was not possible (the Kourou 15 meter antenna was used instead during these times).
A final Radio Science Solar Corona experiment was performed at 4th Conjunction which coincided with the spacecraft's perihelion passage in February/March 1995. This provided a unique scientific opportunity as the Ulysses spacecraft flew nearly perpendicularly through the ecliptic plane at a latitudinal velocity of almost 1° per day.
Continuous tracking was provided by the Deep Space Network for most of the period between 22 February and 14 March. Any gaps in coverage were filled by use of ESA's 15 meter antenna in Kourou. This resulted in increased 1024 bps scientific data return.
Extra coverage was scheduled during early 1998 to support MIDAS (Multi-project Investigation During the Alignment of Spacecraft). The alignment of Ulysses and the ACE, POLAR, WIND and SOHO spacecraft around the Ulysses' 5th Opposition provided a unique opportunity for the study of the Evolution of Coronal Mass Ejections (CMEs), interstellar pickup ions and other phenomena.
SOHO recovery/6th Conjunction
The percentage of 1024bps data returned during the 6th Conjunction period was higher than usual. The overall data return, however, was severely impacted by loss of station time due to the SOHO spacecraft recovery operations.