SOHO high gain antenna status
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SOHO high gain antenna status |
2003 July 1
The SOHO Flight Operations Team commanded the spacecraft telemetry to low rate at 10:29 UT today during a 34-meter DSN contact. Before that, over 50% of the high rate data during the contact were being lost.
The FOT and spacecraft engineers were, however, able to reestablish medium rate downlink through the spacecraft low gain antenna at the 34-meter station, so we should be able to continue to receive scientific data in realtime only during the deepest part of the keyhole.
SOHO engineers successfully tested low, medium, and high rate telemetry through the spacecraft low gain antenna (LGA) and a DSN 70-meter antenna today. We expect to lose the ability to maintain high rate through the 34-m antennas today or tomorrow, and the engineers will test whether it is possible to maintain medium rate through the spacecraft LGA and the DSN 34-meter antennnas during a contact tomorrow.
Scientific telemetry from SOHO continues to be received through DSN 34-meter antennas. Better noise temperature on the 34-meter antennas (as compared to the 26-meter antennas) appears to be prolonging our opportunities for contact. We expect to lose high-rate contact today or tomorrow through 34-meter antennas. (Our single ocntact today is through a 70-meter antenna.)
Spacecraft engineers will begin tests later this week to determine whether we can maintain medium rate (40 kbps) telemetry with the spacecraft low gain antenna through 34-meter stations.
Scientific telemetry from SOHO was lost at approximately 01:50 UT today, during a contact through the DSS-46 26-meter antenna of the Deep Space Network (DSN), as the spacecraft entered a telemetry "keyhole" expected to last until July 14. Until that time, we will be able to monitor spacecraft and instrument health and safety in low-rate telemetry during contacts using DSN 34-meter antennas and one of the spacecraft's low-gain antennas.
13:30 UT: We were able to pick up high-rate telemetry (including scientific telemetry) through a DSN 34-meter antenna. The signal to noise ratio was down to only 2:1 during the last 26-meter contact, though, and we expect to lose scientific telemetry again shortly.
17:40 UT: At the end of the 4-hour 34-meter contact, signal to noise had dropped by almost a factor of two, so we don't expect to be able to bring up high rate on the next 34-meter contact. Spacecraft engineers are looking at whether it would be feasible to acquire medium-rate telemetry through a low-gain antenna during 34-meter contacts.
We expect to lose scientific telemetry sometime late on June 25 or early on June 26, but remain in contact with the spacecraft via a low-gain antenna for the duration of a 19 - 20 day "keyhole" as SOHO passes closest to the Sun-earth line. Unfortunately, we will only be able to obtain "housekeeping" telemetry during the keyhole, not scientific data.
Today's tests attempting to move the HGA Z-axis motor using a single coil at a higher temperature were unsuccessful.
This morning we continued with the troubleshooting of the HGA motor. There were several attempts in high speed mode and one attempt in low speed mode to move the antenna by 30 steps, just enough to get another encoder pulse. None was successful.
The lead we are investigating is the puzzling fact that whenever the antenna was not used for more than 24 hours, i.e., when there were only minimal thermal gradients inside the motor, it moved by about 30 steps (for approx. 5 min in low speed mode) before sticking. The hypothesis is that when the motor is uniformly cool enough, a single set of windings may be enough to drive the gears.
It was decided to increase the temperature of the antenna slightly to about 10C. Antenna motor commanding tests will resume Monday, when all thermal gradients inside the motor should have settled.
Other explanations for the behavior of the antenna motor include debris and/or lubricant thinning, though these have been discounted by the manufacturer.
Although the HGA is now at the "sweet spot", troubleshooting activities continue in an effort better to understand the cause of the problem.
SOHO engineers are currently investigating whether thermal expansion during low-rate driving of the HGA motors might be responsible for the observed behavior. More tests are scheduled for tomorrow morning.
There are now graphics that help to explain:
This afternoon (2003 June 19), SOHO engineers and Flight Operations Team (FOT) were able to use both primary and redundant Z-axis motor drive windings simultaneously to move the high gain antenna (HGA) toward the "sweet spot" in east-west offset that will allow the shortest interruption in scientific telemetry (assuming a 180 ° roll after each "keyhole").
As of 20:20 UT, the FOT is attempting to finish to move to the sweet spot (an E-W offset of -17.7°), which would limit the keyhole in high-rate telemetry to about 19 days (out of every three months), starting late next week.
We will be having a SOHO science Working Team (SWT) telecon tomorrow morning, and plan to spend the next few weeks, including the keyhole, to decide on the course of action that maintains the highest level of scientific return with the minimum risk of more serious interruptions.
A preliminary look at the costs and benefits of changing the SOHO orbit indicate that reducing the ecliptic Y (E-W) extent of the orbit by a factor of ~ 2 would require more fuel than we have on board. There are lots of other options, however, and Flight Dynamics Facility will be examining a wide variety and presenting them to the program[me] office and SWT.
After a spacecraft offpoint maneuver late on the morning of June 18 confirmed that the high gain antenna (HGA) had in fact not moved more than a small amount (in comparison to the steps commanded) since the first offpoint on June 4, we decided to switch to the redundant HGA electronics.
The FOT did so at approximately 19:30 UT, and we commanded an HGA motion about the spacecraft Z axis (i.e., east-west antenna motion) of roughly one degree. Less than one-fifth of that motion took place, as determined from automatic gain control readings at the ground station. We commanded another 131 steps, without any noticable antenna movement.
The similarity of the behavior on both the A and B sides indicates the likelihood of a mechanical problem with the antenna drive motor or mechanisms.
We held a teleconference with the manufacturer of the motor and, following their recommendation, tried to command the antenna in high speed (100 Hz instead of 0.1 Hz) in both directions, without success.
During the night we will increase the temperature of the motor. Tomorrow we will repeat the same back and forth movement hoping to unstick the HGA.
If we are unable to drive the antenna any further back toward the center of its E-W range, we will probably lose high-rate (i.e., scientific) telemetry this weekend or early next week. The spacecraft will continue to maintain contact in low rate via the low-gain antenna.
The HGA east-west angle is near the "sweet spot" that would allow maximum coverage of the plus and minus 25° range of the spacecraft orbit's major axis, with some data loss (perhaps one month out of three) if we roll the spacecraft by 180° once every ~ three months. (The 3 db half-width of the antenna beam is about 7°.) With the help of the NASA Flight Dynamics Facility, we are also exploring the feasibility of changing the orbit in order to optimize the coverage period of the HGA.
The PI teams will be safing their instruments for an off-time of 4-6 weeks.
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