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STEREO SCIENCE CENTER

Important notice about STEREO Behind

Planned Science Data During Reduced STEREO Science Operations

Update: Communications with the STEREO Behind spacecraft were interrupted on October 1, 2014 immediately after a planned reset of the spacecraft performed as part of a test of the solar conjunction operations described below. The spacecraft telemetry indicated an anomaly in the guidance and control system, but this is still being evaluated. No further communications have been successful since Oct. 1st. Follow this link for more information.

 

Periods of reduced telemetry

Between August 2014 and January 2016 one or both STEREO spacecraft will have their high gain antenna pointed at an angle a few degrees away from Earth to keep the antenna feed horn from getting too hot. This will result in greatly reduced telemetry rates while the antennas operate on one of their lower power side lobes instead of the main lobe normally used. However, all the STEREO science instruments will be able to provide a subset of their usual data during these periods.

Antenna lobe pattern
Conceptual diagram demonstrating the relationship between the antenna main and side lobe strengths. Not to scale. The actual lobes are much narrower, and the side lobes are at much smaller angles compared to the main lobe.

The main data limitation during this time is that only real-time data will be downloaded from the spacecraft. There will not be any playback of stored data. Also, because the signals from the spacecraft will be so much weaker than usual, only the largest 70 meter stations in the NASA Deep Space Network (DSN) will be used to bring down the data. Since there are only three 70 meter DSN stations, and they are heavily used by many NASA missions, this will further limit the amount of STEREO data that can be brought down. Taken together, this means that there will only be a few hours of data a day from each spacecraft.

The IMPACT, PLASTIC, and SWAVES instruments will continue to produce their regular space weather beacon telemetry during the real-time passes, along with some additional science and housekeeping telemetry depending on the telemetry rate. The beacon telemetry will be processed in real time by the STEREO Science Center, as is done now. The additional telemetry data will be processed by the instruments as usual. SECCHI will provide images which range in quality from their beacon product to their science product, with emphasis on their EUVI and HI telescopes. All SECCHI images will be processed in real time by the STEREO Science Center, as well as by the SECCHI team as the final data product. More details about the available data from the various STEREO instruments at different points during this period are given below.

A very low rate telemetry stream, consisting of the spaceweather beacon data from the IMPACT, PLASTIC, and SWAVES instruments, along with some housekeeping telemetry from the spacecraft, will be written to the onboard recorder during this period. Once the spacecraft are back to their normal operating mode, in January 2016, these data will be finally read out, and we'll have full 24-hour coverage of these in situ and radio parameters. Unfortunately, there is insufficient room on the recorder to store any SECCHI data.

There are three periods which affect the telemetry rates:

Each of these is described in more detail below. The times when each spacecraft is in one of these three modes is illustrated in the following graphic. Click on the image for a larger version. The dates are still subject to change; check our observations calendar for the most up-to-date information.

Side lobe timeline

First Side Lobe

The first antenna side lobe provides the highest telemetry rates that are available outside of the main lobe. A rate of 10 kbps can be maintained on the first side lobe with the 70 meter DSN stations. Compare this against the much higher 720 kbps that the 70 meter stations are capable of on the main lobe, or even the 120-240 kpbs that the smaller 34 meter stations can support. Of this 10 kbps, 7.4 kpbs is available for instrument science and housekeeping data.

The spacecraft will alternate weekly between two different science modes during operations on the first antenna side lobe. Data will be gathered from all four science instruments during this time, but one mode will give extra telemetry to IMPACT, and the other mode will give extra telemetry to SWAVES. The PLASTIC and SECCHI telemetry allocations will be the same in both modes. Below are details about the science plans for each instrument while on the first side lobe. Please note that these plans are subject to change.

The real-time data described below will be collected during the DSN passes, which are expected to cover on average a few hours each day for each spacecraft. In addition to this, the IMPACT, PLASTIC, and SWAVES instruments will continue to produce their regular space weather beacon telemetry data. During the real-time passes, these beacon data will be brought down along with the other data described below. They will also be stored onboard the spacecraft for eventual download once the period of side-lobe operations is over, and thus will retrospectively provide 24-hour coverage.

IMPACT

  • SWEA/STE-D powered off.
  • MAG data at 4 samples per second.
    • IMPACT-prime: 8 samples per second.
  • HET, LET, and SIT rates. (PHA/Events eliminated.)
    • IMPACT-prime: Some SIT PHA/Events for mass resolution.
  • SEPT-E and SEPT-NS data.

PLASTIC

  • Solar wind H, He bulk parameters.
  • Efficiencies for ion rates.
  • Grouped solar wind ion rates.
  • Grouped suprathermal ion rates.

SECCHI

  • SECCHI scheduling is done on two-hour time blocks. During each two hours, the following images will be brought down:
  • One EUVI 195 Angstrom image every 15 minutes (one 512x512 highly compressed, three 2048x2048 nominal compression).
  • Two sets of high resolution 2048x2048 images at each of the four EUVI wavelengths.
  • One highly compressed COR1 128x128 (beacon) polarization sequence.
  • One highly compressed COR2 256x256 (beacon) total brightness image every 15 minutes.
  • Two losslessly compressed 512x512 HI1 images.
  • One losslessly compressed 512x512 HI2 image.
  • Some of the SECCHI images are taken in between contact times, and stored within the SECCHI instrument internal memory for later downlink. Thus, some of the images described above may be from several hours earlier.

SWAVES

  • The nominal SWAVES science product will be the beacon mode and housekeeping data.
  • In SWAVES-prime mode, full resolution radio spectra and direction finding information will be brought down.

Second Side Lobe

An even lower telemetry rate of 3 kbps, of which 1.7 kbps is available for the instruments, can be supported on the second antenna side lobe. Below are the details of the instrument science plans while on the second side lobe.

As was the case for the first side lobe operations, the real-time data described below will be collected during the DSN passes, which are expected to cover on average a few hours each day for each spacecraft. In addition to this, the IMPACT, PLASTIC, and SWAVES instruments will continue to produce their regular space weather beacon telemetry data. During the real-time passes, these beacon data will be brought down along with the other data described below. They will also be stored onboard the spacecraft for eventual download once the period of side-lobe operations is over, and thus will retrospectively provide 24-hour coverage.

IMPACT

  • SWEA/STE-D powered off.
  • MAG data at 2 samples per second.
  • HET, LET, and SIT rates at lower cadence. (PHA/Events eliminated.)
  • SEPT-E data at lower cadence. (SEPT-NS eliminated.)

PLASTIC

  • Solar wind H, He bulk parameters (< 850 kps), reduced cadence.
  • Grouped solar wind ion rates.
  • Grouped suprathermal ion rates.

SECCHI

  • SECCHI scheduling is done on two-hour time blocks. During each two hours, the following images will be brought down:
  • One highly compressed 512x512 EUVI 195 Angstrom image every 15 minutes.
  • One set of highly compressed 2048x2048 images at each of the four EUVI wavelengths.
  • One highly compressed COR1 128x128 (beacon) polarization sequence.
  • Two highly compressed COR2 256x256 (beacon) total brightness images.
  • One losslessly compressed 256x256 HI1 image.
  • One losslessly compressed 256x256 HI2 image.
  • Some of the SECCHI images are taken in between contact times, and stored within the SECCHI instrument internal memory for later downlink. Thus, some of the images described above may be from several hours earlier.

SWAVES

  • Only SWAVES beacon mode and housekeeping data will be brought down.

Superior Solar Conjunction

The term "superior solar conjunction" is the technical phrase which refers to period when one or the other STEREO spacecraft is on the opposite side of the Sun from Earth. For each spacecraft, there is a period when it is so close to the Sun as seen from Earth that the radio interference from the Sun makes it impossible to communicate at all with the spacecraft. For a few days, it will even be physically blocked by the Sun. Without any possibility of communications, the spacecraft will have to be placed into "safe mode" until contact can be reestablished. Before communications are completely cut off, the spacecraft will start slowly rotating about the axis which points at the Sun. This is done to reduce the load on the reaction wheels which keep the spacecraft pointing stabilized. All the science instruments will be turned off, with the sole exception of the radio instrument SWAVES which will stay on to record beacon mode data on the recorder for eventual playback in November 2015. Technical and safety considerations prevent the other instruments from being on during this time.


Last Revised: Monday, 23-Mar-2015 18:37:43 GMT
Responsible NASA Official: [email address: Joseph.B.Gurman<at>nasa<dot>gov]
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