Use of STEREO High Gain Antenna s Sidelobes at low SPE Angles

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SpaceOps Conferences 16-20 May 2016, Daejeon, Korea SpaceOps 2016 Conference 10.2514/6.2016-2403 Use of STEREO High Gain Antenna s Sidelobes at low SPE Angles Matthew W.

1 SpaceOps Conferences May 2016, Daejeon, Korea SpaceOps 2016 Conference / Use of STEREO High Gain Antenna s Sidelobes at low SPE Angles Matthew W. Cox 1 Johns Hopkins University Applied Physics Laboratory, Laurel, MD, Launched in 2006, NASA s Solar TErrestial RElations Observatory (STEREO) consists of two spacecraft ( Ahead and Behind ) in heliocentric orbits collecting solar observation data. 1,2,3 The orbits of both spacecraft are very similar to that of the Earth s solar orbit, but the orbits drift ahead or behind that of the Earth at a rate of approximately 22 degrees per year. 1,4,5 During the spring of 2014, the STEREO mission ops team noticed that the Behind HGA Feed temperature had increased by 48 C during the 1 st quarter of Similar temperature trends were also noticed on the Ahead spacecraft. It was determined that as the Sun-Probe-Earth (SPE) angle approached zero more solar energy was being focused onto the High Gain Antenna (HGA) Feed Horn. This was a problem for both the Ahead and Behind spacecraft as their projected orbits had both going through superior solar conjunction within the next 12 months. If the HGAs were left in their normal configurations, the temperature placed on each spacecraft s HGA would most likely result in loss of function by the end of solar conjunction. This problem also had the potential to extend the conjunction induced blackout period for solar conjunction to over a year for each spacecraft. The solution to this problem was to shift to the HGA s sibelobes as the SPE angle decreased. An initial test on each spacecraft was conducted with the Deep Space Network (DSN) to map a slice of each HGA antenna pattern to verify the locations and power levels for the sidelobes. These tests proved successful and secondary tests were performed to verify what commanding and data rates the sidelobes could support. Although the sidelobes provided significantly less commanding and data rates, they were extremely stable. The use of these sidelobes would provide safe temperatures for the HGA feed and allow the blackout time period to be reduced to its initial estimate. I. STEREO Overview On October 25 th 2006, NASA launched the twin Solar TErrestial RElations Observatory (STEREO) spacecraft. 2,3,4,5 Both spacecraft were designed, built, and tested by the Space Exploration Sector located at Johns Hopkins University Applied Physics Lab in Laurel, MD (JHUAPL) and NASA Goddard Space Flight Center in Greenbelt, MD. STEREO is the third mission in NASA s Solar Terrestrial Probes Program, and provided stereoscopic data of the Sun with a primary objective of studying solar coronal mass ejections (CME). 1,2,3 One of the spacecraft called Ahead was put into a heliospherical orbit in front of the Earth s orbit. The other spacecraft called Behind was put into a heliospherical orbit that trails behind the orbit of the Earth. The Sun Probe Earth (SPE) angle for each spacecraft changes by approximately 22 degrees per year. 1,4,5, Fig. 1. Each spacecraft is equipped with one 1.2 meter diameter parabolic dish high gain antenna (HGA), and two omni low gain antennas (LGA). The HGA is always pointed toward Earth during nominal operations, while the LGAs are mounted to the +Z and Z sides of the spacecraft, Fig Senior Professional Staff, Space Exploration Sector, Johns Hopkins Rd., Laurel, MD Mail Stop MD Copyright 2016 by the, Inc. Under the copyright claimed herein, the U.S. Government has a royalty-free license to exercise all rights for Governmental purposes. All other rights are reserved by the copyright owner.

Figure 1. North-ecliptic-pole view of the STEREO spacecraft heliocentric orbits. 1,4,5 Figure 2. Location of STEREO antennas with LGA fields of view. II.

2 Figure 1. North-ecliptic-pole view of the STEREO spacecraft heliocentric orbits. 1,4,5 Figure 2. Location of STEREO antennas with LGA fields of view. II. STEREO Thermal Issue on the HGAs and Solar Conjunction The slow changing SPE angle and the science requirement to collect data in the ecliptic plane would result in each spacecraft going into a solar conjunction in early Due to solar conjunction, there would be a long communications blackout period on each spacecraft. For Ahead, the original blackout period was thought to be approximately 3.5 months. While on Behind, the blackout period was thought to be approximately 2 months. These long blackout periods were an issue that was realized in the design phase, but was not simulated or incorporated into the spacecraft design. This decision was due to cost and the fact that issues would not arise until around 6 years after the end of the primary mission. Near the beginning of 2014, the STEREO mission operations team (MOT) began to notice a high temperature alarm on the Behind HGA feed assembly. The feed temperature had been increasing since after launch due to the changing SPE angle. Since the temperature was still in operating spec there was initially no cause for major concern. In April 2014, it was noticed that the feed temperature trend was different than previously thought. At this point it was realized significant thermal energy was being reflecting back toward the feed at lower SPE angles. It was then noticed the Ahead HGA feed assembly was also showing the beginning of the same temperature trend. While initially the problem was more prevalent on Behind, the Ahead HGA feed assembly was going to heat up faster and hotter than Behind s, Fig. 3. 2

3 Figure month HGA feed temperature data for Ahead and Behind. It was believed that if each spacecraft s HGA was left in its current configuration, the heat would cause the HGA to cease functioning before the end of the solar conjunction blackout period. After a thermal analysis of the feed components, it was estimated that the HGA feed assemblies would start be damaged at approximately 150 C. It was also believed that the HGAs would cease normal function at a temperature of 180 C. Multiple thermal models were produced with inconsistent temperature trends, but all showed that the feeds would experience temperatures above the 150 C threshold. Also, the temperature sensor on the feed assembly can only monitor the feed temperature up to 136 C. These conclusions threatened to greatly extend the communications blackout period for each spacecraft to roughly a year. As a way to reduce the potentially long blackout period, the idea of using the HGA sidelobes was introduced. Two RF slices for each HGA were measured before launch, but the STEREO team was not completely certain which slice portion corresponded to the side of the HGA which needed to be pointed away from the Sun. It was also uncertain if the RF pattern had shifted since launch. LGA use was also inquired, but the Earth was near the edge of the LGA antenna pattern for both spacecraft. LGA locations and fields of view can be seen in Fig. 2. At the time it was not certain if the LGAs antenna patterns could be received by DSN at low SPE angles. There were also concerns that the significantly lower data rates used by the LGAs would be more susceptible to solar scintillation effects as the SPE angle decreased. III. Testing the Viability of the HGA Sidelobes Testing on the HGA sidelobes focused around the following questions. The first was to get an accurate look at the RF pattern when the HGA is pointed away from the Sun. Secondly, it needed to be determined what downlink and uplink rates could be supported reliably on the sidelobes and by the spacecraft. A first test was performed on Ahead and Behind to map the HGA RF pattern when the HGA is pointed away Sun. The HGA gimbal can point the HGA only in the Z to +Z direction. The test used a DSN 70 meter station to measure the db loss of both the uplink (U/L) and downlink (D/L) signal. The HGA was moved off of bore sight in 0.2 degree increments every 30 seconds out to 9.2 degrees away from the Sun and back to bore sight. Results from the test on Ahead can be seen in Fig. 4 and Behind in Fig. 5. 3

4 Figure 4. Measured Ahead U/L & D/L RF pattern from 0 to 8 degrees and 0 to -35 db. Figure 5. Measured Behind U/L & D/L RF pattern from 0 to 8 degrees and 0 to -35 db. Several important pieces of information were learned from the first test on Ahead and Behind. First was that the location and magnitude of the sidelobes were successfully measured. Test results also showed high correlation with some data measured prelaunch, which showed no significant change. Although the HGAs on Ahead and Behind should be virtually identically, the test showed that the RF patterns between the two spacecraft were slightly different. By overlaying the uplink and downlink patterns, it can be seen that the lobes do not perfectly align. Incorporating these results, the following conclusions were made by the STEREO team. Implementation of the HGA sidelobes did look like a viable option to reduce the blackout period for solar conjunction. It is also possible to offpoint the HGA by 1 degree and still be significantly high on the main lobe. When offpointing the HGA by only 1 degree the uplink rates could stay the same and the downlink rates would be high enough to provide normal science data for all the instruments. During the time of sidelobe operations, 70 meter DSN stations would be the primary antennas used for daily contact with both spacecraft. This is due to the significant db gain on both the U/L and D/L that the DSN 70 meter stations have when compared to the 34 meter stations. The HGA pointing accuracy would be tightened from within a tenth of a degree to within a twentieth of a degree. To move between lobes the STEREO MOPS team would load a HGA bias parameter to the guidance and control system (G&C). Although the relative db loss was roughly the same for both the U/L and D/L, there was more margin built into the uplink. This resulted in the HGA bias parameter being more focused to the D/L sibelobes. The bias loaded to first sidelobe would be 3.2 degrees for Ahead and 3.4 degrees for Behind. Using rates that were already onboard the spacecraft, the first sidelobe would use a 500 bps U/L rate and a 10 kbps D/L rate. The bias loaded to second sidelobe would be 6.1 degrees for Ahead and 6.2 degrees for Behind. This sidelobe would use a 125 bps U/L rate and a 3 kbps D/L rate. It 4

5 was possible Ahead could have supported higher D/L rates on the sidelobes, but it was felt it would be easier to implement changes if D/L rates stayed the same across both spacecraft. The suggested sidelobe D/L rates were roughly two orders of magnitude less than the D/L rate used for normal operations on a DSN 70 meter contact. Note the nominal science data rates for STEREO when using a 70 meter antenna is 2 kbps for the U/L and 720kbps for the D/L. The sidelobe data rates were unable to support the normal playback data volume. In order to maintain the health and safety of both spacecraft, it was decided that the spacecraft SSR data partitions would be the only partitions played back during sidelobe operations. Although there would be no science playback, the science instruments would be given a significant portion of the D/L time to get real time data. An additional sidelobe test was required to verify that the suggested data rates work near the predicted levels of margin and work consistently. During this test, the HGA spent 30 minutes on each sidelobe and 1 degree off of the main lobe. When the HGA moved, the spacecraft also transitioned to the proper RF configuration for that step. No operation commands (NOOPs) were sent and verified that they were received by the spacecraft. The results for both the Ahead and Behind tests are shown below in Tables 1-4. U/L Margin is defined to be at zero when the bit error rate is 10E-6. No offset 1 deg off 1st sidelobe 2nd sidelobe STB U/L Margin STA U/L Margin Max Margin, db Min Margin, db Range (Max-Min) Average Margin, db CMDs Sent CMDs Received Table 1. Ahead U/L Results from 2 nd sidelobe test. STA D/L Margin No offset 1 deg off 1st sidelobe 2nd sidelobe Max Margin, db Min Margin, db Range (Max-Min) Average Margin, db Estimated Margin, db Table 3. Ahead D/L Results from 2 nd sidelobe test. Table 4. Behind D/L Results from 2 nd sidelobe test. As seen in Tables 1-4 the results were very consistent with predictions for the most part. There was a larger than expected discrepancy during the beginning of the Behind test. This is believed to be the result of heavy rain at the DSN 70 meter station. One unexpected result of this test was the loss of consistent ranging lock on the sidelobes. The ranging data is used by the navigation team to provide accurate ephemerides. Even with the loss of ranging data, Doppler tracking data was sufficient to meet ephemeris accuracy requirements for superior solar conjunction. IV. Implementation of Sidelobe Operations No offset 1 deg off 1st sidelobe 2nd sidelobe Max Margin, db Min Margin, db Range (Max-Min) Average Margin, db CMDS Sent CMDs Received Table 2. Behind U/L Results from 2 nd sidelobe test. STB D/L Margin No offset 1 deg off 1st sidelobe 2nd sidelobe Max Margin (db) Min Margin (db) Range (Max-Min) Average Margin (db) Estimated Margin (db) Sidelobe operations officially began on Ahead on August 20, This was less than 5 months after identifying the severity of HGA problem on Behind. Unfortunately, contact was lost with Behind in October of 2014 at the end of a solar conjunction test due to a double failure in the guidance system. 6 Sidelobe operations officially ended on Ahead on November 9, On Ahead, all RF communications goals using sidelobe operations were obtained. Sidelobe usage allowed for the Ahead solar conjunction blackout period to be restored to its original estimate of approximately 3.5 months. Additional testing was able to show that some DSN 34 meter stations could be used at times in sidelobe operations using a lower data rate to return real-time space weather broadcast data. The real-time science data retrieved during sidelobe operations prevented a potentially yearlong data gap for many of the scientific instruments. The gap was reduced to approximately 3.5 months for most instruments. The Ahead HGA feed temperature never exceeded 125 C through the entirety of solar conjunction and sidelobe operations. The high feed temperature appears to have had no adverse effects on the Ahead HGA. 5

6 V. Conclusion The use of the HGA sidelobes was an adequate solution the HGA feed temperature issue. Had this solution not been implemented, the communication blackout period for STEREO Ahead could have greatly increased to over a year. Due to the Earth being near the edge of both omni LGAs and LGA data rates being more susceptible to solar scintillation, sidelobe usage was the more desired option. To verify the viable of HGA sidelobe usage multiple tests were performed on both Ahead and Behind. The first test involved mapping part of the RF pattern for both the U/L and the D/L. This test provided the necessary data needed to see where the sidelobes are located. It also help to define what U/L and D/L rates could be supported on each sidelobe. After this test, U/L and D/L rates were estimated for the first sidelobe, second sidelobe, and one degree off of the center of the main lobe. A second test was performed on each spacecraft verifying the validity of the suggested U/L and D/L rates for the first sidelobe, second sidelobe, and one degree off of the center of the main lobe. After this test, it was confirmed that sidelobe operations would be used to mediate the high HGA feed temperature. Sidelobe operations were never fully implemented on Behind. This was because contact with the spacecraft was lost before sidelobe operations were needed. Sidelobe operations were implemented on Ahead and worked as predicted. Using the sidelobes also allowed for real time science data to be received. Sidelobe operations were completed on Ahead on November 9, The Ahead HGA feed never exceeded 125 C and has shown no adverse effects to operating at high temperatures for over two years. References 1 N.A.S.A., STEREO booklet, URL: [cited 26 March 2016]. 2 Driesman, A. S., and Denissen, R. A., STEREO The Solar Terestrial Relations Observatory: Guest Editors Introduction, Johns Hopkins APL Technical Digest, Vol. 28, No. 2, 2009, pp Kaiser, M. L., STEREO: Science and Mission Overview, Johns Hopkins APL Technical Digest, Vol. 28, No. 2, 2009, pp Dunham, D. W., Guzmán, J. J., and Sharer, P. J., Reverse Flow Radius in Vortex Chambers, Johns Hopkins APL Technical Digest, Vol. 28, No. 2, 2009, pp Hunt Jr., J. W., and Ray, J. C., STEREO Guidance and Control System On-Orbit Pointing Performance, Johns Hopkins APL Technical Digest, Vol. 28, No. 2, 2009, pp Harman, R. R. et al, Solar Terrestrial Relations Observatory BEHIND (STEREO-B) Loss of Communications Failure Review Board Report, NASA GSFC, March 20,

Saving NASA's STEREO-B the 189-millionmile road to recovery 14 December 2015, by Sarah Frazier

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