Lunar, L1 and L2 A Communications guide based on the James Webb Space Telescope experience

Transcription

1 SpaceOps 2006 Conference AIAA Lunar, L1 and L2 A Communications guide based on the James Webb Space Telescope experience Jonathan Gal-Edd * and Bonnie Seaton GSFC/NASA, Greenbelt, Maryland, 20771, US Curtis Fatig SAIC/GSFC, Greenbelt, Maryland, 20771, US and Alan Johns GSFC/NASA, Greenbelt, Maryland, 20771, US This paper will discuss the operation concept considerations dealing with the characteristics of high data rate, Lunar and Lagrange point (L1, L2) missions (known as near earth missions). Also the paper will discuss the progress in developing the infrastructure to support new Ka-band 26 GHz, such as the flight hardware to support it and the availability of ground stations supporting the new spectra allocation. I. Introduction INCE the year 2000 NASA has focused many of it s new space science mission to Lagrange Points L1 and L2, S(as shown in Figure 1), to move farther away from the environmental effects of the earth. L1 is an excellent point for Sun and Sun Earth science with a permanent orbit point 1.5 million kilometers sunward of the Earth in a halo orbit. None of the periodic eclipses of the Sun are experienced as if the spacecraft was in earths orbit. L2 is used for space astronomy with a permanent halo orbit point 1.75 million kilometers in the Earth s anti-sun position. This orbit avoids the periodic eclipses of the universe that can be experienced if the spacecraft was in the Earth s orbit. L1 and L2 are both relatively close to earth (less than 2 million kilometers) and therefore can support higher data rates than deep space missions. In 2004, the President of the United States called for a return to the Moon. These missions will share some of the same characteristics as the L1 and L2 missions. During the concept phase of developing the James Webb Space Telescope (JWST), it was realized that L2 is not similar to Geosynchronous Figure 1. Lagrange Points. Earth Orbit (GEO) missions and that L2 has a few of the unique operations characteristics: Orbit maintenance requires radiometric information from Ground Stations located the Northern and Southern Hemisphere, Viewing period of one ground station is on average 8 hours depending on latitude and time of year, and L2 spectrum radiates over a big area (as oppose to GEO or LEO) and requires spectra consideration. * JWST Mission Engineer Manager, JWST, GSFC/Code 581, Greenbelt, MD, 20771, US, non-member. JWST Science Operations Manager, JWST, GSFC/Code 581, Greenbelt, MD, 20771, US, non-member. Senior System Engineer, IGSS Development, GSFC/Code 443, Greenbelt, MD, 20771, US, non-member. JWST Ground Segment Manager, JWST, GSFC/Code 443, Greenbelt, MD, 20771, US, non-member. 1 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

2 One of the driving requirements for the JWST mission communications is to download daily the 251 Gigabits (Gb) of science data collected. To deal with this relatively high data rate from a L2 mission the spacecraft and ground systems are to be enhanced. To achieve the required data transfer rates, ground stations are being upgraded to support new frequencies, data lines are being upgraded to support higher transfer rates, the on-board Command and Data Handling (C&DH) computers have been upgraded with more processing power, and the C&DH bus has been upgraded to support data rates of 100 Megabits per second (Mbps). In order to provide greater science efficiency, JWST will provide a file like user interface with priority schemes allowing multiple instruments to write to the same Solid State Recorder (SSR) partition and will utilize an event driven operations capability. In 2004, the President called for the Exploration Initiative. The Exploration Initiative has two (2) phases, Robotic Lunar Exploration Program (RLEP) and manned Crew Exploration Vehicle (CEV). As Goddard Space Flight Center (GSFC) began the work on the Exploration Initiative, it found many similarities between the JWST L2 and Lunar missions. The Lunar Reconnaissance Orbiter (LRO) communications, scheduled to fly in 2008, is an implementation of the concepts developed by JWST. The remaining sections below will address how to develop an operation concepts for these types of missions - discussing what worked, what infrastructure is currently in place, and what infrastructure and technology is planned to be built in the near future. II. The JWST Observatory JWST is a large 6-meter aperture infrared space telescope with a 5-year mission (design goal of 10-years). JWST will continue the National Aeronautics and Space Administration (NASA) tradition of advancing breakthroughs in the understanding of the origins of the earliest stars by detecting the first starlight and will address other questions about the early universe. JWST is currently planned for a 2013 launch and will be passively cooled by using a tennis court size sunshade. The JWST teams are in multiple locations: (1) Project management located at Goddard Space Flight Center (GSFC), (2) Observatory prime contactor, Northrop-Grumman Space Technologies (NGST), located in Redondo Beach, California, (3) Integrated Science Instrument Module (ISIM) located at GSFC, (4) Near Infra Red Camera (NIRCam) instrument (University of Arizona and Lockheed Martin), (5) Near Infrared Multi-Object Spectrometer (NIRSpec) built in Europe, (6) Mid Infrared Instrument (MIRI) built by a joint US and European team, (7) Flight Guidance System (FGS) built in Canada, (8) Deep Space Network, and (9) Science and Operations Center located at the Figure 2. JWST Space Telescope Science Institute (STScI) located in Baltimore MD. An overview of the JWST Observatory can be seen in Figure 2. III. Communications, Orbit Maintenance, and Lunar One of the main challenges for a mission beyond the moon (300,000 km+) is in the satellite-to-earth communications. GEO satellites have straight forward communication concepts as they are stationary over the same spot of the Earth, Low Earth Orbiting (LEO) satellites have many alternatives with ground stations and the Tracking and Data Relay Satellite System (TDRSS), but the challenges for the missions to and beyond the moon tend to share the following characteristics: Due to Earth s rotation, each of the viewing periods of a satellite from a low latitude ground station is between 8-14 hours a day. Ground stations in southern and northern hemisphere are required for ranging in order to provide the angular information needed for determining a ranging solution. A. Communications As can be seen in Figure 3, one (1) ground station cannot provide continuous communication coverage. As part of the JWST spacecraft design and spacecraft operations for the high rate data, a High Gain Antenna (HGA) will be used. A slew of the HGA will occur prior to a Deep Space Network (DSN) contact and will remain stationary until the next DSN contact. In normal operations, the HGA will slew once per day minimizing the impact to science 2

3 observations. The HGA profile in relationship to the Earth and ground station motion is seen in Figure 4. As can be seen, the HGA is slewed into a position that will allow the view of the antenna to trace through the beam of the HGA when the DSN contract occurs. The JWST omni antennas provide S-band forward and low rate return services and in normal operations for JWST are in constant view of the Earth. This is not only true for JWST but currently for all planned lunar, L1 and L2 missions. Further discussions on the uniqueness of the S-band frequency are discussed later in this paper. Figure 3. DSN Contact Profile. In addition to these general characteristics each mission has unique orbit determination requirements that impact Ground Station Motion (0.3 deg) Antenna Boresight Motion (0.06 deg) 0.15 deg Max Distance at Start/End of 8 hour Ground Contact Period Ground Station Motion (0.34 deg) Antenna Boresight Motion (0.8 deg) 0.27 deg Max Distance at Start/End of 8 hour Ground Contact Period Earth Earth 0.42 deg 0.61 deg Figure 4. JWST HGA Path. 3

4 the satellite operation, propellant usage, and ranging requirements. B. Orbit maintenance The JWST requirements for orbit maintenance and determination that present operational challenges are: To allow an accurate orbit determination calculation to converge, only one momentum dump is allowed in a 21-day period, Ranging requires nineteen (19) measurements over 21-day period, using southern and northern hemisphere ground stations, and Ranging accuracy of 15-meters and 8-mm seconds, is required to converge to an orbit determination solution with an accuracy of 50 km and 2 centimeter/seconds (3-sigma). JWST has a large sunshield as shown in Figure 5 that will adversely act like a solar sail. The solar sail effect combined with the fact that no thrusters are allowed on the anti-sun part of the sunshield plus the slow movement of JWST in the L2 halo orbit requires special consideration for maintaining and determining the JWST position. It is anticipated that for JWST, L2 will require a minimum 4-orbit correction maneuvers every 180 days. JWST has a 5-year planned mission with a 5-year follow on and to meet the entire 10-year design goal, efficient use of propellant is required. This will require careful planning of the Delta V maneuvers not only to conserve the amount of propellant used over the life of JWST, but this also must be balance against the science Figure 5. JWST Sunshield View. observation needs. Factoring the L2 environment with the sunshield design, it is anticipated that momentum will need to be dumped every 21 days. This puts further constraints on the science planning system to schedule observations that need a very stable spacecraft within the 21-day period. With JWST, as with other L2 missions, the largest component of the orbital uncertainties for Lagrange orbits is the out-of-plane components. In the past, tracking results from stations located in both the northern and southern hemispheres are necessary for maximum-attainable accuracy in the out-of-plane components. To date, no Lagrange point spacecraft has flown without DSN support, or tracking support from both hemispheres. This supports the capitalization of leasing DSN assets versus building a dedicated ground station for JWST. The science pointing requirements on the orbit knowledge for previous Lagrange missions have tended to be less stringent when compared with JWST. JWST moves very slowly in the L2 halo orbit, so FDF will need many points are needed to determine the orbit solutions. This means Doppler tracking and ranging activities will need to occur during every contact to maintain the fine pointing. Additional accelerometers and guidance hardware on JWST will make the orbit determination easier, but the cost impact to the spacecraft is weight, power, and heat which are all critical issues. The JWST Project is evaluating all the options currently, but adding requirements to the spacecraft is not desirable at this late stage in the spacecraft design. In summary: Get an orbit analyst early in the mission design to define ranging and orbit determination requirements as they have impact on cost and operations. C. Lunar Lunar requirements are more demanding than those in L2. The far side of the moon is obstructed from direct earth viewing coverage and will require a relay satellite to provide navigation and communication. LRO requires continuous radiometric information so the information gathered by LRO instruments mapping the moon can be corralled. LRO is planning to obtain these services with combination of a dedicated mission antenna at White 4

5 Sands, New Mexico and Universal Space Network (USN) assets in Europe and Australia. The CEV mission might require at some point (before landing) precise radiometric information that might require simultaneous ranging from two different ground stations. Missions to the far side of the moon will transfer data back to earth using satellites of Lunar Relay Network schedule to launch at IV. High data rate 26 GHz Ka-band JWST was the first mission to be defined as having a high data rate from L2. JWST was also the first mission that pushed the spectrum allocation group to design a new spectra band in the 26 GHz Ka-band to meet higher data rates of more than 8 Mbps. JWST pushed for enhancements to the DSN capabilities that, at the time of writing this article, are limited to 5 Mbps. JWST requirements are to downlink 270 Gb compressed science and uncompressed engineering data every day. The JWST original concept was to have a daily 8-hour contact using X-band with 8 Mbps downlink rate. 8 Mbps required an allocation of a 20 MHZ X-band frequency. The NASA Spectrum office objected to providing more than a 10 MHZ band in X-band and suggested using Ka-band. The JWST Project decided to move to Ka-band that allowed for an increase in the downlink rate to 28 Mbps and a decrease in the contact time to one (1) 4-hour contact per day for communication and ranging. The next step was finding the infrastructure to support the 26 GHz Ka-band. JWST performed a lease or buy ground station trade study that recommended leasing rather than buying would be the most cost effective. DSN offered to upgrade its existing 34-meter network with Ka-band components and it was agreed that JWST will lease the contact time. The LRO mission is building one 18-meter 26 GHz Ka-band ground station at White Sands, New Mexico to provide high data rate support for recorder dumps of 100 Mbps from the moon and the link is closed with a 40-watts Traveling Wave Tube Amplifier (TWTA). LRO will supplement this ground station with S-band Universal Space Network (USN) ground stations in Europe and Australia for continuous real-time and ranging contacts. Other missions such as the LEO National Polar-Orbiting Operational Environmental Satellite System (NPOESS) and GEO Solar Dynamic Observatory (SDO) also plan to use the 26-GHz band, but due to the nature of GEO and LEO missions they cannot share their resources with Lagrange missions. The Exploration Program is also planning to use the 26 GHz Ka-band. The Tracking and Data Relay Satellite System (TDRSS) supports 26 GHz Ka-band and will provide communication services for the first phase CEV while visiting the International Space Station (ISS) in LEO. CEV missions to the moon are planned to be supported by a new Lunar Communications Network (LCN) infrastructure that is a combination of ground stations and lunar relay satellites. CEV planned downlink data rate is 150 Mbps. Currently there is no plan for JWST to use the TDRSS Ka-band capabilities. Another consideration is for these types of missions are signal strength verses coverage. GEO missions can use high elevation antenna angles that reduce temperature effect and improves the Gain Over Temperature (G/T). Going from 20 degree elevation to 10 degree decreases the link margin by 1.2 db, using the 10 degree elevation significantly increases coverage for L2. Ka-band is more sensitive to weather S-band and that will decrease your link margins even more. For instance, using 20 degree elevation JWST would not be able to communication with Madrid ground station in the June/July timeframe. For Lunar missions to provide continuous coverage using 20 degree elevation would require four antenna sites. As a baseline for planning, JWST suggests using the lower10 degree antenna elevation and compensating by increasing the spacecraft Radio Frequency (RF) power and one high link contact per day using a one 18-meter with Gain Over Temperature (G/T) of 45 db or a DSN 34-meter Ka-band antenna with ground G/T of 55 db. 5

6 A. File Management C&DH, SSR, and CFDP Dealing with large amounts of data requires planning and smart use of resources. The systems need to address issues such as data priority, multiple instruments working at different times, and variances in speed between components (i.e. 100 mbps to the recorder, but lower rates at the C&DH computer and through the ground links) see Figure 6 for an overview of the JWST CD&H subsystem. Figure 6. JWST C&DH. The modern C&DH is based on a 1 Gbps cpci bus and a PowerPC 750. The two emerging standards to move 100 mbps of data on the spacecraft are Spacewire (IEEE 1355) and Firewire (IEEE 1394). Ethernet would have been is desirable, as it would save the cost of using proprietary and expansive ground support equipment for IEEE 1355 and IEEE 1394, however there are few Ethernet implementations on spacecraft. An operation concept of one (1) contact per day requires a relatively large recorder and a method to optimize its use by minimizing maintenance of the fragmentation. The recorder should also be sized to allow for at least one (1) missed contact. JWST has a 470 Gbits SSR with one (1) large partition to collect data from its instruments. JWST and LRO will be using the CCSDS File Delivery Protocol (CFDP), see Figure 7 **, to provide reliable file transfer. To keep up with a high downlink, the recorder data is transmitted directly to the Ka-band transmitter, with CFDP headers being inserted via an FPGA. In other words, the single board PowerPC 750 does not read the file into its internal memory Figure 7. CCSDS CFDP. ** CCSDS G-1 CCSDS FILE DELIVERY PROTOCOL (CFDP) - Part 1 Green Book 6

7 because it will not be able to keep up with the data rate. The files are transmitted to the Ground Station that performs level zero processing stores and forwards these files to the Mission Operations Center. The ground station sends the files to the users Mission Operations Center (MOC). Figure 8 shows the JWST data flow, including CFDP. As part of moving to Ka-band, JWST has decided for software development cost purposes not to have a file system so dumps are performed based on a size, not by a name. LRO uses a file management system and extracts files by name. As a part of CFDP and file implementation, note that the file sizes should be relatively large, 256 Mb to 1 Gb to minimize the CFDP overhead. JWST is using one (1) Gb files sizes. Additionally the large file size was chosen due to the amount of CFDP accounting that must be done on the spacecraft with the propagation delay from L2 is twelve (12) seconds. For each CFDP file, the C&DH holds pointers and number of pointers by grow significantly the smaller the file size. Pointers = 2* (data rate/file size) * propagation delay. Figure 8. JWST Data Flow. In summary dealing with these high data rates requires close coordination with C&DH and a smart design that optimizes the use of resources. V. Launch and Emergencies S-band NASA has a requirement to provide communication coverage during critical events. JWST has performed an initial study to ensure coverage for launch support, see Figure 9, and found that most assets are available at S-band. The dashed lines in figure 9 denote critical JWST launch events. TDRSS provides JWST, LRO and other missions launch support with S-band. Another asset necessary for launch coverage is a tracking ship, as a significant portion of the launch trajectory is over the ocean. Note, United States and Delta launch services interleave the users telemetry in their launch vehicle S-band stream whereas the European Space Agency/Ariane launch vehicle provides a minimal service. The Lunar Program has decided to use the TDRSS architecture for CEV. This means that S-band is here to stay for the foreseeable future. 7

8 Figure 9. Candidate JWST Launch Coverage. In the design phase of the JWST mission, the JWST Project was encouraged to use the X-band frequency on the omni-antennas rather than S-band. Europe is planning to sell the S-band range to cell phone companies and there is pressure to get off S-band. In summary S-band is not dead as a frequency for space missions and is here to stay not only for JWST but for future space missions too. VI. Conclusion The following are good rules of thumb for Lunar, L1, and L2 missions: Users are demanding higher data rates, mbps on Ka-band is the emerging standard but as technology advances so will the need for higher data rates. The data rates that are available to a typical household were unimaginable just 10 years ago. Due to geometry, users should have one 4-hour contact when the average viewing time of a ground station is 8 hours. (LRO uses multiple 45 minutes contacts during view period due to its Lunar orbit). The cost of the additional ground station contact time and the coordination for multiple ground stations makes the shorter contact time highly desirable, until a dedicated L1/L2 communications network is put in place with the similar capabilities as the TDRSS. Ranging requires measurements from both southern and northern hemispheres. This single requirement drove JWST to lease instead of building its own ground station since a second ground station will always be needed. CFDP and other CD&H standards are needed to help deal with these high data rates. Even data products used commercially that involve high data volumes are compressed and transferred reliably. Just like the commercial industry, the L1, L2 and lunar missions are dependant on technology advances to do the compression and reliable data transfers. 8