Interactive comment on “ Space and ground segment performance of the FORMOSAT-3 / COSMIC mission : four years in orbit ”

The paper well describes all what happens to F3/C satellites and payloads during the 4 year in orbit and fit quite well with AMT topics. It does not present novel concepts or ideas since it is a review paper, but the description given allow interested readers to well understand what have been main problems occurred. The performances have been analyzed considering several point of views (mission, constellation, spacecraft, Radio Occultation payload, spacecraft batteries, ground station) and given a very honest overview, highlighting also lacks noticed since mission was planned.


Introduction
The FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) (FS-3/C) Mission is a joint Taiwan-US demonstration satellite mission that was launched in April 2006.The objective of FS-3/C is to demonstrate the value of near-real-time GPS Radio Occultation (GPS-RO) observations in operational numerical weather prediction.FS-3/C is currently providing global GPS-RO data in near-real-time to over 1400 users in more than 52 countries.The GPS-RO data has been demonstrated to be a valuable asset to the climate, meteorology, and space weather communities.The GPS/Meteorology (GPS/MET) experiment (1995)(1996)(1997) showed that the GNSS-RO technique offers great advantages over the traditional passive microwave measurement of the atmosphere by satellites and became the first space-based "proof-of-concept" demonstration of GNSS-RO mission impacts (Ware et al., 1996;Kursinski et al., 1996;Rius et al., 1998;Anthes et al., 2000;Hajj et al., 2000;Kuo et al., 2000).For a more complete history of GNSS-RO see Yunck et al. (2000) and Melbourne (2005).
However, the FS-3/C Mission will reach the end of its five-year design life in 2011, and the critical real-time satellite observing capability will begin to degrade as satellites become no longer operational.As a result, the National Space Organization (NSPO) and National Oceanic and Atmospheric Administration (NOAA) intend to jointly develop the FORMOSAT-7/COSMIC-2 (FS-7/C-2) Mission.FS-7/C-2 will incorporate the next generation GNSS-RO receiver, a significantly improved spacecraft design, and more substantial ground communication network for data download (Chu et al., 2008;Fong et al., 2008cFong et al., , 2009a, b), b).
FS-7/C-2 is intended to provide continuity of the GPS-RO data as well as provide the next generation of GNSS-RO data to the scientific community and the global weather centers.The objective of the FS-7/C-2 Mission is to collect a large amount of atmospheric and ionospheric data primarily for operational weather forecasting and space weather monitoring as well as meteorological, climate, ionospheric, and geodetic research.In addition, the system will allow scientists to collect data over unpopulated and remote regions (such as the polar and oceanic regions) in support of research in these areas.This paper will address the FS-3/C system and mission overview, the spacecraft and ground system performance after four year in orbit, the lessons learned from the encountered technical challenges and observations, and the expected design improvements for the new FS-7/C-2 spacecraft and ground system.

FS-3/C system and mission overview
The FS-3/C space segment includes six Low-Earth-Orbit (LEO) satellites in a constellation-like formation.The FS-3/C satellite constellation was successfully launched into the same orbital plane at 516 km altitude at 01:40 UTC on 15 April 2006.The FS-3/C satellites are equipped with three onboard payloads including a GPS Occultation Receiver (GOX), a Tri-Band Beacon (TBB), and a Tiny Ionospheric Photometer (TIP).The satellite constellation was intended to include six orbit planes at 800 km final mission altitude with 30 degree separation for evenly distributed global coverage.
The FS-3/C system that is in operation today consists of six satellites, a Satellite Operations Control Center (SOCC) in Taiwan, four remote tracking stations (RTSs), two local tracking stations (LTSs), two data processing centers, and a fiducial network.The SOCC uses the real-time telemetry and the back orbit telemetry to monitor, control, and manage the spacecraft state-of-health.There are two LTSs: one located in Chungli, Taiwan and the other in Tainan, Taiwan.There are four RTSs operated by NOAA to support the satellite passes: Fairbanks Command and Data Acquisition Station (FCDAS) in Fairbanks, Alaska and Kongsberg Satellite Services Ground Station (KSAT) in Tromsø, Norway, which are currently the two primary stations for the mission.The Wallops station in Virginia and the McMurdo station in Mc-Murdo, Antarctica provide backup support as needed for the mission (Fong et al., 2008c(Fong et al., , 2009a, b), b).
The science RO data is downlinked from the satellites to the RTS and then transmitted from the RTS via NOAA to the two data processing centers.The two data processing centers are the CDAAC (COSMIC Data Analysis and Archive Center) located in Boulder, Colorado and the TACC (Taiwan Analysis Center for COSMIC) located at the Central Weather Bureau (CWB) in Taiwan.The fiducial GNSS data is combined with the occulted and referenced GNSS data from the GOX payload to remove the satellite clock errors.
The science data collected by the GOX and TIP payloads are processed by the CDAAC and TACC.The results processed by the CDAAC are then passed to the National Environmental Satellite, Data, and Information Service (NESDIS) at NOAA.These data are further routed to the international weather centers including the Joint Center for Satellite Data Assimilation (JSCDA), National Centers for Environment Prediction (NCEP), European Centre for Medium-Range Weather Forecasts (ECMWF), United Kingdom Meteorological Office (UKMO), Japan Meteorological Agency (JMA), Air Force Weather Agency (AFWA), Canadian Meteorological Centre (Canada Met), French National Meteorological Service (Météo France), and Taiwan's CWB.The data are provided to the global weather centers within 180 min to meet the data latency limit required to be assimilated into the numerical weather prediction (NWP) models.

Spacecraft constellation performance
The FS-3/C in-orbit system performance over the last four years is considered to be more than satisfactory in meeting its mission goals.The experimental constellation was defined to have a two-year spacecraft mission life, and a spacecraft design life of five-years.The spacecraft hardware failure and/or degradation are proceeding as anticipated.Although the expectation of the entire 6-satellite constellation continuing operations into the fifth year and beyond is not realistic, a partial constellation with degraded performance is likely to continue for a few more years.It is believed that the lessons learned from the in-orbit operations will provide a solid foundation to migrate the experimental system into a stable and reliable operational system for follow-on missions.
The operation status of the key subsystems for all six satellites after four years in orbit is shown in Table 1.The battery power issue is a common and continuous major degradation problem for all spacecraft.For clarity, the satellites will be referred to as "FMx" where x is 1 to 6. FM4 and FM6 are experiencing significant battery degradations that are causing the payloads to be powered off unexpectedly, even at high battery state-of-charge.In addition, FM2 experienced a sudden significant solar panel power shortage in mid-November 2007.Since then, the output power of FM2 was reduced to one-half of the maximum solar array power capability, from 200 W to 100 W. The root cause of the FM2 power shortage is still undetermined.FM3 encountered a solar array drive mechanism failure at 711 km orbit that prohibited the continuous thrust firing of the FM3.The other five FS-3/C satellites reached their final mission orbit altitude of 800 km by the end of November 2007 (Fong et al., 2008b).FM3 tracked the solar power at reduced duty cycle depending on the power status of the spacecraft.The secondary payloads, TIP and TBB, on FM2 and FM3, as shown in Table 1, have been powered off due to the power shortage issues.Furthermore, FM3 has been in a severe abnormal condition (much more frequent loss of communication and low power status) since July 2010 (Fong et al., 2010).
Figure 1 shows the spacecraft system performance observed over the past four years (since launch) for the GOX mission payload with the duty cycle on, and spacecraft ADCS (Attitude Determination and Control Subsystem) attitude performance vs. spacecraft sun beta angle.The sun beta angle is defined as the angle between the spacecraft orbital plane and the vector from the sun.It determines the percentage of time the spacecraft in low Earth orbit spends in direct sunlight, absorbing solar energy.The GOX payload should be on during the normal operation period except during the constellation deployment phase.
In Fig. 1, it is observed that all spacecraft continue to operate with the GOX payload duty cycle on at high percentage rates even as the spacecraft bus and payload start to show degradation.FM1 has provided good payload performance, however it shows worse attitude performance than the other spacecraft.FM2 started to show reduced GOX payload duty-cycle on operations due to a battery charging efficiencydecreased phenomena that was experienced after the satellite was recovered from lost communication in June 2009.FM3 encountered malfunctions of the solar array drive mechanism starting in August 2007 when it reached a 711 km orbit.FM3 has been kept at that altitude and the GOX payload has been operating at low duty cycle since then.FM4 performed very well during the four year operational period, but recently its battery has shown significant degradation.FM5 has provided good spacecraft performance, however its GOX payload shows low SNR problems resulting in difficulties generating useful data even when the GOX payload is on.FM6 has a similar GOX payload low SNR problem.In September 2007, FM6 experienced loss of communications for 67 days.The satellite resumed contact and recovered on its own after a computer master reset event occurred over the South Atlantic Anomaly (SAA) region.In summary, due to the batteries aging, four out of the six spacecraft have begun to encounter a battery degradation problem.FM4 and FM6 are worse than the other four spacecraft.The major on-orbit performance highlights for all spacecraft are summarized in Table 2.

GOX mission payload performance
Figure 2 shows the four-year statistics for the number of daily occultation events for (a) atmosphere profiles and (b) ionosphere profiles of electron density.The atmosphere and ionosphere occultation profiles contributed by each spacecraft are shown in Table 3.
The GOX payload performance summaries are shown in Table 4.As the primary mission payload, four GOX instruments are being operated at a duty cycle of 100 % and two   other GOX instruments (onboard FM2 and FM3) are being operated based on the state of the power charge at various sun beta angles (due to the power shortages).There are many factors that affect the quality of the occultation data received from the GPS signals.Among them, the low SNR on the occulting precision orbit determination (POD) antenna seems to affect the data quality the most.In this mission the POD antenna has two functions: precision orbit determination, and ionospheric radio occultation processing.The occultation antenna is only used for atmospheric radio occultation processing.Four spacecraft (FM1, FM3, FM5, and FM6) have exihibited a low SNR anomaly on the POD1 antenna for the GOX payload.FM2 exhibited a low SNR anomaly on POD2.
In February 2009, the FM6 GOX payload SNR decreased, however, the GOX payload operating temperature was not over the red high limit (the limit that will shut down the GOX payload power autonomously).The RO profiles decreased to less than 100 per day and FM6 could only generate good RO data while the operating temperature was less than 25 • C.After two months of low RO data generation, the spacecraft was flip-flopped and the FM6 GOX payload recovered on its own and began to operate at full duty cycle."Flip-flop" means the spacecraft is rotated 180 degrees around the nadir (yaw) axis when the spacecraft sun beta angle is changed to 0 degrees, approximately every 57 days.This design allows the solar array to be reduced to half of the required size when compared to a design that does not "flip-flop".

Spacecraft payload on/off performance
The causes of the GOX payload being powered off are categorized as follows: nadir mode due to attitude excursion; stabilized mode after thrust burns; processor reboot/resets; entrance to stabilized/safehold mode; power shortage; derivative of battery molecular to charge (dMdC) anomaly; nadir mode after thrust burns such that spacecraft enters into power contingency; and Power Control Module (PCM) Direct Current (DC) Off anomaly.From the GOX payload duty cycle on values shown in Fig. 1, it is possible to compile the GOX payload duty cycle on statistics for one to four years, which are shown in Fig. 3.It is observed that the FM2 and FM3 power shortages are the main cause of the degraded average GOX duty cycles on those spacecraft.After the completion of the orbit transfer, FM4 and FM5 demonstrate the best GOX payload performance.The drop in FM6 GOX payload duty cycles in the second year is due to the complete loss of communications for 67 days.Additionally, the low SNR issue makes FM6 the 4th best performing satellite among the 6 satellites, following FM4, FM1, and FM5 for GOX payload performance.

Spacecraft Ni-H2 battery performance
There is another payload off phenomenon that did not belong to any category listed in the previous sub-section that is relevant to the battery performance degradation issue.Beginning in April 2009, the operations team has observed the GOX payload unexpectedly turn off while the spacecraft has good power and attitude conditions, where the battery stateof-charge (SOC) indicated is higher than the design value of 5.5 ampere-hours, and the spacecraft is operating nominally at the Nadir-Yaw mode.This phenomenon is beginning to be a frequently recurring event on all six spacecraft.According to the spacecraft design, the payload will be turned off only when any of the following design conditions The battery performance degradation issue has become one of the major triggers of the unexpected payload off phenomenon.The unexpected payload off is categorized as a deviation from the normal payload off that is initiated by either (1) the ground command or (2) autonomous internal command due to insufficient solar power charge to the battery.The S-band transmitter is turned on and needs to draw a substantial amount of power and current with a higher demand priority from the bus during a ground Telemetry Tracking and Control (TT&C) pass.This lowers the bus voltage further if the battery cannot provide sufficient power in time.Consequently, the battery degradation effect may cause the payload to be turned off sometimes during a ground TT&C pass.The battery degradation, as observed, has shown to cause the bus voltage to be lower than 11 Volts (compared to the nominal 14 Volts), but slightly higher than 10 Volts.Since the input voltage requirement for the tank pressure transducer is above 11 Volts, the pressure transducer reading will decrease dramatically and become unreliable to reflect the low bus voltage status when the actual bus voltage falls below 11 Volts.In addition, when the value of bus voltage is below the Power Control Module (PCM) design value of 10 Volts, the payload will be turned off by the PCM internal command due to the internal under voltage protection circuit design (Fong et al., 2010).
Table 5 shows the average variation rate per year of the battery for each spacecraft.FM4 and FM6 have shown the worst battery degradation.The spacecraft battery degradation significantly impacts the spacecraft operational life and the total number of GOX payload occultation profiles.

NSPO ground systems
NSPO was in charge of the mission operations of FS-3/C after launch including the early orbit checkout and initialization, constellation orbit deployment, and normal and contingent satellite operations.The facility used for the mission operations is the SOCC located in Hsin-Chu, Taiwan.The SOCC includes four subsystems: (1) Mission Operation subsystem for the real-time satellite operations during station contact; (2) Flight Dynamics Facility for the orbit determination, prediction and maneuver planning; (3) Science Control subsystem for the science data preprocessing; and (4) Mission Control subsystem for the operation planning and command scheduling.NSPO also provides two TT&C stations, (typically called ground stations) in Taiwan to support the contingent operations of FS-3/C.
In the early orbit checkout phase, the SOCC successfully sent commands to FS-3/C for spacecraft State of Health (SOH) inspection and hardware/software initialization.The measured performance of the in-orbit spacecraft compared to the expected results from the relevent ground tests show the SOH of a specarft in orbit.Some components, such as the GPS receiver and the battery charging parameters, were reconfigured for improved performance.In the constellation orbit deployment phase, the six FS-3/C satellites were maneuvered into the mission orbit altitude one by one in a planned time sequence.Each satellite took 4-6 weeks to maneuver into its mission orbit.The satellite constellation was fully deployed in 19 months.After the deployment, five of the six satellites had successfully reached the predefined mission orbits (except the FM3 whose onboard propulsion function was degraded which prohibited it from reaching its final mission orbit altitude).
In the normal operations phase, the SOCC routinely uplinked the time-tagged command loads to the satellites so that for each scheduled station contact, the satellites would sequentially turn on their transmitter, downlink payload data, downlink SOH data and then turn off their transmitter.On average, there are approximately 80 station contacts per day to dump the onboard payload data for near real-time meteorological research and operational applications.During normal operations some satellite anomalies also occurred, such as FC computer resets, BCR computer resets, ACE computer resets, Master resets and Phoenix resets.Phoenix is an off state of the satellite when satellite is out of battery power and is used to support satellite recovery when power condition is back to stable.Each type of reset was recovered by sending a series of configuration commands so that both the satellite and payload could resume normal operation as soon as possible.
All six satellites have experienced some anomalies in the electric power subsystem and/or payload instrument performance causing onboard electronic power shortages and payload duty-cycle reduction.The SOCC and the operation team used operational methods to reduce the impacts of the anomalies and increase the payload data output.It has proven difficult to maintain the FS-3/C constellation in the current SOH status after four years in operation.

NOAA ground systems
When FS-3/C was launched, ground station support was contracted with the Universal Space Network (USN) through their stations at Poker Flats, Alaska and Kiruna, Sweden.USN performed very well for 2 yr, but in an effort to reduce operational costs NOAA made a decision to employ indigenous resources.NOAA assets were established for FS-3/C at Fairbanks Command & Data Acquisition Station (FCDAS) as well as Wallops Command & Data Acquisition Station (WCDAS), and services were contracted with Kongsberg Satellite Services (KSAT) at their Tromsø Satellite Station through NOAA agreements with the Norwegian Space Centre.Since April 2008, NOAA stations have been providing both uplink and downlink services and Tromsø has been providing downlink services only.Ground station support availability for FS-3/C was required to perform at 90 % or better.Over the course of FS-3/C operations, ground stations services have performed at 95 % or better with only minor interruptions due to occasional equipment issues (hung servers or processors, for example).
FS-3/C command uplink and telemetry downlink activities are coordinated by the NSPO SOCC with the Remote Tracking Stations (RTS).Once upcoming FS-3/C passes have been deconflicted with other ground station activities, SOCC generates spacecraft ephemeris, spacecraft command uploads and ground schedules and distributes the files to the ground stations.All contacts with the spacecraft are established and conducted autonomously via schedules executed at the SOCC and the RTS, with the exception of any realtime commanding conducted by Mission Control personnel at SOCC.During the pass, the spacecraft and ground system are autonomously monitored by SOCC as the data stored on the spacecraft is downlinked to the ground station.After the spacecraft contact has ended, all connections are autonomously terminated and the RTS data server forwards the Contact Report to the SOCC as well as the Payload Data Files to the Data Processing Centers for processing.
FS-3/C mission data is distributed from data servers at the ground stations across the world wide web via Secure File Transfer Protocol (SFTP) to the SOCC and CDAAC.Figure 4 shows the flow of data between the RTS, SOCC and CDAAC.One week prior to real-time, the spacecraft ephemerides (2 line element sets) and RTS pass schedules are made available to the mission team for operations.Timeliness can vary but SFTP has been found to be a very reliable and inexpensive means for distributing the data globally.A typical post contact scenario consists of transferring realtime and non-real-time spacecraft data to SOCC, followed by the transfer of mission files to CDAAC and then to SOCC.Statistics show that mission data arrives at CDAAC for processing within 15 min after spacecraft loss of signal (LOS), which is the end of the scheduled spacecraft contact with the ground station, 97 % of the time.

Science data processing
The COSMIC Data Analysis and Archival Center (CDAAC) at UCAR currently processes COSMIC data in near real-time for operational weather centers and the research community.The CDAAC also reprocesses RO data in a more accurate post-processed mode (within 6 weeks of observation) for COSMIC and other missions such as GPS/MET, CHAMP, SAC-C, GRACE, TerraSAR-X, (and METOP/GRAS in the near future).The data processing at the CDAAC includes: GPS site coordinate and zenith tropospheric delay (ZTD) estimation for a global ground-based reference network, high-rate (30 s) GPS satellite clock estimation, LEO precision orbit determination, computation of L1 and L2 atmospheric excess phases (Schreiner et al., 2009), retrieval of neutral atmospheric bending angles and refractivity for each LEO occultation event (Kuo et al., 2004), estimation of absolute total electron content (TEC), and retrieval of electron density profiles (Schreiner et al., 1999).The CDAAC also provides COSMIC TIP calibrated radiance products.All COSMIC products are made available freely to the community at www.cosmic.ucar.edu.
Since the launch of the FS-3/C constellation in April 2006, COSMIC has provided a large amount of valuable science data to the operational and research communities.As of 1 September 2010, COSMIC and CDAAC have produced over 2.5 million high quality neutral atmospheric and ionospheric sounding profiles, over 2.6 million absolute TEC data arcs, S4 scintillation observations, over 16 000 h of quality controlled TIP radiances, and a significant (but not centrally archived) amount of ground-based TBB observations.On average, COSMIC currently produces around 1000 GPS-RO soundings per day.Approximately ninety percent of these are processed and delivered via the Global Telecommunications System (GTS) to operational centers within three hours.The remaining ten percent have higher latency due to the satellites' inability to downlink every orbit (∼100 min).The COSMIC RTSs are down-linking and forwarding the payload data to the CDAAC in less than 15 min on average.The CDAAC processes a single dump of payload data into profiles and forwards them to the GTS via NOAA in less than 10 min.The average latency of COSMIC data is currently approximately 90 min for single orbit dumps.The reliability of the RTS stations and the CDAAC near real-time processing system have been measured at greater than 95 % and 99.5 %, respectively.

Lessons learned from encountered technical challenges
This section contains highlights of some major challenges encountered and enhancements accomplished after twentyfour satellite-years (4 yr × 6 satellites) of operation in orbit of the FS-3/C mission.There are many lessons learned from the four years of operations, which can be used to improve similar future missions (Fong et al., 2008a(Fong et al., ,b,c, 2009a,b),b).

Mission lessons learned
Table 6 highlights three major mission lessons learned.They are: (1) the determination of the spacecraft communication frequency, (2) the prevention of the Radio Frequency Interference (RFI) among the three different payloads in each spacecraft, and (3) the quantity definition of the radio occultation profiles.

Payload lessons learned
The GOX payloads are performing well and reliabily at the instrument level based on the assessment of the available data  as discussed in Sect.3.2.However, there are some lessons learned from the observed GOX performance at the payload subsystem level.The major lessons learned from the data assessment at the GOX payload subsystem level, as summarized in Table 7, are: (1) GOX POD low SNR problem, (2) GOX OCC low SNR problem, and (3) GOX SNR decrease at high temperature.

Spacecraft system lessons learned
The spacecraft state of health correlates directly to the payload performance.The FS-3/C spacecraft is a modified version of a heritage design of the successful ORBCOMM spacecraft.However, the FS-3/C spacecraft, a micro-grade spacecraft (<100 kg), is not equipped with full comprehensive redundancy for avoiding single critical failure in design and/or high reliability components in critical instruments for durability.Five major spacecraft system lessons learned, as described in 6 Design improvements for the follow-on system

Mission trades and improvements
In order to apply the lessons learned from the FS-3/C program to create an operational constellation, several mission trades have been studied.The results of the FS-7/C-2 mission trade studies are summarized in Fig. 5 (Fong et al., 2010;Yen, 2010;Yen and Fong, 2009).
The FS-7/C-2 satellites will be equipped with the nextgeneration GNSS-RO receiver (TriG) to collect more soundings per receiver.The TriG will have the ability to track GPS, Galileo and GLONASS GNSS systems, which includes 29 operational GPS satellites, 18 planned GLONASS, and 30 planned GALILEO satellites.The TriG mission payload receiver will have the capability to receive the GPS L1/L2/L5 signals, the GALILEO E1/E5/E6 signals, and the GLONASS L1/L2/L5 signals.This payload instrument will significantly improve the amount of data collected, which will lead to improved mission applications.
Figure 6 depicts the proposed FS-7/C-2 mission architecture.The FS-7/C-2 program is planned to have 12 satellites, which will result in collecting 8000 profiles per day.The mission baseline includes 6 satellites at low-inclination-angle orbit and 6 satellites at high-inclination-angle orbit so that the mission will collect more data from the low latitudes over what is currently being collected.Participants on the joint program will work together on the data processing and data utilization to improve the data processing aspect of the system.

Spacecraft trades and improvements
The FS-7/C-2 spacecraft will have improved payload performance, better attitude performance, simplified operation, simplified orbit transfer, increased data storage, and modular design for additional compatible science payloads.The spacecraft bus design intended for the follow-on system vs. the current FS-3/C bus design is shown in Table 9.
NSPO is responsible for the acquisition and management of the spacecraft for the FS-7/C-2 Program.The acquisition goal is to acquire the twelve (12) spacecraft along with the spacecraft design, information on the development, manufacture, assembly, integration, testing, and operations from a spacecraft contractor through a procurement contract.NSPO will integrate the mission payloads onto the contractor-provided spacecraft and perform the required integral system testing at NSPO.Additionally, it is planned that the spacecraft contractor will provide the necessary support to the integral integration and test (I&T) at NSPO, and the launch site operations.The satellite (including spacecraft and payload) major milestones will be developed to incorporate the spacecraft development along with the subsequent production schedule of the spacecraft contractor and the integral satellite I&T at NSPO to meet the intended launch periods as illustrated in the NSPO-NOAA Joint Program Integrated Master Schedule.
NSPO also plans to develop an additional NSPO selfreliant spacecraft along with the RO mission payload to be launched during the second launch of the joint mission.NSPO will be responsible for the system/subsystem design that will meet the satellite System Performance Requirements and perform the integral satellite I&T and the launch site preparation activities.

Ground trades and improvements
The biggest and probably most challenging improvement for the next generation ground system will be meeting the objective latency requirement of 15 min.FS-7/C-2 threshold latency of 45 min is expected to be easily achievable with twice per orbit data dumps in each orbit plane and will be a great improvement over FS-3/C latency.Meeting the objective latency of 15 min is more difficult to achieve.Data recovery trades are currently being evaluated as part of the FS-7/C-2 mission definition to determine feasibility versus affordability.
A ground system solution for FS-7/C-2 that will meet threshold latency requirements will likely employ 10 to 12 ground stations, 2 at each of the poles and 6 to 8 around the equator, to capture data from satellites in both orbit planes.The high-inclination orbit plane will be supported by the existing polar sites at Fairbanks, Wallops, Tromsø, and McMurdo, and    -Singularity occurred at each orbit to the FS-3 magnetic -based controller/estimator -The parameters of the attitude reference system have been tuned to gain better attitude performance -Better performance of attitude sensor, for example star tracker, may be used to improve the ADCS dramatically -Rate sensor, even the coarse rate sensor, will improve the Thrust Mode performance and therefore decrease the duration of the constellation deployment -The three-wheel (or four -wheel) zeromomentum-bias linear control system should be considered in future missions   stations will be required.Conceptually there would be 3-4 ground stations in the Americas and an additional set of 3-4 in Asia-Indonesia.Figure 7 shows an optimized set of potential ground station locations to meet the low-inclination orbit plane threshold latency, as well as providing coverage for some of the high-inclination orbit plane passes.The yellow circles are the 10 degree elevation coverage circle of the potential ground stations, when LEO satellite passes within this yellow circle, then satellite could be acquired by the ground station located in the circle center.In the figure, the upper red line is 24 degree northern latitude, and the lower red line is the 24 degree southern latitude.These two lines are the upper and lower bound of the low-inclination satellite trajectories of the launch #1.Trades are currently being performed to look at existing ground station options versus deploying FS-7/C-2 unique sites that are optimized to meet mission needs.
To meet the objective latency, two options are currently being studied -a more extensive network of ground stations   and crosslink via the National Aeronautic and Space Administration's (NASA) Tracking and Data Relay Satellite System (TDRSS).Both are currently being considered as part of this trade and implementation will depend largely on the total cost to deploy and operate the option.A ground station solution will be difficult to deploy but if stations could be leveraged from existing sites and/or future programs it may be more feasible and very cost effective to operate.On the other hand, TDRSS would be relatively easy to deploy but be potentially expensive for long term service.
Another item in the ground system trade is alternate data transfer options from the ground stations to the data processing centers to better meet latency needs of multiple users.SFTP via the world wide web to multiple users, including potential secondary payload data centers, may not provide adequate latency.Dedicated communication lines may be required to meet the more stringent latency requirements.

Data processing trades and improvements
Data processing architecture for FS-7/C-2 will remain relatively the same as FS-3/C but will require reliable and low latency input data from FS-7/C-2 GNSS-RO payloads and GNSS ground network, updates to data processing software including GNSS (GPS, Galileo, and Glonass) capability, and more computational power to support the improved and additional number of RO instruments.To make data processing more robust for an operational environment, a data processing system (DPS) will be installed at the Environmental Satellite Processing Center (ESPC) in NOAA's Satellite Operations Facility (NSOF) in Suitland, Maryland.ESPC will be the prime data processing center in the United States for FS-7/C-2, providing GNSS-RO data products to the operational weather community.NOAA will provide longterm archive of FS-7/C-2 data in their Comprehensive Large Array-Data Stewardship System (CLASS).

Conclusions
The FS-3/C satellites have performed successfully for over 4 yr.It is not a perfect constellation for an operational system, but it has achieved more than satisfactory results for an experimental system operating in a semi-operational manner.The FS-3/C satellites are degrading as anticipated; however, NSPO assesses these satellites will continue to operate in a reduced capacity for the next few years.The success of the FS-3/C mission has initiated a new era for near real-time operational use of GNSS-RO soundings.NSPO is committed to continuing the FS-3/C satellite constellation operation to collect RO data to minimize the data gap duration between the end of FS-3/C and the beginning of FS-7/C-2.NSPO and NOAA have already begun the FS-7/C-2 joint mission implementation.

Fig. 1 .
Fig. 1.Spacecraft System Performance after Four Years in Orbit.

FM4Fig. 2 .
Figure 2. Four-Year Statistics Showing the Number of Daily Occultation Events (as-of-4/5/2010) for (a) Atmosphere Profiles, and (b) Ionosphere Profiles of Electron Density

Table 1 .
Mission Operation Status of Each Subsystem for All Six Spacecraft.

Table 3 .
Number of Occultation Profiles for Each GOX Instrument after Four Years in Orbit.
are met: (1) the external payload off ground command is sent; (2) the low power spacecraft battery SOC falls below 5.5 ampere-hours; (3) three flight computers (Attitude Control Electronics (ACE), Battery Control Regulator (BCR), or Flight Computer (FC)) have been rebooted or reset; or (4) the spacecraft attitude has entered into stabilize/safehold/thrust mode.

Table 5 .
Average Variation Rate per Year of Each Spacecraft Battery.
Note: SADA = Solar Array Drive Assembly.

Table 8 .
Spacecraft system lessons learned.