SEOSat/Ingenio - Earth Observation Satellite of Spain
Ingenio, Spanish for "ingenuity", is an optical high-resolution imaging mission of Spain - the flagship mission of the Spanish Space Strategic Plan 2007-2011. The mission is devoted to providing high resolution multispectral land optical images to different Spanish civil, institutional and government users, and potentially to other European users in the framework of Copernicus (formerly GMES) and GEOSS. The overall mission objective is to provide information for applications in cartography, land use, urban management, water management, environmental monitoring, risk management and security. The requirements call for panchromatic imagery of 2.5 m and multispectral imagery of 10 m resolution. 1) 2)
Ingenio is also known as SEOSat/Ingenio (Satélite Español de Observación de la Tierra - Spanish System for Earth Observation Satellite).
CDTI (Centro para el Desarrollo Tecnológico Industrial - Spain's Center for Development of Industrial Technology) is funding the mission and is responsible for the programmatic aspects of the program. The project development is overseen by ESA as a national contribution within the framework of Europe. A procurement assistance agreement to this effect was signed between ESA and CDTI in July 2007. Within the agreement, CDTI has entrusted ESA with the technical and contractual management of the industrial activities, thus being in charge of the procurement of the Space Segment and of the Ground Segment of the Ingenio system. 3) 4) 5)
The basic mission requirements are defined according to these main criteria:
• Provision of data services to comply with the Spanish institutional user needs. The global land mission primary objectives call for a "Carpet mapping of Spain"+ image acquisition over main areas of interest (Europe, South America and North of Africa).
• The SEOSat/Ingenio mission is also regarded to provide complementary services to the GMES program, in particular to the objectives of the Sentinel-2 mission.
- SEOSat/Ingenio with the optical payload to serve the civilian users (space segment lead by CDTI/ESA)
- PAZ is an X-band SAR (Synthetic Aperture Radar) spacecraft based on the TerraSAR-X platform, to serve the security and the defense needs. The PAZ mission is a dual-use mission (civil/defense) funded and owned by the Ministry of Defence and managed by HISDESAT (Hisdesat Servicios Estratégicos, S.A., a Spanish mixed private-public communications company providing also its services to the Ministry of Defence (MoD), since 2001.
PNOTS is funded and owned by the government of Spain. INTA (Instituto National de Técnica Aeroespacial), the Space Agency of Spain, is managing the common ground segment of the two missions. Hisdesat together with INTA, will be responsible for the in-orbit operation and the commercial operation of both satellites. EADS CASA Espacio is the prime contractor leading the industrial consortia of both missions.
A major objective of PNOTS is to maximize the common developments, services, and to share the infrastructure between both missions (whenever possible). Both missions will also contribute to the GMES (Global Monitoring for Environment and Security) program of Europe. According to contract, the ESA SEOSat/Ingenio project team must ensure that the European ground segment will allow the SEOSat/Ingenio system to become a candidate national mission contributing to GMES and to participate to the ESA third party mission scheme, within the EO multi-mission environment - and therefore to support HMA (Heterogeneous Mission Access) services.
The PNOTS program schedule calls for:
- March 2009: SSR (System Requirements Review)
- November 2009: PDR (Preliminary Design Review)
- Q4 2011: CDR (Critical Design Review)
- Launch of Ingenio in 2014.
- Launch of the PAZ spacecraft in 2014.
Figure 1: Overview of PNOTS users (image credit: CDTI, INTA)
Table 1: Overview of SEOSat/Ingenio mission parameters
Figure 2: Artist's rendition of the SEOSat/Ingenio spacecraft, deployed (left) and in launch configuration (right), image credit: INTA, ECE
The SEOSat/Ingenio spacecraft contract was awarded to a consortium of Spanish companies led by Airbus Defence and Space (former EADS-CASA Espacio) of Madrid, Spain in 2008. The industrial team comprises a consortium of Spanish and other European industries, including as main partners SENER (Primary Payload Contractor), Thales Alenia Space España (Payload Electronics and Communication Systems), CRISA (Onboard platform electronics) and Airbus Space & Defence France (Avionics and Software).
The new medium-sized AstroSat-L platform (also referred to as AstroSat-250) is being used. The AstroBus-250 of Airbus DS is a standard modular ECSS (European Cooperation for Space Standards) compatible satellite platform compatible with an in-orbit lifetime of up to 10 years, with consumables sizeable according to the mission needs. The platform design is one-failure tolerant and the standard equipment selection is based on minimum Class 2 EEE parts, with compatibility to Class 1 in most cases. The AstroBus-L platform is designed for direct injection into LEO (Low Earth Orbit). Depending on the selection of standard design options, AstroBus-L can operate in a variety of LEOs at different heights and with different inclinations. Further implementations of the AstroBus-L platform are on the missions: Copernicus/Sentinel-2, and EarthCARE. 8)
An essential feature of AstroBus-L is the robust standard FDIR (Failure Detection, Isolation and Recovery) concept, which is hierarchically structured and can easily be adapted to specific mission needs.
The main features are:
- The service module has a box-shaped structure design with fixed solar array panels
- The configuration is based on an aluminum-faced sandwich panel structural concept, with fixed body mounted solar arrays.
- The payload is isostatically mounted on a service module structure, with a star tracker mounted directly onto the payload structure
- The spacecraft is 3-axis stabilized. The AOCS (Attitude and Orbit Control Subsystem) architecture is based on a gyroless solution. Nominal attitude sensing is based on a multihead star sensor, CSS (Coarse Sun Sensors), and magnetometers; actuation is provided by 4 reactions wheels for fine-pointing and 3 magnetorquers (MTQs) for momentum off-loading. In addition, a propulsion subsystem with 4 thrusters is used. Redundant GPS receivers provide the real-time orbit and on-board time. The objective of the AOCS is to perform the functions of inertial attitude estimation, navigation and 3-axes attitude control. 9)
- The propulsion subsystem uses a plug-in concept, packaged on a single and compact module, with chemical monopropellant hydrazine
- The EPS (Electrical Propulsion Subsystem) uses solar panels with a total area of ~ 5.40 m2, a PCDU (Power Control and Distribution Unit), a Li-ion battery of 150 Ah capacity, and an unregulated power bus of 34 VDC
- Payload data handling comprises a >280 Mbit/s X-band data transmission, and a mass memory device of 1Tbit, allowing 33.17 Mpixel/s for the PAN output rate and 8.22 Mpixel/s for the MS output rate, for each optical camera unit
- The total mass of the spacecraft is about 830 kg at launch. 10)
Figure 3: Schematic of AOCS functions (image credit: EADS CASA Espacio)
Figure 4: Schematic view of AOCS and propulsion subsystem elements (image credit: EADS CASA Espacio)
Figure 5: Block diagram of the SEOSat/Ingenio spacecraft (image credit: EADS CASA Espacio)
RF communications: Use of the S-band for TT&C data transmissions. The X-band is used for the download of imagery at 280 Mbit/s. Thales Alenia Space España is responsible to the development and delivery X-band and S-band data communication systems, and the electronics for optical observation instruments. 11)
Figure 6: Artist's rendition of the deployed SEOSat/Ingenio spacecraft (image credit: Airbus DS)
Launch: The launch readiness of the SEOSat/Ingenio spacecraft is planned for 2019. Possible launch vehicle candidates are: Vega, Rockot, Dnepr, Soyuz, and PSLV. 12)
Orbit: Sun-synchronous circular orbit, altitude = 670 km, inclination = 98º, LTDN (Local Time on Descending Node) at 10:30 hours, repeat cycle = 38 days.
SEOSat/Ingenio security system:
The Security System Architecture was defined by the SEOSAT system team, under the technical supervision of ESA. EADS Astrium Crisa contributed to the definition of the system architecture as part of the SEOSAT's system team, and was awarded with the contracts for the development of the DCUs (Deciphering and Ciphering Units) and the associated EGSE (Electrical Ground Support Equipment). The DCU Flight Models (FMs) have been delivered by the end of 2012, and are currently (2013) being integrated in the Satellite at the CASA-Espacio facilities. The security functions of the GS (Ground Segment)for the FOS (Flight Operation Stations) and PDGS (Payload Data Ground Station) have been specified by ESA and are currently under development by the GS subcontractor. 13) 14)
Security system architecture:
The SEOSAT overall system is depicted in Figure 7. The Security System aims at protecting both the S-band and X-band space links between the satellite and the GS. For this purpose, complementary security functions have been allocated at the satellite and GS, namely:
• TC Authenticated Encryption ("Auth Enc") and Decryption ("Auth Dec"), and anti-replay protection
• Housekeeping TM Encryption ("HK-TM Enc") and Decryption ("HK-TM Dec")
• Payload TM Encryption ("PL-TM Enc") and Decryption ("PL-TM Dec").
Figure 7: SEOSAT/Ingenio system overview (image credit: EADS Astrium Crisa)
SEOSat in LEO implies relatively short contact times with the GS. Therefore, the satellite is provided with a relatively high autonomy. Concerning the security system, the satellite is able to autonomously configure the security functions for routine operations. GS intervention is required basically for mission timeline uploads and troubleshooting. At the same time, given the short contact times, operational procedures and protocol overheads associated to the security system management are limited to the minimum necessary.
Besides technical requirements for the security functions, industrial organization constraints were considered for the definition of the security system architecture. The security functions had to be integrated within the satellite's platform without any major impact in the on-going development of other units or system functions. In addition, given the complex subcontractor network, the security perimeter (understood as the human team with direct knowledge about the security functions) had to be carefully limited. Finally, the security functions were the last to be specified and procured, with the subsequent pressure in schedule and cost. Given this scenario, it was decided to implement the security functions by purely hardware (HW) units and without involving the OBSW (On-Board Software) in them.
Security system architecture overview:
SEOSat's security system implements three functional DCUs, respectively, for TC authenticated decryption, HK-TM encryption and PL-TM encryption. The first two are packaged into a common housing, so two physical units have been developed: DCU-S (for S-band DCU) and DCU-X (for X-band DCU). DCU-S includes two hot redundant DCU-S (TC) modules and two cold redundant DCU-S (TM) modules. DCU-X includes two cold redundant modules. Both units are functionally placed in the S-band and X-band chains respectively, as shown in Figure 8.
Figure 8: Schematic view of the SEOSat/Ingenio security system architecture (image credit: EADS Astrium Crisa)
The DCU-S(TC) module processes CLTUs (Communications Link Transmission Units) received from the Transponders (TRSPs) transparently for the OBC (On-Board Computer). The processing is done by a FPGA (Field Programmable Gate Array) and includes: CLTU delimitation, BCH decoding, de-randomization, Security Header extraction, TC Transfer Frame (TF) authenticated decryption and anti-replay control, pseudorandomization and BCH encoding. This module operates nominally at 64 kbit/s, but withstands up to 128 kbit/s.
The DCU-S(TM) module processes CADUs (Channel Access Data Units) received from the OBC transparently for the TRSPs. The processing is also done by a FPGA and includes: SP-L (Split-Phase-Level) line decoding, CADU delimitation, TF encryption and Security Header insertion (within secondary header), Reed-Solomon (RS) encoding, pseudo-randomization and SP-L encoding. VC (Virtual Channel) multiplexing is done by the OBC, so this module is not intended to insert (or remove) any additional bit in the bitstream. This module operates at two possible fixed rates, 1 Mbit/s and 128 kbit/s.
The DCU-X module processes CADUs received from the primary and secondary PDHUs (Payload Data Handling Units). The processing is also done by FPGAs and includes: input flow control, CADU delimitation, VC multiplexing, TF encryption and Security Header insertion (within insert zone), Reed-Solomon (RS) encoding, pseudo-randomization. The VC multiplexing is based on a Round-Robin scheduler, and Idle TFs are inserted automatically if no payload TF is available at the inputs. This module operates at fixed output rate of 280 Mbit/s, and withstands sustained input rates between 1 Mbit/s and 342 Mbit/s.
Crypto algorithms and keys:
All cryptographic algorithms used in SEOSAT are based on AES (Advanced Encryption Standard), and the modes of operation used are in accordance to the CCSDS recommended standard. 15) The algorithms are hard-coded in DCU's FPGAs. A set of cryptographic keys per functional DCU are available on-board. Part of them (session keys) can be updated in flight by means of secure commands, and part of them (master keys) are fixed throughout the mission. Keys are stored in DCU's non-volatile memory and are protected from potential corruption by means of triplication and EDAC (Error Detection And Correction) codes.
The initial key fill is performed while on ground via a dedicated connector on the satellite's skin (Key Fill I/F in Figure 8), and by means of a KFD (Key Fill Device). The KFD is a handheld battery-operated equipment that is used to transport the keys from the KMF (Key Management Facility) to the satellite, and to inject them directly into the DCUs (see also Figure 10). The Key Fill I/F is physically a "data diode", i.e. keys cannot be readout by any means once injected into the Satellite. As for any symmetric key system, the system security relies on the confidentiality of the keys. Anti-tamper mechanisms have been implemented in order to ensure DCU physical integrity while on ground.
Security function management:
Autonomous on-board monitoring of the DCUs is performed by the OBSW through the Mil-STD-1553 bus interfaces, and the associated HK-TM is also reported to GS. The DCUs can be managed from GS by means of the PUS (Packet Utilization Standard) TCs, which are processed by the OBSW. For non-security-related commands, plain TCs are used. For security-related commands (e.g. for key management), authenticated and encrypted messages are embedded into the TCs. When processed by the OBSW, these messages are decapsulated and blindly forwarded to the target DCU. The messages are finally decrypted and authenticated by the DCU's FPGA, and the resulting command is executed. This approach allows excluding OBSW from the security perimeter. Secure commands allow for example OTAR (Over-The-Air-Rekeying), key integrity checks and key disabling.
The security system can be operated in Secure Mode or Clear Mode, with security functions active or inactive, respectively. The satellite and the GS must be configured in the same mode and with the same crypto-parameters (if in Secure Mode), otherwise the TT&C may be interrupted. The security function configuration is nominally initiated by the GS, although in exceptional cases (e.g. autonomous reconfiguration) it can be initiated by the satellite.
Ground Segment constraints and operation:
SEOSAT's GS, specified by ESA and the satellite prime contractor, provides (among other information) the interface control documents, the mission operations concept and the satellite's user manual. For what the Security System and TT&C concerns, several constraints for the GS have been identified in these documents.
In general, TC TFs need to be encrypted, authenticated and sequence number stamped at uplink time while in Secure Mode, which implies that TT&C equipment shall process TC TFs on-the-fly. Optionally, pre-generated CLTUs could be uplinked by auxiliary FOS stations with no crypto-equipment, provided that the embedded TFs have sequential anti-replay stamps and use the COP-1 expedited service. However, the anti-replay acceptance window needs to be sized in order to avoid rejections in case re-transmissions are needed (e.g. due to channel errors). On the other hand, the on-board processing of some secure TCs can take longer than the minimum time specified for the inter-CLTU gap . In this case, the GS must insert a guard time (i.e. extend PLOP-2 CMM-4) after the TC.
Likewise, the HK-TM TFs need to be decrypted at downlink time while in Secure Mode by the TT&C equipment. However this is not strictly necessary, as the CLCWs (Command Link Control Words) in the TF Trailer fields are not encrypted (for any VC) and can therefore be extracted from the encrypted TF stream. Additionally, either in Clear or in Secure Modes, the Idle TFs are left un-encrypted, which allows the possibility of segregating the Idle TFs from the rest before the storage (if any) and decryption process. Regarding the PL-TM, in spite of the higher data rate, the TF processing on ground is less stringent for several reasons, namely: decryption can be performed off-line, the Idle TFs are also left un-encrypted (they can be discarded prior to storage and decryption), and the decryption function can process standalone PL-TM TFs with complete independence of previous TFs in the stream.
The satellite can initiate self-configuration under off-nominal situations, potentially configuring the satellite in an operational mode unexpected for the GS (different from the mode during previous contact). When this occurs outside the GS visibility, the TT&C link may be lost at the following pass. It is important for the operator to plan in advance how to handle such scenarios.
Since the PL-TM data on-board storage is massive, it may happen that a contact time is not long enough to complete the downlink of a big data file. The DCU-X implements input data flow override by command, allowing the operator to split the downlink along several contacts. Downlink in progress can be stopped preserving the encryption context, then resumed during the following contact.
Space data link protocols:
SEOSat's Security Layer is implemented at the Transfer Sublayer of the TC link and HK-TM link applicable protocols, respectively, 16) 17) and at the Data Link Protocol Sublayer of the PL-TM link applicable protocol. 18) The applicable standard for the TM (both HK and PL) Synchronization and Channel Coding Sublayer is provided in reference. 19) The Security Layer was specified ad-hoc for the SEOSat mission during 2009-10 timeframe, when there was still no related standardization available. This situation is well recognized in reference 20), where a summary is given for the recent CCSDS draft recommended standard for SDLS (Space Data Link Security) Protocol.21)
Transfer Sublayer Implementation Aspects:
First, the CLCWs are left un-encrypted so the COP-1 loop can be closed without needing TM decryption (actually with independence of the security mode). Second, the TC authenticated decryption on-board is performed prior to the FARM-1 process, therefore any channel error not detected at Synchronization and Channel Coding Sublayer (via BCH codes), and that could be eventually detected at TC TF level (by means of the ECF field), will result in authentication errors. Third, the TC TF rejection due to authentication errors will not reach the OBC, so from the FOP-1 process viewpoint they cannot be distinguished from channel errors. Fourth, TM TF decryption algorithm is self-synchronizing, so any interruption of the S-band downlink channel does not prevent the re-synchronization of the decryption process on ground. Finally, any potential contingency in the TC and TM links can be alleviated by a proper processing of DCU HK-TM (in order to discriminate errors) and by controlling the size of the TC anti-replay acceptance windows (in order to restrict or relax the allowed number of retransmissions).
Synchronization and Channel Coding Sublayer Implementation Aspects:
First, any loss of bit synchronization at the OBC to DCU-S TM interface may result in the loss of TM at GS. This can be caused by an operator error or by unexpected on-board error (e.g. due to Single Event Effects, SEEs). In this scenario, the DCU-S will generate a pseudo-randomized idle sequence (with no data structure) in order to feed the TRSP with modulation data. Otherwise, the TRSP would transmit an un-modulated carrier potentially violating ITU regulations on power flux density. Secondly, at the time of issuing the standard, no randomization scheme was agreed upon for high data rate TM (above 2 Msymbol/s). SEOSat's X-band symbol rate is 35 Msymbol/s (280 Mbit/s with 4D-8PSK-TCM modulation), so power flux density requirements could not be met during a continuous transmission of Idle TM TFs. The study 22) confirms that the ECSS standard polynomial does not provide enough randomness for long sequences of Idle TFs, resulting in power spectral density spurious above ITU recommendations. To improve randomness, DCU-X implements a configurable pseudo-randomizer (order 32 polynomial) for the Idle TM TF data field, in addition to the CCSDS standard randomizer (order 8 polynomial) for the RS codeblock. This second pseudo-randomizer can be enabled and disabled by command. Both randomizers are shown in Figure 9.
Figure 9: X-band TM pseudo-randomization implementation (image credit: EADS Astrium Crisa)
Comparison with Draft CCSDS SDLS Recommended Standard:
The first CCSDS SDLS protocol draft recommended standard (Ref. 21) was issued in February 2012. It aims at providing a common generic framework for TC, TM and AOS (Advanced Orbiting System) links, by defining additional protocol data unit fields and services. As opposed to it, the SEOSat implementation is ad-hoc and aims at inserting a low-overhead and low-impact Security Layer on the applicable standard protocol stack, and on the on-going system design. Both solutions place the Security Layer at the Transfer Sublayer, with some differences in the TF formats and security processing (encryption/authentication) which are shown in Figure 10.
Figure 10: CCSDS Transfer Frames (left) versus SEOSAT/Ingenio TM Transfer Frames (right), image credit: EADS Astrium Crisa
The CCSDS recommendation defines a common fixed-length Security Header (2-64 octets) and Security Trailer (8-64 octets), which are inserted around the Data Field. The Security Trailer includes just the MAC ( Message Authentication Code), and is optional. For TM TFs, SEOSAT's Security Layer uses the existing Secondary Header and Insert Zone fields in the ECSS and AOS standard TFs, respectively, for inserting (by overwriting specific octets) the Security Header. This solution avoids any overhead and allows keeping the TM TF format generated by the OBC and PDHU. It must be noted that the ECF (Error Control Field) is not used; otherwise it would need to be updated accordingly. The whole TF is error protected by the RS channel encoding (code (255,223) for TM and (255,239) for AOS). SEOSAT does not implement the TM TF authentication, whereas the CCSDS recommendation is to implement authenticated encryption.
Sensor complement: (PP, CSP)
PP (Primary Payload):
The prime contract for the PP instrument was awarded to Sener Ingeniería y Sistemas S. A. (Spain) in November 2008. The primary payload is a pushbroom imager composed of an optical panchromatic (PAN) and multispectral (MS) imaging detector in VNIR (Visible Near Infrared). SENER is the prime contractor for the PP (Primary Payload). The PP industrial consortium is integrated as well by Thales Alenia Space-España, it is responsible for the electronics, and INTA is responsible for the instrument AIV (Assembly, Integration and Verification) activities and for the optical support studies, including the straylight analysis. 23) 24)
PP is based on the following configuration: 25)
• A PAN channel
• Four MS bands in red, green, blue and near-infrared (NIR)
• The GSD corresponds to 2.5 m for the PAN band and 10 m for the MS bands.
• The swath width is ~55 km min (60 km x 60 km image cut) ensuring a repeat cycle of 38 days and a target accessibility of < 3 days on a global scale.
The main driver specification has been the FOV (Field of View). The instrument has a minimum linear swath specified of 55 km, that ensures a frequent revisit period. In addition, there is an emergency mode, providing quick accessibility to any point on Earth by viewing roll angles up to ±35º.
The optical quality has been specified at PP level, before post-processing. The minimum value required for the MTF (Modular Transfer Function) figure of merit is 0.115 for the PAN channel, and 0.3 for the MS channels, at their Nyquist frequencies. This relatively high MTF requirement is a mayor driving requirement of the instrument, and in fact, the most dimensioning parameter to resolve the EP (Entrance Pupil) diameter of the instrument. The maximum distortion required is 2% at the edge of the FOV.
The PP concept is composed of two identical cameras (barrel-mounted), each one covering half of the specified PP swath. The instrument works in pushbroom mode, covering a swath of some 28km in both PAN and MS channels, acquiring images simultaneously, and building the overall instrument swath of 55 km. This leads to a FOV for each camera slightly below 3º. There is a small overlap between the cameras in order to guarantee the full coverage and post processing capabilities, the total FOV of the instrument is ~ 4.75º. The pupil size of each camera is circa 260 mm and the focal length some 3.57m, providing an f/13.7.
Each camera is composed of a Korsch-type TMA telescope (Figure 11), with its optical elements and baffles, a focal plane, including the sensing devices and filters, an electronics chain, two thermal refocusing devices integrated in M2, for a system refocusing on orbit, and a thermal control subsystem. The PP has therefore two telescopes, two focal planes, two electronic chains, two thermal refocusing devices and a thermal control box, all assembled together in high stability structure which provide the necessary thermoelastic performances and stiffness.
Figure 11: Configuration of the PP instrument (image credit: Sener)
Each FPA (Focal Plane Assembly) includes a coplanar image plane, where four detectors are fitted. Two of them are PAN detectors, and the other two are MS detectors.
The PAN detectors have 6000 pixels with a pixel pitch of 13 µm and an integration time of 0.366 ms. The TDI (Time Delay and Integration) operating concept is implemented with a tunable number of stages to improve the SNR (Signal-to-Noise Ratio). The MS detectors have 1500 pixels with a pixel size of 52 µm and an integration time of 1.464 ms; they are composed of four linear arrays, each one corresponding to one spectral band: Blue, Green, Red and NIR.
The two cameras are identical, and are fully decoupled functionally, mechanically, thermally and electromagnetically in order to achieve full qualification of the overall Primary Payload , assemble with a single EQM (Engineering Qualification Model) of one camera and a structural dummy of the other.
Figure 12: Photo of the EQM FPAs with the PAN detectors and filters (left) and the MS detectors and filters (right), image credit: Sener
The detectors, manufactured by E2V, have large pixel's size which implies large focal planes, but they guarantee high values for SNR and MTF, two of the most restringing specifications.
Optical design: The Korsch optical system uses three powered anastigmatic mirrors plus one flat FM (Folding Mirror) used to extract the focal plane and to compact the whole structure. It works on-axis in pupil and field. This design embodies a large Cassegrain focal field and a comparatively small EP (Entrance Pupil).
Some significant benefits of this telescope design are:
• Capability of operating with focal planes of large dimensions.
• Efficient and compact stray-light baffling, due to the presence of an intermediate image, and the possibility of placing a Lyot stop at the EP of the system, which in this design lies close to the FM.
• Pure catoptrical system (but for the marginal effect of filter and detector windows), and hence free of chromatic aberrations, which was a priori a serious concern given the large spectral ranges of operation of the instrument. Also, the instrument will not suffer typical glass ageing effects due to incoming gamma-ray radiation.
• On-axis M3 (tertiary mirror), in contrast to other solutions based on the canonic Korsch design. This feature provides noticeable advantages in terms of lower manufacturing and alignment complexities. Also, it enables the use of a small FM without central hole.
• Focal length of the primary-secondary group of mirrors close to the effective focal length of the complete system. This enables the use of a M3 with magnification close to unity, making this element relatively tolerant to misalignment.
Figure 13: Telescope set-up and optical ray-tracing (image credit: SAGEM - REOSC)
The system is designed with four all-reflective mirrors, enabling a spanning from blue to near infrared without chromatic aberration. M1 is the primary concave aspheric mirror. M2 is the secondary aspheric and convex mirror. M1 & M2 is a stigmatic telescope at the center of field. M3 is the tertiary and concave mirror, a field corrector. MF is the folding mirror.
• The primary mirror (M1) is the aperture stop of the telescope, and its diameter (260 mm) was derived from SNR and MTF specifications to optimize the optical system. This allows the reduction of diffraction effects and maximizes the SNR of the telescope.
• The secondary mirror (M2) is a hyperbolic Cassegrain's conjugate of the nearly parabolic M1. It creates an intermediate image at a plane slightly inside M1 mirror. This naturally creates a field diaphragm when machining a hole in M1 to allow light passage, which is commonly smaller than the M2 obscuration. The huge FOV for a Korsch design (about 3º for each telescope) makes that in SEOSAT/Ingenio design, image forming light beams exceed the M2 obscuration, forcing to machine four holes (slots) into M1 to allow the light passage. These slots complicated the straylight control, and provide SEOSAT/Ingenio with a particular look.
• The tertiary mirror (M3) is a field corrector with a magnification close to 1, which re-images the intermediate image in the focal plane after the deviation of the light beam in the FM. The unusual large size of this mirror is due to the large focal plane (and FOV) of the telescope. The FM is placed close to the EP of the system to minimize the FM size and obscuration, allowing the access to the Lyot stop in the EP itself. This Lyot stop plays a crucial role in the straylight control.
SAGEM-REOSC (France) has been awarded a contract by SENER to manufacture and test the component mirrors of the PP twin telescopes. The project involved manufacturing challenges to meet surface quality, light-weighting and vibration load requirements (lightweighting up to 72% with random acceleration up to 88.1g). SAGEM applied enhanced manufacturing technologies for the implementation of the mirror assemblies.
Figure 14: Longitudinal view of the PP telescope (image credit: Sener)
Figure 15: Filter profile of the FPA (image credit: Sener)
Figure 16: Architecture of the PP instrument (image credit: Sener)
Table 2: Parameters of the PP instrument
Figure 17: Alternate view of the PP instrument with elements defined (image credit: Sener)
Figure 18: Block diagram of the PP instrument (image credit: Sener)
CSP (Complementary Scientific Payload):
The CSP consists of three small and medium size instruments:
• SENSOSOL (Sensor solar de última generación que determina la posición del Sol para la orientación del satélite - A new generation solar sensor for getting the satellite orientation with respect to the Sun position). SENSOSOL has been developed by the University of Sevilla and the UPC (Universitat Politècnica de Cataluña), Barcelona, Spain. INTA has performed the space qualification on the NanoSat-1B mission. 26)
• TTT (The Two Towers) of INTA: An in-orbit dosimeter and radiation spectrometer hosting different sensors (solid-state).
• UVAS (Ultraviolet Visible and near infrared Atmospheric Sounder), PI: Alfonso Saiz-Lopezof CSIC (Consejo Superior de Investigaciones Cientificas) or the Spanish National Research Council, located in Madrid. UVAS is a CCD-based grating spectrometer of CSIC (Consejo Superior de Investigaciones Cientificas). The objectives of the UVAS instrument are to observe atmospheric profiles.
Figure 19: CSP EM/EQM models: Sensosol (left), TTT (center), and UVAS (right), image credit: University of Seville, INTA, CSIC
UVAS (Ultraviolet Visible and near infrared Atmospheric Sounder)
UVAS is conceived to gather global high spatial resolution maps of columns of density of trace gases and aerosols related to air quality and climate change. The instrument focuses on the processes that link atmospheric composition, sources and climate. These processes are of fundamental relevance, and it is important that the key components of the coupled Earth-atmosphere system be further understood, so that uncertainty in prediction of future climate scenarios can be minimized. It is equally important to provide platforms that measure the budgets of gases from anthropogenic emissions into the atmosphere in order to monitor compliance with international agreements. The fundamental objective of the mission is the measurement of atmospheric composition, with high precision and covering the whole height of the atmosphere, to study the feedback between the main anthropogenic gases, their sources, formation and loss mechanisms, and the Earth's climate processes. 27)
The main scientific objectives of the UVAS mission are:
1) Observe with unprecedented high spatial resolution the concentration of air quality monitoring gases including ozone (O3, nitrogen dioxide (NO2), sulphur dioxide (SO2), formaldehyde (HCHO) and glyoxal (CHO-CHO) and aerosols over selected urban areas.
2) Combine high spatial resolution observations with atmospheric models to better quantify sources and sinks of specific gases.
3) Assimilate the remote sensing measurements into global chemistry-climate models to examine the processes linking atmospheric composition and climate.
UVAS instrument: UVAS is a whiskbroom high-resolution spectrometer with two optical modules for its spectral range: 290-490 nm (UV/VIS). These modules are single-point spectrometers (i.e. with linear array sensors) that can be fed by either the calibration module or the Nadir pointing module.
The use of a whiskbroom system imposes some limits to the achievable SNR and swath, employing single-point spectrometers, but on the other hand, it minimizes the instrument budgets (envelope, mass and power) and simplifies the overall instrument design, especially the optical coupling between the instrument telescope and the imaging spectrometers.
The UVAS core consists of two miniaturized spectrometers modules for the Ultraviolet (290-390 nm), and Visible (390-490 nm) spectral ranges. The instrument is structured in the following modules:
• Optomechanical assembly (telescope assembly): comprising all mechanisms and optoelectronic elements that capture, guide and convert the input signal (light) to an electronic signal. This module can be further divided into:
- Two spectral modules (spectrometers) for the UV and VIS spectral ranges, including separated fore-optics, refractive, for optimal image formation and the spectrometer modules. These modules include the sensor arrays and the analog proximity electronics.
- Scanning mirror, to sequentially guide the light coming from different points within the field of view into the instrument. The scanning mirror provides the on-ground spatial resolution of the instrument.
- Zoom mechanism, to provide a higher spatial resolution in the instrument zoom mode. The zoom will provide the capability to change the two basic observation modes: Nominal mode and Zoom mode.
- Nadir shutter, to protect the system from contamination, to perform dark current calibration and to protect the spectrometers and optics from large Sun pointing. The nadir shutter is open during target observations and closed for calibrations.
- Beam distribution module, to guide the beam into the instrument, depolarize the light and separate the suitable spectral bands for each spectral module.
- Calibration module, comprising a shutter for the Sun port, calibration LEDs and lamps, a diffuser and image-forming optics.
- Thermal elements, to provide the adequate thermal environment.
Figure 20: Telescope assembly components (image credit: CSIC)
• Electronic assembly: to provide power conditioning, data processing, optomechanical control and the general instrument management.
• Intra-instrument harness for the interconnection of these assemblies.
Table 3: Main scanning parameters of the UVAS instrument
Figure 21: Schematic view of the UVAS elements (image credit: CSIC)
UVAS will fly on board the SEOSAT satellite oriented into the flight direction and will observe solar radiation back-scattered and reflected from Earth's atmosphere and surface in nadir-viewing (downward looking) geometry. The scanning mirror will scan a swath towards the Earth's surface perpendicularly to the satellite flight direction. The light will be directed through the beam distribution module into the spectrometers; their tasks are to process the light to finally deliver, by means of the system electronics and retrieval algorithms, the atmospheric trace gases concentrations.
In the nominal operations mode, UVAS will perform high spatial resolution measurements with an on-ground pixel size lower than 10 km x 10 km. This spatial resolution exceeds those of any current NASA or ESA instrument, and will be the main advantage and contribution of UVAS to the Atmospheric Remote Sensing field.
UVAS features three in-flight calibration modes:
- Sun calibration: Consists of the acquisition of the Sun spectrum through the Sun port. This operation will be performed at the beginning of a SEOSat orbit, just after exiting the eclipse zone. The calibration shall be used for the absolute radiometric calibration during flight, therefore the sun will be observed via sun port and a diffuser plate.
- Wavelength calibration: Will be performed by means of selected athermal, narrow band fiber-optic LED signals as provided by the UVAS calibration module. This operation will be performed while the Sun & nadir port shutters are closed (dark signal cycle) preferably during eclipse and daily before sun calibration.
- Dark current calibration: A dark signal will be used to check the spectrometer module background dark signal using the same approach as for the wavelength calibration (dark signal cycle) preferably during eclipse. This operation will be performed once per orbit, depending on instrument performances.
Scientific objectives and applications: The UVAS mission will focus on the processes that link atmospheric composition, sources and climate. These processes are of fundamental relevance, and it is important that the key components of the coupled Earth-atmosphere system be further understood, so that uncertainty in prediction of future climate scenarios can be minimized. It is equally important to provide platforms that measure the budgets of gases from anthropogenic emissions into the atmosphere in order to monitor compliance with international agreements. - The primary goal of the mission is the measurement of atmospheric composition to study the feedback between the main anthropogenic gases, their sources, formation and loss mechanisms, and the Earth's climate processes.
The UVAS scientific team comprises researchers from CSIC, INTA and several Spanish Universities, as well as researchers from international scientific institutions like the Harvard-Smithsonian Center for Astrophysics and NASA. The UVAS PI (Principal Investigator) is Alfonso Saiz-López of CSIC.
The UVAS industrial cluster is composed by: CSIC (PM; PA; QA; SE), INTA (thermal and optical designs), EMXYS (electronics, HW and SW), AVS (mechanisms) and AST (mechanical design).
UVAS instrument location and pointing direction on the spacecraft: It has an angle deviation of 30 degrees as a result of a need for the instrument to adapt to the shape and other instruments on the platform ( UVAS is not a dedicated instrument but a secondary scientific payload, hence subject to adaptability and fulfillment with the satellite and prime instrument and mission requirements). This angle deviation is known and approved by ESA and CDTI and the only implication is a change in the shape of the on-ground pixel, which is accounted for by on-ground software processing, but has NO effect at all on the instrument performance and/or scientific objectives of UVAS.
The Ingenio ground segment will be in Spain for the payload data acquisition, processing and dissemination to the users' community of the Ingenio products. Indra Systemas S.A. is the prime contractor of the Ingenio and the PAZ ground segment with contracts from INTA (PAZ) and ESA (Ingenio) to enable maximum commonality. 28) 29)
4) FOS (Flight Operation System), responsible for spacecraft control, spacecraft health monitoring, orbit control and on-board software configuration and maintenance
- The SMCC (Spacecraft Mission Control Center) is located in Torrejón de Ardoz near Madrid. The SMCC includes the nominal FOS and PDGS, a X-band downlink station and a S-band TT&C station.
- The SBUCC (Spacecraft Backup Control Center) is located in Maspalomas on the Canary Islands serving also as a complementary X-band acquisition station.
- In addition, the ground segment will receive data from polar acquisition stations to improve the daily coverage of the system.
5) PDGS (Payload Data Ground Segment), responsible for optical, science and ancillary data acquisition, processing, archiving and distribution, as well as mission planning. The temporary archive will be in Maspalomas, while the processing and long-term archive will be located in Torrejón.
Figure 22: Ground system architecture of SEOSat/Ingenio (image credit: Indra)
Figure 23: Illustration of ground stations for data acquisition (image credit: CDTI, Indra, Ref. 12)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).