Space Qualified Frequency Sources (Clocks) for GNSS Applications: 2012 Update Leo A. Mallette
Affiliated with The Aerospace Corporation El Segundo CA, USA [email protected]
SpectraTime Inc. Neuchatel / Switzerland [email protected]
Ganghua Mei Joe White Space Applications Branch U.S. Naval Research Laboratory Washington, DC USA [email protected]
Abstract—Accurate, stable, and reliable frequency reference sources are critical for communication, navigation, and scientific space applications. Several levels of frequency references are suitable for space applications. This paper discusses similarities and differences among single distributed oscillators for individual units, master oscillator groups for communications systems, and atomic clocks for military and navigation systems. This paper builds on an earlier paper  and broadly describes frequency sources on current and upcoming global navigation satellite systems (GNSS). The systems are the Global Navigation Satellite System (GLONASS), the Global Positioning System (GPS), the Galileo system, the BeiDou/Compass satellite positioning system, the Quasi-Zenith Satellite System (QZSS), and India’s Regional Navigation Satellite System (IRNSS). Keywords – Atomic Clock, BeiDou, Compass, Galileo, GLONASS, GNSS, GPS, IRNSS, QZSS, Scientific Missions
Accurate and stable frequency reference sources are critical for space applications. The first part of this paper describes frequency sources that are suitable for space, including single distributed oscillators, master oscillator groups, and atomic clocks. The second half of this paper builds on a 2007 paper on atomic frequency standards for navigation systems . II.
COMMUNICATIONS SATELLITES, NAVIGATION SATELLITES, AND SCIENTIFIC MISSIONS
Each piece of flight hardware (structure, antenna, solar panel, batteries, propulsion lines, etc.) has requirements that are specific to the hardware function. The generic requirements include size, weight/mass, power consumption (SWAP), thermal density, and reliability. Examples of
Key Laboratory of Atomic Frequency Standards, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences Wuhan 430071, Hubei, China [email protected]
specific requirements might include axial ratio, capacity, dead band, depth of discharge, gain, group delay, connector type, noise figure, output power, pointing accuracy, thermal density, and throughput. Accurate and stable frequency reference sources are critical for commercial, navigation, military, and scientific space applications and the key requirements are phase noise and stability. Crystal oscillator phase noise is often the driver because of the high frequencies used in communication satellite systems. Phase noise in lowfrequency oscillators is increased when the signal is multiplied to higher frequencies – phase noise sidebands are enhanced by 20log(N) when a signal passes through a multiplier of factor N , and N can easily be in the hundreds. All electronic and mechanical hardware will display some sensitivity to many of the space environments. Tests are performed on all flight hardware, including frequency sources, to verify they will survive, and continue to operate, in the environments expected in space. The limits for these tests will depend on the type of launch vehicle (e.g., vibration), the mission duration (life testing), the other units on the satellite (EMI/EMC) and the type of orbit (temperature and radiation). The test environments are for clocks (especially atomic clocks) are described more detail in . III.
The basic crystal oscillator (abbreviated XO) is diagrammed in Fig. 1.
Quartz Crystal Oscillator
The disadvantages are that they are not redundant, require a regulated power supply, have minimal or no telemetry and command capability, and are not coherent with each other when placed in various locations on the satellite. IV.
Figure 1. Crystal Oscillator (XO) Block Diagram. Used with Permission . The advantages of crystal oscillators can be made with a crystal resonator and a handful of components. They are:
small in size: two cubic inches, or less;
inexpensive: a few percent of the cost of a master oscillator group;
low mass: as low as a few ounces;
low power consumption: 200 mA for an XO; 2 to3 watts for an oven controlled crystal oscillators (OCXOs); and
MASTER OSCILLATOR GROUPS
A master oscillator (MO) is a crystal oscillator that is used as the source for most or all frequency references on the satellite. As the name implies, a master oscillator group (MOG) is a group of master oscillators and includes the circuitry needed to support multiple oscillators and amplifiers. The crystal oscillators are usually OCXOs. A MOG centralizes multiple crystal oscillators, and has higher output power, a distribution network with multiple outputs, a power supply, and switching for redundancy in one box or unit. An MOG may have frequency update capability, extensive Telemetry and Command (T&C) capability, and may require special isolation for a hot backup. A typical block diagram is shown in Fig. 2. One possible disadvantage is the need to have coaxial cables to distribute the signal throughout the spacecraft.
can be installed inside of a host unit such as a transmitter, receiver, or a reference frequency generator.
Figure 2. Possible MOG Block Diagram. Used with Permission 
ATOMIC FREQUENCY STANDARDS
Space qualified atomic frequency standards (AFS) are used primarily in navigation satellites but are also found in specialized communication satellites and scientific missions. AFSs have lower drift rate and inherent insensitivity to radiation when compared to conventional crystal oscillators. This makes them preferred for specialized space applications, such as navigation, precise timing, survivability, and for autonomous operation. AFSs use a voltage controlled quartz crystal oscillator (VCXO) that is frequency locked to an atomic resonator (usually called a physics package) as shown in Fig. 3. Space physics packages are controlled by the atomic transitions of either rubidium, cesium, or hydrogen; and mercury is being developed for flight.
The Russian Global Navigation Satellite System (GLONASS or global'naya navigatsionnay sputnikovaya sistema or Uragan) consisted of the full constellation of 24 satellites in three orbital planes in 1996, dropped to a low of seven, and has completed its long trek back to full operational capability with 24 operational satellites in the constellation last December, but Russia intends to keep pushing ahead with its GNSS, said Roscosmos official Sergey Revnivykh at the Munich Satellite Navigation Summit in March [with] plans to eventually expend the constellation to 30 SVs. . The GLONASS system was developed in blocks as described below. A. Block I.
Feedback Atomic Resonator
Quartz Crystal Oscillator
5 MHz Output
The 11 satellites in this pre-operational phase (1982 to 1985) had a design life of one year. The first launch of GLONASS I was on Cosmos 1414 on October 12, 1982. These satellites used the "BERYL" Rubidium AFSs .
B. Block IIa/b/c. Figure 3. Atomic Frequency Standard Block Diagram. Used with Permission  The value of atomic frequency standards (AFS) in space navigation systems is immense. In a 2006 issue of GPS World, Richard Langley stated: Ever since the birth of the marine chronometer, improvements in positioning accuracy have been tied to improvements in clock accuracy. Today we have clocks based on atomic phenomena with extraordinary accuracies. And GPS couldn't exist without its atomic clocks — both those carried by the satellites and those used at the system's monitoring stations.  A general introduction to AFSs can be found in , where the author describes molecular and atomic beam methods, buffered gas cell resonance devices, and masers. Similarly,  provides an excellent introduction to satellite navigation. The previous sections discussed crystal oscillators, master oscillator groups, and atomic frequency standards. The next sections discuss the AFSs that are used in the current and future global navigation satellite systems.
This Block of satellites began launching in December 1985 and had a design life of one year. The GEM frequency standards  were introduced with the six satellites in this Block and used on Blocks IIb and IIc. These cesium AFSs had a stability of 5x10-13 per day . The 12 satellites in Block IIb had a design life of two years and were launched in 1987 and 1988. There were two launch failures and only six of the 12 satellites reached the desired orbit. Block IIc satellites were launched starting in September 1988 and had a design life of three years. The GLONASS constellation was completed on January 18, 1996. The Beryl and GEM AFSs were built by the Russian Institute of Radionavigation and Time . C. GLONASS-M. The 14 modernized satellites (GLONASS-M, also called Uragan-M), weigh 1480 kg, have design lives of 7 years, and have added an L2 civil signal. The Uragan-M satellites have three cesium AFSs developed by the Institute of Radionavigation and Time [11, 12]. They are expected to achieve the one-day stability of 1x10-13  by maintaining the clocks with “temperature stability to +/-1 degree C” [14, p. 42]. The first GLONASS-M satellite was launched in 2003.
D. GLONASS-K (K1, K2, and KM). The unpressurized, lighter-weight (750 kg), longer life (10 years), GLONASS-K (sometimes called K1) satellites will add CDMA, an L3 civil signal, and have cesium AFSs. The clock stability has been quoted as low as 1x10-14 . The first GLONASS-K was launched in February 2011 and the second spacecraft will be launched later in 2012 . Next generation (GLONASS-K2 and -KM) will add additional signals and are planned for later this decade. VII. GLOBAL POSITIONING SYSTEM (GPS) The 24 satellites in the GPS constellation, which was completed in March 1994, are in six orbital planes. The circular 20,200 km orbits have an inclination of 55 degrees. The GPS constellation was developed in blocks as described in this section. A. GPS Block I. The ten Block I satellites were numbered 1 through 11 (7 was a launch failure) in this developmental/system-testing phase and were built by Rockwell International (now Boeing) in Seal Beach California. These satellites were designed for a 4-year life and were launched from Vandenberg AFB, beginning in February 1978. The first 3 GPS Block I satellites were originally known as Navigation Development Satellites, and were launched with three rubidium AFSs on board. These rubidium AFSs consisted of physics packages from Efratom (now Symmetricom), Voltage Controlled Crystal Oscillators (VCXO) from Frequency Electronics, Inc. (FEI) and the integrating electronics was designed and manufactured by Rockwell Autonetics Division (now Boeing). The apparent low reliability of early Rubidium AFSs together with the better overall performance of Cesium AFSs led to flying three Rubidium AFSs and one Cesium AFS in the remaining GPS Block I satellites. The cesium AFS were built by Frequency and Time Systems (now Symmetricom). The first GPS satellite to fly a Cesium AFS was NAVSTAR-4. Further work on the early Rubidium AFS problems resulted in numerous design improvements. One of the significant design improvements was the addition of a temperature controller to the base plate of the Rubidium AFSs to maintain the Rubidium AFS at essentially constant temperature during satellite temperature fluctuations. The temperature controller was used on NAVSTAR 7 and all subsequent satellites. GPS 12 was the qualification vehicle and was not flown. B. GPS Block II/IIA. The 28 Block II/IIA satellites were numbered 13 through 40. They weighed 840/930 kg, were built by Rockwell International (now Boeing) in Seal Beach California, were designed for a 7.5 year life, and were launched beginning in February 1989 . Each GPS Block II/IIA satellites was
flown with two rubidium AFSs and two cesium AFSs, The Rubidium AFSs were the same as Block I. The Cesium standards were built by three companies; the primary supplier for the vast majority of the cesium AFSs was FTS (now Symmetricom). Second source cesium AFSs were produced by Frequency Electronics and Kernco, Inc. The FEI clocks were launched on space vehicles 31 and 32. The Kernco clocks were launched on space vehicles 29, 30, and 34 . For this 2012 update, eleven Block IIA vehicles remain in the active constellation . C. GPS Block IIR and IIR-M The 24 Block IIR (R for replenishment) satellites were built by Lockheed Martin. These satellites were designed for a 10year life and the first GPS IIR satellite was launched in 1997. The GPS IIR satellite uses three rubidium AFSs built by EG&G (now PerkinElmer) . They have excellent reliability and long lifetimes . The last eight of the GPS IIR satellites were modified from the original IIR design to include modernized GPS features. These satellites were designated block IIR-M and added the L5 civil signal, the new military (or M) code and increased output power. At the time of launch these capabilities were not fully realized due to lack of capability in the GPS control segment. The final GPS IIR-M launch was August 17, 2009. All of the Block IIR and IIRM satellites are in the active constellation as of this update with the exception of SVN 42 which was a failed launch. D. Modernization. The GPS block IIR-M (M for modernized) satellites added the L5 downlink signal and increase output power for more robust signals, but modernization did not affect the number AFSs launched. The last eight of the GPS IIR satellites and all of the GPS IIF satellites are modernized. E. GPS Block IIF. The contract for the 12 GPS IIF (F for future) satellites was awarded to Rockwell International (now Boeing) in 1996, and the satellites are expected to launch with two rubidium AFSs and one cesium AFS. The Rubidium AFS is built by PerkinElmer and has better performance than the GPS IIR Rubidium AFS because of the improved physics package design . The cesium AFS is built by Symmetricom and has improved performance compared to the GPS-II/IIA clocks because of design concepts previously demonstrated in commercial digital cesium clocks to reduce environmental sensitivities and to improve manufacturability. Improvement in the cesium beam tube technology from GPS-IIA to GPS-IIF also resulted in improved short term stability and life time. SVN 62, the first Block IIF satellite was launched 28 May, 2010 and SVN 63 was launched 16 July 2011. NRL has reported data on the SVN 62 clocks, figures 4 and 5 comparing the measured on-orbit performance with stability data collected as a part of the life tests of the IIF clocks being conducted at NRL . The instabilities visible in the SVN62 Rb data were
prominent early in the life of the vehicle but have since been reduced with additional operating time. These are not likely to be due to the clock itself. The General Accounting Office reported that "Approximately one month after they were enabled, the second IIF satellite’s Cesium clock—one of three atomic frequency standard clocks onboard that provide GPS accuracy through redundancy—failed. An investigation identified design and manufacturing issues …” .
PerkinElmer will supply rubidium clocks similar to those flown on GPS IIR and IIF . Additionally, Frequency Electronics is developing a space qualified rubidium  and the Jet Propulsion Laboratory is investigating a Mercury Atomic Frequency Standard (MAFS) which is a mercury ion storage clock for future GPS use . GPS related efforts have also been started at the National Institute of Standards and Technology (NIST)  for technology leading to a cold atom space clock and at the United States Naval Observatory (USNO) for an optical clock for space. The first GPS Block III launch is anticipated to be in 2014 . VIII. GALILEO Galileo is a joint initiative of the European Commission and the European Space Agency (ESA) for a state-of-the-art global navigation satellite system, providing a highly accurate and guaranteed global positioning service under civilian control. It will be inter-operable with GPS, GLONASS, BeiDou and IRNSS. The fully deployed Galileo system consists of 30 satellites (27 operational and three active spares). They will be stationed in three circular medium earth orbits (MEO), at an altitude of 23,222 km, with an inclination of 56 degrees to the equator .
Figure 4. SVN 62 Rb27 clock. Comparing the measured onorbit performance with stability data collected as a part of the life tests of the IIF clocks being conducted at NRL .
Figure 5. SVN 62 Cs1010 clock. Comparing the measured onorbit performance with stability data collected as a part of the life tests of the IIF clocks being conducted at NRL . F. GPS III. The GPS III program is the next generation of GPS satellites and control segment. The prime satellite contractor is LockheedMartin with ITT as the subcontractor for the payload.
Atomic clocks have been recognized as critical equipment for the satellite navigation system. The Rubidium Atomic Frequency Standard (Rubidium AFS) and Passive Hydrogen Maser (PHM) are at present the baseline clock technologies for the Galileo navigation payload. All Galileo satellites are planned to be launched with two Rubidium AFSs and two PHMs. The need for dual-technology for the on-board clocks is dictated by the need to insure a sufficient degree of reliability (technology diversity) and to comply with the Galileo lifetime requirement of 12 years. Both developments are based on early studies performed at the Observatory of Neuchâtel from the end of the 1980s and SpectraTime (formerly Temex Neuchâtel Time) since 1995. These studies have been continuously supported by Switzerland within ESA technological programs especially since the European GNSS2 program was formed . The activities related to Galileo System Test Bed (GSTBV2) experimental satellites as well as the implementation of the In Orbit Validation phase are still in progress. An experimental satellite (Galileo In-Orbit Validation Element, GIOVE), GIOVE A, was launched at the end of 2005 and a second satellite (GIOVE B) was launched in April 2008 to secure the Galileo frequency filings. Additionally, the functions of the two GIOVE satellites were to test critical technologies such as the atomic clocks, to perform experimentation on Galileo signals, and to characterize the MEO environment. GIOVE A was launched in December 2005 with two Rubidium AFSs supplied by SpectraTime. The first signals from GIOVE A were transmitted on January 12, 2006  and a review of the performance was
reported on March 2, 2007 the European Space Research and Technology Centre (ESTEC). During the successful commissioning phase, the two onboard rubidium AFSs were tested. The first Rubidium AFS was used for the first 14 months of GIOVE A operations and showed sub-meter positioning capability by providing clock model updates every 100 minutes or so. A few small frequency jumps have been detected and reported - contributing to a position error of up to two meters. Analyses have been completed and improvements have been incorporated into the Rubidium clocks for the next four IOV (In Orbit Validation) satellites. The GIOVE A navigation signals measurements and clock data can be found on the ESA web site . The first signals from GIOVE B were transmitted on May 7, 2008. One PHM and two Rubidium AFS on board the satellite were supplied by Selex Galileo & SpectraTime. The PHM was designed to improve the Galileo system autonomy by a factor of 10 and to provide greatly improved accuracy. This Passive Hydrogen Maser Clock’s stability was estimated and presented at EFTF 2009 . No specific changes have been implemented for the next IOV Passive Hydrogen Masers since long term test data have not shown any limitation of lifetime or degradation of performance . On 21 October 2011 a Soyuz rocket from French Guiana launched first two IOV satellites, with two to follow in 2012: these four Galileo In-Orbit Validation (IOV) satellites represent the operational nucleus of the full 30-satellite constellation . A further PHM technological program is on-going to reduce the mass of this instrument to 12 kg without reduction of performance. To achieve this goal, the physics package weight realized by SpectraTime is 8 kg and demonstrated very similar performance. SpectraTime with other institutional partners and financial support from the Swiss Space Office will start investigating a mercury ion storage clock for future Galileo missions. IX.
QUASI-ZENITH SATELLITE SYSTEM (QZSS)
The quasi-zenith satellite system (QZSS) is a GPS based augmentation system for Japan. The first satellite (MICHIBIKI or QZS-1) was launched in September 2010 with a rubidium AFS and its performance is reviewed in reference . The four  satellites will be in periodic highly elliptical orbits (HEO) and were originally expected to use hydrogen masers and rubidium and cesium AFSs . The hydrogen maser was being developed by the National institute of Information and Communications Technology (NICT) in collaboration with Anritsu Corporation [36, 37]. Future QZSS satellites may use a remote synchronization system for the onboard crystal oscillator (RESSOX) – with an AFS on the ground and crystal oscillators on the satellite(s). Experiments have shown that synchronization within 10 nS to be realizable . See also the discussion on the Engineering Test Satellite VIII (ETS-VIII) in the Scientific Missions section.
China’s navigation satellite system BeiDou (a.k.a COMPASS) was developed according to a three step strategy . The first step was to start constructing the BeiDou Navigation Demonstration System in 1994, and to provide radio determination satellite service (RDSS) in 2000 through the system. The second step was to start constructing the BeiDou Navigation Satellite System in 2004, and to provide radio navigation satellite service (RNSS) for users in China and nearby regions in 2012. The third step involves functional enhancement of the BeiDou Navigation Satellite System to provide high precision RNSS service for the world-wide users in 2020. The demonstration system is designed based on the active bidirectional distance measurement, therefore only two geosynchronous satellites are needed in principle, and the system could provide users with their horizontal position. Two geostationary satellites were launched in 2000, and two others were launched in 2003 and 2004 respectively. Establishment of the BeiDou Navigation Demonstration System provides China with satellite navigation technology in a short time and at a low cost. The BeiDou Navigation Satellite System launched its first satellite in April 2007. From then to December 2011, 10 satellites were launched, including one medium earth orbit (MEO), four geostationary earth orbit (GEO) and five inclined geosynchronous orbit (IGSO) satellites. On 27 December 2011, the Chinese government declared preliminary service of the BeiDou Navigation Satellite System . The service covers most of the area from 84 to 160 in east longitude, and from 55 south latitude to 55north latitude. Currently the positioning precision of the system is 25m in horizontal and 30m in height, the velocity measurement precision is 0.4m/s, and the timing precision is 50ns. Six other satellites will be launched in 2012 (the first of them was launched in February 2012), afterwards the system will have positioning precision of 10m, and the service will cover nearly the whole Asia Pacific region, achieving the goal of the second step of construction of the BeiDou system. Now the third step of construction of the BeiDou system is beginning, and, during this step, more than 30 satellites will be launched. The BeiDou Navigation Satellite System uses design concept similar to that of GPS, GLONASS and Galileo, but incorporates the function of short message communications of the BeiDou Navigation Demonstration System. Research of space rubidium clocks in China for satellite navigation application began in the late 1990’s. At the time of construction of the BeiDou Navigation Demonstration System, the space clock technology in China was not as mature and Chinese clocks were not used in the system. In 2000 several groups were involved in the research, including experts come from CAS (Chinese Academy of Sciences), CASC (China Aerospace Science and technology Corporation), CASIC (China Aerospace Science and Industry Corporation) and Peking University. A milestone advance was achieved in 2006, when all the groups manufactured their space qualified
rubidium clock products. Two key performances of the space clock for satellite navigation application, i.e. the frequency stability per 104 seconds and the stability per day, were better than 310-13, meeting the requirement of the second step of construction of the BeiDou system to achieve 10m positioning precision. All the rubidium clocks contained in the first satellite of the BeiDou Navigation Satellite System launched in 2007 are China-made ones. Satellites in the system launched so far were equipped with products made by Chinese companies and by the Swiss company Temex Neuchâtel Time (now SpectraTime). Performance evaluation of the on-board clock’s behavior concluded that the China-made space rubidium clock matches the demands of BeiDou’s second step of construction either in performance or in reliability.
10.23 MHz navigation reference signals using a heterodyne technique and DDS based synthesizers . The IRNSS
WIPM (Wuhan Institute of Physics and Mathematics at the Chinese Academy of Sciences) is one of manufacturers for space rubidium clock production in China. WIPM started research of space rubidium clocks in 1997. Earlier work focused on realization of performance [41-43] and adaptability for space environment. A test model was built in 2005 and the products were space qualified in 2006. Since then, the clock performance has been improved further . WIPM’s space rubidium clocks were used in the first satellite and following ones of the BeiDou Navigation Satellite System. Statistics of nearly 10 products obtained on-ground showed that one-day frequency stability of the clocks is within 1.5~5.010-14 in Hadamard deviation, comparable with those of the rubidium clocks produced by EG&G (now Excelitas) for GPS Block IIR .
Previous papers have been written about atomic clocks in scientific and communications space missions [52, 53]. These included a hydrogen maser on Gravity Probe-A in 1976, cesium and rubidium clocks on NAVEX in 1985, rubidium clocks on Milstar/AEHF (1995 to present), two rubidium clocks on the Cassini/Huygens planetary mission to Saturn (launched on October 15, 1997, Huygens mission in 2004/5), two active masers and two rubidium clocks on the RadioAstron Project were launched in 2011, and a rubidium clock and two CSACs were placed on ISS in 2011. In order to present a broad view of atomic clocks in space, this section presents the most recent information about atomic clocks in scientific missions.
In order to meet the needs of the third step of the BeiDou system, future research work for space atomic clocks has been planned in China. The work involves further improvement of rubidium clocks and the development of atomic clocks with higher performances, including a passive hydrogen maser and others. XI.
The Indian Regional Navigation Satellite System (IRNSS) is being developed as an independent, indigenous satellite system and builds upon the Indian GPS overlay system known as GPS aided geo-augmented navigation (GAGAN). Gagan is a Hindi word of Saskrit origin for the sky . The GAGAN payloads use oven controlled crystal oscillators (not atomic clocks). The first payload was launched on the GSAT-8 communications satellite in May 2011  and an earlier payload on GSAT-4 failed to reach orbit in 2010. When completed, the IRNSS will have three satellites in geostationary orbit at 32.5°E, 83E, and 131.5°E and two in geosynchronous orbit with crossings at 55°E and 111.5°E with inclinations of 29°. There would be two spare satellites . The IRNSS payload will consist of three redundant atomic clocks . The redundant clock ensemble for IRNSS is manufactured by Astrium GmbH using SpectraTime AFS. The clock ensemble is providing adjustable phase & frequency
… will broadcast GNSS signals – BPSK and BOC (5,2) at L5 and S-band and…the signals will contain data to account for several sources of error, including a lesserknown source within a spacecraft itself: the hardware delay of the signal from its generation to radiation from the satellite antenna.  The first IRNSS satellite will be launched in 2012 . XII. ATOMIC CLOCKS FOR SCIENTIFIC MISSIONS
ETS-VIII The Engineering Test Satellite VIII (ETS-VIII or Kiku 8) was launched in 2006 into geostationary orbit. This satellite was developed by the Japan Aerospace Exploration Agency in cooperation with NICT and NTT Its mission included conducting precise time and frequency transfer experiments using cesium clock on ETS-VIII. The flight clocks are the “same as Cesium atomic clock in GPS in space” . RadioAstron Project The RadioAstron Project is designed by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Science, the Federal Space Agency, and international organizations. This space Radiotelescope is equipped with two active hydrogen masers (manufactured in Russia by Vremya-CH) and two rubidium clocks (manufactured in Switzerland by SpectraTime). The radiotelescope was launched from Baykonur, Kazakhstan on 18 July 2011 . The goal of the project is to carry out investigations of various types of astrophysical objects of the universe with an unprecedented high angular resolution in the centimeter and decimeter wavelength bands. CSAC-SPHERES ISS Two Chip-Scale Atomic Clocks (CSAC) and a ruggedized X72 rubidium clock to serve as the reference clock, were
launched on October 27, 2011 as part of the first program to utilize the enhanced port on the Synchronized Position Hold Engage Reorient Experimental Satellites (SPHERES) research facility on the International Space Station (ISS). The X72 rubidium clock is a standard Symmetricom product. The CSAC was developed at Symmetricom with support from DARPA. Aurora Flight Sciences and MIT´s Space Systems Lab developed the payload interface for the CSAC. The development and operation of the CSAC payload on the SPHERES facility is described in more detail in reference .
The accuracy of satellite based navigation systems is due to the atomic frequency standards (AFS), commonly known as atomic clocks. This paper described publicly available information about current and upcoming navigation systems with AFSs – the Russian Global Navigation Satellite System (GLONASS), the Global Positioning System (GPS), the Galileo system, China’s BeiDou/Compass satellite positioning system, Japan’s quasi-zenith satellite system (QZSS), and India’s regional navigation satellite system (IRNSS). REFERENCES
The ESA mission Atomic Clock Ensemble in Space (ACES) will operate a new generation of atomic clocks on board the International Space Station (ISS) in the 2013-2015 timeframe . The ACES payload will be attached externally to the European Columbus module. The ACES clock signal will reach fractional frequency stability and accuracy of 1 part in 10-16. A GNSS receiver will be connected to the ACES clock signal. Primarily, the GNSS receiver will ensure orbit determination of the ACES clocks using GPS, GALILEO/GIOVE, and possibly GLONASS satellite signals in the L1, L2, and L5/E5a bands. Orbit determination is important for the correct evaluation of relativistic corrections in the space-to-ground comparison of clocks. Secondarily, the receiver offers the potential to support additional functionality for remote sensing applications in the field of GNSS radiooccultation and GNSS reflectometry, exploiting opportunities arising from the new GPS and GALILEO/GIOVE signals . The ACES core scientific objectives include:
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