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SSC14-IX-5 Optical time transfer for future disaggregated small satellite navigation systems John W. Conklin, Nathan Barnwell, Leopoldo Caro, Maria Carrascilla, Olivia Formoso, Seth Nydam, Paul Serra, Norman Fitz-Coy University of Florida Mechanical and Aerospace Engineering, 231 MAE-A Building, P.O. Box 116250, Gainesville, FL 32611-6250; 352-392-9129 [email protected] ABSTRACT Precision time-keeping is a critical requirement of any satellite navigation system, including GPS. Even the most stable space qualified atomic clocks drift over time to the point where they can significantly degrade navigation precision. Periodic re-synchronization of these clocks with respect to terrestrial time standards is therefore required. Time transfer through Earth’s atmosphere using optical frequencies offers improved accuracy due to reduced time delay uncertainties relative to radio frequencies. In this paper we describe the design and laboratory testing of the Optical Precision Time Transfer Instrument, a compact device for real-time terrestrial-to-space clock corrections, using existing satellite laser ranging facilities. This instrument will comprise roughly 1U of a 3U CubeSat mission, sponsored by the Air Force’s University Nanosatellite Program and slated for launch in the 2017 time-frame. The instrument will demonstrate time transfer with a short term accuracy of 100 psec, equivalent to 3 cm of position error, and a long term timing error of 6 nsec over one orbit, limited solely by the frequency stability of the on-board miniature atomic clocks. Future missions using this time transfer technology and equipped with higher stability clocks will enable disaggregated navigation systems, with precision time-keeping components separated from other functionality. INTRODUCTION recently using large satellites and additional experiments are planned in the near future. The Time The problem of time transfer between precision clocks Transfer by Laser Link (T2L2) experiment developed separated by large (global) distances arises in by OCA (Observatoire de la Côte d’Azur) and CNES navigation, communications, networking, fundamental (Centre National d’Études Spatiales), France, was physics experiments and astrophysics. Precise timing is launched in 2008 on the altimetric satellite Jason-23. required for accurate Global Positioning System (GPS) T2L2 is based on the techniques of satellite laser navigation. The transmitted signal from each GPS ranging and time-frequency metrology. It consists of satellite is encoded with its own atomic clock time. By synchronizing ground and space clocks using short geometrically combining the time of flight of signals laser pulses travelling between ground clocks and transmitted by at least four GPS satellites, the receiver satellite equipment. The clock on board Jason-2 is an is able to calculate its position in three dimensions, as ultra-stable oscillator (USO) integrated with the DORIS well as correct its local clock with respect to GPS time. (Doppler Orbitography and Radio-positioning Comparison of precision clocks also provides one Integrated by Satellite) satellite tracking system. The frequency stability (Allan deviation) of this clock is method of testing fundamental physics laws, including –11 the universality of gravitational redshift and local ~ 2×10 over short time scales (1 sec) and improves Lorentz invariance1. In astronomy, the goal may be to with increasing integration time , roughly at a rate correlate observations made by two observatories on proportional to 1/ . two different continents, or to coordinate spacecraft τ flying in precision formation to produce a distributed One-way laser rangingτ to the Lunar Reconnaissance aperture telescope or gravitational wave observatory2. Orbiter (LRO), commissioned in 2009, has been Finally, time transfer is used to evaluate the conducted successfully from NASA's Next Generation performance of the world’s most accurate clocks. The Satellite Laser Ranging System (NGSLR) at Goddard Geophysical and Astronomical observatory (GGAO) in only way this can be done is by comparing one clock 4 with respect to another. Greenbelt, Maryland . A one-way ranging technique is used, where the Earth laser station measures the Several precision time transfer experiments between transmit times of its outgoing laser pulses and the Lunar ground and space, beyond GPS, have been carried out Orbiter Laser Altimeter (LOLA), one of the instruments Conklin 1 28th Annual AIAA/USU Conference on Small Satellites onboard LRO, measures the receive times. The clock are timed with respect to the atomic clock on the associated with LOLA has a frequency stability of ground and are detected by an avalanche photodetector roughly 10–12 at 1000 sec. The time transfer accuracy is on OPTI. An event timer records the arrival time with currently limited to 100 ns at NGSLR. respect to the on-board clock with an accuracy of ~100 ps. At the same time, a retroreflector returns the In the near future, the Atomic Clock Ensemble in Space transmitted beam back to the ground. By comparing the (ACES) mission sponsored by the European Space transmitted and received times on the ground and the 5 Agency will fly aboard the International Space Station . arrival time of the pulses at the satellite, the time ACES is a fundamental physics experiment using a new difference between the ground and space clocks can be generation of atomic clocks operating in the measured. microgravity environment of space, which are compared to a network of ultra-stable clocks on the Unlike the T2L2 mission, OPTI will be incorporated ground. The ACES payload consists of two atomic into a dedicated CubeSat bus whose attitude is dictated clocks: PHARAO, a primary frequency standard based by the requirements of OPTI. An additional new on samples of laser cooled cesium atoms, and the active capability of OPTI is real time clock corrections. The hydrogen maser, SHM. The composite frequency optical link will be used to promptly transmit the timing stability of the ACES clocks is ~10–13 at 1 second, information from the ground to the satellite so that improving roughly as 1/ 2. The ACES clock signal will OPTI’s atomic clock frequency and phase offset can be be transferred between space and ground by a corrected in real time. microwave time and frequencyτ transfer link. ACES is scheduled to launch in 2016. Here, we report on a new compact, low-power Optical Precision Time-transfer Instrument (OPTI) that will simplify the process of correcting for clock drift on spacecraft. Time transfer through Earth’s atmosphere using optical frequencies offers improved accuracy due to reduced time delay uncertainties relative to radio frequencies. The operation of OPTI will be demonstrated on a low Earth orbiting 3U CubeSat in the 2017 time-frame as part of the Air Force’s University Nanosatellite Program. In addition to improving the precision and simplifying the operation of satellite navigation systems, OPTI and its CubeSat demonstration mission CHOMPTT (CubeSat Handling of Multisystem Precision Time Transfer) will aide in the realization of disaggregated satellite navigation systems in the future. Figure 1: CHOMPT Mission Concept A disaggregated navigation system is one in which the Let superscripts ground and space refer to time as precision timing function is separated from the high recorded by the ground and the space clocks gain pseudorange transmission component. A small respectively. Then the goal of OPTI is to accurately number of timing satellites (1-3) would be placed in estimate the clock discrepancy low Earth orbit and transmit timing updates to a larger = over a short period of time in constellation of broadcast satellites in higher orbits. order to푠푝푎푐푒 determine푔푟표푢푛푑 both the relative clock phase and These timing satellites could also provide precision frequency휒 푡 offsets− 푡 . Assume that a light pulse is time to any other space asset that required it, including transmitted from the ground at time (referenced communications satellites and science and astrophysics to the ground clock) and that the light푔푟표푢푛푑 pulse is received missions. 0 at the satellite at time (referenced푡 to the space 푠푝푎푐푒 TIME-TRANSFER CONCEPT clock). Also let 1be the time that the returned 푔푟표푢푛푑푡 The Optical Precision Time-transfer Instrument pulse is received back at the ground. The clock 푡2 concept, shown in Figure 1, is similar to that of the discrepancy is then the difference between the T2L2 instrument. A satellite laser ranging (SLR) measured arrival time of the pulse at the satellite and facility on the ground will transmit ~100 picosecond the expected arrival time based on time measurements laser pulses to the CHOMPTT CubeSat. These pulses made on the ground. The expected time is the average Conklin 2 28th Annual AIAA/USU Conference on Small Satellites of the emitted and received times on the ground plus a Table 1: Comparison of the two OPTI clocks small correction, : Characteristic CSAC6 MAC7 Standard Cesium Rubidium = Δ푡 + (1) –10 –11 푠푝푎푐푒 1 푔푟표푢푛푑 푔푟표푢푛푑 1 sec Allan 2.5×10 5×10 1 2 2 0 Deviation The correction휒 푡 −accounts�푡 for time푡 delays� − causedΔ푡 by 6000 sec Allan 3×10–12 9×10–13 (a) the geometrical offset between the reflection and Deviation detection equivalentΔ푡 locations on OPTI, which depend Average Power 0.12 W 5 W on satellite attitude, (b) relativity, and (c) asymmetry in the atmospheric delay between the outward and return Mass 35 g 85 g path of the laser pulse. Volume 16 cm3 47 cm3 A GPS receiver is also incorporated on the satellite for several reasons. GPS data will be used for orbit determination and for a verification of the optical range measurements. The GPS data will also be used to compare time as recorded by the atomic clocks on the satellite with GPS time. INSTRUMENT DESCRIPTION Overview The main components of OPTI are a pair of the atomic clocks, a pair of event timers and time counters, a pair of avalanche photodetectors, and a single nadir-facing retroreflector. The following subsections describe each of these components.
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