WOrkIng pAperS availability of improved satellite clocks. astronomical observations. These time tions. In the near future, we can expect Let us now look in more detail at what scales and timing systems result from that discussions among GNSS service Envisioning a Future future clocks might look like. the cooperation of about 60 timing providers and metrologists will begin Importance for TOA-Based Systems. laboratories around the world, which on this subject, and numerous propos- GPS, GLONASS, Galileo, and other continuously contribute to the realiza- als concerning applications and design GNSS System of Systems GNSSes are designed to operate on the tion of UTC. concepts will arise as a result. basis of the principle, “one-way time of UTC rarely differs from the inter- The present achievement of 10- GüNtEr W. hEiN, JOSE aNGEl avila rOdriGuEz, StEFaN WallNEr, arrival (TOA) ranging.” Each satellite national average by more than 10 nano- nanosecond accuracy is already an thOmaS paNy, BErNd EiSSFEllEr, aNd philipp hartl Part 2 emits its ranging signals together with a seconds. NIST is one of four laboratories astonishing result of many decades of navigation message that tells the user’s worldwide operating the highest prima- research and developments (see Figure receiver from which satellite, We can expect that over the next 1). However, even that achieve- from which orbital position, and ment still leaves much room for at what time it was broadcast. 20 years or so the advances in timing further scientific progress. We By comparing the time of a accuracy characteristic for the past can expect that over the next 20 signal’s arrival with the time of — namely, to improve the stability by a years or so the advances in tim- its transmission, a pseudorange factor 10 per decade — will continue. ing accuracy characteristic for can be calculated. This one-way the past — namely, to improve TOA principle allows an unlimited ry frequency standards to determine the the stability by a factor 10 per decade number of users to use a GNSS. frequency of UTC. — will continue. However, this method assumes that Before we discuss this point further, all GPS satellite clocks involved in a posi- Improved Clocks — So What? it seems appropriate to judge the present tion solution are fully synchronized with What kind of progress and alternative performance from a different perspec- each other and with the International architecture could one invent, if GNSS tive: that 10-nanosecond time resolution GPS time (or some equivalent reference). receivers’ oscillators had a frequency/ corresponds to three meters considering Moreover, it requires extremely precise phase stability orders of magnitudes the propagation of light. At present, the information about the satellite’s position better than the present ones? Moreover, GPS clock stability in space is still of this in a well-defined reference frame. what would be the practical effect if same order of magnitude. Meanwhile, The current standard for all GNSS GNSS satellites could transmit signals the absolute values of the GPS satellites’ For reasons of political sovereignty, technological competition, policy differences, operational control, and satellites is to have three to four clocks for which the uncertainties in GNSS real-time orbital position coordinates perhaps just plain old national prestige, the planet Earth may have four complete global navigation satellite on board each spacecraft — one of them time were not counted in nanoseconds are on the order of one meter. systems within five or six years. let’s assume that happens. are users and manufacturers destined to work being the master clock and the others, (with 5 ns corresponding to 1.5 meters These statistics make it quite obvious through a labyrinth of competing technical specifications and management regimes in order to take advantage redundant units. Currently those clocks in ranging error), but rather in femtosec- that many interesting effects of nature of the rich GNSS signal resource coming into existence? Or can we shape a better world of GNSS interoperability are of rubidium and/or caesium types. onds (fs): one billionth of one millionth are still hidden inside the GNSS error and cooperation? Galileo will eventually also use of a second (with 5 fs corresponding to budget, and metrology should aim to a hydrogen-maser clock, if tests on 1.5 millimeter ranging error)? detect and exploit such surprising phe- the second Galileo In-Orbit Valida- It would certainly open a lot of new nomena. Scientists dealing with solid GNSS System of Systems? We is considering adopting it as well. They Innovations in Clocks tion Experiment (GIOVE-B) satellite research topics for scientific applica- Earth research and with fundamental began our discussion in the all predominately use middle Earth If there is an area where revolutionary employing this technology turn out to previous column (January/ orbiting (MEO) satellites with constel- developments could occur in the next be successful. Hydrogen masers have A February 2007 issue) with an lations (current or planned) of between few years, then it is in the field of clocks. superior short-term stability compared exploration of the current status, plans, 24 and 30 space vehicles each. The atomic clocks placed on board the to other frequency standards, but lower similarities, and differences among GPS As we concluded in the last column, satellites are probably the most crucial long-term accuracy due to changes in (United States), GLONASS (Russia), the basis for a GNSS system of systems single element to achieve a high-perfor- the properties of the clocks’ microwave Galileo (European Union), and Com- seems strong, building on infrastructure, mance GNSS. cavity over time. pass (People’s Republic of China). operations, and policies that are already Because the development cycle in Atomic clocks are also installed at Consider the similarities: All four in place or under development. Now we clock technology is about 7 years, these ground stations of those GNSS systems. are global systems accessible by users will turn to some concepts further out- newest generation technologies might be GPS clocks in space and on the ground worldwide. They have one or more radio side the realm of the expected — some available on orbit in a timeframe of 20 are in some way synchronized to the frequency bands in common where they speculative possibilities based on inno- years. This would considerably alleviate International Atomic Time (TAI), which broadcast open signals free of charge. vative ideas, but which nevertheless are the challenge of generating predicted is the world’s continuous and stable time They have comparable atomic time and within the realm of the possible. satellite clock corrections. Furthermore, scale. Alternatively (or additionally), the geodetic coordinate frames. And nowhere is there more room one might speculate that, since orbit pre- clocks are also tied to the civil Coordi- Three of them have a common sig- for innovation than in the domain diction already works quite well today, nated Universal Time (UTC). Derived FIGURE 1 Accuracy of terrestrial caesium atomic clocks at the National Institute of Standards and nal technology — code division multiple that underlies GNSS technology itself: differential correction service for GNSS from TAI, but synchronized with the Technology. The NIST-7 is an optically pumped, thermal atomic-beam, microwave caesium spec- trometer and the NIST-F1 is a caesium fountain atomic clock access (CDMA) — and the other system time. systems could become obsolete with the passing of day and night on the basis of 64 InsideGNSS march/april 2007 www.insidegnss.com www.insidegnss.com march/april 2007 InsideGNSS 65 WOrkIng pAperS The two very different types of instru- Atomic Clocks: ments will be synchronized by means not Just for Satellites of a specifically developed “Frequency Improved timing in GNSS receivers is Comparison and Distribution Package” technically already possible today. How- (FCDP), and the results shall be trans- ever, the development of good, stable mitted to ground by two datalinks, a and at the same time extremely cheap laser- and a Ku-band link. reference oscillators has not kept up This experiment will distribute a with progress achieved in other aspects stable and accurate time base for space- of GNSS receiver chip design. Conse- to-ground and ground-to-ground time quently, all types of GNSS receivers still and frequency comparison. Institutes use crystal oscillators for their timing worldwide will participate in comparing reference. their ground-based atomic clocks with Consider, for example, one very the ACES clock signal. Analysis of the interesting development: in 2004 the scientific data will seek to shed light on U.S. National Institute of Standards fundamental relativistic issues, as well as and Technology (NIST) demonstrated FIGURE 2 Color scheme of femtosecond laser frequency comb-synthesizer (Courtesy of Prof. T. W. for time and frequency metrology, geod- a chip-scale atomic clock (CSAC) about Hänsch. For more details refer to the Nobel Lecture in the Additional Resources section) esy, gravimetry, precise orbit determina- the size of “a grain of a rice” and stable tion, and Earth monitoring by means of enough at 10-9. The CSAC’s volume is relativistic issues, for example, offer ground-based clocks will become avail- the very long baseline interferometry less than 10 cubic millimeters (1.5mm highly interesting possibilities for fur- able for space applications a