(EMC) and Final Performance Test (FPT).These measurements were obtained against an ac-tive hydrogen maser and the accuracy of these measurements is therefore expected to be at the 1E-13 level. This figure confirms that the frequency repeatability of the PHM is in the order of a few 1E-12 after satellite vibration.
In orbit, the fractional frequency offset may be estimated by POD techniques. Figure 8.4 presents the estimated fractional frequency offset of the PHM during its live time aboard GIOVE-B. The PHM frequency is estimated against a steered hydrogen maser located at USNO timing laboratory. The absolute frequency of the PHM is 10MHz which is changed later by a frequency synthesizer in the frequency control unit to the nominal value (f0=10.23MHz) from which the navigation signals are derived. The frequency measured on ground was intentionally offset by -4E-10 from this value to account for relativistic effects to 10229999.99590920 Hz. Following the relativity theory the expected observed value in orbit should instead be shifted by +4.718E-10 to +4.718E-10230000.00073570 Hz. Consequently, the expected frequency offset value in-orbit as observed from the ground should be the theoretic minus the precorrected value: +4.718E10 -4.00E-10 = 7.19E-11.
Over the first month of operation, when possible aging effects do not affect the validity of the results, the on-board PHM fractional frequency offset is estimated to be 7.75E-11, as opposed to an expected theoretical value of 7.19E-11 with respect to the measured ground frequency.
Therefore, the PHM allows the measurement of the relativistic frequency shift with an error of 5.58E-12, corresponding to 1.2% accuracy with respect to the measured value. Table 3.8 summarizes the values.
Frequency Freq. Offset A f/f0
f0[Hz] A f[Hz] [s/s] [s/day]
Nominal f0 10230000.0000000 0.00000000 0 0
Ground 10229999.9959092 -0.00409083 -4.00E-10 -3.5E-05 Expected 10230000.0007357 0.00073574 7.19E-11 6.2E-06 In orbit 10230000.0007928 0.00079283 7.75E-11 6.7E-06
Delta 0.00005708 5.58E-12 4.8E-07
Tab. 3.8: GIOVE-B PHM frequency offset and relativity effect
3.8 Conclusions
τ
UTC UTC(k) SYST SAT Rx
TAI TT
UT1 ,TCG,TCB,TDB
Astronomy Users
Absolute Timing users Navigation and Timing users
(7)
(1) (2) (8)
(3) (4) (5) (6)
Fig. 3.15: Traceability between time scales in GNSS
Section 3.2 and 3.3 define the current atomic time scales based on the SI second definition as the basic interval. Elementary time scale is TT derived from the SI second definition on the rotating geoid and as a consequence represents the ideal time of a user on the Earth’s surface.
TAI is a physical realization of TT based on the measurement of atomic frequency standards distributed around the world. It is synchronized with TT apart from a constant offset (1):
TAI−T T =−32.184s [3.28]
TAI drifts slowly from UT1 based on the earth rotation as the earth rotation is slowing down.
Even if most countries have some hours difference with respect to the solar time, UTC was introduced to follow UT1±0.9 seconds requiring periodic integer±1 leap second steps correc-tions (2):
UTC−TAI=±LeapSeconds [3.29]
Section 3.4 explains how GNSS times are created as atomic time scales, maintained by the ground segment and linked to some UTC(k) creation. As UTC(k) contributes to UTC creation the traceability is provided by the BIPM on the CircularT (3):
UTC(k)−UTC=CircularT [3.30]
To support timing users the difference between UTC(k) and system time is provided by the navigation message of each satellite (4):
tsys−UTC(k) =A0+A1(tsys−toc) [3.31]
It also has also to be remarked that other external service providers, as IGS or SBAS, may provide the traceability of the clock in step (5) to their own system time realization. As a con-sequence, the time solution achieved by the user will be referred to this time scale.
Section 3.5 explains how the satellite time is created on board, maintained within the message limits and transmitted to the user through the navigation message (5) :
tsat−tsys=a0+a1(tsys−toc) +a2(tsys−toc)2 [3.32]
The GNSS capabilities to provide traceability between the different time scales,in step (4) and (5), are particularly constrained by the navigation message specifications. The definition lim-its the minimum time to first fix, the maximum accuracy achievable in the time transfer and imposes a limit within which all traced time scales need to be synchronised by a dedicated timekeeping strategy applied from the ground.
Section 3.6 finally explains how the user recovers the satellite Time of Transmission (TOT) in the receiver through the decoding of the navigation message (time-tag information TsysT and frames boundaries TFR and the code delay at the Time of Reception (TOR) in the receiver (Rx). This measure is called pseudorange (PR) since the geometric rangeρ also includes other contributionsξ (6) :
PR(trec) =T OR(trec)−T OT(tsat) =TSY S+TFR+DLL=ρ/c+dtrec(tsys) +ξ [3.33]
In case the position is known, the pseudorange measurement can be corrected to remove the geometric range and the other contributions can then be modelelled or estimated to get the re-ceiver timedtrecoffset to the system time(tsys).
Finally, the definition of the time scales and the time transfer between moving clocks needs to be understood in the framework of the general relativity theory provided in Section 3.7.
Time scales need to be understood in the reference frame and at the position in which they are defined. Astronomical users require that the time be referred to the Geocentric or Barycentric reference time scales (TCG and TCB). Transformation between TT and time scales defined in the geocentric or barycentric reference system (7) are provided in the IAU Resolutions and summarized in [104]. Satellite orbits are provided in the Earth’s fixed frame; users of inertial frames may also be interested in the UT1-UTC difference and the Earth orientation parameters.
This traceability (8) is provided by IERS or the new navigation messages of GLONASS-K and GPS-L5.
In GNSS, the time transfer between moving clocks requires ’relativistic corrections’ in order to refer all clocks to the system time scale used as a reference. These relativistic corrections may be divided into two types: a first group creating a net frequency shift in the clock as observed from ground, and a second group producing a periodic variation to the mean value.
3.8 Conclusions
The frequency shift is normally compensated from the ground while the periodic contributions are corrected by the user. The initial clock frequency offset following its first activation in-orbit has an associated uncertainty, which is compensated from ground together with the relativistic shift. The superior frequency repeatability of the new clock technology provided by the PHM has allowed, within this dissertation, the measurement of the expected relativistic frequency shift (4.718E-10) with an error of 5.58E-12, corresponding to 1.2% of the measured value.
4 Timing signals realization
4.1 Introduction
The generation of any atomic time scale requires Atomic Frequency Standards (AFS). Special AFS are used aboard GNSS satellites due to the low mass, low power consumption and high reliability requirements. AFS represent the core element of the satellite time, being one of the technologies required for GNSS with limited flight experience in other satellites. This technol-ogy is currently only mature enough in some countries with a limited number of suppliers. The potential use by military systems restricts the exportability between countries. The availability of AFS technology by a diversity of reliable manufacturers is a key element for any autonomous GNSS system as demonstrated during GPS lifetime.
The atomic frequency standard signal is further modified by the other units part of the nav-igation payload before transmission to the user receivers. The name ’clock’ is usually applied to the frequency standard on board the satellite even if it does not directly provide any time in-formation. The term ’clock offset’ is also used in the navigation message, or by IGS to refer to the difference between the ground and satellite time scales. However, the term ’timing signal’
rather than ’clock offset’ used by GPS performance reports [120] is more appropriate because the output of the atomic frequency standard is further modified by the electronics before being broadcast by the satellite and only the navigation signal includes time information.
Previous chapters introduced the different time scales in GNSS systems. This chapter intends to produce a more complete understanding of the satellite timing subsystem by examining the physical component, history, state of the art and future trends of its components. This under-standing is absolutely necessary in order to explore the possibilites offered by the new AFS, signals and modulations offered by the upcoming GNSS satellites.