1、 Rep. ITU-R SA.2132 1 REPORT ITU-R SA.2132 Telecommunication characteristics and requirements for space VLBI systems (2008) This Report describes the characteristics of the space VLBI systems. These characteristics form a technical basis for Recommendations related to space VLBI systems. The content
2、s of this Report were originally included in the Recommendation ITU-R SA.1344 Preferred frequency bands and bandwidths for the transmission of space VLBI data, as an annex. That annex material has been removed from the Recommendation and is maintained in this Report. In preparing this Report, many r
3、evisions have been made to the material formerly in the Recommendation and the topics have been rearranged to improve clarity. The Report includes a general description of the space VLBI systems and detailed descriptions of telemetry link for science data and of phase transfer link for time and freq
4、uency synchronization. Also included are the explicit equations for cross-correlation SNR and its degradation due to interference, required carrier frequencies and telemetry bandwidths, an interference criterion for the telemetry channel, effects of noise on the phase-transfer link, the characterist
5、ics of existing and planned space VLBI systems, and the characteristics of earth stations. 2 Rep. ITU-R SA.2132 Contents Page 1 Introduction 3 2 Description of the space VLBI system. 3 2.1 Telecommunication links for space VLBI 4 2.1.1 Earth-to-space (E-s) telecommand link 4 2.1.2 E-s phase transfer
6、 link for time and frequency synchronization 4 2.1.3 Space-to-Earth (s-E) telemetry link for science data . 5 2.1.4 S-E phase transfer link . 5 3 Technical characteristics. 5 3.1 Telemetry link. 5 3.1.1 Space VLBI cross correlation function 5 3.1.2 Cross-correlation SNR degradation . 7 3.1.3 Require
7、d interference criterion for the telemetry link 9 3.1.4 Required bandwidths for the telemetry channel 9 3.1.5 Preferred space-to-Earth telemetry carrier frequencies 10 3.2 Phase-transfer link 10 3.2.1 Phase noise introduced in propagation. 11 3.2.2 Phase noise introduced in carrier recovery 12 4 Pre
8、ferred frequency bands and bandwidths within the space research service (SRS) allocated bands . 13 5 Characteristics of existing and planned space VLBI systems 14 6 Characteristics of earth stations 15 Bibliography. 16 Rep. ITU-R SA.2132 3 1 Introduction Very long baseline interferometry (VLBI) allo
9、ws experimenters to observe radio sources with angular resolutions that cannot be approached by other methods. In addition, VLBI has other scientific and engineering uses. Observations of distant radio sources with two or more VLBI stations can be combined to determine the structure and positions of
10、 extra-galactic radio sources, to determine the geodynamical characteristics of the Earth, to study the Moons libration and tidal response, to determine orientation of the solar system with respect to the extra-galactic inertial frame, to determine the vector separation between antenna sites, and to
11、 provide navigation and tracking of spacecraft. 2 Description of the space VLBI system Space very long baseline interferometry (SVLBI) is a highly useful extension of very long baseline interferometry (VLBI), which in turn is a development from conventional radio interferometry. In all three cases,
12、a specified bandwidth of cosmic or other radio emission is received simultaneously at two or more antennas that are distributed over distances much larger than the size of individual antennas. These bands, which can be described as time-varying spectra, are downconverted to a lower frequency so that
13、 they can be further amplified and then cross-correlated. In conventional interferometry this processing is done in real time. To preserve the amplitude and relative phases of the spectral components the downconversion has to be based upon a common local oscillator or frequency reference. The attrac
14、tion of interferometry is that the angular resolution of the interferometer is related to the separations between the antennas rather than their physical size. However, there is a practical limit to how far antennas can be separated and still use real-time signal transfer, and that the largest conve
15、ntional interferometers lack the angular resolution needed for the investigation of many types of cosmic radio source or determination of the position of distant space probes. The development of ultrastable oscillators, accurate clocks and large-bandwidth data recording systems (using discs or magne
16、tic tape) made it unnecessary to connect the antennas or use common local oscillator references, so the antennas could be moved further apart and the data taken after the experiment to a processing station where they could be synchronized and correlated, yielding a map of the source region. Antenna
17、separations (interferometer baselines) of thousands of kilometres have been used successfully. However the diameter of the Earth sets a hard limit to the usable antenna spacings. Source visibility above the horizon in most cases limits the spacing even further. Space very long baseline interferometr
18、y (SVLBI) removes this limitation by putting one of the interferometer antennas in space. Although in essence the process is still that of conventional ground-based VLBI there are some additional complications. Firstly the spacecraft carrying the spaceborne antenna element is moving at orbital veloc
19、ity, and the motion has to be known quite accurately, and secondly the data have to be downlinked to a ground station for recording. Maintaining accurate time tagging of the data is much more complicated. The configuration of a typical SVLBI experiment is shown in Fig. 1. 2.1 Telecommunication links
20、 for space VLBI The telecommunication links of the space VLBI system are represented in Fig. 1 by the four dashed lines between the space VLBI spacecraft telecommunication antenna and the space VLBI earth station. A description of the radio links follows. 4 Rep. ITU-R SA.2132 2.1.1 Earth-to-space (E
21、-s) telecommand link This radio link is used for reliable transmission of telecommands required for operation and correction of possible spacecraft malfunctions. 2.1.2 E-s phase transfer link for time and frequency synchronization In VLBI accurate knowledge of the time, the signal frequency, and the
22、 signal phase is needed for post-real-time cross-correlation. This requirement is met by using high-stability oscillators, often referred to as “atomic clocks,” at every station and also by utilizing the Global Positioning System (GPS). At present an Earth-to-space phase-transfer link is used to imp
23、art the required time/phase reference to the spacecraft on-board clock and local oscillators. In the future the space VLBI spacecraft may have a space-qualified atomic clock. However, the distant space VLBI station may not be able to utilize the GPS system for time synchronization. Hence, the E-s ph
24、ase transfer link will still be needed for time synchronization. Rep. ITU-R SA.2132 5 2.1.3 Space-to-Earth (s-E) telemetry link for science data The space VLBI spacecraft observes the radio source over a selected bandwidth. This observed spectrum is transmitted to the space VLBI earth station using
25、this s-E telemetry link for science data for recording and subsequent cross-correlation with the spectrum observed by the VLBI earth stations. 2.1.4 S-E phase transfer link This radio link will be a coherent frequency translation of the Earth-to-space phase transfer link described above and will be
26、used to calibrate the phase errors introduced in the Earth-to-space phase transfer link by different causes. This radio link may be dedicated to this phase transfer operation or may be combined with the s-E telemetry link for science data to transfer the observed spectra from the spacecraft as descr
27、ibed in 2.1.3. 3 Technical characteristics A detailed characterization of the space VLBI telemetry link for science data and the phase-transfer link for time and frequency synchronization is given below. In a space VLBI system, we need to consider two issues: firstly the observing system itself, whi
28、ch is similar to a terrestrial VLBI system except that at least one antenna is moving at a high and varying velocity compared with the rest of the network, and consequently establishing the velocity and position vectors is more complicated. This requires the transfer of timing and frequency standard
29、 signals between the ground and the spacecraft. Secondly, the data have to be transferred to the ground receiving station by a telemetry link. There could be quantization effects and also phase scintillation produced by the ionosphere. The space VLBI spacecraft receives the radio source frequency sp
30、ectrum contaminated with background and system noise in the observation bandwidth. This observed spectrum is transmitted to the space VLBI earth station where it is recorded and cross-correlated with the frequency spectrum observed at the earth station of the same radio source. The transmission of t
31、he observed spectrum from the space station to the earth station may be analogue or digital. In digital transmission, the observed analogue signal is first converted to a digital format and then transmitted to the space VLBI earth station for recording. The transmission of a telemetry signal through
32、 space degrades the signal before it is detected at the earth station receiver. In digital transmissions, this degradation increases the errors in detecting the information bits. These degradations, thus, affect the final cross-correlation process of the space VLBI experiment by lowering the cross-c
33、orrelation signal-to-noise ratio (SNR). 3.1 Telemetry link 3.1.1 Space VLBI cross correlation function The basic observables in radio interferometry are the amplitude and relative phase of the cross-correlation of the two observed spectra. This cross-correlation process is usually performed in non-r
34、eal time and may be expressed as: is the estimated mean for the observation over time, x(t) and y(t) are the recorded signals at sites 1 and 2, and gis the wave front time delay. 6 Rep. ITU-R SA.2132 In the cross-correlation function of equation (1), the pre-recorded signals will be contaminated wit
35、h noise from the receiving systems. For each receiving station, we can define an observer signal-to-noise ratio () as: 2,1221= mkTASBkTBASmmmmmmm(2) where: S1: spectral flux density of observed source at antenna 1 (W/(Hz m2) A1: effective area of receive antenna 1 (m2) T1: system noise temperature o
36、f receiver 1 (K) S2: spectral flux density of observed source at antenna 2 (W/(Hz m2) A2: effective area of receive antenna 2 (m2) T2: system noise temperature of receiver 2 (K) k: Boltzmanns constant (= 1.38 1023W/(Hz K) B: observation bandwidth (Hz). Note that equation (2) has a factor 1/2, since
37、the spectral flux densities refer to the total emission from the source, which is almost always largely unpolarized, and any practical antenna system can only receive half of this total. The effective area of a receive antenna can be written as: 2,142= mDAmmm(3) where mis the aperture efficiency and
38、 Dmis the diameter of the antenna (in metres). Using the effective area of the antenna, we can define a system noise equivalent flux density (S*) for each receiver as: 2,1822*= mDkTAkTSmmmmmm(4) Now, using the system noise equivalent flux density for the receivers, we can express the observer signal
39、-to-noise ratios as: 2,1/2*= mSSAkTSmmmmmm(5) It has been shown that the cross-correlation signal-to-noise ratio (X1,2) may be expressed as a function of the two observing signal-to-noise ratios 1and 2as: = BX 2212,1(6) Rep. ITU-R SA.2132 7 where B is the observation bandwidth and is the integration
40、 time of each observation. This cross-correlation SNR can also be expressed in terms of the mean flux density 212,1SSS = of the observed source and the system noise equivalent flux densities of the observing stations as: *2*12,12,12SSBSX= (7) The cross-correlation SNR should be maintained as large a
41、s possible to decrease the measurement error of the gin equation (1). Note that for this space VLBI system with two elements, if we define the noise flux density threshold (Sth) of the cross-correlation as: =BSSSth2*2*1(8) we can then write the cross-correlation SNR as: thSSX2,12,1= (9) This equatio
42、n does not account for the ionospheric scintillation, that is, for the rapidly fluctuating ionospheric delays. The phase error introduced lowers the amplitude of the cross-correlation of a space VLBI system. Thus, the actual, measured cross-correlation SNR with ionospheric scintillation is more accu
43、rately modelled as: 02,12,12,1SSSSgXth= (10) where g is called the coherence factor and S0= Sth/g is the effective sensitivity threshold of the space VLBI system. Note that when coherence factor is one, the VLBI sensitivity threshold equals the cross-correlation noise flux density threshold, an idea
44、l situation. Normally coherence factor will be less than one, and the sensitivity threshold will rise above the noise density threshold. The coherence factor used in equation (10) is determined experimentally. The amount of ionospheric delay fluctuation is represented by an experimentally determined
45、 scintillation index. Low scintillation index means less ionospheric delay fluctuation, whereas high scintillation index means a large ionospheric delay fluctuation. Equation (9) shows that, in order to increase cross-correlation SNR, we need to decrease the space VLBI noise flux density threshold.
46、This we can accomplish by using wider observation bandwidths and longer integration times, and by having stations with lower equivalent flux densities, which in turn means having larger antennas with low system noise temperatures. 3.1.2 Cross-correlation SNR degradation The space VLBI spacecraft rec
47、eives the radio source frequency spectrum contaminated with noise (background, system, etc.) in a selected observing bandwidth, B, at a given observing SNR, ONR1. This observed spectrum has to be transmitted to the space VLBI earth station to be recorded and further processed (cross-correlated). Thi
48、s transmission may be an analogue transmission or the observed analogue signal may be converted to a digital format and transmitted to the space VLBI earth station for recording. 8 Rep. ITU-R SA.2132 The transmission of a telemetry signal through space implies some signal degradation when detected a
49、t the intended receiver. In digital transmissions, this degradation is due to the probability of information bits being in error and is dependent on the received symbol signal-to-noise ratio (SSNR). This link degradation will affect the final process of the space VLBI experiment, i.e. the cross-correlation function of equation (1). For an analogue telemetry link, the cross-correlation SNR that includes the telemetry link losses is shown to be: += BXtlmtlma21211(11) where tlmis the telemetry-link SNR. The first factor is the
