1、136 ITU-R RECMN*SA- LO30 94 4855232 0523205 ObT Rec. ITU-R SA.1030 RECOMMENDATION ITU-R SA. 1030 TELECOMMUNICATION REQUIREMENTS OF SATELLITE SYSTEMS FOR GEODESY AND GEODYNAMICS (Question ITU-R 143/7) ( 1994) The ITU Radiocommunication Assembly, considering a) that satellite systems for geodesy and g
2、eodynamics have unique telecommunication requirements; b) that these requirements affect assignments and other regulatory matters, recommends 1. that the requirements and characteristics described in Annex 1 should to be taken into account in connection with frequency assignments and other regulator
3、y matters concerning satellites systems for geodesy and geodynamics, and their interaction with services other than the Earth exploration-satellite service or the space research service. ANNEX I Telecommunication requirements and characteristics of satellite systems for geodesy and geodynamics 1. In
4、troduction This Recommendation applies to satellite systems in which one or more satellites are linked to earth stations andor to each other by means of high-precision range and range-rate measurements, using radio waves. There are other satellite systems which contribute to the advancement of geode
5、sy and geodynamics: - range measurements by pulsed laser; - VLBI measurements on deep-space probes and celestial sources (see Recommendations on deep-space research); - ocean altimetry using satellite-borne radar (see Recommendations on spaceborne active remote sensing); - microwave radiometry for d
6、etermining the composition of the troposphere and so correcting propagation effects on other measurements (see Recommendations on spaceborne passive remote sensing). These various techniques are often jointly operated, with different equipments on board the same spacecraft and with earth stations co
7、located, that is to say close to each other and close to a geodetic fiducial point. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*SA* 1030 74 4855232 0523206 TTb Rec. ITU-R SA.1030 137 2. Telecommunication requirements fo
8、r range and range-rate measurements 2.1 General Space telecommunication systems for geodesy and geodynamics are generally required to perform three functions: - high-precision orbit determination, - high-precision positioning of points on the Earths surface, and - rapid data distribution (preferably
9、, this function is performed by the system itself). The first and second functions are closely linked. In order to position points in a geocentric reference system, it must be possible to predict or restore the satellite orbit in that reference system with a degree of accuracy comparable to that req
10、uired for the positioning. Consequently, the orbit determination system used for the tracking of geodetic satellites must have better accuracy than that which is generally required for application satellites. Such an orbit determination system typically uses a fairly large number of earth stations (
11、e.g. 10-50) distributed geographically so as to ensure continuous tracking of the satellite(s) which should always be visible from two or more stations. This network may be used also for geodetic applications, Le. to determine parameters relating to the Earths rotation, the geocentric coordinates of
12、 stations and the base lines linking pairs of stations. The second function (precise absolute and relative point positioning) is generally performed with transportable ground stations or networks to be established temporarily in areas of geographical interest, sometimes in clusters of more than 20 s
13、tations within a limited region. With respect to the third function, certain geodetic and satellite orbital parameters must be covered within a relatively short time (approximately one day). It may also be necessary to distribute in situ data gathered locally and orbit prediction data generated at a
14、 central facility. 2.2 Measurement telecommunications Determination of the relative positions of earth stations and satellites or of their variation in relation to the movement of the spacecraft can be based on the measurement of - range, - rangerate, - range difference (e.g. from two satellites to
15、one earth station), - range difference rate, - double differential range (e.g. from each of two satellites to each of two earth stations), - double differential range rate. Classifying of measurement telecommunications may also be based on the number and direction of the links: - one-way space-to-Ea
16、rth, - one-way Earth-to-space, - one-way space-to-space (satellite-to-satellite tracking), - two-way between earth stations and satellites, - two-way between satellites. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMNxSA. 3
17、030 94 4855232 0523207 932 W 138 Rec. ITU-R SA.1030 2.3 Data transmission The measurement systems listed above provide their results at one end of the system. In the case where these data are not extracted at the point where they are needed for further processing or dissemination, they have to be tr
18、ansmitted back to the other end of the system. Furthermore, processing the raw data might entail the addition of auxiliary data available at the other end of the link, for example: - data on propagation conditions measured in the vicinity of the earth stations (atmospheric pressure, temperature, hum
19、idity), and added to the uplink signal; ephemeris data of the satellites, information on the state of the ionosphere, etc., to be distributed to the earth stations. - Three types of information can be transferred within the system: - measurement signals, - measurement results, - auxiliary data. The
20、latter two could be multiplexed with the measurement signal or use separate links for retransmission. 3. Preferred frequency bands 3.1 RF spectrum constraints due to propagation characteristics The usable frequency bands are limited by the characteristics of the media through which the signals pass.
21、 - The troposphere causes both absorption loss and signal delay. Although tropospheric delay causes errors which exceed the accuracy goals of satellite geodesy and which have to be corrected in the parameter recovery process, it is not a criterion for the choice of preferred frequencies. Absorption
22、loss significantly affects link budgets only above about 20 GHz. The ionosphere causes negligible absorption above about 100 MHz. The lower limit of usable frequencies is determined by the phase shift and group delay of the signals used for measurement. - Range measurement errors due to the ionosphe
23、re depend on the total electron content (TEC), which generally varies according to latitude, time, season and solar activity within a range as broad as from 1.4 x 106 to 70 x 10l6 eYm2 and beyond that range in some regions. The direct correction of measurement errors by means of models is not very a
24、ccurate owing to the great variability of the ionosphere. In order to reduce measurement errors caused by inadequate knowledge of the ionosphere, it is necessary either to use fairly high frequencies or to merge the measurement data obtained simultaneously at a number of coherent frequencies. For a
25、mean TEC value of 20 x 10l6 el/m2, the gross error and residual error after correction by the combination of dual-frequency measurements are given in Table 1 for a vertical path completely traversing the ionosphere. For an oblique path inclined at 30“ in relation to the ground horizontal, the values
26、 in Table 1 should be multiplied by 1.8. At elevation angles less than 20 at 400 MHz or less than 10“ at 2000 MHz, the differential curve of the rays causes a rapid increase in residual errors. As shown in Table 1, the combination of dual-frequency measurements considerably reduces the ionospheric e
27、rror. However, if in dual-frequency systems these frequencies are not sufficiently spaced in the radio spectrum, the non- ionospheric errors grow by a factor which is, for example, between 1.2 and 1.6 for the pair 150/400 MHz and attains 3.4 for the pair 1 227/1575 MHz. One major conclusion that can
28、 be drawn from the above considerations is that single-frequency measurement systems are generally inadequate for high-accuracy satellite geodesy and geodynamics missions. Measurement systems for such missions require at least two frequency bands sufficiently spaced in the radio spectrum. COPYRIGHT
29、International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R SA.1030 Frequencies Main frequency Auxiliary frequency (MHz) (MHz) 400 150 2 o00 400 1575 1 227 8000 2000 TABLE 1 Ionospheric error over a vertical path for TEC = 20 x 10l6 Um2 Path meas
30、urement error Residual error Phase delay Group delay Gross error at main frequency 50 m 0.24 m 0.48 m 0.83 cm 2m 0.42 cm 3.2 m 0.15 cm 0.30 cm 12.5 cm 0.005 cm 0.01 cm 139 r f (MW 150 400 2 000 8 O00 3.2 Necessary bandwidth (m) One-way measurements Two-way measurements 2 Af (kHz1 3.2.1 Necessary ban
31、dwidth for Doppler effect measurements Owing to the Doppler shift, the received frequency differs from the emitted frequency by a quantity +Af or -Af depending upon whether the slant range is decreasing or increasing. 2 0.75 0.15 0.0375 9 24 120 480 18 48 240 960 Af = - for one-way measurements, h A
32、f = - for two-way measurements, h v being the rate and h the wavelength. Table 2 gives the necessary bandwidth 2Af for v = 9 kds. TABLE 2 Necessary bandwidth for measuring the Doppler effect corresponding to a range rate of 9 km/s 3.2.2 Necessary bandwidth for ranging Radio ranging consists of measu
33、ring the propagation phase or group delay of signals between the spacecraft and the earth station. However, the measurement is generally not taken on the carrier because of the ambiguity of nh (one-way) or nu2 (two-way). In order to remove the ambiguity, measurements are taken on signals which modul
34、ate the carrier. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services4855232 0523368 3b2 140 Rec ITU-R SA.1030 Two main types of modulation are used. In one case, the phase delay of several sinusoidal signals or tones, modulating the carri
35、er simultaneously or sequentially, is measured. The lowest frequency tone is used to remove the ambiguity, while the highest determines the range resolution. Highest modulating frequencies are typically about I-i0 MHz. However, this technique has the disadvantage of concentrating W energy on spectru
36、m lines and therefore its use may be difficult in some of the bands shared with services requiring protection defined in terms of spectral power- density limits. In the other case, the group delay of a pseudo-noise code, modulated on the carrier, is measured. Here the energy is spread over a band of
37、 some 1-10 MHz. In both cases, after modulation of the carrier, the RF bandwidth is between about 2-20MHz. Larger bandwidths might be used in the future. The Doppler frequency shift (see Table 2) must be added to these values. 3.2.3 Necessary bandwidth for data transmission The data rate of the auxi
38、liary data is in the region of some tens of bit/s. This information may be multiplexed with ranging signals. 3.3 Usable frequency bands Telecommunication systems for satellite geodesy and geodynamics are relevant to the space research service and to the earth exploration-satellite service. Furthermo
39、re, some systems operated by the radionavigation-satellite service can also be exploited for geodesy or geodynamics. Table 3 shows some of the frequency bands currently used or envisaged for satellite geodesy and geodynamics applications. TABLE 3 Frequency bands currently used or envisaged in satell
40、ite telecommunication systems for geodesy and geodynamics Frequency band MW 40 1 -403 1215-1 260 1559-1 610 2025-2110 2 200-2 290 7 190-7 235 8 025-8 400 8 450-8 500 Direction Earth-to-space Space-to-Earth Space-to-Earth Earth-to-space Space-to-Earth Earth-to-space Space-to-Earth Space-to-Earth Allo
41、cation Earth exploration satellite Radionavigation satellite Radionavigation satellite Space research and earth exploration satellite Space research and earth exploration satellite Space research Earth exploration satellite Space research Note I - For geodesy and geodynamics purposes the bands allocated to the radionavigation-satellite service should only be for reception. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services
