1、44 Rec. ITU-R M.1088 RECOMMENDATION ITU-R M. 1088 CONSIDERATIONS FOR SHARING WITH SYSTEMS OF OTHER SERVICES OPERATING IN THE BANDS ALLOCATED TO THE RADIONAVIGATION-SATELLITE SERVICE (Question ITU-R 83/8) ( 1994) The ITU Radiocommunication Assembly, considering a) that the Global Positioning System (
2、GPS) provides worldwide precision navigation information in three dimensions to air, land and sea-based stations; b) that the bands 1215-1 240 MHz and 1 559-1 610 MHz are allocated on a primary basis respectively to the radiolocation and radionavigation-satellite services and to the aeronautical rad
3、ionavigation and radionavigation-satellite services in all three Regions; c) that many administrations additionally allocate the 12151240 MHz band on a primary basis to the fixed, mobile and radionavigation services, and the 1 559-1 610 MHz band on a primary basis to the fixed service; d) that any p
4、roperly equipped earth station may receive navigation information from the GPS on a worldwide basis; e) that because the GPS provides a radionavigation function, it is recognized as performing a safety service and thus requires special measures to ensure its freedom from harmful interference, recomm
5、ends that the characteristics and system description of Annexes 1 and 2 be used in assessing sharing with the GPS. 1. ANNEX 1 GPS receiver characteristics (for typical, low-cost air-navigation receivers) L1 carrier frequency: L2 carrier frequency: P code chip rate: CIA code chip rate: Navigation dat
6、a rate: Undetected bit-error rate: Minimum received power level (L2, P): Minimum received power level (Ll, P): Minimum received power level (Ll, CIA): Preamplifier limiting level: Preamplifier burnout level: 1575.42 MHz 1227.6 MHz 10.23 Mbi/s 1 .O23 Mbi/s 50 bi/s 10-5 -136 dBm -133 dBm -130 dBm -40
7、dBm 30 am, ave. 40 dBm, peak 1s Overload recovery time: RF 3-dB filter bandwidth: f 17 MHz COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMNXM- LO88 94 4855232 0523654 382 = Rec. ITU-R M.1088 RF 45-dB filter bandwidth: Isolat
8、ion between L1 and L2: Receiver noise figure: Normal acquisition YS margin (Ll, CIA): State 5 tracking US margin (151, CIA): State 5 tracking I/S margin (Ll, P): 45 k 50 MHz 40 dB 3 dB 24 dB 31 dB 41 dB ANNEX 2 Technical description and characteristics of the Global Positioning System (GPS) 1. GPS s
9、ystem 1.1 Introduction The Global Positioning System (GPS) will consist of 24 satellite positions with four satellite positions in each of six 55“ inclined equally spaced orbital planes. Each satellite will transmit the same two frequencies for navigational signals. These navigational signals are mo
10、dulated with a predetermined bit stream, containing coded ephemeris data and time, and having a sufficient bandwidth to produce the necessary navigation precision without recourse to two-way transmission or Doppler integration. The system will provide accurate position determination in three dimensi
11、ons anywhere on or near the surface of the Earth. 1.1.1 Frequency requirements The frequency requirements for the GPS system are based upon an assessment of user accuracy requirements, space-to-Earth propagation delay resolution, multipath suppression, and equipment cost and configurations. Two chan
12、nels were selected for GPS operations: 1575.42 MHz (Ll) and 1227.6 MHz (L2). The L1 channel will be used to resolve a users location to within 150 m. A second signal transmitted on both L1 and L2 channels, provides the necessary frequency diversity and wider bandwidth for increased range accuracy fo
13、r Earth-to-space propagation delay resolution and for multipath suppression to increase the total accuracy by an order of magnitude. Telemetry and maintenance signais from United States based control facilities to the satellite and return will be accommodated in the allocated telemetry band in the U
14、nited States of America. GPS will provide a worldwide navigation service. The requirement for navigation safety (refer to Radio Regulation No. 953) demanded by such a service underscores the critical importance that other radio services not cause harmful interference to GPS receivers. 1.2 System ove
15、rview GPS is a space-based, all-weather, continuous radionavigation, positioning and time-transfer system which will provide extremely accurate three-dimensional position and velocity information together with a precise common time reference to suitably equipped users anywhere on or near the surface
16、 of the Earth. The system operates on the principle of passive triangulation. The GPS user equipment first measures the pseudo-ranges to four satellites, computes their positions, and synchronizes its clock to GPS by the use of the received ephemeris and clock correction parameters. It then determin
17、es the three-dimensional user position in a Cartesian Earth- centred, Earth-fixed (ECEF) WGS-84 coordinate system, and the user clock offset from GPS time by essentially calculating the simultaneous solution of four range equations. COPYRIGHT International Telecommunications Union/ITU Radiocommunica
18、tionsLicensed by Information Handling Services ITU-R RECMN*M- 1088 94 4855212 0523b55 219 46 Rec. ITU-R M.1088 Similarly, the three-dimensional user velocity and user clock-rate offset can be estimated by solving four range rate equations given the pseudo-range rate measurements to four satellites.
19、The measurements are termed “pseudo” because they are made by an imprecise user clock and contain fixed bias terms due to the user clock offsets from GPS time. GPS provides two navigation accuracy levels: the Precise Positioning Service (PPS) and the Standard Positioning Service (SPS). For the PPS,
20、the 95 percentile horizontal, vertical and time accuracies are 18 m, 30 m and 170 ns, respectively. The corresponding SPS accuracies are 100 m, 166 m and 330 ns. The velocity accuracy derived from PPS is almost entirely dependent on receiver design and user dynamics, but a 95 percentile accuracy of
21、0.2 m/s per axis can be typically achieved. 1.3. System description The system consists of three major segments: the Space Segment, the Control Segment and the User Segment. The principal function of each segment is as follows. 1.3.1 Space Segment The Space Segment comprises the GPS satellites, whic
22、h function as “celestial” reference points, emitting precisely time-encoded navigation signals from space. As currently planned, the operational constellation of 24 satellites will operate in 12 h orbits with a semi-major axis of about 26 600 km. The satellites will be placed in six orbital planes i
23、nclined 55” relative to the Equator. There will be four satellites per plane. The satellites will be optimally phased to provide visibility of at least five satellites to the users at 5” above the horizon. The satellite is a three-axis stabilized vehicle. The major elements of its principal navigati
24、on payload are the atomic frequency standard for accurate timing, the processor to store navigation data, the pseudo-random noise (PRN) signal assembly for generating the ranging signal, and the 1.U1.6 GHz band transmitting antenna whose shaped-beam gain pattern radiates near-uniform power of signal
25、s at the two 1.2/1.6 GHz band frequencies to users on or near the surface of the Earth. The dual-frequency transmission is to permit correction of ionospheric delays in signal propagation time. 1.3.2 Control Segment The Control Segment performs the tracking, computation, updating and monitoring func
26、tions needed to control all of the satellites in the system on a day-to-day basis. It consists of a Master Control Station (MCS) at Colorado Springs, Colorado (United States of America), where all data processing is performed, and five widely separated monitor stations at Ascension Island, Diego Gar
27、cia, Kwajalein, Colorado Springs and Hawaii. Co-located with three of the monitor stations are the upload ground antennas for satellite maintenance. The monitor stations passively track all satellites in view and accumulate ranging and Doppler data. These data are processed at the MCS for calculatio
28、n of the satellites ephemerides, clock drifts, and propagation delay and then used to generate upload messages. At least three times per day this updated information is transmitted to the satellites for memory storage and subsequent transmission by the satellites as part of the navigation messages t
29、o the users. 1.3.3 User Segment The User Segment is the collection of all user sets and their support equipment. The user set typically consists of an antenna, GPS receiver/processor, computer and input/output devices. It acquires and tracks the navigation signal from four or more satellites in view
30、, measures their RF transit times and Doppler frequency shifts, converts them to pseudo-ranges and pseudo-range rates, and solves for three-dimensional position, velocity, and system time. User equipment will range from relatively simple, light-weight manpack receivers to sophisticated receivers whi
31、ch are integrated with other navigation sensors or systems for accurate performance in highy dynamic environments. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*H* 1088 94 I 4855212 0523656 155 Rec. ITU-R M.1088 47 1.4 GP
32、S signal sructure The GPS navigational signal transmitted from the satellites consists of two modulated carriers: L1 at centre frequency of 1 575.42 MHz (154fo) and L2 at centre frequency of 1 227.6 MHz (120fo), wherefo = 10.23 MHz. fo is the output of the on-board atomic frequency standard to which
33、 all signals generated are coherently related. The L1 signal is modulated with both a precision (P) and a CoarseIacquisition (CIA) pseudo-random noise (PRN) code, each of which is modulo-2 added to a 50 bit/s binary navigation data stream prior to phase modulation. The P code is a long binary pseudo
34、-random sequence of zeros and ones with a clock rate of 10.23 MHz and a period of exactly one week. Every SaturdayISunday midnight, it restarts, serving as a running indicator of time of the week in the space vehicle. The CIA code is a short code, having a clock rate of 1.023 MHz and a period of exa
35、ctly 1 ms. The L2 signal is bi-phase modulated with either the P or CIA code, as selected by ground command. The same 50 bit/s data stream is modulo-2 added to the code prior to phase modulation as is done on the L1 signal. During normal operations, the P code will be transmitted on L2. The operatio
36、n of bi-phase modulation onto the carrier maps the binary PRN code sequences into sequences of plus and minus ones, and turns the modulo-:! addition into multiplication. Thus, the LI and L2 signals transmitted by the satellite can be described as a function of time, The functions of the PRN codes ar
37、e twofold: - they provide good multiple access properties among different satellites since all satellites transmit on the same two carrier frequencies and are differentiated from one another only by the unique pair of P and CIA codes they transmit, and their correlation properties allow precision me
38、asurement of time of arrival and rejection of multipath and interference signals. - The 50 bids data stream provides the navigation message which is formatted in five subframes of 6 s in length. Each subframe, consisting of ten 30-bit words starts with a telemetry word (TLM) and the CIA to P code ha
39、ndover word (HOW). The latter permits the CIA to P transfer to be made at the termination of any 6 s subframe. The first three subframes contain the clock correction and ephemeris data of the particular satellite being tracked. These messages are normally valid for a 4 h period. Subframes 4 and 5 co
40、ntain the almanac information that defines the less precise ephemerides of all the satellites in the constellation, as well as satellite health status, special messages, offset of GPS time from Universal Time Coordination (UTC), etc. There are 25 pages of data each for subframes 4 and 6 and they are
41、 transmitted on a rotating page basis. It therefore takes 6 s to receive one page and 2.5 min to receive all 25 data pages. 1.5 Signalpower and spectra The GPS satellites employ a shaped-beam antenna that radiates near-uniform power to system users. Transmitted signals are right-hand circularly pola
42、rized with the ellipticity for L1 no worse than 0.7 dB and for the L2 no worse than 2.0 dB for the angular range of f14.3“ from boresight. For satellite elevation angle 2 5“. the minimum guaranteed power is specified as -133 dBm for the L1 P code component and -130 dBm for the L1 CIA code component.
43、 The corresponding L2 power level carrying only P code is at least -136 dBm. The actual power received from the satellites is currently 4-5 dB higher than the specified values. 2. Operating frequency Primary operation (Ll) is in a segment of band 9 allocated to the radionavigation-satellite service.
44、 3. Telemetry functions The GPS is a passive system. There is no need for a navigational uplink. Therefore, spectrum is conserved by placing telemetry and housekeeping functions in bands allocated for such use. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Infor
45、mation Handling Services ITU-R RECMN*M= 3088 94 m 4855232 O523657 091 m 48 RW. ITU-R M.1088 4. Receiver characteristics Different GPS receiver configurations are suitable for different applications having various levels of host vehicles dynamics and interference environments. The typical characteris
46、tics of an inexpensive, unsophisticated receiver are given in Annex 1. Typical GPS user equipment is comprised of four principal components: antenna, receiver/processor, computer, and the CDU (control and display unit). The antenna in most cases is a relatively simple element providing hemispheric c
47、overage of both L1 and L2 frequencies. This omnidirectional antenna will have no need for pointing to receive all visible satellite signals, but it will also not have much capability to discriminate spatially against interference. The RF front end of the receiver typically consists of a bandpass fil
48、ter, a preamplifier, and a multi-state down- converter. The bandpass filter is to provide rejection of out-of-band signals. To prevent high-power interference from damaging the receiver, the preampiifierIfilter assembly will also have a diode limiter. After amplification and down-conversion to a con
49、venient IF, the receiver then generates and attempts to match the incoming code pattern for a particular satellite. The process is called correlation or code despreading. After code despreading, the receiver bandwidth is reduced whereas any interference signal will be spread by the locally generated replica code. The normal acquisition is to synchronize to the CIA signal and then transfer to P. This is the most vulnerable operating state of the receiver (state 1) to outside interference because it has not yet locked onto the code. Once the code is acquired, the alignment or synchr
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