IS-GPS-200F (811524), страница 24
Текст из файла (страница 24)
The contents of the SV health,ephemeris representation and accuracy parameters in message types 10 and 11 are defined below, followed bymaterial pertinent to the use of the data. Message type 10 in conjunction with message type 11, provides users withthe requisite data to calculate SV position. The general format of message types 10 and 11 consists of data fields forreference time tags, a set of gravitational harmonic correction terms, rates and rate corrections to quasi-Keplerianelements, and an accuracy indicator for ephemeris-related data.The ephemeris parameters in the message type 10 and type 11 describe the orbit of the transmitting SV during thecurve fit interval of three hours.
The nominal transmission interval is two hours, and shall coincide with the firsttwo hours of the curve fit interval. The predicted period of applicability for ephemeris data coincides with the entirethree-hour curve fit interval. Table 30-I gives the definition of the orbital parameters using terminology typical ofKeplerian orbital parameters; it is noted, however, that the transmitted parameter values are expressed such thatthey provide the best trajectory fit in Earth-Centered, Earth-Fixed (ECEF) coordinates for each specific fit interval.The user shall not interpret intermediate coordinate values as pertaining to any conventional coordinate system.The toe term shall provide the user with a convenient means for detecting any change in the ephemeris representationparameters. The toe is provided in both message type 10 and 11 for the purpose of comparison with the toc term inmessage type 30 - 37.
Whenever these three terms do not match, a data set cutover has occurred and new data mustbe collected. The timing of the toe and constraints on the toc and toe are defined in paragraph 30.3.4.4.Any change in the message type 10 and 11 ephemeris data will be accomplished with a simultaneous change in thetoe value. The CS will assure the toe value for Block IIR-M/IIF and SS will assure the toe value for Block III, for atleast the first data set transmitted by an SV after an upload, is different from that transmitted prior to the cutover.See Section 30.3.4.5 for additional information regarding toe.The CNAV messages contain information that allows users to take advantage of situations when integrity is assuredto the enhanced level.
This is accomplished using a composite integrity assured URA value in conjunction with anintegrity status flag. The composite integrity assured URA (IAURA) value is the RSS of an elevation-dependentfunction of the upper bound value of the URAED component and the upper bound value of the URANED component.The composite IAURA value is assured to the enhanced level only when the integrity status flag is “1”; otherwisethe IAURA value is assured to the legacy level.Bit 272 of Message Type 10 is the Integrity Status Flag (ISF).
A "0" in bit position 272 indicates that the conveyingsignal is provided with the legacy level of integrity assurance. That is, the probability that the instantaneous URE ofthe conveying signal exceeds 4.42 times the current broadcast IAURA value, for more than 5.2 seconds, without anaccompanying alert, is less than 1E-5 per hour. A "1" in bit-position 272 indicates that the conveying signal isprovided with an enhanced level of integrity assurance. That is, the probability that the instantaneous URE of the154IS-GPS-200F21 Sep 2011conveying signal exceeds 5.73 times the current broadcast IAURA value, for more than 5.2 seconds, without anaccompanying alert, is less than 1E-8 per hour.
The probabilities associated with the nominal and lower boundvalues of the current broadcast URAED index, URANED indexes, and related URA values are not defined.In this context, an "alert" is defined as any indication or characteristic of the conveying signal, as specifiedelsewhere in this document, which signifies to users that the conveying signal may be invalid or should not be used,such as the health bits not indicating operational-healthy, broadcasting non-standard code parity error, etc.30.3.3.1.1.1 Transmission Week Number.
Bits 39 through 51 of message type 10 shall contain 13 bits which are amodulo-8192 binary representation of the current GPS week number at the start of the data set transmission interval(see paragraph 6.2.4).30.3.3.1.1.2 Signal Health (L1/L2/L5). The three, one-bit, health indication in bits 52 through 54 of message type10 refers to the L1, L2, and L5 signals of the transmitting SV.
The health of each signal is indicated by:0 = Signal OK,1 = Signal bad or unavailable.The predicted health data will be updated at the time of upload when a new data set has been built by the CS. Thetransmitted health data may not correspond to the actual health of the transmitting SV.Additional SV health data are given in the almanac in messages types 12, 31, and 37. The data given in messagetype 10 may differ from that shown in other messages of the transmitting SV and/or other SVs since the latter maybe updated at a different time.155IS-GPS-200F21 Sep 201130.3.3.1.1.3 Data Predict Time of Week. Bits 55 through 65 of message type 10 shall contain the data predict timeof week (top).
The top term provides the epoch time of week of the state estimate utilized for the prediction ofsatellite quasi-Keplerian ephemeris parameters.30.3.3.1.1.4 Elevation-Dependent (ED) Accuracy. Bits 66 through 70 of message type 10 shall contain theelevation-dependent (ED) component User Range Accuracy (URAED) index for the standard positioning serviceuser.
The URAED index shall provide the ED-related URA index or the current ephemeris curve fit interval. Whilethe ED-related URA may vary over the ephemeris curve fit interval and over the satellite footprint, the URAED index(N) in message type 10 shall correspond to the maximum URAED expected over the entire ephemeris curve fitinterval for the worst-case location within the SV footprint (i.e., nominally two points at the edge of the SVfootprint). At the best-case location within the SV footprint (i.e., nominally directly below the SV along the SVnadir vector), the corresponding URAED is zero.The URAED index is a signed, two’s complement integer in the range of +15 to -16 and has the followingrelationship to the ED URA:URAED IndexURAED (meters)156144.00< URAED(or no accuracy prediction is available)143072.00< URAED≤6144.00131536.00< URAED≤3072.0012768.00< URAED≤1536.0011384.00< URAED≤768.0010192.00< URAED≤384.00996.00 < URAED≤192.00848.00 < URAED≤96.00724.00 < URAED≤48.00613.65 < URAED≤24.0059.65 < URAED≤13.6546.85 < URAED ≤9.6534.85 < URAED6.85≤156IS-GPS-200F21 Sep 201123.40 < URAED≤4.8512.40 < URAED≤3.4001.70 < URAED≤2.40-11.20 < URAED≤1.70-20.85 < URAED ≤1.20-30.60 < URAED≤0.85-40.43 < URAED≤0.60-50.30 < URAED≤0.43-60.21 < URAED≤0.30-70.15 < URAED≤0.21-80.11 < URAED≤0.15-90.08 < URAED≤0.11-100.06 < URAED≤0.08-110.04 < URAED≤0.06-120.03 < URAED≤0.04-130.02 < URAED≤0.03-140.01 < URAED≤0.02-15URAED≤0.01-16No accuracy prediction available-use at own riskFor each URAED index (N), users may compute a nominal URAED value (X) as given by:• If the value of N is 6 or less, but more than -16, X = 2(1 + N/2),• If the value of N is 6 or more, but less than 15, X = 2(N - 2),157IS-GPS-200F21 Sep 2011• N = -16 or N = 15 shall indicate the absence of an accuracy prediction and shall advise the standard positioningservice user to use that SV at his own risk.For N = 1, 3, and 5, X should be rounded to 2.8, 5.7, and 11.3 meters, respectively.The nominal URAED value (X) is suitable for use as a conservative prediction of the RMS ED range errors foraccuracy-related purposes in the pseudorange domain (e.g., measurement deweighting, RAIM, FOM computations).Integrity properties of the IAURAED are specified with respect to the scaled (multiplied by either 4.42 or 5.73 asappropriate) upper bound values of the broadcast URAED index (see 30.3.3.1.1).For the nominal URAED value and the IAURAED value, users may compute an adjusted URAED value as a functionof SV elevation angle (E), for E ≥ 0, as follows:Adjusted Nominal URAEDAdjusted IAURAED= Nominal URAED (sin(E+90 degrees))= IAURAED (sin(E+90 degrees))URAED and IAURAED account for SIS contributions to user range error which include, but are not limited to, thefollowing: LSB representation/truncation error, alongtrack ephemeris errors, and crosstrack ephemeris errors.URAED and IAURAED do not account for user range error contributions due to the inaccuracy of the broadcastionospheric data parameters used in the single-frequency ionospheric model or for other atmospheric effects.30.3.3.1.2 Message Type 10 and 11 Ephemeris Parameter Characteristics.
For each ephemeris parameter containedin message types 10 and 11, the number of bits, the scale factor of the least significant bit (LSB) (which is the lastbit received), the range, and the units are as specified in Table 30-I. See Figures 30-1 and 30-2 for complete bitallocation in message types 10 and 11.30.3.3.1.3 User Algorithm for Determination of SV Position.
The user shall compute the ECEF coordinates ofposition for the SV’s antenna phase center (APC) utilizing a variation of the equations shown in Table 30-II. Theephemeris parameters are Keplerian in appearance; however, the values of these parameters are produced by the CS(Block IIR-M/IIF) and SS (GPS III) via a least squares curve fit of the predicted ephemeris of the SV APC (timeposition quadruples: t, x, y, z expressed in ECEF coordinates). Particulars concerning the applicable coordinatesystem are given in Sections 20.3.3.4.3.3 and 20.3.3.4.3.4.The sensitivity of the SV’s position to small perturbations in most ephemeris parameters is extreme. The sensitivityof position to the parameters A, Crc-n, and Crs-n is about one meter/meter. The sensitivity of position to the angularparameters is on the order of 108 meters/semi-circle, and to the angular rate parameters is on the order of 1012meters/semi-circle/second.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
