Интерфейсный документ GPS, страница 13

The ionospheric model referred to in Figure 20-3 isdiscussed in paragraph 20.3.3.5.2.5 in conjunction with the related data contained in page 18 of subframe 4. TheERDcterm referred to in Figure 20-3 is discussed in paragraph 20.3.3.5.2.6 in conjunction with the related datacontained in page 13 of subframe 4.IS-GPS-200D7 Dec 200491af0, af1, af2, tocTGD*CLOCKCORRECTIONPOLYNOMIAL∆ trESTIMATE OF SVTRANSMISSION TIME∆ tSVTtropoTionoCODE PHASE OFFSET- TRUE SV CLOCK EFFECTS- EQUIPMENT GROUP DELAYDIFFERENTIAL EFFECTS- RELATIVISTIC EFFECTSTROPOSPHERICMODELIONOSPHERICMODEL*αn, βnFILTER ANDCOORDINATECONVERTERPSEUDORANGEDIVIDED BY THESPEED OF LIGHTGPS TIMEERD **cPATH DELAY- GEOMETRIC- TROPOSHERIC- IONOSPHERIC*USER CLOCK BIASGPS TIMEUSER POSITION,VELOCITY, andTIME (CLOCK BIAS)- RANGE DATA FROMOTHER SATELLITES- CALIBRATION DATA- AUXILIARY SENSOR* SINGLE FREQUENCY USER ONLY** OPTIONALFigure 20-3. Sample Application of Correction ParametersIS-GPS-200D7 Dec 20049220.3.3.4 Subframes 2 and 3.

The contents of words three through ten of subframes 2 and 3 are defined below,followed by material pertinent to the use of the data.20.3.3.4.1 Content of Subframes 2 and 3. The third through tenth words of subframes 2 and 3 shall each containsix parity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 ofword ten of each subframe for parity computation purposes. Bits 288 through 292 of subframe 2 shall contain theAge of Data Offset (AODO) term for the navigation message correction table (NMCT) contained in subframe 4(reference paragraph 20.3.3.5.1.9). The remaining 375 bits of those two subframes shall contain the ephemerisrepresentation parameters of the transmitting SV.The ephemeris parameters describe the orbit during the curve fit intervals described in section 20.3.4. Table 20-IIgives the definition of the orbital parameters using terminology typical of Keplerian orbital parameters; it shall benoted, however, that the transmitted parameter values are such that they provide the best trajectory fit in EarthCentered, Earth-Fixed (ECEF) coordinates for each specific fit interval.

The user shall not interpret intermediatecoordinate values as pertaining to any conventional coordinate system.The issue of ephemeris data (IODE) term shall provide the user with a convenient means for detecting any change inthe ephemeris representation parameters. The IODE is provided in both subframes 2 and 3 for the purpose ofcomparison with the 8 LSBs of the IODC term in subframe 1. Whenever these three terms do not match, a data setcutover has occurred and new data must be collected. The timing of the IODE and constraints on the IODC andIODE are defined in paragraph 20.3.4.4.Any change in the subframe 2 and 3 data will be accomplished with a simultaneous change in both IODE words.The CS shall assure that the toe value, for at least the first data set transmitted by an SV after an upload, is differentfrom that transmitted prior to the cutover (reference paragraph 20.3.4.5).A "fit interval" flag is provided in subframe 2 to indicate whether the ephemerides are based on a four-hour fitinterval or a fit interval greater than four hours (reference paragraph 20.3.3.4.3.1).The AODO word is provided in subframe 2 to enable the user to determine the validity time for the NMCT dataprovided in subframe 4 of the transmitting SV.

The related algorithm is given in paragraph 20.3.3.4.4.IS-GPS-200D7 Dec 200493Table 20-II.Ephemeris Data DefinitionsM0Mean Anomaly at Reference Time∆nMean Motion Difference From Computed ValueeEccentricityASquare Root of the Semi-Major AxisΩ0Longitude of Ascending Node of Orbit Plane at Weekly Epochi0Inclination Angle at Reference TimeωArgument of Perigee•ΩRate of Right AscensionIDOTRate of Inclination AngleCucAmplitude of the Cosine Harmonic Correction Term to the Argument of LatitudeCusAmplitude of the Sine Harmonic Correction Term to the Argument of LatitudeCrcAmplitude of the Cosine Harmonic Correction Term to the Orbit RadiusCrsAmplitude of the Sine Harmonic Correction Term to the Orbit RadiusCicAmplitude of the Cosine Harmonic Correction Term to the Angle of InclinationCisAmplitude of the Sine Harmonic Correction Term to the Angle of InclinationtoeReference Time Ephemeris (reference paragraph 20.3.4.5)IODEIssue of Data (Ephemeris)IS-GPS-200D7 Dec 20049420.3.3.4.2 Subframe 2 and 3 Parameter Characteristics.

For each ephemeris parameter contained in subframes 2and 3, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range, and the unitsshall be as specified in Table 20-III.The AODO word (which is not an ephemeris parameter) is a five-bit unsigned term with an LSB scale factor of900, a range from 0 to 31, and units of seconds.20.3.3.4.3 User Algorithm for Ephemeris Determination. The user shall compute the ECEF coordinates of positionfor the phase center of the SVs’ antennas utilizing a variation of the equations shown in Table 20-IV.

Subframes 2and 3 parameters are Keplerian in appearance; the values of these parameters, however, are produced by the CS viaa least squares curve fit of the predicted ephemeris of the phase center of the SVs’ antennas (time-positionquadruples; t, x, y, z expressed in ECEF coordinates). Particulars concerning the periods of the curve fit, theresultant accuracy, and the applicable coordinate system are given in the following subparagraphs.20.3.3.4.3.1 Curve Fit Intervals.

Bit 17 in word 10 of subframe 2 is a "fit interval" flag which indicates the curvefit interval used by the CS in determining the ephemeris parameters, as follows:0 = 4 hours,1 = greater than 4 hours.The relationship of the curve-fit interval to transmission time and the timing of the curve-fit intervals is covered insection 20.3.4.IS-GPS-200D7 Dec 200495Table 20-III.ParameterNo.

of Bits**IODE8Crs16*2-5∆n16*2-43M032*2-31Cuc16*2-29e322-33Scale Factor (LSB)Effective Range***Units(see text)meterssemi-circles/secsemi-circlesradians0.03-29Cus16*2A322-19dimensionlessradiansmeters4604,784toe162Cic16*2-29Ω032*2-31Cis16*2-29i032*2-31Crc16*2-5ω32*2-31Ω24*2-43semi-circles/sec14*-43semi-circles/sec•IDOT******Ephemeris Parameters2secondsradianssemi-circlesradianssemi-circlesmeterssemi-circlesParameters so indicated shall be two's complement, with the sign bit (+ or -) occupying the MSB;See Figure 20-1 for complete bit allocation in subframe;Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor.IS-GPS-200D7 Dec 200496Table 20-IV.

Elements of Coordinate Systems (sheet 1 of 2)µ = 3.986005 x 1014 meters3/sec2•Ω e = 7.2921151467 x 10-5 rad/secA=n0 =(A)2µA3WGS 84 value of the earth's gravitational constant forGPS userWGS 84 value of the earth's rotation rateSemi-major axisComputed mean motion (rad/sec)tk = t - toe*Time from ephemeris reference epochn = n0 + ∆nCorrected mean motionMk = M0 + ntkMean anomalyMk = Ek - e sin EkKepler's Equation for Eccentric Anomaly (may be solvedby iteration) (radians) sin ν k ν k = tan −1  cos ν k True Anomaly 1 − e 2 sin E / (1 − e cos E ) kk= tan −1 ()() cosEe/1ecosE−−kk*t is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light).Furthermore, tk shall be the actual total time difference between the time t and the epoch time toe, and mustaccount for beginning or end of week crossovers.

That is, if tk is greater than 302,400 seconds, subtract604,800 seconds from tk. If tk is less than -302,400 seconds, add 604,800 seconds to tk.IS-GPS-200D7 Dec 200497Table 20-IV. Elements of Coordinate Systems (sheet 2 of 2) e + cos ν k E k = cos −1 1 + e cos ν k Eccentric AnomalyΦ k = νk + ωArgument of Latitude}δuk = cussin2Φk + cuccos2Φkδrk = crssin2Φk + crccos2Φkδik = cissin2Φk + ciccos2ΦkArgument of Latitude CorrectionRadius CorrectionInclination CorrectionSecond Harmonic Perturbationsuk = Φk + δukCorrected Argument of Latituderk = A(1 - e cosEk) + δrkCorrected Radiusik = i0 + δik + (IDOT) tkCorrected Inclinationxk′ = rkcosukyk′ = rksinuk}•Positions in orbital plane.••Ωk = Ω0 + ( Ω - Ω e ) tk - Ω e toexk = xk′cosΩk - yk′cosiksinΩkyk = xk′sinΩk + yk′cosikcosΩkzk = yk′sinikCorrected longitude of ascending node.}Earth-fixed coordinates.IS-GPS-200D7 Dec 20049820.3.3.4.3.2Parameter Sensitivity.The sensitivity of the SV's antenna phase center position to smallperturbations in most ephemeris parameters is extreme.

The sensitivity of position to the parametersCrs is about one meter/meter.A , Crc andThe sensitivity of position to the angular parameters is on the order of 108meters/semicircle, and to the angular rate parameters is on the order of 1012 meters/semicircle/second. Because ofthis extreme sensitivity to angular perturbations, the value of π used in the curve fit is given here. π is amathematical constant, the ratio of a circle's circumference to its diameter. Here π is taken asπ = 3.1415926535898.20.3.3.4.3.3 Coordinate Systems.20.3.3.4.3.3.1 ECEF Coordinate System. The equations given in Table 20-IV provide the SV's antenna phasecenter position in the WGS 84 ECEF coordinate system defined as follows:Origin*=Z-Axis** =Earth's center of massThe direction of the IERS (International Earth Rotation and Reference Systems Service)Reference Pole (IRP)X-Axis=Intersection of the IERS Reference Meridian (IRM) and the plane passing through theorigin and normal to the Z-axisY-Axis=Completes a right-handed, Earth-Centered, Earth-Fixed orthogonal coordinate system* Geometric center of the WGS 84 Ellipsoid** Rotational axis of the WGS 84 EllipsoidIS-GPS-200D7 Dec 20049920.3.3.4.3.3.2 Earth-Centered, Inertial (ECI) Coordinate System.