IS-GPS-800D (797938), страница 8
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At the best-case location within the SV footprint (i.e., nominallydirectly below the SV along the SV nadir 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 thefollowing relationship to the ED URA:URAED IndexURAED (meters)156144.00< URAED143072.00< 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< URAED ≤6.8523.40< URAED ≤4.8544(or no accuracy prediction is available)IS-GPS-800D24 Sep 201312.40< 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 ≤0.02URAED ≤0.01-15-16No 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),• N = -16 or N = 15 shall indicate the absence of an accuracy prediction and shall advise thestandard positioning service 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.45IS-GPS-800D24 Sep 2013The nominal URAED value (X) is suitable for use as a conservative prediction of the RMS EDrange errors for accuracy-related purposes in the pseudorange domain (e.g., measurementdeweighting, RAIM, FOM computations).
Integrity properties of the IAURAED are specifiedwith respect to the scaled (multiplied by either 4.42 or 5.73 as appropriate) upper bound valuesof the broadcast URAED index (see 30.3.3.1.1).For the nominal URAED value and the IAURAED value, users may compute an adjusted URAEDvalue as a function of SV elevation angle (E), for E ≥ 0, as follows:Adjusted Nominal URAED= Nominal URAED (sin(E+90 degrees))Adjusted IAURAED= IAURAED (sin(E+90 degrees))URAED and IAURAED account for SIS contributions to user range error which include, but arenot limited to, the following: LSB representation/truncation error, alongtrack ephemeris errors,and crosstrack ephemeris errors. URAED and IAURAED do not account for user range errorcontributions due to the inaccuracy of the broadcast ionospheric data parameters used in thesingle-frequency ionospheric model or for other atmospheric effects.3.5.3.6 Ephemeris Parameter CharacteristicsFor each ephemeris parameter contained in subframe 2, the bit lengths, scale factors, ranges, andunits are given in Table 3.5-1.
See Figure 3.5-1 for complete bit allocation in subframe 2.3.5.3.6.1 User Algorithm for Determination of SV PositionThe user shall compute the ECEF coordinates of position for the SV’s antenna phase center(APC) utilizing a variation of the equations shown in Table 3.5-2. The ephemeris parameters areKeplerian in appearance; however, the values of these parameters are produced by the SV via aleast squares curve fit of the predicted ephemeris of the SV APC (time-position quadruples: t, x,y, z expressed in ECEF coordinates). Particulars concerning the applicable coordinate systemare given in Sections 20.3.3.4.3.3 and 20.3.3.4.3.4 of IS-GPS-200.The sensitivity of the SV’s position to small perturbations in most ephemeris parameters isextreme.
The sensitivity of position to the parameters A, Crc-n, and Crs-n is about onemeter/meter. The sensitivity of position to the angular parameters is on the order of 108meters/semi-circle, and to the angular rate parameters is on the order of 1012 meters/semicircle/second. Because of this extreme sensitivity to angular perturbations, the value of π used inthe curve fit is given here. π is a mathematical constant, the ratio of a circle’s circumference toits diameter. Here π is taken as 3.1415926535898.46IS-GPS-800D24 Sep 2013Table 3.5-2.
Elements of Coordinate System (part 1 of 2)Element/EquationDescriptionµ = 3.986005 x 10 meters /sec1432WGS 84 value of the earth’s gravitational constant for GPS user•Ωe = 7.2921151467 x 10-5 rad/secWGS 84 value of the earth’s rotation rateA0 = AREF + ∆A *Semi-Major Axis at reference time•Ak = A0 + (A) tkn0 =µA03Semi-Major AxisComputed Mean Motion (rad/sec)tk = t – toe **Time from ephemeris reference time∆nA = ∆n0 +½ ∆n•0 tkMean motion difference from computed valuenA = n0 + ∆nACorrected Mean MotionMk = M0 + nA tkMean AnomalyMk = Ek – en sin EkKepler’s equation for Eccentric Anomaly (radians)(may be solved by iteration) sin ν k νk = tan-1 cos ν k True Anomaly 1 − e 2 sin E / (1 − e cos E ) nknk = tan-1 (cos E k − e n ) / (1 − e n cos E k ) e + cos ν k Ek = cos-1 n1 + e n cos ν k *Eccentric AnomalyAREF = 26,559,710 meters** 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 difference between the time t and the epoch time toe, and must accountfor beginning or end of week crossovers.
That is if tk is greater than 302,400 seconds, subtract 604,800seconds from tk. If tk is less than -302,400 seconds, add 604,800 seconds to tk.47IS-GPS-800D24 Sep 2013Table 3.5-2. Elements of Coordinate System (part 2 of 2)Element/Equation *Φk = νk + ωnDescriptionArgument of Latitudeδuk = Cus-nsin2Φk + Cuc-ncos2ΦkArgument of Latitude Correctionδrk = Crs-nsin2Φk + Crc-ncos2ΦkRadial Correctionδik = Cis-nsin2Φk + Cic-ncos2ΦkInclination Correctionuk = Φk + δukSecond HarmonicPerturbationsCorrected Argument of Latituderk= Ak(1 – en cos Ek) + δrkCorrected Radiusik= io-n + (io-n-DOT)tk + δikCorrected Inclinationxk' = rk cos ukPositions in orbital planeyk' = rk sin uk•••Ω = ΩREF + ∆Ω ***••Rate of Right Ascension•Ωk = Ω0-n + ( Ω − Ωe ) tk – Ωe toeCorrected Longitude of Ascending Nodexk = xk' cos Ωk − yk' cos ik sin Ωkyk = xk' sin Ωk + yk' cos ik cos ΩkEarth-fixed coordinates of SV antenna phase centerzk = yk' sin ik•*** ΩREF = −2.6 x 10-9 semi-circles/second.48IS-GPS-800D24 Sep 20133.5.3.7 Clock Parameter CharacteristicsThe bit lengths, scale factors, ranges, and units of the clock correction parameters shall be asspecified in Table 3.5-1.3.5.3.7.1 User Algorithms for SV Clock Correction DataThe algorithms defined in paragraph 20.3.3.3.3.1 of IS-GPS-200 allow all users to correct thecode phase time received from the SV with respect to both SV code phase offset and relativisticeffects.
However, since the SV clock corrections of equations in paragraph 20.3.3.3.3.1 of ISGPS-200 are estimated by the CS using dual frequency L1 P(Y) and L2 P(Y) codemeasurements, the single-frequency (L1) user and the dual-frequency (L1/L2 and L1/L5) usermust apply additional terms to the SV clock correction equations. These terms are described inparagraph 3.5.3.9. In addition, users shall use toe, provided in bits 39 through 49 of subframe 2,to replace toc in the algorithms in paragraph 20.3.3.3.3.1 of IS-GPS-200.3.5.3.8 Non-Elevation Dependent (NED) Accuracy EstimatesBits 460 through 470 of subframe 2 shall contain the URANED0 Index, URANED1 Index, andURANED2 Index of the SV (reference paragraph 6.2.1) for the user. The following equationstogether with the broadcast URANED0 Index, URANED1 Index, and URANED2 Index shall give theclock-related user range accuracy of IAURANED over the current clock/ephemeris fit interval.While the actual NED-related URA may vary over the satellite footprint, the IAURANEDcalculated using the parameters in message type 10 at each instant during the currentclock/ephemeris fit interval shall bound the maximum IAURANED expected for the worst-caselocation within the satellite footprint at that instant.Non-elevation dependent (URANED) accounts for signal-in-space contributions to user rangeerror that include, but are not limited to, the following: the net effect of clock parameter andcode phase error in the transmitted signal for single-frequency users who correct the code phaseas described in Section 3.5.3.9.1, as well as the net effect of clock parameter, code phase, andintersignal correction error for dual-frequency L1/L2 and L1/L5 users who correct for groupdelay and ionospheric effects as described in Section 3.5.3.9.2 and Section 3.5.3.9.3.The user shall calculate the NED-related URA with the equation (in meters);IAURANED = URANED0 + URANED1 (t - top + 604,800*(WN - WNop))for t - top + 604,800*(WN - WNop) ≤ 93,600 seconds49IS-GPS-800D24 Sep 2013IAURANED = URANED0 + URANED1*(t - top + 604,800*(WN - WNop)) + URANED2*(t - top +604,800*(WN - WNop) - 93,600)2for t - top + 604,800*(WN - WNop) > 93,600 secondswheret is the GPS system timeThe CS shall derive URANED0, URANED1, and URANED2 indexes which, when used together inthe above equations, results in the minimum IAURANED that is greater than the predictedIAURANED during the clock/ephemeris fit interval.The user shall use the broadcast URANED0 index to derive the URANED0 value.
The URANED0index is a signed, two’s complement integer in the range of +15 to -16 and has the followingrelationship to the URANED0 value:URANED0 IndexURANED0 (meters)156144.00< URANED0 (or no accuracy prediction is available)143072.00< URANED0≤6144.00131536.00< URANED0≤3072.0012768.00< URANED0≤1536.0011384.00< URANED0≤768.0010192.00< URANED0≤384.00996.00< URANED0≤192.00848.00< URANED0≤96.00724.00< URANED0≤48.00613.65< URANED0≤24.0059.65< URANED0≤13.6546.85< URANED0≤9.6534.85< URANED0≤6.8523.40< URANED0≤4.8550IS-GPS-800D24 Sep 201312.40< URANED0≤3.4001.70< URANED0≤2.40-11.20< URANED0≤1.70-20.85< URANED0≤1.20-30.60< URANED0≤0.85-40.43< URANED0≤0.60-50.30< URANED0≤0.43-60.21< URANED0≤0.30-70.15< URANED0≤0.21-80.11< URANED0≤0.15-90.08< URANED0≤0.11-100.06< URANED0≤0.08-110.04< URANED0≤0.06-120.03< URANED0≤0.04-130.02< URANED0≤0.03-140.01< URANED0≤0.02URANED0≤0.01-15-16No accuracy prediction available-use at own riskFor each URANED0 index (N), users may compute a nominal URANED0 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),• N = -16 or N = 15 shall indicate the absence of an accuracy prediction and shall advise thestandard positioning service 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.51IS-GPS-800D24 Sep 2013The nominal URANED0 value (X) shall be suitable for use as a conservative prediction of theRMS NED range errors for accuracy-related purposes in the pseudorange domain (e.g.,measurement de-weighting RAIM, FOM computations).
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