Satellite Navigation Integrity and integer ambiguity resolution

Transcription

1 Satellite Navigation Integrity and integer ambiguity resolution Picture: ESA AE4E08 Sandra Verhagen Course , lecture 12 1

2 Today s topics Integrity and RAIM Integer Ambiguity Resolution Study Section (not LMS algorithm in 7.5.3) 2

3 Integrity: performance measures Integrity = ability of a system to provide timely warnings to users when the system should not be used HPE/VPE : Horizontal/Vertical Position Error (not known; due to measurement noise and biases) HPL/VPL : Horizontal/Vertical Protection Level horizontal/vertical position is assured to be within region defined by HPL/VPL HAL/VAL : Horizontal/Vertical Alarm Limit position error that should result in an alarm being raised TTA : Time To Alarm time between occurrence of integrity event (position error too large) and alarm being raised 3

4 Integrity: performance measures HPE/VPE : Horizontal/Vertical Position Error (not known; due to measurement noise and biases) HPL/VPL : Horizontal/Vertical Protection Level horizontal/vertical position is assured to be within region defined by HPL/VPL (can be calculated) HAL/VAL : Horizontal/Vertical Alarm Limit position error that should result in an alarm being raised required: P( XPE > XPL) < integrity risk XPL > XAL alarm, system unavailable 4

5 Stanford plots

6 GPS PRN 23 Anomaly, 1 Jan, East, North and Up differences delf001s.04r North East Up [m] at 18:30 (UT) 3 min Number of epochs Not noticed by US for 3 hours Picked up by EGNOS Alternative: at 10 seconds interval

7 Receiver Autonomous Integrity monitoring up to 14x10 Local Overall (RAIM) 6 Model teststatistic delf001s.04r critical value α = 0.01 immediate response by Overall Model test Number of epochs = full hour period

8 RAIM - Overall model test RAIM: detect and correct for errors in GPS Overall model test: does provide good model? Model: Measurements: Residuals: y = Ax + e ( T ) 1 yy y xˆ = A Q A A Q y yˆ = eˆ= y yˆ Axˆ 1 T 1 yy Mismatch :

9 RAIM - Overall model test RAIM: detect and correct for errors in GPS Overall model test: does provide good model?

10 : number of times (= known) fits into ( = unknown integer) A Simple Ambiguity Resolution Example = = = a p D a p E p, 0 0, σ σ ϕ ρ λ ϕ ϕ Model: integer p λ 1 λ 2 λ 3 λ ( 2) a λ ( 1) a λ a ϕ A B ϕ a λ p : measured fraction distance AB ( = 2 mm) σ ϕ : measured distance AB ( = 20 cm) σ p ρ ρ a : unknown distance AB

11 Relative Positioning: Double Differencing elimination of receiver clock errors elimination of initial receiver phase offsets DD phase ambiguity is an integer number! DD code observation: ( kl) ( ( kl) ) T ( kl) ( kl ) ( kl) ur, i = 1r xur + iiur + Tur + ρ, ur ρ μ ε i l r x ur u k DD phase observation: ( kl ) ( kl ) T ( kl ) ( kl ) ( kl ) ( kl ) ur, i 1r xur iiur Tur inur, i, ur ( ) Φ = μ + + λ + ε Φ i relative receiver position integer DD ambiguity

12 Resolution of the DD ambiguities code observation: dm precision phase observation: mm precision, but: receiver-satellite geometry has to change considerably (long observation time) to solve position with mm-cm accuracy if DD ambiguities are resolved to integers within a short time (or instantaneously), positions (and other parameters) can be solved with mm-cm accuracy

13 Precision of relative GPS positioning code observations two epochs of data: varying time span N E U phase observations float N E U st.dev. [m] 1 st.dev. [m] fixed time [min] time [min]

14 Precision code vs. phase observations code observations phase observations U U E N 1 m E N 0.01 m both RELATIVE positioning phase: provided that the integer ambiguity is KNOWN

15 Ionosphere-fixed, -float, -weighted model Ionosphere-fixed model: no differential ionospheric delay parameters observations may be corrected a priori for ionosphere for short baselines only can already be based on single-frequency data Ionosphere-float model: estimation of differential ionospheric delays no a priori corrections for long baselines based on at least dual-frequency data Ionosphere-weighted model: ionosphere corrections from network RTK subtracted for medium to long baselines

16 Observation equations: GNSS model In book: y= AN+Gδx+ε y= Aa+Bb+e, a Z ; Qyy n y a b Q yy data vector ambiguities baseline coordinates & other unknowns variance-covariance matrix of data

17 SUCCESSFUL INTEGER AMBIGUITY RESOLUTION is the key to FAST and PRECISE GNSS parameter estimation (baseline coordinates, attitude angles, orbit parameters, atmospheric delays)

18 Integer estimation float solution estimate position and carrier ambiguities integer map estimate integer ambiguities fixed solution estimate position (ambiguities fixed) validate float solution validate integer ambiguities

19 Float and fixed solution Ambiguities not fixed Ambiguities fixed Up (meters) 0.0 Up (meters) North (meters) East (meters) North (meters) East (meters)

20 integer map Integer estimation n aˆ R S( aˆ) = a ( Z n no holes & no overlap there will always be ONE solution translation invariant

21 Different choices of integer estimators 2 integer rounding integer bootstrapping integer least-squares after their pull-in region

22 Ambiguity resolution Integer ambiguities are derived from stochastic observations Integer ambiguities are not deterministic but stochastic input (stochastic) n aˆ R S( aˆ) = a ( Z n output (stochastic)

23 Integer estimation Optimal integer estimator: integer least-squares 2 ( a= arg min aˆ z n z Z 2 Q aa ˆˆ

24 Ambiguity search space: a (hyper-) ellipsoid centered at â shape governed by Q aa ˆˆ find all integers z for which T ( a-z ˆ ) Q (a-z ˆ ) -1 aa ˆˆ 2 χ a χ should be set such that search space contains at least one integer vector select the z which provides minimum + a 1 2D example â 1,â 2 (float solution) candidate integer solution

25 Integer ambiguity resolution Float solution: least-squares Integer search: find integer solution with shortest weighted distance to float solution (weighted by variancecovariance matrix of float ambiguites) Search difficult due to correlations LAMBDA: transformation of search space to make it efficient a 2 a 1 + 2D example

26 Example: Ambiguity search space Two dimensions, geometry-free, short baseline â 2 * 16 candidates for a 1, but only one for a 2 such that (a 1, a 2 ) inside search space Search is inefficient â 1

27 Example: Ambiguity search space Two dimensions, geometry-free, short baseline â 2 * After decorrelation Number of candidates INSIDE search space is same Search is efficient â 1

28 Ambiguity estimation and success rate Example based on real data (1000 epochs) Distribution of original ambiguities Distribution of transformed ambiguities 5 1 a2 [cycle] 0 z2 [cycle] a1 [cycle] 1 Success rate: 99% = % of float ambiguities in pull-in region centered at correct integer value z1 [cycle]

29 Integer ambiguity resolution Successful ambiguity resolution depends on precision of float solution, which depends on: baseline length (tropo + iono delays) satellite geometry precision of code and phase observations # frequencies Change in satellite geometry helps (long duration)

30 LAMBDA method Integer estimation: optimal : maximum success rate efficient : (near) real-time LAMBDA Applicable to wide variety of models With or without relative satellite-receiver geometry Stationary or moving receivers With or without atmospheric delays Single- or multi-baseline One, two, three or more frequencies (any GNSS) LAMBDA

31 Baseline models Parameters Ranges N Geometryfree Rovingreceiver Stationaryreceiver Station coordinates N C Ambiguities C C C Ionospheric delays N *) N *) N *) N - New parameter introduced for each observation epoch C - Constant parameter for entire observation period *) - Long baselines only

32 Ambiguity Resolution Methods Search in the 3-dimensional position space (e.g. ambiguity function method); Now deprecated Linear combination of code and phase (using widelane/narrowlane combinations) performance worse with AS has been improved by LAMBDA: 2-dimensional ambiguity resolution/search problem Geometry-free model Search in the n-dimensional ambiguity space Geometry-based model

33 Summary and outlook We covered it all! (except for the applications) Next: Applications: your presentations Exam preparation: check blackboard! 33