Quantifying the Impact of Ionospheric Irregularities on GPS Receiver Carrier Tracking Loop Performance

摘要

A number of environmental factors are known to affectrnthe performance of the Global Positioning System (GPS)rnincluding electromagnetic interference from externalrnsources, atmospheric delays, ionospheric scintillations,rnand multipath, among which the most significant factorrncan be the disturbed ionosphere and associatedrnionospheric scintillation.rnThe main objectives of this paper are to investigate thernimpact of ionospheric scintillations on the GPS receiverrncarrier tracking loop performance, and to quantify theserneffects by defining receiver performance measures as arnfunction of scintillation parameters for different trackingrnloop configurations. To accomplish this, twornmethodologies are employed (see Figure 1): a physicsbasedrnsimulation of scintillations has been developed as arnmeans of providing a variety of test cases for thernsimulated carrier tracking loop. In addition, a widely usedrnstochastic model of scintillation activity is combined withrna tracking model to produce a collection of receiverrnperformance measures. This includes defining the phaserntracking error variance and velocity error variance as arnfunction of amplitude and phase scintillation parameters,rndefining the optimum tracking loop bandwidth for arnminimum probability of losing lock, and determining therneffect of ionospheric scintillations on the correlation ofrndifferent L-band signals employed by the GPS underrndifferent ionospheric conditions. Simulation results arernpresented and compared with theoretical predictions.rnAccording to the simulation results, strong signal fadingrnis almost always followed by a significant change in thernphase of the carrier signal which, in turn, results in largerncarrier tracking errors and loss of lock. For the simulatedrnsecond-order tracking loop, at C/N_0 = 42 dB-Hz, thernoptimum noise bandwidth (Bn1) for minimum probabilityrnof loss of lock is calculated as ～20 Hz. For B_(n1) ≤ 20 Hz,rnthe critical phase scintillation strength is calculated asrnT_(scin) = -10 dB. For higher magnitude scintillation, therntracking loop is expected to lose lock.rnThe scintillation-induced velocity error variance isrncalculated to be of the order of a few tens of (cm/s)~2 whichrncan be of concern for most (precision) applications. Therncorrelation coefficient for pairs of GPS L-bandrnfrequencies (L_1/L_5) is fairly close to unity and reducesrnmarginally as the variance of the electron densityrnfluctuations increases.