An Assisted GPS Acquisition Method using L2 Civil Signal in Weak Signal Environment

Transcription

1 Journal of Global Positioning Systems (004) Vol. 3, No. -: 5-3 An Assisted GPS Aquisition ethod using L Civil Signal in Wea Signal Environment Deu Jae Cho Department of Eletronis, Chungnam National University, Korea panda@slab.nu.a.r; Tel: ; Fax: Chansi Par Shool of Eletrial and Computer Engineering, Chungbu National University, Korea hansp@hungbu.a.r; Tel: ; Fax: Sang Jeong Lee Division of Eletrial and Computer Engineering, Chungnam National University, Korea eesjl@slab.nu.a.r; Tel: ; Fax: Reeived: 5 Nov 004 / Aepted: 3 Feb 005 Abstrat. Reently, there has been inreasing demands on the positioning apability in wea signal environment suh as inside building and urban area. The present assisted GPS tehnology uses GPS L signals only. eanwhile, aording to the GPS modernization plan, Blo IIR- GPS satellite will be first launhed in 005, transmitting the ivil ode in L frequeny as well as in L frequeny with the updated signal struture. Sine the L ivil ode has a worst-ase ross orrelation performane of 45 db (over 5 times better than db ross orrelation performane of the L C/A ode), it will be muh more effetive in wea signal environment. This paper proposes an assisted GPS aquisition method using L ivil signals. It will show that the aquisition suess rate of the proposed assisted GPS aquisition method is better than that of the existing assisted GPS method using L signals in the same environment. The onstellation of the next generation GPS satellites is sheduled to launh in 005. Therefore, in order to design and test the assisted GPS aquiring the L ivil signal, it is neessary to design a signal generator whih an generate the L ivil signal. The signal generator will be designed using the pseudo random noise (PRN) ode generation method and navigation message protool defined in GPS ICD PIRN 00C-007B. Finally, through the simulations using the designed signal transmitter, the suess rate of the proposed assisted GPS aquisition method will be ompared with that of the existing assisted GPS method to show the performane improvements. Key words: Aquisition, L Civil Signal, Wea Signal Introdution A signal proessing of GPS reeiver is omposed of signal aquisition, signal traing and navigation in aordane with funtion. Partiularly, the performane of the signal aquisition has influene on TTFF (Time to First Fix) and RF sensitivity of GPS reeiver. The RF sensitivity of GPS reeiver is defined as the minimum power for aquiring the GPS signal. The GPS L C/A signals and L ivil signals (LCS) in Blo IIR- satellites are guaranteed minimum -8.5dBm and - 3.4dBm signal strength eah into a 3dBi linearly polarized user reeiving antenna at worst normal orientation when the satellite is above a 5-degree elevation angle (ICD PIRN-00C-007B, 00). It is diffiult for GPS reeiver to aquire GPS signals in the ase of being obstales in the line of sight sine the GPS signal strength is very low (Haddrell and Pratt, 00). From this viewpoint, it an be said that the RF sensitivity of GPS reeivers is the dominant fator that has influene on the performane of GPS reeivers. Sine the L ivil ode provides better protetion (4dB) than C/A against ode ross orrelation and ontinuous wave interferene, it will be muh more effetive in wea signal environment. This paper proposes an assisted GPS aquisition method using L ivil signals. In setion, this paper summarizes the struture and the property of L ivil signal

2 6 Journal of Global Positioning Systems omparing with those of L C/A ode. In order to design and test the assisted GPS aquiring the L ivil signal not existing yet, it is neessary to design LCS generator. So setion 3 desribes a software-based LCS generator designed in this paper. Setion 4 proposes an aquisition method for solving the problem of squaring loss in wea signal environment sine the long oherent integration inreases the number of frequeny searh ells and the non-oherent integration of wea GPS signals indues the squaring loss. In setion 5, through the simulations using the designed signal generator, it will show that the aquisition suess rate of the proposed assisted GPS aquisition method is better than that of the existing assisted GPS method using L signals in the same environment. L Civil Signal Struture The new signal struture adds (ilitary) odes and enhanes L ivilian odes (Hartman et al, 000). L ivilian odes are omposed of the L ivil moderate (C) and L ivil long (CL) odes as part of the L ivilian enhanements. The spetrums of urrent and proposed GPS signals are shown in Figure. And Table shows the harateristis of LCS and existing L C/A ode. Present Signal (Blo II/IIA/IIR) Next Generation Of Capability (Blo IIR-) Civil Safety of Life Appliations (Blo IIF and beyond) L5 P() P() P() L LCS LCS P() P() P() L C/A C/A C/A Tab. Cross Correlation Protetion Carrier Frequeny Length Clo Fully Correlation Phases (Hz) (hips) (Hz) Available Protetion,575.4 (L C/A),03.03 Bi-Phase Now > db,7.60 (LCS) 0,30 (C) 767,50 (CL).03 Bi-Phase ~03 > 45 db With the advent of the modernized GPS IIR- satellites there will be an immediate benefit to all ivilian GPS users inluding ivil aviation. This is due to the harateristis of the LC ode on the L frequeny. The LC ode signal is muh more robust than the existing L C/A ode and has muh better ross orrelation properties. The minimum LC ode ross orrelation protetion is 45 db while db for the existing L C/A ode as summarized in Table (Fontana et al., 00). This greater ross orrelation protetion is valuable in many environments where a wea GPS signal may be interfered with by another stronger GPS signal. It is benefiial to emergeny indoor positioning or to personal navigation in wooded areas. Beause of this great ross orrelation protetion, the LC ode signal also has a higher data reovery threshold and a better ode traing performane. The superior ross orrelation properties also enable the GPS reeivers to implement faster aquisition strategies beause it an redue the number of false alarms (Diggelen and Abraham, 00). The LC ode is omposed of two multiplexed ode. Eah of two odes is a disjoint repeating segment of a maximal length ode generated by a 7 bit shift register with 5 taps defined by a oder initial state whih in turn determined by the satellite ID and ode length. The diagram of LC ode generator is shown in Figure (ICD PIRN-00C-007B, 00). The C ode signal is a 0,30 hip sequene repeating every 0ms. The CL ode signal is a 767,50 hip sequene repeating every.5 seonds Hz 7.60 Hz Hz Last state of CL/C Fig. odernized GPS Signal Evolution Tab. Summary of Signal Charateristis Compare L C/A L C L CL L C/CL (TD) Type Gold aximal Length aximal Length aximal Length Chip Rate (hips/se) Length (Chips),03 0,30 767,50,534,500 Repeat Rate (mse) Carrier Frequeny (Hz) Bit Rate 50 bps 5 bps No message 50 sps SB Initial state of CL/C Fig. LC Output The LC NAV (NAVigation) data an be either 50 bps data or 5 bps data, whih is oded with a rate FEC (Forward Error Corretion) onvolutional oder. The oder state history is reset to zero at the beginning of eah data message. The resulting 50 sps (symbol per seond) LSB

3 Cho et al: An Assisted GPS Aquisition ethod using L Civil Signal in Wea Signal Environment 7 L5 - Lie CNAV D essage 5 bits/se Rate ½ FEC D Legay NAV essage 5 Bits/se Legay NAV essage 50 bits/se C C Figure 4 shows the struture of the software-based LC signal generator. In this signal generator, the noise generator has the zero-mean property as shown in Figure 5. And the output of the signal generator is shown in Figure 6. 0,30 Chip C Chip by Chip ultiplexer ½ 767,50 Chip C/A CL B B A Transmitted Signal A.03 Hz Clo Fig. 3 LC Signal Options in IIR- Satellites symbol stream is odulo- added to the C ode. The resultant C, CL bit-trains are ombined using a timedivision multiplex (TD) method starting with the C ode. The ombined bit-trains are used to modulate the L quadrature-phase arrier. The LC NAV will have a flexible message struture ontrollable by the Control Segment. The struture of the navigation message for LC, CNAV, is basially same as that of the L5 signal. It is more ompat and more flexible than that of the urrent NAV message. Instead of a fixed message format, CNAV allows the Control Segment to speify the sequene and timing of eah message omponent onsisting of 300 bit subframe. Sine the data rate of the LC signal is 5bps, eah subframe requires seonds to be transmitted. The LC signal options in IIR- satellites are shown in Figure 3. The signal options are ontrolled by four swithes whose preferred positions are A, B, C, D. Fig. 5 The Output of Noise 3 L Civil Signal Design and Analysis The onstellation of the next generation GPS satellites is sheduled to launh in 005. Therefore, in order to design and test the assisted GPS aquiring the L ivil signal, it is neessary to design a signal generator whih an generate the L ivil signal. The signal generator is designed as shown in Figure 4. Fig. 6 The Output of the Signal 4 A Proposed Assisted GPS Aquisition ethod 4. The Squaring Loss Data Generation C CL L Digitized IF Carrier Rate / FEC + TD X Noise Band-pass Filter AGC A/D In general, in order to enhane RF sensitivity of GPS reeiver, it is neessary to inrease the orrelation integration time over basi orrelation time. Figure 7 shows a previously existing assisted GPS aquisition method using both the oherent integration and the nonoherent integration in wea signal environment. And Equation () and Equation () show the oherent integration and the non-oherent integration, respetively. Fig. 4 Struture of GPS L Civil Signal

4 8 Journal of Global Positioning Systems x( t ) ω r T0 (reeived signal) (despreading mixer) τ ω e Lowpass Filter e = e oherent/non-oherent integration N e Ne = y y η yes signal present η : detetion threshold (generated signal) G( t ) ( ˆ )exp( ˆ = C T0 jωrt ) no signal absent Fig. 7 The Previously Existing Signal Aquisition ethod in Assisted GPS After low pass filtering, the output of the oherent integration and the non-oherent integration are = y = N e e = N e e = i q ( ) + ( ) where e and N e are the number of the oherent integration and the non-oherent integration, respetively, is the output of the lowpass filter, and y are the output of the oherent integration and the non-oherent integration, respetively. Here is omposed of the inphase omponent, i and the quadrature-omponent, q. The oherent integration is a tehnique integrating inphase orrelation result e times as given by Eq. (). Therefore, it is assumed that there is no sign inversion of orrelation by the navigation message bit transition or assistane of sign inversion information during the oherent integration. A relation of the number e of the oherent integration and an allowable arrier frequeny error f e is given by L sin ( πf T ) () () e i Aq = 0log (3) max 0 πf eti where Ti = e Tp is the oherent integration time, T p is the period of integration, and L Aq is the aquisition loss. Eq. (3) shows that the longer integration time requires the less allowable arrier frequeny error for the same signal aquisition loss. Sine there is no navigation bit stream during the initial aquisition time, the oherent integration tehnique is not proper to be adopted in the signal aquisition proess of generi GPS reeivers. For this reason, the generi GPS reeiver performs the oherent integration with demodulating the navigation bit stream after the signal aquisition. Here the purpose of the oherent integration is to enhane the Signal-to-Noise (SNR) and to improve the quality of measurements. The non-oherent integration is a tehnique integrating both the in-phase orrelation result and the quadraturephase orrelation result as shown in Eq. (). Therefore it is not neessary to now the navigation message bit transition. That is, the non-oherent integration is not influened by sign inversion of the navigation message bit during the integration, and an allowable arrier frequeny error is related not to the number of the nonoherent integration but to integration time of orrelation values i and q. Therefore the non-oherent integration tehnique is adopted to enhane RF sensitivity of GPS reeiver. But there is a disadvantage that the nonoherent integration indues the squaring loss for wea GPS signals. Partiularly the squaring loss is the dominant fator among the aquisition losses of assisted GPS dealing with wea GPS signals. The squaring loss is defined as the ratio of the SNR before the non-oherent integration for the SNR after the non-oherent integration. α L sq = (4) αn α = α n f T Γ = p e ( + ) P σ n s α F ; ; 4 π where Γ ( ) is the gamma funtion, ( ) π F is the onfluent hypergeometri funtion, α is the SNR before the non-oherent integration, and α n is the SNR after the non-oherent integration. Therefore, the squaring loss is given by α 4 π L sq = (5) α Γ + F ; ; π And the squaring loss has properties as follows: d L dα sq < 0, Lsq α = 0 = (6)

5 Cho et al: An Assisted GPS Aquisition ethod using L Civil Signal in Wea Signal Environment 9 From Eq. (6), it is explained that Eq. (5) is a monotoni dereasing funtion and the non-oherent integration indues the squaring loss when the Signal-to-Noise Ratio / (SNR) is below 0. As shown in Figure 8, if the SNR / before the non-oherent integration is below 0, the squaring loss exists. But if the SNR before the nonoherent integration is above 0, the squaring loss / does not exist. L sq Squaring loss y = N p = N p =, 0, < 0 where N p is the number of the modified non-oherent integration. When only the noise exists, the modified non-oherent integration method results in ( t ) n ( t ) n ( t ) n ( t ) yn ( t ) ni i + q = (8) E ( y n( t )) = 0 (9) var( y n ( t )) = σ (0) The squaring loss does not our beause the inner produt of adjaent samples has the zero-mean property as given by Eq. (9). q α (SNR before non-oherent integration) [ratio] Fig. 8 The Squaring Loss vs. the SNR before Non-oherent Integration 4. The Proposed Assisted GPS Aquisition ethod The signal aquisition method proposed in this paper is shown in Figure 9. It integrates the inner produts of adjaent two pair of in-phase omponents and quadraturephase omponents sampled at different time instants as given by Eq. (7) (Lee et al., 003). I I + Q = Q (7) 5 Performane Evaluation Test To ompare the signal aquisition performane of the proposed assisted GPS aquisition method using LCS with that of the existing assisted GPS method using L signals in the same environment, this paper performed the signal aquisition test using signal generator designed in setion 3. First of all, this paper evaluates the GPS L signal aquisition performane using the previously existing aquisition method of Figure 7 and then evaluates the L and L ivil signal aquisition performane using the proposed aquisition method. It is assumed that the existing aquisition method of Figure 7 uses the navigation message bit information, and performs the 50 times non-oherent integration after Proposed Aquisition Sheme x( t ) ω r T0 (reeived signal) (generated signal) (despreading mixer) τ ω e Lowpass Filter G( t ) ( ˆ )exp( ˆ = C T0 jωrt ) p p = i i delay p T p ~ Sqr ~ [ Re( )] N p N p = i y i η : detetion threshold yes y η no signal present signal absent Fig. 9 The Proposed Signal Aquisition ethod in Assisted GPS

6 30 Journal of Global Positioning Systems oherent integration for 0 mse. The proposed method does not use the navigation message bit information, and performs the 00 times modified non-oherent integration after the oherent integration for 5 mse. Figure 0 and Figure show the noise distribution of the previously existing aquisition method and the proposed aquisition method respetively. As noted before, the proposed method has a nearly zero mean, but the previously existing method has a non-zero mean. It an be shown that the proposed method solves the squaring loss problem as expeted. From Figure, it an be seen that the aquisition suess rate of the proposed assisted GPS aquisition method is better than that of the existing assisted GPS method using L signals in the same environment. And it is evaluated that the assisted GPS using L ivil signals is muh more available than the assisted GPS using L ivil signals in wea signal environment. Fig. The Signal Aquisition Suess Rate Fig. 0 Noise Distribution of the Existing ethod 6. Conlusion In this paper, the squaring loss of the previously existing assisted GPS is derived and the aquisition method for assisted GPS is proposed for solving the squaring loss problem in wea signal environment. It is shown that the proposed method solves the squaring loss problem by maing the mean of the noise distribution to the integration output zero. Finally, the performane of the proposed aquisition method is verified by signal aquisition test using software based GPS signal generator designed in this paper. It is onluded that the proposed method for wea signal aquisition in assisted GPS shows muh enhaned performane in not only L ivil signal but also L ivil signal. Fig. Noise Distribution of the Proposed ethod Figure shows the wea signal aquisition performane with respet to the input signal levels. Here, the input signal level is verified by omparing input level of the real GPS L signal with output level of the software based L signal generator through the post proessing. And the software based LCS generator designed in this paper adopts this experimental result. The results of Figure exlude the false alarm. Referenes C. Kee; H. Jun; D. un (000): Development of Indoor Navigation System using Asynhronous Pseudolites, Proeedings of the ION GPS-000, F. van Diggelen; C. Abraham (00): Indoor GPS Tehnology, Presented at CTIA Wireless-Agenda, Dallas... Chansarar; L. Garin (000): Aquisition of GPS signals at very low signal to noise ratio, Proeedings of the ION 000 National Tehnial eeting, Rihard D. Fontana; Wai Cheung; Tom Stansell (00): The odernized L Civil Signal, GPS World agazine, 8-34.

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