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More Compass Points used in the monitoring stations of the first Galileo satellite, the Galileo In- Orbit Validation Element–A (GIOVE- A). The articles by W. De Wilde et al and Andrew Simsky et alia listed in the Additional Resources section near the end of this article provide additional details about our receiver technology mentioned here and later in this discus- sion. Figure 1 shows an overview of the receiver. Its front-end receives all Gali- leo and GPS navigation bands (L1/L2/ E6/E5). After splitting and conversion to an intermediate frequency, the L1 and L2 signals are routed to a CA/P-code dual-frequency GPS baseband proces- sor ASIC (application specific integrated circuit). The intermediate frequency (IF) signals of the Galileo bands (L1/E6/E5) More Compass Points enter a field programmable gate array (FPGA) after digitization for baseband FIGURE 1 GPS/Galileo Receiver Block Diagram tracking China’s MEO satellite processing. This FPGA contains eight generic We logged the L1 Band tracking channels. These channels con- IF signal of the front L1 E6 L2 E5 on a Hardware Receiver sist of a mixer for removal of the carrier, end on four random Centre Frequency (MHz) 1575.42 1278.75 1227.6 1191.795 two multicorrelator banks for concur- moments because WiM DE WilDE, On April 14 China launched its first medium-Earth-orbit (MEO) satellite code- rent despreading of data and pilot com- we d id n’t have Bandwidth (MHz) 40 40 25 55 FRank BOOn, named Compass-M1, the first in a planned constellation with global coverage ponents, and two code generators that access to the two- Front-end Type Dual conversion heterodyning j-M slEEWaEgEn, provide associated spreading codes. line element set of Base-band Clock 56 MHz anD FRank WilMs based on the same principles as GPS and Galileo. In order to investigate the The channels were designed to sup- the Compass satel- Number of GPS CA/P channels 9 Septentrio n.V. space vehicle’s characteristics and data structure, researchers modified a port all GIOVE-A signals (including lite. After post-pro- Number of Flexible Channels 8 GPS/Galileo hardware receiver so as to track the Compass MEO satellite. This AltBOC), all open Galileo-signals, GPS cessing in a Matlab TABLE 1. GNSS Receiver Parameters article tells how they did it and presents the first results of their efforts. L1 CA-code, GPS L5, GPS L2C, and all software receiver, GLONASS signals. two out of the four Because most Galileo and modern- log-files clearly showed correlation peaks code is an 11-bit shift register truncated n 2000 China deployed the Beidou- This explains the burst of research a normal hemispherical antenna. ized GPS signals have a significantly after correlation with the codes available Gold code. The overlaying secondary 1 navigation system. Originally this activity after launch of the first Compass Rather than building a receiver from longer spreading code than GPS CA, the from the SU website. This encouraged code is 20 bits long, resulting in a 50 S-band system provided ranging satellite (Compass-M1) in April this scratch, we found we could reuse a search space grows. To support acqui- us to go ahead with the development of Hz data rate. Both frequencies have the Iinformation via geostationary sat- year, particularly because China didn’t generic GNSS platform developed by sition in a reasonable amount of time, real-time tracking software. same primary and secondary code. ellites that operate as transponders. This disclose any details about its operation. our company. we implemented a parallel acquisition To conclude the description of the These concepts have much in com- system design required bulky two-way In an article published the May/June After a software-modification, this unit (PAU) in the FPGA. This unit uses GNSS receiver, table 1 shows its main mon with GPS and Galileo: radios, had a limited capacity, and cover- issue of Inside GNSS, researchers at GPS/Galileo receiver supported track- a combination of a matched filter and an functional parameters. • The chipping rate is a multiple of the age was restricted to East Asia. CNES, the French space agency, pre- ing of the E2 and E5B BPSK(2) signals FFT (fast Fourier transform) algorithm 0.5115 Mcps, just like the rates of all Currently, China is developing the sented dish-antenna measurements of of Compass-M1. This provided a lot of supporting the same signals as the track- Enabling Compass-M1 GPS and Galileo signals. successor to this system, called Beidou- the Compass-M1 signal and unveiled information about signal strength and ing channels. Support • The concept of a truncated Gold code 2 or Compass. The Compass Navigation the main code properties. Subsequently, ranging quality. Because the receiver is The FPGA connects to a large Our modification of the receiver for has already been introduced in GPS Satellite System (CNSS) will consist of a Stanford University (SU) team under- capable of logging navigation symbols, SDRAM (synchronous dynamic random tracking Compass-M1 focused on the L5 and Galileo, though with other 30 medium-Earth-orbit (MEO) satel- took complementary measurements and we were able to figure out the main data access memory) intended for logging of E2 and E5B BPSK(2) signals that are parameters. lites broadcasting code division multiple worked out the spreading code param- structure properties of Compass-M1 as digitized IF samples. The logged samples believed to be the open service signals. • Secondary codes are defined for GPS access (CDMA) signals in the L-band. eters, which are outlined in an article well. can be downloaded over Ethernet and As discussed in the CNES research men- L5 and several Galileo signals. Unlike the relatively large user equip- elsewhere in this issue. used for post-processing. This func- tioned earlier, these codes have a 2.046 It turned out that all Compass-M1 ment of Beidou-1, Compass will sup- The information published by CNES Receiver Fundamentals tionality was very useful to find out the Mcps chipping rate and a primary code signal characteristics were already sup- port global navigation by means of small and SU is sufficient to build a hardware The receiver we used to track Compass Compass-M1 signal was actually avail- length of 2,046 chips. ported in the generic channel design of handheld receivers. receiver able to track the signal through is the commercial version of the receiver able at the antenna. Stanford University figured out this our receiver. 44 InsideGNSS july/august 2007 www.insidegnss.com www.insidegnss.com july/august 2007 InsideGNSS 45 More coMpass pointS 150 I Q code can be sourced via a memory, as of power. Conversion into received iso- 100 required for the non-Gold-code L1BC tropic power results in -153 dBW for E2 and E6BC Galileo signals. and -150 dBW for E5B, compared to -157 50 We chose to use the Gold-code dBW for GPS L1CA. This enables acqui- generator option because this uses the sition at very low elevations and offers 0 hardware more efficiently, envisaging new perspectives for indoor navigation. -50 later implementation in an optimized The second observation is that the product currently in development. Compass ranging quality is similar to -100 The E5B and E2 frequency bands GPS. In the 2 hour to 5 hour time frame Normalized Correlation Amplitude (%) are both within the bandwidth of the in Figure 4 we found a ranging error -150 receiver’s analog front end. Here we need standard deviation as indicated in Table -1.5 -1 -0.5 0 0.5 1 1.5 to remark that the E2 center frequency 2. The contribution due to thermal noise Lab (Chips) happens to be at 1561.098 MHz, rather is displayed in the table as well. FIGURE 5 Compass-M1 Correlation Peak E5B than 1561.1 MHz as stated in earlier These data clearly show that the per- papers. This is exactly equal to the GPS formance is limited by multipath, and center frequency minus 14 times 1.023 we see a similar behavior in this area 30 second periodicity Large peak at 720 seconds MHz and perfectly symmetrical to the for both systems. This indicates that the × 104 E2 ACF E1 band. This may suggest CNSS will higher chipping rate doesn’t have much FIGURE 2 Generic Code Generator 10 support AltBOC(14,2) modulation. influence on the ranging performance. The E2 offset with respect to L1 was This is as could be expected, because 6 E5b RH CP Gain vs. theta L1 RH CP Gain vs. theta well within the pulling-range of the most multipath has a delay much shorter 2 20 20 oscillators in the tracking channel. We than the 500-nanosecond Compass chip -2 theta 0 200 400 600 800 1000 1200 0 0 concluded the the receiver’s hardware length. 4 E5b ACF Seconds × 104 Label didn’t need to be modified in order to Our receiver has a built-in correla- × 10 5 -20 -20 10 Gain (dB) Gain (dB) be Compass-M1–compliant. Enabling tion peak monitoring function. Figure 5 4.5 -40 -40 Compass-M1 reception only required an displays the E5B correlation peak. The 6 4 -60 -60 -180 -120 -60 0 60 120 180 -180 -120 -60 0 60 120 180 upgrade of the embedded software and E2 peak looks very similar. 2 Close3.5 Up theta (*) theta (*) of the graphical user interface. -2 3 0 200 400 600 FIGURE 3 Antenna Gain at E5B and L1 Compass-M1 Data Structure 2.5 4 E2-E5b CCF Seconds tracking Compass-M1 The GNSS receiver can output the data × 10 2 10 C/Nc We hooked the receiver up to a GPS/ symbols received from the satellite.
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