The System: BeiDou Adds Two, Plans Auto Sat Nav

China launched two BeiDou navigation satellites into medium Earth orbit on July 25.

China launched two BeiDou navigation satellites into medium Earth orbit on July 25.

The two new satellites, BeiDou-3 M1 and BeiDou-3 M2, are in orbital slots 1 and 6 of Plane 1 (or A Plane), respectively. The satellites are designated BDS M1-S and M2-S — the “S” may stand for “Test” (in Chinese: 试验 = Shiyan).

On Aug. 14, China stated one satellite was working autonomously and had set up a link with the other satellite, successfully testing the autonomous control technology of the Beidou constellation. The inter-satellite link realizes communication and distance measurement among satellites, bringing autonomous control of the system a step closer.

Autonomous navigation is the project’s key to global operation. It enables satellites to work independently, providing users with more accurate data, according to BeiDou design engineers.

First BeiDou Phase 3 Signals Acquired

By Michele Bavaro and James Curran

Editor’s Note: See the full updated report here.

On Aug. 9, signals from the two new BeidDou satellites were received with a software-defined radio sampler operated at the European Commission’s Joint Research Centre in Ispra, Italy. The sampler is driven by orbit-prediction software that triggers a synchronized acquisition on both 1575.42 MHz and 1278.75 MHz using 1-bit complex samples at 60 megasamples per second (about 60 MHz total bandwidth). The two-line element sets for the orbits were obtained from the CelesTrak website, and predicted positions were computed using code developed following the Simplified General Perturbations Satellite Orbit Model 4 (SGP4) as documented in the U.S. Department of Defense Spacetrack Report No. 3.

To confirm the identity of the satellite being tracked using codeless tracking, we matched the measured Doppler frequency shift with the predicted one. The local oscillator clock drift was modeled using GPS L1 C/A-code signals and taken into account when matching the Doppler shift.

According to a presentation given at Stanford University’s 2014 PNT Symposium by Mingquan Lu and Zheng Yao from Tsinghua University, modernized BeiDou satellites broadcast an MBOC(6,1,1/11) [a multiplexing of BOC(6,1) and BOC(1,1) signals] and a BOC(14,2) signal on the L1 frequency. Neglecting the BOC(6,1) term, side lobes were brought to baseband and cross-correlated by our equipment. In Figure 1, the peak at 1756.41 MHz is BEIDOU-3 M2. This is also confirmed by cross-correlating the lobes of the BOC(14,2) signal, which is quite a unique feature of the new satellites (see Figure 2).

Figure 1. BOC(1,1) cross-correlation.

Figure 2. BOC(14,2) cross-correlation.

On Aug. 10, a 1.8-meter dish was pointed at the satellite, and a Tektronix RSA306 USB Real Time Spectrum Analyzer was used to sample the signal on L1 with 14-bit resolution at 112 megasamples per second. The resulting power spectrum is shown in Figure 3.

Figure 3. Power spectral density of BEIDOU-3 M2 on L1.

The spectrum shows very good overlap between the anticipated BOC(1,1) signal in red, BOC(14,2) in green and BPSK(2) in black. In fact, PRN33 correlates with the low side lobe suggesting that the satellite is also broadcasting a legacy signal on 1561.098 MHz (see Figure 4).

Figure 4. Cross-correlation of a BPSK(2) BeiDou code PRN33 on a 1561.098-MHz carrier.

Meanwhile, tracking by stations participating in the International GNSS Service Multi-GNSS Experiment has established that the second recently launched BeiDou Phase 3 MEO satellite is using PRN code 34, and that the first Phase 3 satellite, BeiDou I1-S launched on March 30, 2015, into an inclined geosynchronous orbit, is using PRN code 31.