Inside the box: New NavIC clock outperforms previous generation

Image: metamorworks/iStock/Getty Images Plus/Getty Images

NVS-01 is the first second-generation satellite of the Indian Navigation Satellite System (IRNSS), also known as Navigation with Indian Constellation (NavIC). It was launched into geostationary orbit on May 20. The satellite is placed at 129.6° eastern longitude and will finally replace IRNSS-1G launched in April 2016.

Whereas the first-generation satellites transmit navigation signals in the L5- and S-band, NVS-01 is the first IRNSS satellite also transmitting in the L1-band. The 1547.42 MHz frequency is also used by other satellite navigation systems, including GPS, Galileo, and BeiDou-3. However, a different modulation is used, namely a Synthesized Binary Offset Carrier (SBOC) signal. The IRNSS L1 SBOC signal has data and pilot components with and without navigation data. Data and pilot signals consist of BOC (1,1) and BOC (6,1) components with sub-frequencies of 1.023 MHz and 6.138 MHz. A quadrature multiplexing is applied for the data and pilot components with a power sharing of 41.82% and 58.18%. The navigation message on the IRNSS L1 signal has a different structure compared to those on the legacy L5- and S-band signals. The new L1 navigation message uses an advanced frame structure and forward error correction inherited from the CNAV-2 message of the GPS/QZSS L1C signal as well as a similar orbit model. Among other things, it provides inter-signal corrections for the L1 data and pilot signals with reference to the S band signal for single-frequency L1 band users.
NVS-01 started signal transmission on June 17, 2023, with the pseudo-random noise (PRN) code I10. The satellite’s L1 and L5 signals were tracked by a Septentrio PolaRx5 receiver located in Tokyo, Japan, with a prototype firmware that is capable of tracking the L1 pilot signal. Figure 1 shows the multipath linear combination of NVS-01’s L1 and L5 pilot signals. Whereas the short-term variations are smaller for L1 compared to L5, the overall RMS is 18 cm for both signals.

Figure 1: Noise- and multipath linear combination for NVS-01’s L1 and L5 pilot signals received on 26 June 2023. (Image: All figures provided by the authors) 

Whereas IRNSS-1’s rubidium clocks were provided by Spectratime, NVS-01 is the first satellite operating a new type of rubidium atomic frequency standard (RAFS) developed in India. The short-term performance of GNSS satellite clocks can be evaluated with the one-way carrier phase method. The receiver is connected to a highly stable external clock, e.g., a hydrogen maser. Thus, the receiver clock error is negligible. Measurement biases as well as the delays of ionosphere and troposphere on short time scales are removed by fitting a fourth-order polynomial. If no external clock is available, as is the case for the station in Tokyo, the precise clock information can be transferred from another station by a reference satellite jointly tracked by both receivers.

The Allan deviation based on this three-way carrier phase (TWCP) analysis is shown in Figure 2. The hydrogen maser of the IGS station USUD in Usuda, Japan, is used as the reference clock. At short integration times up to 20 s, the Allan deviation computed from the TWCP analysis is dominated by the GNSS measurement noise hiding the true clock performance. Above 20 s, the TWCP demonstrates that the NVS-01’s RAFS stability meets the performance of the ground tests and even exceeds them for longer integration times. At all integration times, the new RAFS outperforms the first generation IRNSS clocks.

Figure 2: IRNSS clock performance obtained from three-way carrier phase analysis as well as ground tests.

Manufacturers

GNSS data used in this article were collected with a Septentrio PolaRx5 receiver.

Further Reading

Bandi TN, Arora R (2019) Indigenous Atomic Clock and Monitoring Unit for NavIC. ICG-14, https://www.unoosa.org/documents/pdf/icg/2019/icg14/WGD/icg14_wgd_09.pdf

ISRO (2022) NavIC Signal in Space ICD for Standard Positioning Service in L1 Frequency, Version 1.0. U.R. RAO Satellite Centre, Indian Space Research Organization, Bangalore, https://www.isro.gov.in/media_isro/pdf/SateliteNavigation/Draft_NavIC_SPS_ICD_L1_Oct_2022.pdf

Peter Steigenberger and Oliver Montenbruck are scientists at the German Space Operations Center of the German Aerospace Center (DLR), where they conduct research in the field of new satellite navigation systems.

Jean‑Marie Sleewaegen is Lead Architect at Septentrio, Belgium, where he has been responsible for GNSS signal processing, system design and technology development since the company’s inception in 1999.