QZSS CLAS tropospheric delay augmentation information for remote islands in Japan

categories: gnss
tags: clas qzss

Introduction

The quasi-zenith satellite system (QZSS), petnamed Michibiki, not only broadcasts positioning signals such as GPS, but also sends signals that improve its positioning accuracy. Improving positioning accuracy by some means is called “augmentation” in satellite positioning. Michibiki broadcasts three types of augmentation signals, but here, the centimeter-level augmentation service (CLAS) for Japan is used, especially on remote islands in Japan. Let’s think about what we can do with QZS L6 Tool.

Troposphere delay information for CLAS regional augmentation

Positioning augmentation by CLAS is roughly divided into those related to satellites and those related to the region of Japan. The latter is managed by the compact network identification (NID) and the “grid” that belongs to it. I tried plotting this grid coordinates on a map

The CLAS receiver first determines the NID that belongs to that coordinate from the rough receiver coordinates, and then searches for the closest 3 to 4 grids among the multiple grids that belong to it. Then, by weighting the grid value by those distances, the augmentation peculiar to the region is performed.

By the way, according to the revision history of the current CLAS specification IS-QZSS-L6-004 (issued on 2021-07-14), NIDs of 13 or above have been deleted in the current 4th edition. So, before that, was augmentation information broadcast for areas with NID 13 and above?

Current CLAS tropospheric delay augmentation information

Therefore, let’s check the augmentation information currently being broadcast. Download the QZS L6 Tool from GitHub and run the following command in its ` python`` directory.

cat ../sample/2022001A.l6 | ./qzsl62rtcm.py -t 1| grep NID | sort | uniq

This is to analyze the information including NID for the CLAS augmentation information for 1 hour from 00:00 UTC on January 1, 2022 in the sample with the QZS L6 Tool. The file name 001 represents the number of days from January 1st to that date, and A represents midnight UTC time. If it is B, it means time 01:00 UTC, and if it is X, it means time 23:00 UTC. To display the tropospheric delay augmentation information, set the QZS L6 Tool trace option -t to 1 and observe the detailed information. CLAS augmentation information conveys a set of information in 30 seconds, so this 1-hour augmentation information contains 120 sets of information. Therefore, I extracted the lines containing the character string NID with grep, sorted them in alphabetical order with sort, and removed the duplicates with uniq. When you do this, you get the following results:

ST11 NID=1
ST12 tropo=0b11 stec=0b11 NID=1 ngrid=8
ST12 tropo=0b11 stec=0b11 NID=10 ngrid=23
ST12 tropo=0b11 stec=0b11 NID=11 ngrid=19
ST12 tropo=0b11 stec=0b11 NID=12 ngrid=2
ST12 tropo=0b11 stec=0b11 NID=2 ngrid=11
ST12 tropo=0b11 stec=0b11 NID=3 ngrid=32
ST12 tropo=0b11 stec=0b11 NID=4 ngrid=15
ST12 tropo=0b11 stec=0b11 NID=5 ngrid=15
ST12 tropo=0b11 stec=0b11 NID=6 ngrid=27
ST12 tropo=0b11 stec=0b11 NID=7 ngrid=22
ST12 tropo=0b11 stec=0b11 NID=8 ngrid=20
ST12 tropo=0b11 stec=0b11 NID=9 ngrid=18

Subtype 12 (ST12) augmentation information includes all NIDs 1-12. For example, when focusing on the line with NID=1, the bit image of the troposphere delay type is 11. This means that the highest definition augmentation is selected according to Table 4.1.2-36 on page 53 of the CLAS specification IS-QZSS-L6-004. The same applies to bit images of the slant total electron content (STEC) type (page 55, Table 4.1.2-38). The number of grids (ngrid) is 8. None had a NID greater than 13. For NID=1, which represents the remote islands of Okinawa, “Compact SSR GNSS Combined Correction” of subtype 11 (ST11) was also added.

Troposphere delay augmentation information around the time of CLAS specification 3rd edition

So, was the tropospheric delay augmentation information in these areas transmitted before NIDs 13-19 were removed? To confirm that, we will use the QZS archive data. On this page, enter a date before July 14, 2021, for example, 0:00 UTC on January 1, 2021, or use the API (application interface) to enter curl or wget. Download the L6 file.

curl https://sys.qzss.go.jp/archives/l6/2021/2021001A.l6 -o 2021001A.l6

Let’s extract the tropospheric delay augmentation information from this L6 file in the same way.

cat 2021001A.l6 | ./qzsl62rtcm.py -t 1 | grep NID | sort | uniq
ST11 NID=1
ST12 tropo=0b11 stec=0b11 NID=1 ngrid=8
ST12 tropo=0b11 stec=0b11 NID=10 ngrid=23
ST12 tropo=0b11 stec=0b11 NID=11 ngrid=19
ST12 tropo=0b11 stec=0b11 NID=12 ngrid=2
ST12 tropo=0b11 stec=0b11 NID=2 ngrid=11
ST12 tropo=0b11 stec=0b11 NID=3 ngrid=32
ST12 tropo=0b11 stec=0b11 NID=4 ngrid=15
ST12 tropo=0b11 stec=0b11 NID=5 ngrid=15
ST12 tropo=0b11 stec=0b11 NID=6 ngrid=27
ST12 tropo=0b11 stec=0b11 NID=7 ngrid=22
ST12 tropo=0b11 stec=0b11 NID=8 ngrid=20
ST12 tropo=0b11 stec=0b11 NID=9 ngrid=18

There was no information for NIDs 13-19 here. By this time, it seems that the augmentation services for the remote islands of Hokkaido, Okinawa, and Shimane had already ended.

Try it at other dates and times. Let’s download and analyze the L6 file 2020234A.l6 in the same way at 0:00 UTC on August 21, 2020, the day after the CLAS specification 3rd edition was published.

curl https://sys.qzss.go.jp/archives/l6/2020/2020234A.l6 -o 2020234A.l6
cat 2020234A.l6 | ./qzsl62rtcm.py -t 1 | grep NID | sort | uniq
ST11 NID=1
ST9 correct_type=1 correction_range=0 NID=1 quality=1 ngrid=8
ST9 correct_type=1 correction_range=0 NID=10 quality=13 ngrid=23
ST9 correct_type=1 correction_range=0 NID=10 quality=14 ngrid=23
ST9 correct_type=1 correction_range=0 NID=10 quality=15 ngrid=23
ST9 correct_type=1 correction_range=0 NID=10 quality=16 ngrid=23
ST9 correct_type=1 correction_range=0 NID=11 quality=10 ngrid=19
ST9 correct_type=1 correction_range=0 NID=11 quality=11 ngrid=19
ST9 correct_type=1 correction_range=0 NID=11 quality=8 ngrid=19
ST9 correct_type=1 correction_range=0 NID=11 quality=9 ngrid=19
ST9 correct_type=1 correction_range=0 NID=2 quality=13 ngrid=11
ST9 correct_type=1 correction_range=0 NID=2 quality=14 ngrid=11
ST9 correct_type=1 correction_range=0 NID=2 quality=15 ngrid=11
ST9 correct_type=1 correction_range=0 NID=2 quality=16 ngrid=11
ST9 correct_type=1 correction_range=0 NID=2 quality=17 ngrid=11
ST9 correct_type=1 correction_range=0 NID=2 quality=18 ngrid=11
ST9 correct_type=1 correction_range=0 NID=3 quality=10 ngrid=32
ST9 correct_type=1 correction_range=0 NID=3 quality=11 ngrid=32
ST9 correct_type=1 correction_range=0 NID=3 quality=9 ngrid=32
ST9 correct_type=1 correction_range=0 NID=4 quality=11 ngrid=15
ST9 correct_type=1 correction_range=0 NID=4 quality=12 ngrid=15
ST9 correct_type=1 correction_range=0 NID=4 quality=13 ngrid=15
ST9 correct_type=1 correction_range=0 NID=4 quality=14 ngrid=15
ST9 correct_type=1 correction_range=0 NID=5 quality=11 ngrid=15
ST9 correct_type=1 correction_range=0 NID=5 quality=12 ngrid=15
ST9 correct_type=1 correction_range=0 NID=5 quality=13 ngrid=15
ST9 correct_type=1 correction_range=0 NID=6 quality=10 ngrid=27
ST9 correct_type=1 correction_range=0 NID=6 quality=11 ngrid=27
ST9 correct_type=1 correction_range=0 NID=7 quality=6 ngrid=22
ST9 correct_type=1 correction_range=0 NID=7 quality=7 ngrid=22
ST9 correct_type=1 correction_range=0 NID=7 quality=8 ngrid=22
ST9 correct_type=1 correction_range=0 NID=7 quality=9 ngrid=22
ST9 correct_type=1 correction_range=0 NID=8 quality=10 ngrid=20
ST9 correct_type=1 correction_range=0 NID=8 quality=11 ngrid=20
ST9 correct_type=1 correction_range=0 NID=8 quality=12 ngrid=20
ST9 correct_type=1 correction_range=0 NID=8 quality=13 ngrid=20
ST9 correct_type=1 correction_range=0 NID=8 quality=9 ngrid=20
ST9 correct_type=1 correction_range=0 NID=9 quality=10 ngrid=18
ST9 correct_type=1 correction_range=0 NID=9 quality=11 ngrid=18
ST9 correct_type=1 correction_range=0 NID=9 quality=9 ngrid=18
ST9 correct_type=1 correction_range=1 NID=12 quality=0 ngrid=2
ST9 correct_type=1 correction_range=1 NID=14 quality=0 ngrid=1
ST9 correct_type=1 correction_range=1 NID=15 quality=0 ngrid=1
ST9 correct_type=1 correction_range=1 NID=16 quality=0 ngrid=1
ST9 correct_type=1 correction_range=1 NID=17 quality=0 ngrid=1
ST9 correct_type=1 correction_range=1 NID=18 quality=0 ngrid=1
ST9 correct_type=1 correction_range=1 NID=19 quality=0 ngrid=1

The tropospheric delay is expressed in subtype 9 (Compact SSR Gridded Correction Message) instead of subtype 12.

There was also information on NIDs 14-19! However, along with NID 12 (Ogasawara Islands), correction_range = 1 and quality = 0. According to page 45 of IS-QZSS-L6-004, both STEC and tropospheric delay resolution correction_range are as low as 7 bits (usually 16 bits). Also, according to section 5.4.3 on page 87 and section 5.4.2 on page 84, this atmospheric correction quality indication quality is undefined (quality indication in other NIDs is 9-16, and it means the user range accuracy of 2-8 millimeters). The smaller the quality is other than zero, the higher the accuracy.

I couldn’t find any augmentation information on NID 13. There are multiple rows representing the same NID because the value of quality has changed over the last hour.

I also tried the one on January 1, 2019 at 0:00 UTC. The quality was 1 to 12 and the user range accuracy was even better, probably because it was winter and the ionospheric activity was not active. Again, there was no augmentation information for NID 13, and the quality of the augmentation information for NIDs 12-19 showed zero.

In addition, it seems that NID and their grid coordinate definitions have changed in the draft version on March 28, 2016 and August 31, 2018, and the first version (November 5, 2018). We need to be careful when we analyze data before these dates. The CLAS message does not include the version number for the grid coordinate definition.

Conclusion

Of the Michibiki CLAS augmentation information, I confirmed the augmentation information about the area using the QZS archive data. Previously, although the user range accuracy was undefined, it was found that augmentation information for remote islands of Shimane and Okinawa was also broadcast. The NID for the remote islands of Hokkaido has beed defined, but as far as I can confirm, it was not broadcast.

In addition, augmentation information for NID 12 (Ogasawara Islands) is still being broadcast, but user range accuracy has not been defined.

By changing from the conventional format of subtypes 8 and 9 to the compression format of subtype 12, the number of satellites to be augmented has increased. Perhaps the removal of infrequently used NIDs when changing the format to subtype 12 also contributed to the increase in the number of satellites to be augmented.

Since the maximum number of satellites to be augmented is not defined in the CLAS specifications, I think it may be possible to increase the number of satellites with the current message format.