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Satoshi Takahashi

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Health information expression for QZSS 7-satellite configuration

category: gnss
tags: mosaic oem729 qzsl6tool qzss

Introduction

Currently, four satellites of Quasi-Zenith Satellite System (QZSS), or Michibiki, are in operation. In the future, there are plans to increase the number to seven, and then to 11.

On the other hand, navigation messages (digital information sequences including satellite coordinates and time) broadcast by positioning satellites include health information indicating that the satellite is available. I was aware that this health information representation would be changed in preparation for the 7-plane Michibiki system, but I hadn’t actually checked carefully.

Michibiki health information

The ephemeris (information about the satellite itself) in the navigation message broadcast by Michibiki and GPS includes 6-bit health information. Galileo and BeiDou ephemeris also contain health information. I assumed that all of this health information would be zero when everything was working properly.

When I checked the status of the positioning satellites, I found that all Michibiki’s health information was 1 in decimal format.

QZSS Health Information

I thought that Michibiki 2nd satellite J02, Michibiki 3rd satellite J07, Michibiki 4th satellite J03, and Michibiki 1st satellite replacement J04 were all unhealthy.

However, the NovAtel OEM729 receiver and the Septentrio mosaic-go X5 receiver demodulate the signal from the Michibiki satellites.

QZSS reception with OEM729 receiver QZSS reception with mosaic-go X5 receiver

It can be said that the QZS L6 Tool that I have published is incorrectly displayed as unhealthy. However, according to the RTCM 3 specification I have (RTCM 10403.3, published 2020-04-28), 6-bit all-zero health information can be read as a normal state.

QZSS health description in RTCM standard 10403.3

However, with the transition to the Michibiki’s 7-satellite configuration, the L1C/A signal that the signal is transmitted from Michibiki satellites and most widely used will be changed to the L1C/B signal. In accordance with this, health information will also be used for this signal identification (the document is written in Japanese).

According to the Revision History of Michibiki’s Satellite Positioning Service Specification, IS-QZSS-PNT-005, the health information expression was changed in the 4th edition. It was written that. This is a specification issued three years ago.

004 January 25, 2021
3,6,7,8,15,16,37, 39,41,50,118,119,120,125,126,130,135,138,143,144
Change in Tables, Figures and explanations with addition of the new signal “L1C/B”

Decoding health information

I decided to display Michibiki’s health information as follows:

  1. If the 1st bit or any bit from the 3rd bit to the 5th bit of the health information contains 1, it is unhealthy. The signal name corresponding to each bit up to the 6th bit is also displayed.
  2. When the 6-bit health information is all zero, it is normal and nothing is displayed as health information.
  3. If the 1st bit of the health information is 0, there is a possibility of a new signal usage indication. If the 2nd or 6th bit is 1, it is normal but contains a new signal usage indication, so that signal name will be displayed.

Writing these in Python code looks like this: It’s like Fizz Buzz problem.

if (r.svh[0:1]+r.svh[2:5]).u:  # determination of QZSS health including L1C/B is complex, ref.[2], p.47, 4.1.2.3(4) 
    unhealthy = ''
    if r.svh[1]: unhealthy += ' L1C/A'
    if r.svh[2]: unhealthy += ' L2C'
    if r.svh[3]: unhealthy += ' L5'
    if r.svh[4]: unhealthy += ' L1C'
    if r.svh[5]: unhealthy += ' L1C/B'
    msg += self.trace.msg(0, f' unhealthy ({unhealthy[1:]})', fg='red')
elif not r.svh[0]:                # L1 signal is healthy
    if r.svh[1]: msg += ' L1C/B'  # transmitting L1C/B
    if r.svh[5]: msg += ' L1C/A'  # transmitting L1C/A

Past navigation messages of Michibiki recorded on 2022-12-13 have all-zero health information as normal operation. I think the health information expression was changed when Navigation message transmission pattern change announcement released on 2023-12-21.

QZSS PNT tarnsmission pattern change announcement

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