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Satoshi Takahashi

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Galileo Timing Service Message

category: gnss
tags: galileo qzsl6tool rtklib

Introduction

Satellite positioning is designed to be accomplished by only receiving its radiowaves. These radiowaves include ranging signals (used to measure the distance from the satellite to the receiver) and navigation message signals (digital information sequences such as satellite positions for positioning calculations).

This navigation message may also provide services other than positioning. In April 2024, the European positioning satellite Galileo published a specification, Galileo Timing Service Message Operational Status Definition (TSM OSD) Issue 1.0, that defines the Timing Service Message (TSM) for time synchronization.

I haven’t been able to see this Galileo TSM on my receiver yet.

Galileo Timing Service Message

Positioning satellite receivers are sometimes used as high-precision time providing servers.

In the initial state, the latitude, longitude, and ellipsoid height of the positioning receiver are unknown. Since the receiver calculates the distance from the satellite to the receiver based on the difference in propagation delays of radiowaves, the current time is also unknown. Assuming perfect time synchronization among the positioning satellites, it is possible to estimate the coordinates and current time simultaneously from radiowave reception from four different satellites.

However, different satellites may differ in their satellite internal clock accuracy. Galileo TSM attempts to provide this accuracy information. TSM will broadcast the following 8-level (actually 4-level) accuracy evaluation results for all Galileo satellites. Based on this information, users will select satellites to use depending on the required time accuracy.

service leveldescrioption
1Not OK
2Timing Service Level 1 (max tolerable error of 1000 ns)
3Timing Service Level 2 (max tolerable error of 100 ns)
4Reserved for Timing Service Level 3 (max tolerable error of 15 ns)
5Spare
6Spare
7Spare
8Monitoring not available

In the navigation message I/NAV (integrity navigation) that would also broadcasts TSM messages, the message type is distinguished by a 6-bit “word type.” According to the specification TSM OSD, the word type number of TSM is 44.

Positioning satellites use their own time system as the system time, rather than Coordinated Universal Time (UTC). Galileo’s time system is GST (Galileo System Time).

Galileo provides GST time information in word type 5 messages and more precise conversion parameters between GST and UTC in word type 6 messages. The spare word (word type 0) also contains GST time information.

Receiving Galileo navigation messages

Therefore, the command line application str2str of RTKLIB, and the ubxread. py and galinavread.py of QZS L6 Tool are used to survey the Galileo navigation messages. Extracts the I/NAV navigation message from the raw data stream of the u-blox ZED-F9P receiver and displays its contents.

Galileo I/NAV messages are broadcast at a rate of 1 message every 2 seconds, and the cycle is 30 seconds. Therefore, it is necessary to observe for more than 30 seconds.

This output includes the satellite number (e.g. E15), the SSP (secondary synchronization pattern) number (repeating 1 to 3), the word number, and the time within a 30 second period estimated from the SSP number and word number. As of this time, word type 44 was not found yet.

$ str2str -in ntrip://ntrip.phys.info.hiroshima-cu.ac.jp:80/F9PR 2>/dev/null | ubxread.py -i | galinavread.py
E15 SSP2 Word 16 (15)
E03 SSP2 Word 16 (15)
E05 SSP2 Word 16 (15)
E13 SSP2 Word 16 (15)
E30 SSP2 Word 16 (15)
E34 SSP2 Word 16 (15)
E08 SSP2 Word 16 (15)
E27 SSP2 Word 16 (15)
E21 SSP2 Word 16 (15)
E03 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E05 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E13 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E30 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E34 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E08 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E27 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E21 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E15 SSP3 Word  0 (17)  2024-04-06 08:26:04 (1284 548777)
E15 SSP1 Word  0 (19)  2024-04-06 08:26:06 (1284 548779)
E03 SSP1 Word  0 (19)  2024-04-06 08:26:06 (1284 548779)
E05 SSP1 Word  0 (19)  2024-04-06 08:26:06 (1284 548779)
E13 SSP1 Word  0 (19)  2024-04-06 08:26:06 (1284 548779)
E30 SSP1 Word  0 (19)  2024-04-06 08:26:06 (1284 548779)
E34 SSP1 Word  0 (19)  2024-04-06 08:26:06 (1284 548779)
...

Note that in this example, SSP numbers have appeared for all satellites, but older satellites (e.g. E12) do not display SSP numbers because they do not broadcast SSP. The launch date of the Galileo satellite can be confirmed on the Quasi-Zenith Satellite System homepage. In addition, ARAIM (advanced receiver autonomous integrity monitoring, word number 22) message defined in Galileo Open Service Signal-In-Space Interface Control Document (OS SIS ICD) Issue 2.1 has also not been confirmed yet. I look forward to broadcasting your new message.

By the way, the Quasi-Zenith Satellite (QZSS) uses L1S signals to provide augmenatation information that improves positioning accuracy to the submeter level, as well as a disaster and crisis management report (DCR) that conveys disaster information. Furthermore, from April 2024, QZSS started providing DCX (DCR extended information), which is extended information of DCR to worldwide. The message type (MT) for DCR is 43 and the MT for DCX is 44. This DCX is scheduled to be standardized with Galileo disaster messages, so I was expecting it to be standardized as word type 44 in the Galileo disaster information transmission. I am somewhat disappointed that Galileo I/NAV word type 44 was assigned to TSM.

Positioning satellite receivers continue to output positioning results as they have in the past, but their content continues to evolve. I enjoy being able to easily come into contact with new technology.

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