Galileo down - July 2019 (Part 2)

The first part of the news is available here.

17/7/2019 UPDATE

This morning we noticed that the ephemerides in most Galileo signals visible over Torino seem to be valid again and then usable by GNSS receivers.

A test done with a ublox EVK-M8T receiver confirmed that almost all the visible Galileo satellites were usable by the receiver to compute a valid 3D fix, as shown in the in the screenshot below (taken around 07:32 UTC).

A further inspection on the data logged by the receiver during the previous 24 hours demonstrated that the receiver was already able to compute a valid fix since yesterday  evening. The receiver started to output a valid fix on the 16/07/2019 around 19:04 UTC (screenshot below).

At that time, only 4 Galileo satellites over Torino (i.e. PRN 2, 3, 7, 15) were providing updated ephemerides and were used by the receiver. Few minutes after this first fix also other Galileo satellites started to broadcast valid ephemerides. In conclusion, this commercial receiver has computed valid PVT solutions without interruptions from the evening of 16/07/2019, using up to 7 Galileo satellites.

We further investigated these results with our NGene software receiver. The analysis of the decoded Galileo navigation message leads to the conclusion that ephemerides and clock correction data were updated around 19:00 UTC of 16/07/2019: namely, PRN 3 and 15 changed Issue Of Data (IOD) from 958 to 17 at Galileo Signal Time TOW 241855, which corresponds to 19:01:25. We report in the table below the time when the first updated IOD was been received over Torino for each PRN.

Galileo PRN

New IOD value

GST

TOW (s)

Time and date (UTC, as received in Torino)

1

51

263185

01:06:25, 17/07/2019

2

18

241915

19:11:55, 16/07/2019

3

17

241285

19:01:25, 16/07/2019

4

51

264625

01:30:25, 17/07/2019

5

79

282295

06:24:55, 17/07/2019

7

57

265675

01:47:55, 17/07/2019

8

17

241855

19:10:55, 16/07/2019

9

69

272695

03:44:55, 17/07/2019

12

41

256075

23:07:55, 16/07/2019

13

21

243895

19:44:55, 16/07/2019

14

102

293785

09:36:25, 17/07/2019

15

17

241285

19:01:25, 16/07/2019

18

62

268705

02:38:25, 17/07/2019

19

41

256165

23:09:25, 16/07/2019

21

42

256555

23:15:55, 16/07/2019

24

89

285325

07:15:25, 17/07/2019

25

23

245935

20:18:55, 16/07/2019

26

21

243835

19:43:55, 16/07/2019

27

31

250105

21:28:105, 16/07/2019

30

22

244375

19:52:55, 16/07/2019

31

65

273925

04:05:25, 17/07/2019

33

107

295315

10:01:55, 17/07/2019

The following table reports the status of the navigation message as received by the NGene software receiver, updated at 12:00:00 of 17/07/2019: a renewed navigation message has been received from all the visible satellites, including PRNs 14 and 18 which are still characterized by the flag “Signal Component Currently in Test”.

An interesting observation is that the SISA flag in the last column is 255, i.e., according to the OS SDD, the accuracy status is “No Accuracy Prediction Available (NAPA)”. This means that the status of the broadcast SIS must be intended as “Marginal”. Differently than the EVK-M8T, the Xiaomi Mi8 Pro smartphone seems still excluding the Galileo satellites from its PVT fix.

Galileo PRN

t0c

t0e

Issue Of Data, IOD

GST of the last Word 5 analyzed

E1B Data Validity Status

E1B Signal Health Status

E1B Signal In-Space Accuracy Index

WN

TOW

1

294600

294600

107

1038

295375

0

0

255

2

300000

300000

116

1038

302575

0

0

255

3

244200

244200

23

1038

246205

0

0

255

4

302400

302400

120

1038

303415

0

0

255

5

283800

283800

89

1038

288205

0

0

255

7

298800

298800

114

1038

299635

0

0

255

8

255600

255600

42

1038

256795

0

0

255

9

285000

285600

92

1038

286435

0

0

255

11

294000

294000

106

1038

296665

0

0

255

12

286800

286800

94

1038

287545

0

0

255

13

283800

283800

89

1038

287125

0

0

255

14

302400

302400

120

1038

303415

0

3

255

15

276000

276000

76

1038

277195

0

0

255

18

274200

274200

73

1038

274975

0

3

255

19

300000

300000

116

1038

301945

0

0

255

21

285600

285600

92

1038

286615

0

0

255

24

302400

302400

120

1038

303415

0

0

255

25

302400

302400

120

1038

303415

0

0

255

26

294600

294600

107

1038

296215

0

0

255

27

275400

275400

75

1038

276175

0

0

255

30

265200

265200

58

1038

266275

0

0

255

31

301200

301200

118

1038

302065

0

0

255

33

302400

302400

120

1038

303415

0

0

255

36

303600

303600

122

1038

304645

0

0

255

A final remark is that the use of external ephemerides to process the Galileo signals during the “system outage”, as suggested by some experts, has given accurate PVT solutions. The figure below shows two different navigation solutions obtained by processing a data collection taken in Friday 12th July at 10.00 UTC (12.00 Local time). The red dots indicate few fixes obtained by demodulating the navigation message transmitted by the Galileo satellites and showing a remarkable bias w.r.t. the reference antenna location. The green dots are the navigation solution obtained correcting the satellites positions according to precise orbits data provided by IGS network. The two results were obtained by processing the same satellites signals, thus proving that their quality was still sufficient to get an acceptable positioning solution during the Galileo service outage period. This suggests us a confirmation that, during the outage, only the ephemerides updates were affected by problems, while the other SIS components appeared sound and usable.