NOTAM Discrepancy - 2021-074 Launch

As per routine, China issued a Notice to Airmen (NOTAM) to cover a drop zone for stage 1 of the CZ 4B that launched a pair of Tianhui 2 satellites from Taiyuan 2021 August 18. Launch NOTAMS usually cover a time period commencing 7-8 minutes before lift-off. In this case the time was 22:35 UTC so launch at 22:42 was expected. When the post-launch press release came, it showed lift-off occurred a full ten minutes early.


Taiyuan Launches to Sun-synchronous Orbit

Launches to similar inclinations from a given site follow similar ground tracks as they depart so it is possible to estimate the Local Time at the orbit's Descending Node (LTDN) once the launch time is known. In the case of Taiyuan, for a launch to about 600 kilometres, adding 7 hours and 8 minutes to the launch time (in UTC) gives an approximate LTDN. Higher-altitude Sun-synchronous lauches require slightly higher inclinations so the increment can drop as low as about 7 hours and 4 minutes.

Occasionally, Taiyuan launches result in LTDN that computes as lift-off + 7 hpurs and 18 minutes, some ten minutes later than than might be expected. The reason is use of a dog-leg in the launch trajectory dictated by using an alternative drop zone for the launch vehicle second stage.


Tianhui 2 - 2021 August 18

A Tianhui launch at the predicted 22:42 UTC would have resulted in LTDN 22:50 UTC, exactly what was required for putting the new satellites into the same orbit plane as the previous pair of satellites, orbited in 2019.

When the launch announcement was published by Xinhua it gave the time as 22:32 UTC that should have resulted in LTDN 22:40. It was definitely not consistent with the NOTAM but it still produced an actual LTDN near 22:50 UTC. It was one of the 'dog-leg' missions but it revealed an obvious mistake in the times published in the pre-launch NOTAM.


Vietnam Navigation Warning

In accordance with international treaties, China informed Vietnam air traffic controllers that debris from the launch would fall in the sea at about 15° north latitude a few hundred miles off the country's coast. The Ho Chi Minh Flight Information Region accordingly issued its own NOTAM on August 17.

On August 18, Vietnam issued a revised NOTAM with times advanced by ten minutes and a note pointing out the change. China had obviously recognised the error, had informed Vietnam, but it did not update its own NOTAM.


Ascent to Orbit

The map above shows the launch trajectories for both the actual lift-off at 22:32 UTC using the dog-leg manoeuvre, and a launch direct to the target inclination.

The CZ 4B departed Taiyuan in a southerly direction as if it were heading for orbit at about 91° inclination. Both the first and second stages fired along that vector. When finished, the second stage followed a ballistic trajectory and eventually re-entered the atmosphere over the sea near Vietnam.

When the third stage ook over and, as well as pushing on to orbital velocity it imparted some sideways thrust that had the effect of making the rocket follow a track that curved to the west. It resulted in inclination 97°.5 at orbital injection.

A secondary effect was to place the Right Ascension of the orbit plane's Ascending Node (RAAN) 2.5° to the east of where a direct launch would have put it. In order to match the orbit plane with the 2019 satellites, lift-off had to be advanced by ten minutes to counter the Earth's rotation.


The 2019 Tianhui 2 launch

The earlier satellite pair was also launched using a CZ 4B. However, it followed the 'direct' route. In that case, The Laos Flight Information Region issued a NOTAM in advance for a debris impact near its borders with Cambodia and Vietnam.


The Mix-up

What happened seems to be that the launch time for the 'direct' route got applied to the NOTAM for the 'dog-leg' route. Maybe the relative timings used in the NOTAM were a 'cut and paste' from the 2019 launch where the compiler was unaware of the significance of the two slightly different launch azimuths.

At some point the discrepancy was noticed and Vietnam's air traffic authorities were supplied with revised times.

The Chinese NOTAM was not updated. One possibility is that the expected impact of debris from a 20:32 UTC lift-off just squeezed into the already-published window so a decision was made not draw attention to the error by issuing revised times.


Launch NOTAM - China

The dates and times are shown numerically in the format YYMMDDHHMM with the start and finish as items B and C respectively.

A2020/21 NOTAMN
Q) ZHWH/QRTCA/IV/BO/W/000/999/3010N11034E021
A) ZHWH
B) 2108182235
C) 2108182302
E) A TEMPORARY RESTRICTED AREA ESTABLISHED BOUNDED BY:
N303018E1102534-N302830E1104717-N295049E1104308-N295234E1102129,BACK TO START. ALL ACFT ARE
FORBIDDEN TO FLY INTO THE TEMPORARY RESTRICTED AREA.
VERTICAL LIMITS:GND-UNL.
F) GND G) UNL


Launch NOTAM - Vietnam, issued August 17

A1653/21 NOTAMN
Q) VVHM/QAFXX/IV/NBO/W/000/999/1432N10958E032
A) VVHM
B) 2108182216
C) 2108182333
E) DUE TO AEROSPACE FLIGHT ACTIVITIES FROM CHINA, THE FLIGHT SAFETY OF THE AIRCRAFT IN THE
FOLLOWING AREAS MAY BE AFFECTED:
N150029E1094236 - N145844E1101601 - N140434E1101254 -
N140618E1093940, FOUR-POINT CONNECTION RANGE.
VERTICAL ALTITUDE: SFC-UNL.
DURING THIS ACTIVITY, ATS ROUTES/SEGMENTS AS MENTION BELOW ARE NOT AVBL FOR ALL TFC INCOMING
AND OUTGOING FM HO CHI MINH FIR:
- G221 (PCA-ASUKU)
- L628 (PCA-DAMEL)
- L642 (VEPAM-EXOTO)
RMKS:
- ALL ACFT SHALL AVOID THIS AREA OR SUBJ ATC CLR.
- THE AEROSPACE FLIGHT ACTIVITIES CONDUCTED BY CHINA AFFECTING HO CHI MINH FIR WITHOUT PRIOR
ARRANGEMENT AND COORDINATION WITH ATS AUTHORITIES OF VIET NAM DID NOT MEET ICAO REQUIREMENTS
AS SPECIFIED IN ANNEX 11, 15 OF CHICAGO CONVENTION.
- THIS NOTAM ISSUED IN ACCORDANCE WITH ICAO REQUIREMENTS FOR THE SAFETY OF ACFT OPERATION
WI FIRS OF VIET NAM.


Revised Launch NOTAM - Vietnam, issued August 18

A1661/21 NOTAMN
Q) VVHM/QAFXX/IV/NBO/W/000/999/1432N10958E032
A) VVHM
B) 2108182206
C) 2108182333
E) DUE TO AEROSPACE FLIGHT ACTIVITIES FROM CHINA, THE FLIGHT SAFETY OF THE AIRCRAFT IN THE
FOLLOWING AREAS MAY BE AFFECTED:
N150029E1094236 - N145844E1101601 - N140434E1101254 -
N140618E1093940, FOUR-POINT CONNECTION RANGE.
VERTICAL ALTITUDE: SFC-UNL.
DURING THIS ACTIVITY, ATS ROUTES/SEGMENTS AS MENTION BELOW ARE NOT AVBL FOR ALL TFC INCOMING
AND OUTGOING FM HO CHI MINH FIR:
- G221 (PCA-ASUKU)
- L628 (PCA-DAMEL)
- L642 (VEPAM-EXOTO)
RMKS:
- ALL ACFT SHALL AVOID THIS AREA OR SUBJ ATC CLR.
- THE AEROSPACE FLIGHT ACTIVITIES CONDUCTED BY CHINA AFFECTING HO CHI MINH FIR WITHOUT PRIOR
ARRANGEMENT AND COORDINATION WITH ATS AUTHORITIES OF VIET NAM DID NOT MEET ICAO REQUIREMENTS
AS SPECIFIED IN ANNEX 11, 15 OF CHICAGO CONVENTION.
- THIS NOTAM ISSUED IN ACCORDANCE WITH ICAO REQUIREMENTS FOR THE SAFETY OF ACFT OPERATION
WI FIRS OF VIET NAM.
- THIS NOTAM REPLACES NOTAM A1653/21 DUE TO THE CHANGE IN EFFECTIVE TIME OF AEROSPACE FLIGHT
ACTIVITIES FROM CHINA.