此文章中涉及的故障调查,本人觉得已经接近尾声,但是仍然没有明确定论,仅供参考。
近期,公司机队频发NAV ADS-B RPTG x FAULT警告,并伴随有MMR 1+2 (40RT1+2)/ ATC 1 (1SH1)故障。此故障在所有配备ADS-B out的飞机上都有发生,并且飞机落地后做BITE测试,设备都是正常的。
在此插段介绍:ADS-B其实是ATC Mode S的加强版,其多加出一截数字信号段,用于告知对方更多飞机的信息,这些信息包括:GPS经度,GPS纬度,GPS速度等等。
公司的工程技术科室人员向空客的技术服务中心咨询,空客人员答复认为可能是GPS信号不好,希望我们统计下故障发生的地点和时间。 registation number | date(UTC) | time(UTC) | flight phase | location | B-16xx | 2019/3/23 | 11:41 | approaching | xxHA | B-16xx | 2019/3/28 | 1:33 | approaching | xxHA | B-16xx | 2019/4/3 | 16:44 | approaching | ZxxD | B-xx2N | 2019/3/13 | 3:18-3:24(multiply) | cruise | NOT SURE | B-xx2N | 2019/4/6 | 13:49 | approaching | ZLJQ | B-xx2P | 2019/3/17 | 16:26 | approaching | ZxxD | B-xx2P | 2019/4/9 | 00:39/00:44(twice) | takeoff | ZLLL | B-xx2P | 2019/4/9 | 7:09 | takeoff | ZSWZ | B-xx3V | 2019/3/12 | 17:03 | approaching | ZxxD | B-xx3V | 2019/3/20 | 4:42 | approaching | ZGNN | B-xx3V | 2019/3/21 | 06:46-06:56(multiply) | approaching | ZSNB | B-xx3V | 2019/3/23 | 16:21 | approaching | ZxxD | B-xx3V | 2019/4/1 | 2:18 | takeoff | ZSQZ | B-xx4Q | 2019/3/13 | 16:26 | approaching | ZxxD | B-xx4Q | 2019/3/22 | 11:45 | takeoff | ZGNN | B-xx4Q | 2019/3/22 | 16:17 | approaching | ZxxD | B-xx4Q | 2019/3/26 | 16:36 | approaching | ZxxD | B-xx4Q | 2019/4/2 | 6:55 | approaching | VxxD | B-xx4Q | 2019/4/9 | 6:29 | approaching | VxxD | B-xx4R | 2019/3/20 | 6:53 | approaching | ZSNB | B-xx4R | 2019/3/21 | 11:58 | takeoff | ZGNN | B-xx4R | 2019/3/23 | 16:16 | approaching | ZxxD | B-xx4R | 2019/3/25 | 16:31 | approaching | ZxxD | B-xx4R | 2019/4/3 | 16:18 | approaching | ZxxD |
正好本人最近的本职工作不是太忙,抽空把故障信息统计了一下。统计信息如下(地点、机号做过处理):
其中,我还调出了飞机巡航时出现故障时的DFDR数据,发现飞机自动驾驶正常接通,飞机平稳飞行。ZxxD为我们公司的主基地,在UTC(12:00-14:00)也有装备有ADS-B out 的飞机降落,但从未出过警告信息。
从表中得出两个结论: 1、飞机经常在进近和起飞时出现该故障; 2、飞机在某个地点的某个时段很容易出现该故障。
基于此,关于该故障我有三个假设 1、云层的存在可能会干扰GPS信号,所以飞机在进近和起飞时容易出现警告。 2、GPS卫星每12小时走一个周期,所以每经过24小时,从地球上某个特定位置观察卫星,GPS卫星在天上的位置是一样的,当某个时段在该特定点上,GPS卫星的数目特别少,容易造成信号丢失。 3、飞机MMR与FWC的硬件与软件因素,比如最新的FWC驱动软件H2F10D对该故障警告需要5秒钟的确认时长,而我司用的H2F9D仅需要3秒钟。所以新版本的FWS软件报告该故障的频率会大大降低。
我把邮件发给技术服务中心的老外后,老外姐姐还挺认可我的研究成果,并且把我的邮件转给的了空客的飞机设计办(Design Office)。设计办的专家给技术服务中心的姐姐回了封邮件,原文如下: Thank you for your message sharing customer feedback.
From Design Office perspective, we know several cases where we can have a lack of GNSS signal reception which could involve ADS-B OUT Failure: 1) Quality of the Constellation number of satellites and position (Constellation geometry). It is true to say that every 24 h constellation geometry is quit the same. For information, you can find 2 simulations at Qingdao Liuting International Airport on 15thof April. Simulate Constellation at 36.26 LAT – 120.337 LONG at 16:10 Simulate Constellation at 36.26 LAT – 120.337 LONG at 16:50 You can see that we have a lot of satellite with a low Azimuth (they can be tracked but they can be easily lost too) and 4 satellites with a good visibility. To be in Navigation mode we need 5 satellites at least. So this constellation is not the best and it could explain some issues reported.
2) Signal masking and Multipath effect due to buildings, mountains, aircraft attitude itself (Roll, pitch, speed …) or other aircrafts/obstacles In the simulation above satellites with low azimuth could be lost easily regarding these effects.
3) Spatial weather (ionosphere) according to solar activity could affect GPS signal in space. It is necessary to consult specialist on this domain to have the impact on GPS signal at the day and the time of the issue.
4) System Architecture (like technician from Qingdao Airlines describes in the factor 3 part) could have an impact. The Installation quality itself (Antenna, coax, plugs) too. Finally, an equipment issue has to be considered as well. The End to End GPS reception must be checked (Antennas, installation, and equipment). For example, antenna should be clean, connectors without any oxidation and no damage on coaxial cable.
5) Finally, and not the minor thing, GPS Interferences (Jamming or Spoofing) is a contributor which is, unfortunately, more and more present in the world. Regarding the Factor 1 (clouds), there is no effect on GPS signal reception base on our in service experience. We think this factor has not to be considered.
To help in your investigation, I suggest you to notice, exactly, when the issue occurs, the Aircraft position, time and day. By this way it will be possible to identify a GPS jamming source if it is the case. You can also check the constellation as I did.
Hope it helps you in your investigation, don’t hesitate to contact us if needed.
总的来说,就是承认我的后两条假设有道理,并且补充了三点: 1、大气电离层对GPS信号有影响; 2、飞机机体结构阻挡对信号有影响; 3、GPS收发器之间可能会存在相互干扰(不确定)。
收到反馈后,我又抽空调查了下天气记录。发现出警告时居然是晴天居多、雨天较少。说明云确实不是阻挡信号的因素。
最终我的结论是:GPS的星座分(可见卫星较少)部是导致该故障的主要因素,飞机自身机体阻挡信号为次要因素。同时飞机做滚转时,可能会使某些GPS信号被机体阻隔,加剧GPS信号丢失。
建议空客可以考虑在飞机机体下表面加装个辅助GPS天线,用来捕获一些进地平线的卫星信号。
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