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GPS信号丢失在运行中经常会碰到：
某737-800飞机，短停上客后机组反映两部GPS无信号，GPS故障灯亮，重置MMR1和MMR2跳开关无效，报签派后按MEL放行。
后续在推出滑行阶段，GPS自动恢复正常，后续航段均正常。
可能细心的人会发现，GPS仅仅是信号丢失，又没有故障，GPS故障灯为什么会亮呢？
之前我们经常会说：GPS如果仅仅是信号丢失，但系统本身并没有故障，GPS故障灯不会亮。
▲2016年1月空地交流
包括厂家资料里也是这么说的：
▲737 FOTB:21-04
Query about the GPS signal loss cause trigger terrain alert
The MMR unit, which provides GPS functionality, is designed to continuously monitor its internal hardware and the GPS antenna for hard faults. If the MMR unit detects a hard fault, it will set a failure bit on the A429 data bus and is received by the IRS mode select panel. The IRS mode select panel will then illuminate the GPS fault light when GPS functions for both MMRs are declared faulted, or when the recall button is pressed and one MMR is declaring a GPS fault.
If GPS signal is lost, the MMR unit will not declare a hard fault (GPS fault light will not illuminate), but the GPS position will not be available on the CDU position reference page and the scratchpad may display the message "GPS L/R INVALID". Additionally, the Captains and F/O's clocks will show dashes, and TAWS related messages may display during GPS signal loss for both MMR units.
GPS signal loss is not a hardware fault, so the ground test will pass since there is nothing wrong with the hardware and the GPS system usually automatically recovers after GPS signal is regained.
但是在实际中，我们经常会碰到一些飞机GPS信号丢失时（非硬件故障），GPS故障灯也会亮：只有1侧GPS信号丢失，GPS故障灯再现亮；双GPS信号丢失时，GPS故障灯自动亮。
但是在公司大部分737NG飞机上，GPS信号丢失时GPS故障灯并不会亮。
我自己之前也关注过这个现象，发现是在较新引进的一些飞机上，GPS信号丢失与GPS故障的现象是一样，所以怀疑是MMR件号不一样导致的差异。
最近咨询了厂家，也证实了这一点：
The planes listed by A are all equipped withMMR P/N 822-1821-001. B has learned that this MMR box responds to a loss of GPS data (NCD outputs, not failed outputs), which can occur from things like GPS interference, by setting the GPS mode to failed.
The GPS Fault light is based off of GPS declaring the mode failure and when both MMRs are declaring a failure the GPS Fault Light illuminates. Once GPS signal comes back the mode failure is cleared along with the fault light clearing.
当然，这些飞机上安装的MMR件号其实是822-1821-002，并不是822-1821-001。-001已经不再生产，被-002替换，两者可以互换和混装：
▲757-SL-34-175：822-1821-001早就已经不再生产了
由此可见，GPS信号丢失后，GPS故障灯会不会亮，与安装的MMR件号有关。
如果飞机上安装的是件号822-1821-001、822-1821-002的MMR，GPS信号丢失和GPS故障是一样的现象，都会使GPS故障灯亮：
后面再碰到GPS信号丢失后GPS故障灯亮时，可以看看飞机上安装的MMR件号，留意一下其他件号的MMR是否也存在这种情况。
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发表于 2023-4-23 16:30:19
