Critical DME
在FAA中，Critical DME的定义：
地址：http://www./air_traffic/flight_info/aeronav/criticaldme/
A critical Distance Measuring Equipment (DME) is a DME facility that, when notavailable, results in navigation service which is not sufficient forDME/DME/IRU operations along all or portions of a specific route or procedure.The required performance assumes an aircraft's RNAV system meets the minimumstandard (baseline) for DME/DME RNAV systems found in AC 90-100A, appendix 1,or the minimum standard for DME/DME/IRU systems found in AC90-100A, appendix 2.For example, terminal RNAV DPs and STARs may be published with only two (2)DMEs, in which case, both are critical.
If any critical DME facilities exist on an RNAV DP or STAR, they are identifiedon the relevant chart. NOTAMs should be checked to verify the operation ofcritical DMEs for navigation relying on DME. Pilots should assess theircapability to navigate (potentially to an alternate destination) in case offailure of critical DME while airborne. The critical DME list is updated ascharts are published. This list is searchable to determine which facilities arecritical for portions or all of a procedure and/or route.
在google里关于DME GAP的说法：
DME Gap is the distance where you wonthave any signal coverage from the navaids the procedure is based upon. If youhave any other equipment onboard, i.e. GPS/GPS you can still fly the procedureaccurate.
Critical DME has to do with your RNAV Capabilities and availability for theSTAR/SID. Japan has not quite embraced pure GPS navagation so for RNAV aircraftyou must also have ground based navaids for backup position recognition. So. Toanswer your question Lets go though the chart. Lets take the Celes Rnav Arrivalto RWY 34.
In this case the box on the lower left hand corner (if using Jepps) shows theCritical DME. Let's take the Kajiki VOR for example. it has a DME Gap of 12.5NM, so basically if the KGE VOR is inop, somewhere in this arrival there wouldbe a 12.5nm gap in coverage of the ground based navaids to calculate DME/DMEfor your RNAV equipment. Obviously this is probably not a problem because mosttransport catagory aircraft use a FMS that has GPS/GPS or GPS/DME calculationsso you're navigation performance is not degraded significantly. Most operatorsin Japan have a DME Gap limit for each type of aircraft flown. My aircraft forexample has a max DME Gap of 14.0 nm, so if the KGE VOR (which is mostrestrictive) is INOP I am still ok to fly this arrival.
RNAV运行不是一定需要借助GNSS系统，它完全可以利用地基导航系统进行定位并完成RNAV运行。
所以在PBN运行中，如果GNSS数据不可用，则飞机还可以依赖其他传感器或导航设备进行定位，从而完成PBN运行。在RNAV1和RNAV2中，飞机可以依赖DME/DME/IRU定位。此时机组必须确认关键DME台工作正常。RNAV1经常用于进离场程序，在杰普逊航图中的进离场图中，会经常用一个表格列出Critical DME和DME GAP。如下图：（以下表格识读方法为笔者推测，准确与否还有待考证）
ROUTE SEGMENT表示该Critical DME所处的位置和DME GAP影响的航段，而DME GAP表示该Critical DME信号无法覆盖到的范围。但还有下列一种表格：
这里的DME GAP所表示的意义仅为在该范围内没有除了GNSS以外的任何导航设施的信号覆盖。但是这并不代表在该范围内无法继续执行RNAV程序实施进近，因为有GNSS可以继续进行定位导航。关于DME GAP的产生原理：根据《在航路和终端区实施RNAV1和RNAV2的运行指南》里的“附件1 DME/DME RNAV系统的最低（基准）性能标准”里的规定：
由此可知，只有满足以上规定范围的DME才被作为有效的DME。这也很好理解，因为DME所测距离为斜距，因此越靠近DME上方，由于航空器和DME的高度差的影响，其所测距离越不准确。但要说明另外一种情况，在进近着陆的时候DME不可用的范围要比上述规定的范围要小很多，因为即将着陆的航空器与DME的高度差变得很小，所以其所提供的斜距仍具有较高的精确度。
