How to add a PlaceNode to a camera?

[MRdesperado1]

Hello, there was a problem when I used osgEarth to create another camera.
How to add a PlaceNode to a camera?
I used the camera to create a thumbnail, just like in a large game. I expect some key points to be marked on the thumbnail, such as moving aircraft, etc. However, my PlaceNode cannot be displayed on the thumbnail, but when I use Root to add these created PlaceNode, they can be displayed normally on the big map. What is the reason?
The code for creating PlaceNode is as follows：

osgEarth::Style styleairfly;
osgEarth::Symbology::TextSymbol* textstyle2 = styleairfly.getOrCreateSymbolosgEarth::Symbology::TextSymbol();
styleairfly.getOrCreateosgEarth::Symbology::TextSymbol()->encoding() = osgEarth::Symbology::TextSymbol::ENCODING_UTF8;
textstyle2->fill()->color() = osg::Vec4f(1.0, 0.0, 0.0, 0.5);
textstyle2->halo()->color() = osg::Vec4f(1.0, 0.8, 0.8, 1.0);
textstyle2->font() = "simsun.ttc";
textstyle2->size() = 16.0;
textstyle2->alignment() = osgEarth::Symbology::TextSymbol::ALIGN_CENTER_BOTTOM;
ffLabel = new osgEarth::Annotation::PlaceNode(osgEarth::GeoPoint(mapNode->getMapSRS(), osg::Vec3d(109.1347, 34.3834, 100)), transcode::transferunicode("F35C-1"), styleairfly, jichang);

The code for creating Camera is as follows：

radar = new osg::Camera;
radar->setName("radar");
radar->setClearColor(osg::Vec4(0.2, 0.2, 0.2, 1.0));
radar->setRenderOrder(osg::Camera::POST_RENDER);
radar->setAllowEventFocus(false);
radar->setClearMask(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
radar->setReferenceFrame(osg::Transform::ABSOLUTE_RF);
radar->setViewport(0,0,256,256);
radar->setProjectionMatrixAsPerspective(30.0f, static_cast(256) / static_cast(256), 1.0, 1000.0);

The code for adding placenode to camera is as follows：

radar->addChild(mapNode);
radar->addChild(ffLabel);
mRoot->addChild(radar);

The effect after adding is shown in the figure：

Add it directly to the big map, and the effect is as shown in the figure：

What is the reason?
Thank you very much. And I want to ask you. My airport displays floating in the air. As the viewpoint moves, the airport will also move. How should this situation be solved?
I use OSG version 3.6.3 and osgearth version 2.10.

[gwaldron]

You might need to disable small feature culling on your inset camera:

radar->setSmallFeatureCullingPixelSize(-1.0f);

[MRdesperado1]

I tried to do as you said, but it didn't work. PlaceNode is still not displayed. Is there any other way?