Just woken up here in the UK.
I agree with @craigcurtin - you need to split out the string to get the 'lat' and 'lon' values.
I've very quickly modified my Wildfires flow and turned it into a very simple position indicator for the ISS.
Once you have split out your values you could probably make use of the last two nodes in this flow.
[{"id":"605c9ce313e4a5a7","type":"ui_worldmap","z":"a9b12d67e974b017","group":"d4d2c5a78449ad8a","order":1,"width":18,"height":14,"name":"fires","lat":"51","lon":"0","zoom":"4","layer":"EsriS","cluster":"","maxage":"","usermenu":"show","layers":"show","panit":"true","panlock":"false","zoomlock":"false","hiderightclick":"true","coords":"none","showgrid":"false","allowFileDrop":"false","path":"/worldmap","overlist":"CO,RA,DN","maplist":"EsriS,EsriT,NatGeo","mapname":"","mapurl":"","mapopt":"","mapwms":false,"x":830,"y":440,"wires":[]},{"id":"060a72676e03f9d3","type":"inject","z":"a9b12d67e974b017","name":"Trigger every 5-secs","props":[{"p":"headers","v":"{\"Accept\":\"text/json\"}","vt":"json"},{"p":"url","v":"http://api.open-notify.org/iss-now.json","vt":"str"}],"repeat":"5","crontab":"","once":true,"onceDelay":0.1,"topic":"","x":200,"y":440,"wires":[["3b88474842c4750d"]]},{"id":"3b88474842c4750d","type":"http request","z":"a9b12d67e974b017","name":"Get ISS coordinates","method":"GET","ret":"obj","paytoqs":"ignore","url":"","tls":"","persist":false,"proxy":"","authType":"","senderr":false,"x":440,"y":440,"wires":[["6b32ca2b8bafb6cf"]]},{"id":"6b32ca2b8bafb6cf","type":"function","z":"a9b12d67e974b017","name":"Extract lat, lon","func":"let out = []; // Declare an array\n\nlet id = \"ISS\";\nlet lat = \"\";\nlet lon = \"\";\n\nlon = msg.payload.iss_position.longitude;\nlat = msg.payload.iss_position.latitude;\n\nout.push({\"name\": id, \"lat\": lat, \"lon\": lon, \"layer\": \"ISS track\", \"icon\": \"fa-space-shuttle\"});\n\nmsg.payload = out;\nreturn msg;\n\n","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":660,"y":440,"wires":[["605c9ce313e4a5a7"]]},{"id":"d4d2c5a78449ad8a","type":"ui_group","name":"World_map","tab":"47ec854f298fe1a9","order":1,"disp":true,"width":"18","collapse":false,"className":""},{"id":"47ec854f298fe1a9","type":"ui_tab","name":"Wild_fires","icon":"dashboard","order":40,"disabled":false,"hidden":false}]