Creation of Function Node

Acording to the PDF with sample data you could so this...

which matches this...
image

using buffer parser set as per the info you provided...

Demo flow...

[{"id":"99b86a91.cc8f18","type":"inject","z":"b489a0e4.4dd7e","name":"test data AgH5e5xFaIdRRA==","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"data\":\"AgH5e5xFaIdRRA==\"}","payloadType":"json","x":820,"y":100,"wires":[["88fc873f.c086c8"]]},{"id":"2bb51028.cbdab","type":"buffer-parser","z":"b489a0e4.4dd7e","name":"","data":"payload.data","dataType":"msg","specification":"spec","specificationType":"ui","items":[{"type":"floatle","name":"lat","offset":2,"length":1,"offsetbit":0,"scale":1,"mask":""},{"type":"floatle","name":"lon","offset":6,"length":1,"offsetbit":0,"scale":1,"mask":""}],"swap1":"","swap2":"","swap3":"","swap1Type":"swap","swap2Type":"swap","swap3Type":"swap","msgProperty":"payload","msgPropertyType":"str","resultType":"keyvalue","resultTypeType":"output","multipleResult":false,"setTopic":true,"x":1110,"y":160,"wires":[["2057e639.f3a49a"]]},{"id":"b4228108.10405","type":"debug","z":"b489a0e4.4dd7e","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":1290,"y":100,"wires":[]},{"id":"2057e639.f3a49a","type":"debug","z":"b489a0e4.4dd7e","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":1290,"y":160,"wires":[]},{"id":"363f26cc.7e8fda","type":"inject","z":"b489a0e4.4dd7e","name":"click me","props":[{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","x":760,"y":160,"wires":[["2b6a66d1.86484a"]]},{"id":"2b6a66d1.86484a","type":"function","z":"b489a0e4.4dd7e","name":"test data 02 01 F9 7B 9C 45 68 87 51 44","func":"msg.payload = { data: Buffer.from(\"0201F97B9C4568875144\", \"hex\") }\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":860,"y":200,"wires":[["2bb51028.cbdab","29572a95.8942b6"]]},{"id":"88fc873f.c086c8","type":"base64","z":"b489a0e4.4dd7e","name":"","action":"","property":"payload.data","x":1100,"y":100,"wires":[["b4228108.10405","2bb51028.cbdab"]]},{"id":"29572a95.8942b6","type":"debug","z":"b489a0e4.4dd7e","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":1290,"y":200,"wires":[]}]