GPS Back-up

The lightest travel freindly accurate kit to back-up GPS is a do it yourself mini Bris sextant combined with a 1 page Pub 249 sun long term almanac / 10' natural trig table and the back to the future Casio calculator watch CA - 53W. If there is extra room in the duffle bag then add a Davis MK 3 plastic sextant for completeness.