GPS is sensitive to jamming etcand interferance because the signal levels the system levels are so low<ref>http://tf.nist.gov/sim/2010_Seminar/SIM_2010_GPS_Lombardi.ppt</ref> and can easily be swamped by other sources, that can be accidental or deliberate.<ref>http://www.pnt.gov/advisory/recommendations/2010-11-jammingwhitepaper.pdf</ref>. Also since GPS depends on line of sight signals can be disrupted by [[Urban canyon]] effects, making GPS only available to some locations at certain times of the day, for example.
A GPS outage however is not initially an issue because clocks can go into Holdover holdover<ref>http://www.syncuniversity.org/drsync/q45.php</ref>, allowing the interference to be alleviated as much as the stability of the oscillator providing holdover will allow.<ref>http://www.gmat.unsw.edu.au/snap/publications/khan&dempster2007b.pdf</ref> The more stable the oscillator, the longer the system can operate without GPS.
GPS suffers from interference which can be alleviated by a GPSDO, up to the stability of the GPSDO<ref>http://www.gmat.unsw.edu.au/snap/publications/khan&dempster2007b.pdf</ref>
