How many caesium clocks are there on each GPS satellite ? Is there any other atomic clock on-board? I am trying to find how many caesium atomic clocks are carried by each of the GPS satellites. And whether the satellites also carry other atomic clocks, such as rubidium or hydrogen clocks.
 A: https://science.nasa.gov/science-news/science-at-nasa/2002/08apr_atomicclock/: "Each of the 24 GPS satellites carries 4 atomic clocks on board." Maybe something has changed since then.
EDIT (10/01/17):http://space.skyrocket.de/doc_sdat/navstar-2.htm : "GPS-2 (Global Positioning System) or Navstar-2 (Navigation System using Timing And ranging) are improved satellites of the second generation of the GPS navigation system...Each spacecraft carried 2 rubidium and 2 cesium clocks."
EDIT (10/01/17): @MartinBeckett : the following document seems recent: ftp://tycho.usno.navy.mil/pub/gps/gpsb2.txt "The design life of the Block II/IIA satellite is 7.3 years; each contain four atomic clocks:  two Cesium (Cs) and two Rubidium (Rb)...The design life of the Block IIR satellite is 7.8 years; each contains three Rb atomic clocks"
EDIT (Feb 23, 2022):

There are currently several dozen GNSS satellites in orbit. GPS and
GLONASS satellites carry Rb (rubidium) and Cs (caesium) frequency
standards, whereas Galileo satellites are equipped with PHM (passive
hydrogen maser) and Rb clocks. In the case of BeiDou, the clocks used
are of either Rb or HMAC (Hydrogen Maser Atomic Clock) standards

(https://www.mdpi.com/1424-8220/21/7/2396/htm )
See also https://qzss.go.jp/en/technical/satellites/index.html Looks like the answer to your question may depend on the generation of the satellite system.
