Why different type of atomic clocks used ?

Hi!

I have a question:
Why are different atomic clocks used on the satellites ?

II/IIA satellites have 2 cs and 2 RB clocks
IIR-M have 3 RB clocks.
IIF will have 2 CS and 1 RB clock.


I mean, why did they change from 2cs/2rb to 3RB and "back" to 2cs/1RB ?


Matthias

Our ability to make atomic clocks improved over time. Rubidium atomic
clocks on the GPS Block IIR(M) have proven to be very accurate. See
http://tycho.usno.navy.mil/ptti/ptti2003/paper15.pdf

RB clocks do suffer from frequency drift but this can be (and is)
corrected for. Cesium atomic clocks tend to remain close to the ideal
cesium clock frequency and have almost zero frequency drift.

Why three atomic clocks? In the 19th century, sailors had a saying,
"Take one clock to sea or three". If you brought two and they
disagreed, how could you tell which was wrong? With three you can
vote.


The Boeing built Block IIF satellites incorporate a substantial
upgrade to the cesium atomic clock technology. "The improvement is the
latest result of long-standing development work on frequency standard
technologies by the U.S. Naval Research Laboratory (NRL), Washington,
D.C." http://www.fas.org/spp/military/program/nav/970505-2f.htm

[snip]

--Mike Jr