Doug Ritter wrote an article on strobes a while ago on this site:
http://www.equipped.org/signal.htm#BatteryPoweredAttraction

A good strobe is visible 3-5 miles in all directions under optimal conditions.

You can see which ones are USCG approved by going here:
http://cgmix.uscg.mil/Equipment/EquipmentSearch.aspx
and using the pulldowm menu to select:
Lifejacket Light (SOLAS) - 161.112

The ACR 4F/4G seems to have been the second type (of 67 total) to ever get approval, assuming the approval numbers were issued serially.

The US Coast Guard site discusses the tests [1] a unit has to pass to get USCG/SOLAS* approval.


[1] http://www.uscg.mil/hq/cg5/cg5214/docs/Lifejacket_Light_Guidelines.pdf

[2] SOLAS == International Convention for the Safety of Life at Sea
http://en.wikipedia.org/wiki/International_Convention_for_the_Safety_of_Life_at_Sea

[3] Evaluation of Night Vision Goggles (NVG) for Maritime Search and Rescue: HH-65A Sweep Width Verification and Laser Illuminator Evaluation
http://handle.dtic.mil/100.2/ADA376017
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)