Here is the Gato sns file from RFB 2.0. The RFB manual says that the SD Air search radar becomes available in Dec. 1941, with the SD-1 Air search radar becoming available from June 1942. My current campaign date is mid April, 1942.
Presumably [Sensor 15] is the SD in question(?), in which case the end date is not until March '43, suggesting that it should still be available.
Also, it looks to me as if [Sensor 16] is the SD-1 radar, in which case the availability dates for that do not match those in the manual...
; sensor configuration file
;lookout on conning tower
[Sensor 1]
NodeName=O01
LinkName=VisualSubWatch
StartDate=19380101
EndDate=19451231
;atack periscope
[Sensor 2]
NodeName=O02
LinkName=Periscope
StartDate=19380101
EndDate=19451231
;Observation periscope
[Sensor 3]
NodeName=O03
LinkName=Periscope
StartDate=19380101
EndDate=19451231
;Hydrophone
[Sensor 4]
NodeName=H01
LinkName=NSS_Hydrophone_Ball_02
StartDate=19380101
EndDate=19430430
[Sensor 5]
NodeName=H04
LinkName=NSS_Hydrophone_Ball_01
StartDate=19380101
EndDate=19430430
;Hydrophone2
[Sensor 6]
NodeName=H02
LinkName=NSS_Hydrophone_Head
StartDate=19380101
EndDate=19451231
;Sonar
[Sensor 7]
NodeName=N01
LinkName=NULL
StartDate=19380101
EndDate=19451231
;Radar
[Sensor 8]
NodeName=R01
LinkName=NULL
StartDate=19380101
EndDate=19420704
[Sensor 9]
NodeName=R01
LinkName=NSS_EarlySJ_Mount
StartDate=19420705
EndDate=19430630
[Sensor 10]
NodeName=R01
LinkName=NSS_LateSJ_Mount
StartDate=19430701
EndDate=19450831
[Sensor 11]
NodeName=R01
LinkName=NSS_EarlySJ_Mount
StartDate=19450901
EndDate=19451231
;RadioDF
[Sensor 12]
NodeName=D01
LinkName=NULL
StartDate=19380101
EndDate=19451231
;Radar Warning
[Sensor 13]
NodeName=C01
LinkName=NULL
StartDate=19380101
EndDate=19451231
[Sensor 14]
NodeName=R02
LinkName=NULL
StartDate=19380101
EndDate=19411224
[Sensor 15]
NodeName=R02
LinkName=NSS_EarlySD
StartDate=19411225
EndDate=19430311
[Sensor 16]
NodeName=R02
LinkName=NSS_LateSD
StartDate=19430312
EndDate=19450731
[Sensor 17]
NodeName=R02
LinkName=NSS_SS
StartDate=19450801
EndDate=19451231
[Sensor 18]
NodeName=R03
LinkName=NSS_IFF_01
StartDate=19380101
EndDate=19451231
[Sensor 19]
NodeName=D02
LinkName=NULL
StartDate=19380101
EndDate=19451231
[Sensor 20]
NodeName=H01
LinkName=NSS_Hydrophone_Ball_04
StartDate=19430501
EndDate=19451231
[Sensor 21]
NodeName=H04
LinkName=NSS_Hydrophone_Ball_03
StartDate=19430501
EndDate=19451231
I did read somewhere of an instance where someone had to do 1 patrol without it after upgrading his sub, and then was able to purchase it his next time in port. I'm hoping this will happen to me