It looks like there are a couple of options depending on exactly which model of 959 you have and whether or not you want a heading sensor. See below:
Humminbird AS GPS HS
•Precision GPS Antenna w/Heading Sensor
•Standard w/ 360 Imaging™
•View Compass Information & Boat orientations While Moving Or Sitting
•Accurate Course Over Ground Readings
•Works In Lower Signal Areas
For Humminbird Models: •ONIX Series: All
•HELIX Series: 9, 10, 12
•800 Series: 859ci HD Combo, 859ci HD DI Combo, 859ci HD XD Combo, 899ci HD SI Combo
•900 Series: 959ci HD Combo, 959ci HD DI Combo, 959ci HD XD Combo, 999ci HD SI Combo
•1100 Series: 1159ci HD Combo, 1159ci HD DI Combo, 1159ci HD XD Combo, 1199ci HD SI Combo
•AS 360SSI, AS 360 TM
Humminbird AS GRP
•Precision GPS Antenna
•Fast & Precise Position Fixes
•More Accurate Than Industry Standard GPS
•Works In Lower Signal Areas
For Humminbird Models: •ONIX Series: All
•HELIX Series: 9, 10, 12
•800 Series: All
•900 Series: All
•1100 Series: All