Aircraft Avionics

GPSS Retrofits: Automation to the Max

Nearly every proposal for a new autopilot installations-which these days means S-TEC upgrades-should include a GPSS option. And optional is the key word because unless you buy a flagship S-TEC 55X autopilot, GPSS wont be included with the base system. For most customers looking to upgrade GPS and autopilot equipment, GPSS hardware is grossly misunderstood. Although hardly a major system, GPSS is considered an accessory that plays a huge part in total autopilot automation. Impressively, it emulates the tight performance found with big-airplane inertial navigational systems.

Larry Anglisano
Verified

Nearly every proposal for a new autopilot installations-which these days means S-TEC upgrades-should include a GPSS option. And optional is the key word because unless you buy a flagship S-TEC 55X autopilot, GPSS wont be included with the base system. For most customers looking to upgrade GPS and autopilot equipment, GPSS hardware is grossly misunderstood.

GPSS Retrofits

Although hardly a major system, GPSS is considered an accessory that plays a huge part in total autopilot automation. Impressively, it emulates the tight performance found with big-airplane inertial navigational systems.

Upgrading to GPSS is pretty easy, since there are several aftermarket GPSS systems to choose from. They can be interfaced into a new autopilot installation or tacked on to an existing 30-year-old autopilot. Theyll work in a limited way with older GPS navigators as we’ll as newer ones.

Lets do a brief rundown with a simple explanation of GPSS along with a look at the current market offerings. To cut to the chase, if you buy a new Aspen PFD or Garmin G500, GPSS is included as a bonus.

GPSS 101

Its easy to see why GPSS systems cause so much confusion. To understand the theory of operation, you need to understand the relationship between an analog and a digital interface. Most general aviation autopilots (with the exception of Garmins integrated GFC700 thats part of the G1000 suite) are analog-based systems.

This 1980s and earlier analog circuitry knows nothing about high-speed databuses or RS232 serial waypoint data that stream from most modern GPS units. Instead, when autopilots track a course, they follow the analog left/right needle action from a mechanical CDI or HSI. Analog autopilots chase the needle just like a hand-flying pilot would. But this game of needle-scalloping cat and mouse can be decisively

Larry Anglisano

Editor in Chief Larry Anglisano has been a staple at Aviation Consumer since 1995. An active land, sea and glider pilot, Larry has over 30 years’ experience as an avionics repairman and flight test pilot. He’s the editorial director overseeing sister publications Aviation Safety magazine, IFR magazine and is a regular contributor to KITPLANES magazine with his Avionics Bootcamp column.