USALS and H-H mounts
The screw jack actuators are usually rated for 24 or 36 pulses per inch of travel.
Unfortunately, the way they are mechanically linked to the dish, one inch of travel on either side of the arc is not the same as one inch of travel in the middle.
In Horizon to Horizon motors, if you have 20 pulses per degree of travel on the east side of the arc, you have it everywhere.
Each motor is different, and by changing out the magnet wheel with one having more magnets, you increase the accuracy.
One of the mods for the Birdview mount, was to change out an 8 magnet wheel, for one with 24 magnets.
Where the magnet wheel is located in the drive chain, determines the ultimate accuracy.
From the perspective of the user, two birds which are 2° apart at the horizon, appear closer together than two birds which are 2° apart near your true south.
So, even though the H-H motor has constant number of counts per degree of motor-travel, that doesn't map to a constant number of counts between satellites.
That's what USALS is designed to calculate.
It's all done in the receiver.
Unfortunately, there is no Gbox compatible with USALS receivers.