A modern GPS does a few things to test itself and verify that everything works. Once the system loads there are a few things for you to do to make sure it is ready to go. For this post, I will use a Garmin 530. Other GPS systems may vary somewhat in how they operate.
Make sure the battery and avionics master switches are turned on. Then verify that the GPS is turned on. If it is off, turn it on using the comm volume knob near the top left of the unit. The GPS will begin to start.
When it is ready it will show you the status of the database (see image below). In this example unit notice that the obstacle database is not current and it hasn’t been for almost 10 years! “OK?” is highlighted so press ENT to continue.
Then the instrument self-test will begin immediately. The left side is showing the output from the unit. We will check this against the instrument it is transmitting to. On the right, we can change fuel settings so the unit will remind us to change tanks. There is also a button to use checklists stored in the Garmin if desired.
To verify that our GPS is operating correctly I look at the HSI, and we are particularly concerned with the yellow needle right now, representing our position relative to an imaginary path the GPS has placed to our right.
Looking at the self-test page it is calling for LCDI: Half-left which means that the course deviation indicator (yellow needle) is halfway to the left.
Then it has LFLG:Out of view meaning that the lateral navigation warning flag is NOT displayed. Notice that the HDG flag is displayed below indicating that the instrument is not working for displaying heading. There is no vacuum at the moment so the gyro hasn’t spun up yet. This is expected and not part of the GPS self-test.
Next, we verify VCDI: Half-up and VFLG: Out of view. The vertical course deviation indicator is the small yellow arrow on the left in the image below and you can see that it is half up and there is no flag shown.
The To indication is shown as a white arrow near the middle of the instrument.
This plane is not equipped with outputs for the rest of the items on the list except for DTK, which stands for direct track.
When I took a picture of the GPS the DTK was 150 but I turned the HSI for the picture below so the GPS DTK changed from 150, where the needle was pointing, to 290 as shown below.
Once these are verified press ENT to continue and begin using the GPS.