Minimum Sector Altitude
Minimum Sector Altitude (MSA) diagrams graphically depict safe altitudes to fly around the Approach reference NAVAID, in this case, GVO VORTAC.
The CIRCLE shows us the radius that the safe altitude applies to, in this case 25nMiles surrounding GVO VORTAC.
In the centre of the circle the Approach Reference Navaid sets. From it, may be SECTORS drawn inward cutting the circle into pie-like sections. These show directions of approach that have different safe altitudes. Inside the small BOXES are numbers indicating Altitude in Feet MSL.
Example : If you were flying an inbound course of 076°, you would fall between the 360° and 140° sectors and would have a MSA of 4600 ft. MSL.
If you were flying inbound at 270°, your MSA would be 8000 ft. MSL.
MSA ensures you have 1000 ft. obstacle clearance within a 25nMile radius, but not necessarily adequate navaid reception like the MAA's, MEA's offer on enroute charts.