If bit 110 is set to “1”, the offset position data which is applied to the coarse position data provided in bits 59 through 85 of PDF-1 is encoded into the National Location Protocol beacons in bits 113 to 126 as indicated below. In order to minimise the frequency of updates of the position data in PDF-1, the magnitude of the offsets has a range of +/- 2 times the resolution provided by PDF-1.
The initial offset to PDF-1 encoded in bits 113 to 126 of PDF-2 is selected so that when summed with the coarse position it produces a finer position that is as close as possible to the actual position of the beacon. Subsequent position updates are then encoded by retaining the coarse position (in PDF-1) and changing only the offset, provided that the required value is within the range of the offset.
If bit 110 is set to “0”, the structure of PDF-2 must be defined by the responsible authority of the Country identified by the country code (bits 27 – 36).
The default settings to be used if offset location data is not available are as follows:
- Bits 113 to 119 = “1 00 1111”
- Bits 120 to 126 = “1 00 1111”
The table below provides an example of how the initial coarse and offset values are determined.
| Real Position | Coarse Position Encoded in PDF-1 | |
| 43 deg : 31 min : 56 sec North | 43 deg : 32 min North | – 0 min : 4 sec |
| 01 deg : 25 min : 32 sec East | 01 deg : 26 min East | – 0 min : 28 sec |
In the example above the 43 deg: 31 min: 56 sec North, 1 deg: 25 min: 32 sec East would be the resulting position encoded in the beacon. The allocation of bits for this example is depicted here.