# What is the best pair selection algorithm?

Distance Measuring Equipment selection criteria is said “those closest to the aircraft and which intersect at an angle between 30 degrees and 150 degrees" (also said one whose geometry is closest to 90 degrees") (Not sure range or angle is more important in operation)

So question is what should be the best pair selection algorithm for coding in operation?

When using a DME pair, the location of the receiver can be determined with some accuracy associated.

The time measurement is performed with some uncertainty leading to a slant range that is within a ring area (the double circles).

Observation

When looking at the intersection of the two rings with different positions of the DMEs:

• The hatched area must be eliminated using some method
• The blue area is reduced when the two DME bearings are at a right angle
• As the width of a ring is proportional to the radius (the uncertainty is a percentage of the range), the smallest blue area is obtained with DME at right angle and the closest to the aircraft (last diagram).

Proposed algorithm

Divide the area around the aircraft into, say 8 sectors of 45° (the number of sectors impacts the accuracy but also the number of computations to be done, in an exponential way).

1. Look for the two closest stations, write down the sector(s) they belong to

2. Compute the intersection area

3. Find the next closest station D, note its sector S

5. Else within the already known sectors, find sector SP closest to 90° of S. Get station DC of this sector.

6. Compute intersection area for pair D-DC

7. If this area is smaller than the previous smallest area, keep it as new smallest

8. Repeat from step 3 until all sectors have been tested (one station per sector) or the radius has exceeded some predetermined limit.

9. Determine the new aircraft position using the best pair found.

10. Restart at step 1 using the new aircraft position (the time interval before restarting the cycle may be determined from the distance the aircraft moved. If the aircraft hasn't moved, then the next search for a pair is likely to find the same pair).

Example

• Step 1: The two closest stations are in sectors S3 and S7 (fig A).
• Step 2: Their intersection area is X (fig. A).
• Step 3: The next closest is D3 in sector S2 (fig B).
• Step 4: S2 has not yet been visited, so let's look at D3.
• Step 5: In {S3, S7}, S7 is the sector closest to 90° of S2
• Step 6: D3-D6 intersection area is Y (fig. B)
• Step 7: Y is smaller than X, so our candidates are now D3-D6
• Step 3: The next closest is D5 in sector S5 (fig C).
• Step 4: S5 has not yet been visited, so let's look at D5.
• Step 5: In {S2, S3, S7}, S3 and S7 are the sector closest to 90° of S5, we chose S3 because D4 is closer than D6.
• Step 6: D4-D5 intersection area is Z.
• Step 7: Z is smaller than Y, so our candidates are now D4-D5
• Step 3: The next closest is D2 in sector S2
• Step 4: S2 has already been visited, we ignore D2.
• Step 3: The next closest is D1 in sector S1
• Step 4: S1 has not yet been visited, so let's look at D1 (fig D).
• Step 5: In {S2, S3, S5, S7}, S3 and S7 are the sector closest to 90° of S1, we chose S3 because D4 is closer than D6.
• Step 6: D4-D5 intersection area is Z.
• Step 7: Z is smaller than Y, so our candidates are now D4-D5
• Step 3: The next closest is D1 in sector S1 (fig D).
• Step 4: S1 has not yet been visited, so let's look at D1.
• Step 5: In {S2, S3, S5, S7}, S3 and S7 are the sector closest to 90° of S1, we chose S3 because D4 is closer than D6.
• Step 6: D1-D4 intersection area is T.
• Step 7: T is larger than Z, so our candidates still D4-D5
• Step 8: Our maximum radius has been reached, our pair is D4-D5
• @Efes06: At step 3, by range, at step 5 by sector azimuth (as mentioned). For the latter, as sectors are, in this example, 45° wide, you use the mean azimuth of each sector, not the exact azimuth of the stations. "30/150° constraint"? If you mean D3-D6 pair in fig. B: I know I made a mistake in fig. B (I should have selected pair D3-D4 because D4 is closer than D6), but I needed to update the picture, and as it is barely visible and has no incidence on the result, I left it with the error. – mins Dec 24 '17 at 10:42