That is correct. Basically it consists of aircraft that are confirmed as Mode S equipped and tied-up via official or sighting reports.
You must remember the vast majority of registered machines (including Gliders, balloons and Microlights) still remain non-Mode S equipped.
Here is the CAA stats
http://www.caa.co.uk/docs/56/REG%20TOTALS%20WG%20010111.pdf- there are circa 20,000 registered in the UK and each has been allocated a nominal code by the CAA. However at least 50% of those nominally allocated codes will be for machines that will not have been fitted with Mode S.
Of the 20,000:
4,000 are microlights
2,300 are Gliders
1,700 are Balloons
300 are gyroplanes
280 are Self Launch Gliders
That makes 8,500 with a miniscule Mode S take-up probably less than 0.05 percent.
Another 3,000 GA aircraft under 750kg
there is probably currently only a 15% mode S equippage in this area.
You can now see that over 50% of the UK register does not currently need to be in our database as they are not Mode-S equipped or are stored/derelict etc and therefore will not be picked up by users.
We currently have 8662 UK tie-ups in the DB which equates to circa 43% of the register. Of course until CAA publishes Mode S equippage stats, we will never know how close this is to the total equipped, but we are definately getting there with regards to the ones actually getting airborne and broadcasting a Mode S code. Of course as more aircraft are confirmed as Mode S equipped they will be added in the normal way.
