Passive Stations were installed as part of the Global Positioning System (GPS).
The OS Passive GPS station database:
Ordnance Survey's network of passive GPS stations throughout Great Britain allows GPS surveyors to precisely position their survey stations in the ETRS89 coordinate system. OS passive GPS stations are geodetic quality ground marks in user-accessible locations. A typical survey site in Great Britain will have several OS passive stations within 20-35 km (distances may be greater in the Scottish Highlands). The main differences between this network and the Active station network are that passive stations must be occupied by the user's own GPS reference receiver during the survey; and that typical distances from a survey site to the nearest passive stations are smaller than for Active stations, making required observation times shorter and allowing the use of single-frequency GPS receivers.
The OS Passive GPS station database (List or Search) is a searchable database of all passive stations, including precise geodetic coordinates, station descriptions, access information, and recent maintenance and monitoring details. Many, but not all, station records include a site photograph and location sketch. The database allows GPS surveyors to plan control surveys based on reliable OS control stations. To use the National GPS Network passive stations, two or more survey GPS receivers (preferably dual frequency) and processing software are required. The resulting ETRS89 GPS coordinates can be precisely converted to National Grid coordinates and Newlyn height coordinates using OS precise transformations. The passive station database is no longer maintained. An explanation of the passive station naming scheme is available.
Physically, Passive Stations take a number of different forms:
- Berntsen (102)
- Bolt (84), often but not always located in a concrete block
- Buried Block (3) i.e. Gallow Hill, Bolnhurst and Hemingstone.
- Cut (3) i.e. Hugh Town, Down Barn Farm and Cumrue.
- Fundamental Benchmark (200)
- Pillar (233)
- Rivet (142)
- Surface Block (240)
- Total 1007
In additon, as a result of a FOI request to the OS by James S, a further 44 stations have been discovered that were originally intended to be Passive Stns, but never appeared in the PSD for reasons unknown. At least 16 of these stations are extant as of Jun 2023; 11 are probably destroyed; the remaining 17 are of unknown status. They comprise: Bolts x13, Buried Blocks x1, Cut x1, Pillars x14, Rivets x4, Spiders x1, Surface Blocks x3, Unknown type x7.
- Grand Total 1051
It should be noted that the OS list 844 passive stations in their current database. It is not known why 163 have been deleted from the original list of 1007, possibly they have been destroyed through change of land use, although it is known that at least 876 are extant, as of July 2023.
All of the Fundamental Bench Marks are designated Passive Stns (albeit only 83 are listed in the current OS PSD), with the exception of 6 that are recorded as Destroyed. And all of the 102 Berntsens listed on T:UK are Passives. FBMs often have another Passive in fairly close proximity, typically consisting of a Berntsen and/or a Block.
An example of an FBM with Surface Block and Berntsen close together is the Oswestry FBM H1SJ2724 with nearby Oswestry Aux 1 Berntsen H2SJ2724 and Llynclys surface block C1SJ2824; they are 0.4km from each other.