UPS – Universal Polar Stereographic

Polar position reference

UPS stands for Universal Polar Stereographic and is a metric grid coordinate system for the regions around the North and South Poles. It supplements UTM beyond the limits of its normal coverage. A position is described by an easting and a northing in the appropriate polar grid; MGRS can express that position using polar designation letters, a 100-kilometre square and two numeric groups. For personnel isolated during military operations, UPS provides an interpretable position reference in polar regions which can also be used in a personnel recovery position report when communication is available.

The normal UPS grid applies north of 84° north and south of 80° south. UTM covers the area between these limits. Being in the Arctic therefore does not by itself mean that a map uses UPS. A map sheet at a junction may show different grids. The grid designation and reference information for the part of the map actually being used determine the correct reference.

North and south grids

UPS has a separate grid for each pole. At the respective geographic pole, both the easting and the northing are 2,000,000 metres. This numerical offset shifts the coordinate origin and avoids negative values within the intended polar coverage. For two locations in the same polar grid, the difference between their eastings is the separation along the easting axis; the corresponding northing difference gives the separation along the northing axis. Distances calculated in the grid may differ from actual ground distances because of projection distortion.

The axes have fixed directions. Moving away from either pole, eastings increase along the 90° east meridian. Northings increase away from the North Pole along the 180° meridian, and away from the South Pole along the 0° meridian. On the 180° meridian in the north polar grid, an increasing northing therefore leads geographically south. “Easting” and “northing” name the two grid axes; they are not instructions to move geographically east or north.

A full numerical UPS coordinate pair also requires its north or south polar grid and geodetic datum. Two metre values without this context cannot be assigned unambiguously to a location. Depending on the agreed representation, the polar MGRS grid designation can supply the polar context.

UPS in MGRS format

Polar MGRS omits the UTM zone number. Its first letter identifies the geographic western or eastern half of the relevant polar region. Two letters identifying the 100-kilometre square follow. The remaining digits are split into two groups of equal length: easting within that square first, then northing. Crossing into another polar half or square changes this letter context; the numbers alone recur in other squares.

A numerical reading example uses a full easting of 1,735,000 metres and a northing of 2,243,000 metres in the north polar grid. Its MGRS reference at kilometre resolution is YXK3543: Y identifies the western north polar half, XK the 100-kilometre square, 35 an easting offset of 35 kilometres within that square and 43 a northing offset of 43 kilometres. The reference identifies a grid cell with a side length of one kilometre.

One to five digits per numeric group specify cells with side lengths of 10 kilometres, one kilometre, 100 metres, 10 metres or one metre. Leading zeros are retained. To express an MGRS reference at a coarser resolution, the rightmost digits of each group are truncated separately. Rounding can select an adjacent cell. A full numerical UPS easting, by contrast, is not an MGRS offset within a square; its first digits are not a UTM zone number either.

Checking the position and map reference

The isolated person first needs to establish where the position came from. A terrain feature positively identified on the map, a current satellite fix and a position estimated forward from the last known point have different uncertainties. Someone who has not located themselves on the map cannot obtain a current position by reading arbitrary grid numbers. The last confirmed point, its time and movement since then remain the useful starting information.

On a UPS map, values are read from the corresponding grid. The map margin and grid reference box provide the grid designation, polar designation, square letters and geodetic reference information; the scale determines how finely the point can be read. Lines or marginal ticks for latitude and longitude belong to a different representation. Where several grids appear, numbers, square designations and direction information must come from the same grid.

An available GPS receiver must actually support the required output format and be configured accordingly. Display format and geodetic datum are checked separately. WGS 84 describes the Earth reference, while UPS, MGRS or latitude and longitude specify how the position is represented. A coordinate conversion changes the representation; a datum transformation changes the geodetic reference. Giving unchanged numbers a new system name causes a false location assignment. If the conversion cannot be checked with the available means, the existing reference is retained in full and identified as that format in a report.

Grid direction and actual orientation

In polar regions, geographic meridians converge on the pole while the grid axes keep their fixed orientation. Grid north is the direction of increasing northings along an easting line. True north follows the local meridian. A direction measured on the map therefore needs an explicit grid-north or true-north reference. The diagram in the map margin shows their angular relationship for the stated reference point; a value for the sheet centre must not be transferred without checking to a distant location.

Magnetic north has another reference. A magnetic compass aligns with the local horizontal component of the Earth's magnetic field. Near the magnetic poles, that component becomes so weak that compass direction is unreliable in magnetic blackout zones; it can also be significantly affected in the surrounding caution zones. These regions do not coincide with the entire UPS area. Transferring a direction requires the relationship between magnetic, grid and true north appropriate to the location and time, together with a direction-finding instrument usable there. A correctly read UPS position alone does not determine which way the person is facing or moving.

Position uncertainty and time

Grid resolution describes the cell identified by the character string. Actual accuracy depends on how the position was established. Adding digits does not improve a location only approximately identified on a map. A GPS fix can also be affected by poor satellite geometry, blocked or reflected signals and interference. The person therefore records the position source and any known or estimated uncertainty. A receiver's accuracy indication relates to that fix and does not prove that the person is inside the small cell named by the reference.

The coordinate is accompanied by the day, time and time reference used. It is also identified as the current position, the last confirmed position or an intended destination. Someone who moves after taking a fix must not report the old fix as their present position. Recorded movement intentions and subsequent deviations help interpret the old reference even if contact cannot initially be established.

An unambiguous position report

When a report to friendly or recovery forces is actually possible, the agreed coordinate format is used. In polar MGRS, the polar designation, square letters and both numeric groups stay together. For full UPS metre values, the polar grid and easting-then-northing order are identified explicitly. Datum, position source, uncertainty and time supplement the location according to the applicable procedure. Someone with only latitude and longitude available identifies that representation, its precise angular format and its direction indicators; it is not reported as UPS. An adversary who intercepts an unprotected position report can use the same location reference; transmission therefore follows the applicable procedure for protecting position information.

If readback is possible, the comparison follows the format used: for polar MGRS, the polar designation, square letters and each numeric group are compared separately with the person's own record; for full UPS metre values, the north or south polar grid and the full easting and northing are compared. Any transmitted datum, time, position source and uncertainty are also checked. A different designation, a lost zero, reversed values or a different datum reference are resolved against the original map or receiver reference. Especially when reporting across unit or national boundaries, the recipient must not add a missing UTM zone or an assumed polar half. A fully understood reference enables further location verification; the person's identity is checked using the prescribed authentication procedures. Subsequent pickup depends on available forces, access, threats and the actual pickup procedures.