Photon ASCII files (.pha) have a header that describes their content (see 
example below).  For grids created in the SeisWare software product, 
"MinX" and "MinY" define the lower-left corner of the grid in map units 
(usually meters), whereas "IncrementX" and "IncrementY" define the grid-cell 
size (also meters).  "Points [ N ]" is the total number of points included 
(nulls are typically left out of the file for compactness). Grid values are 
given in meters.

To convert to XY, add the product of Row and IncrementX to MinX to get X, 
and add the product of Column and IncrementY to MinY to get Y.

For the project used in this work (Putzig et al. 2009), the projection used 
is polar stereographic, centered on the north pole, and defined by an ellipsoid 
with a semi-major axis of 3396000 m and a semi-minor axis of 3376000 m.

Example of a Photon ASCII header:

#<SeisWare Grid Export - Photon ASCII Compatible>

struct Grid {
    Name
    MinX IncrementX
    MinY IncrementY
    RMSError NullValue
    Rows Columns
    Points[ N ] {
            Value Row Column
    }
}

Grid "C thickness (Putzig et al. 2009)"
-904734.710275 2688.181794
-902590.799342 2689.813666
0.000000 1.701410e+038
672 674
108211
3.000489 477 316
3.001465 215 505
...

Reference:
Putzig, N. E., Phillips, R. J., Campbell, B. A., Holt, J. W., Plaut, J. J., 
Carter, L. M., Egan, A. F., Bernardini, F., Safaeinili, A., Seu, R., 2009. 
Subsurface structure of Planum Boreum from Mars Reconnaissance Orbiter Shallow 
Radar soundings. Icarus 204, 443-457, doi: 10.1016/j.icarus.2009.07.034.