diff --git a/process.py b/process.py
index a7ae779..5599700 100755
--- a/process.py
+++ b/process.py
@@ -21,6 +21,7 @@
 from stvid.fourframe import FourFrame
 from stvid.fourframe import Observation
 from stvid.fourframe import AstrometricCatalog
+from stvid.fourframe import format_satno
 
 from astropy.utils.exceptions import AstropyWarning
 
@@ -143,8 +144,11 @@ def process_loop(args):
                       "catalogname": ident.catalogname}
 
         # Save track
-        t.save(f"{ff.froot}_{ident.satno:05d}_{ident.catalogname}.csv", ff)
-
+        t.save(
+            f"{ff.froot}_{format_satno(ident.satno)}_"
+            f"{ident.catalogname}.csv",
+            ff,
+        )
         # Measure single position
         m = t.measure_single_position(ff)
         iod_line = m.to_iod_line(ff, ident)
@@ -185,10 +189,16 @@ def process_loop(args):
     screenoutput_idents = []
     for o in obs:
         # Open file
-        outfname = f"{ff.froot}_{o.satno:05d}_{o.catalogname}.dat"
+        outfname = (
+            f"{ff.froot}_{format_satno(o.satno)}_"
+            f"{o.catalogname}.dat"
+        )
         with open(outfname, "w") as fp:
             fp.write(f"{o.iod_line}\n")
-        outfname = f"{ff.froot}_{o.satno:05d}_{o.catalogname}_m.dat"
+        outfname = (
+            f"{ff.froot}_{format_satno(o.satno)}_"
+            f"{o.catalogname}_m.dat"
+        )
         with open(outfname, "w") as fp:
             for iod_line in o.iod_lines:
                 fp.write(f"{iod_line}\n")
diff --git a/stvid/fourframe.py b/stvid/fourframe.py
index 693654b..3c4516f 100644
--- a/stvid/fourframe.py
+++ b/stvid/fourframe.py
@@ -134,8 +134,8 @@ def to_iod_line(self, ff, ident):
         tstr = (
             nfd.replace("-", "").replace("T", "").replace(":", "").replace(".", "")
         )
-        iod_line = "%05d %-9s %04d G %s 17 25 %s 37 S" % (
-            ident.satno,
+        iod_line = "%s %-9s %04d G %s 17 25 %s 37 S" % (
+            format_satno(ident.satno),
             ident.cospar,
             ff.site_id,
             tstr,
@@ -749,7 +749,7 @@ def diagnostic_plot(self, predictions, track, obs, cfg):
             iod_line = obs.iod_line
             satno = obs.satno
             catalogname = obs.catalogname
-            outfname = f"{self.froot}_{satno:05d}_{catalogname}.png"
+            outfname = f"{self.froot}_{format_satno(satno)}_{catalogname}.png"
         else:
             iod_line = ""
             outfname = f"{self.froot}_0.png"
@@ -807,7 +807,7 @@ def diagnostic_plot(self, predictions, track, obs, cfg):
             ax.text(
                 track.x0,
                 track.y0,
-                f" {satno:05d}",
+                f" {format_satno(satno)}",
                 color=color_detected,
                 ha="center",
                 in_layout=False,
@@ -890,7 +890,7 @@ def plot_prediction(self, p, ax, tlefiles, colors, dt=2.0, w=10.0):
 
         if self.in_frame(xs[0], ys[0]):
             ax.text(
-                xs[0], ys[0], f" {p.satno:05d} ", color=color, ha=ha, in_layout=False
+                xs[0], ys[0], f" {format_satno(p.satno)} ", color=color, ha=ha, in_layout=False
             )
 
         for state, linestyle in zip(
@@ -975,6 +975,43 @@ def decode_line(line):
     return ax, ay, az, bx, by, bz, n
 
 
+# TLE/IOD Alpha-5 satellite-number alphabet.
+# Positions 10 through 33 correspond to A-Z, omitting I and O.
+_ALPHA5 = "0123456789ABCDEFGHJKLMNPQRSTUVWXYZ"
+
+
+def format_satno(satno):
+    """
+    Format a numeric NORAD catalogue number as a five-character field.
+
+    Examples:
+        25544  -> "25544"
+        100000 -> "A0000"
+        102554 -> "A2554"
+        270438 -> "T0438"
+
+    Satellite numbers remain integers internally. Alpha-5 is only used
+    when writing a fixed-width textual field.
+    """
+    satno = int(satno)
+
+    if satno < 0:
+        raise ValueError(f"Invalid satellite number: {satno}")
+
+    if satno < 100000:
+        return f"{satno:05d}"
+
+    prefix = satno // 10000
+    suffix = satno % 10000
+
+    if prefix >= len(_ALPHA5):
+        raise ValueError(
+            f"Satellite number {satno} is outside the Alpha-5 range"
+        )
+
+    return f"{_ALPHA5[prefix]}{suffix:04d}"
+
+
 # IOD position format 2: RA/DEC = HHMMmmm+DDMMmm MX   (MX in minutes of arc)
 def format_position(ra, de):
     # Right ascension