diff --git a/core/domain/src/main/java/com/rtbishop/look4sat/core/domain/wavelog/GridEntry.kt b/core/domain/src/main/java/com/rtbishop/look4sat/core/domain/wavelog/GridEntry.kt
new file mode 100644
index 00000000..a16b6853
--- /dev/null
+++ b/core/domain/src/main/java/com/rtbishop/look4sat/core/domain/wavelog/GridEntry.kt
@@ -0,0 +1,115 @@
+/*
+ * Look4Sat. Amateur radio satellite tracker and pass predictor.
+ * Copyright (C) 2019-2026 Arty Bishop and contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.rtbishop.look4sat.core.domain.wavelog
+
+/**
+ * Checks a typed counterpart grid before it reaches the log.
+ *
+ * Separate from qthToPosition's validator, which requires six characters and is private. Plenty of
+ * satellite operators exchange only the four-character square, so requiring six would reject
+ * perfectly good entries - and this follows the same rule the callsign field settled on: refuse only
+ * what is certainly wrong, warn about the rest, never silently discard.
+ *
+ * The reason to check at all is that an unchecked value goes into the ADIF GRIDSQUARE field, and a
+ * malformed one is stored by Wavelog as-is. It then pollutes grid statistics and VUCC award
+ * tracking, where a wrong square is worse than a missing one.
+ */
+object GridEntry {
+
+    /** What a typed grid amounts to. */
+    sealed interface Verdict {
+
+        /** Usable. [normalised] is what should be logged - upper case for the pair, lower for the subsquare. */
+        data class Acceptable(val normalised: String, val warning: Warning? = null) : Verdict
+
+        /** Certainly not a grid. [reason] says which rule it broke. */
+        data class Unusable(val reason: Reason) : Verdict
+
+        /** Nothing typed. The QRZ lookup should run instead. */
+        data object Empty : Verdict
+    }
+
+    /** Worth mentioning but not worth refusing. */
+    enum class Warning {
+        /** Four characters, so the location is only accurate to about 100km. */
+        SQUARE_ONLY
+    }
+
+    /** Why an entry cannot be a grid. */
+    enum class Reason {
+        /** Not 4, 6 or 8 characters. Maidenhead has no other lengths. */
+        WRONG_LENGTH,
+
+        /** First pair outside A-R. S-X would decode past the poles. */
+        FIELD_OUT_OF_RANGE,
+
+        /** Second pair is not two digits. */
+        SQUARE_NOT_DIGITS,
+
+        /** Third pair outside A-X. */
+        SUBSQUARE_OUT_OF_RANGE
+    }
+
+    /**
+     * Judge a typed entry.
+     *
+     * Case is normalised on the way out rather than demanded on the way in - an operator typing
+     * one-handed outdoors should not have to care, and the conventional rendering is upper case for
+     * the field, digits, then lower case for the subsquare.
+     */
+    fun check(entry: String): Verdict {
+        val text = entry.trim()
+        if (text.isEmpty()) return Verdict.Empty
+        if (text.length !in VALID_LENGTHS) return Verdict.Unusable(Reason.WRONG_LENGTH)
+
+        val upper = text.uppercase()
+        if (upper[0] !in FIELD_RANGE || upper[1] !in FIELD_RANGE) {
+            return Verdict.Unusable(Reason.FIELD_OUT_OF_RANGE)
+        }
+        if (!upper[2].isDigit() || !upper[3].isDigit()) {
+            return Verdict.Unusable(Reason.SQUARE_NOT_DIGITS)
+        }
+        if (text.length >= SUBSQUARE_LENGTH) {
+            if (upper[4] !in SUBSQUARE_RANGE || upper[5] !in SUBSQUARE_RANGE) {
+                return Verdict.Unusable(Reason.SUBSQUARE_OUT_OF_RANGE)
+            }
+        }
+        if (text.length == EXTENDED_LENGTH && (!upper[6].isDigit() || !upper[7].isDigit())) {
+            return Verdict.Unusable(Reason.SQUARE_NOT_DIGITS)
+        }
+
+        return Verdict.Acceptable(
+            normalised = normalise(upper),
+            warning = if (text.length == SQUARE_LENGTH) Warning.SQUARE_ONLY else null
+        )
+    }
+
+    /** `OL72ap` - upper case field, digits, lower case subsquare, as the convention renders it. */
+    private fun normalise(upper: String): String = buildString {
+        append(upper.take(SQUARE_LENGTH))
+        if (upper.length >= SUBSQUARE_LENGTH) append(upper.substring(4, 6).lowercase())
+        if (upper.length == EXTENDED_LENGTH) append(upper.substring(6, 8))
+    }
+
+    private const val SQUARE_LENGTH = 4
+    private const val SUBSQUARE_LENGTH = 6
+    private const val EXTENDED_LENGTH = 8
+    private val VALID_LENGTHS = setOf(SQUARE_LENGTH, SUBSQUARE_LENGTH, EXTENDED_LENGTH)
+    private val FIELD_RANGE = 'A'..'R'
+    private val SUBSQUARE_RANGE = 'A'..'X'
+}
diff --git a/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/wavelog/GridEntryTest.kt b/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/wavelog/GridEntryTest.kt
new file mode 100644
index 00000000..96f1530e
--- /dev/null
+++ b/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/wavelog/GridEntryTest.kt
@@ -0,0 +1,167 @@
+package com.rtbishop.look4sat.core.domain.wavelog
+
+import com.rtbishop.look4sat.core.domain.utility.qthToPosition
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * A typed grid goes into the ADIF GRIDSQUARE field and Wavelog stores whatever arrives, where a
+ * wrong square is worse than a missing one - it pollutes grid statistics and VUCC tracking.
+ *
+ * The rule follows the callsign field: refuse only what is certainly wrong. Four characters is a
+ * legitimate exchange on satellites, so requiring six would reject good entries.
+ */
+class GridEntryTest {
+
+    /** Real grids, from stations and from this project's own test data. */
+    private val realGrids = listOf(
+        "OL72AP",   // Shenzhen, the user's own
+        "FN31pr",   // W1AW, mixed case as conventionally written
+        "JO31",     // four-character exchange
+        "PM95uq",
+        "IO91vl39", // eight-character extended
+        "gf15vc",   // all lower case
+        "RR73"      // the highest legal field pair
+    )
+
+    @Test
+    fun `real grids are accepted`() {
+        for (grid in realGrids) {
+            val verdict = GridEntry.check(grid)
+            assertTrue("$grid must be usable, got $verdict", verdict is GridEntry.Verdict.Acceptable)
+        }
+    }
+
+    /**
+     * Cross-check against the app's own converter: anything this accepts at six characters or more
+     * must also decode to a real position, or the two disagree about what a grid is.
+     */
+    @Test
+    fun `accepted grids of six or more decode to a position`() {
+        for (grid in realGrids.filter { it.length >= 6 }) {
+            val verdict = GridEntry.check(grid)
+            assertTrue(verdict is GridEntry.Verdict.Acceptable)
+            assertNotNull(
+                "$grid was accepted here but qthToPosition rejects it",
+                qthToPosition((verdict as GridEntry.Verdict.Acceptable).normalised)
+            )
+        }
+    }
+
+    /** Four characters is a real exchange, flagged rather than refused. */
+    @Test
+    fun `a four character square is acceptable with a warning`() {
+        val verdict = GridEntry.check("JO31")
+        assertEquals(
+            GridEntry.Verdict.Acceptable("JO31", GridEntry.Warning.SQUARE_ONLY),
+            verdict
+        )
+    }
+
+    @Test
+    fun `six characters carry no warning`() {
+        val verdict = GridEntry.check("OL72AP") as GridEntry.Verdict.Acceptable
+        assertNull(verdict.warning)
+    }
+
+    /** Upper field, lower subsquare - the conventional rendering, whatever was typed. */
+    @Test
+    fun `case is normalised rather than demanded`() {
+        assertEquals("OL72ap", (GridEntry.check("ol72ap") as GridEntry.Verdict.Acceptable).normalised)
+        assertEquals("OL72ap", (GridEntry.check("OL72AP") as GridEntry.Verdict.Acceptable).normalised)
+        assertEquals("OL72ap", (GridEntry.check("oL72Ap") as GridEntry.Verdict.Acceptable).normalised)
+    }
+
+    @Test
+    fun `surrounding whitespace does not matter`() {
+        assertEquals("OL72ap", (GridEntry.check("  OL72AP  ") as GridEntry.Verdict.Acceptable).normalised)
+    }
+
+    /** Nothing typed means the QRZ lookup should run, which is a different outcome from bad input. */
+    @Test
+    fun `an empty entry is neither acceptable nor unusable`() {
+        assertEquals(GridEntry.Verdict.Empty, GridEntry.check(""))
+        assertEquals(GridEntry.Verdict.Empty, GridEntry.check("   "))
+    }
+
+    /** Maidenhead has no odd lengths and no two-character form in this context. */
+    @Test
+    fun `only 4 6 or 8 characters can be a grid`() {
+        for (bad in listOf("OL", "OL7", "OL72A", "OL72APX", "OL72AP123")) {
+            val verdict = GridEntry.check(bad)
+            assertEquals(
+                "$bad must be refused for length, got $verdict",
+                GridEntry.Verdict.Unusable(GridEntry.Reason.WRONG_LENGTH),
+                verdict
+            )
+        }
+    }
+
+    /**
+     * The first pair runs A-R only. S-X decodes past the poles, which is how a plausible-looking
+     * entry produces a position that cannot exist.
+     */
+    @Test
+    fun `a field pair past R is refused`() {
+        for (bad in listOf("SS12AA", "ZZ99ZZ", "TT34bb")) {
+            assertEquals(
+                "$bad decodes outside the world",
+                GridEntry.Verdict.Unusable(GridEntry.Reason.FIELD_OUT_OF_RANGE),
+                GridEntry.check(bad)
+            )
+        }
+    }
+
+    @Test
+    fun `the square pair must be digits`() {
+        for (bad in listOf("OLAAAP", "OL7AAP", "ABCDEF")) {
+            assertEquals(
+                "$bad has no square digits",
+                GridEntry.Verdict.Unusable(GridEntry.Reason.SQUARE_NOT_DIGITS),
+                GridEntry.check(bad)
+            )
+        }
+    }
+
+    /** The subsquare runs A-X. Y and Z do not exist. */
+    @Test
+    fun `a subsquare past X is refused`() {
+        assertEquals(
+            GridEntry.Verdict.Unusable(GridEntry.Reason.SUBSQUARE_OUT_OF_RANGE),
+            GridEntry.check("OL72YZ")
+        )
+    }
+
+    /** A callsign is the most likely wrong thing to be typed into this field. */
+    @Test
+    fun `a callsign is not a grid`() {
+        for (call in listOf("BG7NTA", "W1AW", "JA1ABC", "DL1ABC")) {
+            val verdict = GridEntry.check(call)
+            assertTrue(
+                "$call must not read as a grid, got $verdict",
+                verdict is GridEntry.Verdict.Unusable
+            )
+        }
+    }
+
+    /** Digits alone are a frequency or a report, not a grid. */
+    @Test
+    fun `numbers alone are not a grid`() {
+        for (bad in listOf("123456", "5959", "14550")) {
+            assertTrue(GridEntry.check(bad) is GridEntry.Verdict.Unusable)
+        }
+    }
+
+    /** The extended form ends in two digits. */
+    @Test
+    fun `an eight character grid needs digits at the end`() {
+        assertTrue(GridEntry.check("IO91vl39") is GridEntry.Verdict.Acceptable)
+        assertEquals(
+            GridEntry.Verdict.Unusable(GridEntry.Reason.SQUARE_NOT_DIGITS),
+            GridEntry.check("IO91vlab")
+        )
+    }
+}
diff --git a/core/presentation/src/main/res/values-zh/strings.xml b/core/presentation/src/main/res/values-zh/strings.xml
index 8359625f..5d313408 100644
--- a/core/presentation/src/main/res/values-zh/strings.xml
+++ b/core/presentation/src/main/res/values-zh/strings.xml
@@ -278,6 +278,8 @@
     <string name="wavelog_call_hint">对方呼号</string>
     <string name="wavelog_grid_hint">网格 (可选)</string>
     <string name="wavelog_grid_help">对方报给你的网格。留空则从 QRZ 查询。</string>
+    <string name="wavelog_grid_bad">不是有效网格</string>
+    <string name="wavelog_grid_square_only">仅方格 - 精度约 100 公里</string>
     <string name="wavelog_mode_hint">模式</string>
     <string name="wavelog_mode_edit">修改模式</string>
     <string name="wavelog_time_hint">时间</string>
diff --git a/core/presentation/src/main/res/values/strings.xml b/core/presentation/src/main/res/values/strings.xml
index b0358b64..7272880f 100644
--- a/core/presentation/src/main/res/values/strings.xml
+++ b/core/presentation/src/main/res/values/strings.xml
@@ -309,6 +309,8 @@
     <string name="wavelog_call_hint">Callsign</string>
     <string name="wavelog_grid_hint">Grid (optional)</string>
     <string name="wavelog_grid_help">What they sent you. Left empty, it is looked up on QRZ.</string>
+    <string name="wavelog_grid_bad">Not a grid square</string>
+    <string name="wavelog_grid_square_only">Square only - accurate to about 100 km</string>
     <string name="wavelog_mode_hint">Mode</string>
     <string name="wavelog_mode_edit">Edit the mode</string>
     <string name="wavelog_time_hint">Time</string>
diff --git a/feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/LogTab.kt b/feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/LogTab.kt
index 30e01720..1173aeb4 100644
--- a/feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/LogTab.kt
+++ b/feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/LogTab.kt
@@ -81,6 +81,7 @@ import com.rtbishop.look4sat.core.domain.model.SatRadio
 import com.rtbishop.look4sat.core.domain.predict.OrbitalPos
 import com.rtbishop.look4sat.core.domain.utility.DopplerFrequencyCalculator
 import com.rtbishop.look4sat.core.domain.qrz.QrzGrid
+import com.rtbishop.look4sat.core.domain.wavelog.GridEntry
 import com.rtbishop.look4sat.core.domain.wavelog.PassClock
 import com.rtbishop.look4sat.core.domain.wavelog.CallsignEntry
 import com.rtbishop.look4sat.core.domain.wavelog.WavelogQso
@@ -337,7 +338,9 @@ private fun ExpandedLogInput(
         val tx = radio.uplinkLow ?: radio.downlinkLow ?: 0L
         val rx = radio.downlinkLow ?: radio.uplinkLow ?: 0L
         val qsoId = UUID.randomUUID().toString()
-        val typedGrid = gridEntry.trim().uppercase()
+        // Only a usable grid is logged. A malformed one is dropped rather than stored, and the
+        // field has been saying so while it was typed.
+        val typedGrid = (GridEntry.check(gridEntry) as? GridEntry.Verdict.Acceptable)?.normalised ?: ""
         queue.add(
             WavelogQso(
                 id = qsoId,
@@ -455,16 +458,33 @@ private fun ExpandedLogInput(
         // could previously only arrive by scraping QRZ - which needs a cookie, and returns nothing
         // for a station with no locator on file. Typed here it also skips the lookup entirely,
         // because what the operator heard beats what a web page says.
+        val gridVerdict = GridEntry.check(gridEntry)
         OutlinedTextField(
             value = gridEntry,
-            onValueChange = { gridEntry = it.take(6).uppercase() },
+            // 8 characters, not 6: the extended form exists and truncating it would silently
+            // change the location. Case is normalised on commit rather than while typing, so the
+            // cursor does not jump under the operator's thumb.
+            onValueChange = { gridEntry = it.take(8) },
             label = { Text(stringResource(id = R.string.wavelog_grid_hint)) },
             supportingText = {
                 Text(
-                    text = stringResource(id = R.string.wavelog_grid_help),
+                    // Says what is wrong while there is still time to fix it. A malformed grid is
+                    // stored by Wavelog as-is and then pollutes grid statistics and VUCC tracking,
+                    // where a wrong square is worse than a missing one.
+                    text = when (gridVerdict) {
+                        is GridEntry.Verdict.Unusable -> stringResource(id = R.string.wavelog_grid_bad)
+                        is GridEntry.Verdict.Acceptable ->
+                            if (gridVerdict.warning == GridEntry.Warning.SQUARE_ONLY) {
+                                stringResource(id = R.string.wavelog_grid_square_only)
+                            } else {
+                                stringResource(id = R.string.wavelog_grid_help)
+                            }
+                        GridEntry.Verdict.Empty -> stringResource(id = R.string.wavelog_grid_help)
+                    },
                     style = MaterialTheme.typography.bodySmall
                 )
             },
+            isError = gridVerdict is GridEntry.Verdict.Unusable,
             singleLine = true,
             keyboardOptions = KeyboardOptions(imeAction = ImeAction.Done),
             keyboardActions = KeyboardActions(onDone = { submit() }),