diff --git a/docs/cli_commands.md b/docs/cli_commands.md
index bc30adbd48c..864f3db6fe7 100644
--- a/docs/cli_commands.md
+++ b/docs/cli_commands.md
@@ -1078,7 +1078,7 @@ region save
 ---
 
 #### GPS anti-spoofing guard (When built with `GPS_GUARD`)
-With `GPS_GUARD`, a fix becomes the node's last fix (position, telemetry, adverts) only after it passes plausibility checks: at least `GPS_GUARD_MIN_SATS` satellites, HDOP ≤ `GPS_GUARD_MAX_HDOP10`/10, inside the optional build-time region (`GPS_GUARD_LAT_MIN/MAX`, `GPS_GUARD_LON_MIN/MAX`), within the fence around the node's configured `lat`/`lon` (if set), no jump faster than `GPS_GUARD_MAX_SPEED_KMH`, and `GPS_GUARD_STABLE_FIXES` consistent fixes in a row. An unverified position is never reported.
+With `GPS_GUARD`, a fix becomes the node's last fix (position, telemetry, adverts) only after it passes plausibility checks: a position on Earth (latitude within ±90°, longitude within ±180°), at least `GPS_GUARD_MIN_SATS` satellites, HDOP ≤ `GPS_GUARD_MAX_HDOP10`/10, inside the optional build-time region (`GPS_GUARD_LAT_MIN/MAX`, `GPS_GUARD_LON_MIN/MAX`), within the fence around the node's configured `lat`/`lon` (if set), no jump faster than `GPS_GUARD_MAX_SPEED_KMH`, and `GPS_GUARD_STABLE_FIXES` consistent fixes in a row. An unverified position is never reported.
 
 GPS time is written to the clock only while such a fix is current, never earlier than the firmware build date, and, once the clock holds a plausible time (from the build date to 10 years after it), only within `GPS_GUARD_TIME_MAX_STEP` seconds for the first sync and within clock drift (`GPS_GUARD_TIME_DRIFT_BASE` s + `GPS_GUARD_TIME_DRIFT_PPM`) afterwards. Setting the clock manually (`time`, `clock sync`) resets that anchor. A clock outside that window (e.g. a bogus date in 2080) is corrected by the first trusted fix.
 
diff --git a/src/helpers/sensors/GpsGuard.h b/src/helpers/sensors/GpsGuard.h
index 46b1936caae..7108939ca3b 100644
--- a/src/helpers/sensors/GpsGuard.h
+++ b/src/helpers/sensors/GpsGuard.h
@@ -144,7 +144,9 @@ class GpsGuard {
 
     const char* why = NULL;
     double dist = 0;
-    if (sats < GPS_GUARD_MIN_SATS) {
+    if (lat_e6 < -90000000L || lat_e6 > 90000000L || lon_e6 < -180000000L || lon_e6 > 180000000L) {
+      why = "coord";   // not a position on Earth (e.g. a garbled sentence that still passes its checksum)
+    } else if (sats < GPS_GUARD_MIN_SATS) {
       why = "sats";
     } else if (hdop10 == 255 || hdop10 > GPS_GUARD_MAX_HDOP10) {
       why = "hdop";
diff --git a/test/mocks/RTClib.h b/test/mocks/RTClib.h
new file mode 100644
index 00000000000..05805271838
--- /dev/null
+++ b/test/mocks/RTClib.h
@@ -0,0 +1,27 @@
+#pragma once
+
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+
+// Just enough of RTClib's DateTime for GpsGuard: the firmware build date as an epoch.
+class DateTime {
+  uint32_t _unix = 0;
+
+public:
+  // __DATE__ ("Mmm dd yyyy") and __TIME__ ("hh:mm:ss")
+  DateTime(const char* date, const char* time) {
+    static const char* months = "JanFebMarAprMayJunJulAugSepOctNovDec";
+    int mon = 1;
+    for (int i = 0; i < 12; ++i) if (strncmp(date, months + 3 * i, 3) == 0) mon = i + 1;
+    const int day = atoi(date + 4), year = atoi(date + 7);
+    // days from civil (Howard Hinnant)
+    const int y = year - (mon <= 2);
+    const int era = y / 400, yoe = y - era * 400;
+    const int doy = (153 * (mon + (mon > 2 ? -3 : 9)) + 2) / 5 + day - 1;
+    const int doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
+    const long days = (long)era * 146097 + doe - 719468;
+    _unix = (uint32_t)(days * 86400L + atoi(time) * 3600L + atoi(time + 3) * 60L + atoi(time + 6));
+  }
+  uint32_t unixtime() const { return _unix; }
+};
diff --git a/test/test_gps_guard/test_gps_guard_range.cpp b/test/test_gps_guard/test_gps_guard_range.cpp
new file mode 100644
index 00000000000..ae413f42701
--- /dev/null
+++ b/test/test_gps_guard/test_gps_guard_range.cpp
@@ -0,0 +1,44 @@
+#include <gtest/gtest.h>
+#include <helpers/sensors/GpsGuard.h>
+
+// A fix is judged on its position alone here: 8 satellites, HDOP 0.9, no reference
+// position, no earlier fix. Positions are in micro-degrees, as MicroNMEA reports them.
+static bool rejectedAsCoord(long lat_e6, long lon_e6) {
+  GpsGuard guard;
+  guard.acceptFix(lat_e6, lon_e6, 8, 9, 1800000000UL);
+  char status[64];
+  guard.formatStatus(status, sizeof(status));
+  return strstr(status, "rej coord") != nullptr;
+}
+
+TEST(GpsGuardRange, ordinaryPositionIsNotRejected) {
+  EXPECT_FALSE(rejectedAsCoord(47268723L, -122559463L));
+  EXPECT_FALSE(rejectedAsCoord(0L, 0L));
+}
+
+TEST(GpsGuardRange, limitsThemselvesAreAccepted) {
+  EXPECT_FALSE(rejectedAsCoord(90000000L, 180000000L));
+  EXPECT_FALSE(rejectedAsCoord(-90000000L, -180000000L));
+}
+
+TEST(GpsGuardRange, latitudeOutsideRangeIsRejected) {
+  EXPECT_TRUE(rejectedAsCoord(90000001L, 0L));
+  EXPECT_TRUE(rejectedAsCoord(-90000001L, 0L));
+  EXPECT_TRUE(rejectedAsCoord(100666665L, 181650000L));   // what a garbled "9999.9999,N" reports
+}
+
+TEST(GpsGuardRange, longitudeOutsideRangeIsRejected) {
+  EXPECT_TRUE(rejectedAsCoord(0L, 180000001L));
+  EXPECT_TRUE(rejectedAsCoord(0L, -180000001L));
+}
+
+TEST(GpsGuardRange, guardOffAcceptsEverything) {
+  GpsGuard guard;
+  guard.setEnabled(false);
+  EXPECT_TRUE(guard.acceptFix(100666665L, 181650000L, 8, 9, 1800000000UL));
+}
+
+int main(int argc, char** argv) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}