diff --git a/src/georinex/nav3.py b/src/georinex/nav3.py
index d0758c3..45d5448 100644
--- a/src/georinex/nav3.py
+++ b/src/georinex/nav3.py
@@ -232,6 +232,8 @@ def _sparefields(cf: list[str], sys: str, N: int) -> list[str]:
     elif sys == "I":
         if N == 28:
             cf = cf[:28]
+        if N == 26: # Only one middle spare field
+            cf = cf[:22] + cf[23:26] + cf[27:28]
 
     if N != len(cf):
         raise ValueError(f"System {sys} NAV data is not the same length as the number of fields.")
diff --git a/src/georinex/tests/data/BRDC00IGS_R_20250570000_01D_MN.rnx.gz b/src/georinex/tests/data/BRDC00IGS_R_20250570000_01D_MN.rnx.gz
new file mode 100644
index 0000000..aec9442
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diff --git a/src/georinex/tests/data/BRDC00IGS_R_20250870000_01D_MN.rnx.gz b/src/georinex/tests/data/BRDC00IGS_R_20250870000_01D_MN.rnx.gz
new file mode 100644
index 0000000..c4f9827
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diff --git a/src/georinex/tests/test_nav3.py b/src/georinex/tests/test_nav3.py
index 13554a4..55d2016 100755
--- a/src/georinex/tests/test_nav3.py
+++ b/src/georinex/tests/test_nav3.py
@@ -48,6 +48,24 @@ def test_spare(fname):
     assert (G01["FitIntvl"] == approx([4.0, 4.0])).all()
 
 
+@pytest.mark.parametrize(
+    "fname",
+    [
+        "BRDC00IGS_R_20250870000_01D_MN.rnx.gz",
+        "BRDC00IGS_R_20250570000_01D_MN.rnx.gz",
+    ],
+)
+def test_mixed_igs(fname):
+    """
+    Test for mixed IGS files with multiple systems if all spare fields are detected
+    """
+    fn = ir.files(f"{__package__}.data") / fname
+    nav = gr.load(fn)
+
+    assert isinstance(nav, xarray.Dataset)
+    assert set(nav.svtype) == {"C", "E", "G", "R", "I", "S", "J"}
+
+
 @pytest.mark.parametrize("fname", ["ELKO00USA_R_20182100000_01D_MN.rnx.gz"])
 def test_mixed(fname):
     fn = ir.files(f"{__package__}.data") / fname