diff --git a/libraries/AP_NavEKF3/derivation/generate_1.py b/libraries/AP_NavEKF3/derivation/generate_1.py
index 4d54105604..882d1524fe 100755
--- a/libraries/AP_NavEKF3/derivation/generate_1.py
+++ b/libraries/AP_NavEKF3/derivation/generate_1.py
@@ -1,10 +1,16 @@
 #!/usr/bin/env python3
 # Copied from https://github.com/PX4/ecl/commit/264c8c4e8681704e4719d0a03b848df8617c0863
 # and modified for ArduPilot
+from sympy import __version__ as __sympy__version__
 from sympy import *
 from code_gen import *
 import numpy as np
 
+# version required to generate the exact code currently present in ArduPilot.
+# sympy version upgrades must ensure generated code doesn't pose any problems
+# and must not have any other changes to the generator.
+assert __sympy__version__ == "1.9", "expected sympy version 1.9, not "+__sympy__version__
+
 # q: quaternion describing rotation from frame 1 to frame 2
 # returns a rotation matrix derived form q which describes the same
 # rotation
diff --git a/libraries/AP_NavEKF3/derivation/generate_nix b/libraries/AP_NavEKF3/derivation/generate_nix
new file mode 100755
index 0000000000..cbb5e6ecb2
--- /dev/null
+++ b/libraries/AP_NavEKF3/derivation/generate_nix
@@ -0,0 +1,13 @@
+#!/usr/bin/env nix-shell
+#! nix-shell --pure -i bash -p "python3.withPackages (p: [ p.numpy p.sympy ])"
+#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/eabc38219184cc3e04a974fe31857d8e0eac098d.tar.gz
+
+# above pins Python 3.9.13, Numpy 1.21.2, and Sympy 1.9 (and deps)
+# using the last nixos-21.11 branch commit
+
+cd "$(dirname "$0")"
+rm -rf generated # ensure generated directory exists and is empty
+mkdir -p generated
+
+# explicitly invoke python3 to use interpreter from nix-shell
+python3 ./generate_1.py