diff --git a/README.md b/README.md
index 0d6ecb1..4b265c0 100644
--- a/README.md
+++ b/README.md
@@ -64,6 +64,10 @@ The harness writes both simulations to temporary directories and compares the
 logical ROOT contents exactly. Set `MUSRSIM_ROOT_COMPARATOR` when the comparator
 is not available under `tests/` beside the candidate executable.
 
+The same differential comparison is available for the GPS Mark-II setup:
+
+    tests/compare_gps_versions.sh /path/to/baseline/musrSim /path/to/candidate/musrSim 500
+
 ### Running the simulations ###
 
 Prepare a file, `filename.mac`, with your beamline and spectrometer design following the instructions in the [musrSim manual](https://bitbucket.org/muonspin/musrsim/src/master/doc/musrSim.pdf). Create a folder `data` and then run the simulation 
diff --git a/tests/compare_gps_versions.sh b/tests/compare_gps_versions.sh
new file mode 100755
index 0000000..9b3f027
--- /dev/null
+++ b/tests/compare_gps_versions.sh
@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+script_directory=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+source_directory=$(cd "$script_directory/.." && pwd)
+
+exec "$script_directory/compare_musr_versions.sh" \
+  "$source_directory/run/GPS/template_Mark-II.mac" \
+  95000 \
+  500 \
+  "$@"
diff --git a/tests/compare_lem_versions.sh b/tests/compare_lem_versions.sh
index 3528d87..a526415 100755
--- a/tests/compare_lem_versions.sh
+++ b/tests/compare_lem_versions.sh
@@ -2,94 +2,11 @@
 
 set -euo pipefail
 
-usage() {
-  echo "usage: $0 <baseline-musrSim> <candidate-musrSim> [events]" >&2
-  echo "set MUSRSIM_ROOT_COMPARATOR if the comparator is not beside the candidate build" >&2
-}
-
-if [[ $# -lt 2 || $# -gt 3 ]]; then
-  usage
-  exit 2
-fi
-
-baseline_binary=$(realpath "$1")
-candidate_binary=$(realpath "$2")
-events=${3:-500}
-
-if [[ ! -x "$baseline_binary" || ! -x "$candidate_binary" ]]; then
-  echo "both musrSim paths must be executable" >&2
-  exit 2
-fi
-if [[ ! "$events" =~ ^[1-9][0-9]*$ ]]; then
-  echo "event count must be a positive integer" >&2
-  exit 2
-fi
-
 script_directory=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 source_directory=$(cd "$script_directory/.." && pwd)
-lem_directory="$source_directory/run/LEM"
-source_macro="$lem_directory/4000_15kV_ModUniform_SR-10_ModRot0deg.mac"
 
-if [[ -n ${MUSRSIM_ROOT_COMPARATOR:-} ]]; then
-  comparator=$(realpath "$MUSRSIM_ROOT_COMPARATOR")
-else
-  candidate_directory=$(dirname "$candidate_binary")
-  comparator="$candidate_directory/tests/musrRootOutputCompare"
-fi
-if [[ ! -x "$comparator" ]]; then
-  echo "ROOT comparator not found at $comparator" >&2
-  echo "build the musrRootOutputCompare target or set MUSRSIM_ROOT_COMPARATOR" >&2
-  exit 2
-fi
-
-work_directory=$(mktemp -d "${TMPDIR:-/tmp}/musrsim-lem-compare.XXXXXX")
-trap 'rm -rf "$work_directory"' EXIT
-mkdir -p "$work_directory/baseline" "$work_directory/candidate"
-
-make_macro() {
-  local output_directory=$1
-  local output_macro=$2
-  awk -v events="$events" -v output_directory="$output_directory" '
-    /^[[:space:]]*\/run\/beamOn[[:space:]]+/ {
-      ++beam_on_count
-      print "/musr/command rootOutputDirectoryName " output_directory
-      print "/musr/run/runID 94000"
-      print "/run/beamOn " events
-      next
-    }
-    { print }
-    END {
-      if (beam_on_count != 1) {
-        print "expected exactly one active /run/beamOn command" > "/dev/stderr"
-        exit 2
-      }
-    }
-  ' "$source_macro" > "$output_macro"
-}
-
-run_version() {
-  local label=$1
-  local binary=$2
-  local output_directory="$work_directory/$label"
-  local macro="$work_directory/$label.mac"
-  local log="$work_directory/$label.log"
-
-  make_macro "$output_directory" "$macro"
-  if ! (cd "$lem_directory" && "$binary" "$macro") >"$log" 2>&1; then
-    echo "$label simulation failed; final log lines:" >&2
-    tail -40 "$log" >&2
-    exit 1
-  fi
-
-  local root_file="$output_directory/musr_94000.root"
-  if [[ ! -f "$root_file" ]]; then
-    echo "$label simulation did not create $root_file" >&2
-    exit 1
-  fi
-  echo "$root_file"
-}
-
-baseline_output=$(run_version baseline "$baseline_binary")
-candidate_output=$(run_version candidate "$candidate_binary")
-
-"$comparator" "$baseline_output" "$candidate_output"
+exec "$script_directory/compare_musr_versions.sh" \
+  "$source_directory/run/LEM/4000_15kV_ModUniform_SR-10_ModRot0deg.mac" \
+  94000 \
+  500 \
+  "$@"
diff --git a/tests/compare_musr_versions.sh b/tests/compare_musr_versions.sh
new file mode 100755
index 0000000..4dbd0ec
--- /dev/null
+++ b/tests/compare_musr_versions.sh
@@ -0,0 +1,104 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+usage() {
+  echo "usage: ${MUSRSIM_COMPARE_COMMAND:-$0} <baseline-musrSim> <candidate-musrSim> [events]" >&2
+  echo "set MUSRSIM_ROOT_COMPARATOR if the comparator is not beside the candidate build" >&2
+}
+
+if [[ $# -lt 5 || $# -gt 6 ]]; then
+  usage
+  exit 2
+fi
+
+source_macro=$(realpath "$1")
+run_id=$2
+default_events=$3
+baseline_binary=$(realpath "$4")
+candidate_binary=$(realpath "$5")
+events=${6:-$default_events}
+
+if [[ ! -f "$source_macro" ]]; then
+  echo "source macro not found: $source_macro" >&2
+  exit 2
+fi
+if [[ ! -x "$baseline_binary" || ! -x "$candidate_binary" ]]; then
+  echo "both musrSim paths must be executable" >&2
+  exit 2
+fi
+if [[ ! "$run_id" =~ ^[1-9][0-9]*$ ]]; then
+  echo "run ID must be a positive integer" >&2
+  exit 2
+fi
+if [[ ! "$events" =~ ^[1-9][0-9]*$ ]]; then
+  echo "event count must be a positive integer" >&2
+  exit 2
+fi
+
+macro_directory=$(dirname "$source_macro")
+
+if [[ -n ${MUSRSIM_ROOT_COMPARATOR:-} ]]; then
+  comparator=$(realpath "$MUSRSIM_ROOT_COMPARATOR")
+else
+  candidate_directory=$(dirname "$candidate_binary")
+  comparator="$candidate_directory/tests/musrRootOutputCompare"
+fi
+if [[ ! -x "$comparator" ]]; then
+  echo "ROOT comparator not found at $comparator" >&2
+  echo "build the musrRootOutputCompare target or set MUSRSIM_ROOT_COMPARATOR" >&2
+  exit 2
+fi
+
+macro_name=$(basename "$source_macro" .mac)
+work_directory=$(mktemp -d "${TMPDIR:-/tmp}/musrsim-${macro_name}-compare.XXXXXX")
+trap 'rm -rf "$work_directory"' EXIT
+mkdir -p "$work_directory/baseline" "$work_directory/candidate"
+
+make_macro() {
+  local output_directory=$1
+  local output_macro=$2
+  awk -v events="$events" -v output_directory="$output_directory" -v run_id="$run_id" '
+    /^[[:space:]]*\/run\/beamOn[[:space:]]+/ {
+      ++beam_on_count
+      print "/musr/command rootOutputDirectoryName " output_directory
+      print "/musr/run/runID " run_id
+      print "/run/beamOn " events
+      next
+    }
+    { print }
+    END {
+      if (beam_on_count != 1) {
+        print "expected exactly one active /run/beamOn command" > "/dev/stderr"
+        exit 2
+      }
+    }
+  ' "$source_macro" > "$output_macro"
+}
+
+run_version() {
+  local label=$1
+  local binary=$2
+  local output_directory="$work_directory/$label"
+  local macro="$work_directory/$label.mac"
+  local log="$work_directory/$label.log"
+
+  make_macro "$output_directory" "$macro"
+  if ! (cd "$macro_directory" && "$binary" "$macro") >"$log" 2>&1; then
+    echo "$label simulation failed; final log lines:" >&2
+    tail -40 "$log" >&2
+    exit 1
+  fi
+
+  local root_file="$output_directory/musr_${run_id}.root"
+  if [[ ! -f "$root_file" ]]; then
+    echo "$label simulation did not create $root_file" >&2
+    exit 1
+  fi
+  echo "$root_file"
+}
+
+baseline_output=$(run_version baseline "$baseline_binary")
+candidate_output=$(run_version candidate "$candidate_binary")
+
+"$comparator" "$baseline_output" "$candidate_output"