diff --git a/cable-delay-calc.xlsx b/cable-delay-calc.xlsx
new file mode 100644
index 0000000..778a112
Binary files /dev/null and b/cable-delay-calc.xlsx differ
diff --git a/notes-scratch-todo.txt b/notes-scratch-todo.txt
new file mode 100644
index 0000000..2e371d0
--- /dev/null
+++ b/notes-scratch-todo.txt
@@ -0,0 +1,45 @@
+rtc module
+    ds3231
+        product page: https://www.adafruit.com/product/3013
+        product tutorial: https://learn.adafruit.com/adafruit-ds3231-precision-rtc-breakout/overview
+        raspberry pi RTC tutorial: https://learn.adafruit.com/adding-a-real-time-clock-to-raspberry-pi/set-rtc-time
+        pinout: https://learn.adafruit.com/adafruit-ds3231-precision-rtc-breakout/pinouts
+        battery:  CR1220
+        chipset product page: https://www.analog.com/en/products/ds3231.html
+        chipset datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/DS3231.pdf
+        chipset reliability report: https://www.analog.com/media/en/technical-documentation/reliability-data/ds3231.pdf
+        notes:
+            https://serverfault.com/questions/1093101/chrony-hosting-time-with-rtc-as-authoritative-clock-on-an-air-gapped-isolated?__cf_chl_tk=G_sZX4ClltOv3fQJpH1q2a65tx4O2s_H_FObeBQ2UyU-1749831141-1.0.1.1-W0v5fl8cCrf0tIpB0pAqIb93L5MuoWeebiHm7iLGAlo
+            https://stackoverflow.com/questions/49730407/how-to-do-one-shot-time-sync-using-chrony
+    add bat
+    wire up
+    do oneshot sync
+    script set every x seconds
+        `sudo hwclock -r`
+change gps to use 5v if possible so rtc can use 3v
+
+cable delay
+    cable is 10ft? RG-174?
+        14.3
+    
+velocity factors of cable forms
+RG-58: VF ≈ 0.66 (66% of the speed of light)
+RG-174: VF ≈ 0.70
+LMR-200: VF ≈ 0.83
+LMR-400: VF ≈ 0.85
+
+formula
+    CD = time in nanoseconds of cable delay
+    L = Length of cable in UNIT
+    DF = delay factor in UNIT )speed of light in a vaccum used in nanoseconds per UNIT)
+            3.33 = ns (nanoseconds per meter)
+            1.016 = feet (feet per meter)
+    VF = velocity factor of cable (above)
+        CD ≈ (L*DF)/VF
+        CD ≈ (Length (meters) * 3.33 (baseline light speed in a vaccum ns/m)) / Velocity Factor of cable (VF)
+
+    derriving the speed of light offset
+        t(ns) = time in nanoseconds
+        c = speed of light in a vaccum in meters per second (3*(10^8) m/s)
+        d(m) = distance to travel in meters
+            t(ns) = d(m)/c 
\ No newline at end of file