Training - Communications: SARNEG

A SARNEG (Search and Rescue Number Encryption Grid) is a simple, low-tech encryption method used in radio communications, primarily in military or search and rescue (SAR) operations, to securely transmit sensitive information, such as the location of a downed pilot or crew, over non-secure radio channels.

It uses a pre-shared 10-letter code word with no repeating letters to encode numerical coordinates (e.g., grid references or latitude/longitude) into a format that appears nonsensical to unauthorized listeners.

Description of SARNEG

• Purpose: SARNEG obscures numerical data (e.g., coordinates) by converting digits into letters based on a daily or mission-specific code word, making it difficult for adversaries to interpret without knowing the code word.
• Structure: The code word is a 10-letter sequence (e.g., "CRAZYHORSE") where each letter corresponds to a digit from 0 to 9. For example:
  • C = 0, R = 1, A = 2, Z = 3, Y = 4, H = 5, O = 6, R = 7, S = 8, E = 9
  • Note: If a letter repeats in the code word, it’s invalid, as each digit must map to a unique letter.
• Transmission: Coordinates are broken into groups of digits, each digit is replaced with its corresponding letter, and the resulting letter groups are transmitted. The recipient, knowing the code word, reverses the process to decode the numbers.

Example

Using the code word CRAZYHORSE:

• Assign digits: C=0, R=1, A=2, Z=3, Y=4, H=5, O=6, R=7, S=8, E=9
• Suppose the coordinates to transmit are 26345, 77135.
• Encoding:
  • 2 = A, 6 = O, 3 = Z, 4 = Y, 5 = H → 26345 becomes AOZYH
  • 7 = R, 7 = R, 1 = R, 3 = Z, 5 = H → 77135 becomes RRRZH
• Transmission: The operator says, “USING SARNEG, location is AOZYH, RRRZH.”
• Decoding: The recipient, knowing CRAZYHORSE, converts AOZYH back to 26345 and RRRZH back to 77135.

Without the code word, an adversary intercepting “AOZYH, RRRZH” cannot easily deduce the coordinates.

Implementation in Radio Communications

To implement SARNEG in radio communications, follow the steps below.

Select a Code Word

• Choose a 10-letter word with no repeating letters (e.g., “STUPENDOUS”).
• Assign each letter to a digit (0–9). For example: S=0, T=1, U=2, P=3, E=4, N=5, D=6, O=7, R=8, U=9.
• Ensure all operators (transmitter and receiver) have the code word in advance, typically distributed securely before the mission (e.g., via encrypted channels or in-person briefings).
• Change the code word daily or per mission to maintain security.

Prepare the Coordinates

• Convert the location (e.g., grid coordinates or latitude/longitude) into a numerical format, typically as two groups of five digits. For example:
  • Grid: 12345 67890
  • Latitude/Longitude: Convert to a 10-digit format (e.g., 12.345°N, 67.890°W becomes 12345 67890).
• If coordinates have fewer digits, pad with zeros (e.g., 123 becomes 00123).

Encode the Message

• Replace each digit with its corresponding letter from the code word.
• Group the letters into pronounceable chunks (typically 5 letters per group) for clarity during transmission.
• Example: For 12345 using STUPENDOUS (S=0, T=1, U=2, P=3, E=4, N=5), 12345 becomes TUPEN.

Transmit the Message

• Use standard radio communication protocols for clarity (e.g., NATO phonetics, clear enunciation).
• Preface the transmission with “USING SARNEG” to indicate the encryption method.
• Example: “USING SARNEG, location is TUPEN, ORSPD.”
• Follow radiotelephony procedures (e.g., “Over” to indicate the end of transmission).

Receive and Decode

• The recipient, knowing the code word, writes down the transmitted letters.
• Convert each letter back to its corresponding digit using the code word.
• Reconstruct the coordinates for use in navigation or rescue operations.

Operational Considerations

• Security: Ensure the code word is kept secret and only shared with authorized personnel. If compromised, select a new code word.
• Training: Train all operators on SARNEG procedures to ensure accurate encoding/decoding and proper radio etiquette.
• Frequency Selection: Use appropriate radio frequencies (e.g., VHF for short-range SAR, HF for long-range) based on the mission environment. HF is often used for SAR due to its ability to reflect off the ionosphere for long-distance communication.
• Backup Systems: In case of radio failure or interference, have alternative communication methods (e.g., satellite phones) ready.

Regulatory Compliance

• Ensure radio operations comply with international and national regulations (e.g., ITU, FCC, or NTIA in the U.S.) to avoid interference with other services. Obtain necessary licenses for radio frequencies.
• Use allocated SAR frequencies (e.g., 121.5 MHz or 243.0 MHz for emergency beacons) when applicable.

Equipment

• Use reliable radio transceivers (e.g., handheld VHF radios for short-range or HF radios for long-range). Ensure they are compatible with the frequency band used.
• Equip with directional or omnidirectional antennas based on the operational need (e.g., omnidirectional for broadcast SAR signals).

Practical Tips

• Clarity: Use phonetic pronunciation for letters (e.g., “Alpha” for A) to avoid confusion.
• Verification: After decoding, verify coordinates with a secondary method (e.g., map check) to ensure accuracy.
• Interference Mitigation: Be aware of potential signal propagation issues (e.g., reflection, diffraction, or absorption) that could affect transmission clarity, especially in urban or rugged environments.
• Practice: Conduct regular drills to ensure operators are proficient in encoding, transmitting, and decoding SARNEG messages.

Limitations

• Security Level: SARNEG is a basic encryption method and not highly secure against determined adversaries with code-breaking capabilities. It relies on the secrecy of the code word.
• Complexity: Requires training to avoid errors in encoding/decoding, especially under stress.
• Range: Limited by the radio’s power and frequency band. For example, VHF is line-of-sight (up to 64 km), while HF can achieve intercontinental ranges via skywave propagation.

By implementing SARNEG with proper training, equipment, and adherence to radio protocols, it can be an effective tool for secure communication in SAR operations over non-secure channels.