Sometimes, non-standard ICD numbers (like a hypothetical 153) refer to Control Segment documents (how the Air Force controls the satellites) rather than the User Segment (how users receive signals).
ICD-GPS-100 usually refers to the Operational Control Segment.
ICD-GPS-300 often refers to the User Segment requirements.
There is no publicly listed "ICD-GPS-153" in the classified index ranges currently available to public contractors.
Why Should You Care? The Strategic Role of the 153 Protocol
Despite the rise of GNSS (Galileo, GLONASS, BeiDou), the GPS P(Y) code defined by ICD-GPS-153 provides three irreplaceable advantages: icd-gps-153 protocol
Assured Positioning in Contested Spectrum: While civilians lose lock in a jammer, a 153-conformant receiver using L1/L2 P(Y) can maintain track because the signal power is higher and the processing gain is 10x better.
Time Transfer: Financial markets, power grids, and telecom backbones use GPS time. The 153 protocol provides UTC(USNO) accuracy within 5 nanoseconds—critical for high-frequency trading.
Nuclear Detonation Detection (NDS): U.S. GPS satellites carry payloads that detect nuclear flashes. The 153 data message includes this NDS data.
3. The Navigation (Data) Message
ICD-GPS-153 defines a distinct data message uplinked from the Control Segment. Key differences from IS-GPS-200 include: Feature draft — ICD-GPS-153 protocol
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Higher Speed: 50 bps (same as civilian), but with different frame structures.
Ephemeris: Ultra-precise orbital parameters (sub-meter accuracy).
Clock Correction: Higher-order polynomial coefficients for the atomic clocks.
URA (User Range Accuracy): Values as low as 0.01 meters (vs ~3 meters for civilian).
Status & Health: Encrypted constellation status including anti-spoofing (AS) activation flags.
Message Structure
The protocol uses a binary packet structure, not ASCII text (unlike NMEA). Each message consists of: Why Should You Care
| Field | Size (Bytes) | Description |
| :--- | :--- | :--- |
| Sync | 2 | Unique start-of-message characters (e.g., 0xAA, 0x55) |
| Message Type | 1 or 2 | Defines the payload content (e.g., position, time, almanac) |
| Length | 2 | Length of the payload (excluding header and checksum) |
| Payload | Variable | Encrypted or unencrypted data fields |
| Checksum | 2 | CRC-16 or similar for error detection |