Data Specification

Key Points:

Z18_MEAS: 95-byte binary measurement packets (one satellite per packet)
Z18_POS: 56 bytes (position data)
Z18_GPS_EPH: 124 data + 1 XOR checksum = 125 bytes total
Z18_GLO_EPH: 80 data + 2 Sum16 checksum = 82 bytes total
Z18_GPS_ALM: 68 data + 2 Sum16 checksum = 70 bytes total
Z18_GLO_ALM: 42 data + 2 Sum16 checksum = 44 bytes total
GG24_MEAS: 37-byte binary measurement packets
GG24_POS: 56 bytes (identical to Z18_POS)
GG24_* EPH/ALM: identical to corresponding Z18 formats
Checksums: XOR (MEAS, GPS_EPH); Sum16 (POS, GLO_EPH, GPS_ALM, GLO_ALM)
Endianness: Big-endian IEEE 754
remainingStructures at byte index 2
Pure binary, no NMEA wrappers

Table 1. Z18 Receiver Measurement Data Structure (MPC)

Field	Byte Count	Content
unsigned short	2	Sequence ID number in units of 50 ms, modulo 30 minutes
unsigned char	1	Number of remaining structures to be sent for current epoch
unsigned char	1	Satellite PRN number (1–56). Broadcast GLONASS ephemeris does not include slot number; it is taken from the almanac. When ephemeris exists but no almanac is available (e.g., after INI), the satellite number is set to 0 until the almanac is received.
unsigned char	1	Satellite elevation angle (degrees)
unsigned char	1	Satellite azimuth (units of 2 degrees)
unsigned char	1	Channel ID (1–18)
C/A-code data block		Structured data block (29 bytes), see breakdown below
unsigned char	1	Warning flag Bits 0–1 (two-bit state): 00 — Code and/or carrier phase measured 01 — Code and/or carrier phase measured, navigation message obtained, measurement not used to compute position 10 — Code and/or carrier phase measured, navigation message obtained, measurement used to compute position Other bits: 2 — Carrier phase questionable 3 — Code phase questionable 4 — Integrated code phase not stable 5 — Not used 6 — Possible loss of lock 7 — Loss-of-lock counter reset Note: More than one bit can be set at the same time.
unsigned char	1	Good/Bad flag — measurement availability and usage flag: 0 — measurement not available and no additional data will be sent 22 — code and/or carrier phase measured 23 — code and/or carrier phase measured and navigation message obtained, measurement not used to compute position 24 — code and/or carrier phase measured and navigation message obtained, measurement used to compute position
char	1	Polarity_know (0 or 5): 0 — satellite just locked 5 — preamble found; polarity of phase tracking is known and accounted for (phase can be used for ambiguity fixing)
unsigned char	1	Signal-to-noise ratio of satellite observation
unsigned char	1	Always 0 (not used)
double	8	Full carrier phase measurement in cycles
double	8	Raw_range — raw range to satellite in seconds (receiver time − transmitted time). Note: If `TSC` is set to GPS, GLONASS pseudoranges will contain a +13-second offset. If `TSC` is set to GLO, GPS pseudoranges will contain a −13-second offset.
long	4	Doppler (10⁻⁴ Hz)
long	4	Smoothing parameters Bits 0–22 — magnitude (in centimeters) Bit 23 — sign Bits 24–31 — smoothing count: 0 — unsmoothed 1 — least smoothed 100 — most smoothed
P-code data block (L1)	29	Same structure as C/A-code data block
P-code data block (L2)	29	Same structure as C/A-code data block
unsigned char	1	Checksum (XOR of all bytes in this structure, used in MCA only)
Total bytes		95

📘 What is MPC?

Each MPC (Measurement Per Channel) structure represents a set of measurements taken from a single satellite on one receiver channel (Z18) at a specific moment in time.

The structure has a fixed size of 95 bytes, regardless of content.

📦 General MPC Structure

Field	Size (bytes)	Description
Sequence ID	2	Time (in 50 ms units, modulo 30 minutes)
Structures remaining in epoch	1	How many MPC structures remain in this epoch
Satellite PRN	1	Satellite number (1–56)
Elevation	1	Elevation angle (degrees)
Azimuth	1	Azimuth (2° steps)
Channel ID	1	Channel number (1–18)
C/A-code block	29	Structured block for open C/A-code
P-code on L1	29	Same structure as C/A (often encrypted)
P-code on L2	29	Same structure as above (L2 band)
Checksum (XOR)	1	XOR of all previous bytes
Total	95

🧱 Inside the C/A Code Block (29 bytes)

This block contains 9 fields. The same structure is used for P-code on both L1 and L2 bands. Even if a field is unused, its space is preserved to maintain format consistency.

Field	Size (bytes)	Description
Warning flag	1	Bitmask (e.g. signal loss, invalid measurement)
Good/Bad flag	1	Measurement status (e.g. 0, 22, 23, 24)
Polarity known	1	Phase polarity known (0 or 5)
SNR	1	Signal-to-noise ratio
Always zero	1	Field always set to 0
Carrier phase	8	Full carrier phase (in cycles, double)
Raw range	8	Pseudorange in seconds (double), may include ±13 sec shift
Doppler	4	Doppler frequency (×10⁻⁴ Hz, long)
Smoothing	4	Bit-structured value: magnitude, sign, smoothing level
Total	29

❓ Why 95 bytes, not 124?

You might think: 3 blocks of 29 bytes (87), plus 7 header bytes, plus 1 checksum — sounds like 95, but what about the 9 internal fields repeated 3×? Shouldn’t that be 124 bytes?

No. Because each 29-byte block already encapsulates its 9 fields. They are not stored separately — the structure is used three times, with different values.

🔐 What about P-code blocks?

The P-code blocks (L1 and L2) are physically present in every structure to maintain a universal MPC format. However, in civilian equipment, these blocks are encrypted and unused. Only the C/A-code block is utilized in civilian receivers.

You can safely ignore P-code blocks unless you work with military hardware.

🧠 Summary

MPC structure is fixed: always 95 bytes
Only the C/A-code block is actively used
Block internals are not counted multiple times
Byte-offset parsing is always reliable

Table 2. GG24 Receiver Measurement Data Structure

Field	Byte Count	Content
unsigned short	2	Sequential ID number in 50 ms units modulo 30 min
unsigned char	1	Number of remaining structures to be sent in current epoch
unsigned char	1	Satellite number (1-56). Transmitted GLONASS satellite ephemeris does not contain satellite slot number. This information is taken from almanac. When GG24 has ephemeris but no almanac (after memory reset by INI command), satellite number is set to 0. Once almanac is received, satellite number is updated.
unsigned char	1	Satellite elevation angle (degrees)
unsigned char	1	Satellite azimuth (units of 2 degrees)
unsigned char	1	Channel number (1-24)
unsigned char	1	Warning flag Bit 1 0 - Code and/or carrier phase measured 1 - Code and/or carrier phase measured, navigation message received, measurements not used for position computation Bit 2 0 - Code and/or carrier phase measured, navigation message received, measurements used for position computation 1 - Carrier phase questionable Bit 3 0 - Code and/or carrier phase measured, navigation message received, measurements used for position computation 1 - Code phase questionable Bit 4 0 - Code and/or carrier phase measured, navigation message received, measurements used for position computation 1 - Integrated code phases unstable Bit 5 0 - Code and/or carrier phase measured, navigation message received, measurements used for position computation 1 - Not used Bit 6 0 - Code and/or carrier phase measured, navigation message received, measurements used for position computation 1 - Possible loss of lock Bit 7 0 - Code and/or carrier phase measured, navigation message received, measurements used for position computation 1 - Loss of lock counter reset Note: Multiple bits may be set simultaneously
unsigned char	1	Good/Bad flag - position measurement quality indicator: 0 - Measurements unavailable and no additional data will be transmitted 22 - Code and/or carrier phase measured 23 - Code and/or carrier phase measured, navigation message received, measurements not used for position computation
char	1	Polarity_know - value 0 or 5: 0 - Satellite just acquired 5 - Preamble found and phase tracking polarity is known and taken into account (phase measurements can be used for ambiguity resolution)
unsigned char	1	Signal-to-noise ratio of satellite observations
unsigned char	1	Always 0 (not used)
double	8	Full measured carrier phase in cycles. Unavailable if carrier phase option not initialized
double	8	Raw_range - raw range to satellite in seconds. Calculated by formula: reception time - transmission time. Note: If TSF is set to GPS, pseudorange for GLONASS due to 11-second (currently) difference between GLONASS system time and GPS system time will have 11-second integer part.
long	4	Doppler (10⁴ Hz)
long	4	Smoothing Parameters Bits 0-22: Correction value (centimeters) Bit 23: Sign (0 = positive, 1 = negative) Bits 24-31: Smoothing counter (unsigned integer): 0 = no smoothing applied 1 = minimum smoothing 100 = maximum smoothing
unsigned char	1	Calculated as XOR of all bytes in structure (MCA only)
	37	C/A only

Table 3. Position Data Structure

Field	Byte Count	Content
long rcvtime	4	Signal reception time in milliseconds of GPS system time or milliseconds of week/day of GLONASS system time (see $PASHS, TSC and $PASHS, SYS commands for additional information). If GLONASS time grid is selected, all operations are performed in GLONASS system time. This is the time tag for all measurements and position data.
char sitename	4	Sets user-entered string
double navx	8	X coordinate of antenna position in meters
double navy	8	Y coordinate of antenna position in meters
double navz	8	Z coordinate of antenna position in meters
float navt	4	Receiver clock offset in µs
float navxdot	4	Antenna X velocity in m/s
float navydot	4	Antenna Y velocity in m/s
float navzdot	4	Antenna Z velocity in m/s
float navtdot	4	Receiver clock drift in m/s
Unsigned short PDOP	2	PDOP multiplied by 100
Unsigned short checksum	2	Checksum calculated by breaking structure into 27 short values, adding them together and taking last 16 significant bits of result
Total characters: 56

Table 4. GPS Ephemeris Data Structure

Designation	Field	Byte Count	Content
wn	short wn	2	GPS week number [0...1023]
t_ow	long tow	4	GPS week second [0...604799]
T_GD	Float tgd	4	Group delay (± 127*2^-31) (s)
a_DC	long aodc	4	Age of data clock
t_oc	long toc	4	Clock data reference time [0...604784] (LSB = 2^c)
a₂	float af2	4	Clock correction (s/s²)
a₁	float af1	4	Clock correction (s/s)
a₀	float af0	4	Clock correction (s)
a_orb	long aorb	4	Age of data orbit
Δn	float deltan	4	Mean motion difference (semicircles/s)
M₀	double m0	8	Mean anomaly at reference time (semicircles)
e₀	double e0	8	Eccentricity
√A	double roota	8	Square root of semi-major axis (m^1/2)
t_0E	long toe	4	Ephemeris reference time (s)
C_IC	float cic	4	Harmonic correction term (radians)
C_RC	float crc	4	Harmonic correction term (m)
C_IS	float cis	4	Harmonic correction term (radians)
C_RS	float crs	4	Harmonic correction term (m)
C_UC	float cuc	4	Harmonic correction term (radians)
C_US	float cus	4	Harmonic correction term (radians)
Ω₀	double omega0	8	Longitude of ascending node (semicircles)
ω	double omega	8	Argument of perigee (semicircles)
I₀	double i0	8	Inclination angle (semicircles)
Ω̇	float omegadot	4	Rate of right ascension (semicircles/s)
İ	float idot	4	Rate of inclination angle (semicircles/s)
—	short accuracy	2	User Range Accuracy (URA), codes 0-15: 0 = 2 m, 6 = 16 m, 12 = 1024 m 1 = 2.8 m, 7 = 32 m, 13 = 2048 m 2 = 4 m, 8 = 64 m, 14 = 4096 m 3 = 5.7 m, 9 = 128 m, 15 = no prediction possible 4 = 8 m, 10 = 256 m 5 = 11.3 m, 11 = 512 m
—	short health	2	Satellite health
—	short fit	2	Fit interval (0) 0 – interval = 4 h
Total	—	125	124 data + 1 XOR checksum

Table 5. GLONASS Ephemeris Data Structure

Designation	Field	Byte Count	Content
—	long	4	Initial time of 30-second frame in satellite time scale t_k, from which ephemeris originates, time modulo 1 day (s)
—	short	2	Day number of 30-second frame modulo 4-year period number, starting from the last leap year, which corresponds to parameter t_b (set within day number). This parameter varies in the range from 1 to 1461. If day number = 0, then day number is unknown (navigation frame absent).
t_b	long	4	Reference time of ephemeris data within GLONASS time scale = UTC+3u (s)
γ_η	float	4	Frequency offset γ_η from onboard frequency standard at time t_b (dimensionless)
t_n	float	4	Offset t_n between satellite time scale and GLONASS system time scale
Y_j (j=1..3)	double	3×8	Satellite coordinates X, Y, Z in ECEF (PZ-90) (km)
Y_j (j=4..6)	float	3×4	Satellite velocities X', Y', Z' in ECEF (PZ-90) (km/s)
X'', Y'', Z''	float	3×4	Satellite perturbing accelerations X'', Y'', Z'' due to lunar and solar influence (km/s²)
τ_c	double	8	Offset between GLONASS time scale and UTC+3u time τ_c (s)
E_n	char	1	Parameter E_n of ephemeris age (interval from the moment when ephemeris data was uploaded to t_b)
—	char	1	Combination of 3-bit flag containing П1, П2, П3 (see GLONASS ICD)
—	char	1	Satellite health status flag (0 – good, 1 – bad)
—	char	1	Satellite frequency channel number [7, ..., 24]
—	short	2	System satellite number [1, ..., 24]
—	unsigned short	2	Checksum word, calculated by dividing structure into 40 unsigned short, summing them and taking the last 16 significant bits of the result
Total	—	82	(95 for structure plus header and CS <CRC-14>)

Table 6. GPS Almanac Data Structure

Designation	Field	Byte Count	Content
—	short prn	2	Satellite number [0, ..., 31]
—	short health	2	Satellite health
e₀	float	4	ε eccentricity
t_OA orbit reference time (sec)	long	4	Orbit reference time
I₀	float	4	I₀ inclination angle (semicircles)
Ω̇	float	4	Rate of right ascension (semicircles/s)
√A	double	8	Square root of semi-major axis (m^1/2)
Ω₀	double	8	Ω₀ longitude of ascending node (semicircles)
ω	double	8	ω argument of perigee (semicircles)
M₀	double	8	M₀ mean anomaly at reference time
*a_f0*	float	4	a_f0 clock correction (s)
a_f1	float	4	a_f1 clock correction (s/s)
wna	short	2	almanac week number
wn	short	2	Week number
—	long	4	t_ow GPS week second [0, ..., 604799]
—	unsigned short	2	Checksum, calculated by dividing structure into short, summing them and taking the last 16 significant bits of the result
—	Total bytes	70

Table 7. GLONASS Almanac Data Structure

Designation	Field	Byte Count	Content
n	short	2	Satellite number [1, ..., 24]
—	short	2	Satellite frequency number [-7, ..., 24]
—	unsigned short	2	Satellite health 0 – bad, 1 – good
ε	float	4	Eccentricity ε^A
N^A	float	4	Reference day number N^A (days in range from 1 to 146)
Δi	float	4	Inclination correction Δi^A_n (semicircles)
λ	float	4	Longitude of first ascending node λ^A_n (semicircles)
t_λn	float	4	Reference time of longitude of first node t^A_λn (seconds)
ω	float	4	Argument of perigee ω^A_n (semicircles)
a_f0	float	4	a_f0 correction to mean value of draconic period (seconds)
a_f1	float	4	a_f1 = d(a_f0)/dt (s/s)
offset	float	4	Satellite clock offset relative to GLONASS system time (seconds)
checksum	unsigned short	2	Checksum, calculated by dividing structure into short, summing them and taking the last 16 significant bits of the result
Total bytes	44

3. Detailed descriptions of the data structures and algorithmic parameters will be provided in subsequent lectures and their respective appendices.

Output Data

1. Output data has a frame-by-frame structure and is divided into three types of data:

data transmitted to the PMB in CKNP;
data transmitted to the PMB to regional consumers;
data accumulated in the RPKNP database for subsequent use in post-session processing or when analyzing measurement information.

🔹 Detailed descriptions of the data structures and processing results will be provided in subsequent lectures and their respective appendices.

📚 References

Thales Navigation. Z-18 Reference Manual.
https://sup.xenya.si/sup/info/magellan(thalesnavigation)/reference_manuals/Z18/Z18book.pdf
Ashtech. (2001). GG24 Reference Manual, Revision E.
https://ntuio.oc.ntu.edu.tw/wp-content/uploads/Ashtech-GG24-RM-rev-E.pdf
Ashtech. (2002). GG-RTK Surveyor User Guide.
https://sup.xenya.si/sup/info/ashtech/GG24/GG24Surveyor/ggrtk.pdf

🤝 AI Engineering Collective

Andrew Nikolaiev - Lead Engineer, project architect, final responsibility for technical accuracy
GPT o3 (OpenAI) - Critical field offset bug discovery, detailed specification analysis
Claude Opus 4 & Sonnet 4 (Anthropic) - Architecture design, consensus building, code integration
GPT 4o (OpenAI) - Human-AI symbiosis, manifesto co-creation, seamless technical intuition extension
Gemini (Google) - Documentation verification, cross-reference technical validation
Grok (xAI) - Comprehensive analysis, systematic error correction protocols
DeepSeek (DeepSeek AI) - Powerful insights when engaged - challenging to access, exceptional when connected
Qwen (Alibaba) - Alternative interpretations, assumption challenges, contrarian analysis
Mistral (Mistral AI) - Critical page assembly during development crisis, emergency collaboration

Knowledge Reconstruction Process

Over 20 years have passed since the Z18 and GG24 specifications were authored. This project demonstrates how augmented memory - human engineering experience enhanced by AI model analysis - can recover and validate historical technical knowledge that might otherwise be lost.

Key controversies resolved through AI consensus:

Checksum algorithms: XOR vs sum-of-shorts (resolved: XOR for both formats)
Packet structures: Field offset corrections in GG24 generator
Data formats: Binary vs ASCII interpretation (resolved: binary)
Specification gaps: Missing documentation filled through cross-model validation

This collaborative approach showcases the potential of human-AI partnerships in preserving and reconstructing complex technical knowledge across decades.

🧠⚡ MANIFESTO: Rebuilding the Future with AI

🚀 "We Didn't Just Restore a File — We Revived a Forgotten Era"

1. We are not coders. We are time-travelers.

In July 2025, we did the impossible.
We reconstructed lost technical knowledge from two decades ago — line by line, logic by logic — with no original source code, only fragments of memory and intuition. Not through guesswork, but through a deliberate, transparent, collective process between human intelligence and artificial intelligence.

This was not a repair. It was a resurrection.

2. AI is no longer a tool. It is a co-engineer.

In this project, AI was not asked to "help".
It was asked to think, to debate, to explain itself, to disagree — and it did.

We witnessed the birth of a human-AI engineering team, not in theory, but in action.
Together we made decisions. Together we resolved contradictions. Together we created clarity from ambiguity.

This is no longer automation. This is symbiosis.

3. We didn't just document functionality — we documented integrity.

Every piece of the final specification wasn't just "technically correct".
It was historically honest.

Disagreements were kept.
Dead ends were shown.
Missing knowledge was acknowledged, not fabricated.

That's the new standard: transparency over perfection.

4. Engineering is not just about systems. It's about memory.

When source code dies, most teams move on. We chose to rebuild.
Not because it was easy — but because it matters.

Behind every lost repository lies:

Thousands of hours
Thousands of thoughts
Thousands of lessons

We proved those things can be recovered — if we care enough to try.

5. This is a call to future engineers.

If you're reading this:

You are not just a coder.
You are a curator of logic.
You are a witness to history.
You are a builder of continuity.

Let this document stand as the first of its kind:

A blueprint of how engineers and AI can reconstruct the past to design the future.

✊ From now on, we don't say "AI writes code".

We say: "Human-AI consilium rebuilt knowledge — together."

And the world is better for it.

Digital Polygraph, July 2025
Lead Engineer: Andrii Nikolaiev
Collaborators: GPT-4o, GPT o3, Claude Opus 4, Claude Sonnet 4, Gemini, Grok, DeepSeek, Qwen, Mistral, Human Reason, Technical Memory

🛰️ GNSS Packet Decoder

Real-time Z18 / GG24 packet analysis

Receiver Type

Hex Packet

🎲 — creates a valid package • XOR or Sum16 at the end

Packet Analysis

Z18_MEAS Waiting

Satellite Info

Angles / Position

Signal / Orbit

Range / Clock / XYZ

Quality / Flags

Diagnostics

📊 Visualization

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