GT502MMG GNSS Module Specifications
GT502MMG is a multi-constellation GNSS (Global Navigation Satellite System) receiver module that provides accurate positioning and timing information. It serves as the planned timing and location reference system for the OSECHI cosmic ray detector.
Module Overview
What is GNSS?
GNSS (Global Navigation Satellite System) is a generic term for satellite-based positioning systems that provide location (latitude, longitude, altitude) and time information anywhere on Earth.
Multi-Constellation Support
The GT502MMG supports five GNSS constellations simultaneously:
- GPS - United States
- GLONASS - Russian Federation
- BeiDou - People's Republic of China
- Galileo - European Union
- SBAS - Satellite-based augmentation systems
Benefits:
- Higher satellite visibility (urban canyons, mountainous terrain)
- Faster time-to-first-fix (TTFF)
- Improved accuracy and reliability
- Redundancy and fault tolerance
Hardware Specifications
| Item |
Specification |
| Module Name |
GT502MMG |
| Manufacturer |
GlobalTop Technology |
| Form Factor |
SMD (Surface Mount Device) |
| Size |
25 mm × 25 mm × 5.8 mm |
| Weight |
~0.8 g |
Electrical Specifications
| Item |
Value |
Notes |
| Supply Voltage |
2.8V - 4.3V |
3.3V typical |
| Operating Current |
~35 mA |
Tracking mode (average) |
| Peak Current |
~50 mA |
Initial acquisition |
| Standby Current |
~5 mA |
Low-power mode |
| Shutdown Current |
<0.1 mA |
Power-down mode |
Communication Interface
| Item |
Specification |
| Primary Interface |
UART (Serial) |
| Default Baud Rate |
9600 bps |
| Supported Rates |
4800, 14400, 19200, 38400, 57600, 115200 bps |
| Data Format |
8 data bits, 1 stop bit, no parity |
| Protocol |
NMEA 0183 standard |
| Buffer Size |
256 bytes |
Reception Specifications
| Item |
Specification |
| Total Channels |
66 (multi-constellation) |
| GPS Channels |
16 |
| GLONASS Channels |
16 |
| BeiDou Channels |
16 |
| Galileo Channels |
16 |
| SBAS Channels |
2 |
Signal Reception
| Item |
Specification |
| RF Frequency |
1575.42 MHz (GPS L1) |
| Cold Start Sensitivity |
-155 dBm |
| Hot Start Sensitivity |
-162 dBm |
| Tracking Sensitivity |
-160 dBm |
| Reacquisition Sensitivity |
-161 dBm |
Positioning Accuracy
| Condition |
Accuracy |
| Latitude/Longitude |
±2.5 m (95% confidence) |
| Altitude |
±5 m (95% confidence) |
| Horizontal (RMS) |
±1.5 m (typical) |
Timing Accuracy
| Item |
Specification |
| Pulse-per-second (PPS) Width |
100 ms (default) |
| PPS Frequency Accuracy |
±1 ppm (typical) |
| Time-to-First-Fix (Cold) |
45 seconds |
| Time-to-First-Fix (Warm) |
5 seconds |
| Time-to-First-Fix (Hot) |
<1 second |
Velocity Measurement
| Item |
Specification |
| Maximum Speed |
515 m/s (1854 km/h) |
| Velocity Accuracy |
0.05 m/s |
Satellite Tracking
| Item |
Specification |
| Minimum Elevation Angle |
5 degrees (configurable) |
| Update Rate |
1 Hz (default) |
| Alternative Rates |
0.5 Hz, 2 Hz, 5 Hz, 10 Hz |
| Minimum Satellites |
4 (for 3D position) |
Operating Conditions
Environmental Specifications
| Item |
Specification |
| Operating Temperature |
-40 to +85°C |
| Storage Temperature |
-50 to +100°C |
| Operating Humidity |
5% to 95% (non-condensing) |
| Vibration Tolerance |
3G (10 Hz - 500 Hz) |
Power Modes
| Mode |
Current |
Wake Time |
Purpose |
| Normal |
~35 mA |
N/A |
Full tracking and positioning |
| Standby |
~5 mA |
2-3 seconds |
Low-power, quick wake |
| Shutdown |
<0.1 mA |
Variable |
Deep sleep, minimal current |
Output Protocols
NMEA 0183 Sentences
The GT502MMG outputs standard NMEA 0183 sentences:
| Sentence |
Description |
| GGA |
Global Positioning System Fix Data |
| GLL |
Geographic Position (latitude, longitude) |
| GSA |
GPS DOP and active satellites |
| GSV |
Satellites in view (signal strength) |
| RMC |
Recommended minimum navigation info |
| VTG |
Track and ground speed |
| GBS |
GNSS Satellite Fault Detection |
| ZDA |
Time and date |
PMTK Protocol
Proprietary MTK protocol for module configuration:
- Baud rate adjustment
- Update rate configuration
- SBAS enable/disable
- WAAS/DGPS settings
- Constellation selection
Example NMEA Output
$GPRMC,123519,4807.038,N,01131.000,E,022.4,084.4,230394,003.1,W*6A
$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47
$GPGSA,A,3,04,05,,09,12,,,24,,,,,2.5,1.3,2.1*30
Antenna Specifications
Typical Requirements
| Item |
Specification |
| Antenna Type |
Active GPS antenna (recommended) |
| Frequency Range |
1575.42 MHz ±5 MHz |
| Impedance |
50 Ohms |
| Gain |
>20 dB |
| Noise Figure |
<2 dB |
| Connector |
SMA or MCX |
| Cable Length |
<10 m (typical) |
Reception Environment
- Line of sight: Required for optimal performance
- Obstruction: Avoid large metallic structures directly above
- Multipath: Minimize reflections from buildings
- Shielding: Copper shielding recommended for industrial use
Pin Configuration
UART Interface Pinout
| Pin |
Function |
Description |
| 1 |
VCC |
Power supply (2.8V - 4.3V) |
| 2 |
TX |
Serial transmit data |
| 3 |
RX |
Serial receive data |
| 4 |
GND |
Ground |
| 5 |
PPS |
Pulse-per-second output (optional) |
| 6 |
GND |
Ground |
Typical UART Connection
GT502MMG ESP32
────────────────────────
TX (pin 2) → RX (GPIO3)
RX (pin 3) ← TX (GPIO1)
VCC (pin 1) ── 3.3V
GND (pin 4) ── GND
PPS (pin 5) ── GPIO (optional)
OSECHI Integration
Current Status
- Implementation: Currently unimplemented (designed for future integration)
- Use Case: Provide synchronized timing and location data for cosmic ray event correlation
Planned Features
- Timestamp Synchronization
- Link cosmic ray detection events to absolute GNSS time
- Enable multi-detector event correlation across locations
-
Provide RTC reference timing
-
Spatial Information
- Record detector location (latitude, longitude, altitude)
- Store environmental parameters with geographic context
-
Enable location-based cosmic ray analysis
-
Data Collection
- Capture GNSS quality metrics (DOP, satellite count)
- Monitor signal reception quality over time
- Detect accuracy degradation
Hardware Interface Plan
GT502MMG GNSS Module
├── UART0 (shared with PC communication)
│ ├── TX → GPIO1 (multiplexed)
│ └── RX → GPIO3 (multiplexed)
├── PPS Signal → GPIO36 (input-only)
└── Power 3.3V ← Regular supply
Configuration Examples
PMTK Configuration Commands
// Set baud rate to 115200
$PMTK251,115200*1F
// Set update rate to 5 Hz
$PMTK220,200*2C
// Enable all constellations
$PMTK353,1,1,1,1,1*2B
Typical Communication Sequence
- Power-on: Module boots, outputs NMEA sentences at default rate
- Configuration: Send PMTK commands to adjust settings
- Acquisition: Wait for satellite lock (10-45 seconds cold start)
- Operation: Receive periodic position/time updates at configured rate
- Power-down: Hold enable pin low or send shutdown command
Factors Affecting Accuracy
- Satellite Geometry
- Minimum 4 visible satellites for 3D position
- GDOP (Geometric Dilution of Precision) value
-
Higher satellite count improves accuracy
-
Signal Quality
- Signal strength (dBm values in GSV sentences)
- Multipath reflections (urban vs. open sky)
-
Atmospheric conditions
-
Environmental Factors
- Tree canopy coverage
- Urban canyon effects
- Dense building structures
- Underground/indoor operation (impossible)
Accuracy Improvement Techniques
- SBAS Augmentation: Improves accuracy to ±1 meter
- DGPS: Requires reference station
- RTK: Centimeter-level accuracy (external base station needed)
- Multi-constellation: Using all 5 constellations simultaneously
Specification Summary
| Category |
Specification |
| Module |
GT502MMG (GlobalTop) |
| Constellations |
GPS, GLONASS, BeiDou, Galileo, SBAS |
| Total Channels |
66 |
| Supply Voltage |
2.8V - 4.3V (3.3V typical) |
| Operating Current |
~35 mA (tracking) |
| UART Baud Rate |
9600-115200 bps |
| Position Accuracy |
±2.5 m (95% confidence) |
| TTFF (Cold) |
45 seconds |
| TTFF (Hot) |
<1 second |
| Antenna |
Active GPS antenna (external) |
| Operating Temperature |
-40 to +85°C |
References