Last time, we covered the implementation design of the barometer and countermeasures against the three major disturbances (airflow, temperature, and electrical noise). In this article, which is FC11, we will cover GPS/GNSS antenna placement design, which is the key to autonomous flight for UAS.
GNSS reception performance is not determined simply by "choosing a high-performance module." Physical and electrical integration design, such as electromagnetic interference (EMI) inside the airframe, antenna placement, and shielding due to changes in the aircraft's attitude, is the core of reception quality. In this article, we will explain design guidelines for antenna placement that maximize GNSS reception performance and are directly linked to stabilizing RTK positioning and improving the Fix rate.
Why GNSS antenna placement is the most important aspect of FC design
Because GNSS (GPS, GLONASS, Galileo, BeiDou, etc.) satellite signals reach the ground from space at an altitude of approximately 20,000 km, the signal strength at the time of reception is extremely weak (approximately -130 dBm to -160 dBm). This is close to the thermal noise level, and even slight noise interference or shielding can easily cause signal loss or multipath (errors due to radio wave reflection).
In particular, for RTK (Real-Time Kinematic) positioning, which realizes high-precision automatic flight and hovering, it is necessary to accurately measure the carrier phase, so even higher signal quality (higher C/N0 ratio: carrier-to-noise ratio) is required than in normal standalone positioning.
ArduPilot's EKF3 (Extended Kalman Filter) highly integrates position and velocity information obtained from GNSS with data from the IMU (Inertial Measurement Unit). If the GNSS C/N0 ratio drops and cycle slips (phase discontinuities) occur frequently, the EKF lowers the reliability of the GNSS, and the standard deviation of position estimation (predicted value of position error) increases. In the worst case, there is a risk that autonomous flight cannot be maintained, and the system will be forced to transition to AltHold mode or Land mode.
Types of major GNSS antennas and selection criteria
The following two types of antennas are mainly used in FC design and the selection of peripheral modules.
1. Patch antenna (ceramic patch)
This is the most common square ceramic antenna.
- Features: High directivity, with high gain for radio waves from directly above the antenna (zenith direction).
- Selection points: The larger the size (25mm x 25mm, 35mm x 35mm, etc.), the higher the gain and the more stable the C/N0 ratio. For industrial aircraft and RTK operations, it is standard to select a patch antenna of at least 25mm square to ensure sufficient gain, or a 35mm square patch antenna, or a multi-frequency patch antenna that supports two frequencies (L1/L2 or L1/L5) if space permits.
2. Helical antenna (quadrifilar helical, etc.)
This is a cylindrical antenna.
- Features: Relatively wide directivity, with characteristics that make it easy to pick up radio waves from satellites at low elevation angles (directions close to the horizon). Another advantage is that reception performance is less likely to drop even when the aircraft is tilted significantly.
- Selection points: This is a strong option for fixed-wing aircraft, highly maneuverable multicopters, or when mounting on the tip of a vertical stabilizer where horizontal mounting space cannot be secured.
Three factors that hinder GNSS reception
Disturbances that should be eliminated when considering antenna placement are classified into three categories: "electrical noise (high-frequency EMI)," "physical shielding (shadowing)," and "multipath (reflected waves)."
[Primary Noise and Shielding Sources]
[Propellers / Motors] ── Magnetic field noise generated by high current draw
[Camera / VTX] ────── Radiated emission from high-speed signals (e.g., MIPI), 5.8 GHz radio frequency (RF) noise
[Carbon Fiber Frame] ─ Electromagnetic shielding and signal reflection characteristics
1. Electrical noise (high-frequency EMI from components)
Inside the airframe, there are many powerful noise sources that can instantly wipe out weak GNSS signals.
- Digital high-frequency noise: Harmonic noise radiated from MCU (STM32, etc.) clock signals, high-speed buses of companion computers (Jetson, etc.), camera MIPI ribbon cables, and SD card write lines interferes with the GNSS L1 band (around 1.575 GHz) and L2 band (around 1.227 GHz).
- Switching noise: Switching frequencies and their harmonics from ESCs (Electronic Speed Controllers) and DC-DC converters.
- Transmitters (VTX/Telemetry): If video/data transmission antennas for the 1.2 GHz, 2.4 GHz, or 5.8 GHz bands are near the GNSS antenna, the front-end LNA (Low Noise Amplifier) will saturate, causing significant sensitivity degradation (desensitization).
2. Physical Shielding (Shadowing)
Because GNSS radio waves have high directivity, they cannot be received if there is an object blocking the signal between the antenna and the satellite. In particular, carbon fiber (carbon fiber reinforced plastic), which is the primary material for drones, is conductive and completely blocks and reflects radio waves. Also, large-capacity LiPo batteries act as massive radio wave shields.
3. Multipath (Reflections from the ground or airframe structure)
If the antenna picks up not only the radio waves arriving directly from the satellites (direct waves) but also radio waves that arrive late after reflecting off the ground or large carbon plates on the airframe (reflected waves), the phase calculation will be incorrect, causing positioning errors of several to tens of meters and leading to a persistent RTK Float state (not fixed).
Countermeasure 1: Measures against electrical noise—Physical isolation and mast structure
The simplest and most effective EMI (Electromagnetic Interference) countermeasure is to "physically separate it from the noise source."
- Adoption of a GPS mast (tower): In industrial drones, the fact that the GNSS module (or the antenna alone) is raised several to over ten centimeters above the airframe by a carbon mast is not a design choice, but a clear noise countermeasure. By moving it upward away from the center of the airframe (the area where the FC, wiring, battery, and camera are concentrated), the impact of radiated noise, which attenuates in proportion to the square of the distance, can be dramatically reduced.
- Utilization of shield cases: The GNSS receiver board itself directly under the antenna, as well as the LNA area, must be covered with a metal shield can. Also, on the FC board side, covering noise sources such as the MCU and DC-DC converters with shield cases suppresses the radiation of noise into space itself.
Countermeasure 2: PCB layout and chassis design improvements—Ensuring a ground plane
To maximize the performance of a patch antenna, it is necessary to place a ground plane of an appropriate size (ideally a solid GND layer) directly under the antenna.
- Gain improvement and reflection blocking by the ground plane: A patch antenna functions as an antenna in pairs with the GND plane directly beneath it. Generally, a continuous GND plane of about 70mm x 70mm (at least one size larger than the antenna) without deformations or slits ensures that the antenna gain is improved and also functions as a shield that physically blocks reflected waves (multipath and noise) from below (inside the airframe or from the ground).
- Prohibition of direct attachment to carbon frames: It is strictly forbidden to attach a GNSS antenna directly to a carbon plate under the assumption that "carbon can serve as a GND." Due to the non-uniform conductivity and impedance characteristics of carbon, the resonant frequency of the antenna will shift, significantly reducing reception sensitivity. Always use plastic or fiberglass (FR4/G10) spacers, or mount a dedicated antenna board (with a GND plane) on the mast.
Countermeasure 3: Redundant GNSS (Dual GNSS) and GPS compass (moving base) design
In industrial and defense UAS where reliability is required, the installation of two GNSS modules (redundancy) has become the de facto standard configuration.
1. Avoiding common failure modes through different frequencies and different manufacturers
When installing two GNSS units, rather than just lining up identical modules, you can increase resistance to specific satellite system anomalies or local jamming (radio interference) by making one "L1 single-frequency" and the other "L1/L2/L5 multi-frequency (RTK compatible)," or by combining receiver chips from different manufacturers (e.g., u-blox and Hexagon/NovAtel).
2. Utilization of GPS Heading (Moving Base RTK)
Magnetic compasses are highly susceptible to magnetic interference from the airframe's metal structure, high-current wiring, and surrounding structures (such as steel frames). Therefore, we design a GPS compass (moving-base RTK) that determines the aircraft's 'Heading' from the relative positional relationship between two GNSS antennas placed in a straight line at a fixed distance (baseline: at least 20–30 cm apart) on the airframe. This enables extremely robust heading estimation that is unaffected by magnetic interference, without using a magnetic compass at all.
An example of a dual GNSS (SPI and UART connection) definition in ArduPilot's hwdef.dat is as follows.
Plaintext
Primary GNSS 1 (Connected via UART4)
UART_ORDER OTG1 UART4
define HAL_GPS_TYPE_DEFAULT 1 # Auto detect
Redundant GNSS 2 (Assumes a multi-module connected via I2C/UART or SPI)
Note: Custom secondary definitions or connections via CAN (DroneCAN) are the current industry standard
define HAL_CAN_DRIVER_DEFAULT 1
Validation of Reception Performance (Log Analysis Method)
Whether antenna placement and noise countermeasures have been implemented appropriately can be quantitatively evaluated by analyzing flight logs (.bin files). Focus on the following parameters in the ArduPilot logs.
- GPS.NSats (Number of satellites tracked): Check if 15 or more satellites (or 25–30 or more for multi-GNSS/dual-frequency) are consistently tracked in an open sky environment.
- GPS.HDop / VDop (Dilution of Precision): Ideally, HDop (horizontal) should be stable at 0.6 or less and VDop (vertical) at 1.0 or less.
- GPA.Delta (Time difference in received data): Check for any update delays.
- C/N0 ratio (Carrier-to-Noise density ratio): This is the most important metric. Check if the C/N0 ratio of major satellites is maintained at 45 dB-Hz or higher (nearly 50 dB-Hz during RTK). If the C/N0 ratio drops by 5–10 dB-Hz or more overall when the motors are spinning (when throttle is increased) or the moment the camera/VTX is powered on, it is evidence that high-frequency noise from those components is leaking into (desensitizing) the GNSS antenna, and additional shielding or mast extension is required.
Summary
The essence of GNSS antenna design lies in how to protect weak signals from space from the 'sea of noise' within the airframe. Physical isolation using a mast, ensuring a solid ground plane directly under the antenna, and introducing a GPS compass (moving-base) to neutralize magnetic interference are essential approaches for ensuring the success of autonomous UAS flight in harsh environments.
In the next installment, [FC12], we will delve deeper into multi-GNSS design (constellation selection and frequency allocation) to efficiently operate multiple satellite systems simultaneously and further enhance anti-jamming performance.