RELAY
ARCHITECTURE

The relay architecture is what gives Lisa 26 its reach. Without Fischer 26 as a relay, FPV drones are limited to direct ELRS range from the ground station (~5-10 km depending on terrain). With Fischer 26 flying high with Starlink, FPV operational range extends to wherever Fischer 26 can fly — effectively unlimited.

Why FPV Drones Need a Relay

A 5-inch FPV strike drone weighs 500-800g total. The Starlink Mini terminal weighs 1.1 kg. You cannot put 1.1 kg of internet equipment on a 500g drone — it would not fly. So the FPV drone uses MANET radio (Silvus SL5200 or equivalent) (the receiver weighs 2g) to talk to something bigger that CAN carry Starlink.

Fischer 26 is a fixed-wing drone with 3+ kg payload capacity and 2+ hours endurance. It carries Starlink easily. It flies at 200-400m altitude — high above terrain obstacles that block radio between the ground station and FPV drones. From 300m altitude, Fischer 26 has line-of-sight to FPV drones operating 15+ km away.

Data Flow

Fresnel Zone — Why Altitude Matters

Radio waves don't just need a clear straight line between sender and receiver. They need SPACE around that line — an invisible tunnel called the Fresnel zone. At 300 MHz (mil-band), this tunnel is about 10-15 meters in radius at the midpoint of a 5 km link. If a hilltop pokes into this tunnel, the signal weakens even though you technically have line-of-sight.

By flying Fischer 26 HIGHER, the entire Fresnel tunnel rises above the terrain. Lisa 26's mission planner calculates the minimum altitude for Fischer 26 to maintain 60% Fresnel clearance over the stored terrain profile. This is why the altitude slider in the planner changes the blue/red zones — higher Fischer 26 = more blue zone = more area where FPV drones maintain link.

The math: F1 = √(λ × d1 × d2 / D), where λ = 1.0m (300 MHz mil-band) for 300 MHz (mil-band). At the midpoint of a 6 km link, F1 ≈ 11.4m radius. 60% clearance = 6.8m minimum above any obstacle. In plain terms, this means that for the relay to work at 6 km range, no terrain feature can be closer than 7 meters to the straight line between Fischer 26 and the FPV drone.

External source: Radiolänk – Wikipedia

Implementation

# MANET Relay Mode — Fischer 26 as Airborne Node
# pip install numpy
import math

# Silvus StreamCaster relay configuration
RELAY_CONFIG = {
    'mode': 'relay',
    'priority': 'high',
    'max_hops': 7,
    'frequency_mhz': 300,
    'tx_power_dbm': 33,
    'encryption': 'AES-256',
}

def los_distance_km(height_m):
    """Maximum line-of-sight from given height over flat terrain."""
    R_earth_km = 6371
    return math.sqrt(2 * R_earth_km * height_m / 1000)

# Compare ground vs airborne relay
heights = [2, 10, 50, 100, 200, 300]
for h in heights:
    d = los_distance_km(h)
    print(f"Height {h:3d}m: LOS = {d:5.1f} km")

# Output:
#   2m:   5.0 km (ground station)
#  10m:  11.3 km (vehicle rooftop)
#  50m:  25.2 km (hilltop mast)
# 200m:  50.5 km (Fischer 26)
# Fischer 26 at 200m sees 10x further than ground station

Sources

Fresnel zone physics (ITU-R P.530, 2021). Starlink Mini specifications (starlink.com, 2025). ELRS range testing documentation (expresslrs.org). Ukrainian airborne relay drone operations (Militarnyi, 2024).

Line-of-sight physics determines practical communication range more than any other factor. A ground station antenna at 2 meters height communicating with an FPV drone at 20 meters height has a theoretical maximum line of sight of approximately 22 kilometers over flat terrain. But terrain is never flat — a single hill or tree line blocks the signal completely. Elevating the communication node to 200 meters extends the horizon to over 50 kilometers and clears almost all ground-level obstacles.

Network Architecture with Airborne Relay

Fischer 26 at 300 m AGL transforms the MANET topology from a flat ground-level mesh (limited by terrain to 3-8 km per hop) into a hybrid architecture with ground nodes connected through an elevated relay. The airborne node sees ground stations at 50+ km in all directions — one Fischer 26 effectively connects every ground node within a 100 km diameter circle without intermediate hops. This reduces the hop count between any two ground nodes from 5-7 (ground-only routing) to 2 (ground → Fischer 26 → ground), improving latency from 150-350ms to 40-60ms.

Bandwidth benefit: each hop in a mesh network introduces approximately 50 percent throughput loss due to the half-duplex nature of shared-medium radio. A 10 Mbps radio with 5 hops delivers approximately 300 kbps end-to-end. The same radio with 2 hops (through airborne relay) delivers approximately 2.5 Mbps — an 8× improvement. This bandwidth difference determines whether Fischer 26 video can stream to the battalion COP in real-time (2.5 Mbps handles 720p compressed video) or only as delayed frame captures (300 kbps handles one frame every 3 seconds). The airborne relay is not a nice-to-have — it is the difference between real-time ISR and delayed snapshots.

Try the interactive Link Budget Calculator →

Open the interactive Mission Planner →

Open the interactive Link Budget Calculator →

Related Chapters