Signal Strength FAQ
Low Earth Orbit vs Geostationary Satellites
Geostationary Satellites (e.g. Inmarsat)
Geostationary satellites orbit 35,786 km above Earth and travel at the same speed as Earth's rotation — hence why they appear stationary in the sky. If your antenna is designed to communicate with a geostationary satellite, it must be precisely aimed at a fixed point in the sky (the 'look angle'). Once aligned, and as long as the antenna doesn't move, it will maintain a stable connection.
Low Earth Orbit Satellites (e.g. Iridium)
Iridium operates 66 satellites orbiting at approximately 780 km altitude, travelling at around 27,000 km/h. At this altitude, a single satellite takes about seven minutes to pass from horizon to horizon. In most locations, two or three satellites are visible at any given time.
This architecture brings a significant advantage: antennas don't need to be aimed. Iridium antennas are omnidirectional — they can communicate at virtually any angle, and as long as there's a clear view of the sky, one of the passing satellites will handle your transmission.
The trade-off is that if your device doesn't have an unobstructed view of the sky, there will be moments when no satellite is visible, causing gaps in connectivity.
Working with Low Earth Orbit Satellites
Satellite Movement Patterns
Iridium satellites complete a horizon-to-horizon pass in under 10 minutes, and at any given moment the active satellite can be positioned anywhere in the sky — meaning low-elevation connections are a routine part of normal operation rather than an edge case.
Elevation Statistics
The majority of Iridium signal activity happens at relatively shallow angles: 90% of satellite contacts occur below 45° elevation. At equatorial locations, connections are split roughly evenly above and below 22° elevation.
What Does a Clear View of the Sky Mean?
Coverage is a key consideration when choosing a satellite network for your application. At its most basic level, you need to ensure that the network you choose has orbiting satellites that cover the area from which you need to transmit data.
However, being able to provide coverage on paper is not the same as being able to do so in practice. You also need to ensure that your terminal's antenna can communicate with the satellite — and to do so, it needs a clear line of sight.
Optimal installation requirements
An unobstructed line of sight maximises satellite connectivity. For best performance, your transceiver needs a clear 360° view of the sky, with minimal blockages above 8.2° above the horizon in all directions. This applies to trees, buildings, mountains, and similar obstacles. Seasonal changes in vegetation and potential future development in the area should also be factored in.
For permanent installations where obstructions or RF interference may be present, a site survey is recommended.
To quickly assess obstructions in the field, extend your arm with your fist resting on the horizon. Any obstruction should not exceed the height of your fist.
Application-specific interference sources
Specific applications may introduce additional factors to consider during antenna installation. The table below lists potential interference sources by application type.
| Aviation | Military | Maritime | Industrial |
|---|---|---|---|
| Airport radar systems | High-powered radar installations | Ship radar systems | High-power electrical equipment |
| Ground-based navigation equipment | Electronic warfare equipment | Marine VHF radio equipment | Radio frequency welding equipment |
| Aircraft communication systems | Communication jamming systems | Navigation and positioning systems | Industrial process control systems |
| VSAT terminals |
Antenna placement clearance requirements
The antenna requires an unobstructed view of the sky in all directions above the horizontal plane. Beyond that, two specific clearance zones apply:
- Horizontal clearance: Maintain a minimum of 8.0 m (26.2 ft) clear on all sides of the antenna at mounting surface level, minimising obstructions where possible.
- Vertical clearance: At least 2.0 m (6.6 ft) of clearance is required between the antenna and any metallic or conductive surface directly below it.
- Material restrictions: No metallic or concrete obstructions should be present in the area immediately beneath and to the sides of the antenna. Outside this zone, obstructions should be minimised but are tolerable.
The clearance requirements above apply specifically to Iridium Certus IP data connections. For IMT and SBD messaging services, the same general principles apply but the tolerances are considerably more relaxed — antennas can be mounted directly on metal surfaces such as a ship deck or vehicle roof. We do recommend a minimum of 30 cm of clear air between your Iridium antenna and any other RF antenna to avoid co-location interference.
Why Signal Strength Varies During a Satellite Pass
Signal strength changes throughout a satellite pass, and the relationship isn't as straightforward as "higher is better." While basic physics suggests that satellites directly overhead should provide the strongest signals (shorter distance, less atmosphere), real-world performance tells a more nuanced story.
In practice, signals are often strongest at mid-elevations (roughly 15–50°) rather than at zenith. This is primarily due to how ground antennas are designed.
Ground Antenna Patterns
Most Iridium antennas use radiation patterns specifically optimized for lower-to-mid elevation angles. These antennas are engineered to provide peak gain where satellites spend most of their time: between the horizon and mid-sky.
This design choice makes sense: satellites spend the majority of their 7-minute pass at lower elevations, moving relatively slowly across your field of view. They only spend a few seconds directly overhead at zenith. By optimizing antenna gain for 15–50° elevation angles, designers maximize usable communication time during each pass.
What This Means in Practice
- Best performance: 15–50° elevation — where antenna gain is optimized and satellites linger longest
- Good performance: Near zenith (80–90°) — brief but workable, though antenna gain may be reduced
- Minimum performance: Very low angles (<10–15°) — approaching the system's 8.2° minimum elevation threshold
Iridium Satellite Signal Strength
Understanding "Clear View of the Sky" for LEO Satellite Communications
The animation above demonstrates a typical satellite pass. Notice how obstructions like trees degrade signal quality (orange beam), while solid obstacles like buildings and mountains completely block communication (red dashed line).
The Key Takeaway
Obstructions at lower elevations (below ~35°) are particularly critical because they block satellites during the portion of their pass where:
- Ground antenna gain is highest
- Satellites spend the most time
- Communication windows are longest
This is why achieving a clear view of the sky at lower elevation angles — not just directly overhead — is essential for reliable Iridium connectivity.
Frequently Asked Questions
What can block a clear view of the sky?
Common obstructions include:
- Buildings — Urban environments with high-rise structures
- Vegetation — Trees and forests, particularly deciduous trees in full leaf
- Terrain — Mountains, valleys, cliffs, and hillsides
- Structures — Vehicle roofs, ship superstructures, masts
Any object above approximately 8° elevation can potentially block satellite signals.
What's the impact of an obstructed view?
The effects depend on how much sky is blocked:
- Intermittent connectivity — Short messages (SBD/IMT) may need multiple retry attempts; some transmissions will succeed when satellites pass through gaps in the obstructions.
- Reduced throughput — Streaming connections (e.g. Iridium Certus) will experience interruptions and degraded quality.
- Extended gaps — In severely restricted locations (like a deep canyon), you may wait up to 120 minutes between successful connections — the time it takes for Earth's rotation to bring another satellite to your narrow window of sky.
How can I check if I have a clear view?
Our engineers recommend the "crocodile mouth" test:
- Stand where you plan to use the device
- Extend your arms in front of you, hands together
- Raise one arm to create roughly a 30° angle between your hands
- Slowly turn 360°, looking through the gap
- Note any obstructions that break the line of sight
If you see obstacles between your hands at any point, you don't have a completely clear view of the sky. The areas where obstructions appear indicate directions where satellite communication may be interrupted.
The "Crocodile Mouth" Test
First-person view: checking for a clear view of the sky
What are my options if I don't have a clear view?
Several approaches can help:
-
Accept partial coverage — If your application tolerates occasional delays, a compromised view may still work. Messages will get through when satellites pass through clear sections of sky.
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Elevate the antenna — Mounting on a pole or mast is the most common solution, raising the antenna above nearby obstructions.
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Relocate if possible — Even moving a few metres can significantly improve sky visibility.
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Use a relay system — In extreme cases (e.g. deep canyons or mine sites), technologies like LoRaWAN can relay data from a challenging location to a better-positioned satellite transceiver.
Should I check signal strength before sending data?
No. We recommend transmitting immediately and implementing a retry strategy if it fails.
Iridium satellites cross the sky in about 7 minutes, and a signal strength check takes valuable seconds. A satellite showing strong signal may move behind an obstruction before your transmission completes. Conversely, a reading of zero bars doesn't mean failure — another satellite may become visible during the transmission window.
Recommended retry strategy:
- Attempt transmission
- If it fails, retry within 0–5 seconds (repeat twice)
- If still unsuccessful, wait a random 0–30 seconds before retrying (repeat twice)
- If these attempts fail, increase the delay to several minutes
You're only charged for successful transmissions, so failed attempts cost nothing. This approach handles network variability far more effectively than pre-checking signal.
What's the minimum elevation angle for Iridium?
Iridium is designed to work with satellites as low as 8.2° above the horizon. This means any obstruction taller than about 8° elevation in a given direction can block communication when a satellite passes through that part of the sky.
For reference, at arm's length, 8° is roughly the width of your fist held horizontally.
How long does a satellite pass last?
A single Iridium satellite takes approximately 7 minutes to cross from horizon to horizon. During a typical pass, you'll see signal strength rise as the satellite climbs, peak when it's at maximum elevation, and fall as it descends.
In most locations, multiple satellites are visible simultaneously, providing continuous coverage — assuming a clear view of the sky.
We're happy to discuss your project and recommend the best antenna placement and satellite service for your requirements.
Email: help@groundcontrol.com