BağLAN RF TOOLS · MARINE
Radio Horizon Calculator: Base Station Range at Sea
Use this radio horizon calculator to see what limits the link between a coastal base station and your vessel. Calculate the radio horizon, Fresnel clearance, PRACH (Physical Random Access Channel) format, 5G TDD (Time Division Duplex) guard period and uplink link budget on a single screen.
Radio Horizon Calculator
Results update instantly as you change the values. Heights are in metres above sea level.
Geometry
Network
Most operator sites run the default format (~14.5 km).
n78, 30 kHz SCS: 1 symbol ≈ 35.7 µs. Typical value: 2.
Uplink link budget
Approximate value for a narrow uplink allocation at the cell edge.
| Parameter | Value | Notes |
|---|
The horizon is the real limit
Path loss over the sea is low; even at 50 km on 700 MHz it is around 125 dB. Most of the time the link is cut by the curvature of the Earth. Raising the vessel antenna increases range directly.
Band choice
Long range comes from low FDD bands (700 MHz n28, 800 MHz B20, 900 MHz B8). Because of the TDD guard period, 3.5 GHz (n78) is limited to ~9 km with typical settings.
Field verification
The TA (Timing Advance) value reported by the modem gives the distance to the serving site: TA × 78 m in LTE. Read it together with RSRP (Reference Signal Received Power) and SINR (Signal to Interference plus Noise Ratio) to see which coastal site is serving you.
TECHNICAL GUIDE
Base Station Range Parameters and Coverage at Sea
Timing Advance, PRACH formats, TDD guard period, radio horizon and maritime applications in GSM, LTE, NB-IoT and 5G NR.
1. The basic relationship
The base station tells the UE (User Equipment: modem, router, phone) how early it must transmit so that its uplink signal fits the base station's time frame. This value is the signal's round-trip time, so it is halved when converted to distance.
Example: the round trip for 40 km is ≈ 267 µs and for 50 km ≈ 333 µs. These values are below one millisecond; a TA value of “1500 ms” would correspond to 225,000 km and does not exist in any system. A parameter in the millisecond range is usually the timeAlignmentTimer: how long the TA stays valid (500, 750, 1280, 1920, 2560, 5120, 10240 ms or infinity in LTE/NR).
2. Four limits that set the range
No single parameter sets a cell's radius. Whichever of the following limits comes first defines the range:
| Limit | What it determines | When it typically dominates |
|---|---|---|
| TA range | The largest delay the protocol can correct | Rarely; in practice not a limit except for GSM's 35 km |
| PRACH format (CP/GT) and Ncs | Whether a distant UE's initial access preamble is detected | The real limit at most sites; default format ~14.5 km |
| Link budget | Especially uplink: UE output 23 dBm (0.2 W) | Over land, in rough terrain and indoors |
| TDD guard period (GP) | Timing of the distant UE at the DL→UL switch | TDD bands such as 3.5 GHz (n78) |
3. GSM
In GSM, TA is given in steps of one bit period (3.69 µs); each step is ≈ 553 m. The 6-bit field takes values 0–63, giving a maximum range of ≈ 35 km.
- Extended cell: Two consecutive timeslots are reserved for the access burst to increase the guard time; depending on the vendor, ~70–120 km is reached. The cost: capacity drops on that TRX.
- GSM900 is still the “last resort” layer for coastal and rural coverage; POS, telemetry and emergency call traffic falls back to it.
4. LTE
In LTE the TA step is 16·Ts ≈ 0.52 µs (≈ 78 m), and the 11-bit command in the RAR (Random Access Response) ranges from 0–1282 (≈ 667 µs, ~100 km). The practical limit, however, is set by the PRACH format:
| PRACH format | CP (µs) | Sequence (µs) | GT (µs) | Max radius | Use |
|---|---|---|---|---|---|
| 0 | 103.1 | 800 | 96.9 | ≈ 14.5 km | Default; urban, suburban, most rural sites |
| 1 | 684.4 | 800 | 515.6 | ≈ 77 km | Long range, sea/desert |
| 2 | 203.1 | 1600 | 196.9 | ≈ 29.5 km | Medium range + better detection (double sequence) |
| 3 | 684.4 | 1600 | 715.6 | ≈ 100 km | Longest range (reaches the TA limit) |
| 4 (TDD only) | 14.6 | 133 | – | ≈ 1.4 km | Small cell, inside UpPTS |
- Ncs / zeroCorrelationZoneConfig: As the radius grows, the cyclic shift grows and fewer preambles can be derived from each root sequence. A 40–50 km cell needs more root sequences; PRACH collisions with neighbour cells need care in planning.
- High-speed set: Intended for large Doppler cases such as aircraft and high-speed trains; vessel speeds do not require it.
- LTE-M has the same timing as LTE; repetitions (coverage enhancement) add up to +15 dB of coverage.
5. NB-IoT
| NPRACH format | CP | Max radius | Note |
|---|---|---|---|
| 0 | 66.7 µs | ≈ 10 km | Default |
| 1 | 266.7 µs | ≈ 40 km | Rural / coastal |
| 2 (Rel-15) | 800 µs | ≈ 120 km | Very long range |
With coverage enhancement of up to +20 dB (MCL ≈ 164 dB), NB-IoT provides the longest reach for low data-rate applications such as buoys, ocean sensors and port telemetry.
6. 5G NR
6.1 The TA step depends on subcarrier spacing (SCS)
In NR the TA unit is 16·64·Tc/2µ; the 12-bit command in the RAR ranges from 0–3846. The theoretical range is wider than LTE, but in practice the PRACH format and the TDD guard period still set the limit.
| SCS | TA step | Distance step | Theoretical range via RAR | Typical band |
|---|---|---|---|---|
| 15 kHz | 0.52 µs | ≈ 78 m | ≈ 300 km | n28 (700 MHz FDD) |
| 30 kHz | 0.26 µs | ≈ 39 m | ≈ 150 km | n78 (3.5 GHz TDD) |
| 120 kHz | 0.065 µs | ≈ 9.8 m | ≈ 37 km | mmWave (FR2) |
6.2 PRACH formats
| Format | Preamble SCS | Max radius | Use |
|---|---|---|---|
| Long 0 | 1.25 kHz | ≈ 14.5 km | Equivalent to LTE Format 0, default macro |
| Long 1 | 1.25 kHz | ≈ 100 km | Long range, maritime coverage |
| Long 2 | 1.25 kHz | ≈ 22 km | Better detection (4 repetitions) |
| Long 3 | 5 kHz | ≈ 14.5 km | High speed (Doppler tolerance) |
| Short A1–C2 | 15/30/60/120 kHz | ~1–9 km at 15 kHz; half at 30 kHz | Small cells, n78, mmWave |
Long formats exist only in FR1. 5G cells that need long range (e.g. n28) use Long Format 1.
6.3 TDD guard period: why n78 is not a long-range band
In TDD the UE transmits its uplink early by the TA amount. The guard period (GP) in the special slot must therefore cover the round-trip time of the farthest UE plus a ≈ 13 µs switching margin (NTA,offset). At 30 kHz SCS one OFDM symbol is ≈ 35.7 µs.
GP symbols carry no data, and because of inter-operator synchronisation they cannot be changed freely at a single site. Result: long range comes from low FDD bands (n28, B20, B8); n78 is a capacity layer.
| GP (symbols) | GP duration | Practical max range | Comment |
|---|---|---|---|
| 2 | 71 µs | ≈ 8.7 km | Typical in common special-slot patterns |
| 4 | 143 µs | ≈ 19 km | Extended GP for rural TDD |
| 6 | 214 µs | ≈ 30 km | Noticeable capacity loss |
| 8 | 285 µs | ≈ 41 km | The order needed for 40–50 km |
| 10 | 357 µs | ≈ 51 km | Not used in practice |
7. Example: bands and typical operator values in Türkiye
In Türkiye, 5G spectrum was awarded in the October 2025 auction in the 700 MHz and 3.5 GHz bands; each of the three operators received 2×10 MHz at 700 MHz, and 5G service has been offered since 1 April 2026. The 26 GHz (mmWave) band was not included in the auction.
| Band | 3GPP | Duplex | Technology | Role |
|---|---|---|---|---|
| 700 MHz | n28 | FDD | 5G NR | Wide area, rural and coastal coverage |
| 800 MHz | B20 | FDD | LTE | Coverage layer; anchor band in NSA |
| 900 MHz | B8 | FDD | GSM / LTE | Coverage, GSM last layer |
| 1800 MHz | B3 | FDD | LTE (GSM) | Main capacity layer |
| 2100 MHz | B1 | FDD | LTE | Capacity |
| 2600 MHz | B7 | FDD | LTE | Urban capacity |
| 3.5 GHz | n78 | TDD | 5G NR | High capacity, short range |
The values below reflect general industry practice; they vary by operator, vendor and site:
| Environment | Band | Cell radius | Antenna height / tilt | PRACH |
|---|---|---|---|---|
| City centre | n78, B3/B1/B7 | 0.2–1 km | 20–30 m, 6–10° downtilt | LTE F0 / NR short or Long 0 |
| Suburban | B3, B20, n28 | 1–5 km | 25–35 m, 3–6° | Format 0 |
| Rural | B20/B8, n28, GSM900 | 5–15 km (up to ~30 km) | 30–50 m, 0–3° | Format 0, Format 2 if needed |
| Coastal / sea | B20/B8, n28, GSM900 | 20–50+ km | Hilltop site, 0° or slight uptilt | LTE F1/F3, NR Long 1, GSM ext. |
Macro sites typically use 58–65 dBm EIRP per sector. Because most sites run the default format, ~14.5 km is the de facto limit; longer formats are enabled only at dedicated sites.
8. Propagation over the sea and the radio horizon
The sea surface is flat and conductive; there are no building or vegetation losses. For example, at 700 MHz and 50 km the free-space loss is ≈ 123 dB, and the plane-earth (two-ray) model gives ≈ 124 dB for 150 m / 10 m antenna heights; both are comfortably within a macro link budget. That is why the real limit over the sea is the line of sight, and a radio horizon calculator is the first step in maritime coverage planning. For field measurements, see 4G/5G signal range at sea.
| Base station (m) | Vessel antenna 3 m | Vessel antenna 10 m | Vessel antenna 15 m |
|---|---|---|---|
| 50 | 36 km | 42 km | 45 km |
| 150 (coastal hill) | 58 km | 64 km | 66 km |
| 400 (mountain site) | 90 km | 95 km | 98 km |
- Near the horizon and Fresnel: Diffraction loss rises quickly near the horizon (≈ 6 dB extra at the horizon line, tens of dB beyond); in practice reliable range is taken as ≈ 70–80% of the radio horizon (details: Fresnel zone and marine antennas). With low antennas the first Fresnel zone touches the sea surface early (the two-ray breakpoint); beyond it loss rises by 40 dB/decade but the link continues.
- Sea reflection: Interference between the direct and reflected rays creates deep fades depending on distance and wave height. Remedy: two-antenna MIMO or height diversity (vertical spacing between antennas).
- Tropospheric ducting: Especially in summer over the Aegean and the Mediterranean, signals can travel hundreds of kilometres. This is why a modem “sometimes connects from very far away”, but it is not reliable coverage; in TDD (n78) it causes remote base station interference (3GPP Rel-16 RIM).
9. Maritime: operator side
- Sectors facing the sea are installed on high coastal hills, using narrow vertical-beam, high-gain (17–18 dBi) antennas with 0° tilt or slight uptilt.
- Access parameters are tuned to the range: PRACH Format 1/3 in LTE, Long Format 1 in NR, extended cell in GSM, large Ncs and a suitable root sequence plan.
- Border coordination: In border areas, frequency coordination with neighbouring countries limits the field strength at the border; the regulatory limit may set the range before the technical one.
- Vessels move slowly, so Doppler is not an issue; high-speed settings (high-speed set, Long 3) are not needed.
- There are dedicated deployments worldwide, e.g. offshore LTE networks installed on platforms in the North Sea.
10. Maritime: vessel side
- Antenna height: By the horizon formula every metre counts; mount the antenna as high as possible and away from mast and radar shadows (marine antenna installation).
- Omni antenna, moderate gain (5–8 dBi): Very high-gain omnis have a narrow vertical beam and lose the signal during roll and pitch. Directional antennas lose their target as the vessel turns.
- MIMO: 2–4 elements; cross-polarised or physically separated placement.
- Cable loss: Place the router close to the antenna. At 900 MHz LMR400 ≈ 0.13 dB/m, LMR200 ≈ 0.29 dB/m; 20 m of LMR200 alone means ≈ 6 dB loss (a quarter of the signal).
- Band locking: Locking to B20/B8 or n28 offshore prevents unstable cell changes between 1800/2600 and n78.
- PLMN locking: Prevents roaming charges from attaching to a foreign network in border areas and international waters.
- Hybrid connectivity: Cellular near the coast, satellite offshore (Starlink Maritime etc.); Peplink MAX routers (Peplink marine solutions) provide automatic failover or link bonding with SpeedFusion. On the antenna side, Poynting marine omni and MIMO antennas are a typical part of this setup; see BağLAN Marine for solution examples.
- UE power class: Cellular modems usually operate at Power Class 3 (23 dBm); since the uplink budget is limited, the uplink often sets the range. HPUE (PC2, 26 dBm) is mainly available in TDD bands such as n78/n41.
11. Practical expectations
With a properly installed vessel setup (high antenna, short cable, low-band priority), typical expectations by distance from shore are:
12. Field checklist
- Calculate the radio horizon for the target distance; multiply by 70–80% for a reliability margin.
- Find out the band (low FDD band?) and the PRACH format of the coastal site providing coverage; if it is Format 0, ~14.5 km is the de facto limit.
- Raise the vessel antenna, shorten the cable; configure low-band priority on the router and a PLMN lock if needed.
- Read RSRP, SINR and TA together in the field: TA × 78 m (LTE) gives the approximate distance and confirms which site you are connected to.
- If regular use beyond 35 km is expected, plan a hybrid setup with satellite.
13. Abbreviations
| Abbreviation | Expansion and meaning |
|---|---|
| 3GPP | 3rd Generation Partnership ProjectBody that writes mobile communication standards |
| ASL | Above Sea LevelHeight measured from sea level |
| BS / BTS | Base Station / Base Transceiver StationCellular base station |
| CP | Cyclic PrefixGuard copy at the start of a symbol or preamble |
| DL / UL | Downlink / UplinkBTS → device / device → BTS direction |
| EIRP | Effective Isotropic Radiated PowerTransmit power plus antenna gain |
| FDD | Frequency Division DuplexSeparate frequencies for DL and UL |
| FR1 / FR2 | Frequency Range 1 / 25G below 7.125 GHz / mmWave |
| GP | Guard PeriodDL→UL switching gap in TDD |
| GSM | Global System for Mobile Communications2G mobile system |
| GT | Guard TimeEmpty time at the end of a PRACH preamble |
| HPUE | High Power User EquipmentPower Class 2 device, 26 dBm |
| LOS | Line of SightUnobstructed path between antennas |
| LTE | Long Term Evolution4G mobile system |
| LTE-M | LTE for Machines (Cat-M1)LTE variant for machine communication |
| MCL | Maximum Coupling LossLargest path loss a link can tolerate |
| MIMO | Multiple Input Multiple OutputMulti-antenna transmission |
| mmWave | Millimeter WaveBands above 24 GHz |
| Abbreviation | Expansion and meaning |
|---|---|
| NB-IoT | Narrowband Internet of ThingsNarrowband cellular IoT |
| Ncs | Cyclic Shift (NCS)Preamble cyclic shift value |
| NPRACH | Narrowband PRACHNB-IoT random access channel |
| NR | New Radio5G radio interface |
| NSA | Non-Standalone5G anchored on LTE |
| OFDM | Orthogonal Frequency Division MultiplexingMulticarrier modulation used in LTE/NR |
| PC | Power ClassDevice output power class |
| PLMN | Public Land Mobile NetworkMobile network (operator identity) |
| PRACH | Physical Random Access ChannelInitial access channel |
| RAR | Random Access ResponseBase station reply carrying the TA |
| RIM | Remote Interference ManagementTDD remote interference handling |
| RSRP | Reference Signal Received PowerReceived reference signal level |
| SCS | Subcarrier SpacingOFDM subcarrier spacing |
| SINR | Signal to Interference plus Noise RatioSignal quality metric |
| TA | Timing AdvanceUplink timing offset ∝ distance |
| TDD | Time Division DuplexSame frequency, time-shared DL/UL |
| TRX | TransceiverTransmitter-receiver unit |
| UE | User EquipmentModem, router or phone |
Frequently asked questions
How is the radio horizon calculated?
In a standard atmosphere (k = 4/3) the radio horizon is d (km) ≈ 4.12 × (√hBS + √hvessel), with heights in metres. For example, between a coastal site at 150 m and a vessel antenna at 10 m the radio horizon is ≈ 64 km.
How far does 4G/5G reach at sea?
With a properly installed vessel setup, expect reliable service at 0–15 km and good low-band service (800/900 MHz, 700 MHz n28) at 15–35 km. 35–60 km is only possible with specially configured high coastal sites; beyond 60 km satellite is required.
How do I get distance from the Timing Advance (TA) value?
In LTE each TA step is ≈ 78 m; multiply the TA value reported by the modem by 78 m to get the approximate distance to the serving base station. In GSM the step is ≈ 553 m.
Why does 3.5 GHz 5G have short range at sea?
3.5 GHz (n78) is a TDD band; the guard period (GP) between downlink and uplink must cover the round-trip time of the farthest device. With a typical 2-symbol GP the practical range is ≈ 9 km. Long range comes from 700/800/900 MHz FDD bands.
Sources: 3GPP TS 36.211 / 36.213 (LTE physical layer, PRACH, TA), TS 38.211 / 38.213 (NR), TS 36.331 / 38.331 (timeAlignmentTimer), TS 45.010 (GSM timing), ITU-R P.453 and P.526 (refraction, horizon, diffraction), 3GPP TR 38.866 (remote interference management). Türkiye 5G auction: news reports of the BTK auction results (October 2025). In the calculator, path loss is the larger of the free-space and plane-earth (two-ray) models; transient propagation such as ducting is not included. Values are rounded; operator and vendor parameters should govern site design.