Telemetry for remote hydromet stations: link budgets for GSM, LoRa and satellite

A flood station that cannot report is worth less than one that is not installed, because it gives false confidence. The choice of telemetry, how a station's readings reach a server, is therefore a core design decision, not an accessory. This article treats it as engineering: the arithmetic of a radio link, what mountains do to it, the rules on India's licence-free band, how much energy each option costs, and the patterns that keep data flowing when a storm is cutting the network.

The three families of link

India's RTDAS programme combines INSAT satellite telemetry with GSM and GPRS links, for exactly this reason: no single link works everywhere [1].

The link budget

Every radio link follows the same accounting. The power that arrives at the receiver is:

P_rx = P_tx + G_tx + G_rx − L_cable − L_path − L_extra

in decibels, where P_tx is the transmit power, G the antenna gains, L_cable the feed losses, L_path the path loss and L_extra any obstruction or fading. The link works when P_rx exceeds the receiver's sensitivity by a safety margin. Rearranged: the largest path loss you can afford is P_tx + G_tx + G_rx − L_cable − sensitivity − margin.

Free-space path loss

In open air with a clear path, the loss grows with distance and frequency:

FSPL (dB) = 32.44 + 20 · log10(d in km) + 20 · log10(f in MHz)

Each doubling of distance costs 6 dB. A lower frequency loses less, which is one reason sub-GHz links reach further than cellular bands.

What LoRa gives you

LoRa trades data rate for sensitivity using a spreading factor (SF). A higher SF sends each bit more slowly and more robustly. For a 125 kHz channel, typical receiver sensitivities are [2][3]:

Each step up in SF adds about 2.5 dB of link budget and roughly doubles the time on air, so SF12 takes about 25 times as long as SF7 and about 20 to 25 times the energy per transmission [3][4]. The best SF is "not necessarily the highest or the lowest one, but most likely somewhere in between", because retransmissions of failed messages can cancel the saving [3].

A worked budget: +14 dBm transmit power, 5 dBi of total antenna gain, 1 dB cable loss, SF9 (−129 dBm) and a 20 dB fade margin gives an allowed path loss of 14 + 5 − 1 + 129 − 20 = 127 dB. In free space that is about 60 km. Free space is not the limit. Terrain is.

Mountains: Fresnel clearance and obstruction loss

A radio wave does not travel along a line. It occupies an ellipsoid around it, the first Fresnel zone, whose radius at the midpoint of a link of length D is:

r1 ≈ 17.3 · √( D / (4 · f) )       (D in km, f in GHz, r1 in metres)

At 868 MHz, the first Fresnel radius at the middle of a link, and the 60 % of it that should be kept clear of terrain, are:

If a ridge or building cuts into that zone, the signal suffers diffraction loss. A common approximation for a single knife-edge obstruction of height h above the line of sight, at the midpoint of a 5 km link at 868 MHz, is:

These losses compare directly with the 20 dB margin in the example: a ridge 30 m above the line of sight consumes nearly all of it. In a deep valley there may be several obstructions, and the loss adds up. The remedy is geometric: raise the antenna, move the station or the gateway, or add a repeater on a ridge, so the link is two clear hops. The knife-edge model comes from ITU-R recommendation P.526 and is a first estimate, not a substitute for a site survey with a real receiver [5].

India's licence-free band: duty cycle

LoRa in India uses the 865 to 868 MHz band, delicensed for short-range devices. The 2021 notification on low-power equipment in this band lists categories with different limits. The category for tracking, tracing and data acquisition devices, which explicitly includes sensors for water and meteorology, allows up to 500 mW effective radiated power with adaptive power control, channels up to 200 kHz, and a duty cycle of at most 2.5 % for ordinary end devices (10 % for network access points). A non-specific short-range device category is more restrictive, at 25 mW and a 1 % duty cycle [6]. Always check the current notification and confirm how a given product is classified.

A 2.5 % duty cycle means 90 seconds of transmission an hour. With the 20-byte message above:

So even at SF12 a station reporting every 5 minutes is far inside the limit. The limit bites when a station sends frequently, uses long payloads or retransmits repeatedly. Keep messages short (a binary payload of about 10 to 20 bytes, not a text record of 200) and design urgent messages carefully, since a flood is exactly when you want to send more.

Energy: where the watts go

The energy cost of a transmission is power times time on air. From the station power budget in how to specify a river water level station, a cellular modem drawing about 2 W for about 20 seconds per report dominates the budget at roughly 3.2 Wh a day for 288 reports. Most of that 20 seconds is not data: it is the modem registering on the network and setting up a connection. A LoRa transmission of about 0.2 s at about 0.15 W is 0.03 J, which is about 2.4 mWh a day for 288 messages, around a thousand times less [3]. The comparison is only half the story, because the energy has moved to the gateway, which needs power and its own backhaul (usually cellular or satellite). LoRa is attractive where one gateway can serve many stations, so the energy of its backhaul is shared, and where stations are in places that cellular does not reach.

Cellular stays the default when coverage exists, because it needs no infrastructure of your own. Reduce its cost by sending fewer, batched messages in normal conditions and a faster, shorter message when a threshold is crossed.

Reliability patterns that matter more than the radio

1. Store and forward. The logger keeps all readings and sends them in order when a link returns. 30 days of 5-minute data is only 8,640 records.
2. Acknowledged delivery. Use a protocol with acknowledgement (for example MQTT with at-least-once delivery), and let the server remove duplicates by timestamp.
3. Local alarms. The station decides when to alarm and when to switch to a fast reporting rate, without waiting for the server.
4. Retry with back-off. Retrying at once and forever drains the battery and clogs the network. Back off, and cap the attempts.
5. Redundancy at key sites. Two SIMs on different operators, or cellular plus satellite or LoRa, so one failure does not blind the site.
6. Watchdog and reboot. A modem that hangs is common. A hardware watchdog that power-cycles it restores the link without a visit.
7. Health in every message. Battery, panel current, signal strength, temperature and the age of the oldest unsent record.
8. Clock discipline. Synchronise time from the network or GPS and carry it in every record.
9. Antenna and lightning care. The antenna should be as high and as clear as is practical, and cable runs short and protected against surges. Cable and connector losses subtract directly from the link budget.

Choosing: a short decision guide

Whatever you choose, test on site with a real device at the real antenna height, in weather, before fixing the design. A signal-strength reading taken from the road is not a prediction for a station on a riverbank below it.

Where ClimMaTech fits

All ClimMaTech devices report over GSM (2G / 3G / 4G), with LoRa and other options chosen by site, and work from -40 °C to +70 °C. Our data logger keeps recording and runs its alerts locally during an outage, then sends the stored data when the connection returns, and DeviceCloud brings the readings together. For the surrounding design, see RTDAS explained and river monitoring in hill catchments.

References

1. Water Resources Department, Nagaland, National Hydrology Project: RTDAS components (automated rain gauge, river gauge and reservoir stations with INSAT and GSM/GPRS telemetry). wrd.nagaland.gov.in.
2. LoRa receiver sensitivity by spreading factor and link budget (module selection guide). zbotic.in.
3. The Things Network, How spreading factor affects LoRaWAN device battery life. thethingsnetwork.org.
4. Choosing a LoRa module by designing the whole radio link, DigiKey forum (airtime example for a 20-byte payload). forum.digikey.com.
5. ITU-R Recommendation P.526, Propagation by diffraction. itu.int.
6. Government of India, Ministry of Communications, Use of Low Power Equipment in the Frequency Band 865 to 868 MHz for Short Range Devices (Exemption from Licence) Rules, 2021. thc.nic.in.

Worked values (path loss, Fresnel radius, knife-edge loss, airtime and duty-cycle arithmetic, and energy) are computed from the stated relations with stated assumptions. They are illustrations, not product specifications, and should be replaced with figures for the equipment and sites in question.