LTE Desense of GNSS in Small Trackers

Illustration

LTE transmit bursts can desensitize a GNSS receiver when antenna isolation and front-end filtering fall short. Doubling the Band 13 and Band 14 uplink ranges gives 1554 to 1574 MHz and 1576 to 1596 MHz, which you compare with your GNSS band. Log C/N0 against modem transmit state on your own sample to see the effect.

A tracker that holds a fix on the bench and loses it when the modem sends a report is usually fighting coexistence between two radios sharing a small board. This guide explains the mechanism with the numbers from public documents, shows the arithmetic you can rerun for your own band, and lists what to log on your own sample. It is written for an RF-aware hardware engineer who sees GNSS lock fail or drift whenever the modem transmits.

Source figures are as of October 31, 2026. We have no measured desense data for a specific tracker, so this page reports no test results. It gives the method and a log template.

Why can an LTE transmitter hurt a GNSS receiver at all?

The two radios are on different frequencies, so the cause is not obvious. u-blox explains that even out-of-band signals can affect GNSS receiver performance. If antenna and front-end filter attenuation are not enough, the receiver's automatic gain control (AGC) still regulates the jamming signal, and very high levels can cause non-linear effects in the pre-amplifier stages, which desensitizes the whole receiver (u-blox application note, as of October 31, 2026).

In a compact tracker the LTE antenna and the GNSS antenna can sit a few centimeters apart. That makes isolation between them the first number to look at.

Which LTE harmonics could reach the GNSS band?

A transmitter produces energy at multiples of its frequency. The second harmonic of an uplink at frequency f sits at 2 × f. The table doubles the Band 13 and Band 14 uplink ranges from 3GPP TS 36.101 and lists the results. Compare them with the L1 frequency in the GPS interface specification and with the GNSS band your receiver tracks.

Inputs: uplink ranges from Table 5.5-1 of 3GPP TS 36.101 V14.5.0, published by ETSI (source, as of October 31, 2026). The harmonic columns are the uplink range multiplied by 2.

Whether either harmonic range matters depends on its distance from the GNSS signals you track, on your modem's harmonic level (which the module vendor may or may not publish) and on your filtering.

Rerun it for your own channel

• Harmonic frequency = 2 × f_UL, in MHz, where f_UL is the uplink center frequency of the channel your modem actually uses. Read the channel from the modem's own status report.
• Offset = (2 × f_UL) − f_GNSS, in MHz, where f_GNSS is the center of the GNSS signal you track, taken from its interface specification. A small absolute offset means the harmonic sits close to that signal.
• Worked example (assumed values): an uplink at 790 MHz inside Band 14 gives 2 × 790 = 1580 MHz.

The fundamental can also matter on its own through the front-end effects described above, in addition to the harmonics. That is why the isolation budget below uses the transmit power itself.

How much isolation does the fundamental need?

u-blox gives a design rule for a cellular transmitter next to a GNSS antenna: plan a minimum isolation of 40 dB, and note that about 20 dB is reachable in a handheld design with careful placement, which is not sufficient, so an additional input filter is needed on the GNSS side (u-blox application note, as of October 31, 2026). That guidance is written for GSM, so use it as an analogous example and set the LTE number from your own measurement.

Formula: power at the GNSS port (dBm) = transmit power (dBm) − antenna-to-antenna isolation (dB).

Inputs: isolation values from the u-blox application note. The 23 dBm transmit power is an assumption you replace with your modem's actual maximum for the band and power class. The result is the level before any filter on the GNSS side. Compare it with the input-level figures in your own receiver's datasheet. Those figures are damage limits. Desense starts at lower levels, per the u-blox mechanism above, and has to be measured with the C/N0 test below.

The second row shows the point of the u-blox guidance. With 20 dB of isolation the fundamental arrives at the GNSS port 20 dB higher than with 40 dB, so the filter is carrying the design.

What do the vendors claim?

What breaks in a small tracker?

These items are engineering reasoning from the documents above, with no measured results to report.

• Antennas too close for the isolation you assumed. u-blox says roughly 20 dB is what careful placement gives in a handheld, which falls short of the 40 dB planning value. A tracker is smaller than a handheld, so do not assume you will do better without measuring.
• No filter on the GNSS side. The same guidance says an additional input filter is needed when isolation falls short. A board that relies on placement alone has no second line of defense.
• A band chosen without checking its harmonic. By the arithmetic above, Band 14 uplink harmonics fall in the 1576 to 1596 MHz range. A product that enables a band for coverage reasons should have its harmonic position checked before layout is frozen.
• A vendor suppression figure treated as a guarantee. The Quectel 20 dB statement has no stated test conditions. Your antenna, enclosure and cable routing are different from theirs.
• Testing only with the modem idle. A GNSS sensitivity test with the modem off or attached but silent will not see desense, because the problem appears when the modem transmits.
• Testing at only one channel. The offset from the GNSS signal changes with the uplink channel, so a result on one channel does not cover the others the network may assign.

What should you test on your own board?

Only your hardware shows how much desense you have. Run these on a sample with the final antennas, enclosure and cabling. We have not run them on your product, and this page reports no results. Set the pass line yourself from your product requirements and record the basis for it.

Use this template for the log, one row per capture:

This is an empty template. No values are reported on this page.

What should you decide before choosing a layout and a filter?

• Which LTE bands will the product enable, and what is each band's second harmonic position relative to the GNSS bands you track?
• What antenna-to-antenna isolation can the enclosure give, and what did you assume when you chose the GNSS filter?
• Does the module vendor publish harmonic levels for your bands, and under what test conditions?
• Is a GNSS-side input filter in the design from the start, and what is its insertion loss on the signals you need?
• Which GNSS constellations must the receiver track, since that sets which part of the band must stay clean?
• Where will C/N0 and modem transmit state be logged, and who owns the pass line and the basis for it?
• Which carriers and regions will the product ship to, and which bands do they use?

This page is an engineering summary of public documents. For certification or legal questions, consult the relevant specialists.

Planning an LTE and GNSS tracker and want the antenna layout and filter plan reviewed against your bands? Tell us about your device. For how the work is staged, see how it works, or read about LTE and GNSS tracker development.

Primary sources

All pages below were accessed on 2026-10-31. Confirm figures on the live pages.

• u-blox, GNSS antennas application note (UBX-15030289) (content.u-blox.com), accessed 2026-10-31.
• ETSI, 3GPP TS 36.101 V14.5.0 (etsi.org), accessed 2026-10-31.
• Quectel, LC29H GNSS module product page (quectel.com), accessed 2026-10-31.