Update: GPS/GNSS Interference From Iridium Data Transmissions
The Iridium satellite communication system broadcasts in the
1610 to 1626.5 MHz band. UNAVCO Inc. uses iridium communication links to
download GPS data from remote GPS stations. The L1 frequencies broadcast by GPS,
Galileo and GLONASS satellites are 1575.42 MHz, 1575.42 MHz and 1602 MHz + n × 0.5625 MHz, respectively (each GLONASS satellite uses a unique frequency).
The proximity of the Iridium frequency band with the L1 frequencies of the GPS,
Galileo and GLONASS systems leaves GNSS receivers susceptible to interference
from Iridium data transmissions. Interference from Iridium transmissions can
cause cycle slips and loss of lock on the carrier and code phases, thereby
degrading the quality of GNSS observations. In 2008, UNAVCO staff members observed that the % of slips/#
of observations increased as the distance between a Trimble GPS choke ring
antenna (TRM29659.00) and an Iridium antenna decreased. From those observations
they suggested that Iridium and GPS antennas should be separated by >30 m to
minimize cycle slips caused by the interference from Iridium data transmissions.
A second test conducted in 2/09 using a newer Trimble GNSS choke ring antenna (TRM59800.00)
and a Trimble NetRS receiver showed similar results to the previous test
despite the wider frequency range of the newer antenna. Cycle slips were
observed to be decreasing as the GPS antenna was placed further away from a
bank of Iridium antennas. Cycle slips decreased to zero over an hour-long session
when the GPS antenna position was a distance of 15 m from the bank of Iridium
The purpose of this update is to present new observations
showing GNSS tracking interference from Iridium data transmissions. These results
show that two Galileo enabled receiver models (Leica GR10 and Trimble NetR9),
when connected to GNSS compatible antennas, have increased sensitivity to
interference from Iridium transmissions compared to older receivers (NetRS, NetR5,
NetR8, etc.). Frequent loss of
lock indicators and ionospheric delay slips were detected by UNAVCO’s
pre-processing software, TEQC, with both the GR10 and NetR9 receivers.The distance between the Iridium transmission
source and the GNSS antenna was ~35 m for all of the observations. Our results indicate that the old
recommendation of a >30m antenna separation will not prevent interference
from occurring with newer Galileo enabled receiver designs. Future testing will
be necessary to determine the minimum antenna separation distance to prevent Iridium
interference from degrading the quality of GNSS observations. We will also need
to search for alternates to antenna separation for mitigating the RF interference,
as large separation distances may be impractical in the field.
Antennas: A mix of ~10 iridium antennas (green stick, puck and marine
antenna).These antennas are
transmitting data all of the time.
The observations used in this report were collected on the
roof of the UNAVCO facility. The GNSS antenna test location is shown below with
a red circle. The approximate location of the Iridium antenna bank is indicated
with a yellow circle. The separation of the antennas is ~35 m.
GPS Data Processing
The GNSS data in this report were processed using TEQC.
Leica files were translated from the Leica .m00 format to RINEX using TEQC.
Trimble data must first be translated using the Trimble translator program runpkr00
before being translated to RINEX using TEQC. Navigation files were extracted
from the data using the +nav flag. We used the +qc flag to quality check each
RINEX file.For this report we
only use the TEQC summary file with the .10S filename extension. We use two pieces of information from
the summary file to detect possible iridium interference in our GNSS data, the
“qc plot” and the obs/slip number.
All of the receivers in this report were set with 0 degree
elevation mask. Both the number of sessions and the frequency of the data vary
from receiver to receiver. Although, there were primarily 15 sec, 1 Hz and 5 Hz
Iridium Off vs. On
on at approximately 22:30:00
Receiver Type: Trimble NetR9
Antenna Type: Trimble GNSS Choke
This quality check plot from TEQC shows a large increase in
both ionospheric delay (phase) slips (I) and loss of lock indicators (L) after
the Iridium system was turned on at ~22:30:00. This is from a 1 Hour session
with a 5 Hz measurement interval. Both GLONASS and GPS signals were affected. TEQC
results from a Leica GR10 receiver sharing the same antenna via a splitter showed
a similar increase in ionospheric delay slips and loss of lock indicators after
the Iridium system was turned on.
Testing Antenna Types
Iridium: Was transmitting for the duration of this session
We tested several antenna models commonly used by UNAVCO to
determine which are more susceptible to interference from Iridium signals. We
observed a significant decrease in both loss of lock indicators (L) and
ionospheric delay slip indicators (I) for tracked GPS satellites when we
changed the antenna from a L5 and Galileo enabled Trimble Zephyr Geodetic 2 to the
older Trimble Zephyr Geodetic antenna. This decrease occurred in recorded observables from both the
GR10 and NetR9 receivers. Other antenna/receiver combinations were also tested
and those tests are summarized in Table 1.
Loss of lock and ionospheric delay slip indicators in the
GLONASS observables did not decrease as significantly from the antenna change.
We suspect that the proximity of the GLONASS L1 frequency to the Iridium
frequency band makes it more susceptible to interference, even when using an older
narrower band antenna.
A 30 m distance between a Galileo and L5 enabled
receiver/antenna and an Iridium antenna is not “safe” and the GNSS data may be
significantly degraded by the RF interference.
A new “safe” distance must be determined for
Galileo/ L5 enabled receiver/antenna combinations when they are near an Iridium
transmitter to prevent adverse effects to the data.
We suspect that the newer Galileo/L5 enabled
receivers/antennas have a much wider bandwidth making them more susceptible to
interference from other communication bands.
An inline notch-type filter that rejects 1616-1626.5 MHz will reduce interference from Iridium
transmissions. However, this may reduce the receiver’s ability to track certain
GNSS signals (e.g. GLONASS).
Using an older non-Galileo/L5 antenna in
combination with a Galileo and L5 enabled receiver significantly reduces the
interference from Iridium transmissions in the GPS data. It does not; however,
significantly reduce interference from Iridium transmissions in the GLONASS
We suspect the improvement in the GPS data when
using an older antenna is due to the narrower bandwidth of older antennas.
Using a 20 dB attenuator does not reduce
interference from Iridium transmissions.
Other receiver brands and models may need to be
tested for susceptibility to interference from Iridium signals. It is likely
that any Galileo/L5 enabled receiver may be susceptible to increased
interference from Iridium transmissions.
This table shows receiver/antenna combinations with known
increased sensitivity to interference from Iridium Transmissions. A distance of
30 m between iridium and GNSS antennas with these combinations will not prevent the degradation of GNSS
Trimble GNSS Choke
Trimble Zephyr Geodetic 2
Trimble Zephyr Geodetic
(Rev 2 board)
Further testing of additional receiver models and mitigation techniques is ongoing.
Comment #1 (Posted by shylohjacobs
Thank you for sharing this information! I find it to be very useful! I want to take my <a href="http://www.ar10t.com">ar10</a> for a spin but with the area I have in mind, I need some major GPS training and experience. Thank you again, I will keep the things you have said here in mind.
Comment #2 (Posted by max_kaufman
We have RF interference affecting a NetR9 and Zephyr2 on the NW side of Kachemak Bay, Alaska. We are near a cell tower, but don't know about any nearby IRIDIUM antennae.
We will likely change the receiver/antenna combo. Getting lots of cycle slips, especially at 5 min past the hour, hourly.
Comment #3 (Posted by Frederick Blume
At the Kachemak Bay station mentioned in Comment #2, an Iridium modem was found to be present at a nearby SNOTEL station. It's operator confirmed that the unit was set to transmit data hourly at 5 minutes past the hour. Separation between the Iridium and GNSS antennas is approximately 75 meters.