LR1110
Application Note:
GNSS Antenna
Performance Optimization
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Table of Contents
1.
Overview ................................................................................................................................................................................ 5
2.
The Fundamentals of GNSS with LR1110 ................................................................................................................... 6
2.1
GNSS Scan Principle of Operation ......................................................................................................................... 6
2.2
GNSS Scan Mode RF Performance ......................................................................................................................... 6
2.3
Satellite Visibility .......................................................................................................................................................... 7
2.4
GNSS Performance Metrics....................................................................................................................................... 8
2.4.1
Received Signal Strength Indicator (RSSI) ................................................................................................. 8
2.4.2
Output SNR........................................................................................................................................................... 9
2.4.3
Carrier to Noise Density Ratio: C/N0 ........................................................................................................... 10
Aside: GNSS Signal Power Conversions ........................................................................................................................... 10
2.5
3.
Number of Space Vehicles Visible........................................................................................................................ 10
Antenna Placement and Environment ..................................................................................................................... 11
3.1
SV Location and Dilution of Precision ................................................................................................................ 11
3.2
Radio Environment & Weather Conditions....................................................................................................... 12
4.
Antenna Specifications & Selection ........................................................................................................................... 13
Aside: Polarization, Gain and Polarization Loss ............................................................................................................. 13
4.1
Antenna Polarization ................................................................................................................................................ 14
4.2
Antenna Gain............................................................................................................................................................... 14
4.3
Antenna Radiation Efficiency ................................................................................................................................ 15
5.
Antenna Selection ............................................................................................................................................................ 16
5.1
Ceramic Patch Antennas ........................................................................................................................................ 16
5.2
PCB Antennas .............................................................................................................................................................. 16
5.3
Miniaturized Ceramic Antennas ........................................................................................................................... 17
5.4
Antenna Selection Summary ................................................................................................................................. 18
6.
General GNSS Antenna Design Considerations..................................................................................................... 18
6.1
Board Size ..................................................................................................................................................................... 18
6.2
Antenna Placement .................................................................................................................................................. 19
6.3
Filtering ......................................................................................................................................................................... 21
7.
Experimental Setup to Verify GNSS Antenna Performance .............................................................................. 22
7.1
The Setup ...................................................................................................................................................................... 22
7.2
The Measurement Process ..................................................................................................................................... 23
7.2.1
Autonomous Mode Testing.......................................................................................................................... 23
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7.2.2
Assisted Mode Testing ................................................................................................................................... 24
8.
Conclusion........................................................................................................................................................................... 25
9.
Useful Links ......................................................................................................................................................................... 26
10.
References ........................................................................................................................................................................... 26
11.
Revision History ................................................................................................................................................................. 26
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List of Figures
Figure 1. The LR1110 Geolocation Process ............................................................................................................................ 6
Figure 2. Assisted mode (green and blue) gives better SV visibility than autonomous mode (blue) .............. 7
Figure 3. GNSS Signal Power Metrics ....................................................................................................................................... 8
Figure 4. Measured RSSI Response of the LR1110 with External 2 dB NF LNA ......................................................... 8
Figure 5. Output SNR Response of the LR1110 to both GPS (left) and Beidou (right) Signals ............................ 9
Figure 6. Example of satellite geometry influencing localization accuracy ............................................................. 11
Figure 7. Ground Clutter Reflections Reversing the Polarity of Circular Polarization and reducing
Satellite Visibility ........................................................................................................................................................................... 11
Figure 8. An example of the Space Weather Forecast from [3]. ................................................................................... 12
Figure 9. Schematic and views of the live test setup ....................................................................................................... 14
Figure 10. Localization is reliably possible down to -3 dBic .......................................................................................... 15
Figure 11. An example passive GNSS patch antenna from Taoglas [5]. .................................................................... 16
Figure 12. An example PCB flex antenna from Molex [6]. .............................................................................................. 16
Figure 13. The LoRa Edge Tracker features a printed folded F-antenna. .................................................................. 17
Figure 14. An example ceramic antenna from Johanson Technologies [7]. ............................................................ 17
Figure 15. Comparison of antenna gain vs board size..................................................................................................... 18
Figure 16. Example omnidirectional (left) and directional (right) radiation patterns. ......................................... 19
Figure 17. Antenna placement and orientation................................................................................................................. 20
Figure 18. A SAW filter is recommended if the radio may operate close to other radio systems .................... 21
Figure 19. LR1110 GNSS Testing Setup ................................................................................................................................. 22
Figure 20. The results of a GNSS scan using the EVK ........................................................................................................ 23
Figure 21. Assisted mode: Number of visible SVs for a PCB (top) vs Ceramic Patch Antenna (below) .......... 24
List of Tables
Table 1. RF performance of the LR1110 by GNSS constellation and operating mode ........................................... 6
Table 2. Antenna Cross-Polarization Loss............................................................................................................................. 14
Table 3: Performance of common antenna types ............................................................................................................. 18
Table 4. The signal strength scale of SNR and C/N readings ......................................................................................... 23
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1. Overview
One of the revolutionary features of the LR1110 is the ability to perform localization using the GPS
and Beidou constellations of global navigation satellite systems (GNSS) at much lower consumption
than a conventional GNSS receiver. This is accomplished by performing a GNSS scan and relaying the
received satellite data to a cloud service that decodes and then solves the location of the LR1110.
Because this process is much lower energy consumption, different RF performance constraints apply
when compared with conventional and legacy GNSS receivers. These performance considerations
extend also to the antenna, for which specific performance limitations apply.
The localization performance of the LR1110 is dependent upon the performance of the GNSS
antenna, the placement of the antenna and the attendant sky view (so satellite visibility) that
placement affords.
In this application note we take you through the antenna performance required for GNSS operation,
how to select a suitable GNSS antenna and how to place your antenna in your final application.
Moreover, we introduce practical measurement setup and performance metrics that can help you
evaluate and improve your design.
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2. The Fundamentals of GNSS with LR1110
2.1 GNSS Scan Principle of Operation
The GNSS scan briefly receives and records the satellites visible to the LR1110 and their timing
information, then offloads the localization calculation to the cloud as illustrated below.
Geolocation
Solver
NAV Message
GNSS
LoRa
LR1110
Almanac
Time
Coarse Location
Gateway
Figure 1. The LR1110 Geolocation Process
2.2 GNSS Scan Mode RF Performance
There are two GNSS scan modes available to the LR1110: autonomous mode and assisted mode,
each with different requirements and performance levels. For the solver to determine an
approximate location, autonomous mode does not require any information other than the NAV
message that is output by the LR1110.
Alternatively, assisted mode requires information, including an estimate of the position to within
150 km (for example this could be the location of the closest LoRa gateway or the last known
position), the current time (accurate to within 10 to 30 s and a maximum of 120 s) and satellite
almanac data (which must be less than 15 weeks old). This information must be transmitted to the
LR1110, but improves the receiver sensitivity.
The additional information provided to the receiver in assisted mode translates into a significant
increase in the GNSS receiver sensitivity: the sensitivity performance in each mode by GNSS
constellations are shown below.
Table 1. RF performance of the LR1110 by GNSS constellation and operating mode
Autonomous Mode
Assisted Mode
GPS
-134 dBm
-141 dBm
Beidou
-131 dBm
-138 dBm
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2.3 Satellite Visibility
The enhanced RF sensitivity performance of the LR1110 in assisted mode is important because it
permits the reception of more heavily attenuated space vehicle (SV) signals. As illustrated in Figure 2,
assisted mode increased sensitivity (green and blue) gives better SV visibility than autonomous
mode (blue).
-134 dBm
-141 dBm
Figure 2. Assisted mode (green and blue) gives better SV visibility than autonomous mode (blue)
This ability to receive more satellites has two effects:
1. The number of visible SVs is increased. A minimum of 5 SVs required (but 8 to 10 is
recommended) to permit a successful localization, as this number increases, the localization
accuracy generally increases (although there are diminishing returns for high numbers of
visible SVs).
2. The quality of the GNSS signals increases, an increase in the power of the received signal
relative to the noise in the channel, the signal to noise ratio (SNR), which improves the
accuracy of the localization estimate.
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2.4 GNSS Performance Metrics
Given the importance of the signal strength received by the LR1110, it is necessary to understand the
signal power metrics available to the LR1110. The diagram below shows the GNSS receiver path of
the LR1110. Three signal power metrics are available: the RSSI, SNR and C/N0.
LR1110
LNA
RF Front End
Gain, NF
Baseband &
Correlator
RSSI
SNRIN
C/N0
Down
Conversion
ADC
Demod &
Processing
SNROUT
Theoretical 1 Hz Noise
Figure 3. GNSS Signal Power Metrics
2.4.1 Received Signal Strength Indicator (RSSI)
RSSI is the received input signal power at the input of the receiver chain – in this case with an
external 2 dB noise figure LNA with a gain of 15 dB.
Because the GNSS signals are received below the noise floor, this quantity will rarely be of use for a
real application or assessment of the device using GNSS signals from a space vehicle. The measured
response of the RSSI is shown below: note that the response is limited by the thermal noise in the 1
MHz wide GNSS channel below -95 dBm.
Figure 4. Measured RSSI Response of the LR1110 with External 2 dB NF LNA
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2.4.2 Output SNR
The output SNR gives a measure of the signal power at the output of the correlator, so the signal
power that is to be demodulated. (This shouldn’t be confused with the SNR at the input of the signal
chain which is a negative value, below the noise floor).
14
14
12
12
10
10
Output SNR [dB]
Output SNR [dB]
The graphs below show the measured output SNR for both Beidou and GPS constellation scans as a
function of the GNSS signal strength. Note also that there is some variability in these measured
signal quantities as evidenced by the ~1 dB standard deviation variability of the measurement. Note
that the RSSI was also subject to the same error (not illustrated).
8
6
4
6
4
2
-144
8
2
-142
-140
-138
-136
-134
-132
-130
-128
-126
-140
-138
Input Power [dBm]
-136
-134
-132
-130
-128
-126
-124
-122
Input Power [dBm]
Figure 5. Output SNR Response of the LR1110 to both GPS (left) and Beidou (right) Signals
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2.4.3 Carrier to Noise Density Ratio: C/N0
This is a popular signal power metric in GNSS systems as it is independent of the bandwidth. The
C/N0 is another signal-to-noise ratio but simply normalized to 1 Hz of bandwidth from the 1 MHz
Aside: GNSS Signal Power Conversions
To convert between the input SNR, output SNR and C/N0 signal quantities the relationship is
straightforward:
C/N0 = SNROUT + 30 dB
SNRIN = SNROUT - 30 dB
Example:
If we take the example the GPS SNR response of Figure 5:
At an input power of -131 dBm we see that we have an SNROUT of +11 dB.
This therefore corresponds to an SNRIN of -21 dB, and a C/N0 of 41 dBHz.
GNSS channel bandwidth.
2.5 Number of Space Vehicles Visible
The final performance metric of interest is the number of space vehicles (satellites) visible from the
constellation under scan. It can be of help to determine the potential quality of a GNSS fix. However,
as we will discuss in Section 3, satellite visibility is affected by many factors, but when combined with
a signal power metric it is useful for characterizing the real-world performance of our GNSS antenna.
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3. Antenna Placement and Environment
3.1 SV Location and Dilution of Precision
It is important to note that more satellite visibility does not always improve the quality of a
localization result. In the example below, we have 2 satellites, each with the lines of constant delay to
the earth’s surface (for a hypothetical GNSS receiver location) projected below onto the Earth’s
surface. In this simplified view, on the left, we see that the intersection of the two distance results
represents the GNSS-derived location. On the right, because of the geometry of the satellites, we see
that this intersecting area is expanded, increasing the geometric dilution of precision (GDOP). In
these figures, the shaded area represents the position estimation error based upon the location of
the two satellites. The geometry of the satellites influences the resulting accuracy despite the
measurement precision of the individual timing measurements being the same in both cases.
Figure 6. Example of satellite geometry influencing localization accuracy
From the image above, two consequences can be inferred. Firstly, at the extremes of latitude, the
satellite constellations can periodically experience unfavorable geometries (for an example see [1]).
Secondly, the placement of the antenna, relative to objects on the ground, could also influence the
resulting visibility, so geometry of the visible satellites, as shown below.
RHCP
LHCP
Figure 7. Ground Clutter Reflections Reversing the Polarity of Circular Polarization and reducing
Satellite Visibility
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Another very important consideration for GNSS systems is the influence of reflected signals. The
signal transmitted by the satellite is sent with a circular polarization to help overcome a
phenomenon known as Faraday rotation which imparts circular rotation on signals traversing the
atmosphere from space.
Consequently, the optimal direct reception of any GNSS signal will require an identical, right-handed,
circular polarization (RHCP). However, as shown in the previous image, indirect GNSS signals that are
reflected also see a reversal of the polarization (so RHCP becomes left-hand circularly polarization,
LHCP). The mitigation of this effect is something to consider when selecting the GNSS antenna.
3.2 Radio Environment & Weather Conditions
Space weather can have a significant impact on the signal strength of GNSS signals passing through
the earth’s atmosphere. It is caused by the sun’s interaction with the Earth’s magnetic field and varies
both cyclically and instantaneously.
The Planetary K index, or Kp-index, is a figure of merit used to characterize the magnitude of
geomagnetic storms, and forecasts of this value are available from various sources including NOAAs
space weather prediction center [3]. Kp is an excellent indicator of disturbances in the Earth's
magnetic field (it maps directly to a specific value of field strength fluctuation) with values above 5
indicating a geomagnetic storm. The influence of such storms is to add unpredictable path length
distortion and attenuation to the GNSS signal, with obvious implications for satellite visibility and
location prediction.
Figure 8. An example of the Space Weather Forecast from [3].
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4. Antenna Specifications & Selection
Aside: Polarization, Gain and Polarization Loss
Polarization influences the power of the signal received by an antenna. To illustrate this, in the
upper images of the figure below we have a hypothetical GNSS antenna under test (AUT). The
AUT have the same 0 dBi gain but only the polarization changes, they are measured by a
rotating linearly polarized measurement antenna (MA). The red trace shows the projection of
the linear and circular polarization on the x-y measurement plane.
RHCP
Linear Polarisation
y
y
y
x
AUT
AUT
x
x
y
AUT
y
0 dBi
Polarisation Direction
Polarisation Pattern
x
y
0 dBi
0 dBi
x
x
In the lower row of images, the blue trace is a plot of the AUT gain seen by the MA.
•
•
In the linear polarization case, we see that the full 0 dBi gain of the AUT is seen when the
polarizations of the antennas are aligned.
In the circular polarization case, we see that a constant gain is seen by the MA, but of -3 dBi.
If we used an RHCP measurement antenna to receive the RHCP signal we would see the full
0 dBi circularly polarized gain. When we use a mix of linear and circular polarization we incur 3
dB polarization loss. There are hence two antenna gain quantities of interest for GNSS
antennas:
dBi which is the gain of the antenna relative to a linearly polarized isotropic radiator, and
dBic which is the antenna gain relative to an isotropic radiator with circular polarization.
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4.1 Antenna Polarization
The polarization of the incoming GNSS signal is RHCP unless scattered or reflected, in which case the
polarization can become corrupted. In most applications where the LR1110 is employed, a direct sky
view is recommended for successful satellite detection and the best localization performance.
The table below shows the polarization loss between the various possible polarization combinations.
Attention should be paid to this when considering antennas for GNSS applications. One of the key
questions for an application is whether it is better to have a circularly polarized antenna and reject
reflected (LHCP) signals, or to have a linearly polarized antenna that will indiscriminately receive all
signals.
Table 2. Antenna Cross-Polarization Loss
Polarisation Loss
[all units in decibels]
Circular
Left-Hand
Right-Hand
Linear
Vertical
Horizontal
Circular
Left-Hand Right-Hand
0
20
20
0
3
3
3
3
Linear
Vertical
Horizontal
3
3
3
3
0
20
20
0
For applications with LR1110 with direct sky view, an RHCP antenna is recommended where
possible.
4.2 Antenna Gain
The antenna gain is a critical specification for GNSS systems using the LR1110, especially where the
lower sensitivity autonomous mode will be used in the final application. Understanding that we
need 5 satellites in view to successfully perform a localization, we derived a minimum antenna gain
specification for LR1110 experimentally.
This specification was measured using the setup below, featuring a Taoglas antenna [4], to assess the
link budget margin needed to establish a given satellite visibility by artificially reducing the link
budget with an attenuator.
Taoglas Passive
GPS Antenna
0 to 20 dB
LR1110
Kit
PC
Data
Collection
Figure 9. Schematic and views of the live test setup
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The resulting gain specification is shown in Figure 10. The satellite visibility versus antenna gain in
dBic for an environment with a clear sky view as a function of the number of visible satellites.
Understanding that we will have higher sensitivity in assisted mode, we nonetheless propose the
same gain specification for both assisted and autonomous modes of operation. The rationale for this
is twofold:
i)
ii)
Assisted mode still needs to see at least one satellite with the signal strength required for
operation in autonomous mode.
The higher link budget performance of assisted mode can still be employed to provide a
faster and higher precision localization.
10
9
8
7
Number of Visible Space Vehicles
6
5
4
3
2
1
0
-10
-8
-6
-4
-2
0
2
Antenna Gain [dBic]
Figure 10. Localization is reliably possible down to -3 dBic
4.3 Antenna Radiation Efficiency
The efficiency of the antenna is the ratio of the power radiated by the antenna to the input power, in
the case of a transmit antenna. By reciprocity this also applies to the antenna in receive operation.
Typically, efficiencies below approximately -3 dB (50%) are not recommended for GNSS designs.
As with any of the figures of merit and design parameters mentioned here, because efficiency of
often specified for a specific board size, care should be taken that the datasheet efficiency is still
realized in the final application environment.
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5. Antenna Selection
Although many antenna types and technologies are available, the three most commonly employed
in miniaturized applications are ceramic patch antennas, PCB antennas and miniaturized ceramic
antennas, each of which comes with its own benefits and limitations.
5.1 Ceramic Patch Antennas
The ceramic patch antenna provides circular polarization and a
typical gain in excess of 2 dBic. This makes it ideal for designs which
will have a reliable orientation of the antenna towards the sky and
where the size of the patch can be accommodated.
Patch antennas typically come in two main types: active and
passive. The active antenna features the benefit of including
additional gain and filtering onboard the antenna which removes
the need to implement the LNA on the PCB. However, the flexibility
to rematch the antenna is lost. The passive antenna, comes at lower
cost but responsibility for the RF signal chain implementation rests
with the designer.
Figure 11. An example
passive GNSS patch antenna
from Taoglas [5].
In both cases, one of the main difficulties with patch antennas
arises where detuning occurs. For a specific design, should the
antenna become significantly detuned because of the placement
within your design, the patch must itself be retuned by
modification of the metallization of the patch and the creation of a
specific part number for your design.
5.2 PCB Antennas
Many antenna types can be implemented in planar form on the
PCB substrate. For brevity we restrict our considerations to two
types of printed antenna: off the shelf types and custom PCB
designs.
Premade off the shelf antennas feature the advantage of not
requiring any initial design effort. Many come connectorized
with a U.FL connection and are self-adhesive to be mounted
within the enclosure of the end device. Whilst low-profile, such
antennas do have a large surface area. They can also typically
require custom matching in a specific application environment.
Figure 12. An example PCB
flex antenna from Molex [6].
Custom PCB antennas will always incur some preliminary
design and simulation or prototyping effort to ensure that the
antenna functions as intended in the end device. Custom PCB
antenna designs can be highly varied but electronic designs
are typically implemented on a multi-layer FR4 design. For our
general comparison we’ll consider the LR1110 LoRa Edge
Tracker reference design.
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The GNSS antenna on the tracker is labelled in the image below. Here we see one of the main
advantages of the custom PCB antenna: although the FR4 substrate does not allow significant
antenna miniaturization, the antenna itself can be tailored to a space constrained design. The cost is
also low – only requiring some additional PCB surface.
GNSS Antenna
Figure 13. The LoRa Edge Tracker features a printed folded F-antenna.
5.3 Miniaturized Ceramic Antennas
Figure 14. An example
ceramic antenna from
Johanson Technologies [7].
This category includes all ceramic antennas that are implemented on a
ceramic substrate, except for large ceramic patch antennas. The main
interest in using a ceramic substrate is the very high permittivity it affords.
This allows for drastic miniaturization of the antenna, as the wavelength
within the substrate material is much reduced. However, the high
permittivity of the substrate does come at the cost of reduced efficiency. It
should also be noted that the performance of miniaturized antennas are
often depend heavily on the surrounding PCB size.
Other benefits include some immunity to detuning and the provision of
good reference designs. However, as with all antennas, care must be taken
to ensure that the antenna remains tuned and matched when
implemented in a custom design.
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5.4 Antenna Selection Summary
The table below gives an indication of the overall performance of the most common antenna types.
Table 3: Performance of common antenna types
Antenna Technology
Typ. Gain
Polarization
Size(mm)
Cost
Ceramic patch (active)
2 dBic
RHCP
25 x 25 x 7.5
*****
Ceramic patch (passive) [4]
2 dBic
RHCP
12 x 12 x 4
****
PCB (flex)
1.1 dBi
Linear
56 x 20 x 0.1
***
0 to 3 dBi
Linear
15.5 x 8 x 0.035
*
1 dBi
Linear
15 x 4 x 3.2
**
PCB (tracker)
Miniaturised Ceramic
6. General GNSS Antenna Design Considerations
6.1 Board Size
The question of circuit board size is extremely important in the consideration of antenna gain. All
antennas are specified for a specific board size. As can be seen in the image below, three different
board sizes using the same miniaturized ceramic antenna produce increasing gain as a function of
size.
The area in red highlights gain below our minimum specification (0 dBi): implying a minimum board
size and limited suitable orientations for some board sizes.
Figure 15. Comparison of antenna gain vs board size
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6.2 Antenna Placement
To minimize unforeseen detuning issues, the placement of the GNSS antenna on the board should
be the first design consideration when laying out the PCB. The antenna pattern must be oriented
towards the sky, without obstruction, when in normal operating orientation.
The two most common antenna patterns in miniaturized applications are weakly directional gain
(such as seen with the small ceramic patch antenna) and omnidirectional gain (for any miniaturized
linearly polarized antenna). In omnidirectional gain, the pattern will feature substantial minima in the
radiation pattern, which must avoid being oriented towards the sky as it could prevent GNSS
reception. Ideally, once placed, the antenna gain pattern should be measured to ensure the correct
orientation.
Figure 16. Example omnidirectional (left) and directional (right) radiation patterns.
With antenna placement and orientation on the end device determined, care must also be taken
during use in the application, that the antenna orientation is still respected, and that the placement
of the antenna is not obscured by proximate objects in the environment.
As we saw in the previous section, the PCB and the antenna component work together to form the
entire antenna. The same is also true of any other conductive objects near to the antenna, including
ground planes, which can also modify the antenna pattern away from the expected form.
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Figure 17 illustrates the simulated influence on a ceramic patch antenna location on a wooden cargo
pallet. On the left we see the stand-alone antenna radiation pattern, the center pattern shows the
radiation pattern of the same antenna but mounted on the red location on the pallet. Here, the
presence of the ground under the pallet and the 90° tilt of the antenna conspire to reduce the
antenna gain by 5 to 10 dB. The rightmost antenna pattern in the antenna pattern when positioned
in the green location. Here, the influence of the ground alone introduces reflections and side lobes
in the antenna pattern, but without compromising the gain at zenith.
Antenna Only
Red Location
Green Location
Figure 17. Antenna placement and orientation
Note also that the material to which the device is mounted can also have an effect on the tuning of
the antenna. Much care should be taken to ensure that the antenna is still correctly tuned and
realized the design antenna gain when housed in the final application packaging and mounted in
the final application environment.
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6.3 Filtering
Most practical applications may operate in or around other radio services, which could cause
interference to the GNSS receiver and reduce its sensitivity.
It is important to realize that the frequency of these interfering links can be far removed from the
GNSS operating frequency yet still have an impact if the interfering power is high enough.
If the LR1110 is known to be operating in proximity to other radio links then the inclusion of a SAW
filter is strongly recommended, following the LNA as shown below. The reason for this placement is
the insertion loss of the SAW filter and its influence on the receiver noise figure.
LR1110
Preselector
SAW
Figure 18. A SAW filter is recommended if the radio may operate close to other radio systems
The noise figure of a radio receiver is a measure of the noise the receiver itself adds to the signal
being received. The noise figure is dominated by the noise figure of the first elements of the receive
chain, this arrangement of SAW and LNA minimizes this impact.
In environments where very high signal powers are expected in close proximity to the GNSS receiver,
a preselect filter can also be used to help prevent saturation of the LNA. These come at the expense
of increased insertion loss and lower sensitivity.
The choice of when to incur the potential size, cost and performance penalties of such filtering is
hence dependent upon the specific application environment in which the radio will be deployed.
Note that no protection is required from the other radios within the LR1110 as they cannot be used
simultaneously.
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7. Experimental Setup to Verify GNSS Antenna
Performance
Antenna optimization can be a real challenge, especially in the case of receiver antennas where there
is no transmitted signal to measure. It can be useful to use GNSS received signal metrics to
determine how well a given design is performing and whether that performance is adequate for your
application.
7.1 The Setup
In the absence of a controlled environment and test signals where an absolute measurement of the
antenna performance can be performed, we instead propose the relative measurement process as
shown below.
alpha
d
Semtech
EVK
DUT
Figure 19. LR1110 GNSS Testing Setup
The principle behind this test setup is to compare the measured Device Under Test (DUT) with the
known performance of the Semtech Evaluation Kit. In this relative measurement the influence of the
environment will be identical in both systems: exposed to both the same SV visibility and the same
radio (interference) and ambient (temperature, humidity) environment. With the evaluation kit
giving ideal reference with which to compare your implementation: allowing a direct performance
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comparison of the number of satellites each system can see and the signal strength seen by each
system.
The DUT should be capable of performing a GNSS scan and outputting the C/N0 (or SNR) of the
satellite constellation seen. A distance “d” greater than 20 cm and a clear sky view with angle to the
horizon, alpha, of at least 30° is recommended. To simplify this operation, the GNSS scan process can
be performed in autonomous mode (assisted mode needs a working internet connection for various
updates).
7.2 The Measurement Process
7.2.1 Autonomous Mode Testing
There are two metrics of interest: the first is the number of space vehicles visible, and the second is
the C/N0 values reported for each satellite. Both of these figures determine the accuracy of the GNSS
localization.
The image below shows the results of a GNSS scan using the LR1110 EVK – these should be
compared with the results of the system under test. Similarly, the modem-E API provides a scan
which can output the number of SVs and the respective signal strength.
Figure 20. The results of a GNSS scan using the EVK
The range of values in Table 4 compares the signal strength with the anticipated quality of the
localization output. As we can see from this scale, the measured values are in the range good-toexcellent.
Table 4. The signal strength scale of SNR and C/N readings
Signal Quality
Excellent
SNR
Above 12 dB
C/N
Available Mode
Above 42 dB-Hz
Assisted / Autonomous
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Good
8 to 9 dB
Poor
2 dB or below
38 to 39 dB-Hz
Assisted / Autonomous
32 dB-Hz or below
Assisted only
Another useful indicator is the lack of any Beidou signals – because the Beidou sensitivity of the
LR1110 is 3 dB lower than GNSS, this could also point to an environmental problem if neither DUT
nor EVK receive such signals.
7.2.2 Assisted Mode Testing
If the device under test is equipped to perform an assisted mode scan, then a direct comparison can
be performed between antenna technologies. The plots below show a 24 hour test during which the
SV visibility was evaluated for the reference Ceramic patch antenna (top) and a PCB antenna
(bottom).
Note the natural variation in the number of SVs that are visible at any given time as the satellites
orbit the earth. For this reason, it is important that all comparative SV visibility tests be performed at
the same time.
Figure 21. Assisted mode: Number of visible SVs for a PCB (top) vs Ceramic Patch Antenna (below)
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8. Conclusion
We have presented the main antenna specifications of interest for a miniaturized GNSS system and
the signal strength metrics available in LR1110. The main factors that can influence the performance
an available link budget of the GNSS link were also highlighted, including some aspects to consider
in the final application environment.
Using the output SNR or C/N0 we showed a practical setup that demonstrated how to compare a
new design against the LR1110 reference implementation of the evaluation kit. Autonomous mode
can be employed for this testing with no requirement for internet connectivity during testing. This
technique can be used to evaluate the performance and suitability of a candidate antenna type to a
specific application and application environment.
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9. Useful Links
LoRa Edge™ Tracker Evaluation Kit Quick Start Guide | DEVELOPER PORTAL (semtech.com)
10. References
[1] Mohd Hafiz Yahya and md Kamarudin, "Analysis of GPS Visibility and Satellite-Receiver Geometry
over Different Latitudinal Regions", January 2008, Conference: International Symposium on
Geoinformation (ISG 2008)
[2] Sreeja, V. Impact and mitigation of space weather effects on GNSS receiver performance. Geosci.
Lett. 3, 24 (2016). https://doi.org/10.1186/s40562-016-0057-0
[3] https://www.swpc.noaa.gov/products/planetary-k-index
[4] https://www.taoglas.com/product/gp-1575-25-2-a-02-gps-1575-42mhz-patch-antenna-252mm2/
[5] https://www.taoglas.com/product/gp-1575-12-4-a-02-2/
[6] https://www.molex.com/pdm_docs/ps/2065600050-PS.pdf
[7] https://www.johansontechnology.com/datasheets/1575AT54A0010/1575AT54A0010.pdf
11. Revision History
Version
ECO
Date
Changes
1.0
060217
Jan 2022
First version
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