The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 or more satellites in medium Earth orbit (MEO). A GPS Receiver is a device that captures those signals and provides accurate location and time information. GPS works in any weather condition, anywhere in the world, 24 hours a day. There is no subscription fees or setup charges to use GPS.

The 24 satellites that make up the GPS space segment are orbiting the earth about 20,180 km above us. They are constantly moving, making two complete orbits in less than 24 hours with speeds of about 14,000 km/hour.
GPS satellites circle the earth in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use trilateration[1] to calculate the user's exact location.

A GPS receiver must be locked on to the signal of at least 3 satellites to calculate a 2-D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3-D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there is no solar power. Small rocket boosters on each satellite help keep them flying in the correct path.

### GPS Signals

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz (10.23 MHz × 154) in the UHF band. Military designated GPS uses the L2 frequency of 1227.60 MHz (10.23 MHz × 120). These signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

Civilian Global Navigation Satellite Systems (GNSS) signals broadcast by the U.S. GPS system, the Russian GLONASS system, the European Galileo system, the Chinese BeiDou system and all of the SBAS systems use Right-Hand Circularly Polarized (RHCP) signals.

### Printed Circuit Board Design

Our chosen receiver is u-blox MAX-M8Q. The datasheet suggests a basic layout design on Page 9[4]. We are going to adapt a Passive Antenna based design. RF trace will be the most crucial part to reproduce on a physical board as it requires exact 50 ohm impedance. It is computed using microwave theory formulae but it depends on Board Stack-Up of the PC Board manufacturer. Then we will decide the topology of the RF front end; whether the LNA comes first or SAW filter or the SAW filter comes in between two LNAs. We will look at Noise Figure calculations after finalizing our LNA and SAW filter.

Components:

Let us see in detail about all the components required to build a working GPS Receiver:

u-blox is a leading manufacturer of high accuracy positioning modules. With it's detailed receiver protocol description and resourceful datasheet, it is the module of choice. There are modules available from other companies like Telit (Jupiter JF2) which follow similar design principles.

We are going to design a GPS receiver based on the u-blox MAX-M8Q module to take advantage of all the above mentioned features. In fact, it should be called a GNSS receiver as it supports almost all the available satellite constellations. Let's look at the features below and then we will dive deep into designing the GNSS receiver.

• 72-channel u-blox M8 engine
• Concurrent reception of GPS/QZSS, GLONASS, BeiDou
• Up to 18 Hz Navigation Rate
• Circular Error Probability (CEP): 2.5m
• Time-To-First-Fix for GPS & GLONASS (Cold Start): 26s (Open Sky)

Antenna

An antenna is used for capturing the weak signals from GPS satellites. Antennas are the main interface between the GNSS Space Segment (satellite constellations) and the User Segment (GNSS receivers), as they are responsible for capturing the L-band signals transmitted from space. Based on the receiver capabilities and below mentioned characteristics, the designer may choose a compatible antenna.

The wavelengths of GPS carriers are 19 cm (L1), 24 cm (L2) and 25 cm (L5) and antennas that are a quarter or half wavelength tend to be the most efficient. So, GPS antenna elements can be as small as 4 cm or 5 cm. Most of the receiver manufacturers use a microstrip antenna. These are also known as patch antennas. Microstrip patch antennas are durable, compact, have a simple construction with a flat, rectangular and low profile.

Due to the characteristics of GNSS systems, receiver antennas are typically Right-Hand Circularly Polarized. Their spacial reception pattern is near hemispherical. This dome shaped pattern with large beamwidth enables user reception of satellite signals in any azimuthal direction, and from zenith to horizon, thus maximizing satellite visibility.

There are two choices while selecting an antenna:

• Active Antenna - It has in built Low Noise Amplifier, thus requiring a power supply to power the internal LNA, hence called active. The supply voltage is fed to the antenna through a coaxial RF cable. Inside the antenna, DC component on the inner conductor will be separated from the RF signal and routed to the supply pin of the LNA. Usually the best choice when the PCB is inside some enclosure and antenna is to be mounted outside the enclosure or if the RF-cable length between the receiver and antenna exceeds about 10 cm. It makes the design of PCB considerably easy as the designer would only need to put a u.Fl or SMA connector on the PCB and it will work without any extra RF components or tuning.
• Passive Antenna - It does not contain an inbuilt Low Noise Amplifier. So, the designer has to include an LNA and other components like SAW Filter in the PCB design. It requires initial testing and calibration. If the PCB is to fitted inside an enclosure, then some modifications may be required to match the center frequency of the antenna with GPS signals (1575.42 MHz). Tuning is usualy done by precisely scratching the top silver electrode of the antenna. Then, there are different types of patch antennas like, ceramic patch, flex, vehicle mount, external, internal, etc.

Design Note: Microstrip patch antennas consist of a metal patch on a grounded substrate. The patch radiates from fringing fields around its edges, and its pattern is maximum at its broadside. Our antenna of choice is the Taoglas Ceramic Patch Antenna CGGBP.35.6.A.02. Some of the features are listed below:

• BEIDOU 1561MHz / GPS 1575MHz / GLONASS 1602MHz
• Center Frequency: 1594MHz +/- 6MHz
• Bandwidth: 57MHz Return Loss @-10dB
• Some major points to consider while designing with a passive patch antenna:

• Effects of ground plane size -strong> As the ground plane size (in sq. mm) is increased, the gain and bandwidth increases. The center frequency will increase in proportion to the ground plane size.[6]
• Shift in Center Frequency -strong> Even the double sided tape below antenna can shift the center frequency down by 1 MHz to 3 MHz. For physical tuning of patch antenna, shape of top silver electrode can be changed or feed-point can be moved. The patch antenna impedance decreases as the feed location approaches the center.
• No components should be placed close to the patch antenna.
• No signal lines should pass under or near the antenna.
• A four-layer PCB with a dedicated ground plane will provide better performance than a two-layer board.

Low Noise Amplifier

GPS signal power levels are weak and below the noise floor at -155 dBm. An RF Apmplifier can help in amplifying these weak GPS signals from satellites.

Design Note: Our application calls for the best available LNA in the market at a good price point and low component count. A good LNA should have low Noise figure (in dB) and high insertion power gain (in dB). Here, we have chosen BGA 725L6 E6327 from Infineon. It covers full GNSS L1 band, from 1550 MHz to 1615 MHz, i.e., support for major Global Navigation Satellite Systems (GNSS) like, GPS, GLONASS and even regional systems like Galileo and Compass (BeiDou).

Features of Infineon LNA:

• Covers full GNSS L1 band, from 1550 MHz to 1615 MHz
• Low Noise figure (NF) = 0.65 dB
• Insertion Power Gain 20.0 dB
• Supply voltage 1.5 V to 3.6 V
• Low current consumption of 3.6 mA
• RF output internally matched to 50 Ω
• Requires only one external SMD component
• Small 6-pin TSLP-6-2 leadless package 0.7 mm × 1.1 mm

Here, VCC = 3.3V
PON = 3.3V (Enable pin is tied to Vcc.)
AI = RF trace from Antenna (Input)
AO = RF trace to SAW Filter (Output)
C1 = DC Blocking Capacitor - Murata GRM1555C1H102FA01D - 1.0nF
L1 = Input Matching Inductor - Murata LQW15AN7N5G00D - 7.5nF
C2 = RF Bypass Capacitor - 1uF (any type)

Surface Acoustic Wave (SAW) filter

It is a Bandpass filter which allows to pass only the required band of signals inside the module. A good SAW filter should have high attenuation and low insertion loss, typically, less than 1.4 dB. An ideal SAW filter should have excellent selectivity and offer high out-of-band rejection performance.

Design Note:: We have chosen B39162B8813P810 (B8813) SAW Filter from EPCOS/TDK. Most of the SAW filters are common from circuit design point of view as they share a similar footprint. So, it is possible to make a generic footprint in the PCB design software and then use any SAW filter which is easily available. GND pad design is important as shown in figure. We will see about the RF trace width and those vias later. properly grounded. In the layout of the front-end evaluation board the suppliers’ recommendations have been followed. See Fig 5, please note that every GND pin has its own ground-via and there is a ground path between the input and the output.

Features of EPCOS / TDK SAW Filter:

• Simultaneous usage of GPS, COMPASS and GLONASS bands
• Usable passbands: 2.0 MHz for GPS, 4.092 MHz for COMPASS and 8.34 MHz for GLONASS
• Center Frequency: 1582.47 MHz
• Maximum insertion attenuation of 1.9 dB
• Package Size: 1.1 mm × 0.9 mm
• No matching network required for operation at 50 Ω

Here,
1 = Input (Unbalanced - 50 Ω)
2 = Ground
3 = Ground
4 = Output (Unbalanced - 50 Ω)
5 = Ground

Backup Battery

A backup battery may be used to store orbital information between operations and to enable faster start-up. If the main supply voltage fails and there is a backup battery available, parts of the receiver switch off but the RTC still runs providing a timing reference for the receiver. This operating mode is called Hardware Backup Mode, which enables all relevant data to be saved in the Battery Backed RAM (BBR) to allow a hot or warm start later.

This is not a mandatory requirement but can be used for hot starts, i.e., getting a fix without waiting for a long duration of time. During application testing, it is suggested to use a backup battery as there can be frequent power supply interruptions (maybe, for testing different power supplies). Without a backup battery, the gps receiver will always perform a cold start and hence will take time to get valid data.

Design Note:: We have chosen MS621 compatible rechargeable lithium battery with a capacity of 5.5 mAh. To charge this battery, follow the circuit from manufacturer's datasheet. We have to decrease the charging voltage as our V_backup voltage is 3.3V. If we add a RB751V40T1G Schottky diode with low forward voltage drop of approximately 300mV[10], we can get the suggested value of charging voltage to 3.0V (3.3V - 0.3V drop). A 1K resistor is also added in series to limit charge current.[11]

Memory (EEPROM)

Configuration settings can be modified with UBX configuration messages or directly with a software called u-center for Windows. The modified settings remain effective until power-down or reset. If these settings have been stored in battery-backup RAM, then the modified configuration will be retained, as long as the backup battery supply is not interrupted. When the backup cattery has discharged, all the configuration settings inside the module will be erased. Hence, to save all the configuration messages by the user like message format and rate of update, an external EEPROM is required.

Design Note:: We can select from different memory sizes. Optimal size would be a 32 Kb (4 KB) Serial I2C memory in SOT-23-5 footprint like 24AA32AT-I/OT from Microchip. There are different packages available with higher pin count but those extra pins are used for address modification to connect multiple memory devices. Default configuration of u-blox modules is to keep all the address bits to be zero (0). In SOT-23-5 package, all address pins - A0, A1, A2 and A3 are internally connected to ground. Thus SOT-23-5 can be the footprint of choice.

### Frequency Range of GNSS Signals

If the application is for a multi-GNSS solution, all the components like, Antenna, LNA and SAW filter must feature a wide range of bandwidths to accomodate the necessary constellation's signals. Different GNSS Frequency ranges are as follows:

GNSS Frequency Range
Frequency Range Bandwidth GNSS Type Centre Frequency
1574.42 MHz to 1576.42 MHz 2 MHz GPS 1575.42 MHz
1559.05 MHz to 1563.15 MHz 4.092 MHz COMPASS 1561.098 MHz
1573.37 MHz to 1577.47 MHz 4.092 MHz GALILEO 1575.42 MHz
1597.78 MHz to 1605.66 MHz 7.88 MHz GLONASS 1602 MHz

As shown in the table above, each GNSS constellation has a different centre frequency with the exception of GPS and BeiDou (COMPASS).

### Selecting a Topology for the RF Section

Before going towards impedance matching of the RF trace, we have to select a topology for the chosen LNA and SAW Filter. As the GPS signal field strength is very low, a high sensitivity of the receiver system is mandatory. This can be achieved by choosing the proper front-end topology and a suitable front-end filter with low insertion loss. Placement of these components is crucial in deciding the performance of the final receiver. The overall noise figure of receiver is primarily established by the noise figure of its first amplifying stage. The noise figure contribution of the subsequent stages is small and have a diminishing effect on signal-to-noise ratio as we will see below.

Noise Figure of cascaded elements. Following figures show the difference between two common topologies for GPS RF Front end:

Each topology has a different noise figure / sensitivity (based on components). First topology has a high gain LNA with low noise figure (thus, low insertion loss).

We can calculate the Noise figure of a cascaded system by using Friis' Equation:

$$F=F_{1}+\frac{F_{2}-1}{G_{1}}+\frac{F_{3}-1}{G_{1}\ast G_{2}}+\frac{F_{4}-1}{G_{1}\ast G_{2}\ast G_{3}}$$

Where, $$F$$ = Total Noise Figure as seen by the Receiver
$$F_{1}$$ = Noise Figure for first stage component,
$$F_{2}$$ = Noise Figure for second stage component, and so on...
$$G_{1}$$ = Gain from first stage,
$$G_{2}$$ = Gain from second stage, and so on...

From the respective components' datasheets, we can find the above required values in dB:

For our chosen LNA from Infineon, Gain and Noise Figure from datasheet are 20.0dB and 0.65dB respectively.

For our chosen SAW Filter from EPCOS/TDK, given the maximum insertion attenuation $$\alpha_{max}$$ of the SAW filter to be 1.9dB (maximum) would result in a "gain" of about –1.9dB (maximum) and a noise figure of 1.9dB. Linear passive devices have noise figure equal to their loss. Expressed in dB, the Noise Figure is equal to -S21(dB). Something with one dB loss has one dB noise figure.

We have to convert the dB values (logarithmic) from datasheet to Linear values (Antilog) using the below equation:

Linear Value $$= 10^{\frac{dB}{10}}$$.

Thus, $$F_{LNA} = 10^{\frac{0.65dB}{10}}$$

Or, $$F_{LNA} = 1.16$$

Similarly, we can calculate linear data for the remaining valuvalueses

$$G_{LNA} = 100.00$$

$$F_{SAW} = 1.55$$

$$G_{SAW} = 0.65$$

Now, we have the values required to calculate Noise Figures of various topologies. Let us consider them one by one.

Topology 1 - SAW-LNA - When SAW filter is connected to the Patch antenna before LNA

$$F_{Total}=F_{SAW}+\frac{F_{LNA}-1}{G_{SAW}}$$

$$F_{Linear}=1.55+\frac{1.16-1}{0.65}$$

$$F_{Linear}=1.80$$

$$F_{Total}=2.55dB$$ (Converting back to logarithmic scale.)

Topology 2 LNA-SAW - When LNA is connected to the Patch antenna before SAW filter

$$F_{Linear}=F_{LNA}+\frac{F_{SAW}-1}{G_{LNA}}$$

$$F_{Linear}=1.16+\frac{1.55-1}{100.00}$$

$$F_{Linear}=1.167$$

$$F_{Total}=0.67dB$$ (Converting back to logarithmic scale.)

Topology 3 - LNA-SAW-LNA - When SAW filter is connected between two LNAs:

$$F_{Linear}=F_{LNA1}+\frac{F_{SAW}-1}{G_{LNA1}}+\frac{F_{LNA2}-1}{{G_{LNA1}\ast G_{SAW}}}$$

$$F_{Linear}=F_{LNA1}+\frac{F_{SAW}-1}{G_{LNA1}}+\frac{F_{LNA2}-1}{{G_{LNA1}\ast{G_{SAW}}}}$$

$$F_{Linear}=1.16+\frac{1.55-1}{100.00}+\frac{1.16-1}{100.00\ast{0.65}}$$

$$F_{Linear}=1.16+\frac{1.55-1}{100.00}+\frac{1.16-1}{64.57}$$

$$F_{Linear}=1.169$$

$$F_{Total}=0.68dB$$ (Converting back to logarithmic scale.)

As we can see, Topology 1 (SAW-LNA) produces a high Noise Figure as compared to the next two topologies, so we can safely ignore the first one. Considering Topology 3 (LNA-SAW-LNA), however, we find that this solution is more robust as even if the minimum insertion loss of SAW filter is higher, since an LNA is connected before SAW filter, its effect will be minimised. As compared to Topology 2 (LNA-SAW) there is not much difference by adding an extra LNA in Topology 3. Hence, we can select topology 2 (LNA-SAW).

### Calculations for RF Trace and Impedance Matching

After finalizing all the necessary components, next step is to calculate the RF trace width. To match the impedance of the RF trace which will carry this 1.575GHz signal is the most important step. If there are any sharp turns (> 45degree) incorporated during board design, then there will be reflections and thereby signal loss. Also, if the RF trace impedance is not matched with 50 ohm, then the maximum power transfer will not take place between driver and load. Thus, it is of utmost importance that we calculate proper trace width for the microstrip line to get as close as possible to 50 ohms.

Smith Charts are used to calculate matching networks like pi-networks to get exact 50 ohm impedance. Here, we will look at a some ready made calculators.

You will need board stack-up from your PC board manufacturer. Without that, further calculations can not be done. Once you get all the details like Dielectric and layer wise thickness of the board, then proceed forward to calculations below.

We have received the following specifications for our four-layer board from our manufacturer:
Dielectric Constant (Er) = 4.3 for a normal FR4 PCB material.
Conductor Height (H) = 0.35mm (Distance from nearest copper plane.)
Trace Thickness = .035mm or 35um for 1oz. copper
Frequency (MHz) = 1575MHz
Characteristic Impedance (Zo) = 50ohm
Trace Width (W) = To be calculated

Both the above calculators show a Trace Width (W) of 0.648mm to be used to achieve 50 ohm characteristic impedance (Zo). Hence, we can conclude Impedance Matching in this section.

Board Stack-Up - For a four-layer board, it is necessary to use a four-layer design for designing GPS receiver as the RF microstrip line requires a continuous ground plane reference below it. An internal layer on a four layer PC board can be reserved for this purpose. Also, RF trace width will be considerably wide if a two layer board is used as the dielectric thickness is much greater (internal reference planes are absent).

Ground Plane Significance - Any ground plane near the microstrip line (on the same layer) should be three times the width of the microstrip line (3 x Width) away as a general rule of thumb.

Via Stitching - As can be seen in the board design image, we will have to use multiple return path vias on the whole PCB. Particularly near the microstrip line, these are of utmost importance. There is a large current density area flow under the return path of RF trace. It can increase chances of reflections. It can even change the characteristic impedance that we planned so carefully.

By adding multiple ground vias, the current density area flow is minimised. It is necessary for return currents that are induced in the top layer. They are given a short path to the underlying ground layer.

Other important components can be chosen as per the application requirements:

• Power Supply - A low dropout regulator is well suited for this application. We have chosen MIC5365 LDO. It is a 3.3V output device with 150mA current output and working voltage range of 3.3V to 5.5V.
• Adding a Logic Level converter on board will help users to connect the GPS receiver to 5V devices as well as with 3.3V devices. UART Tx, Rx pins can be made 5V ready using an on-board mosfet logic level circuitry.
• Power LED - Can be used to indicate that the device is powered up.
• Timepulse LED - It shows the time pulse 1 signal. The LED starts flashing one pulse per second during a GNSS fix. If there is no GNSS fix, the LED will only light, without flashing.

Conclusion: With the above knowledge, you are all set to start designing your own GPS receiver. You can choose from many different manufacturers' gps modules. Just watch out for the RF trace width though.

We designed a GPS receiver following the above design guidelines. The receiver is able to provide position with more than 20 GPS and GLONASS satellites in view.

• 44 square mm Ground Plane for better antenna gain and directivity.
• External I2C EEPROM for receiver configuration storage
• Backup battery for efficient warm start.
• Power Input: 3.3V to 5V Input. Or, If connected to an IMU 9 DOF AHRS, may be powered by a Single-cell Lithium Polymer battery.
• 1 TTL UART port (Tx, Rx pins are 5V ready).
• Power LED - Red and Timepulse LED - Blue
• JST-SH six pin horizontal connector - 1.0mm pitch (EM-406, EM-506 compatibile).
• Dimensions: 44 mm x 44 mm x 8.5 mm
• Weight: 35 grams
• Fully assembled and ready to use.
• Supports direct connection with Explore Labs IMU 9DOF AHRS.
• Supports AssistNow Online and AssistNow Offline A-GPS services.
• Fully assembled and ready to use

Applications:

• Can be used in UAVs or Hot Air Balloon (HAB) projects.
• Can be used for evaluating the ublox MAX positioning module.
• Vehicle Tracking System and as Fleet Management for Logistics.
• Geocaching - Treasure Hunt.

Some Working Screenshots:

Explore Labs GPS / GNSS Receiver can get a 3D Fix using any of the GPS or GLONASS satellites. This can be controlled via the u-center software for Windows. By default, concurrent reception of GPS with SBAS and GLONASS is enabled.

The module here is showing seven different types of messages with tons of information about satellites in view as well as latitude and longitude. The module is capable of providing the data like, number of satellites in view, position, satellite status, speed over ground, date, time, etc. All these message outputs can be configured by u-center for Windows. Messages can be turned on or off. Custom message formats are also available under UBX configuration section[17].

Further Resources: Why use a ready made GPS module when you can design your own receiver from scratch? Find out more at [18] and [19]

References: