July 17, 2018
|By: Jim Harrison
Guest Blogger, Lincoln Technology Communications
The Global Positioning System (GPS) is a wonderous thing. It helps us navigate from here to there, lets our friends know where we are, and finds my iPhone when I lose it under the bushes in the garden.
It also could allow the authorities to know not only where we are at any given moment, but also everywhere we have been, for, like, forever. But let’s not go there right now. Note, however, that recently the Supreme Court issued a decision requiring police to obtain a warrant from a judge in order to track individuals through cellphone records. So that tracking is a bit harder for government now.
Whenever you're driving, GPS is listening. That’s how your car navigation system gets traffic info: by reading everyone’s speed on the roadways with GPS, constantly.
I use a hiking app on my iPhone (AllTrails). If I download the trail map before heading out, I can keep track of where I am on that trail even though I have no Wi-Fi or cellular connection. That’s because I do receive GPS. How much power will the GPS receiver take and, therefore, how long can I hike or backpack using the guide? It varies, but certainly a whole day. Operating current for GPS chips is around 15mA—but they stay in standby (< 1mA) most of the time.
Global Navigation Satellite Systems (GNSS), such as GPS, are an essential element of the global information infrastructure. The free, open, and dependable nature of GPS/GNSS has led to the development of hundreds of applications affecting every aspect of modern life. GPS technology is now in everything from cellphones and wristwatches, to bulldozers, shipping containers, and ATMs.
And perhaps it should be in your next product design. GNSS does not cost much and it allows you, or your customer, to track the device wherever it may go. Should the unit get stolen or lost, you can find it. Almost any camera would like to know where an image was taken and that holds true for many portable measuring devices. But, understand that GPS and other global tracking devices are receive only. So, your device can find out where it’s located, but it must figure a way to transmit this information to you. Automobiles primarily use cellular networks for this connection.
GNSS is a general term describing any satellite constellation that provides positioning, navigation, and timing (PNT) services on either a global or regional basis. GPS is a U.S.-owned GNSS utility with satellites and satellite control systems that enable users. The U.S. Air Force develops, maintains, and operates the space and control systems.
GPS and other global navigation satellite systems (GNSSs), such as Europe’s Galileo, Japan’s QZSS, and Russia’s GLONASS, allow a receiver to determine its position by calculating its distance from three or more satellites. All GNSS satellites—even the oldest generation still in use—broadcast a message called the L1 signal, which includes the satellite’s location, the time, and an identifying signature pattern. This L1 signal is at 1575.42MHz. A newer generation broadcasts a more complex signal called L5 at a different frequency (1176.45MHz) in addition to the L1. The receiver uses these signals to fix its distance from each satellite based on how long it takes the signal to go from satellite to receiver. There are advanced systems that use L5 on the market now that are generally for industrial purposes, such as oil and gas exploration. Mass-market chips that use both L1 and L5 are starting to come to market. At this point, there are about 30 satellites with the L5 signal in orbit, counting a set that only flies over Japan and Australia.
Figure 1. The basic GNSS system. Photo courtesty of www.gps.gov.
The U.S. government currently claims four-meter average horizontal accuracy for civilian GPS. Vertical accuracy is worse. Mind you, that's the worst case. Some devices/locations reliably (95% of the time or better) can get three-meter accuracy. Some high-end cell phones with L5 capability are due out in 2018. They should offer an accuracy of about ±four centimeters.
Some cellphone makers use a technology called Hybridized Emergency Location (HELO) to figure out where you are located if you make an emergency call on the phone. The technology estimates a mobile 911 caller’s location using cell towers and on-device data sources like GPS and WiFi access points. On the iPhone, you can turn off 'location' for any app except this one. Interestingly, cellphones also use barometric data to figure out location.
The MAX2769 GNSS receiver IC from Maxim covers GPS, GLONASS, and Galileo satellites. The chip’s single-conversion, low-IF GNSS receiver yields high performance for a wide range of applications, including mobile handsets. It has the complete receiver chain including a dual-input LNA and mixer, image-rejected filter, programmable gain amplifier, VCO, fractional-N frequency synthesizer, crystal oscillator, and a 50Ms/s multibit A/D. The total cascaded noise figure of the receiver is as low as 1.4dB. The MAX2769 implements on-chip IF filters requiring only a few external components. Its 20-bit sigma-delta fractional-N synthesizer allows the device to tune to a required VCO frequency with an accuracy of approximately ±40Hz.
Figure 2. Top view of the GPS reference design that features the MAX2769 receiver. Reference Circuit 4275.
The IC has two RF inputs for passive and active antennas, dual uncommitted LNAs, and an antenna switch. It uses a 2.7V to 3.3V supply and comes in a 5mm x 5mm, 28-pin thin QFN package with an exposed paddle. It’s also available in die form. An evaluation kit is available. The chip is priced at about $3.60 in 100 quantity.
Another chip from Maxim relevant to GPS designs is the MAX2679B GPS/GNSS ultra-low current low-noise amplifier. This device targets 1575MHz designs for GPS L1, Galileo, and GLONASS with a claim-to-fame of an optimized input-referred 1dB compression point and 3rd-order intercept point. The IC consumes only 650µA while providing an excellent 1.03dB noise figure and 16.5db of gain. It operates from a 1.08V to 1.98V single supply and comes in a 4-bump 0.83mm square WLP package.
Figure 3. Functional diagram for the MAX2679B ultra-low current LNA.
GPS, Galileo, QZSS, and Glonass GNSSs are incredible assets that we engineers should be taking maximum advantage of. The free, open, and dependable GPS/GNSS systems are currently used in hundreds of applications affecting aspects of almost every person on the planet and many aspects of the majority of us.