Mobile ADS-B at the spot: reception with a smartphone, SDR stick and power bank

You are standing at the fence, the camera is ready, and a landing light blinks on the horizon. Is that the A350 you have been waiting for, or just another A320? Flight tracker apps usually answer the question – provided the mobile network cooperates and the data arrives without much delay. With mobile ADS-B reception at the spot you become independent of all that: a small SDR stick, an antenna a few centimeters long, your smartphone and a power bank add up to a pocket radar that plucks the aircraft’s signals straight out of the air. This guide shows you which variants exist, what you really need and how to avoid typical mistakes at the airport.

The short answer: what you need for ADS-B on the go

A mobile ADS-B kit consists of the same four building blocks as a fixed station: antenna, receiver, decoder and power. The only difference is who does the decoding. On Android devices the phone itself can process the raw signals from the SDR stick. On an iPhone, and for everyone who wants to spare their phone, a small computer such as a Raspberry Pi does it and sends the finished map to your display over Wi-Fi.

How ADS-B works in principle – transponders, 1090 MHz, position reports – is explained in the basics article Flight tracking with ADS-B for spotters. For the mobile kit one idea is enough: every aircraft with ADS-B Out broadcasts its position, altitude, speed and identification by itself. Anyone within radio range who has a suitable receiver listens in – with no server, subscription or mobile network.

Why your own receiver makes sense at the spot

Flight tracker apps are convenient, but at the fence they have three weaknesses. First, they depend on the mobile network, and especially at remote spotting positions or on crowded observation decks the connection is often sluggish. Second, the data runs through servers before it reaches you. Depending on the service and subscription, the display can lag a bit behind reality – how that plays out is covered in the article How accurate are ETA predictions on flight tracker sites?. Third, some services filter certain aircraft out of the public view – more on that in the article Why an aircraft does not show up on Flightradar.

Your own receiver, by contrast, shows exactly what your antenna is hearing right now, practically without delay. At the spot that means: you see when the approach turns onto final, when an aircraft lifts off and which registration is just rolling onto the runway – provided the antenna has a clear view. For the question “When do I need to raise the camera?” that is worth its weight in gold.

There is one limitation you should know: aircraft that transmit only Mode S without ADS-B often appear on big flight tracker maps via MLAT. For that, servers evaluate the time differences between several receivers. On your own at the spot you do not have that computing basis, so you miss such positions. The background is in the article MLAT/multilateration explained.

Option 1: SDR stick directly on an Android phone

The leanest setup: antenna on the stick, stick on the phone via an OTG adapter, start the app. Many Android devices support USB OTG, meaning operation as a USB host. Whether yours is one of them you can find in the data sheet or, very practically, by trying a USB flash drive on the adapter.

An example of software for this setup is Avare ADS-B (also known as ADSB Receiver). According to the description on rtl-sdr.com, the app works with common RTL-SDR sticks based on the RTL2832U and an OTG cable, runs entirely on the device and needs neither cloud nor fees. Besides 1090 MHz it can also decode the 978 MHz UAT, which is used only in the US for general aviation and therefore does little for you in Europe, and it offers adjustable gain, bias tee for active antennas, filters, proximity alerts and data recording. There are other Android apps for the same purpose; the selection changes regularly, so before buying a pro version check the current reviews in the store and whether your stick is supported.

One detail makes the difference: the stick draws its power from the phone. Users report that the battery shrinks fast as a result. An OTG cable with an additional charging input solves this, because through it the power bank supplies phone and stick at the same time. When buying, make sure the cable explicitly supports OTG and charging in parallel.

Option 2: Raspberry Pi as a Wi-Fi radar

If you have an iPhone, or want to keep your phone free for camera apps, navigation and the community, you outsource the decoding. A Raspberry Pi with an SDR stick receives the signals, decodes them with readsb and shows the map through tar1090. The phone only has to open a web page. According to the project documentation you reach tar1090 at the Pi’s IP address with the path /tar1090. How to set up both is described in the guide Installing readsb and tar1090.

There are two common approaches for getting the data to your phone:

• The Pi joins your phone’s hotspot. You set up the Wi-Fi at home beforehand. At the spot you switch on the hotspot, the Pi connects, and you open its address in the browser. Advantage: map tiles keep loading over your mobile network.
• The Pi creates its own Wi-Fi network. Then it is completely independent of the network. Stratux comes preconfigured for this, for example – an open-source project for general aviation pilots. According to the project page it runs on a Raspberry Pi, receives 1090 MHz and optionally OGN signals on 868 MHz, opens its own Wi-Fi with a web interface and, according to the developers, is explicitly suitable for use in Europe as well. It is intended for cockpit apps, but its web interface also shows you the traffic at the spot.

Which computers besides the classic one are suitable is compared in the article ADS-B reception without a Raspberry Pi. For mobile use, what counts most is low power consumption, a small case and built-in Wi-Fi.

The two options compared

Criterion	Stick on an Android phone	Raspberry Pi with Wi-Fi
Suitable smartphones	Android with USB OTG	any with a browser, including iPhone
Luggage	stick, antenna, OTG cable	Pi, stick, antenna, cable, case
Setup time at the spot	very short	short if preconfigured
Power needs	from phone battery or power bank via charging cable	separate power bank for the Pi
Phone battery	heavily drained	spared, only display and Wi-Fi
Multiple viewers	only one display	several devices on the same Wi-Fi
Data recording	depends on the app	possible via readsb/tar1090

For a spontaneous trip to the observation deck, option 1 is enough. If you regularly spend whole days at the spot or go with friends who all want to look at the same map, the Pi is worth it.

The antenna: small, but decisive

The stick is quickly bought, but the antenna decides the quality of reception. The good news for on the go: for 1090 MHz the wavelength is about 27.5 centimeters, so a quarter-wave element measures about 6.9 centimeters. If your stick has a telescopic antenna, extend it to exactly that length instead of pulling it out fully. If the element stands on a metal surface – magnetic base on a car roof, tin can, metal plate in the camera bag – that surface forms the counterpoise.

If you like tinkering, you will find building instructions for ground plane and spider antennas in the article Build your own 1090 MHz antenna. For mobile use a few extra rules apply:

• Set it up high and in the open. ADS-B needs line of sight. Even your own car, an earth embankment or your body shadows the antenna. On the lid of the camera bag or on a small pole it works better than in your trouser pocket.
• Short cables. Long, thin coax cables swallow noticeable signal at 1090 MHz. On the go, a short piece between antenna and stick is usually enough.
• Align it vertically. ADS-B is vertically polarized. An antenna lying at an angle receives worse.

How to optimize a fixed location systematically is explained in Finding the best antenna location. On the go, though, you can almost always just walk a few meters up or to the side – an advantage a rooftop antenna does not have.

Gain at the airport: less is more

At home many feeders turn the gain up to catch distant aircraft. At an airport the situation is the opposite. The aircraft roll past you a few hundred meters away, and on final approach they are often only a few hundred meters high. Their signals arrive so strongly that a stick with the gain set too high becomes overloaded: messages get garbled, strong signals drown out weak ones, and aircraft jump around or disappear on the map.

This is especially noticeable with sticks that have a built-in preamplifier. What provides range on the roof is often too much of a good thing next to the runway. Here is how to proceed:

You will find more on gain, filters and preamplifiers in the article Boosting ADS-B range. You normally do not need an additional LNA at the spot – on the contrary, it makes the overload problem worse.

Power: choosing the right power bank

The underrated part of the mobile kit is the power supply. An SDR stick works constantly, and decoding keeps the processor busy. A few basic rules:

• Capacity with reserve. Plan so that the power bank gets phone and receiver through your whole spotting day – including the display you keep switching on, and the camera battery if you charge it over USB as well.
• Stable voltage for the Pi. A Raspberry Pi is sensitive to undervoltage, which can show up as crashes or memory access errors. Use a power bank that can deliver the current the manufacturer recommends for your model, and a short, decent USB cable.
• No auto shutoff. Some power banks switch off when little current flows. With frugal setups this can happen, right in the middle of an approach. Test this at home.
• Frugal hardware. A small, low-power Pi lasts much longer on the same power bank than a powerful model. For ADS-B alone, little computing power is needed.

If you travel to your spotting trip by plane: power banks are subject to their own rules in carry-on luggage, which the article Power banks and lithium batteries on planes summarizes.

How a spotting day with a pocket radar goes

At the spot itself the receiver mainly helps you with timing and selection. A few examples from practice:

• At the Munich Airport visitors’ hill you see on the map which of the two parallel runways the next approach is lining up for, and can reposition in time.
• At the Zeppelinheim viewing platform at Frankfurt Airport the altitude readout helps you tell arrivals from departing traffic.
• At the Affenfelsen in Hamburg you can tell from the registration whether the aircraft just touching down is the special livery you are waiting for.

Which runway is in use depends on the wind – your pocket radar shows you that live. How the choice of runway direction works is explained in Understanding runway direction in use at airports. And if after the spotting day you want to know what you actually photographed, the article Matching sightings with ADS-B logs helps: the recording from your own receiver is an excellent source for that.

Which option suits you?

If you are still undecided which stick suits you, the comparison of SDR sticks for ADS-B helps. Many spotters use the same stick alternately at home and on the go.

What to keep in mind

• Respect the rules at the spot. An antenna on a tripod or a Pi at the fence can prompt questions from security staff. Stay on public areas, do not attach anything to the airport fence and explain in a friendly way what you are doing. What is allowed around the fence is covered in Is plane spotting legal? and in the article on the airport operator’s house rules.
• Receiving is not the same as publishing. Whether and how you may pass on received data depends on the country. The article on the legal situation of ADS-B reception gives an overview.
• Download offline maps. Without a network the map background stays gray. You still see aircraft, but orientation is easier with a preloaded map.
• Secure the cables. A torn-off OTG adapter or a stick slipping out of the socket ends the session. A short extension cable takes the strain off the phone’s port.
• Do not put the kit above the camera. The pocket radar is a tool. Anyone who only stares at the display misses the moment the aircraft actually passes by.

Conclusion

Mobile ADS-B reception turns your spotting position into a small radar station. Android users get there fastest with an SDR stick, an OTG cable and an app like Avare ADS-B. iPhone users, and everyone who wants to spare their phone battery, rely on a Raspberry Pi with readsb and tar1090 or a ready-made project like Stratux that delivers the map over Wi-Fi. In both cases the same applies: short, open antenna, low gain at the airport, power bank with reserve – and test everything at home before you step up to the fence. If you are just starting out, Plane spotting for beginners and the equipment checklist provide the right framework.