Piksi Technical Specs:

• Centimeter level positioning (RTK)
• Fast (50 Hz) position/velocity/time updates
• Open source software and board design
• Low power consumption : 500mW / 100mA typical
• Small form factor : 53 x 53 mm
• Low cost : $900 for a complete RTK system

For full technical specifications, check out the Piksi datasheet. User guides, code documentation, software and hardware repositories, and CAD models are all available on the Documentation Wiki.

A flexible platform

From the start, we wanted Piksi to be an indispensable tool for GPS experimentation. Whether you want to test out a new algorithm, receive signals from new constellations, more closely integrate and tune your receiver for your application, or teach yourself about GPS, Piksi gives you the flexibility, power, and transparency to do it.

The software that runs on Piksi is open source, as are the PCB schematics and layout. The development toolchain is supported on Linux, Windows, and OSX.

We’ve also developed an open source GPS post-processing tool, Peregrine, that provides a high-level interface to the same open source GPS library as used by the Piksi firmware. Raw GPS samples can be passed through Piksi over USB to a PC and post-processed with Peregrine. Being written in Python, Peregrine is well-suited for rapid development of new algorithms that can then be quickly transitioned to running standalone on the Piksi hardware.

How does RTK work?

A GPS receiver determines its position by measuring its distance to four or more GPS satellites. By comparing the relative phase offsets of unique ‘codes’ continually transmitted by the satellites, the receiver can determine the relative distance to each satellite. Each bit of the codes is about 300 meters in length, which in practice limits the precision to which the receiver can measure the code phase to a few meters. This is one reason that a normal receiver cannot achieve centimeter level accuracy.

Another important source of error for GPS receivers is ionospheric delay. When GPS signals travel through the ionosphere, they are slowed, adding a few meters of error to the distance measurement. The amount the signal is slowed varies over time and location, and is difficult to predict.

An RTK GPS receiver achieves centimeter level accuracy by mitigating these two sources of error.

First, in addition to measuring the code phase, an RTK GPS receiver measures the phase of the carrier wave that the code is modulated upon. The carrier has a wavelength of about 19 centimeters. This makes it possible to measure to a much greater degree of accuracy than the 300 meter code, but there is a catch – there are an unknown number of whole carrier wavelengths between the satellite and receiver. Clever algorithms are required to resolve this “integer ambiguity” by checking that the code and carrier phase measurements lead to a consistent position solution as the satellites move and the geometry of the problem changes.

Second, an RTK GPS receiver is able to reduce the ionospheric error with the help of an additional reference receiver. The ionospheric delay varies only slowly with location, so with a nearby reference receiver, the delay is almost the same for both receivers and can largely be cancelled out. This is why an RTK GPS system uses two receivers.

Project status

We currently have 25 pre-production Piksi receivers (identical to the production ones) assembled and ready to ship. The Piksi firmware currently supports the functionality of a normal GPS receiver, without RTK, and we’ve started the implementing the RTK functionality. We’ve also written a host of PC-side development tools to make it easy to interact with the hardware. The development toolchain is supported on Linux, Windows, and OSX.

We’ll use the Kickstarter funding to pay for development costs that we incur while finishing the RTK functionality. We’ll also be refining the development tools and adding more documentation to make using Piksi a delight for developers and end users of any background.

Rewards

We are offering two main rewards, the PIKSI and the RTK KIT. The PIKSI is simply a single Piksi receiver for people who only need one receiver. As we explained in our technical section above you need two receivers to do RTK so the PIKSI reward on its own won’t allow you to get centimeter level precision.

The RTK KIT reward is the main deal. It contains two Piksi receivers and everything else you need to do centimeter level RTK positioning. Have a look at this diagram which shows how it all fits together:

We are offering two versions of our PIKSI and RTK KIT rewards. The Developer Edition and the Production Edition. These two versions both contain identical hardware.

The difference is that the Developer Edition rewards will be shipped from the small batch of Piksi receivers that we already have assembled and will ship immediately after the Kickstarter campaign ends. Please be aware that Developer Edition rewards will ship before the RTK software development is complete.

We will be starting a new full production run of hardware for the Production Edition rewards which will be ready to ship in December when the RTK software development is complete.

PIKSI rewards include:

• 1 Piksi
• 1 Swift Navigation retractable Micro-USB cable
• 2 cable assemblies for connecting devices to Piksi’s UART headers

RTK KIT rewards include: 

• 2 Piksi receivers
• 2 Swift Navigation retractable Micro-USB cables
• 4 cable assemblies for connecting devices to the Piksi UART headers
• 2 XBee radios
• 2 cable assemblies for connecting the XBee radios to the Piksi receivers

Who are Swift Navigation?

We previously worked at a company named Joby Energy where we successfully developed an RTK GPS system for high-altitude wind turbines. This system was used to guide UAV’s in highly dynamic environments (greater than 8g accelerations, over 100mph). We’ve both been working on GPS full time for the past 2-3 years, and were working on our own independent GPS projects before that. See our Kickstarter bio for more information. And for those interested, here’s a presentation we gave at Defcon 2012 on GPS.

Risks and challengesLearn about accountability on Kickstarter

We have already built a small batch of Piksi receivers that are ready to ship and have locked down all part sourcing and manufacturing for further batches, so there are unlikely to be any unanticipated delays in the delivery of Piksi hardware.

However, it’s difficult to know exactly how long the RTK functionality will take to implement – software development schedules seem to always run over their anticipated delivery dates, even when you take into account Hofstadter’s Law. We feel the goals we’re proposing to accomplish with this campaign are reasonable – adding a new set of software functionality (which we successfully implemented on a previous platform) upon an existing base of stable hardware and software.

We’ve planned out the development schedule with these facts in mind, giving ourselves enough time to have the new features finished by the delivery date.