GPS without limits

At Copenhagen Suborbitals we have been using GPS receivers from the very beginning for all our flying projects. GPS is indeed a relatively cheap and very simple to use instrument for determining the position of an object in 3 dimensions.

By default all GPS units have some built-in limits (so-called COCOM limits) that prevents them from providing any data if the velocity exceeds 515 m/s at altitudes above 18 km. These artificial limits are built into GPS receivers to prevent bad guys from using them in missiles and other nasty stuff.

We are going to fly beyond 100 km altitude and our velocity may reach 1000 m/s, depending on which technology we are going to choose, so these limits are of serious concern to us.

A GPS receiver that implements the COCOM limits correctly will still function at altitudes above 18 km, as long as the velocity stays below 515 m/s. Such GPS unit may still be useful to us for determining the apogee of the spacecraft because at this point the velocity is very low.

Unfortunately, we often find even more limitations listed in GPS data sheets, for example:

  • Max. acceleration: 4g (some have higher values)
  • Max. altitude: 50 km

This additional altitude limitation is clearly a problem if we want to use the GPS for measuring the apogee.

When we have a rocket with active guidance we also have instruments that allow us to estimate the altitude within few hundred meters. This may sound like very inaccurate but this accuracy is sufficient for this purpose. Nevertheless, a GPS receiver is still desirable because it can give us an independent measurement of the altitude and this could be very useful during development flights.

So, we went out looking for GPS receivers that might work at 100 km altitude and beyond. We have done this during the design of the avionics for HEAT-2X and in 2013 we have actually acquired two different GPS units that we expect to work at these altitudes.

One of them is called Poxa, the other one is called Piksi (not kidding).

 

POXA1328-003

This GPS receiver is a sponsorship from Acte A/S and comes with a special firmware where the altitude limit has been disabled. They could do that for us since we are a peaceful organization.

POXA1328-003 GPS unit mounted on a test PCB

Figure 1: POXA1328-003 GPS unit mounted on a test PCB. Photo: Thomas Scherrer.

 

This GPS unit has been previously mentioned in the Rocket Shop blog on Wired back in September 2013.

It is a simple to use GPS unit that gives us the usual NMEA sentences over an UART connection. Datasheet with specs is available on the web.

 

Piksi

Piksi is an open source GPS receiver from Swift Navigation launched during a Kickstarter campaign.

Piksi GPS receiver.

Figure 2: Piksi GPS receiver. Photo: Swift Navigation.

Most of the firmware is available on the web and we could disable the altitude limit ourselves.

Piksi is more than just a GPS. It contains amongst other an STM32F4 processor that can run your own applications to the extent there is memory and CPU cycles available. You can find more info in the data sheet or in the extensive documentation in their wiki.

Piksi costs slightly more that a normal GPS unit; however, the money is well spent when you get the firmware source code.

This was a brief overview of what GPS units we have at our disposal when we will fly high.

We are now going to create a simple test setup where Poxa & Piksi can be tested side by side using a GPS simulator. A GPS simulator will allow us to test specific velocities and altitudes, as well as run pre-programmed flight profiles.

More about that later as we proceed.

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