We mostly focussed on the 2.4GHz high-speed bi-directional communication system at this meeting. This system (COM) is supposed to give us a high-speed data link to the rocket - we've discussed 100kbps as a target speed but it's all very flexible. Basically, we'll consider anything over 19.2kbps as high-speed.
According to Glenn, we have two routes to go:
1) Build our own radio with a digital radio chipset
We could use a chipset to build a QPSK (Quadrature Phase Shift Keying) modem (two bits of data transmitted for every signal transition). Advantages are that we'll get exactly what we want and understand the entire system - the disadvantages are that we have to build the system up from the ground level and that will take time - it's doubtful that we could get anything working well by the September launch. In some senses, this is the "right" way to proceed since we want to "role our own" as much as possible… but right now, we just don't have the resources to build any radio system like this quickly.
2) Use 802.11 wireless Ethernet
If it's possible that we could just use an "off the shelf" solutions then we discussed that we should try that first. Such a system would be to use the wireless Ethernet standard, 802.11. The 802.11 standard does everything we want - multiple data speeds, error correction, etc.etc. The transceiver boards even come in PCMCIA card formats, so we could literally plug in a PCMCIA card to the CANmegaBOX flight computer (we think it has a Type 2 slot) and have a working link.
The big problem with 802.11 is transmitted power. According to the FCC, 802.11 devices falls under the FCC Class 15 device requirements, which means it can't transmit more than 1W EIRP (Effective Isotropic Radiation Pattern - i.e., using the radiated pattern of a point source). Usually, we'd just power amplify the heck out the card, but because 802.11 uses 2.400 - 2.483 GHz - which is outside the amateur bands - we can't legally do that.
There are also other questions - where do we hook in the power amp? Do we rip open the PCMCIA card and use our own transmit/receive switch? Do we try and use an external only device? Are there Linux drivers for all of this? Is it possible to shift the 802.11 signal down to below 2.400GHz (which is an amateur band)?
Our strategy is to run with the 802.11 solution until we hit a brick wall. It'll be an interesting exercise if nothing else, but we all feel like this might actually work and might work fast. Action items are:
- Glenn is going to spend the next few weeks reading up on 802.11 and trying to figure out how we might amplify it - and if it's possible to change the frequency which is unlikely.
- Glenn is also going to contact people about our antenna conundrum to see if anyone has any better ideas.
- Larry is going to make the first few contacts to the FCC and Congressman Wu to see if it's possible for us to get a FCC "waiver" or permission to amplify a Part 15 device.
- Andrew will arrange for a meeting with Binkley, a CS prof/networking guy who actually knows something about 802.11.
In other news:
- Andrew and Tim (and Scott) are working on the spin table for IMU calibration.
- We're still in negotiations with the CANmegaBOX people about price and features.
A few interesting URLs for 802.11: