[Ovmsdev] SIM808/SIM908 GPS accuracy

HONDA S-2000 s2000 at audiobanshee.com
Wed Feb 8 14:25:14 HKT 2017

On Feb 7, 2017, at 4:30 AM, Edward Cheeseman <cheesemanedward at gmail.com> wrote:
>> On 7/02/2017, at 8:21 PM, HONDA S-2000 wrote:
>> On Feb 6, 2017, at 10:43 PM, Edward Cheeseman wrote:
>>> Seconds * 3600 = degrees. So native units would be degrees * 3600 * 2048.
>> I see. While intermediate values are calculated in float, the return value is a fixed-point, signed 32-bit long value (a.k.a. Q19.11).
>> That allows for a maximum of ±145 degrees (?).
> I think its more like Q20.11? One bit sign, 11 bits of fractions of a second, 20bits of seconds/3600 = +/-291 degrees

Oops. I somehow subtracted the sign bit twice. Something about the number 11 must have freaked me out ;-)

That's good news, because Q20.11 can handle longitude better than the mythical Q19.11, i.e., ±291 degrees, as you point out. That's enough for up to 180°E and 180°W.


p.s. The rest of your comments are also correct. It seems that we don't have a lot of Flash space left for readable code solutions. Your idea to use comments to explain the source is a good one. That's assuming that any rewrite turns out to be an improvement, otherwise the code could stay as it was. Let us know what you find out.

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