GPS accuracy

This question is motivated by the recent discussion about deriving
polars from flight logs.

We know that GPS fixes have some amount of (absolute) error. What is
their accuracy relative to each other? In other words, when I'm
analyzing flight logs, how accurately can I calculate something like
sink rate using just the GPS fixes ("B" records)?

I'm thinking about taking an early morning tow sometime this winter
when the pressure is high and the air is stable (at least vertically)
so I can calculate my actual polar, but I don't want to waste time and
money on this if the flight log isn't good enough. (And a few cockpit
notes of course -- IAS at various times, etc).

Prove the altitude variance to yourself, stand still and watch the
altitude change slowly up and down. It can really move around, maybe
enough to make a GPS polar useless.

1 meter per second change while dead still is seen frequently on my
GPS.

Bob



wrote:
This question is motivated by the recent discussion about deriving
polars from flight logs.

We know that GPS fixes have some amount of (absolute) error. What is
their accuracy relative to each other? In other words, when I'm
analyzing flight logs, how accurately can I calculate something like
sink rate using just the GPS fixes ("B" records)?

I'm thinking about taking an early morning tow sometime this winter
when the pressure is high and the air is stable (at least vertically)
so I can calculate my actual polar, but I don't want to waste time
and
money on this if the flight log isn't good enough. (And a few cockpit
notes of course -- IAS at various times, etc).

Ah. I knew that. I'd have to use the pressure altitudes from the B
records.

But these have to be adjusted .. the OAT must be recorded at the
various altitudes and used to adjust the pressure altitudes to
SL/standard atmosphere. Can anyone fill me in on how to do that?

"But these have to be adjusted .. the OAT must be recorded at the
various altitudes and used to adjust the pressure altitudes to
SL/standard atmosphere. Can anyone fill me in on how to do that?"

Why? You're not concerned with absolute (True) altitude; just a delta
between contiguous points. As long as you're flying through a sharp
frontal boundary during your measurements, I can't see how this will
matter. It's been a while since I actually did these manually, but Rho
(density) cancels out in a ratio of Lift to Drag, no?

L = 1/2RhoV2 CL S
D = 1/2 Rho V2 CD S

In any case, there are a number of places to get the gas law equations
to convert to ISA. Google on ISA conversion formula or somesuch - I
got several.

P3

D=

Should have writtent "NOT flying through a sharp frontal boundary..."

writes:

This question is motivated by the recent discussion about deriving
polars from flight logs.

We know that GPS fixes have some amount of (absolute) error. What is
their accuracy relative to each other? In other words, when I'm
analyzing flight logs, how accurately can I calculate something like
sink rate using just the GPS fixes ("B" records)?

A standard logger is not much chop. You really need to record raw pseudo-range
data, and have a unit at a fixed place as well. Then with post processing
you can get to a few feet with good single frequency units, and to a hand
full of cm with a dual frequency one. I've heard mutterings of mm accuracy
with P-code over a few meters.

Get a reference on GPS surveying and look at the tollerence they quote.

I'm thinking about taking an early morning tow sometime this winter
when the pressure is high and the air is stable (at least vertically)
so I can calculate my actual polar, but I don't want to waste time and
money on this if the flight log isn't good enough. (And a few cockpit
notes of course -- IAS at various times, etc).



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+61 (08) 9257-1001 Kalamunda.
West Australia 6076
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