Sacramento Surveyors Visit the NGS Airborne Gravimeter - CLSA

Positioning using GPS and CORS
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on the market and assigned very specific abbreviations
denoting the manufacturer, model and version of a particular
antenna. If you are processing GPS data using your own
proprietary software, the offsets for each antenna model
used in your survey must be properly applied. If you are
submitting your GPS data in RINEX format to an online
service, the proper abbreviations for your antenna model(s)
must be used in order for the proper offsets to be assigned
by the online service. To see a list of NGS antenna models
and their observed offsets, go to
http://www.ngs.noaa.gov/ANTCAL/.
Typical GPS antenna diagram from NGS Site (NOV702_3.00).
Common Times
When talking about the time a GPS receiver is collecting
satellite data, it is important to remember that, for relative
positioning techniques, data only counts when at least
two receivers are running simultaneously. If you start
Receiver 1, then go to a second point and start Receiver 2,
the time used to process baselines starts when both
Receiver 1 and 2 are collecting data. For a short base line
(2-5 km long), dual frequency receivers can probably collect
enough data in 5 or 6 minutes However, in practice,
such a short time brings a high probability of a failed solution.
It is customary to collect at least 15 minutes of common
data with dual frequency receivers. Single frequency
receivers usually need to collect data simultaneously for 20
to 30 minutes.
Observation Rate
In doing the field work you must set the epoch - or
sampling rate - for your receiver to collect data. CGPSS
have a variety of sampling rates from 1 to 30 seconds. A 5
second sample is a good rate to use on your receiver as it
will provide parity with the available array of CORS observations
rates, while still being thrifty with your receiver
memory. If the local CGPSS are sampling at 1 second
rates, and your receiver has the memory capacity to store
the data for the desired duration, set your collection rate at
1 second.
Using CORS Data
Online vs. DIY CORS Options for Base Positions
For this article, a base station is any point for which you
want to determine a position based on CORS sites. This
processing can be done using either an on-line service
such as OPUS, SCOUT or PPP, or by doing it yourself using
your own post processing software. Both methods will be
discussed, for if you do a lot of this, you will probably do
both. Again, like all tools, there is no one perfect tool, and
if you use them long enough you will eventually smack your
thumb. To use CGPSS for rapid static you should wait for
at least one day before you can download the CGPSS data.
This is because although NGS provides data in hourly
increments, it is easier to download data for a full 24 hour
interval which is available online the following day. The non-
NGS sites only provide data for a full 24 hour period.
CORS Data
CORS data are provided in 24-hour blocks for a particular
“ordinal day.” Sometimes these days are referred to as
the “Julian” day, which isn’t technically correct. The ordinal
day is basically a day’s number in the year: January 1 is day
001, and December 31 is day 365 in a non-leap year. An
ordinal day begins on 0000 hours Universal Coordinated
Time beginning at midnight in Greenwich, England. So,
when you started your GPS receiver in California starting at
4:01pm Pacific Standard Time on day 050 (February 19),
you will use CORS data posted for day 051 (February 20),
since it was 0001 hrs on February 20 UTC when you began
to record data. For a useful printable chart of the ordinal
days for both leap years and non-leap years, go to:
http://amsu.cira.colostate.edu/julian.html and
http://amsu.cira.colostate.edu/leap.html.
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