Upcoming Webinar and DoD Releases New GPS Specification Documents

August 23, 2010 By: Eric Gakstatter

Survey Scene, August 2010

I received some engaging e-mails about the Solar Activity discussion in my last newsletter. Since that newsletter, I’ve announced a webinar I’ll be conducting Tuesday, August 31. The primary subject I will discuss during the webinar will be the effect of solar activity on GPS users. I don’t pretend to be an expert on the subject, so I rely on my studies over the past several years as well as vetting my presentation with a couple of people who’ve made space weather a career.

The reason I keep covering the space weather topic is two-fold:

1. Space weather is a broad topic. GPS users are only affected by a certain types of space weather. Since the next solar cycle is beginning to ramp up, it’s important for GPS users to understand how solar activity might affect their GPS operations.
2. The mainstream media paints solar activity with a very broad brush, which can be very misleading. It usually throws cell phone networks, utility infrastructure and GPS in the same bucket.

My primary goal with the webinar is to paint a clear picture on the effects of solar activity specifically for GPS users. You can sign up for the free webinar by clicking here.

The second topic of the webinar will be an update on SBAS, more specifically WAAS, with an update on the geosynchronous satellite (PRN-135) failure last April and where the U.S. Federal Aviation Administration (FAA) stands on replenishing the WAAS GEOs.

The third topic will be an update on the GPS 24+3 satellite configuration.

I’ll be warming up my voice this week because it’s a lot of information to cover in 60 minutes. It will be fast paced and full of good information. As usual with my webinars, I’ll address some questions from the audience at the end. Those questions I don’t have time to address, I’ll tackle in my early September newsletter. By the looks of the questions I’ve received already, this webinar will be a very good one.

New Department of Defense (DoD) GPS Specification Documents Released

Just a couple of weeks ago, on August 13, The U.S. government released three documents pertaining to GPS:

1. Interface Specification IS-GPS-200 (Rev. E).

This document describes the interface between the GPS segment and our GPS receivers for both L1 and L2 frequencies.

2. Interface Specification IS-GPS-705 (Rev. A).

This document describes the interface between the GPS segment and our GPS receivers for the L5 frequency.

3. Interface Specification IS-GPS-800 (Rev. A).

This document describes the interface between the GPS segment and our GPS receivers for the L1C signal.

The IS-GPS-200 document (formerly named ICD-GPS-200) is a long-standing, well-known document that is critical for GPS receiver designers. The purpose of the latest release is to incorporate GPS-IIIA information.

It was first released in January 1983. Sort of like the Federal Radionavigation Plan, it’s rather dry reading (and far more technical) but has some sections you might be interested in.

Page 16 – Signal Polarization. The transmitted signal shall be right-hand circularly polarized (RHCP).

This may not seem significant, but wait until our discussion about solar activity in my next webinar.

Page 35 – GPS Time and SV Z-Count. GPS time is established by the Control Segment and is referenced to Coordinated Universal Time (UTC) as maintained by the U.S. Naval Observatory (UTC (USNO)) zero time-point defined as midnight on the night of January 5, 1980/morning of January 6, 1980. The largest unit used in stating GPS time is one week defined as 604,800 seconds. GPS time may differ from UTC because GPS time shall be a continuous time scale, while UTC is corrected periodically with an integer number of leap seconds. There also is an inherent but bounded drift rate between the UTC and GPS time scales. The OCS shall control the GPS time scale to be within one microsecond of UTC (modulo one second).

Page 44 – GPS Week Number. The GPS week numbering system is established with week number zero (0) being defined as that week which started with the X1 epoch occurring at midnight UTC(USNO) on the night of January 5, 1980/morning of January 6, 1980. The GPS week number continuously increments by one (1) at each end/start of week epoch without ever resetting to zero.

Page 95 – ECEF Coordinate System. The equations given in Table 20-IV provide the SV's antenna phase center position in the WGS 84 ECEF coordinate system defined as follows:

Origin* = Earth's center of mass

Z-Axis** = The direction of the IERS (International Earth Rotation and Reference Systems Service)
Reference Pole (IRP)

X-Axis = Intersection of the IERS Reference Meridian (IRM) and the plane passing through the
origin and normal to the Z-axis

Y-Axis = Completes a right-handed, Earth-Centered, Earth-Fixed orthogonal coordinate system

* Geometric center of the WGS 84 Ellipsoid
** Rotational axis of the WGS 84 Ellipsoid

Page 113 – Ionospheric Model. The "two frequency" (L1 and L2) user shall correct the time received from the SV for ionospheric effect by utilizing the time delay differential between L1 and L2 (reference paragraph 20.3.3.3.3.3). The "one frequency" user, however, may use the model given in Figure 20-4 to make this correction. It is estimated that the use of this model will provide at least a 50 percent reduction in the single - frequency user's RMS error due to ionospheric propagation effects. During extended operations, or for the SVs in the Autonav mode if the CS is unable to upload the SVs, the use of this model will yield unpredictable results.

Page 118 – Speed of Light. The speed of light used by the CS for generating the data described in the above paragraphs is

c = 2.99792458 x 10^8 meters per second

which is the official WGS 84 speed of light. The user shall use the same value for the speed of light in all computations.

Thanks, and see you next time.

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