GPS detector app Miami Beach Coral Gables.

GPS detector app Miami Beach Coral Gables.

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GPS detector app Miami Beach Coral Gables.

GPS detector app Miami Beach Coral Gables.

The GPS satellite transmits a Right Hand Circularly Polarized (RHCP) L-band signal known as L1 at 1575.42 MHz. This signal is transmitted with enough power to ensure a minimum signal power level of -160 dBw at the Earth’s surface. The SPS signal generation and transmission process are represented in Figure 1-1, in Section 1.5. GPS detector app Miami Beach Coral Gables. The GPS satellite also transmits a second ranging signal known as L2 at 1227.6 Mhz. This signal is transmitted with enough power to ensure a minimum signal power level of -166 dBw at the Earth’s surface. GPS detector app Miami Beach Coral Gables. This signal is not considered by the DOD to be a part of the SPS. However, we note that many civil receivers have incorporated carrier tracking and cross-correlation technology into their design that enables them to use L2 to support two-frequency corrections. Neither these signal characteristics nor the SPS performance standards (Annex A) and characteristics (Annex B) are predicated upon the use of L2.

L1 is Bipolar-Phase Shift Key (BPSK) modulated with a Pseudo Random Noise (PRN) 1.023 MHz code known as the Coarse/Acquisition (C/A) code. This C/A code sequence repeats each millisecond. GPS detector app Miami Beach Coral Gables. The transmitted PRN code sequence is actually the Modulo-2 addition of a 50 Hz navigation message and the C/A code. The SPS receiver demodulates the received code from the L1 carrier and detects the differences between the transmitted and the receiver-generated code. GPS detector app Miami Beach Coral Gables. The SPS receiver uses an exclusive-or truth table to reconstruct the navigation data, based upon the detected differences in the two codes.

An Overview of the GPS Navigation Message. GPS detector app Miami Beach Coral Gables.

GPS detector app Miami Beach Coral Gables.

GPS detector app Miami Beach Coral Gables.

Each GPS satellite provides data required to support the position determination process. Figure 2-1 provides an overview of the data contents and structure within the navigation message. GPS detector app Miami Beach Coral Gables. The data includes information required to determine the following:

  • Satellite time of transmission. GPS detector app Miami Beach Coral Gables.
  • Satellite position. GPS detector app Miami Beach Coral Gables.
  • Satellite health · Satellite clock correction. GPS detector app Miami Beach Coral Gables.
  • Propagation delay effects. GPS detector app Miami Beach Coral Gables.
  • Time transfer to UTC. GPS detector app Miami Beach Coral Gables.
  • Constellation status. GPS detector app Miami Beach Coral Gables.

Minimum Usage Conditions. GPS detector app Miami Beach Coral Gables.

Although the DOD specifies and controls the characteristics and performance of the GPS ranging signals, SPS performance must be specified in the positioning domain. GPS detector app Miami Beach Coral Gables. However, since the definition of SPS receiver design requirements is not within the scope of this document, certain minimum assumptions concerning receiver design and usage must be made in order to map ranging signal performance characteristics into the positioning domain. GPS detector app Miami Beach Coral Gables. These assumptions establish the minimum position and time determination capabilities which an SPS receiver must possess to meet the minimum performance standards, as they are specified in Annex A. Users whose receiver designs do not meet these assumptions may not experience performance in accordance with the performance standards. GPS detector app Miami Beach Coral Gables.

Satellite Tracking and Selection. GPS detector app Miami Beach Coral Gables.

The SPS receiver must provide the capability to track and generate a position solution based on measurements and data taken from at least four satellites. GPS detector app Miami Beach Coral Gables. No other assumptions are made regarding the SPS receiver’s channel architecture or ranging signal measurement strategy. The SPS receiver must be capable of tracking and using satellites down to a 5° mask angle with respect to the local horizon. GPS detector app Miami Beach Coral Gables. The local horizon is defined for the purposes of this Signal.

Specification to be equivalent to the local tangent plane, with respect to the ellipsoid model used in the position solution. GPS detector app Miami Beach Coral Gables. Performance standards do not take into consideration the presence of obscure above the 5° mask angle. GPS detector app Miami Beach Coral Gables. The SPS receiver must be able to compensate for dynamic Doppler shift effects on nominal SPS ranging signal carrier phase and C/A code measurements.

The SPS receiver manufacturer is responsible for ensuring that the receiver compensates for Doppler shift behavior unique to the receiver’s anticipated application. GPS detector app Miami Beach Coral Gables. Doppler shift behavior is a function of expected satellite-to-user relative velocities, where the primary uncertainty is the dynamics of the user platform. Satellite selection must be based upon the minimum Position Dilution of Precision (PDOP). GPS detector app Miami Beach Coral Gables. The performance standard definitions are based upon an assumption that the SPS receiver will recompute the optimum PDOP every five minutes, or whenever a satellite used in the position solution sets below the 5° mask angle. GPS detector app Miami Beach Coral Gables.

The SPS receiver must have the capability to read the health field and status bits in the navigation message and exclude unhealthy satellites from the position solution. Note that the Subframe 1 health field takes precedence over the almanac health field. GPS detector app Miami Beach Coral Gables. Each time the SPS receiver is powered on, it must ensure that it is using up-to-date ephemeris and clock data for the satellites it is using in its position solution. GPS detector app Miami Beach Coral Gables. The SPS receiver designer is encouraged to monitor the Issue of Data, Clock (IODC)/Issue of Data, Ephemeris (IODE) values, and to update ephemeris and clock data based upon a detected change in one or both of these values.

At a minimum, the SPS receiver must update its ephemeris and clock data for a given satellite no more than two hours after it last updated its data for that satellite. GPS detector app Miami Beach Coral Gables. The SPS receiver must ensure that the datasets it uses in the position solution process are internally consistent for a given satellite, and are not mixes of old and new data. GPS detector app Miami Beach Coral Gables.

SPS Receiver Design and Usage Contributions to Position Solution Error

The SPS receiver’s error contribution to the SPS ranging error is not taken into consideration in the definition of SPS performance standards. SPS accuracy standards reflect only the error characteristics of the signal-in-space. GPS detector app Miami Beach Coral Gables. Atmospheric propagation path effects on single-frequency range measurement accuracy are taken into consideration in the positioning accuracy performance standard development. GPS detector app Miami Beach Coral Gables.

The positioning accuracy performance standard development assumes that the SPS receiver design implements the satellite position estimate, measured range computation, ionospheric correction, and satellite time correction algorithms in accordance with this Signal Specification. GPS detector app Miami Beach Coral Gables. The performance standards do not consider the possible effects of multipath on position solution accuracy, other than the specification of a 5° mask angle. Platform dynamics are not explicitly taken into consideration in performance standard development. GPS detector app Miami Beach Coral Gables. However, receivers that are designed to operate under medium dynamic conditions should not experience degradations in service availability or accuracy. The term medium dynamic conditions are defined here to mean SPS user motion which does not:

1) impart acceleration or jerk effects on frequency, phase or code measurements in excess of those experienced by a stationary user, or

2) change the receiver antenna’s nominal orientation with respect to local horizontal. GPS detector app Miami Beach Coral Gables.

The SPS receiver must implement the Universal Coordinated Time (UTC) corrections supplied in the navigation message, in order to experience position solution time, transfer accuracies as specified in the accuracy performance standard.

Position Fix Dimensions. GPS detector app Miami Beach Coral Gables.

The GPS architecture provides the inherent capability to solve for a four-dimensional solution. The specific coordinate system used to define the position solution’s output dimensions will be unique to a given SPS receiver’s design and user’s needs. GPS detector app Miami Beach Coral Gables. However, GPS operates in a well-defined set of coordinate systems, and all performance standard definitions assume their usage. The satellite position and geometric range computations must be accomplished in the World Geodetic Survey 1984 (WGS-84) Earth-Centered, Earth-Fixed (ECEF) coordinate system. In order for the user to experience performance consistent with the performance standards, the position solution must be accomplished in WGS-84 local coordinates, or in a local coordinate system which meets the following conditions:

  • The coordinate system must have an accepted mathematical relationship with the WGS- 84 ECEF coordinate system. GPS detector app Miami Beach Coral Gables.
  • Latitude must be defined with respect to the equator of a documented ellipsoid model.
  • Longitude must be defined with respect to the Greenwich meridian, or another reference that has a documented relationship with the Greenwich meridian. GPS detector app Miami Beach Coral Gables.
  • Local horizontal must be defined as a plane perpendicular to a documented ellipsoid model’s local radius of curvature, or tangent to the ellipsoid surface at the user’s location. GPS detector app Miami Beach Coral Gables.
  • Local vertical must be defined to be parallel with a documented ellipsoid model’s local radius of curvature, or perpendicular to the local horizontal plane. GPS detector app Miami Beach Coral Gables.

Position Fix Rate. GPS detector app Miami Beach Coral Gables.

SPS accuracy measurement algorithms (defined in Annex C) are based upon a position fix rate of once per second, to support high confidence interval evaluations. GPS detector app Miami Beach Coral Gables. However, the use of different fix rates is not precluded in the performance standard definition, since the instantaneous position solution predictable error is independent of the fixed rate.

Position Solution Ambiguity. GPS detector app Miami Beach Coral Gables.

SPS performance standards (as specified in Annex A) assume no ambiguities in the position solution process. The formal derivation of the GPS position solution does, however, admit the possibility of position determination ambiguities due to bifurcate solutions, although the probability is nil for users on or near the surface of the Earth. GPS detector app Miami Beach Coral Gables. The potential for ambiguity arises from the occurrence of very specific and rare conditions in the position solution geometry. The probability of an ambiguity occurring is completely dependent on how the receiver manufacturer’s position solution implementation deals with bifurcate solution conditions.

Ranging Signal Carrier Characteristics. GPS detector app Miami Beach Coral Gables.

The L-band carrier is modulated by a bit train which is a composite generated by the Modulo-2 addition of a Pseudo Random Noise (PRN) ranging code and downlink system data (referred to as navigation data or the navigation message). GPS detector app Miami Beach Coral Gables.

The L-band SPS ranging signal is contained within a 2.046 MHz band centered about L1. The carrier frequency of the L1 signal is coherently derived from a frequency source within the satellite. GPS detector app Miami Beach Coral Gables. The nominal frequency of this source — as it appears to an observer on the ground — is 1.023 MHz. To compensate for relativistic effects, the output frequency of the satellite’s frequency standard — as it would appear to an observer located at the satellite — is 10.23 MHz offset by a Df/f = -4.4647 x 10-18 or a Df = -4.567 x 10-3 Hz. This frequency offset results in an output of 10.22999999543 MHz, which is frequency divided to obtain the appropriate carrier modulation signal (1.022999999543 MHz). GPS detector app Miami Beach Coral Gables. The same output frequency source is also used to generate the nominal L1 carrier frequency (fo) of 1575.42 MHz.

Correlation Loss. GPS detector app Miami Beach Coral Gables.

Correlation loss is defined as the difference between the satellite power received in a 2.046 MHz bandwidth and the signal power recovered in a nominal correlation receiver of the same bandwidth. GPS detector app Miami Beach Coral Gables. On the L1 channel, the correlation loss apportionment is as follows: · Satellite modulation imperfections 0.6 dB · Ideal user receiver waveform distortion 0.4 dB.

Carrier Phase Noise. GPS detector app Miami Beach Coral Gables.

The phase noise spectral density of the unmodulated carrier is such that a phase locked loop of 10 Hz one-sided noise bandwidth is able to track the carrier to an accuracy of 0.1 radians RMS.

Spurious Transmissions. GPS detector app Miami Beach Coral Gables.

In-band spurious transmissions are at least 40 dB below the unmodulated L1 carrier over the allocated channel bandwidth.

Equipment Group Delay. GPS detector app Miami Beach Coral Gables.

Equipment group delay is defined as the delay between the L-band radiated output of a specific satellite (measured at the antenna phase center) and the output of that satellite’s onboard frequency source; the delay consists of a bias term and an uncertainty. GPS detector app Miami Beach Coral Gables. The bias term is of minimal concern to the SPS user since the majority of its value is included in clock correction parameters relayed in the navigation data, and is therefore accounted for by the user computations of system time (reference paragraph 2.5.5.2). The SPS receiver manufacturer and the user should note that a C/A code epoch may vary up to 10 nanoseconds (2 s) with respect to the clock correction parameters provided in the navigation message.

Signal Polarization. GPS detector app Miami Beach Coral Gables.

The transmitted signal is right-hand circularly polarized. The ellipticity for L1 will not exceed 1.2 dB for the angular range of ±14.3 degrees from boresight.

C/A Code Generation and Timing. GPS detector app Miami Beach Coral Gables.

The SPS PRN ranging code is known as the Coarse/Acquisition (C/A) code. Appropriate code-division-multiplexing techniques allow differentiating between the satellites even though they all transmit on the same L-band frequency.

The characteristics of the C/A code are defined below in terms of its structure and the basic method used for generating it. GPS detector app Miami Beach Coral Gables. The C/A code consists of 1.023 Mbps Gi (t) patterns with Modulo 2 addition of the navigation data bit train, D(t), which is clocked at 50 bps. The resultant composite bit train is then used to BPSK modulate the L-band carrier. The user receiver is then required to independently generate and synchronize with the satellite transmitted C/A code and perform Modulo 2 additions in order to decode and interpret the navigation message. GPS detector app Miami Beach Coral Gables.

Signal Coverage and Power Distribution. GPS detector app Miami Beach Coral Gables.

The minimum power of the near-ground user-received L1 signal as a function of satellite elevation angle using the following assumptions:

(a) the signal is measured at the output of a 3 dBi linear polarized receiving antenna,

(b) the satellite is at or above a 5-degree elevation angle, GPS detector app Miami Beach Coral Gables.

(c) the received signal levels are observed within the in-band allocation defined in paragraph 2.1.1,

(d) the atmospheric path loss is 2.0 dB, and (e) the satellite attitude error is 0.5 degrees (towards reducing signal level). GPS detector app Miami Beach Coral Gables.

Higher received signal levels can be caused by such factors as satellite attitude errors, mechanical antenna alignment errors, transmitter power output variations due to temperature variations, voltage variations, and power amplifier variations, and due to a variability in link atmospheric path loss. GPS detector app Miami Beach Coral Gables. The maximum received L1 C/A signal levels as a result of these factors is not expected to exceed -153.0 dBw. This estimate assumes that the receiving antenna characteristics are as described above, the atmospheric loss is 0.6 dB and the satellite attitude error is 0.5 degrees (towards increased signal level).

GPS Time and the Satellite Z-Count. GPS detector app Miami Beach Coral Gables.

GPS time is established by the Control Segment and is used as the primary time reference for all GPS operations. GPS time is referenced to a UTC (as maintained by the U.S. Naval Observatory) zero time-point defined as midnight on the night of January 5, 1980/morning of January 6, 1980. GPS detector app Miami Beach Coral Gables. 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 is 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. GPS detector app Miami Beach Coral Gables. The GPS time scale is maintained to be within one microsecond of UTC (Modulo one second). The navigation data contains the requisite data for relating GPS time to UTC.

Message Structure. GPS detector app Miami Beach Coral Gables.

The navigation message is transmitted by the satellite on the L1 data link at a rate of 50 bps. The following sections define the navigation data format and contents. Implementation algorithms for this data are provided later.

As shown in Figure 2-8, the message structure utilizes a basic format of a 1500 bit long frame made up of five subframes, each subframe being 300 bits long. Subframes 4 and 5 are sub commutated 25 times each so that a complete data message will require the transmission of 25 full frames. GPS detector app Miami Beach Coral Gables. The 25 versions of subframes 4 and 5 are referred to as pages 1 through 25 of each subframe. Each subframe will consist of ten words, each 30 bits long; the MSB of all words is transmitted first. Each subframe and/or page of a subframe starts with a Telemetry (TLM) word and a Handover word (HOW) pair. GPS detector app Miami Beach Coral Gables.

The TLM word is transmitted first, immediately followed by the HOW. The latter is followed by eight data words. Each word in each frame contains parity. GPS detector app Miami Beach Coral Gables. At end/start of week (a) the cyclic paging to subframes 1 through 5 will restart with subframe 1 regardless of which subframe was last transmitted prior to end/start of week, and (b) the cycling of the 25 pages of subframes 4 and 5 will restart with page 1 of each of the subframes, regardless of which page was the last to be transmitted prior to the end/start of week. GPS detector app Miami Beach Coral Gables. All upload and page cutovers will occur on frame boundaries (i.e., Modulo 30 seconds relative to end/start of the week); accordingly, new data in subframes 4 and 5 may start to be transmitted with any of the 25 pages of these subframes.

Data Parity. GPS detector app Miami Beach Coral Gables.

Words one through ten of subframes 1-5 each contain six parity bits as their LSBs. In addition, two non-information bearing bits are provided as bits 23 and 24 of words two and ten for parity computation purposes.

Default Navigation Data Transmission. GPS detector app Miami Beach Coral Gables.

Under certain conditions, GPS satellites can transmit default navigation data in place of valid data in the navigation message. The default navigation data is defined as follows:

  • A pattern of alternating ones and zeros in words 3 through 10, GPS detector app Miami Beach Coral Gables.
  • The two trailing bits of word 10 will be zeros, to allow the parity of subsequent subframes to be valid, and
  • The parity of affected words will be invalid. GPS detector app Miami Beach Coral Gables.

If the condition is a lack of a data element, only those subframes supported by that data element will transition to this condition. GPS detector app Miami Beach Coral Gables. Other conditions can cause all the subframes to transition to default navigation data and cause the subframe ID in the HOW to equal one.

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