Fixing the GPS interference problems

Posted in LightSquared, Operators, Regulatory, Spectrum at 10:50 am by timfarrar

One of the key issues for LightSquared is that the downlink transmissions from its base stations are expected to interfere with a wide range of GPS devices, which operate in adjacent frequencies. LightSquared will be using part of the L-band downlink frequencies (1525-1559MHz), while GPS (and GLONASS) operate within the 1559-1610MHz band. Though LightSquared’s base stations will be fitted with filters which cut off the signal abruptly at the top of the L-band, so LightSquared’s signals do not leak into the GPS band, the filters on most GPS devices do not have such a strict cutoff at the bottom of the GPS band, and so can be overwhelmed by the very high power LightSquared terrestrial transmissions in the adjacent L-band frequencies. A good illustration of the impact is given in this chart from a Deere & Company submission to the FCC:

As this chart shows, the types of receivers that are most affected are high precision receivers used in applications such as farming and surveying, although lower precision receivers such as those incorporated in automobile navigation and even in cellphones may be impacted closer to the LightSquared base stations. Aeronautical navigation is an area of particular concern, given the safety critical nature of this application. Testing is now ongoing to determine the extent of interference, and early estimates of the impact vary greatly, ranging from a few hundred meters or less up to several miles for low precision receivers, and potentially tens of miles for high precision receivers.

It appears that LightSquared expects that GPS manufacturers should “fix” their devices, in order to mitigate these interference issues, although unsurprisingly this is being resisted strongly by the GPS community, because the costs would be very significant. PRTM estimates that it will only cost 30 cents per device to fit filters to the “40M standalone GPS devices” made worldwide each year for a total of $12M. PRTM also assumes that no additional filters will be needed for the much larger number of GPS-enabled cellphones sold each year, despite Qualcomm telling the FCC in January 2011 that it plans to use a filter to prevent self-interference in L-band enabled cellphones, and had “not determined whether this filter provides sufficient protection to avoid interference to the GPS receiver from LTE base stations operating on the L band.”

In reality, whether or not cellphone manufacturers ultimately decide an additional filter is needed to protect their GPS receivers, the overall cost impact would be far, far greater than PRTM indicate. To take a directly analogous situation, LightSquared is paying Inmarsat $250M to fit filters to its L-band satellite terminals on up to 10K aircraft and perhaps 50K ships. If we assume these filters cost $30 rather than 30 cents each, then following PRTM’s calculations the cost of solving the problem would be less than $2M. However, Inmarsat expects to spend the vast majority of the $250M it is receiving on actually fixing the problem, and the filters themselves are less than 1% of the total cost. Instead, the bulk of the expenditure will go on securing approvals (including from safety authorities) for replacement equipment, then going out and fitting this equipment on ships and aircraft.

Across the GPS industry the same considerations would apply – dramatically increased costs for testing, safety approvals, retrofits of existing equipment, etc. not to mention the markups that would apply to the filter component costs as they flow through to an increased total cost of the devices sold. Many of these costs would be concentrated in lower volume and safety critical applications such as the aeronautical market, and if some GPS users experienced a permanent loss of accuracy, then there could be additional indirect costs to consumers (e.g. reduced crop yields leading to higher food costs). Just to give one example, the National Association of Wheat Growers indicated that its members have invested $3B in GPS equipment for precision farming, in order to increase the productivity and efficiency of farm processes.

Thus it is more credible to look at the total cost impact on manufacturers and consumers as being of order $1B+ per year over the next decade, as tens of billions of dollars of equipment needs to be upgraded or replaced. As in the Inmarsat situation, PRTM’s estimate of the filter hardware costs (for what was in any case only a subset of the overall GPS equipment market) likely represents no more than 1% of the total bill. Given that such a large cost impact might well outweigh the value of freeing up additional L-band spectrum, it would be very interesting to see a detailed cost-benefit analysis of these issues, so that economic rationality can play some part in the ultimate decision.

Nevertheless, despite the significant cost impact on the GPS industry and end users, the FCC might still decide to impose “receiver standards” on future GPS devices. However, it would still take considerable time before these standards became effective. For example, the FCC could easily take 12-18 months (or longer) to decide on what receiver standards to mandate, and then it might require that all new GPS receivers manufactured after say the end of 2014 were capable of withstanding potential interference. Then there would need to be several more years for older devices to be replaced or updated, with a sunset date perhaps as late as the end of 2018 or 2019 (or beyond).

Assuming that this is the path the FCC decides to follow, it is still unclear what spectrum LightSquared would then be able to use for its network in the near term. The Phase 0 spectrum which is currently available to LightSquared has its downlink between 1550 and 1555MHz, which is the channel closest to the GPS band, and so its use would likely be heavily restricted or completely prohibited until GPS receiver standards came into force. LightSquared gains access to an additional 2x5MHz channel (the Phase 1A spectrum) sometime between February and November 2012, depending on how quickly this is cleared by Inmarsat. This channel is at the bottom end of the band (1526-1531MHz downlink) and so is the least likely to interfere with GPS. LightSquared then adds 2x10MHz of additional spectrum (Phase 2) with downlinks at 1531-1536MHz and 1545-1550MHz at the end of July 2013. However, it is uncertain whether and under what conditions the use of the 1545-1550MHz band would be permitted before any receiver standards came into force.

This timeline indicates that (if Channel 1 is usable) LightSquared should have access to 2x5MHz of spectrum sometime in 2012 and at least 2x10MHz of spectrum from the end of July 2013. However, it is far from clear (even assuming LightSquared has a network contractor in place) how the company expects to offer service by the end of 2011.

More broadly, the outcome of the GPS interference testing and FCC deliberations also remains in doubt. PRTM characterized this as “a situation where the neighbor [GPS] built the fence too far over the property line and may not have realised it at the time. Now the other neighbor wants to build a pool and there is not enough space. So the question is: who has to pay to move the fence?”. However, I look at the analogy somewhat differently – regardless of where the fence is, if you have protected butterflies [defense and aviation systems] living at the bottom of both gardens, will the government let you build a pool at all?

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